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MATHEMATICAL 
KECREATIONS AND ESSAYS 



MACMILLAN AND CO., Limited 

LONDON • BOMBAY • CALCUTTA • MADRAS 
MELBOURNE 

THE MACMILLAN COMPANY 

NEW YORK • BOSTON • CHICAGO 
DALLAS • SAN FRANCISCO 

THE MACMILLAN CO. OF CANADA, Ltd. 

TORONTO 



MATHEMATICAL 
RECREATIONS AND ESSAYS 



BY 

W. W ROUSE BALL, 

FELLOW OF TRINITY COLLEGE, CAMBRIDGE. 



SEVENTH EDITION. 



MACMILLAN AND CO., LIMITED 

ST MARTIN'S STREET, LONDON 

1917 

[All rights reserved] 



Copyright 

First Edition, February, 1892. 
Second Edition, May, 1892. 
Third Edition, 1896. 
Fourth Edition, 1905. 
Fifth Edition, 1911. 
Sixth Edition, 1914. 
Seventh Edition, 1917. 



GIFT 

J 



63 
19/7 



PREFACE. 

rpHE earlier part of this book contains an account of certain 
-*- Mathematical Recreations : this is followed by some Essays 
on subjects most of which are directly concerned with historical 
mathematical problems. I hasten to add that the conclusions 
are of no practical use, and that most of the results are not 
new. If therefore the reader proceeds further he is at least fore- 
warned. At the same time I think I may say that many of the 
questions discussed are interesting, not a few are associated 
with the names of distinguished mathematicians, while hitherto 
several of the memoirs quoted have not been easily accessible 
to English readers. A great deal of new matter has been added 
since the work was first issued in 1892, but insertions made 
since 1911, when the book was stereo tjrped, have had to be 
placed where room for them could best be found. 

The book is divided into two parts, but in both parts 
I have excluded questions which involve advanced mathematics. 

The First Part now consists of ten chapters, in which are 
described various problems and amusements of the kind usually 
termed Mathematical Recreations. Several of the questions 
mentioned in the first five chapters are of a somewhat trivial 
character, and had they been treated in any standard English 
work to which I could have referred the reader, I should have 
left them out: in the absence of such a work, I thought it 
better to insert them and trust to the judicious reader to 
omit them altogether or to skim them as he feels inclined. 
I may add that in discussing problems where the complete 

ivi855y49 



VI PREFACE 

solutions are long or intricate I have been generally content 
to indicate memoirs or books in which the methods are set 
out at length, and to give a few illustrative examples. In 
several cases I have also stated problems which still await 
solution. 

The Second Part now consists of twelve chapters, mostly 
dealing with Histoncal Questions. To make room for my dis- 
cussion of Calculating Prodigies and Arithmetical Calculating 
Machines I have, in this (the seventh) edition, cut out the 
essay on the History of the Mathematical Tripos. It is with 
some hesitation that I have inserted papers on String Figures, 
Astrology, and Ciphers, but I think they may be interesting to 
my readers, even though the subjects are only indirectly con- 
nected with Mathematics. 

I have inserted detailed references, as far as I know them, 
to the sources of the various questions and solutions given; 
also, wherever I have given only the result of a theorem, 
I have tried to indicate authorities where a proof may be found. 
In general, unless it is stated otherwise, I have taken the 
references direct from the original works ; but, in spite of 
considerable time spent in verifying them, I dare not suppose 
that they are free from all errors or misprints. I shall be 
grateful for notices of additions or corrections which may occur 
to any of lAy readers. 

W. W. ROUSE BALL. 

Trinity College, Cambridge. 
Juli/, 1917. 



Vll 



TABLE OF CONTENTS. 
PART I. 

iMatbematiral »ecreaticinsf* 

Chapter I. Arithmetical Recreations. 

PAGE 

Baciet. Ozanam. Montucla 2 

Elenentary Questions on Numbers 8 

Determination of a number selected by someone .... 4 

Prediction of the result of certain operations .... 7 

Problems involving two numbers 10 

Miscellaneous Problems 12 

Probems with a Series of Numbered Things 14 

Medival Problems 18 

The osephus Problem. Decimation 23 

Addedum on Solutions 27 

Chapter II. Arithmetical Recreations (Continued). 

Arithietical Fallacies c ... 28 

Paraoxical Problems 31 

Permtation Problems 32 

Bachk's Weights Problem . .34 

Prob'jms in Higher Arithmetic 36 

Primes. Mersenne's Numbers. Perfect Numbers ... 37 

Unsolved Theorems by Euler, Goldbach, Lagrange ... 39 

Fermat's Theorem on Binary Powers 39 

Fermat's Last Theorem 40 



VIU TABLE OF CONTENTS 



Chapter III. Geometrical Recreations. 

PAGK 

Geometrical Fallacies. 44 

Paradoxical Problems . . * 52 

Map-Colouring Theorem 54 

Physical Configuration of a Country 59 

Addendum on a Solution . . . . . <1 

Chapter IV. Geometrical Recreations {Continued). 

Statical Games of Position .62 

Three-in-a-row. Extension to ^-in-a-row 62 

Tesselation. Anallagmatic Problems 64 

Colour- Cube Problem 67 

Tangrams 69 

Dynamical Games of Position . 69 

Shunting Problems 69 

Ferry-Boat Problems 71 

Geodesic Problems 73 

Problems with Counters or Pawns 74 

Geometrical Puzzles with Rods '. . 80 

Paradromic Kings 80 

Addendum on Solutions • • • 81 

Chapter V. Mechanical Recreations. I 

Paradoxes on Motion 84 

Laws of Motion 87 

Force, Inertia. Centrifugal Force .87 

Work. Stability of Equilibrium 89 

Perpetual Motion 93 

Models 97 

Sailing quicker than the wind 98 

Boat Moved by a Rope inside the Boat 101 

Hauksbee's Law 101 

Cut on Tennis-Balis, Cricket-Balls, Golf-Balls 103 

Flight of Birds 106 

Curiosa Physica .•• 107 

Chapter VI. Chess-Board Recreations. 



Chess-Board Notation 
Relative Value of the Pieces 
The Eight Queens Problem 
Maximum Pieces Problem . 
Minimum Pieces Problem . 
Analogous Problems 



109 
110 
113 

119 
119 
121 



TABLE OF CONTENTS It 

PAGE 

Ke-Entrant Paths on a Chess-Board 122 

Knight's Ee-Entiant Path 122 

King's Ee-Entrant Path 133 

Rook's Re-Entrant Path 133 

Bishop's Re-Entrant Path 134 

Miscellaneous Problems 134 

Chapter VII. Magic Squares. 

Notes on the History of Magic Squares 138 

Construction of Odd Magic Squares 139 

Method of De la Loub^re and Bachet 140 

Method of De la Hire 142 

Construction of Even Magic Squares . . . . * . . . 144 

First Method 145 

Method of De la Hire and Labosne 149 

Composite Magic Squares 162 

Bordered Magic Squares 152 

Magic Squares involving products 154 

Magic Stars, etc 154 

Unsolved Problems 155 

Hyper-Magic Squares 156 

Pan-Diagonal Squares 156 

Symmetrical Squares 162 

Doubly-Magic and Trebly-Magic Squares 163 

Magic Pencils 163 

Magic Puzzles 166 

Magic Card Square. Euler's Officers Problem .... 166 

Reversible Square 167 

Domino Squares . , 168 

Coin Squares 169 

Addendum on a Solution 169 

Chapter VIII. Unicursal Problems. 

Euler's Problem 170 

Definitions 172 

Euler's Results ... 172 

Illustrations 17G 

Number of Ways in which a Unicursal Figure can be described . . 177 

Mazes 182 

Rules for traversing a Maze 183 

The History of Mazes 183 

Examples of Mazes 186 

Geometrical Trees 188 

The Hamiltouian Game . . - 189 



TABLE OP CONTENTS 



Chapter IX. Kirkman's School-Girls Problem. 

PAGK 

History of the Problem 193 

Solutions by One-Step Cycles 195 

Examples when w=3, 9, 27, 33, 51, 57, 75, 81, 99 . . . 196 

Solutions by Two-Step Cycles 199 

Examples when n=15, 27, 39, 51, 63, 75, 87, 99 . . . 200 

Solutions by Three-Step Cycles 203 

Examples n = 9, 21, 27, 33, 39, 45, 51, 57, 63, 69, 75, 81, 87, 92, 99 205 

Solutions by the Focal Method 209 

Examples when n = 33, 51 . 210 

Analytical Methods 211 

Application when n = 27 with 13 as base 212 

Examples when n = 15, 39 216 

Number of Solutions 217 

Harison's Theorem 218 

Problem of n' Girls in n groups (Peirce) 219 

Examples when w= 2, 3, 4, 5, 7, 8 219 

Example when n is prime 220 

Kirkman's Problem in Quartets, <fec . 221 

A Bridge Problem. Arrangements in Pairs ....'.. 221 

Sylvester's Corollary to Kirkman's Problem 222 

Steiner's Combinatorische Aufgabe 223 



Chapter X. Miscellaneous Problems. 

The Fifteen Puzzle 224 

The Tower of Hanoi 228 

Chinese Rings 229 

Algebraic Solution 230 

Solution in Binary Scale of Notation 232 

Problems connected with a Pack of Cards 234 

Shuffling a Pack 235 

Arrangements by Rows and Columns 237 

Bachet's Problem with Pairs of Cards 238 

The Three Pile Problem 240 

Gergonne's Generalization 241 

The Mouse Trap. Treize 245 



TABLE OF CONTENTS XI 



PART 11. 
Chapter XI. Calculating Prodigies. 

PAGE 

Calculating Prodigies. Authorities 248 

John Wallis, 1616-1703 249 

Buxton, circ. 1707-1772 249 

Problems solved by 250 

Methods of .251 

Fuller, 1710-1790 252 

Ampere, Gauss, Whately 262 

Colburn, 1804-1840 253 

Problems solved by 253 

Power of Factorizing Numbers 254 

Bidder, 1806-1878 255 

Career of 255 

Problems solved by 256 

Bidder's Relations 259 

Mondeux, Mangiamele 260 

Dase, 1824-1861 260 

Problems solved by 261 

Scientific Work of 262 

Safford, 1836-1901 262 

Zamebone, Diamandi, Eiickle 268 

Inaudi, 18G7- 263 

Problems solved by 264 

Expression of Numbers by Four Squares 2G4 

Nature of Public Performances 264 

Types of Memory of Numbers 265 

Bidder's Analysis of Methods used 266 

Preferably only one step at a time 267 

Multiplication. Examples of 267 

Division. Digit-Terminals 269 

Digital Method for Division and Factors 269 

Square Roots. Higher Roots 270 

Compound Interest 272 

Logarithms 273 

Suggested Law of Rapidity of Calculation 274 

Requisites for Success 275 

Chapter XII. Arithmetical Calculating Machines. 

Modern Machines for Elementary Processes 276 

Addition Machines, Types of 277 

Totalisers 279 



Xll TABLE OF CONTENTS 

PAGE 

Subtraction worked by Addition Machines 280 

Multiplication and Division 280 

Steiger's and Hamann's Instruments 281 

Typewriters combined with Addition Machines 282 

Inventions of Pascal, Leibnitz, Babbage 282 



Chapter XIII. Three Classical Geometrical Problems. 

Statement of the Problems 284 

The Duplication of the Cube. Legendary Origin of the Problem . . 285 

Hippocrates's Lemma 287 

Solutions by Archytas, Plato, Menaechmus, ApoUonius, and Diodes . 287 

Solutions by Vieta, Descartes, Gregory of St Vincent, and Newton . 290 

The Trisection of an Angle 291 

Ancient Solutions quoted by Pappus ....... 291 

Solutions by Pappus, Descartes, Newton, Clairaut, and Chasles . . 292 

The Quadrature of the Circle , . 293 

Incommensurability of ir . . . . 294 

Definitions of tt .295 

Origin of symbol tt 296 

Methods of approximating to the numerical value of ir. . . . 296 

Geometrical methods of Approximation ....... 296 

Besults of Egyptians, Babylonians, Jews 297 

Eesults of Archimedes and other Greek Writers .... 297 

Eesults of Eoman Surveyors and Gerbert 298 

Eesults of Indian and Eastern Writers 298 

Eesults of European Writers, 1200-1630 300 

Theorems of Wallis and Brouncker 302 

Analytical methods of Approximation. Gregory's series . . . 303 

Eesults of European Writers, 1699-1873 303 

Geometrical Approximation 305 

Approximations by the Theory of Probability 305 



Chapter XIV. The Parallel Postulate. 

The Problem 307 

The Earliest Proof. Thales , 308 

Pascal's Proof 308 

Pythagorean and Euclidean Proof 310 

Features of the Problem 311 

Ptolemy's Proof of the Postulate 313 

Proclus's Proof of the Postulate 314 

Wallis's Proof of the Postulate .314 

Bertrand's Proof of the Postulate 315 



TABLE OF CONTENTS Xlll 

PAGE 

Play fair's Proof of the Postulate 316 

Playfair's Eotational Proof of the Proposition . . . , . 317 

Legendre's First Proof of the Proposition . , '. , . . 318 

Legendre's Analytical Proof of the Proposition 318 

Lagrange's Memoir . 320 

Other Parallel Postulates 320 

Definitions of Parallels 322 

Non-Euclidean Systems 323 

Comparison of Elliptic, Parabolic and Hyperbolic Geometries . 325 

Chapter XV. Insolubility op the Algebraic Quintic. 

The Problem 827 

Lagrange's Examination of Solutions of Algebraic Equations , . 327 

Demonstration that the Quintic is Insoluble. Abel. Galois . . 329 

Order of the Badicals in any Solution 330 

Chapter XYI. Mersenne's Numbers. 

Mersenne's Enunciation of the Question . . . . . . 333 

Kesults already known 334 

Cases awaiting verification 335 

Table of Besults 336 

History of Investigations. . 337 

Methods used in attacking the Problem 340 

Lucas's Test by Residues ........ 340 

By trial of divisors of known forms 341 

By indeterminate equations 343 

By properties of quadratic forma 344 

By the use of a Canon Arithmeticus 344 

By properties of binary powers 345 

By the use of the binary scale of notation . . . . . 346 

By the use of Format's Theorem ...... 346 

Mechanical methods of Factorizing Numbers 847 

Chapter XVTI. String Figures, 

Two Types of String Figures or Cat's-Cradles 348 

Terminology. Rivers and Haddon 349 

European Varieties of Cat's-Cradles 350 

The Cradle 351 

Snuffer-Trays 352 

Cat's-Eye 352 

Fish-in-a-Dish 353 

Pound-of-Candles, Hammock, Lattice- Work, Other Forms . . 354 

Sequence of Forms 356 



XIV TABLE OF CONTENTS 

PAGE 

Oceanic Varieties of Cat's- Cradles 357 

Openings A and B. Movement T 357 

A Door 858 

Climbing a Tree 359 

Throwing the Spear 360 

Triple Diamonds 361 

Quadruple Diamonds 362 

Multiple Diamonds 363 

Many Stars 364 

Owls .365 

Single Stars. W. W .366 

The Setting Sun 367 

The Head Hunters 368 

The Parrot Cage 368 

See-Saw 369 

Lightning 369 

A Butterfly 370 

String Tricks 371 

The Lizard Trick 372 

The Caroline Catch 372 

Threading the Needle 373 

The Yam Thief or Mouse Trick 374 

Cheating the Halter 374 

The Fly on the Nose 375 

The Hand-Cuff Trick 375 

The Elusive Loop 376 

The Button-Hole Trick 377 

The Loop Trick 378 

The Waistcoat Puzzle . 378 

Knots, Lashings 379 



Chapter XVIII. Astrology. 

Astrology. An Ancient Art 380 

Two Branches of Medieval Astrology: Natal and Horary* . . 381 

Bales for Casting and Beading a Horoscope 381 

Houses and their Significations 381 

Planets and their Significations 384 

Zodiacal Signs and their Significations 386 

Knowledge that rules were worthless 388 

Notable Lastances of Horoscopy 389 

Lilly's prediction of the Great Fire and Plague .... 390 

Flamsteed's guess 390 

Cardan's Horoscope of Edward VI 391 



TABLE OF CONTENTS XV 
Chapter XIX. Cryptographs and Ciphers. 

PAGE 

Authorities 395 

Cryptographs. DeiBnition. Illustration 395 

Ciphers. Definition. Illustration 396 

Essential Features of Cryptographs and Ciphers 397 

Cryptographs of Three Types 397 

Transposition Type. The Eoute Method 397 

Use of non-significant symbols. The Grille .... 400 

Use of broken symbols. The Scytale 402 

Ciphers. Use of arbitrary symbols unnecessary 403 

Ciphers of Four Types 403 

Substitution Alphabets. Illustrations 403 

Conrad's Tables of Frequency 405 

Ciphers of the Second Type. Illustrations 406 

Shifting Alphabets. Illustrations 409 

Gronfeld's and the St Cyr Methods 409 

The Playfair Cipher 411 

Ciphers of the Fourth Type. Illustrations 412 

Desiderata in a good Cipher or Cryptograph 413 

Cipher Machines 414 

On the Solution of Cryptographs and Ciphers 414 

Example 415 

Historical Ciphers 418 

Julius Caesar, Augustus, Bacon, Morse Code .... 418 

Charles I 419 

Pepys 420 

De Rohan, Marie Antoinette 421 

The Five Digit Code Dictionary 421 

Chapter XX. Hyper-Spacb. 

Authorities 424 

Two subjects of speculation ...» 425 

Space of Two Dimensions 425 

Space of One Dimension 426 

Space of Four Dimensions 426 

Existence in such a world 427 

Arguments in favour of the existence of such a world . . 428 

Use of, to explain physical phenomena 430 

Non-Euclidean Geometries 433 

Euclid's Axioms and Postulates. The Parallel Postulate . . . 433 

Hyperbolic and Elliptic Geometries of Two Dimensions , . . 433 

Non-Euclidean Geometries of Three or more Dimensions . . . 435 

Non-Archimedian Geometries of Two Dimensions 436 

Non-Legendrian Geometry 436 

Semi-Euclidean Geometry 436 

Truth of Axioms , 437 



xvi TABLE OF CONTENTS 

Chapter XXI. Time and its Measurement. 

PAOE 

Difficulty of the Subject 438 

Simultaneity 438 

Units for measuring Durations (days, weeks, months, years) . . 439 

The Civil Calendar (Julian, Gregorian, &c.) 443 

The Ecclesiastical Calendar (Date of Easter) 446 

Day of the Week corresponding to a given date 449 

Means of measuring Time 450 

Styles, Sun-dials, Sun-rings 450 

Water-clocks, Sand-clocks, Graduated Candles .... 453 

Clocks 454 

Watches •. . . 455 

Watches as Compasses 457 

Chapter XXII. Matter and Ether Theories. 

Subject of Chapter. Views formerly held about Matter , . . 459 

Hypothesis of Continuous Matter 460 

Atomic Theories ' . . 460 

Popular Atomic Hypothesis ...,.,,. 460 

Boscovich's Hypothesis 461 

Hypothesis of an Elastic Solid Ether. Labile Ether . . . 462 

Dynamical Theories 462 

The Vortex King Hypothesis .463 

The Vortex Sponge Hypothesis 464 

The Ether-Squirts Hypothesis 465 

The Electron Hypothesis 465 

Speculations due to investigations on Kadio-Activity . . . 466 

Le Bon's Suggestion 468 

Mendelejev's Hypothesis 468 

The Bubble Hypothesis . . .469 

Eequisites in a Theory 470 

Conjectures as to the Cause of Gravity 471 

Conjectures to explain the finite number of species of Atoms . , 475 

Size of the Molecules of Bodies 477 

Mass of a Body affected by its Motion . 480 

Theory of Relativity 480 

Index 483 

KoiicBS OF Books 493 



p 



PAET L 
iMatfefmatical ^Retreatwnsi* 

" Les hommes ne sont jamais plus ingenieux que dans V invention 
desjeux; V esprit s'y trouve h son aise....Apres lesjeux qui dependent 
uniquement des nomhres viennent les jeux ou entre la situation — 
Apres les jevM oil n^entrent que le nombre et la situation viendraient 
les jeux oib entre le mouvement....Enjin il serait a souhaiter qu^on 
eUt un cours entier des jeuXf traites mathematiquement." (Leibnitz: 
letter to De Montmort, July 29, 1715.) 



B. B. 



CHAPTER I. 

ARITHMETICAL RECREATIONS. 

I commence by describing some arithmetical recreations. 
The interest excited by statements of the relations between 
numbers of certain forms has been often remarked, and the 
majority of works on mathematical recreations include several 
such problems, which, though obvious to any one acquainted 
with the elements of algebra, have to many who are ignorant 
of that subject the same kind of charm that mathematicians 
find in the more recondite propositions of higher arithmetic. 
I devote the bulk of this chapter to these elementary 
problems. 

Before entering on the subject, I may add .that a large 
proportion of the elementary questions mentioned here are 
taken from one of two sources. The first of these is the classical 
Prohlemes plaisans et d^lectables, by Claude Caspar Bachet, 
sieur de Meziriac, of which the first edition was published in 
1612 and the second in 1624 : it is to the edition of 1624 that 
the references hereafter given apply. Several of Bachet's 
problems are taken from the writings of Alcuin, Pacioli di 
Burgo, Tartaglia, and Cardan, and possibly some of them are 
of oriental origin, but I have made no attempt to add such 
references. The other source to which I alluded above is 
Czanam's Bdcreations mathematiques et physiques. The greater 
portion of the original edition, published in two volumes at 
Paris in 1694, was a compilation from the works of Bachet, 
Mydorge, and Leurechon : this part is excellent, but the same 
cannot be said of the additions due to Ozanam. In the 
Biographie Universelle allusion is made to subsequent editions 



CH. l] ARITHMETICAL RECREATIONS 3 

issued in 1720, 1735, 1741, 1778, and 1790; doubtless these 
references are correct, but the following editions, all of which 
I have seen, are the only ones of which I have any knowledge. 
In 1696 an edition was issued at Amsterdam. In 1723 — six 
years after the death of Ozanam — one was issued in three 
volumes, with a supplementary fourth volume, containing, 
among other things, an appendix on puzzles. Fresh editions 
were issued in 1741, 1750 (the second volume of which bears 
the date 1749), 1770, and 1790. The edition of 1750 is said to 
have been corrected by Montucla on condition that his name 
should not be associated with it ; but the edition of 1790 is 
the earliest one in which reference is made to these corrections, 
though the editor is referred to only as Monsieur M***. 
Montucla expunged most of what was actually incorrect in the 
older editions, and added several historical notes, but un- 
fortunately his scruples prevented him from striking out the 
accounts of numerous trivial experiments and truisms which 
overload the work. An English translation of the original 
edition appeared in 1708, and I believe ran through four 
editions, the last of them being published in Dublin in 1790. 
Montucla's revision of 1790 was translated by C. Hutton, and 
editions of this were issued in 1803, in 1814, and (in one 
volume) in 1840 : my references are to the editions of 1803 
and 1840. 

I proceed to enumerate some of the elementary questions 
connected with numbers which for nearly three centuries 
have formed a large part of most compilations of mathe- 
matical amusements. They are given here largely for their 
historical — not for their arithmetical — interest ; and perhaps a 
mathematician may well omit this chapter. 

Many of these questions are of the nature of tricks or puzzles, 
and I follow the usual course and present them in that form. 
I may note however that most of them are not worth proposing, 
even as tricks, unless either the method employed is disguised 
or the result arrived at is different from that expected ; but, as 
I am not writing on conjuring, I refrain from alluding to the 

1—2 



4 ARITHMETICAL RECREATIONS [CH. 1 

means of disguising the operations indicated, and give merely 
a bare enumeration of the steps essential to the success of the 
method used, though I may recall the fundamental rule that 
no trick, however good, will bear immediate repetition, and that, 
if it is necessary to appear to repeat it, a different method of 
obtaining the result should be used. 

To FIND A NUMBER SELECTED BY SOME ONE. There are 
innumerable ways of finding a number chosen by some one, 
provided the result of certain operations on it is known. I 
confine myself to methods typical of those commonly used. 
Any one acquainted with algebra will find no difficulty in 
framing new rules of an analogous nature. 

First Method*, (i) Ask the person who has chosen the 
number to treble it. (ii) Enquire if the product is even or 
odd : if it is even, request him to take half of it ; if it is odd, 
request him to add unity to it and then to take Jialf of it. 
(iii) Tell him to multiply the result of the second step by 8. 
(iv) Ask how many integral times 9 divides into the latter 
product: suppose the answer to be n. (v) Then the number 
thought of was 2n or 2^ + 1, according as the result of step (i) 
was even or odd. 

The demonstration is obvious. Every even number is of 
the form 2?^, and the successive operations applied to this give 
(i) Qn, which is even; (ii) J6w = 3n; (iii) 3x3?i = 9?i; (iv) 
^^n=n\ (v) 2n. Every odd number is of the form In + 1, and 
the successive operations applied to this give (i) 67i + 3, which 
is odd; (ii) J(6n + 3 + 1) = StiH- 2 ; (iii) 3(3/i+ 2) = 9/i + 6; 
(iv) "I (9^1 + 6) = ri + a remainder ; (v) 2n + 1. These results 
lead to the rule given above. 

Second Method^, Ask the person who has chosen the 
number to perform in succession the following operations, 
(i) To multiply the number by 5. (ii) To add 6 to the product, 
(iii) To multiply the sum by 4. (iv) To add 9 to the product, 
(v) To multiply the sum by 5. Ask to be told the result of 
the last operation: if from this product 165 is subtracted, and 

* Bacbet, ProhUmes, Lyons, 1624, problem i, p. 53. 
t A similar rule was given by Bachet, problem iv, p. 71. 



CH. l] ARITHMETICAL RECREATIONS 5 

then the remainder is divided by 100, the quotient will be the 
number thought of originally. 

For let n be the number selected. Then the successive 
operations applied to it give (i) 5n ; (ii) 5rH- 6 ; (iii) 20n 4- 24 ; 
(iv) 2071 + 38 , (v) lOOw + 165. Hence the rule. 

Third Method*. Request the person who has thought of 
the number to perform the following operations, (i) To 
multiply it by any number you like, say, a. (ii) To divide the 
product by any number, say, h. (iii) To multiply the quotient 
by c. (iv) To divide this result by d. (v) To divide the final 
result by the number selected originally, (vi) To add to the 
result of operation (v) the number thought of at first. Ask for 
the sum so found : then, if ac/hd is subtracted from this sum, 
the remainder will be the number chosen originally. 

For, if n was the number selected, the result of the first four 
operations is to form nac/bd; operation (v) gives ac/bd; and 
(vi) gives n + (ac/bd\ which number is mentioned. But ac/bd 
is known; hence, subtracting it from the number mentioned, 
n is found. Of course a, b, c, d may have any numerical values 
it is liked to assign to them. For example, if a— 12, 6 = 4, 
c = 7, d — S it is sufficient to subtract 7 from the final result in 
order to obtain the number originally selected. 

Fourth Method f. Ask some one to select a number less 
than 90. Request him to perform the following operations, 
(i) To multiply it by 10, and to add any number he pleases, 
a, which is less than 10. (ii) To divide the result of step (i) 
by 3, and to mention the remainder, say, b. (iii) To multiply 
the quotient obtained in step (ii) by 10, and to add any number 
he pleases, c, which is less than 10. (iv) To divide the result 
of step (iii) by 3, and to mention the remainder, say d, and 
the third digit (from the right) of the quotient; suppose 
this digit is e. Then, if the numbers a, b, c, d, e are known, 
the original number can be at once determined. In fact, if 
the number is 9x -f y, where a? :[> 9 and yif^> ^^^ if ^ is the 



* Bachet, problem v, p. 80. 

t Educational Times, London, May 1, 1895, vol. xi.viii, p. 234. 



6 ARITHMETICAL RECREATIONS [CH. I 

remainder when a — b + ^ (c — d) is divided by 9, we have 
x = e, 2/ = 9 — r. 

The demonstration is not diflBcult. Suppose the selected num- 
ber is 9^7+3/. Step (i) gives 90a7 + 10i/4-a. Let y -i- a = Sn -\- b, 
then the quotient obtained in step (ii) is SOx -^ Sy -^ n. Step 
(iii) gives SOOa; + 30y + lOn + c. Let n + c = 3m + d, then the 
quotient obtained in step (iv) is 100^ + lOy -\-Sn-\-m, which I 
will denote by Q. Now the third digit in Q must be x, because, 
since y :\^ 8 and a :)► 9, we have ti :[> 5 ; and since n :|> 5 and c :^ 9, 
we have m :|> 4 ; therefore lOy + 3n + m :^ 99. Hence the third 
or hundreds digit in Q is a?. 

Again, from the relations y-\-a = Sn-\-b and n + c = 3m + d, 
we have 9m — y = a — b-bS(c — d) : hence, if r is the remainder 
when a — b + S(c — d) is divided by 9, we have y = 9 — r. [This 
is always true, if we make r positive ; but if a — b-\-S(c — d) 
is negative, it is simpler to take y as equal to its numerical 
value ; or we may prevent the occurrence of this case by 
assigning proper values to a and c] Thus x and y are both 
known, and therefore the number selected, namely dx + y, is 
known. 

Fifth Method*. Ask any one to select a number less 
than 60. Request him to perform the following operations, 
(i) To divide it by 3 and mention the remainder ; suppose it 
to be a. (ii) To divide it by 4, and mention the remainder; 
suppose it to be b. (iii) To divide it by 5, and mention the 
remainder; suppose it to be c. Then the number selected is 
the remainder obtained by dividing 40a. + 456 + 36c by 60. 

This method can be generalized and then will apply to any 
number chosen. Let a', b', c\ ... be a series of numbers prime 
to one another, and let p be their product. Let n be any 
number less than p, and let a, 6, c, ... be the remainders 
when n is divided by a, b\ c', ... respectively. Find a number 
A which is a multiple of the product b'c'd^ . . . and which 
exceeds by unity a multiple of a'. Find a number B which is 
a multiple of obdd' ... and which exceeds by unity a multiple 

* Bachet, problem vi, p. 84 : Bachet added, on p. 87, a note on the previous 
history of the problem. 



CH. l] ARITHMETICAL RECREATIONS 7 

of b', and similarly find analogous numbers G, D, .... Rules 
for the calculation of A, B, G,... are given in the theory of 
numbers, but in general, if the numbers a\ b', c', ... are small, 
the corresponding numbers A, B, C, ... can be found by in- 
spection. I proceed to show that n is equal to the remainder 
when Aa-\- Bh+ Gc-\- ... is divided by p. 

Let N = Aa -\- Bb + Gc -{- ..., and let M (x) stand for a 
multiple of x. Now A = M(a') + 1, therefore Aa = M{a') + a. 
Hence, if the first term in N, that is Aa, is divided by a\ the 
remainder is a. Again, jB is a multiple of a'c'df .... Therefore 
Bb is exactly divisible by a Similarly Gc, Dd,... are each 
exactly divisible by a'. Thus every term in j^^^, except the first, 
is exactly divisible by a'. Hence, if N is divided by a\ the 
remainder is a. Also if n is divided by a', the remainder is a. 

Therefore N—n=^M (a). 

Similarly N-n = M{b'), 



But a\ b\ c ,... are prime to one another. 

.-. N -n = M {a'b'c' ...)=-- M {p), 
that is, N=M (p) + n. 

Now n is less than p, hence if N is divided by p, the 
remainder is n. 

The rule given by Bachet corresponds to the case of a' = 3, 
6' = 4, c'=5, p = 60, ^ = 40, B = 4>5, 6' =36. If the number 
chosen is less than 420, we may take a' = 3, b' = 4>, c' = 5, d' = 7, 
p = 420, A = 280, B = 105, G = 336, D = 120. 

To FIND THE RESULT OF A SERIES OF OPERATIONS PER- 
FORMED ON ANY NUMBER (unknown to the operator) without 
ASKING ANY QUESTIONS. All rules for solving such problems 
ultimately depend on so arranging the operations that the 
number disappears from the final result. Four examples will 
suffice. 

First Example*. Request some one to think of a number. 
Suppose it to be n. Ask him (i) to multiply it by any number 
you please, say, a ; (ii) then to add, say, b ; (iii) then to divide 
* Bachet, problem viii, p 102. 



8 ARITHMETICAL RECREATIONS [CH. I 

the sum by, say, c. (iv) Next, tell him to take ajc of the 
number originally chosen; and (v) to subtract this from the 
result of the third operation. The result of the first three 
operations is {na + 6)/c, and the result of operation (iv) is najc : 
the difference between these is hjc, and therefore is known to 
you. For example, if a = 6, 6=12, c = 4, then ajc = IJ, and the 
final result is 3. 

Second Example^. Ask A to take any number of counters 
that he pleases : suppose that he takes n counters, (i) Ask 
some one else, say B, to take p times as many, where p is 
any number you like to choose, (ii) Request A to give q of 
his counters to B, where q is any number you like to select, 
(iii) Next, ask B to transfer to A a number of counters equal 
to p times as many counters as A has in his possession. Then 
there will remain in B's hands q{p + l) counters: this number 
is known to you; and the trick can be finished either by 
mentioning it or in any other way you like. 

The reason is as follows. The result of operation (ii) is 
that B has pn + q counters, and A has n — q counters. The 
result of (iii) is that B transfers p{n-q) counters to A : hence 
he has left in his possession {pn + q) — p(n — q) counters, that 
is, he has q(p-\- 1). 

For example, if originally A took any number of counters, 
then (if you chose p equal to 2), first you would ask B to take 
twice as many counters as A had done ; next (if you chose q 
equal to 3) you would ask A to give 3 counters to B ; and then 
you would ask B to give to JL a number of counters equal to 
twice the number then in A's possession ; after this was done 
you would know that B had 3(2 + 1), that is, 9 left. 

This trick (as also some of the following problems) may be 
performed equally well with one person, in which case A may 
stand for his right hand and B for his left hand. 

Third Example. Ask some one to perform in succession 
the following operations, (i) Take any number of three digits, 
in which the difference between the first and last digits exceeds 
unity, (ii) Form a new number by reversing the order of 

* Bacliet, problem xiii, p. 123 : Bachet presented the above trick in a form, 
somewhat more general, but less effective in practice. 



CH. l] ARITHMETICAL RECREATIONS 9 

the digits, (iii) Find the difference of these two numbers, 
(iv) Form another number by reversing the order of the digits 
in this difference, (v) Add together the results of (iii) and (iv). 
Then the sum obtained as the result of this last operation will 
be 1089. 

An illustration and the explanation of the rule are given 
below. 

100a + 106 + c 
100c + 10b+a 



(i) 


237 


(ii) 
(iii) 


732 
495 


(iv) 


594 


(v) 


1089 



lOO(a-c-l) + 90 + (10 + c-a) 
100(10 + c-a)+ 90-i-(a-c-l) 
900 +180 + 9 



The result depends only on the radix of the scale of notation in 
which the number is expressed. If this radix is r, the result is 
(r - 1) (r + 1)^ ; thus if r = 10, the result is 9 x ll^ that is, 1089. 

Fourth Example*. The following trick depends on the 
same principle. Ask some one to perform in succession the 
following operations, (i) To write down any sum of money 
less than £12, in which the difference between the number of 
pounds and the number of pence exceeds unity, (ii) To 
reverse this sum, that is, to write down a sum of money ob- 
tained from it by interchanging the numbers of pounds and 
pence, (iii) To find the difference between the results of 
(i) and (ii). (iv) To reverse this difference, (v) To add to- 
gether the results of (iii) and (iv). Then this sum will be 
£12. 185. lid. 

For instance, take the sum £10. 17^. od.] we have 

£ s. d. 

(i) 10 17 5 

(ii) 5 17 10 

(iii) 4 19 7 

(iv) 7 19 4 

(v) 12 18 11 

• Educational Times BeprinUf 1890, vol. liii, p. 78, 



10 ARITHMETICAL RECREATIONS [CH. I 

The following analysis explains the rule, and shows that the 
final result is independent of the sum written down initially. 

£ 8. d. 

(i) a b c 

(ii) c h a 

(iii) a-c- 1 19 c-a + 12 

(iv) c~a+12 19 a-c- 1 

(v) 11 38 11 

Mr J. H. Schooling has used this result as the foundation 
of a slight but excellent conjuring trick. The rule can be 
generalized to cover any system of monetary units. 

Problems involving Two Numbers. I proceed next to 
give a couple of examples of a class of problems which involve 
two numbers. 

First Example*. Suppose that there are two numbers, one 
even and the other odd, and that a person A is asked to select 
one of them, and that another person B takes the other. It is 
desired to know whether A selected the even or the odd number. 
Ask A to multiply his number by 2, or any even number, and B 
to multiply his by 3, or any odd number. Request them to add 
the two products together and tell you the sum. If it is even, 
then originally A selected the odd number, but if it is odd, then 
originally A selected the even number. The reason is obvious. 

Second Example^. The above rule was extended by Bachet 
to any two numbers, provided they are prime to one another 
and one of them is not itself a prime. Let the numbers be 
m and n, and suppose that n is exactly divisible by p. Ask A 
to select one of these numbers, and B to take the other. Choose 
a number prime to p, say q. Ask A to multiply his number by 
q, and B to multiply his number hy p. Request them to add the 
products together and state the sum. Then A originally selected 
m or n, according as this result is not or is divisible by p. The 
numbers, m = 7, n = 15, p = 3, g' = 2, will illustrate the rest. 

Problems depending on the Scale of Notation. Many 
of the rules for finding two or more numbers depend on the 
* Bachet, problem ix, p. 107. t Bachet, problem xi, p. 113. 



CH. l] ARITHMETICAL RECREATIONS 11 

fact that in arithmetic an integral number is denoted by 
a succession of digits, where each digit represents the product 
of that digit and a power of ten, and the number is equal 
to the sum of these products. For example, 2017 signifies 
(2 X 100 + (0 X lO'') + (1 X 10) 4- 7 ; that is, the 2 represents 
2 thousands, i.e. the product of 2 and 10^, the represents 

hundreds, i.e. the product of and 10^; the 1 represents 

1 ten, ie. the product of 1 and 10, and the 7 represents 7 units. 
Thus every digit has a local value. The application to tricks 
connected with numbers will be understood readily from three 
illustrative examples. 

First Example*. A common conjuring trick is to ask a boy 
among the audience to throw two dice, or to select at random 
from a box a domino on each half of which is a number. The 
boy is then told to recollect the two numbers thus obtained, to 
choose either of them, to multiply it by 5, to add 7 to the 
result, to double this result, and lastly to add to this the other 
number. From the number thus obtained, the conjurer sub- 
tracts 14, and obtains a number of two digits which are the 
two numbers chosen originally. 

For suppose that the boy selected the numbers a and h. 
Each of these is less than ten — dice or dominoes ensuring this. 
The successive operations give (i) 5a; (ii) 5a 4- 7; (iii) 10a+ 14; 
(iv) lOa + 14 + 6. Hence, if 14 is subtracted from the final 
result, there will be left a number of two digits, and these 
digits are the numbers selected originally. An analogous trick 
might be performed in other scales of notation if it was thought 
necessary to disguise the process further. 

Second Example-^. Similarly, if three numbers, say, a, h, c, 
are chosen, then, if each of them is less than ten, they can be 

• Some similar questions were given by Bachet in problem xii, p. 117 ; by 
Oughtred or Leake in the Mathe malic all Recreations, commonly attributed to 
the former, London, 1653, problem xxxiv; and by Ozanam, part i, chapter x. 
Probably the Mathematicall Recreations were compiled by Leake, but as the 
work is usually catalogued under the name of W. Oughtred, I shall so describe 
it: it is founded on the similar work by J. Leurechon, otherwise known as 
H. van Etten, published in 1626. 

t Bachet gave some similar questions in problem xii, p. 117. 



12 ARITHMETICAL RECREATIONS [CH. I 

found by the following rule, (i) Take one of the numbers, say, a, 
and multiply it by 2. (ii) Add 3 to the product, (iii) Multiply 
this by 5, and add 7 to the product, (iv) To this sum add the 
second number, b. (v) Multiply the result by 2. (vi) Add 3 
to the product, (vii) Multiply by 5, and, to the product, add 
the third number, c. The result is 100a+106 + c + 235. Hence, 
if the final result is known, it is sufficient to subtract 235 from 
it, and the remainder will be a number of three digits. These 
digits are the numbers chosen originally. 

Third Example*. The following rule for finding the age 
of a man born in the 19th century is of the same kind. Take 
the tens digit' of the year of birth ; (i) multiply it by 5 ; (ii) to 
the product add 2 ; (iii) multiply the result by 2 ; (iv) to this 
product add the units digit of the birth-year ; (v) subtract the 
sum from 120. The result is the man's age in 1916. 

The algebraic proof of the rule is obvious. Let a^ and h be 
the tens and units digits of the birth -year. The successive opera- 
tions give (i) 5a; (ii) 5a + 2; (iii) lOa + 4; (iv) 10a + 4H-5; 
(v) 120 - (10a + h), which is his age in 1916. The rule can be 
easily adapted to give the age in any specified year. 

Fourth Example^. Anqther such problem but of more 
difficulty is the determination of all numbers which are in- 
tegral multiples of their reversals. For instance, among 
numbers of four digits, 8712 = 4 x 2178 and 9801 = 9 x 1089 
possess this property. 

Other Problems with numbers in the denary scale. 
I may mention here two or three other problems which seem 
to be unknown to most compilers of books of puzzles. 

First Problem. The first of them is as follows. Take any 
number of three digits : reverse the order of the digits : sub- 
tract the number so formed from the original number : then, if 
the last digit of the difference is mentioned, all the digits in 
the difference are known. 

* a similar question was given by Laisant and Perrin in their Algebre, Paris, 
1892; and in U Illustration for July 13, 1895. 

t L' Intermediaire des Mathematiciens, Paris, vol. xv, 1908, pp. 228, 278; 
vol. XVI, 1909, p. 34; vol. xix, 1912, p. 128. 



CH. l] ARITHMETICAL RECREATIONS 13 

For suppose the number is 100a + 106 + c, that is, let a be 
the hundreds digit of the number chosen, h be the tens digit, 
and c be the units digit. The number obtained by reversing 
the digits is 100c + 106 + a- The difference of these numbers is 
equal to (100a + c) - (100c + a), that is, to 99 (a - c). But a~c 
is not greater than 9, and therefore the remainder can only be 
99, 198, 297, 396, 495, 594, 693, 792, or 891— in each case the 
middle digit being 9 and the digit before it (if any) being 
equal to the difference between 9 and the last digit. Hence, if 
the last digit is known, so is the whole of the remainder. 

Second Problem. The second problem is somewhat similar 
and is as follows, (i) Take any number ; (ii) reverse the digits ; 
(iii) find the difference between the number formed in (ii) and 
the given number ; (iv) multiply this difference by any number 
you like to name; (v) cross out any digit except a nought; 
(vi) read the remainder. Then the sum of the digits in the 
remainder subtracted from the next highest multiple of nine 
will give the figure struck out. This is clear since the result 
of operation (iv) is a multiple of nine, and the sum of the digits 
of every multiple of nine is itself a multiple of nine. This and the 
previous problem are typical of numerous analogous questions. 

Empirical Problems. There are also numerous empirical 
problems, such as the following. With the ten digits, 9, 8, 7, 6, 
5, 4, 3, 2, 1, 0, express numbers whose sum is unity : each digit 
being used only once, and the use of the usual notations for 
fractions being allowed. With the same ten digits express 
numbers whose sum is 100. With the nine digits, 9, 8, 7, 6, 6, 
4, 3, 2, 1, express four numbers whose sum is 100. To the 
making of such questions there is no limit, but their solution 
involves little or no mathematical skill. 

Four Digits Problem. I suggest the following problem as 
being more interesting. With the digits 1, 2, ... n, express the 
consecutive numbers from 1 upwards as far as possible, say to 
p : four and only four digits, all different, being used in each 
number, and the notation of the denary scale (including 
decimals), as also algebraic sums, products, and positive 
integral powers, being allowed. If the use of the symbols 



14 ARITHMETICAL RECREATIONS [CH. 1 

for square roots and factorials (repeated if desired a finite 
number of times) is also permitted, the range can be ex- 
tended considerably, say to q consecutive integers. If n = 4, 
we have j9 = 88, 9 = 264 ; if n = 5, ^ = 231, g = 790 ; with the 
four digits 0, 1, 2, 3, _p = 36, q = 40. 

Four Fours Problem. Another traditional recreation is, 
with the ordinary arithmetic and algebraic notation, to express 
the consecutive numbers from 1 upwards as far as possible in 
terms of four " 4's." Everything turns on what we mean by 
ordinary notation. If (a) this is taken to admit only the use of 
the denary scale (ex. gr. numbers like 44), decimals, brackets, 
and the symbols for addition, subtraction, multiplication and 
division, we can thus express every number up to 22 inclusive. 
If (/3) also we grant the use of the symbol for square root 
(repeated if desired a finite number of times) we can get 
to 30; but note that though by its use a number like. 2 can be 
expressed by one " 4," we cannot for that reason say that '2 is 
so expressible. If (7) further we permit the use of symbols for 
factorials we can express every number to 112. Finally, if (5) 
we sanction the employment of integral indices expressible by 
a "4" or "4's" and allow the symbol for a square root to be 
used an infinite number of times we can get to 156; but if (e) 
we concede the employment of integral indices and the use of 
sub-factorials* we can get to 877. These interesting problems 
are typical of a class of similar questions f for four (or n) 
"2's,""3V &c. 

Problems with a series of things which are numbered. 
Any collection of things numbered consecutively lend themselves 
to easy illustrations of questions depending on elementary pro- 
perties of numbers. As examples I proceed to enumerate a 
few familiar tricks. The first two of these are commonly 
shown by the use of a watch, the last four may be exemplified 
by the use of a pack of playing cards. 

* Sub-factorial n is equal to w! (1 -1/1 ! + 1/2! - 1/3 ! + ... ±l/n!). On the 
use of this for the four "4's" problem, see the Mathematical Gazette, May, 1912. 

t With four " 2's," we can under a get to 27 and under 7 to 36 ; with 
four " 3'8," under a to 21 and under 7 to 43; with four "5's," under a to 30, 
under 7 to 36, and with the use of indices to 67; and with four " 9's" under a 
to 12, uoder /3 to 37, under 7 to 66, and with the use of indices to 132. 



CH. l] ARITHMETICAL RECREATIONS 15 

First Example*. The first of these examples is connected 
with the hours marked on the face of a watch. In this 
puzzle some one is asked to think of some hour, say, m, and 
then to touch a number that marks another hour, say, n. 
Then if, beginning with the number touched, he taps each 
successive hour marked on the face of the watch, going in the 
opposite direction to that in which the hands of the watch 
move, and reckoning to himself the taps as m, (m 4- 1), &c., 
the (n + 12)tn tap will be on the hour he thought of For 
example, if he thinks of V and touches ix, then, if he taps 
successively ix, Vlil, Vli, vi, . . . , going backwards and reckoning 
them respectively as 5, 6, 7, 8, ... , the tap which he reckons as 
21 will be on the v. 

The reason of the rule is obvious, for he arrives finally at 
the (n + 12 — m)th hour from which he started. Now, since he 
goes in the opposite direction to that in which the hands of 
the watch move, he has to go over (n — m) hours to reach the 
hour m : also it will make no difierence if in addition he goes 
over 12 hours, since the only effect of this is to take him 
once completely round the circle. Now (n + 12 — m) is always 
positive, since n is positive and m is not greater than 12, and 
therefore if we make him pass over (n+ 12 — m) hours we can 
give the rule in a form which is equally valid whether m is 
greater or less than n. 

Second Example. The following is another well-known 
watch-dial problem. If the hours on the face are tapped suc- 
cessively, beginning at vii and proceeding backwards round the 
dial to VI, V, &c., and if the person who selected the number 
counts the taps, beginning to count fi-om the number of the hour 
selected (thus, if he selected x, he would reckon the first tap 
as the 11th), then the 20th tap as reckoned by him will be on 
the hour chosen. 

For suppose he selected the nth hour. Then the 8th tap 
is on XII and is reckoned by him as the (n + 8)th ; and the tap 

• Bachet, problem xx, p. 155; Oughtred or Leake, Mathematicall Eccrea- 
tion$, London, IQod, p. 28. 



16 ARITHMETICAL RECREATIONS [CH. I 

which he reckons as (n + p)th. is on the hour (20— p). Hence, 
putting p = 20 — n, the tap which he reckons as 20th is on the 
hour n. Of course the hours indicated by the first seven taps 
are immaterial : obviously also we can modify the presentation 
by beginning on the hour viii and making 21 consecutive taps, 
or on the hour ix and making 22 consecutive taps, and so on. 

Third Example. The following is another simple example. 
Suppose that a pack of n cards is given to some one who is asked 
to select one out of the first m cards and to remember (but not to 
mention) what is its number from the top of the pack ; suppose 
it is actually the ^th card in the pack. Then take the pack, 
reverse the order of the top m cards (which can be easily 
effected by shuffling), and transfer y cards, where y <n — m, 
from the bottom to the top of the pack. The effect of this is 
that the card originally chosen is now the {y-\-m — x + l)th 
from the top. Return to the spectator the pack so rearranged, 
and ask that the top card be counted as the (ii; + l)th, the next 
as the {x + 2)th, and so on, in which case the card originally 
chosen will be the (3/ + m + l)th. Now y and m can be chosen 
as we please, and may be varied every time the trick is per- 
formed ; thus any one unskilled in arithmetic will not readily 
detect the method used. 

Fourth Example^. Place a card on the table, and on it 
place as many other cards from the pack as with the number 
of pips on the card will make a total of twelve. For example, 
if the card placed first on the table is the five of clubs, then 
seven additional cards must be placed on it. The court cards 
may have any values assigned to them, but usually they are 
reckoned as tens. This is done again with another card, and 
thus another pile is formed. The operation may be repeated 
either only three or four times or as often as the pack will 
permit of such piles being formed. If finally there are p such 
piles, and if the number of cards left over is r, then the sum 
of the number of pips on the bottom cards of all the piles will 
be 13(p~4) + r. 

For, if X is the number of pips on the bottom card of a pile, 
* A particular case oi thia problem was given by Bachet, problem xvn, p. 138. 



CH. l] ARITHMETICAL RECREATIONS 17 

the number of cards in that pile will be 13 — a;. A similar 
argument holds for each pile. Also there are 52 cards in the 
pack ; and this must be equal to the sum of the cards in the 
p piles and the r cards left over. 

/. (13-a?i) + (13-^2)+... + (13-a;p) + r = 52, 

.*. a?! + 572 + • • • + ^p = 13|) — 52 + r 
= 13(p-4)+r. 

More generally, if a pack of n cards is taken, and if in each 
pile the sum of the pips on the bottom card and the number of 
cards put on it is equal to m, then the sum of the pips on the 
bottom cards of the piles will be (m + 1)^ + r — w. In an ecarte 
pack 7i= 32, and it is convenient to take m= 15. 

Fifth Example. It may be noticed that cutting a pack 
of cards never alters the relative position of the cards provided 
that, if necessary, we regard the top card as following im- 
mediately after the bottom card in the pack. This is used in 
the following trick*. Take a pack, and deal the cards face 
upwards on the table, calling them one, two, three, &c. as you 
put them down, and noting in your own mind the card first 
dealt. Ask some one to select a card and recollect its number. 
Turn the pack over, and let it be cut (not shuffled) as often as 
you like. Enquire what was the number of the card chosen. 
Then, if you deal, and as soon as you come to the original first 
card begin (silently) to count, reckoning this as one, the 
selected card will appear at the number mentioned. Of course, 
if all the cards are dealt before reaching this number, you 
must turn the cards over and go on counting continuously. 

Sixth Example. Here is another simple question of this 
class. Kemove the court cards from a pack. Arrange the 
remaining 40 cards, faces upwards, in suits, in four lines thus. 
In the first line, the 1, 2, ... 10, of suit -4; in the second line, the 
10, 1, 2, ... 9, of suit 5; in the third line, the 9, 10, 1, ... 8, of 
suit C; in the last line, the 8, 9, 10, 1, ... 7, of suit D. Next 
take up, face upwards, the first card of line 1, put below it the 

* Bacbet, problem xiz, p. 152. 
B. R. 2 



18 ARITHMETICAL RECREATIONS [CH. I 

first card of line 2, below that the first card of line 3, and below 
that the first card of line 4. Turn this pile face downwards. 
Next take up the four cards in the second column in the same 
way, turn them face downwards, and put them below the first 
pile. Continue this process until all the cards are taken up. 
Ask someone to mention any card. Suppose the number of 
pips on it is 7?. Then if the suit is ^, it will be the 4nth card 
in the pack ; if the suit is B, it will be the (4?i + 3)th card ; if 
the suit is C, it will be the (4?^ + 6)th card ; and if the suit is D, 
it will be the {^n + 9)th card. Hence by counting the cards, 
cyclically if necessary, the card desired can be picked out. It is 
easy to alter the form of presentation, and a full pack can be 
used if desired. The explanation is obvious. 

Medieval Problems in Arithmetic. Before leaving the 
subject of these elementary questions, I may mention a few 
problems which for centuries have appeared in nearly every 
collection of mathematical recreations, and therefore may claim 
what is almost a prescriptive right to a place here. 

First Example. The following is a sample of one class of 
these puzzles. A man goes to a tub of water with two jars, 
of which one holds exactly 3 pints and the other 5 pints. How 
can he bring back exactly 4 pints of water ? The solution 
presents no difficulty. 

Second Example*. Here is another problem of the same 
kind. Three men robbed a gentleman of a vase, containing 
24 ounces of balsam. Whilst running away they met a 
glass-seller, of whom they purchased three vessels. On reaching 
a place of safety they wished to divide the booty, but found 
that their vessels contained 5, 11, and 13 ounces respectively. 
How could they divide the balsam into equal portions ? 
Problems like this can be worked out only by trial. 

Third Example^. The next of these is a not uncommon 

* Some similar problems were given by Baciiet, Appendix, problem iii, 
p. 206; problem ix, p. 233: by Oughtred or Leake in the Mathematicall 
Recreations, p. 22: and by Ozanam, 1803 edition, vol. i, p. 174; 1840 edition, 
p. 79. Earlier instances occur in Tartaglia's writings. 

t Bacliet, problem xxii, p. 170. 



CH. l] ARITHMETICAL RECREATIONS 19 

game, played by two people, say A and B. A begins by 
mentioning some number not greater than (say) six, B may 
add to that any number not greater than six, A may add 
to that again any number not greater than six, and so on. 
He wins who is the first to reach (say) 50. Obviously, if A 
calls 43, then whatever B adds to that, A can win next time. 
Similarly, if A calls 36, B cannot prevent A'a calling 43 the 
next time. In this way it is clear that the key numbers are 
those forming the arithmetical progression 43, 36, 29, 22, 15, 
8, 1 ; and whoever plays first ought to win. 

Similarly, if no number greater than m may be added at 
any one time, and n is the number to be called by the victor, 
then the key numbers will be those forming the arithmetical, pro- 
gression whose common difference is m + 1 and whose smallest 
term is the remainder obtained by dividing n by m + 1. 

The same game may be played in another form by placing 
p coins, matches, or other objects on a table, and directing each 
player in turn to take away not more than m of them. Who- 
ever takes away the last coin wins. Obviously the key numbers 
are multiples of m + 1, and the first player who is able to leave 
an exact multiple of (m + 1) coins can win. Perhaps a better 
form of the game is to make that player lose who takes away 
the last coin, in which case each of the key numbers exceeds 
by unity a multiple of m-\-l. 

Another variety* consists in placing p counters in the form 
of a circle, and allowing each player in succession to take away 
not more than m of them which are in unbroken sequence: 
m being less than p and greater than unity. In this case 
the second of the two players can always win. 

These games are simple, but if we impose on the original 
problem the restriction that each player may not add the 
same number more than (say) three times, the analysis 
becomes by no means easy. I have never seen this ex- 
tension described in print, and I will enunciate it at length. 
Suppose that each player is given eighteen cards, three 
of them marked 6, three marked 5, three marked 4, three 
* S. Loyd, Tit-Bits, London, July 17, Aug. 7, 1897. 

2—2 



20 ARITHMETICAL RECREATIONS [CH. I 

marked 8, three marked 2, and three marked 1. They play 
alternately ; A begins by playing one of his cards ; then B plays 
one of his, and so on. He wins who first plays a card which 
makes the sum of the points or numbers on all the cards 
played exactly equal to 50, but he loses if he plays a card 
which makes this sum exceed 50. The game can be played 
by noting the numbers on a piece of paper, and it is not 
necessary to use cards. 

Thus suppose they play as follows. A takes a 4, and scores 4; 
B takes a 3, and scores 7; A takes a 1, and scores 8; B takes a 6, 
and scores 14; J. takes a 3, and scores 1*7; B takes a 4, and scores 
21; A takes a 4, and scores 25 ; B takes a 5, and scores 30 ; A 
takes a 4, and scores 34 ; B takes a 4, and scores 38 ; A takes a 

5, and scores 43. B can now win, for he may safely play 3, 
since A has not another 4 wherewith to follow it; and if A 
plays less than 4, B will win the next time. Again, suppose 
they play thus. A, 6; B, 3; ^, 1; B, 6; A, S; B, 4; A, 2; 
B,5;A,1;B,5;A,2;B,5;A,2;B,S. A is now forced to 
play 1, and B wins by playing 1. 

A slightly different form of the game has also been sug- 
gested. In this there are put on the table an agreed number 
of cards, say, for example, the four aces, twos, threes, fours, fives, 
and sixes of a pack of cards — twenty-four cards in all. Each 
player in turn takes a card. The score at any time is the sum 
of the pips on all the cards taken, whether by A or B. He 
wins who first selects a card which makes the score equal, say, 
to 50, and a player who is forced to go beyond 50 loses. 

Thus, suppose they play as follows. A takes a 6, and scores 
6; B takes a 2, and scores 8; A takes a 5, and scores 13; 
B takes a 2, and scores 15; A takes a 5, and scores 20; B 
takes a 2, and scores 22 ; A takes a 5, and scores 27 ; B takes 
a 2, and scores 29; A takes a 5, and scores 34; B takes a 

6, and scores 40 ; A takes a 1, and scores 41 ; B takes a 4, and 
scores 45 ; A takes a 3, and scores 48 ; B now must take 1, and 
thus score 49 ; and A takes a 1, and wins. 

In these variations the object of each player is to get to 
one of the key numbers, provided there are sufficient available 



CH. l] ARITHMETICAL llECREATIONS 21 

remaining numbers to let him retain the possession of each 
subsequent key number. The number of cards used, the points 
on them, and the number to be reached can be changed at will ; 
and the higher the number to be reached, the more difficult 
it is to forecast the result and to say whether or not it is an 
advantage to begin. 

Fourth Example. Here is another problem, more difficult 
and less well known. Suppose that m counters are divided 
into n heaps. Two players play alternately. Each, when his 
turn comes, may select any one heap he likes,. and remove from 
it all the counters in it or as many of them as he pleases. 
That player loses who has to take up the last counter. 

To solve it we may proceed thus*. Suppose there are a, 
counters in the rth heap. Express a^ in the binary scale, and 
denote the coefficient of 2^ in it by drp. Do this for each 
heap, and let Sp be the sum of the coefficients of 2^ thus 
determined. Thus Sp = d^p + dc^-\- d^p-\- .,., Then either >S^o> 
81,82,... are all even, which we may term an A arrangement, 
or they are not all even, which we may term a B arrangement. 

It will be easily seen that if one player, P, has played so as 
to get the counters in any A arrangement (except that of an 
even number of heaps each containing one counter), he can 
force a win. For the next move of his opponent, Q, must bring 
the counters to a 5 arrangement. Then P can make the next 
move to bring the counters again to an A arrangement, other 
than the exceptional one of an even number of heaps each con- 
taining only one counter. Finally this will leave P a winning 
position, ex. gr. two heaps each containing 2 counters. 

If that player wins who takes the last counter, the rule is 
easier. For if one player P has played so as to get the counters 
in any B arrangement, he can force a win, since the next move 
of his opponent Q must bring the counters to an A arrangement. 
Then P can make the next move to bring the counters again to 
a B arrangement. Finally this will leave P a winning position. 

Fifth Example. The following medieval problem is some- 
what more elaborate. Suppose that three people, P, Q, B, 
* From a letter to me by Mr B. K. Morcom, August 2, 1910. 



22 ARITHMETICAL RECREATIONS [CH. I 

select three things, which we may denote by a, e, i respec- 
tively, and that it is desired to find by whom each object was 
selected *. 

Place 24 counters on a table. Ask P to take one counter, 
Q to take two counters, and R to take three counters. Next, 
ask the person who selected a to take as many counters as he 
has already, whoever selected e to take twice as many counters 
as he has already, and whoever selected i to take four times as 
many counters as he has already. Note how many counters 
remain on the table. There are only six ways of distributing 
the three things among P, Q, and R ; and the number of counters 
remaining on the table is different for each way. The remainders 
may be 1, 2, 3, 5, 6, or 7. Bachet summed up the results in 
the mnemonic line Par fer (1) Cesar (2) jadis (3) devint (5) si 
grand (6) prince (7). Corresponding to any remainder is a 
word or words containing two syllables: for instance, to the 
remainder 5 corresponds the word devint The vowel in the 
first syllable indicates the thing selected by P, the vowel in 
the second syllable indicates the thing selected by Q, and of 
course R selected the remaining thing. 

Extension. M. Bourlet, in the course of a very kindly 
notice f of the second edition of this work, gave a much neater 
solution of the above question, and has extended the problem 
to the case of n people, Po, Pi, Pa, ..., Pn-i, each of whom 
selects one object, out of a collection of n objects, such as 
dominoes or cards. It is required to know which domino or 
card was selected by each person. 

Let us suppose the dominoes to be denoted or marked by 
the numbers 0, 1, ..., n — 1, instead of by vowels. Give one 
counter to Pj, two counters to Pg, and generally k counters to 
Pfc. Note the number of counters left on the table. Next 
ask the person who had chosen the domino to take as many 
counters as he had already, and generally whoever had chosen 
the domino h to take n^ times as many dominoes as he had 
already: thus if Ph had chosen the domino numbered h, 

* Bacliet, problem xxv, p. 187. 

t Bulletin des Sciences Mathemitigiues, Paris, 1893, vol. xvii, pp. 105—107. 



CU. l] ARITHMETICAL RECREATIONS 23 

he would take n^k counters. The total number of counters 
taken is ^n^c. Divide this by w, then the remainder will be 
the number on the domino selected by Pq\ divide the quotient 
by n, and the remainder will be the number on the domino 
selected by Pi ; divide this quotient by n, and the remainder 
will be the number on the domino selected by Pj ; and so on. 
In other words, if the number of counters taken is expressed in 
the scale of notation whose radix is n, then the {h'+ l)th digit 
from the right will give the number on the domino selected by P;^. 

Thus in Bachet's problem with 3 people and 3 dominoes, 
we should first give one counter to Q, and two counters to R, 
while P would have no counters; then we should ask the 
person who had selected the domino marked or a to take 
as many counters as he had already, whoever had selected the 
domino marked 1 or e to take three times as many counters as 
he had already, and whoever had selected the domino marked 2 
or i to take nine times as many counters as he had already. 
By noticing the original number of counters, and observing 
that 3 of these had been given to Q and P, we should know 
the total number taken by P, Q, and R. If this number were 
divided by 3, the remainder would be the number of the 
domino chosen by P ; if the quotient were divided by 3 the re- 
mainder would be the number of the domino chosen by Q; and the 
final quotient would be the number of the domino chosen by R. 

Exploration Problems. Another common question is con- 
cerned with the maximum distance into a desert which could 
be reached from a frontier settlement by the aid of a party of 
n explorers, each capable of carrying provisions that would last 
one man for a days. The answer is that the man who reaches 
the greatest distance will occupy nal{n + 1) days before he 
returns to his starting point. If in the course of their journey 
they may make depots, the longest possible journey will occupy 
ia(l + i + J+... + l/n)days. 

The Josephus Problem. Another of these antique problems 
consists in placing men round a circle so that if every mth man 
is killed, the remainder shall be certain specified individuals. 
Such problems can be easily solved empirically. 



24 ARITHMETICAL RECREATIONS [CH. I 

Hegesippus* says that Josephus saved his life by such 
a device. According to his account, after the Romans had 
captured Jotapat, Josephus and forty other Jews took refuge 
in a cave. Josephus, much to his disgust, found that all 
except himself and one other man were resolved to kill them- 
selves, so as not to fall into the hands of their conquerors. 
Fearing to show his opposition too openly he consented, but 
declared that the operation must be carried out in an orderly 
way, and suggested that they should arrange themselves round 
a circle and that every third person should be killed until but 
one man was left, who must then commit suicide. It is alleged 
that he placed himself and the other man in the 31st and 16th 
place respectively. 

The medieval question was usually presented in the following 
form. A ship, carrying as passengers 15 Turks and 15 
Christians, encountered a storm, and, in order to save the ship 
and crew, one-half of the passengers had to be thrown into the 
sea. Accordingly the passengers were placed in a circle, and every 
ninth man, reckoning from a certain point, was cast overboard. 
It is desired to find an arrangement by which all the Christians 
should be saved "|*. In this case we must arrange the men thus : 
GGGGTTTTTGCTGGGTGTTGGTTTGTTGGT, 
where G stands for a Christian and T for a Turk. The order 
can be recollected by the positions of the vowels in the follow- 
ing line : From numbers' aid and art, never will fame depart, 
where a stands for 1, e for 2, i for 3, o for 4, and u for 5. Hence 
the order is o Christians, u Turks, &c. 

If every tenth man were cast overboard, a similar mnemonic 
line is Rex paphi cum gente bona dat signa serena. An oriental 
setting of this decimation problem runs somewhat as follows. 
Once upon a time, there lived a rich farmer who had 30 children, 
15 by his first wife who was dead, and 15 by his second wife. The 
latter woman was eager that her eldest son should inherit the 
property. Accordingly one day she said to him, " Dear Husband, 

* De Bello Judaico, bk. iii, chaps. 16 — 18. 

t Bachet, problem xxm, p. 174. The same problem had been previously 
enunciated by Tartaglia. 



CH. l] ARITHMETICAL RECREATIONS 25 

you are getting old. We ought to settle who shall be your 
heir. Let us arrange our 30 children in a circle, and counting 
from one of them remove every tenth child until there remains 
but one, who shall succeed to your estate." The proposal 
seemed reasonable. As the process of selection went on, the 
farmer grew more and more astonished as he noticed that the . 
first 14 to disappear were children by his first wife, and he ob- 
served that the next to go would be the last remaining member 
of that family. So he suggested that they should see what 
would happen if they began to count backwards from this lad. 
She, forced to make an immediate decision, and reflecting that 
the odds were now 15 to 1 in favour of her family, readily 
assented. Who became the heir? 

In the general case n men are arranged in a circle which is 
closed up as individuals are picked out. Beginning anywhere, 
we continually go round, picking out each mth man until only 
r are left. Let one of these be the man who originally occu- 
pied the pth place. Then had we begun with n+1 men, he 
would have originally occupied the (p + m)th place when 
^ + m is not greater than 71+ 1, and the (p + m — n — l)th 
place when p + m is greater than ?^ + 1. Thus, provided there 
are to be r men left, their original positions are each shifted 
forwards along the circle m places for each addition of a single 
man to the original group*. 

Now suppose that with n men the last survivor (r = l) 
occupied originally the pth place, and that with (n + x) men 
the last survivor occupied the yth place. Then, if we confine 
ourselves to the lowest value of x which makes y less than m, 
we have y — {p-\- mx) — (n + x). 

Based on this theorem we can, for any specified value of n, 
calculate rapidly the position occupied by the last survivor of 
the company. In effect, Tait found the values of n for which 
a man occupying a given position p, which is less than m, 
would be the last survivor, and then by repeated applications 
of the proposition, obtained the position of the survivor for 
intermediate values of n. 
* P. G. Tait, Collected Udentijic Papers, Cambridge, vol. n. 1900, pp. 432—436. 



26 ARITHMETICAL RECREATIONS [CH. I 

For instance, take the Josephus problem in which m = 3. 
Then we know that the final survivor of 41 men occupied 
originally the 31st place. Suppose that when there had been 
(41 + x) men, the survivor occupied originally the yth. place. 
Then, if we consider only the lowest value of x which makes 
y less than m, we have y = (SI + Sx) - (4^1 + x) = 2x - 10. 
Now, we have to take a value of x which makes y positive and 
less than m, that is, in this case equal to 1 or 2. This is a; = 6 
which makes y == 2. Hence, had there been 47 men the man 
last chosen would have originally occupied the second place. 
Similarly had there been (47 + x) men the man would have 
occupied originally the yth place, where, subject to the same 
conditions as before, we have y = (2 4- Sx) — (47 -{- x) = 2x — 45. 
If x = 23, y = 1. Hence, with 70 men the man last chosen 
would have occupied originally the first place. Continuing 
the process, it is easily found that if n does ngt exceed 
2000000 the last man to be taken occupies the first place when 
n = 4, 6, 9, 31, 70, 105, 355, 799, 1798, 2697, 9103, 20482, 30723, 
69127, 155536, 233304, 349956, 524934, or 787401 ; and the 
second place when n = 2, 3, 14, 21, 47, 158, 237, 533, 1199, 4046, 
6069, 13655, 46085, 103691, 1181102, or 1771653. From these 
results, by repeated applications of the proposition, we find, for 
any intermediate values of n, the position originally occupied 
by the man last taken. Thus with 1000 men, the 604th place ; 
with 100000 men, the 92620th place ; and with 1000000 men, 
the 637798th place are those which would be selected by a 
prudent mathematician in a company subjected to trimation. 

Similarly if a set of 100 men were subjected to decimation, 
the last to be taken would be the man originally in the 26th 
place. Hence, with 227 men the last to be taken would be the 
man originally in the first place. 

Modifications of the original problem have been suggested. 
For instance* let 5 Christians and 5 Turks be arranged round 
a circle thus, TGTGGTGTCT. Suppose that, if beginning 
at the ath man, every /ith man is selected, all the Turks will be 
picked out for punishment ; but if beginning at the 6th man, 
♦ H. E. Dudeney, Tit-Bits, London, Oct. 14 and 28, 1905. 



CH. l] ARITHMETICAL RECREATIONS 27 

every A;th man is selected, all the Christians will be picked out 
for punishment. The problem is to find a, b, h, and k. 

I suggest as a similar problem, to find an arrangement of c 
Turks and c Christians arranged in a circle, so that if beginning 
at a particular man, say the first, every hth man is selected, all 
the Turks will be picked out, but if, beginning at the same man, 
every kth man is selected, all the Christians will be picked out. 
This makes an interesting question because it is conceivable 
that the operator who picked out the victims might get confused 
and take k instead of h, or vice versa, and so consign all his 
friends to execution instead of those whom he had intended to 
pick out. The problem is, for any given value of c, to find an 
arrangement of the men and the corresponding suitable values 
of h and k. Obviously if c = 2, then for an arrangement like 
TGCT a. solution is A = 4, A; = 3. If c = 3, then for an arrange- 
ment like TGTCGT a solution is A = 7, A; = 8. If c = 4, then 
for an arrangement like TGTTGTGG a solution is A = 9, 
/j = 5 ; and so on. Is it possible to give similar arrangements 
for higher numbers ? 

ADDENDUM. 

Note. Page 13. Solutions of the ten digit problems are 
35/70 + 148/296=1, or '01234 + -98765 = 1; 
and 60 + 49 + 1/2 + 38/76 = 100. 

A solution of the nine digit problem is 

l-234 + 98-765=100, or 97 + 8/12 + 4/6 + 5/3 = 100; 
but if an algebraic sum is permissible a neater solution is 

123-45-67 + 89 = 100, 
where the digits occur in their natural order. 

Note. Page 18. There are several solutions of the division of 24 ounces 
under the conditions specified. One of these solutions is as follows : 

The vessels can contain 24 oz. 13 oz. 11 oz. 5 oz. 

Their contents originally are... 24 

First, make their contents 

Second, make their contents ... 16 
Third, make their contents ... 16 
Fourth, make their contents ... 3 

Fifth, make their contents 3 

Lastly, make their contents .« 8 
Note. Pages 26 — 27. The simplest solution of the five Christians and five 
Turks problem is a=l, /i = ll, 6 = 9, & = 29. 






.. 0.. 


0... 


8 


.. 11 .. 


5 ... 


8 


.. 0.. 


0... 





.. 8 .. 


... 


13 


.. 8 .. 


0... 


8 


.. 8 .. 


5 ... 


8 


.. 8 .. 


0... 



28 



CHAPTEK TI. 

ARITHMETICAL RECREATIONS CONTINUED. 

I devote this chapter to the description of some arithmetical 
fallacies, a few additional problems, and notes on one or two 
problems in higher arithmetic. 

Arithmetical fallacies. I begin by mentioning some 
instances of demonstrations* leading to arithmetical results 
which are obviously impossible. I include algebraical proofs as 
well as arithmetical ones. Some of the fallacies are so patent 
that in preparing the first and second editions I did not think 
such questions worth printing, but, as some correspondents 
expressed a contrary opinion, I give them for what they are 
worth. 

First Fallacy. One of the oldest of these — and not a very 
interesting specimen — is as follows. Suppose that a = 6, then 
ah^a\ .'. ah-¥ = a^-b\ .'. b (a-b) = {a + b)(a-b). 
.'. b=^a-\-b. .-. 6 = 26. .-. 1 = 2. 

* Of the fallacies given in the text, the first and second are well known ; 
the third is not new, but the earliest work in which I recollect seeing it is 
my Algebra, Cambridge, 1890, p. 430; the fourth is given in G. Chrystal's 
Algebra, Edinburgh, 1889, vol. ii, p. 15'.) ; the sixth is due to G. T. Walker, 
and, I believe, has not appeared elsewhere than in this book; the seventh 
is due to D'Alembert ; and the eighth to F. Galton. It may be worth re- 
cording (i) that a mechanical demonstration that 1 = 2 was given by R. Chartres 
in Knowledge, July, 1891; and (ii) that J. L. F. Bertrand pointed out that 
a demonstration that 1=-1 can be obtained from the proposition in the 
Integral Calculus that, if the limits are constant, the order of integration is 
indifferent ; hence the integral to x (from a; = to a; = 1) of the integral to y (from 
y = to y = l) of a function should be equal to the integral to y (from y=0 to 
y = l) of the integral to x (from x = to a;=:l) of 0, but if <p^ix''^-y^)l(x^ + y^)\ 
this gives \ir= -{tt. 



CH. Il] ARITHMETICAL RECREATIONS 29 

Second Fallacy. Another example, the idea of which is due 
to John Bernoulli, may be stated as follows. We have (— 1)'* = 1. 
Take logarithms, /. 2 log (- 1) = log 1 = 0. .'. log (- 1) = 0. 
.-. -l = e°, .-. -1 = 1. 

The same argument may be expressed thus. Let x be a 
quantity which satisfies the equation e*= — 1. Square both 
sides, 

.-. e^^l. .-. 2a;=0. .-. 57 = 0. .'. 6^=6". 

Bute^=-1 and e"=l, .-. -1 = 1. 

The error in each of the foregoing examples is obvious, but 
the fallacies in the next examples are concealed somewhat 
better. 

Third Fallacy. As yet another instance, we know that 

\og{\+x)^x-\x'^-\-la?- .... 
If x—ly the resulting series is convergent; hence we have 
log2 = l-i + J-i + i-i + f-J + i-.... 
.-. 21og2 = 2-l+f-i + f-J + ^-i + |-.... 
Taking those terms together which have a common denominator, 
we obtain 

21og2 = l+i-i + i + f-i+i... 

= l-i + i-i + i- 

= log 2. 
Hence 2 = 1. 

Fourth Fallacy. This fallacy is very similar to that last 
given. We have 

log2 = l-i + 4-i + i-J + ... 

= {(1 +*+*+. ..)+(i+i + i + ...)!-2(i + i+H.-) 

={l+i+i+...l-(l+i+J+...) 

= 0. 
Fifth Fttllacy. We have 

Va X VS = ^ab. 
Hence V^ x V^ = \/(^I)(^T), 

therefore, (V^)' = VI, that is, -1 = 1. 



80 ARITHMETICAL RECREATIONS [CH. II 

Sixth Fallacy. The following demonstration depends on 
the fact that an algebraical identity is true whatever be the 
symbols used in it, and it will appeal only to those who are 
familiar with this fact. We have, as an identity, 

\/x — y = i^/y—x (i), 

where i stands either for + V— 1 or for — V— 1. Now an 
identity in x and y is necessarily true whatever numbers x and 
y may represent. First put a? = a and y = 6, 

.*. '\/a^h — i^b — a (ii). 

Next put ic = 6 and y = a, 

.*. "Jh — a^i^a — h (iii). 

Also since (i) is an identity, it follows that in (ii) and (iii) the 
symbol i must be the same, that is, it represents + V— 1 or 
— V—l in both cases. Hence, from (ii) and (iii), we have 

Va — 6 V6 — a = 1*2 \/6 — a Va — 6, 

that is, 1 = ~ 1. 

Seventh Fallacy. The following fallacy is due to D'AIem- 
bert*. We know that if the product of two numbers is equal 
to the product of two other numbers, the numbers will be in 
proportion, and from the definition of a proportion it follows 
that if the first term is greater than the second, then the third 
term will be greater than the fourth : thus, if ad — be, then 
a:h = c:d, and if in this proportion a>b, then c> d. Now if 
we put a = d = l and 6 = c = — 1 we have four numbers which 
satisfy the relation ad = be and such that a>b; hence, by the 
proposition, c> d, that is, — 1 > 1, which is absurd. 

Eighth Fallacy. The mathematical theory of probability 
leads to various paradoxes: of these one specimenf will suffice. 
Suppose three coins to be thrown up and the fact whether each 
comes down head or tail to be noticed. The probability that 
all three coins come down head is clearly (1/2)*, that is, is 1/8 ; 

* opuscules Mathematiques, Paris, 1761, vol. i, p. 201. 

+ See Nature, Feb. 15, March 1, 1894, vol. xlix, pp. 365—366, 413. 



CH. Il] ARITHMETICAL RECREATIONS 31 

similarly the probability that all three come down tail is 1/8 ; 
hence the probability that all the coins come down alike (i.e. 
either all of them heads or all of them tails) is 1/4. But, of 
three coins thus thrown up, at least two must come down alike : 
now the probability that the third coin comes down head is 1/2 
and the probability that it comes down tail is 1/2, thus the 
probability that it comes down the same as the other two coins 
is 1/2 : hence the probability that all the coins come down 
alike is 1/2. I leave to my readers to say whether either of 
these conflicting conclusions is right, and, if so, which is 
correct. 

Arithmetical Problems. To the above examples I may add 
the following standard questions, or recreations. 

The first of these questions is as follows. Two clerks, A and 
B, are engaged, ^ at a salary commencing at the rate of (say) 
£100 a year with a rise of £20 every year, 5 at a salary 
commencing at the same rate of £100 a year with a rise of £5 
every half-year, in each case payments being made half-yearly ; 
which has the larger income ? The answer is B ; for in the 
first year A receives £100, but B receives £50 and £55 as 
his two half-yearly payments and thus receives in all £105. In 
the second year A receives £120, but B receives £60 and £65 
as his two half-yearly payments and thus receives in all £125. 
In fact B will always receive £5 a year more than A. 

Another simple arithmetical problem is as follows. A hymn- 
board in a church has four grooved rows on which the numbers 
of four hymns chosen for the service are placed. The hymn- 
book in use contains 700 hymns. What is the smallest number 
of single figured numerals which must be kept in stock so 
that the numbers of any four different hymns selected can be 
displayed? How will the result be afiected if an inverted 6 
can be used for a 9 ? The answers are 86 and 81. 

As another question take the following. A man bets 1/nth 
of his money on an even chance (say tossing heads or tails 
with a penny): he repeats this again and again, each time 
betting l/?ith of all the money then in his possession. If, 
finally, the number of times he has won is equal to the number 



32 ARITHMETICAL RECREATIONS [CH. II 

of times he has lost, has he gained or lost by the transaction ? 
He has, in fact, lost. 

Here is another simple question to which not unfrequently 
I have received incorrect answers. One tumbler is half-full 
of wine, another is half- full of water : from the first tumbler 
a teaspoonful of wine is taken out and poured into the 
tumbler containing the water: a teaspoonful of the mixture 
in the second tumbler is then transferred to the first tumbler. 
As the result of this double transaction, is the quantity of 
wine removed from the first tumbler greater or less than the 
quantity of water removed from the second tumbler ? In my 
experience the majority of people will say it is greater, but 
this is not the case. 

Here is another paradox dependent on the mathematical 
theory of probability. Suppose that a player at bridge or whist 
asserts that an ace is included among the thirteen cards dealt 
to him, and let p be the probability that he has another ace 
among the other cards in his hand. Suppose, however, that 
he asserts that the ace of hearts is included in the thirteen 
cards dealt to him, then the probability, q, that he has another 
ace among the other cards in his hand is greater than was the 
probability p in the first case. For, if r is the probability that 
when he has one ace it is the ace of hearts, we have p=r.q, 
and since p, q, r are proper fractions, we must have q greater 
than p, which at first sight appears to be absurd. 

Permutation Problems. Many of the problems in per- 
mutations and combinations are of considerable interest. As 
a simple illustration of the very large number of ways in which 
combinations of even a few things can be arranged, I may note 
that there are 500,291833 different ways in which change for a 
sovereign can be given in current coins*, including therein the 
obsolescent double-florin, and crown; also that as many as 
19,958400 distinct skeleton cubes can be formed with twelve 
differently coloured rods of equal length f; while there are no less 
than (52!)/(13!y, that is, 53644,737765,488792,839237,440000 

* The Tribune, September 3. 1906. 

t Mathematical Tripos, Cambridge, Part I, 1804. 



CH. Il] ARITHMETICAL RECREATIONS 83 

possible different distributions of hands at bridge or whist with 
a pack of fifty-two cards. 

Voting Problems. As a simple example on combinations 
I take the cumulative vote as affecting the representation of 
a minority. If there are p electors each having r votes of which 
not more than s may be given to one candidate, and n men 
are to be elected, then the least number of supporters who can 
secure the election of a candidate must exceed prl{ns + r). 

The Knights of the Round Table. A far more difficult 
permutation problem consists in finding as many arrangements 
as possible of n people in a ring so that no one has the same 
two neighbours more than once. It is a well-known proposition 
that n persons can be arranged in a ring in {n — l)!/2 different 
ways. The number of these arrangements in which all the 
persons have different pairs" of neighbours on each occasion 
cannot exceed {n — l)(7i — 2)/2, since this gives the number of 
ways in which any assigned person may sit between every 
possible pair selected from the rest. But in fact it is always 
possible to determine (n — l)(w — 2)/2 arrangements in which 
no one has the same two neighbours on any two occasions. 

Solutions for various values of n have been given. Here 
for instance (if n = 8) are 21 arrangements* of eight persons. 
Each arrangement may be placed round a circle, and no one 
has the same two neighbours on any two occasions. 

1.2.5.6.8.7.4.3; 1.2.7.8.4.3.5.6] 

1.3.7.4.6.8.2.5; 1.3.8.6.2.5.7.4; 

1.4.3.8.7.5.6.2; 1.4.5.7.6.2.3.8; 

1.5.7.3.8.2.6.4; 1.5.8.2.4.6.3.7 

1.6.3.5.8.4.2.7; 1.6.4.8.2.7.3.5 

1.7.6.3.2.4.5.8; 1.7.8.5.6.3.4.2; 

1.8.4.5.3.2.7.6; 1.8.6.7.4.5.2.3. 

The methods of determining these arrangements are lengthy, 
and far from easy. 

* Communicated to me by Mr E. G. B. Bergholt, May, 1906, see The Secretary 
and The Queen, August, 1906. Mr Dudeney had given the problem for the case 
when n = 6 in 190c), and informs me that the problem has been solved by 
Mr E. D. Bewley when n is even, and that he has a general method applicable 
when n is odd. Various memoirs on the subject have appeared in the mathe- 
matical journals. 

B. n. 3 



1.2.3.4.5.6.7.8 
1.3.5.2.7.4.8.6 
1.4.2.6.3.8.5.7 
1.5.6.4.3.7.8.2 
1.6.2.7.5.3.8.4 
1.7.4.2.5.8.6.3 
1.8.2.3.7.6.4.5 



34 ARITHMETICAL RECREATIONS [CH. II 

The Mi^nage Problem*. Another difficult permutation 
problem is concerned with the number x of possible arrange- 
ments of n married couples, seated alternately man and woman, 
round a table, the n wives being in assigned positions, and the 
n husbands so placed that a man does not sit next to his wife. 

The solution involves the theory of discordant permutations f, 
and is far from easy. I content myself with noting the results 
when n does not exceed 10. When n = 3, a; = 1 ; when ti = 4, 
a? = 2 ; when ti = 5, a; = 13 ; when ?i = 6, a; = 80 ; when n — *J, 
^ = 579; when 7i = 8, a? = 4738; when /i = 9, a; = 43387; and 
when w = 10, a? = 439792. 

Bachet's Weights Problem {. It will be noticed that a 
considerable number of the easier problems given in the last 
chapter either are due to Bachet or were collected by him in 
his classical Prohlhnes. Among the more difficult problems 
proposed by him was the determination of the least number of 
weights which would serve to weigh any integral number of 
pounds from 1 lb. to 40 lbs. inclusive. Bachet gave two 
solutions: namely, (i) the series of weights of 1, 2, 4, 8, 16, 
and 32 lbs. ; (ii) the series of weights of 1, 3, 9, and 27 lbs. 

If the weights may be placed in only one of the scale-pans, 
the first series gives a solution, as had been pointed out in 
1556 by Tartagliag. 

Bachet, however, assumed that any weight might be placed 
in either of the scale -pans. In this case the second series gives 
the least possible number of weights required. His reasoning 
is as follows. To weigh 1 lb. we must have a 1 lb. weight. To 
weigh 2 lbs. we must have in addition either a 2 lb. weight or 
a 3 lb. weight ; but, whereas with a 2 lb. weight we can weigh 
1 lb., 2 lbs., and 3 lbs., with a 3 lb. weight we can weigh 
1 lb., (3 - 1) lbs., 3 lbs., and (3 + 1) lbs. Another weight of 
9 lbs. will enable us to weigh all weights from 1 lb. to 13 lbs. ; 
and we get thus a greater range than is obtainable with any 

* Theorie des Nomhres, by E. Lucas, Paris, 1891, pp. 215, 491—495. 
+ See P. A. MacMahon, Combinatory Analysis, vol. i, Cambridge, 1915, 
pp. 253—256. 

+ Bachet, Appendix, problem v, p. 215. 

§ Trattato de' numeti e misure, Venice, 1556, voL ii, bk. i, chap, xvi, art. 32. 



CH. II] ARITHMETICAL RECREATIONS 35 

weight less than 9 lbs. Similarly weights of 1, 3, 9, and 
27 lbs. suffice for all weights up to 40 lbs., and weights of 
1, 3, 3^ ..., 3^~^ lbs. enable us to weigh any integral number 
of pounds from 1 lb. to (1+ 3 + 3^ + . . . 3'^-^ lbs., that is, to 
i(3'*-l)lbs. 

To determine the arrangement of the weights to weigh any 
given mass we have only to express the number of pounds in 
it as a number in the ternary scale of notation, except that in 
finding the successive digits we must make every remainder 
either 0, 1, or — 1 : to effect this a remainder 2 must be written 
as 3 — 1, that is, the quotient must be increased by unity, in 
which case the remainder is — 1. This is explained in most 
text-books on algebra. 

Bachet's argument does not prove that his result is unique 
or that it gives the least possible number of weights required. 
These omissions have been supplied by Major MacMahon, 
who has discussed the far more difficult problem (of which 
Bachet's is a particular case) of the determination of all possible 
sets of weights, not necessarily unequal, which enable us to 
weigh any integral number of pounds from 1 to n inclusive, 
(i) when the weights may be placed in only one scale-pan, and 
(ii) when any weight may be placed in either scale-pan. He 
has investigated also the modifications of the results which are 
necessary when we impose either or both of the further condi- 
tions (a) that no other weighings are to be possible, and (b) that 
each weighing is to be possible in only one way, that is, is to 
be unique*. 

The method for case (i) consists in resolving l-\-x+ar-\-.., + a)^ 
into factors, each factor being of the form l+as^ + a^^ -{-,.. -^ a;"'<»; 
the number of solutions depends on the composite character of 
n + 1. The method for case (ii) consists in resolving the expres- 
sion a?-" + aj-'^+i -I- . . . + a?-i -I- 1 -h a; -i- . . . -f x^'^ + a?" into factors, 
each factor being of the form x~"^ + . . . + ar<^ + l+x'^-\- ,.. + x'^\ 
the number of solutions depends on the composite character of 
2/1 -hi. 

* See his article in the Quarterly Journal of Mathematics^ 1886, vol. xxi, 
pp. 367—373. An account of the method is given in Nature, Dec. 4, 1890, 
vol. xm, pp. 113—114. 

8—2 



36 ARITHMETICAL RECREATIONS [CH. II 

Bachet's problem falls under case (ii), n = 40. MacMahon's 
analysis shows that there are eight such ways of factorizing 
^_4o ^ ^-39 + . , , + 1 + .,. 4. ^n9 + ^40^ First, there is the expres- 
sion itself in which a = 1, m=40. Second, the expression is 
equal to (1 — a^^ya^*^ (1 — x), which can be resolved into the 
product of (l-x^)lx(l-x) and {1 -x^^) 1x^^(1 --a^)] hence it 
can be resolved into two factors of the form given above, in 
one of which a = l, m = 1, and in the other a = 3, m = 13. 
Third, similarly, it can be resolved into two such factors, in 
one of which a = l, m = 4, and in the other a =9, m = 4. 
Fourth, it can be resolved into three such factors, in one of 
which a = l, m = l, in another a = 3, m=l, and in the other 
a = 9, m = 4. Fifth, it can be resolved into two such factors, 
in one of which a = 1, m = 13, and in the other a = 27, m — 1. 
Sixth, it can be resolved into three such factors, in one of 
which a = l, m = l, in another a = 3, m = 4, and in the other 
a = 27, m = 1. Seventh, it can be resolved into three such 
factors, in one of which a = 1, m = 4, in another a = 9, m = 1, 
and in the other a = 27, m= 1. Eighth, it can be resolved into 
four such factors, in one of which a = 1, m = 1, in another a = 3, 
m = 1, in another a = 9, m = 1, and in the other a = 27, m = 1. 

These results show that there are eight possible sets of 
weights with which any integral number of pounds from 1 to 
40 can be weighed subject to the conditions (ii), (a), and (b). 
If we denote p weights each equal to w by w^, these eight 
solutions are P«; 1, 3^^; 1*, 9*; 1, 3, 9^; V\ 27; 1, 3^ 27; 
1^ 9, 27; 1, 3, 9, 27. The last of these is Bachet's solution: 
not only is it that in which the least number of weights are 
employed, but it is also the only one in which all the weights 
are unequal. 

Problems in Higher Arithmetic. Many mathematicians 
take a special interest in the theorems of higher arithmetic: 
such, for example, as that every prime of the form 4?i + 1 and 
every power of it is expressible as the sum of two squares*, and 
that the first and second powers can be expressed thus in only 
one way. For instance, 13 = 3^* + 2^, 13^ = 12-' + 5^, 13^ = 46^ + 9», 

• Fermat's Diopliantus, Toulouse, 1670, bk. m, prop. 22, p. 127; or 
Brassinne's Precis, Paris, 1853, p. 65. 



CH. Il] ARITHMETICAL RECREATIONS 37 

and so on. Similarly 41 = 52-f-4», 41^= 402 + 9«, 41»= 236^+115', 
and so on. Propositions such as the one just quoted may be 
found in text-books on the theory of numbers and therefore 
lie outside the limits of this work, but, there are one or two 
questions in higher arithmetic involving points not yet 
quite cleared up which may find a place here. I content 
myself with the facts and shall not give the mathematical 
analysis. 

Primes. The first of these is concerned with the possibility 
of determining readily whether a given number is prime or not. 
No test applicable to all numbers is known, though of course 
we can get tests for numbers of certain forms. It is difficult to 
believe that a problem which has completely baffled all modern 
mathematicians could have been solved in the seventeenth 
century, but it is interesting to note that in 1643, in answer 
to a question in a letter whether the number 100895,598169 
was a prime, Fermat replied at once that it was the product of 
898423 and 112303, both of which were primes. How many 
mathematicians to-day could answer such a question with 
ease? 

Mersenne's Numbers*. Another illustration, confirmatory 
of the opinion that Fermat or some of his contemporaries had 
a test by which it was possible to find out whether numbers of 
certain forms were prime, may be drawn from Mersenne's Cogitata 
Physico-Mathematica which was published in 1644. In the 
preface to that work it is asserted that in order that 2^ — 1 may 
be prime, the only values of p, not greater than 257, which are 
possible are 1, 2, 3, 5, 7, 13, 17, 19, 31, 67, 127, and 257. To 
these numbers 89 and 107 must be added. Some years ago I 
gave reasons for thinking that the number 67 is a misprint 
for 61. With these corrections the statement appears to be 
true, and it has now been verified for all except fourteen values^ 
of ^: namely, 101, 103, 109, 137, 139, 149, 157, 167, 193, 199, 
227, 229, 241, and 257. Of these values, Mersenne asserted 
that p = 257 makes 2^ — 1 a prime, and that the other values 

* For references, see chapter xvi below. 



38 ARITHMETICAL RECREATIONS [CH. II 

make 2^ — 1 a composite number. The demonstration of the 
prime character of 2^— 1 when p = 12l has not been published : 
and the verification in this case has not been corroborated by 
independent work. 

Mersenne's result could not have been obtained empirically, 
and it is impossible to suppose that it was worked out for every 
case ; hence it would seem that whoever first enunciated it was 
acquainted with certain theorems in higher arithmetic which 
have not been re-discovered. 

Perfect Numbers*. The theory of perfect numbers de- 
pends directly on that of Mersenne's Numbers. A number is 
said to be perfect if it is equal to the sum of all its integral 
subdivisors. Thus the subdivisors of 6 are 1, 2, and 3 ; the 
sum of these is equal to 6 ; hence 6 is a perfect number. 

It is probable that all perfect numbers are included in the 
formula 2^'"^ (2^ — 1), where 2^ — 1 is a prime. Euclid proved 
that any number of this form is perfect; Euler showed that 
the formula includes all even perfect numbers; and there is 
reason to believe — though a rigid demonstration is wanting — 
that an odd number cannot be perfect. If we assume that the 
last of these statements is true, then every perfect number is 
of the above form. It is easy to establish that every number 
included in this formula (except whenp=2) is congruent to 
unity to the modulus 9, that is, when divided by 9 leaves a 
remainder 1; also that either the last digit is a 6 or the last 
two digits are 28. 

Thus, if |)= 2, 3, 5, 7,13, 17, 19, 31, 61, then by Mersenne's rule 
the corresponding values of 2^ — 1 are prime; they are 3, 7, 31, 
127, 8191, 131071, 524287, 2147483647, 2305843009213693951; 
and the corresponding perfect numbers are 6, 28, 496, 8128, 
33550336, 8589869056, 137438691328, 2305843008139952128, 
and 2658455991569831744654692615953842176. 



• On the theory of perfect numbers, see bibliographical references by 
H. Brocard, L'lnterm^diaire des Matliematiciens, Paris, 1895, vol. ii, pp. 52—54; 
and 1905, vol. xii, p. 19. The first volume of the second edition of the French 
translation of this book contains (pp. 280—294) a summary of the leading 
investigation on Perfect Numbers, as also some remarks on Amicable Numbers. 



CH. Il] AlUTHMETICAL RECREATIONS 89 

Euler's Biquadrate Theorem*. Another theorem, 
believed to be true but as yet unproved, is that the arith- 
metical sum of the fourth powers of three numbers cannot 
be the fourth power of a number; in other words, we cannot 
find values of x, y, z, v, which satisfy the equation x* -{■ y* -\- z* = v*. 
The proposition is not true of an algebraical sum, for Euler gave 
more than one solution of the equation x* + y* =^ z* + v*, for 
instance, x = 542, y = 103, z = 359, v = 514. 

Goldbach's Theorem. Another interesting problem in 
higher arithmetic is the question whether there are any even 
integers which cannot be expressed as a sum of two primes. 
Probably there are none. The expression of all even integers 
not greater than 5000 in the form of a sum of two primes has 
been effected f, but a general demonstration that all even 
integers can be so. expressed is wanting. 

Lagrange's Theorem:]:. Another theorem in higher arith- 
metic which, as far as I know, is still unsolved, is to the effect 
that every prime of the form 4n — 1 is the sum of a prime of 
the form 4n + 1 and of double another prime also of the form 
4?i + 1 ; for example, 23 = 13 + 2 x 5. Lagrange, however, added 
that it was only by induction that he arrived at the result. 

Fermat's Theorem on Binary Powers. Fermat enriched 
mathematics with a multitude of new propositions. With one 
exception all these have been proved or are believed to be 
true. This exception is his theorem on binary powers^ in which 
he asserted that all numbers of the form 2"* + 1, where m= 2", 
are primes §, but he added that, though he was convinced of 
the truth of this proposition, he could not obtain a valid 
demonstration. 



* See Euler, Commentationes Arithmeticae Collectae, Petrograd, 1849, vol. i, 
pp. 473_47G ; yoL ii, pp. 450—456. 

t Transactions of the Halle Academy {Naturforschung), vol. lxxii, Halle, 
1897, pp. 5—214 : see also V Inter m€dinire des MatMmaticiens, 1903, vol. x, 
and 1904, vol. xi. 

X Nouveaux MSmoires de VAcademie Royale des Sciences, Berlin, 1775, p. 356. 

§ Letter of Oct. 18, 1G40, Opera^ Toulouse, 1679, p. 162 : or Brassinne's 
Pricis, p. 143. 



40 A.RITHMETICAL RECREATIONS [CH. II 

It may be shown that 2"* + 1 is composite if m is not a power 
of 2, but of course it does not follow that 2'^ + 1 is a prime 
if m is a power of 2, say, 2**. As a matter of fact the theorem 
is not true. In 1732 Eulor* showed that if 7i= 5 the formula 
gives 4294,967297, which is equal to 641 x 6,700417 : curiously 
enough, these factors can be deduced at once from Fermat's 
remark on the possible factors of numbers of the form 2"* ± 1, 
from which it may be shown that the prime factors (if any) 
of 2^2 4-1 must be primes of the form 64n + 1. 

During the last thirty years it has been shownf that the 
resulting numbers are composite when ?i= 6, 7, 8, 9, 11, 12, 18, 
23, 36, 38 and 73. The digits in the last of these numbers are 
so numerous that, if the number were printed in ftdl with the 
type and number of pages used in this book, many more 
volumes would be required than are contained in all the public 
libraries in the world. I believe that Eisenstein asserted that 
the number of primes of the form 2"* + 1, where m = 2", is 
infinite: the proof has not been published, but perhaps it 
might throw some light on the general theory. 

Fermat's Last Theorem. I pass now to another assertion 
made by Format which hitherto has not been proved. This, 
which is sometimes known as Fermat's Last Theorem, is to the 
effect| that no integral values of a?, y, z can be found to satisfy 
the equation x^ + y^ — ^'*, if n is an integer greater than 2. 

* Commentarii Academiae Scientiarum Petropolitanae, Petrograd, 1738, 
vol. VI, p. 104; see also Novi Conim. Acad. Sci. Petrop., Petrograd, 1764, 
vol. IX, p. 101 : or Gommentationes Arithmeticae Collectae, Petrograd, 1849, 
vol. I, pp. 2, 357. 

t For the factors and bibliographical references, see A. J. C. Cunningham 
and A. E. Western, Transactions of the London Mathematical Society, 1903, 
series 2, vol. i, p. 175 ; and J. 0. Morehead and A. E. Western, Bulletin of the 
American Mathematical Society, 1909, vol. xvi, pp. 1 — 6. 

X Fermat's enunciation will be found in his edition of Diophantus, Toulouse, 
1670, bk. II, qu. 8, p. 61 ; or Brassinne's Precis, Paris, 1853, p. 53. For 
bibliographical references, see the article on the theory of numbers in the 
Encyclopedie des Sciences Mathematiques : considerable additions are embodied 
in the French translation of this book which is therefore generally preferable to 
the German original. See also L'Intermediaire des Mathematiciens, Paris, 1908, 
vol. XV, p. 234. 



CH. Il] ARITHMETICAL RECREATIONS 41 

This proposition has acquired extraordinary celebrity from the 
fact that no general demonstration of it has been given, but 
there is no reason to doubt that it is true. 

Fermat seems to have discovered its truth first* for the case 
71 = 3, and then for the case n = 4. His proof for the former 
of these cases is lost, but that for the latter is extant f , and a 
similar proof for the case of n = 3 was given by Euler}. These 
proofs depend upon showing that, if three integral values of 
X, y, z can be found which satisfy the equation, then it will be 
possible to find three other and smaller integers which also 
satisfy it: in this way finally we show that the equation must 
be satisfied by three values which obviously do not satisfy it. 
Thus no integral solution is possible. It would seem that this 
method is inapplicable except when n = 3 and n = 4. 

Fermat's discovery of the general theorem was made later. 
A demonstration can be given on the assumption that every 
number can be resolved in one and only one way into the 
product of primes and their powers. This assumption is true 
of real numbers, but it is not necessarily true when complex 
factors are admitted. For instance 21 can be expressed as the 
product of 3 and 7, or of 4 -j- \/(— 5) and 4 — \l{— 5), or of 
1 + 2 V(- 5) and 1-2 V(- 5). It is possible that Fermat 
made some such erroneous supposition, though it is perhaps 
more probable that he discovered a rigorous demonstration. 
At any rate he asserts definitely that he had a valid proof — 
demonstratio mirabilis sane — and the fact that no theorem on 
the subject which he stated he had proved has been subse- 
quently shown to be false must weigh strongly in his favour ; 
the more so because in making the one incorrect statement in 
his writings (namely, that about binary powers) he added that 
he could not obtain a satisfactory demonstration of it. 

It must be remembered that Fermat was a mathematician 
of quite the first rank who had made a special study of the 
theory of numbers. The subject is in itself one of peculiar 

* See a Letter from Fermat quoted in my History of Mathematics, London, 
chapter xv. 

t Fermat's Diophantus, note on p. 339; or Brassinne's PrScis, p. 127. 
i Euler's Algebra (English trans. 1797), vol. n, chap, xv, p. 217. 



42 ARITHMETICAL RECREATIONS [CH. II 

interest and elegance, but its conclusions have little practical 
importance, and since his time it has been discussed by only 
a few mathematicians, while even of them not many have made 
it their chief study. This is the explanation of the fact that 
it took more than a century before some of the simpler results 
which Fermat had enunciated were proved, and thus it is not 
surprising that a proof of the theorem which he succeeded, in 
establisliing only towards the close of his life should involve 
great difficulties. 

In 1823 Legendre* obtained a proof for the case of n = 5; 
in 1832 Lejeune Dirichlet"[* gave one for yi=14; and in 1840 
Lame and Lebesgue l gave proofs for n = 7. 

The proposition appears to be true universally, and in 1849 
Kummer§, by means of ideal primes, proved it to be so for all 
numbers except those (if any) which satisfy three conditions. 
The proof is complicated and difficult, and there can .be little 
doubt is based on considerations unknown to Fermat. It is not 
known whether any number can be found to satisfy these con- 
ditions. It was shown a considerable time ago, that there is no 
number less than 100 which does so, and recently the limit 
has been extended by L. E. Dickson ||. But mere numerical 
verifications have little value; no one doubts the truth of 
the theorem, and its interest lies in the fact that we have 
not yet succeeded in obtaining a rigorous general demonstra- 
tion of it. The general problem was also attacked on other 
lines by Sophie Germain, who showed that it was true for all 
numbers except those (if any) which satisfied certain defined con- 
ditions. I may add that to prove the truth of the proposition 
when n is greater than 4, obviously it is sufficient to confine 



* Reprinted in his Theorie des Nombres, Paris, 1830, vol. n, pp. 361—368 : 
see also pp. 5, 6. 

t Crelle's Journal, 1832, vol. ix, pp. 390—393. 

X Liouville's Journal, 1841, vol. v, pp. 195—216, 276—279, 348—349. 

§ References to Kummer's Memoirs are given in Smith's Report to the 
British Association on the Theory of Numbers, London, 1860. 

II See L'Intermediaire des Mathematiciens, Paris, 1908, vol. xv, pp. 247—248 ; 
Messenger of Mathematics, Cambridge, 1908, vol. xxxviii, pp. 14—32; and 
Quarterly Journal of Mathematics, Cambridge, 1908, vol. xl, pp. 27—45. 



CH. Il] ARITHMETICAL RECREATIONS 43 

ourselves to cases where n is a prime. A prize * of 100,000 marks 
has been offered for a general proof, to be given before 2007. 

Naturally there has been much speculation as to how Fer- 
mat arrived at the result. The modern treatment of higher 
arithmetic is founded on the special notation and processes 
introduced by Gauss, who pointed out that the theory of 
discrete magnitude is essentially different from that of con- 
tinuous magnitude, but until the end of the last century the 
theory of numbers was treated as a branch of algebra, and such 
proofs by Fermat as are extant involve nothing more than 
elementary geometry and algebra, and indeed some of his 
arguments do not involve any symbols. This has led some 
writers to think that Fermat used none but elementary 
algebraic methods. This may be so, but the following remark, 
which I believe is not generally known, rather points to the 
opposite conclusion. He had proposed, as a problem to the 
English mathematicians, to show that there was only one 
integral solution of the equation a?'^ 4- 2 = 3/^ : the solution 
evidently being x = o,y = S. On this he has a notef to the 
effect that there was no difficulty in finding a solution in 
rational fractions, but that he had discovered an entirely new 
method — sane pulcherrima et subtilissima — which enabled him 
to solve such questions in integers. It was his intention to 
write a work j on his researches in the theory of numbers, but 
it was never completed, and we know but little of his methods 
of analysis. I venture however to add my private suspicion 
that continued fractions played a not unimportant part in his 
researches, and as strengthening this conjecture I may note 
that some of his more recondite results — such as the theorem 
that a prime of the form 4/1 + 1 is expressible as the sum of 
two squares — may be established with comparative ease by 
properties of such fractions. 

• L'IntermSdiaire des MatMmaticiens, vol. xv, pp. 217—218, for references 
and details. 

t Fermat's Diophantus, bk. vi, prop. 19, p. 320; or Brassinne's Precis, 
p. 122. 

X Fermat's Diophantus, bk. iv, prop. 31, p. 181 ; or Brassinne's Pr^cU, p. 82. 



4^ 



CHAPTER TIL 

GEOMETRICAL RECREATIONS. 

In this chapter and the next one I propose to enumerate 
certain geometrical questions the discussion of which will not 
involve necessarily any considerable use of algebra or arithmetic. 
Unluckily no writer like Bachet has collected and classified 
problems of this kind, and I take the following instances from 
my note-books with the feeling that they represent the subject 
but imperfectly. Most of this chapter is devoted to questions 
which are of the nature of formal propositions : the next chapter 
contains a description of various trivial puzzles and games, 
which the older writers would have termed geometrical. 

In accordance with the rule I laid down for myself in the 
preface, I exclude the detailed discussion of theorems which 
involve advanced mathematics. Moreover (with one or two 
exceptions) I exclude any mention of the numerous geomet- 
rical paradoxes which depend merely on the inability of the 
eye to compare correctly the dimensions of figures when their 
relative position is changed. This apparent deception does 
not involve the conscious reasoning powers, but rests on the 
inaccurate interpretation by the mind of the sensations derived 
through the eyes, and I do not consider such paradoxes as 
coming within the domain of mathematics. 

Geometrical Fallacies. Most educated Englishmen are 
acquainted with the series of logical propositions in geometry 
associated with the name of Euclid, but it is not known so 
generally that these propositions were supplemented originally 
by certain exercises. Of such exercises Euclid issued three 



CH. Ill] 



GEOMETRICAL RECREATIONS 



45 



series: two containing easy theorems or problems, and the 
third consisting of geometrical fallacies, the errors in which 
the student was required to find. 

The collection of fallacies prepared by Euclid is lost, and 
tradition has not preserved any record as to the nature of the 
erroneous reasoning or conclusions; but, as an illustration of 
such questions, I append a few demonstrations, leading to 
obviously impossible results. Perhaps they may amuse any one 
to whom they are new. I leave the discovery of the errors to 
the ingenuity of my readers. 

First Fallacy*. To prove that a right angle is equal to an 
angle which is greater than a right angle. Let A BCD be a 
rectangle. From A draw a line AE outside the rectangle, 
equal to -45 or DC and making an acute angle with AB, as 




indicated in the diagram. Bisect CB in H, and through H 
draw HO at right angles to CB. Bisect CE in K^ and through 
K draw KG at right angles to CE. Since CB and CE are not 
parallel the lines HO and KO will meet (say) at 0. Join OA, 
OE, OC, and OD. 

The triangles ODC and DAE are equal in all respects. 
For, since KO bisects CE and is perpendicular to it, we have 

* I believe that this and the fourth of these fallacies were first published 
in this book. They particularly interested Mr C. L. Dodgson ; see the Lewis 
Carroll Picture Book, London, 1899, pp. 264, 266, where they appear in the 
form in which I originally gave them. 



46 



GEOMETRICAL RECREATIONS 



[CH. Ill 



OG = OE. Similarly, since HO bisects GB and DA and is per- 
pendicular to them, we have OD — OA. Also, by construction, 
LG = AE. Therefore the three sides of the triangle ODG are 
equal respectively to the three sides of the triangle DAE. 
Hence, by Euc. I. 8, the triangles are equal ; and therefore the 
angle ODG is equal to the angle OAE. 

Again, since HO bisects DA and is perpendicular to it, v^^e 
have the angle ODA equal to the angle OAD. 

Hence the angle ADG (which is the difference of ODG and 
ODA) is equal to the angle DAE (which is the difference of 
OAE and OAD). But ADG is a right angle, and DAE is 
necessarily greater than a right angle. Thus the result is 
impossible. 

Second Fallacy*. To prove that a part of a line is equal to 
the whole line. Let ABG be a triangle; and, to fix our ideas, 
let us suppose that the triangle is scalene, that the angle B is 




acute, and that the angle A is greater than the angle G. From 
A draw AD making the angle BAD equal to the angle C, and 
cutting BG in D. From A draw AE perpendicular to BG. 

The triangles ABC, ABD are equiangular; hence, by Euc. 
VI.19, AABG : AABD = AG': AD\ 

Also the triangles ABG, ABD are of equal altitude; hence, by 

Euc. VI. 1, 

AABG : AABD=BG :BD, 

.'. AG':AD^==BG : BD. 

AG'_AD' 

•'• BG " BD' 

* jSee a note by M. Ooccoz in L* Illustration, Paris, Jan. 12, 1895. 



CH. Ill] GEOMETRICAL RECREATIONS 47 

Hence, by Euc. ii. 13, 

AB' + BC' -2BG.be ^ ABl -h BD^ - 2BD . BE 

BG ~ BD ' 

AB^ AB* 

/. ^ + 5(7 - 2B^ = ^ + 5i) - 2BE. 

. AB^ j.r._AB^ 
AB'-BG.BD AB^-BG.BD 



BG BD 

.-. BG^BD, 

a result which is impossible. 

Third Fallacy*. To prove that the sum of the lengths of two 
sides of any triangle is equal to the length of the third side. 
A^ , , , ,0 




Let ABG be a triangle. Complete the parallelogram of 
which AB and BG are sides. Divide AB into rn- 1 equal 
parts, and through the points so determined draw n lines 
parallel to BG. Similarly, divide BG into n + l equal parts, 
and through the points so determined draw n lines parallel to 
AB. The parallelogram ABCD is thus divided into (n + 1)^ 
equal and similar parallelograms. 

I draw the figure for the case in which n is equal to 3, 
then, taking the parallelograms of which ^0 is a diagonal, as 
indicated in the diagram, we have 

AB + BG=-AG-\'HJ-^KL-\-MN 
-^GH + JK + LM + NG. 
A similar relation is true however large n may be. Now 
let n increase indefinitely. Then the lines AG, GH, &c. will 

• The Canterbury Puzzles, by H. E. Dudeney, London, 1907, pp. 26—28. 



48 GEOMETRICAL RECREATIONS [CH. Ill 

get smaller and smaller. Finally the points G, J", Z, ... will 
approach indefinitely near the line AG, and ultimately will lie 
on it; when this is the case the sum oi AG and GH will be 
equal to AH, and similarly for the other similar pairs of lines. 
Thus, ultimately, 

AB + BG =:^AH + HK + KM + MG 
= AG, 

a result which is impossible. 

Fourth Fallacy. To prove that every triangle is isosceles. 
Let ABG be any triangle. Bisect BG in D, and through D 
draw DG perpendicular to BG. Bisect the angle BAG by AG. 

First. If BG and ^40 do not meet, then they are parallel. 
Therefore ^0 is at right angles to BG. Therefore AB=-AG. 

Second. If DG and AG meet, let them meet in 0. Draw 
GE perpendicular to AG. Draw GF 
perpendicular to AB. Join GB, GG, 

Let us begin by taking the case 
where is inside the triangle, in 
which case E falls on AG and F on 
BG. 

The triangles AGF and AGE are 
equal, since the side ^0 is common, 

angle GAF— angle GAE, and angle GFA. = angle GEA. Hence 
AF=AE. Also, the triangles BOF and GGE are equal. For 
since GJ) bisects BG a>t right angles, we have GB = GG; also, 
since the triangles AGF and AGE are equal, we have 
GF— GE; lastly, the angles at F and E are right angles. 
Therefore, by Euc. I. 47 and i. 8, the triangles BOF and GGE 
are equal. Hence FB = EG. 

Therefore AF + FB ^ AE -h EG, that is, AB = AG. 

The same demonstration will cover the case where BG and 
AG meet at D, as also the case where they meet outside BG 
but so near it that E and F fall on AG and AB and not on 
AG and AB produced. 

Next take the case where DO and AG meet outside the 
triangle, and E and F fall on AG and AB produced. Draw 




CH. Ill] 



GEOMETRICAL RECREATIONS 



49 



OE perpendicular to AG produced. Draw OF perpendicular 
to AB produced. Join OB, 00. 

Following the same argument as before, from the equality 
of the triangles AOF and AOE, we obtain AF=AE\ and, 
from the equality of the triangles BOF and GOE^ we obtain 
FB = EG. Therefore AF- FB = AE -EG, that i3,AB=^A G. 




Thus in all cases, whether or not DO and AO meet, and 
whether they meet inside or outside the triangle, we have 
AB—AG: and therefore every triangle is isosceles, a result 
which is impossible. 

Fifth Fallacy*. To prove that irj^ is equal to tt/S. On the 
hypothenuse, BG, of an isosceles right-angled triangle, DBG, 
describe an equilateral triangle ABG, the vertex A being on 
the same side of the base as D is. On GA take a point H so 
that GH = GD. Bisect BD in K. Join HK and let it cut GB 
(produced) in L. Join DL. Bisect DL at M, and through 
M draw MO perpendicular to DL. Bisect HL at N, and 
through N draw NO perpendicular to HL. Since DL and HL 
intersect, therefore MO and NO will also intersect ; moreover, 
since BDG is a right angle, MO and NO both slope away from 
DG and therefore they will meet on the side of DL remote 
from A, Join OG, OD, OH, OL. 

The triangles OMD and OML are equal, hence OD = OL, 
Similarly the triangles ONL and ONH are equal, hence 
OL = OH. Therefore OD = OH. Now in the triangles OGD 
and OGH, we have OD = OH, GD = GH (by construction), and 

• This ingenious fallacy is due to Captain Turton : it appeared for the first 
time in the third edition of this work. 

s. & 4 



50 



GEOMETRICAL RECREATIONS 



[CH. Ill 



OG common, hence (by Euc. I. 8) the angle OGD is equal to the 
angle OCH. Hence the* angle BCD is equal to the angle BGH\ 
that is, 7r/4 is equal to 7r/3, which is absurd. 

Sixth Fallacy^. To prove that, if two opposite sides of a 
quadrilateral are equal, the other two sides must he parallel. 
Let ABGD be a quadrilateral such that ^5 is equal to DG. 
Bisect AD in M, and through M draw MO at right angles to 
AD. Bisect BG in N, and draw NO at right angles to BG. 

If MO and NO are parallel, then AD and BG (which are at 
right angles to them) are also parallel. 

If MO and NO are not parallel, let them meet in ; then 
must be either inside the quadrilateral as in the left-hand 







B 

diagram or outside the quadrilateral as in the right-hand 
diagram. Join OA, OB, OG, OD. 

Since OM bisects AD and is perpendicular to it, we have 
OA = OD, and the angle 0AM equal to the angle ODM. 
Similarly OB = OG, and the angle OBN is equal to the angle 
OGN. Also by hypothesis AB = DG, hence, by Euc. i. 8, the 
triangles OAB and ODG are equal in all respects, and therefore 
the angle AOB is equal to the angle DOG, 

Hence in the left-hand diagram the sum of the angles 
AOMy AOB is equal to the sum of the angles DOM, DOG; 
and in the right-hand diagram the difference of the angles 
AOM,AOB is equal to the difference of the angles DOM, DOG; 
and therefore in both cases the angle MOB is equal to the 
angle MOG, i.e. OM (or OM produced) bisects the angle BOG, 
But the angle NOB is equal to the angle NOG, i.e. ON bisects 
the angle BOG; hence OM and ON coincide in direction, 

* Mathesis, October, 1893, series 2, vol. ni, p. 224. 



CH. Ill] GEOMETiUCAL RECREATIONS 51 

Therefore AD and BG, which are perpendicular to this direc- 
tion, must be parallel. This result is not universally true, 
and the above demonstration contains a flaw. 

Seventh Fallacy. The following argument is taken from a 
text-book on electricity, published in 1889 by two distinguished 
mathematicians, in which it was presented as valid. A given 
vector OP of length I can be resolved in an infinite number 
of ways into two vectors OMy MP, of lengths l\ I", and we 
can make Vjl" have any value we please from nothing to 
infinity. Suppose that the system is referred to rectangular 
axes Ox, Oy\ and that OP, OM, MP make respectively angles 
6, 6', 6" with Ox. Hence, by projection on Oy and on Ox, 
we have 

Zsin^ = rsin^' + rsinl9", 

Z cos ^ = Z' cos ^' + r cos ^". 

. a n sin 6' 4- sin 6" 

.*. tan 6' = ^7- ^,, 

71 cost/ -hcosa 

where n = r/V\ This result is true whatever be the value of n. 
But n may have any value (ex. gr. w = oo , or w = 0), hence 
tan 6 — tan B' = tan 6", which obviously is impossible. 

Eighth Fallacy*. Here is a fallacious investigation of the 
value of tt: it is founded on well-known quadratures. The 
area of the semi-ellipse bounded by the minor axis is (in the 
usual notation) equal to ^irah. If the centre is moved off to 
an indefinitely great distance along the major axis, the ellipse 
degenerates into a parabola, and therefore in this particular 
limiting position the area is equal to two-thirds of the circum- 
scribing rectangle. But the first result is true whatever be 
the dimensions of the curve. 

/. \'iTah = fa X 2h, 

.-. 7r=8/3, 

a result which obviously is untrue. 

Ninth Fallacy. Every ellipse is a circle. The focal distance 
of a point on an ellipse is given (in the usual notation) in terms 

♦ This was communicated to me by Mr R. Chartres. 

4—2 



52 GEOMETRICAL RECREATIONS [CH. Ill 

of the abscissa by the formula r=a+ex. Hence drjdx^e. 
From this it follows that r cannot have a maximum or minimum 
value. But the only closed curve in which the radius vector 
has not a maximum or minimum value is a circle. Hence, every 
ellipse is a circle, a result which obviously is untrue. 

Geometrical Paradoxes. To the above examples I may 
add the following questions, which, though not exactly falla- 
cious, lead to results which at a hasty glance appear impossible. 

First Paradox. The first is a problem, sent to me by 
Mr W. Ronton, to rotate a plane lamina (say, for instance, a 
sheet of paper) through four right angles so that the effect is 
equivalent to turning it through only one right angle. 

Second Paradox. As in arithmetic, so in geometry, the 
theory of probability lends itself to numerous paradoxes. 
Here is a very simple illustration. A. stick is broken at 
random into three pieces. It is possible to put them together 
into the shape of a triangle provided the length of the 
longest piece is less than the sum of the other two pieces 
(c/! Euc. I. 20), that is, provided the length of the longest 
piece is less than half the length of the stick. But the 
probability that a fragment of a stick shall be half the 
original length of the stick is 1/2. Hence the probability that 
a triangle can be constructed out of the three pieces into 
which the stick is broken would appear to be 1/2. This is not 
true, for actually the probability is 1/4. 

Third Paradox. The following example illustrates how 
easily the eye may be deceived in demonstrations obtained by 
actually dissecting the figures and re-arranging the parts. In 
fact proofs by superposition should be regarded with consider- 
able distrust unless they are supplemented by mathematical 
reasoning. The well-known proofs of the propositions Euclid 
I. 32 and Euclid I. 47 can be so supplemented and are valid. 
On the other hand, as an illustration of how deceptive a non- 
mathematical proof may be, I here mention the familiar paradox 
that a square of paper, subdivided like a chessboard into 
64 small squares, can be cut into four pieces which being put 



CH. Ill] 



GEOMETRICAL RECREATIONS 



63 



together form a figure containing 65 such small squares*. This 
is effected by cutting the original square into four pieces in the 
manner indicated by the thick lines in the first figure. If these 
four pieces are put together in the shape of a rectangle in the 
way shown in the second figure it will appear as if this rectangle 
contains 65 of the small squares. 

This phenomenon, which in my experience non-mathema- 
ticians find perplexing, is due to the fact that the edges of 
the four pieces of paper, which in the second figure lie along 




A 



the diagonal AB, do not coincide exactly in direction. In 
reality they include a small lozenge or diamond-shaped figure, 
whose area is equal to that of one of the 64 small squares in 
the original square, but whose length AB is much greater than 
its breadth. The diagrams show that the angle between the 
two sides of this lozenge which meet at A is tan~i| — tan~^|, 
that is, is tan"^:^, which is less than 1J°. To enable the eye 
to distinguish so small an angle as this the dividing lines in the 
first figure would have to be cut with extreme accuracy and the 
pieces placed together with great care. 

This paradox depends upon the relation SxlS-S'^sl. 
Similar results can be obtained from the formulae 

13x34-2P=:l, 34x89-552=1,...; 
or from the formulae 

.52-3x8 = 1, 13'^-8x21 = l, 34^-21 x55 = T,.... 

* I do not know who discovered this paradox. It is given in variona raodern 
books, but I cannot find an earlier reference to it than one in the Zeitschrift 
filr Mathematik und Physik, Leipzig, 1868, vol. xiii, p. 162. Some similar 
paradoxes were given by Ozanam, 1803 edition, vol. i, p. 299. 



54 GEOMETRICAL RECREATIONS [CH. Ill 

These numbers are obtained by finding convergents to the con- 
tinued fraction 

111 
^■^14-1+1 + '**' 

Dissection Problems. The above paradoxes naturally sug- 
gest the consideration of dissection problems. An excellent 
typical example is to cut a square into 20 equal triangles, and 
conversely to construct a square of 20 such triangles. 

There is an interesting historical example of such a problem. 
Two late Latin writers, Victorinus and Fortunatianus, describe 
an Archimedean toy composed of 14 ivory polygons which fitted 
exactly into a square box, and they suggest that the puzzle was 
to fit the pieces into the box. A recent discovery * has shown 
that its association with the name of Archimedes is due to the 
fact that he gave a construction for dividing a square into 14 
such pieces (namely, 11 triangles, 2 scalene quadrilaterals, and 
one pentagon) so that the area of each piece is a rational 
fraction of the area of the square. His construction is as 
follows: let A BOB be the square, and E, F, G, H, the mid- 
points of the sides AB, BG, GD, DA. Draw HB, HF, EG, and 
let J, K, L be the mid-points of these lines : draw A KG cutting 
HB in M, and let N be the mid-point of AM, and F the mid- 
point of 5jP. BmwBN. Draw ^P cutting 5^5 in Q. DrawP/. 
Draw BL, and produce it to cut DC in P. Draw FL cutting 
AG in S. Draw LG. Rub out the lines AQ and BL. The 
remaining lines will give a division as required, each figure 
being an integral multiple of l/48th of the square. Why 
Archimedes propounded so peculiar a division it is impossible 
to guess, but no doubt the problem has a history of which we 
are ignorant. 

Colouring Maps. I proceed next to mention the geo- 
metrical proposition that not more than four colours are neces- 
sary in order to colour a map of a country (divided into districts) 

* H. Suter, Zeitschrift filr Mathematik und Fhysik, Abhandlungen zur 
Gesch. der Math. 1899, vo]. xliv, pp. 491—499. 



CH. Ill] 



GEOMETRICAL RECREATIONS 



65 



in such a way that no two contiguous districts shall be of the 
same colour. By contiguous districts are meant districts having 
a common line as part of their boundaries : districts which touch 
only at points are not contiguous in this sense. 

The problem was mentioned by A. F. Mobius* in his 
Lectures in 1840, but it was not until Francis Guthrie f com- 
municated it to De Morgan about 1850 that attention was 
generally called to it : it is said that the fact had been familiar 
to practical map-makers for a long time previously. Through 
De Morgan the proposition then became generally known ; and 
in 1878 Cayleyl recalled attention to it by stating that he could 
not obtain a rigorous proof of it. 

Probably the following argument, though not a formal 
demonstration, will satisfy the reader that the result is true. 

Let Ay B, be three contiguous 
districts, and let X be any other 
district contiguous with all of them. 
Then X must lie either wholly out- 
side the external boundary of the 
area ABC or wholly inside the in- 
ternal boundary, that is, it must 
occupy a position either like X or 
like X'. In either case there is no 
possible way of drawing another 
area Y which shall be contiguous 
with Ay By G, and X. In other 
words, it is possible to draw on a 
plane four areas which are contiguous, but it is not possible 
to draw five such areas. If A, B, G are not contiguous, each 
with the other, or if X is not contiguous with A, By and G, it 
is not necessary to colour them all differently, and thus the 
most unfavourable case is that already treated. Moreover any 

* Leipzig Transactions [Math.-phys. Glasse), 1885, vol. xxxvii, pp. 1 — 6. 

+ See Proceedings of the Royal Society of Edinburgh, July 19, 1880, vol. x, 
p. 728. 

X Proceedings of the London Mathematical Society, 1878, vol. ix, p. 148, and 
Proceedings of the Royal Geographical Society, London, 1879, N.S., vol. i, 
pp. 259 — 261, where some of the diflficulties are indicated. 




56 GEOMETRICAL RECREATIONS [CH. Ill 

of the above areas may diminish to a point and finally disappear 
without affecting the argument. 

That we may require at least four colours is obvious from 
the above diagram, since in that case the areas A, B, G, and X 
would have to be coloured differently. 

A proof of the proposition involves difficulties of a high 
order, which as yet have baffled all attempts to surmount them. 
This is partly due to the fact that if, using only four colours, we 
build up our map, district by district, and assign definite colours 
to the districts as we insert them, we can always contrive the 
addition of two or three fresh districts which cannot be coloured 
differently from those next to them, and which accordingly 
upset our scheme of colouring. But by starting afresh, it 
would seem that we can always re-arrange the colours so as 
to allow of the addition of such extra districts. 

The argument by which the truth of the propdsition was 
formerly supposed to be demonstrated was given by A.B. Kempe* 
in 1879, but there is a flaw in it. 

In 1880, Tait published a solution f depending on the 
theorem that if a closed network of lines joining an even 
number of points is such that three and only three lines meet 
at each point then three colours are sufficient to colour the 
lines in such a way that no two lines meeting at a point are of 
the same colour; a closed network being supposed to exclude 
the case where the lines can be divided into two groups 
between which there is but one connecting line. 

This theorem may be true, if we understand it with the 
limitation that the network is in one plane and that no line 

* He sent his first demonstration across the Atlantic to the American Journal 
of Mathematics, 1879, vol. ii, pp. 193 — 200 ; but subsequently he communicated 
it in simplified forms to the London Mathematical Society, Transactions^ 1879, 
vol. X, pp. 229—231, and to Nature, Feb. 26, 1880, vol. xxi, pp. 399—400. The 
flaw in the argument was indicated in articles by P. J. Heawood in the Quarterly 
Journal of Mathematics, London, 1890, vol. xxiv, pp. 332—338; and 1897, 
vol. XXXI, pp. 270—285. 

t Proceedings of the Boyal Society of Edinburgh, July 19, 1880, vol. x, 
p. 729 ; Philosophical Magazine, January, 1884, series 5, vol. xvii, p. 41 ; and 
Collected Scientific Pajjers, Cambridge, vol. ii, 1890, p. 93. 



CH. Ill] GEOMETRICAL RECREATIONS 57 

meets any other line except at one of the vertices, which is all 
that we require for the map theorem; but it has not been proved. 
Without this limitation it is not correct. For instance the 
accompanying figure, representing a closed network in three 
dimensions of 15 lines formed by the sides of two pentagons 
and the lines joining their corresponding angular points, 
cannot be coloured as described by Tait. If the figure is in 
three dimensions, the lines intersect only at the ten vertices 
of the network. If it is regarded as being in two dimensions, 
only the ten angular points of the pentagons are treated as 
vertices of the network, and any other point of intersection of 



y 

— y 

— ^^ 




y ^ — 




the lines is not regarded as such a vertex. Expressed in tech- 
nical language the difiiculty is this. Petersen* has shown that 
a graph (or network) of the 2nth order and third degree and 
without offshoots {or feuilles) can be resolved into three graphs 
of the 27ith order and each of the first degree, or into two graphs 
of the 2/ith order one being of the first degree and one of the 
second degree. Tait assumed that the former resolution was 
the only one possible. The question is whether the limitations 
mentioned above exclude the second resolution. 

Assuming that the theorem as thus limited can be estab- 
lished, Tait's argument that four colours will suffice for a map 
is divided into two parts and is as follows. 

• See J. Petersen of Copenhagen, L'Intermediaire den MatMmaticiem, vol. v, 
1898, pp. 225—227; and vol. vi, 1899, pp. 36—38. Also Acta MatJiematica, 
Stockholm, vol. xv, 1891, pp. 193—220. 



58 GEOMETRICAL RECREATIONS [CH. Ill 

First, suppose that the boundary lines of contiguous dis- 
tricts form a closed network of lines joining an even number 
of points such that three and only three lines meet at each 
point. Then if the number of districts is n + 1, the number of 
boundaries will be Sn, and there will be 2n points of junction; 
also by Tait's theorem, the boundaries can be marked with three 
colours /5, 7, 8 so that no two like colours meet at a point of 
junction. Suppose this done. Now take four colours, A,B,G, D, 
wherewith to colour the map. Paint one district with the colour 
A; paint the district adjoining A and divided from it by the 
line y8 with the colour B ; the district adjoining A and divided 
from it by the line y with the colour G; the district adjoining A 
and divided from it by the line B with the colour D. Proceed 
in this way so that a line jS always separates the colours A 
and B, or the colours G and D; a line 7 always separates 
A and G, or D and B; and a line B always separates-^ and B, 
or B and G. It is easy to see that, if we come to a district 
bounded by districts already coloured, the rule for crossing each 
of its boundaries will give the same colour : this also follows 
from the fact that, if we regard ft 7, B as indicating certain 
operations, then an operation like B may be represented as 
equivalent to the effect of the two other operations /3 and 7 
performed in succession in either order. Thus for such a map 
the problem is solved. 

In the second case, suppose that at any point four or more 
boundaries meet, then at any such point introduce a small 
district as indicated below; this will reduce the problem to 
the first case. The small district thus introduced may be 



-¥- i< 



coloured by the previous rule; but after the rest of the map is 
coloured this district will have served its purpose, it may be 
then made to contract without limit to a mere point and will 
disappear leaving the boundaries as they were at first. 

Although a proof of the four-colour theorem is still wanting, 
no one has succeeded in constructing a plane map which requires 



CH. Ill] GEOMETRICAL RECREATIONS 59 

more than four tints to colour it, and there is no reason to 
doubt the correctness of the statement that it is not necessary 
to have more than four colours for any plane map. The 
number of ways in which such a map can be coloured with four 
tints has been also considered*, but the results are not suffi- 
ciently interesting to require mention here. 

I believe that in the corresponding question with solids in 
space of three dimensions not more than six tints are required 
to colour the exposed surfaces, but I have never seen any 
attempt to prove this extension of the problem. 

Physical Configuration of a Country. As I have been 
alluding to maps, I may here mention that the theory of 
the representation of the physical configuration of a country 
by means of lines drawn on a map was discussed by Cayley 
and Clerk Maxwell f. They showed that a certain relation 
exists between the number of hills, dales, passes, &c. which 
can co-exist on the earth or on an island. I proceed to give a 
summary of their nomenclature and conclusions. 

All places whose heights above the mean sea level are equal 
are on the same level. The locus of such points on a map is 
indicated by a contour-line. Roughly speaking, an island is 
bounded by a contour-line. It is usual to draw the successive 
contour-lines on a map so that the difference between the 
heights of any two successive lines is the same, and thus the 
closer the contour-lines the steeper is the slope, but the 
heights are measured dynamically by the amount of work to 
be done to go from one level to the other and not by linear 
distances. 

A contour-line in general will be a closed curve. This 
curve may enclose a region of elevation: if two such regions 

* See A. 0. Dixon, Messenger of Mathematics, Cambridge, 1902-3, vol. xxxii, 
pp. 81—83. 

t Cayley on • Contour and Slope Lines,^ Philosophical Magazine, London, 
October, 1859, series 4, vol. xvni, pp. 264 — 268; Collected Works, vol. iv, 
pp. 108—111. J. Clerk Maxwell on 'Hills and Dales,' Philosophical Magazine, 
December, 1870, series 4, vol. xl, pp. 421—427; Collected Works, vol. ii, 
pp. 233—240. 



60 GEOMETRICAL RECREATIONS [CH. Ill 

meet at a point, that point will be a crunode (i.e. a real double 
point) on the contour-line through ib, and such a point is 
called a pass. The contour-line may enclose a region of de- 
pression: if two such regions meet at a point, that point 
will be a crunode on the contour-line through it, and such 
a point is called a fork or bar. As the heights of the corre- 
sponding level surfaces become greater, the areas of the regions 
of elevation become smaller, and at last become reduced to 
points: these points are the summits of the corresponding 
mountains. Similarly as the level surface sinks the regions of 
depression contract, and at last are reduced to points: these 
points are the bottoms, or immits, of the corresponding valleys. 

Lines drawn so as to be everywhere at right angles to 
the contour-lines are called lines of slope. If we go up a line 
of slope generally we shall reach a summit, and if we go 
down such a line generally we shall reach a bottom': we may 
come however in particular cases either to a pass or to a fork. 
Districts whose lines of slope run to the same summit are 
hills. Those whose lines of slope run to the same bottom are 
dales. A watershed is the line of slope from a summit to a 
pass or a fork, and it separates two dales. A watercourse is 
the line of slope from a pass or a fork to a bottom, and it 
separates two hills. 

If /I + 1 regions of elevation or of depression meet at a 
point, the point is a multiple point on the contour-line drawn 
through it; such a point is called a pass or a fork of the 
nth order, and must be counted as n separate passes (or forks). 
If one region of depression meets another in several places at 
once, one of these must be taken as a fork and the rest as 



Having now a definite geographical terminology we can 
apply geometrical propositions to the subject. Let h be the 
number of hills on the earth (or an island), then there will be 
also h summits ; let d be the number of dales, then there will 
be also d bottoms ; let p be the whole number of passes, pi that 
of single passes, pa of double passes, and so on ; let / be the 
whole number of forks, /i that of single forks, f^ of double 



CII. Ill] GEOMETRICAL RECREATIONS 61 

forks, and so on ; let w be the number of watercourses, then 
there will be also w watersheds. Hence, by the theorems of 
Cauchy and Euler, 

eZ = l+/, +2/,+..., 

and ^=2(;)i4-/i) + 3(;)2+/2) + .... 

These results can be extended to the case of a multiply- 
connected closed surface. 

ADDENDUM. 

Note. Page 52. The required rotation of the lamina can be effected 
thus. Suppose that the result is to be equivalent to turning it through 
a right angle about a point 0. Describe on the lamina a square OABC, 
Rotate the lamina successively through two right angles about the 
diagonal OB as axis and through two right angles about the side OA as 
axis, and the required result will be attained. 



62 



CHAPTER IV. 

GEOMETRICAL RECREATIONS CONTINUED. 

Leaving now the question of formal geometrical proposi- 
tions, I proceed to enumerate a few games or puzzles which 
depend mainly on the relative position of things, but I post- 
pone to chapter X the discussion of such amusements of 
this kind as necessitate any considerable use of arithmetic or 
algebra. Some writers regard draughts, solitaire, chess, and 
such like games as subjects for geometrical treatment in the 
same way as they treat dominoes, backgammon, and games 
with dice in connection with arithmetic : but these discussions 
require too many artificial assumptions to correspond with the 
games as actually played or to be interesting. 

The amusements to which I refer are of a more trivial 
description, and it is possible that a mathematician may like to 
omit this chapter. In some cases it is difficult to say whether 
they should be classified as mainly arithmetical or geometrical, 
but the point is of no importance. 

Statical Games of Position. Of the innumerable statical 
games involving geometry of position I shall mention only 
three or four. 

Tkree-in-a-row. First, I may mention the game of three- 
in-a-row, of which noughts and crosses, one form of merrilees, 
and go-bang are well-known examples. These games are 
played on a board — generally in the form of a square con- 
taining V? small squares or cells. The common practice is for 
one player to place a white counter or piece or to make a cross 
on each small square or cell which he occupies : his opponent 



CH. IV] GEOMETRICAL RECKEATIONS 63 

similarly uses black counters or pieces or makes a nought on 
each cell which he occupies. Whoever first gets three (or any 
other assigned number) of his pieces in three adjacent cells 
and in a straight line wins. There is no difficulty in giving 
the complete analysis for boards of 9 cells and of 16 cells: but 
it is lengthy and not particularly interesting. Most of these 
games were known to the ancients*, and it is for that reason 
I mention them here. 

Three-in-a-row. Extension. I may, however, add an elegant 
but difficult extension which has not previously found its way, 
so far as I am aware, into any book of mathematical recreations. 
The problem is to place n counters on a plane so as to form as 
many rows as possible, each of which shall contain three and 
only three counters f. 

It is easy to arrange the counters in a number of rows 
equal to the integral part of {n — ly/S. This can be effected by 
the following construction. Let P be any point on a cubic. 
Let the tangent at P cut the curve again in Q. Let the tangent 
at Q cut the curve in A. Let PA cut the curve in B, QB cut 
it in G, PC cut it in D, QD cut it in E, and so on. Then the 
counters must be placed at the points P, Q, A, B, .,„ Thus 9 
coun-ters can be placed in 8 such rows ; 10 counters in 10 rows ; 
15 counters in 24 rows ; 81 counters in 800 rows ; and so on. 

If however the point P is a pluperfect point of the nth 
order on the cubic, then Sylvester proved that the above con- 
struction gives a number of rows equal to the integral part of 
(n — l){n— 2)16. Thus 9 counters can be arranged in 9 rows, 
which is a well-known and easy puzzle ; 10 counters in 12 rows ; 
15 counters in 30 rows ; and so on. 

Even this however is an inferior limit and may be ex- 
ceeded — for instance, Sylvester stated that 9 counters can be 
placed in 10 rows, each containing three counters ; I do not 
know how he placed them, but one way of so arranging them is 

* Becq de Fouqui^res, Les Jeux des Amiens^ second edition, Paris, 1873, 
chap. xvm. 

t Educational Times Reprints, 1868, vol. viii, p. 106 ; Ibid. 1880, vol. xlv, 
pp. 127—123. 



64 GEOMETRICAL RECREATIONS [CH. IV 

by putting them at points whose coordinates are (2, 0), (2, 2), 
(2, 4), (4, 0), (4, 2), (4, 4), (0. 0), (3, 2), (6, 4); another way 
is by putting them at the points (0, 0), (0, 2), (0, 4), (2, 1), 
(2, 2), (2, 3), (4, 0), (4, 2), (4, 4) ; more generally, the angular 
points of a regular hexagon and the three points (at infinity) of 
intersection of opposite sides form such a group, and therefore 
any projection of that figure will give a solution. At present it 
is not possible to say what is the maximum number of rows of 
three which can be formed from n counters placed on a plane. 

Extension to p-in-a-row. The problem mentioned above at 
once suggests the extension of placing n counters so as to form 
as many rows as possible, each of which shall contain p and only 
p counters. Such problems can be often solved immediately by 
placing at infinity the points of intersection of some of the lines, 
and (if it is so desired) subsequently projecting the diagram 
thus formed so as to bring these points to a finite distance. One 
instance of such a solution is given above. 

As examples I may give the arrangement of 10 counters in 
5 rows, each containing 4 counters ; the arrangement of 16 
counters* in 15 rows, each containing 4 counters; the arrange- 
ment of 18 counters in 9 rows, each containing 5 counters ; and 
the arrangement of 19 counters in 10 rows, each containing 5 
counters. These problems I leave to the ingenuity of my readers. 

Tesselation. Another of these statical recreations is known 
as tesselation, and consists in the formation of geometrical 
designs or mosaics covering a plane area by the use of tiles of 
given geometrical forms. 

If the tiles are regular polygons, the resulting forms can be 
found by analysis. For instance, if we confine ourselves to the 
use of like tiles each of which is a regular polygon of n sides, we 
are restricted to the use of equilateral triangles, squares, or hex- 
agons. For suppose that to fill the space round a point where 
one of the angles of the polygon is situated we require m poly- 
gons. Each interior angle of the polygon is equal to (n— 2)7r/w. 
Hence m (n - 2) ir/n = 27r. Therefore (m - 2) (n - 2) = 4. Now 
from the nature of the problems m is greater than 2, and so 
is w. If m = 3, n = 6. If wi > 3, then n < 6, and since n>2, 



CH. IV] GEOMETRICAL RECREATIONS 65 

we have in this case only to consider the vahies n — 3, n — 4f, 
and 71 = 5. If n = 3 we have m = 6. If n = 4 we have m = 4. 
If n = 5, m is non-integral, and this is impossible. Thus the 
only solutions are m = 3 and n = 6, m = 4 and ti = 4, m = 6 
and 71 = 3 *. 

If, however, we allow the use of unlike equilateral tiles 
(triangles, squares, &c.), we can construct numerous geometrical 
designs covering a plane area; though it is impossible to 
do so by the use of such starred concave polygons f. If at 
each point the same number and kind of polygons are used, 
analysis similar to the above shows that we can get six possible 
superposable arrangements, namely when the polygons are 
(i) 3-sided, 12-sided, 12-sided; (ii) 4-sided, 6-sided, 12-sided; 
(iii) 4-sided, 8-sided, 8-sided; (iv) 3-sided, 3-sided, 6-sided, 
6-sided; (v) 3-sided, 4-sided, 4-sided, 6-sided; (vi) 3-sided, 
3-sided, 3-sided, 4-sided, 4-sided. 

The use of colours introduces new considerations. One 
formation of a pavement by the employment of square tiles of 
two colours 'is illustrated by the common chess-board ; in this 
the cells are coloured alternately white and black. Another 
variety of a pavement made with square tiles of two colours was 
invented by SylvesterJ, who termed it anallagmatic. In the 
ordinary chess-board, if any two rows or any two columns are 
placed in juxtaposition, cell to cell, the cells which are side by 
side are either all of the same colour or all of different colours. 
In an anallagmatic arrangement, the cells are so coloured (with 
two colours) that when any two columns or any two rows are 
placed together side by side, half the cells next to one another 
are of the same colour and half are of different colours. 

Anallagmatic pavements composed of m^ cells or square 
tiles can be easily constructed by the repeated use of the four 
elementary anallagmatic arrangements given in the angular 

* Monsieur A. Hermann has proposed an analogous theorem for polygons 
covering the surface of a sphere. 

t On this, see the second edition of the French translation of this work, 
Paris, 1908, vol. ii, pp. 26—37. 

X See Mathematical Questions from the Educational Times, London, vol. x, 
1868, pp. 74—76 ; vol. lvi, 1892, pp. 97—99. The results are closely connected 
with theorems in the theory of equations. 

B. U. 5 



GEOMETRICAL RECREATIONS 



[CH. IV 



spaces of the accompanying diagram. In these fundamental 
forms A represents one colour and B the other colour. The 
diamond-shaped figure in the middle of the diagram represents 
an anallagmatic pavement of 256 tiles which is symmetrical 
about its diagonals. In half the rows and half the columns 
each line has 10 white tiles and 6 black tiles, and in the 
remaining rows and columns each line has 6 white tiles and 



A 


B 


B 


B 



B 


A 


B 


B 




B 


B 


A 


B 



B 


B 


B 


A 



An Anallagmatic Isochromatic Pavement. 

10 black tiles. Such an arrangement, where the difference 
between the number of white and black tiles used in each line 
is constant, and equal to Vm, is called isochromatic. If m is 
odd or oddly even, it is impossible to construct anallagmatic 
boards which are isochromatic. 

Interesting problems can also be proposed when the tiles are 
triangular, whether equilateral or isosceles right-angled. Two 
equal isosceles right-angled tiles of different colours can be 



CH. IV] 



GEOMETRICAL RECREATIONS 



67 



put together so as to make a square tile as shown in the 
margin We can arrange four such tiles in 
no less than 256 different ways, making 64 
distinct designs. With the use of more tiles 
the number of possible designs increases with 
startling rapidity*. I content myself with 
giving two illustrations of designs of pave- 
ments constructed with sixty-four such tiles, all exactly alike. 




mdrwmmmm 









Examples of Tesselated Pavements, 

If more than two colours are used, the problems become 
increasingly difficult. As a simple instance take sixteen square 
tiles, the upper half of each being yellow, red, pink, or blue, 
and the lower half being gold, green, black, or white, no two 
tiles being coloured alike. Such tiles can be arranged in the 
form of a square so that in each vertical, horizontal, and diagonal 
line there shall be 8 colours and no more ; or so that there 
shall be 6 colours and no more; or 5 colours and no more; 
or 4 colours and no more. 

Colour-Cube Problem. As an example of a recreation 
analogous to tesselation I will mention the colour-cube problem f. 
Stripped of mathematical technicalities the problem may 
be enunciated as follows. A cube has six faces, and if six 
colours are chosen we can paint each face with a different 
colour. By permuting the order of the colours we can obtain 

* On this, see Lucas, Recreations Mathematiques, Paris, 1882-3, vol. ii, part 4 ; 
hereafter I shall refer to this work by the name of the author. 

+ By P. A. MaoMahon ; an abstract of his paper, read before the London 
Mathematical Society on Feb. 9, 1893, was given in Nature, Feb. 23, 1893, 
vol. xLvii, p. 406, 

6—2 



68 . GEOMETllICAL EECREATIONS [CH. IV 

thirty such cubes, no two of which are coloured alike. Take 
any one of these cubes, K, then it is desired to select eight out 
of the remaining twenty-nine cubes, such that they can be 
arranged in the form of a cube (whose linear dimensions are 
double those of any of the separate cubes) coloured like the 
cube K, and placed so that where any two cubes touch each 
other the faces in contact are coloured alike. 

Only one collection of eight cubes can be found to satisfy 
these conditions. These eight cubes can be determined by the 
following rule. Take any face of the cube K: it has four 
angles, and at each angle three colours meet. By permuting the 
colours cyclically we can obtain from each angle two other 
cubes, and the eight cubes so obtained are those required. A 
little consideration will show that these are the required cubes, 
and that the solution is unique. 

For instance suppose that the six colours are indicated 
by the letters a, 6, c, d, e, /. Let the cube K be put on a 
table, and to fix our ideas su[»pose that the face coloured / 
is at the bottom, the face coloured a is at the top, and the 
faces coloured 6, c, d, and e front respectively the east, north, 
west, and south points of the compass. I may denote such an 
arrangement by (/; a; 6, c, d, e). One cyclical permutation 
of the colours which meet at the north-east corner of the top 
face gives the cube (/; c; a, 6, d, e), and a second cyclical 
permutation gives the cube (/; 6; c, a, d, e). Similarly 
cyclical permutations of the colours which meet at the north- 
west corner of the top face of ^ give the cubes (/; d] b,a,c,e) 
and (/; c ; byd, a, e). Similarly from the top south-west corner 
of K we get the cubes (/; e; h,G, a, d) and (/; d) b,c, e, a) : 
and from the top south-east corner we get the cubes 
(J;e;a, c, d, b) and (/; b; e, c, d, a). 

The eight cubes being thus determined it is not difficult to 
arrange them in the form of a cube coloured similarly to jBT, 
and subject to the condition that faces in contact are coloured 
alike; in fact they can be arranged in two ways to satisfy 
these conditions. One such way, taking the cubes in the 
numerical order given above, is to put the cubes 3, 6, 8, and 2 



CH. IV] GEOMETRICAL RECREATIONS 69 

at the SE, NE, N W, and SW corners of the bottom face ; of 
course each placed with the colour /at the bottom, while 3 and 
6 have the colour b to the east, and 2 and 8 have the colour d 
to the west : the cubes 7, 1, 4, and 5 will then form the SE, NE, 
NW, and SW comers of the top face; of course each placed 
with the colour a at the top, while 7 and 1 have the colour b to 
the east, and 5 and 4 have the colour d to the west. If K is 
not given, the difficulty of the problem is increased. Similar 
puzzles in two dimensions can be made. 

Tangrams. The formation of designs by means of seven 
pieces of wood, namely, a square, a rhombus, and five triangles, 
known as tans, of fixed traditional shapes, is one of the oldest 
amusements in the East. Many hundreds of figures represent- 
ing men, women, birds, beasts, fish, houses, boats, domestic 
objects, designs, &c. can be made, but the recreation is not 
mathematical, and I reluctantly content myself with a bare 
mention of it. 

Dynamical Games of Position. Games which are played 
by moving pieces on boards of various shapes — such as merrilees, 
fox and geese, solitaire, backgammon, draughts, and chess — 
present more interest. In general, possible movements of the 
pieces are so numerous that mathematical analysis is not 
practicable, but in a few games the possible movements are 
sufficiently limited as to permit of mathematical treatment; 
one or two of these are given later: here I shall confine 
myself mainly to puzzles and simple amusements. 

Shunting Froblems. The first I will mention is a little 
puzzle which I bought some years ago and which was described 
as the " Great Northern Puzzle." It is typical of a good many 
problems connected with the shunting of trains, and though it 
rests on a most improbable hypothesis, I give it as a specimen 
of its kind. 

The puzzle shows a railway, BEFy with two sidings, DBA 
and FGAy connected at A. The portion of the rails at A 
which is common to the two sidings is long enough to permit) 
of a single wagon, like P or Q, running in or out of it ; but is 
too short to contain the whole of an engine, like R. Hence, if 



70 



GEOMETRICAL RECREATIONS 



[CH. IV 



an engine runs up one siding, such as DBA, it must come back 
the same way. 

Initially a small block of wood, P, coloured to represent 
a wagon, is placed at -B ; a similar block, Q, is placed at G ; and 
a longer block of wood, jR, representing an engine, is placed at 
E. The problem is to use the engine R to interchange the 
wagons P and Q, without allowing any flying shunts. 




Another shunting puzzle, on sale in the streets in 1905, 
under the name of the " Chifu-Chemulpo Puzzle," is made as 
follows. A loop-line BQE connects two points B and ^ on a 
railway track AF, which is supposed blocked at both ends, as 
shown in the diagram. In the model, the track AF is 9 inches 
long, AB = EF= If inches, and AH==FK=^ BO =DE = i inch. 

G 




H K 

On the track and loop are eight wagons, numbered succes- 
sively 1 to 8, each one inch long and one-quarter of an 
inch broad, and an engine, e, of the same dimensions. 
Originally the wagons are on the track from A to F and in the 
order 1, 2, 8, 4, 5, 6, 7, 8, and the engine is on the loop. The 
construction and the initial arrangement ensure that at any one 
time there cannot be more than eight vehicles on the track. 
Also if eight vehicles are on it only the penultimate vehicle 
at either end can be moved on to the loop, but if less than eight 
are on the track then the last two vehicles at either end can be 
moved on to the loop. If the points at each end of the loop- 
line are clear, it will hold four, but not more than four, vehicles. 
The object is to reverse the order of the wagons on the track. 



CH. IV] GEOMETRICAL RECREATIONS 71 

SO that from A to F they will be numbered successively 8 to 1 ; 
and to do this by means which will involve as few transferences 
of the engine or a wagon to or from the loop as is possible. 
Twenty-six moves are required, and there is more than one 
solution in 26 moves. 

Other shunting problems are not uncommon, but these two 
examples will suffice. 

Ferry-Boat Problems, Everybody is familiar with the story 
of the showman who was travelling with a wolf, a goat, and a 
basket of cabbages ; and for obvious reasons was unable to leave 
the wolf alone with the goat, or the goat alone with the cabbages. 
The only means of transporting them across a river was a boat 
so- small that he could take in it only one of them at a time. The 
problem is to show how the passage could be effected*. 

A somewhat similar problem is to arrange for the passage of 
a river by three men and three boys who Irave the use of a boat 
which will not carry at one time more than one man or two 
boys. Fifteen passages are required f. 

Problems like these were proposed by Alcuin, Tartaglia, and 
other medieval writers. The following is a common type of such 
questions. Three J beautiful ladies have for husbands three 
men, who are young, gallant, and jealous. The party are 
travelling, and find on the bank of a river, over which they 
have to pass, a small boat which can hold no more than two 
persons. How can they cross the river, it being agreed that, in 
order to avoid scandal, no woman shall be left in the society of 
a man unless her husband is present? Eleven passages are 
required. With two married couples five passages are required. 
The similar problem with four married couples is insoluble. 

Another similar problem is the case of n married couples 
who have to cross a river by means of a boat which can be 
rowed by one person and will carry n — 1 people, but not more, 
with the condition that no woman is to be in the society of a 
man unless her husband is present. Alcuin's problem given 

♦ Ozanam, 1803 edition, vol. i, p. 171 ; 1840 edition, p. 77. 
t H. E. Dudeney, The Tribune, October 4, 1906. 
X Bachet, Appendix, problem iv, p. 212, 



72 GEOMETRICAL RECREATIONS [CH. IV 

above is the case of ?^ = 3. Let y denote the number of passages 
from one bank to the other which will be necessary. Then it 
has been shown that if n = 3, y = 11 ; if ?i = 4, 3/ = 9 ; and if 
7i > 4, 2/ = 7. 

The following analogous problem is due to the late E. 
Lucas*. To find the smallest number x of persons that a boat 
must be able to carry in order that n married couples may by 
its aid cross a river in such a manner that no woman shall 
remain in the company of any man unless her husband is 
present ; it being assumed that the boat can be rowed by one 
person only. Also to find the least number of passages, say y, 
from one bank to the other which will be required. M. Delannoy 
has shown that ii n=2, then a? = 2, and y = 5. If ti = 3, then 
x = % and 2/ = 11. If n = 4, then a; =3, and 3/ = 9. If 7i = 5, 
then x—Z, and 3/ = 11. And finally if n>5, then a? = 4, and 

M. De Fonteney has remarked that, if there was an island 
in the middle of the river, the passage might be always effected 
by the aid of a boat which could carry only two persons. If there 
are only two or only three couples the island is unnecessary, and 
the case is covered by the preceding method. His solution, 
involving 8w — 8 passages, is as follows. The first nine passages 
will be the same, no matter how many couples there may be : 
the result is to transfer one couple to the island and one couple 
to the second bank. The result of the next eight passages is 
to transfer one couple from the first bank to the second bank, 
this series of eight operations must be repeated as often as 
necessary until there is left only one couple on the first bank, 
only one couple on the island, and all the rest on the second 
bank. The result of the last seven passages is to transfer all the 
couples to the second bank. It would however seem that if n 
is greater than 3, we need not require more than 6/i — 7 passages 
from land to land. 

M. G. Tarry has suggested an extension of the problem, 
which still further complicates its solution. He supposes that 

* Lucas, vol. I, pp. 15—18, 237—238. 



CH. IV] GEOMETRICAL RECREATIONS 73 

each husband travels with a harem of m wives or concubines ; 
moreover, as Mohamnaedan women are brought up in seclusion, 
it is reasonable to suppose that they would be unable to row a 
boat by themselves without the aid of a man. But perhaps the 
difficulties attendant on the travels of one wife may be deemed 
sufficient for Christians, and I content myself with merely 
mentioning the increased anxieties experienced by Moham- 
medans in similar circumstances. 

Geodesies. Geometrical problems connected with finding 
the shortest routes from one point to another on a curved 
surface are often difficult, but geodesies on a flat surface or flat 
surfaces are in general readily determinable. 

I append one instance*, though I should have hesitated to 
do so, had not experience shown that some readers do not readily 
see the solution. It is as follows. A room is 30 feet long, 
12 feet wide, and 12 feet hiorh. On the middle line of one of 
the smaller side walls and one foot from the ceiling is a wasp. 
On the middle line of the opposite wall and 11 feet from the 
ceiling is a fly. The wasp catches the fly by crawling all the 
way to it: the fly, paralysed by fear, remaining still. The 
problem is to find the shortest route that the wasp can 
follow. 

To obtain a solution we observe that we can cut a sheet of 
paper so that, when folded properly, it will make a model to 
scale of the room. This can be done in several ways. If, when 
the paper is again spread out flat, we can join the points repre- 
senting the wasp and the fly by a straight line lying wholly on 
the paper we shall obtain a geodesic route between them. 
Thus the problem is reduced to finding the way of cutting out 
the paper which gives the shortest route of the kind. 

Here is the diagram corresponding to a solution of the 
above question, where A represents the floor, B and D the 
longer side-walls, G the ceiling, and W and F the positions on 
the two smaller side-walls occupied initially by the wasp and fly. 

* This is due to Mr H. E. Dudeney. I heard a sunilar question propounded 
at Cambridge in 1903, but I first saw it in print in the Daily Mail, London, 
February 1, 1905. 



74 



GEOMETRICAL RECREATIONS 



[CH. IV 



In the diagram the square of the distance between W and F is 
(32)2 + (24)2 J ]^Q^QQ the distance is 40 feet. 



W 



^. 



Problems with Counters placed in a row. Numerous 
dynamical problems and puzzles may be illustrated with a 
box of counters, especially if there are counters of two colours. 
Of course coins or pawns or cards will serve equally well. 
I proceed to enumerate a few of these played with counters 
placed in a row. 

Fio^st Problem with Counters. The following problem must 
be familiar to many of my readers. Ten counters (or coins) are 
placed in a row. Any counter may be moved over two of those 
adjacent to it on the counter next beyond them. It is required 
to move the counters according to the above rule so that they 
shall be arranged in five equidistant couples. 

If we denote the counters in their initial positions by the 
numbers 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, we proceed as follows. Put 
7 on 10, then 5 on 2, then 3 on 8, then 1 on 4, and lastly 9 on 
6. Thus they are arranged in pairs on the places originally 
occupied by the counters 2, 4, 6, 8, 10. 

Similarly by putting 4 on 1, then 6 on 9, then 8 on 3, then 
10 on 7, and lastly 2 on 5, they are arranged in pairs on the 
places originally occupied by the counters 1, 3, 5, 7, 9. 

If two superposed counters are reckoned as only one, 
solutions analogous to those given above will be obtained by 



CH. IV] GEOMETRICAL RECREATIONS 75 

putting 7 on 10, then 5 on 2, then 3 on 8, then 1 on 6, and 
lastly 9 on 4 ; or by putting 4 on 1, then 6 on 9, then 8 on 3, 
then 10 on 5, and lastly 2 on 7 *. 

There is a somewhat similar game played with eight counters, 
but in this case the four couples finally formed are not equi- 
distant. Here the transformation will be effected if we move 

6 on 2, then 3 on 7, then 4 on 1, and lastly 6 on 8. This form 
of the game is applicable equally to (8 + ^n) counters, for if we 
move 4 on 1 we have left on one side of this couple a row of 
(8 + 27i — 2) counters. This again can be reduced to one of 
(8 + 271 — 4) counters, and in this way finally we have left eight 
counters which can be moved in the way explained above. 

A more complete generalization would be the case of n 
counters, where each counter might be moved over the m 
counters adjacent to it on to the one beyond them. For instance 
we may place twelve counters in a row and allow the moving 
a counter over three adjacent counters. By such movements we 
can obtain four piles, each pile containing three counters. Thus, 
if the counters be numbered consecutively, one solution can be 
obtained by moving 7 on 3, then 5 on 10, then 9 on 7, then 12 
on 8, then 4 on 5, then 11 on 12, then 2 on 6, and then 1 on 2. 
Or again we may place sixteen counters in a row and allow 
the moving a counter over four adjacent counters on to the 
next counter available. By such movements we can get four 
piles, each pile containing four counters. Thus, if the counters 
be numbered consecutively, one solution can be obtained by 
moving 8 on 3, then 9 on 14, then 1 on 5, then 16 on 12, then 

7 on 8, then 10 on 7, then 6 on 9, then 15 on 16, then 13 on 1, 
then 4 on 15, then 2 on 13, and then 11 on 6. 

Second Problem with Counters. Another problem f, of a 
somewhat similar kind, is of Japanese origin. Place four florins 
(or white counters) and four halfpence (or black counters) 
alternately in a line in contact with one another. It is required 

* Note by J. Fitzpatrick to a French translation of the third edition of this 
work, Paris, 1898. 

t Bihliotheca Mathematica, 1896, series 3, vol. vi, p. 323 ; P. G. Tait, PhUo^ 
tophical Magazine, London, January, 1884, series 6, vol. xvu, p. 39; or Collected 
Scientific Papen, Cambridge, vol. ii, 1890, p. 93. 



76 GEOMETRICAL RECREATIONS [CH. IV 

in four moves, each of a pair of two contiguous pieces, without 
altering the relative position of the pair, to form a continuous 
line of four halfpence followed by four florins. 

This can be solved as follows. Let a florin be denoted by a 
and a halfpenny by h, and let x x denote two contiguous blank 
spaces. Then the successive positions of the pieces may be 
represented thus; 

Initially y. x ah ah ah ah. 

After the first move . . baahahaxxh. 

After the second move . haahxxaahb. 

After the third move . . bxxhaaaahb. 

After the fourth move . hhhhaaaax x. 

The operation is conducted according to the following rule. 
Suppose the pieces to be arranged originally in circular order, 
with two contiguous blank spaces, then we always move to the 
blank space for the time being that pair of coins which occupies 
the places next but one and next but two to the blank space on 
one assigned side of it. 

A similar problem with 2n counters — n of them being white 
and n black — will at once suggest itself, and, if n is greater 
than 4, it can be solved in n moves. I have however failed to 
find a simple rule which covers all cases alike, but solutions, due 
to M. Delannoy, have been given* for the four cases where n is 
of the form 4m, 4m + 2, 4m 4- 1, or 4m + 3 ; in the first two cases 
the first ^n moves are of pairs of dissimilar counters and the 
last ^n moves are of pairs of similar counters; in the last two 
cases, the first move is similar to that given above, namely, 
of the penultimate and antepenultimate counters to the be- 
ginning of the row, the next J (w — 1) moves are of pairs of 
dissimilar counters, and the final J (n — 1) moves are of similar 
counters. 

The problem is also capable of solution if we substitute the 
restriction that at each move the pair of counters taken up must 
be moved to one of the two ends of the row instead of the 
condition that the final arrangement is to be continuous. 

* La Nature, June, 1887, p. 10. 



CH. IV] GEOMETRICAL RECREATIONS 77 

Tait snsfgested a variation of the problem by making it 
a condition that the two coins to be moved shall also be made to 
interchange places ; in this form it would seem that five moves 
are required ; or, in the general case, w + 1 moves are required. 

Problems on a Chess-board with Counters or Pawns. The 
following three problems require the use of a chess-board as well 
as of counters or pieces of two colours. It is more convenient 
to move a pawn than a counter, and if therefore I describe them 
as played with pawns it is only as a matter of convenience and 
not that they have any connection with chess. The first is 
characterized by the fact that in every position not more than 
two moves are possible ; in the second and third problems not 
more than four moves are possible in any position. With these 
limitations, analysis is possible. I shall not discuss the similar 
problems in which more moves are possible. 

First Problem with Pawns*. On a row of seven squares 
on a chess-board 3 white pawns (or counters), denoted in the 
diagram by "a"s, are placed on the 3 squares at one end, and 
3 black pawns (or counters), denoted by " b "s, are placed on the 
3 squares at the other end — the middle square being left vacant. 
Each piece can move only in one direction ; the " a " pieces can 
move from left to right, and the " 6 " pieces from right to left. 
If the square next to a piece is unoccupied, it can move on 



a 


a 


a 




b 


b b 



to that ; or if the square next to it is occupied by a piece of the 
opposite colour and the square beyond that is unoccupied, then 
it can, like a queen in draughts, leap over that piece on to the 
unoccupied square beyond it. The object is to get all the white 
pawns in the places occupied initially by the black pawns and 
vice versa. 

The solution requires 15 moves. It may be effected by 
moving first a white pawn, then successively two black pawns, 
then three white pawns, then three black pawns, then three 
white pawns, then two black pawns, and then one white pawn. 
We can express this solution by saying that if we number the 

* Lucas, voL n, part 5, pp. 141—143. 



78 



GEOMETRICAL RECREATIONS 



[CH. IV 



cells (a term used to describe each of the small squares on a 
chess-board) consecutively, then initially the vacant space 
occupies the cell 4 and in the successive moves it will occupy 
the cells 3, 5, 6, 4, 2, 1, 3, 5, 7, 6, 4, 2, 3, 5, 4. Of these moves, 
six are simple and nine are leaps. 

Similarly if we have n white pawns at one end of a row of 
2?i + l cells, and n black pawns at the other end, they can 
be interchanged in n (n+ 2) moves, by moving in succession 
1 pawn, 2 pawns, 3 pawns, ... n — l pawns, n pawns, n pawns, 
n pawns, n — 1 pawns, ... 2 pawns, and 1 pawn — all the pawns 
in each group being of the same colour and different from that 
of the pawns in the group preceding it. Of these moves 2n are 
simple and n^ are leaps. 

Second Problem with Pawns*. A similar game may be 
played on a rectangular or square board. The case of a square 
board containing 49 cells, or small squares, will illustrate this 
sufficiently: in this case the initial position is shown in the 
annexed diagram where the " a "s denote the pawns or pieces 



a 


a 
a 
a 
a 
a 
a 
a 


_«_ 
a 
a 
a 
a 
a 
a 


a 
a 
a 

b 
T 


6 
b 
b 
b 
b 
b 


b 
b 
b 
b 
b 
b 
b 


b 


a 


b 


a 
a 
a 
a 
a 


b 
b 
b 
b 
b 



of one colour, and the " b "s those of the other colour. The '* a" 
pieces can move horizontally from left to right or vertically 
down, and the " b " pieces can move horizontally from right to 
left or vertically up, according to the same rules as before. 

The solution reduces to the preceding case. The pieces 
in the middle column can be interchanged in 15 moves. In the 
course of these moves every one of the seven cells in that 
column ia at some time or other vacant, and whenever that 

* Lucas, vol. II, part 5, p. 144. 



CH. IV] 



GEOMETRICAL RECREATIONS 



79 



is the case the pieces in the row containing the vacant cell 
can be interchanged. To interchange the pieces in each of 
the seven rows will require 15 moves. Hence to interchange all 
the pieces will require 15 + (7 x 15) moves, that is, 120 moves. 

If we place 2n(n4-l) white pawns and 2n (rH- 1) black 
pawns in a similar way on a square board of (2n+l)'^ cells, 
we can transpose them in 2n(n-\-l)(n + 2) moves : of these 
4n(n + l) are simple and 2n^ (^+1) are leaps. 

Third Problem with Pawns. The following analogous 
problem is somewhat more complicated. On a square board 
of 25 cells, place eight white pawns or counters on the cells 



a 
d 


b 
e 


c 
1 







9 


h 


« 


H 


G 






G 


E 
B 


D 
A 



denoted by small letters in the annexed diagram, and eight 
black pawns or counters on the cells denoted by capital letters : 
the cell marked with an asterisk (*) being left blank. Each 
pawn can move according to the laws already explained — the 
white pawns being able to move only horizontally from left 
to right or vertically downwards, and the black pawns being 
able to move only horizontally from right to left or vertically 
upwards. The object is to get all the white pawns in the 
places initially occupied by the black pawns and vice versa. 
No moves outside the dark line are permitted. 

Since there is only one cell on the board which is unoccupied, 
and since no diagonal moves and no backward moves are 
permitted, it follows that at each move not more than two 
pieces of either colour are capable of moving. There are how- 
ever a very large number of empirical solutions. In previous 
editions 1 have given a symmetrical solution in 48 moves, but 
the following, due to Mr H. E. Dudeney, is efifected in 46 moves : 
Hhg*Ffc*CBHh*GDF/ehbag*GABUEF/dg*Hhbc*CFf*Gnh* 



80 GEOMETRICAL RECREATIONS [CH. IV 

the letters indicating the cells from which the pieces are suc- 
cessively moved. It will be noticed that the first twenty-three 
moves lead to a symmetrical position, and that the next twenty- 
two moves can be at once obtained by writing the first twenty- 
two moves in reverse order and interchanging small and capital 
letters. Similar problems with boards of various shapes can be 
easily constructed. 

Probably, were it worth the trouble, the mathematical theory 
of games such as that just described might be worked out by the 
use of Van derm onde's notation, described later in chapter vi, or 
by the analogous method employed in the theory of the game of 
solitaire *. 

Problems on a Chess-board with Chess-pieces. There are 
several mathematical recreations with chess-pieces, other than 
pawns. Some of these are given later in chapter vi.. 

Geometrical Puzzles with Rods, etc. Another species 
of geometrical puzzles, to which here I will do no more than 
allude, are made of steel rods, or of wire, or of wire and string. 
Numbers of these are often sold in the streets of London for a 
penny each, and some of them afford ingenious problems in the 
geometry of position. Most of them could hardly be discussed 
without the aid of diagrams, but they are inexpensive to 
construct, and in fact innumerable puzzles on geometry of position 
can be made with a couple of stout sticks and a ball of string, or 
with only a box of matches : several examples are given in 
various recent English works. Most of them exemplify the 
difficulty of mentally realizing the effect of geometrical altera- 
tions in a figure unless they are of the simplest character. 

Paradromic Kings. The fact just stated is illustrated 
by the familiar experiment of making paradromic rings by 
cutting a paper ring prepared in the following manner. 

* On the theory of the solitaire, see Eeiss, 'Beitrdge zur Theorie des Solitdr- 
SpielSy' GrelWs Journal^ Berlin, 1858, vol. uv, pp. 344 — 379 ; and Lucas, vol. i, 
part V, pp. 89 — 141, 



CH. IV] GEOMETRICAL RECREATIONS 81 

Take a strip of paper or piece of tape, say, for convenience, 
an inch or two wide and at least nine or ten inches long, rule a 
line in the middle down the length AB of the strip, gum one 
end over the other end B, and we get a ring like a section of 
a cylinder. If this ring is cut by a pair of scissors along the 
ruled line we obtain two rings exactly like the first, except that 
they are only half the width. Next suppose that the end A is 
twisted through two right angles before it is gummed to B 
(the result of which is that the back of the strip at A is gummed 
over the front of the strip at B), then a cut along the line will 
produce only one ring. Next suppose that the end A is twisted 
once completely round (i.e. through four right angles) before it 
is gummed to B, then a similar cut produces two interlaced 
rings. If any of my readers think that these results could be 
predicted oflf-hand, it may be interesting to them to see if they 
can predict correctly the effect of again cutting the rings formed 
in the second and third experiments down their middle lines in 
a manner similar to that above described. 

The theory is due to J. B. Listing* who discussed the case 
when the end A receives m half-twists, that is, is twisted through 
WITT, before it is gummed to B. 

If m is even we obtain a surface which has two sides and two 
edges, which are termed paradromic. If the ring is cut along 
a line midway between the edges, we obtain two rings, each 
of which has m half-twists, and, which are linked together \m 
times. 

If m is odd we obtain a surface having only one side and 
one edge. If this ring is cut along its mid-line, we obtain only 
one ring, but it has 2m half-twists, and if m is greater than 
unity it is knotted. 



ADDENDUM. 

Note. Page 64. One method of arranging 16 counters in 15 lines, as 
stated in the text, is as follows. Draw a regular re-entrant pentagon 
vertices ^i, A^j A^, A^, A^, and centre 0. The sides intersect in five 

* Vorstudien zur Topologiet -Die Studien^ GSttingen, 1847, part x. 
an. 6 



82 GEOMETRICAL RECREATIONS [CH. IV 

points Bif ... Z?5. These latter points may be joined so as to form a 
smaller regular re-entrant pentagon whose sides intersect in five points 
Ci, ... Cs- The 16 points indicated are arranged as desired {The Canter- 
bury Puzzles, 1907, p. 140). 

An arrangement of 18 counters in 9 rows, each containing 5 counters, 
can be obtained thus. From one angle, A of an equilateral triangle 
A A' A", draw lines AD, A E inside the triangle making any angles with 
A A'. Draw from A' and A" lines similarly placed in regard to A' A" and 
A" A. Let A'D' cut A"E" in F, and A'E' cut A"D" in G. Then AFG is 
a straight line. The 3 vertices of the triangle and the 15 points of inter- 
section oiAD^A E, AF, with the similar pencils of lines drawn from A\ A!\ 
will give an arrangement as required. 

An arrangement of 19 counters in 10 rows, each containing 5 counters, 
can be obtained by placing counters at the 19 points of intersection of the 
10 lines x=±.a, x= ±.l>y y = ±a, y = ±h^ y = ±x : of these points two are 
at infinity. 

Note. Page 69. The Great Northern Shunting Problem is effected thus, 
(i) R pushes P into A. (ii) R returns, pushes ^ up to P in A, couples 
Q to P, draws them both out to F, and then pushes them to E. (iii) A 
is now uncoupled, R takes Q back to J., and leaves it there, (iv) R returns 
to P, takes P back to (7, and leaves it there, (v) R running successively 
through F, D, B comes to A^ draws Q out, and leaves it at B. 

Note. Page 70. One solution of the Chifu-Chemulpo Puzzle is as 
follows. Move successively wagons 2, 3, 4 up, i.e. on to the loop line. 
[Then push 1 along the straight track close to 5 ; this is not a " move."] 
Next, move 4 down, i.e. on to the straight track and push it along to 1. 
Next, move 8 up, 3 down to the end of the track and keep it there tempo- 
rarily, 6 up, 2 down, e down, 3 up, 7 up. [Then push 5 to the end of the 
track and keep it there temporarily.] Next, move 7 down, 6 down, 2 up, 
4 up. [Then push e along to 1.] Next, move 4 down to the end of the track 
and keep it there temporarily, 2 down, 5 up, 3 down, 6 up, 7 up, 8 down 
to the end of the track, e up, 5 down, 6 down, 7 down. In this solution 
we moved e down to the track at one end, then shifted it along the track, 
and finally moved it up to the loop from the other end of the track. We 
might equally well move e down to the track at one end, and finally 
move it back to the loop from the same and. In this solution the pieces 
successively moved are 2, 3, 4, 4, e, 8, 7, 3, 2, 6, 5, 5, 6, 3, 2, 7, 2, 5, 6, 3, 
7, e, 8, 5, 6, 7. 



83 



CHAPTER V. 
MECHANICAL RECREATIONS. 

I proceed now to enumerate a few questions connected with 
mechanics which lead to results that seem to me interesting 
from a historical point of view or paradoxical. Problems in 
mechanics generally involve more difficulties than problems in 
arithmetic, algebra, or geometry, and the explanations of some 
phenomena — such as those connected with the flight of birds — 
are still incomplete, while the explanations of many others of an 
interesting character are too difficult to find a place in a non- 
technical work. Here I exclude all transcendental mechanics, 
and confine myself to questions which, like those treated in the 
preceding chapters, are of an elementary character. The 
results are well-known to mathematicians. 

I assume that the reader is acquainted with the funda- 
mental ideas of kinematics and dynamics, and is familiar with 
the three Newtonian laws; namely, first that a body will 
continue in its state of rest or of uniform motion in a straight 
line unless compelled to change that state by some external 
force : second, that the change of momentum per unit of time 
is proportional to the external force and takes place in the 
direction of it: and third, that the action of one body on 
another is equal in magnitude but opposite in direction to the 
reaction of the second body on the first. The first and second 
laws state the principles required for solving any question on 
the motion of a particle under the action of given forces. The 
third law supplies the additional principle required for the 
solution of problems in which two or more particles influence 
one another. 

6—2 



84 MECHANICAL RECREATIONS [CH. V 

Motion. The difficulties connected with the idea of motion 
have been for a long time a favourite subject for paradoxes, 
some of which bring us into the realm of the philosophy of 
mathematics. 

Zenos Paradoxes on Motion. One of the earliest of these 
is the remark of Zeno to the effect that since an arrow cannot 
move where it is not, and since also it cannot move where it is 
(that is, in the space it exactly fills), it follows that it cannot 
move at all. This is sometimes presented in the form that at 
each instant a flying arrow occupies a fixed position ; but 
occupying a fixed position at a given instant means that it is 
then at rest. Hence the arrow is at rest at every instant of its 
flight, and therefore is not in motion. The usual answer is that 
the very idea of the motion of the arrow implies the passage 
from where it is to where it is not. 

Zeno also asserted that the idea of motion was itself incon- 
ceivable, for what moves must reach the middle of its course 
before it reaches the end. Hence the assumption of motion 
presupposes another motion, and that in turn another, and so 
ad infinitum. His objection was in fact analogous to the 
biological difficulty expressed by Swift: — 

" So naturalists observe, a flea hath smaller fleas that on him prey. 
And these have smaller fleas to bite 'em. And so proceed ad infinitum." 

Or as De Morgan preferred to put it 

"Great fleas have little fleas upon their backs to bite 'em, 
And little fleas have lesser fleas, and so ad infinitum. 
And the great fleas themselves, in turn, have greater fleas to go on; 
While these have greater still, and greater still, and so on." 

Achilles and the Tortoise. Zeno's paradox about Achilles 
and the tortoise is known even more widely. The assertion 
was that if Achilles ran ten times as fast as a tortoise, yet 
if the tortoise had (say) 1000 yards start it could never be 
overtaken. To establish this, Zeno argued that when Achilles 
had gone the 1000 yards, the tortoise would still be 100 yards 
in front of him ; by the time he had covered these 100 yards, 
it would still be 10 yards in front of him ; and so on for ever. 
Thus Achilles would get nearer and nearer to the tortoise 
but would never overtake it. Zeno regarded this as confirming 
his view that the popular idea of motion is self-contradictory. 



CH. V] MECHANICAL RECREATIONS 85 

The fallacy lies in the use of the word "never." The 
argument shows that during the time occupied by the motion 
described Achilles will not reach the tortoise. It does not deal 
with what happens after that time, and in fact Achilles would 
then overtake and pass the tortoise. Probably Zeno would 
have stated that the argument and explanation alike rest on 
the assumption, which he would not have admitted, that space 
and time are infinitely divisible. 

Zends Paradox on Time. Zeno seems further to have 
contended that while, to an accurate thinker, the notion of 
the infinite divisibility of time was impossible, it was equally 
impossible to think of a minimum measure of time. For 
suppose, he argued, that t is the smallest conceivable interval, 
and suppose that three horizontal lines composed of three 
consecutive spans ahc, a'h'c\ a"h"G" are placed so that a, a', a" 
are vertically over one another, as also 6, 6', h" and c, c', c". 
Imagine the second line moved as a whole one span to the right 
in the time t, and simultaneously the third line moved as a 
whole one span to the left. Then 6, a', c" will be vertically over 
one another. And in this duration t (which by hypothesis is 
indivisible) a' must have passed vertically over the space a"h" 
and the space h"c'\ Hence the duration is divisible, contrary 
to the hypothesis. 

The Paradox of Tristram Shandy. Mr Russell has enun- 
ciated* a paradox somewhat similar to that of Achilles and the 
Tortoise, save that the intervals of time considered get longer 
and longer during the course of events. Tristram Shandy, as 
we know, took two years writing the history of the first two days 
of his life, and lamented that, at this rate, material would 
accumulate faster than he could deal with it, so that he could 
never finish the work, however long he lived. But had he 
lived long enough, and not wearied of his task, then, even if his 
life had continued as eventfully as it began, no part of his 
biography would have remained unwritten. For if he wrote the 
events of the first day in the first year, he would write the 

* B. A. W. Russell, Principles of Mathematics, Cambridge, 1903, vol. i, p. 868. 



86 MECHANICAL RECREATIONS [CH. V 

events of the nth. day in the nth year, hence in time the events 
of any assigned day would be written, and therefore no part of 
his biography would remain unwritten. This argument might 
be put in the form of a demonstration that the part of a 
magnitude may be equal to the whole of it. 

Questions, such as those given above, which are concerned 
with the continuity of space and time involve difficulties of a 
high order. Many of the resulting perplexities are due to the 
assumption that the number of things in a collection of them is 
greater than the number in a part of that collection. This 
is axiomatic for a finite number of things, but must not be 
assumed as being necessarily true of infinite collections. 

Angular Motion. A non-mathematician finds additional 
difficulties in the idea of angular motion. For instance, there 
is a well-known proposition on motion in an equiangular spiral 
which shows that a body, moving with uniform velocity and 
as slowly as we please, may in a finite time whirl round a fixed 
point an infinite number of times. To a non- mathematician 
the result seems paradoxical if not impossible. 

The demonstration is as follows. The equiangular spiral 
is the trace of a point P, which moves along a line OP, the 
line OP turning round a fixed point with uniform angular 
velocity while the distance of P from decreases with the 
time in geometrical progression. If the radius vector rotates 
through four right angles we have one convolution of the 
curve. All convolutions are similar, and the length of each 
convolution is a constant fraction, say l/nth, that of the con- 
volution immediately outside it. Inside any given convolu- 
tion there are an infinite number of convolutions which get 
smaller and smaller as we get nearer the pole. Now suppose 
a point Q to move uniformly along the spiral from any point 
towards the pole. If it covers the first convolution in a seconds, 
it will cover the next in a/n seconds, the next in a/n^ seconds, and 
so on, and will finally reach the pole in 

(a + a/n -\- a/n"^ + a/n^ ■\- ) 

eeconds, that is, in an/{n — 1) seconds. The velocity is uniform. 



CH. V] MECHANICAL RECREATIONS 87 

and yet in a finite time, Q will have * traversed an infinite 
number of convolutions and therefore have circled round the 
pole an infinite number of times*. 

Simple Relative Motion. Even if the philosophical diffi- 
culties suggested by Zeno are settled or evaded, the mere idea 
of relative motion has been often found to present difficulties, 
and Zeno himself failed to explain a simple phenomenon 
involving the principle. As one of the easiest examples of 
this kind, I may quote the common question of how many 
trains going from B to A a. passenger from ^ to -B would 
meet and pass on his way, assuming that the journey either 
way takes 4^ hours and that the trains start from each end 
every hour. The answer is 9. Or again, take two pennies, 
face upwards on a table and edges in contact. Suppose that 
one is fixed and that the other rolls on it without slipping, 
making one complete revolution round it and returning to its 
initial position. How many revolutions round its own centre 
has the rolling coin made ? The answer is 2. 

Laws of Motion, I proceed next to make a few remarks on 
points connected with the laws of motion. 

The first law of motion is often said to define force, but 
it is in only a qualified sense that this is true. Probably 
the meaning of the law is best expressed in Clifford's phrase, 
that force is "the description of a certain kind of motion" — 
in other words it is not an entity but merely a convenient 
way of stating, without circumlocution, that a certain kind of 
motion is observed. 

It is not difficult to show that any other interpretation 
lands us in difficulties. Thus some authors use the law to 
justify a definition that force is that which moves a body or 
changes its motion ; yet the same writers speak of a steam- 
engine moving a train. It would seem then that, according 
to them, a steam-engine is a force. That such statements 
are current may be fairly reckoned among mechanical 
paradoxes. 

• The proposition is put in this form in J, Richard's Philosophie des 
Mathimatiques, Paris, X903, pp. 119—120. 



88 MECHANICAL RECREATIONS [CH. V 

The idea of force is difficult to grasp. How many people, 
for instance, could predict correctly what would happen in a 
question as simple as the following? A rope (whose weight 
may be neglected) hangs over a smooth pulley ; it has one end 
fastened to a weight of 10 stone, and the other end to a sailor 
of weight 10 stone, the sailor and the weight hanging in the 
air. The sailor begins steadily to climb up the rope ; will the 
weight move at all ; and, if so, will it rise or fall ? In fact, it 
will rise. 

It will be noted that in the first law of motion it is asserted 
that, unless acted on by an external force, a body in motion 
continues to move (i) with uniform velocity, and (ii) in a 
straight line. 

The tendency of a body to continue in its state of rest 
or of uniform motion is called its inertia. This tendency 
may be used to explain various common phenomena and 
experiments. Thus, if a number of dominoes or draughts are 
arranged in a vertical pile, a sharp horizontal blow on one of 
those near the bottom will send it out of the pile, and those 
above will merely drop down to take its place — in fact they 
have not time to change their relative positions before there 
is sufficient space for them to drop vertically as if they were 
a solid body. On this principle depends the successful per- 
formance of numerous mechanical tricks and puzzles. 

The statement about inertia in the first law may be taken 
to imply that a body set in rotation about a principal axis 
passing through its centre of mass will continue to move with 
a uniform angular velocity and to keep its axis of rotation fixed 
in direction. The former of these statements is the assumption 
on which our measurement of time is based as mentioned below 
in chapter XX. The latter assists us to explain the motion of 
a projectile in a resisting fluid. It affords the explanation of 
why the barrel of a rifle is grooved ; and why, similarly, anyone 
who has to throw a flat body of irregular shape (such as a card) 
in a given direction usually gives it a rapid rotatory motion 
about a principal axis. Elegant illustrations of the fact just 
mentioned are afforded by a good mau) of the tricks of acrobats, 



CH. V] MECHANICAL KECREATIONS 89 

though the full explanation of most of them also introduces 
other considerations. Thus it is a common feat to toss on to 
the top surface of an umbrella a penny so that it alights on its 
edge, and then, by turning round the stick of the umbrella 
rapidly, to cause the coin to rotate. By twisting the umbrella 
at the proper rate, the coin can be made to appear stationary 
and standing upright, though the umbrella is moving away 
underneath it, while by diminishing or increasing the angular 
velocity of the umbrella the penny can be made to run forwards 
or backwards. This is not a difficult trick to execute : it was 
introduced by Japanese conjurers. 

The tendency of a body in motion to continue to move 
in a straight line is sometimes called its centrifugal force. 
Thus, if a train is running round a curve, it tends to move in 
a straight line, and is constrained only by the pressure of the 
rails to move in the required direction. Hence it presses on 
the outer rail of the curve. This pressure can be diminished 
to some extent both by raising the outer rail, and by putting 
a guard rail, parallel and close to the inner rail, against which 
the wheels on that side also will press. 

An illustration of this fact occurred in a little known inci- 
dent of the American civil war*. In the spring of 1862 a 
party of volunteers from the North made their way to the 
rear of the Southern armies and seized a train, intending to 
destroy, as they passed along it, the railway which was the 
main line of communication between various confederate corps 
and their base of operations. They were however detected 
and pursued. To save themselves, they stopped on a sharp 
curve and tore up some rails so as to throw the engine which 
was following them ofif the line. Unluckily for themselves 
they were ignorant of dynamics and tore up the inner rails of 
the curve, an operation which did not incommode their 
pursuers, who were travelling at a high speed. 

The second law gives us the means of measuring mass, 
force, and therefore work. A given agent in a given time can 
do only a definite amount of work. This is illustrated by the 
• Capturing a Locomotive by W. Pittenger, London, 1882, p. 104. 



90 MECHANICAL RECREATIONS [CH. V 

fact that although, by means of a rigid lever and a fixed 
fulcrum, any force however small may be caused to move any 
mass however large, yet what is gained in power is lost in 
speed — as the popular phrase runs. 

Montucla* inserted a striking illustration of this principle 
founded on the well-known story of Archimedes who is said to 
have declared to Hiero that, were he but given a fixed fulcrum, 
he could move the world. Montucla assumed that a man could 
work incessantly at the rate of 116 foot-lbs. per second, which 
is a very high estimate. On this assumption it would take 
over three billion centuries, i.e, 3 x 10^^ years, before a particle 
whose mass was equal to that of the earth was moved as much 
as one inch against gravity at the surface of the earth : to move 
it one inch along a horizontal plane on the earth's surface 
would take about 6,000 centuries. 

Stability of Equilibrium. It is known to all those who 
have read the elements of mechanics that the centre of gravity 
of a body, which is resting in equilibrium under its own weight, 
must be vertically above its base : also, speaking generally, we 
may say that, if every small displacement has the effect of 
raising the centre of gravity, then the equilibrium is stable, 
that is, the body when left to itself will return to its original 
position; but, if a displacement has the effect of lowering the 
centre of gravity, then for that displacement the equilibrium 
is unstable; while, if every displacement does not alter the 
height above some fixed plane of the centre of gravity, then 
the equilibrium is neutral. In other words, if in order to cause 
a displacement work has to be done against the forces acting 
on the body, then for that displacement the equilibrium is stable, 
while if the forces do work the equilibrium is unstable. 

A good many of the simpler mechanical toys and tricks 
afford illustrations of this principle. 

Magic Bottles^. Among the most common of such toys are 
the small bottles — trays of which may be seen any day in the 
streets of London — which keep always upright, and cannot 

* Ozanam, 1803 edition, vol. ii, p. 18 ; 1840 edition, p. 202. 
t Ozanam, 1803 edition, vol. ir, p. 15 ; 1840 edition, p. 201. 



CH. V] MECHANICAL RECREATIONS 91 

be upset until their owner orders them to lie down. Such a 
bottle is made of thin glass or varnished paper fixed to the 
plane surface of a solid hemisphere or smaller segment of 
a sphere. Now the distance of the centre of gravity of a 
homogeneous hemisphere from the centre of the sphere is 
^hree-eighths of the radius, and the mass of the glass or 
varnished paper is so small compared with the mass of the 
lead base that the centre of gravity of the whole bottle is 
still within the hemisphere. Let us denote the centre of the 
hemisphere by G, and the centre of gravity of the bottle by G. 

If such a bottle is placed with the hemisphere resting on a 
horizontal plane and GO vertical, any small displacement on the 
plane will tend to raise Gy and thus the equilibrium is stable. 
This may be seen also from the fact that when slightly dis- 
placed there is brought into play a couple, of which one force 
is the reaction of the table passing through G and acting 
vertically upward, and the other the weight of the bottle 
acting vertically downward at G. If G is below (7, this couple 
tends to restore the bottle to its original position. 

If there is dropped into the bottle a shot or nail so heavy 
as to raise the centre of gravity of the whole above C, then 
the equilibrium is unstable, and, if any small displacement is 
given, the bottle falls over on to its side. 

Montucla says that in his time it was not uncommon to 
S3e boxes of tin soldiers mounted on lead hemispheres, and 
when the lid of the box was taken off the whole regiment 
sprang to attention. 

In a similar way we may explain how to balance a pencil 
in a vertical position, with its point resting on the top of one's 
finger, an experiment which is described in nearly every book 
of puzzles*. This is effected by taking a penknife, of which 
one blade is opened through an angle of (say) 120°, and sticking 
the blade in the pencil so that the handle of the penknife is 
below the finger. The centre of gravity is thus brought below 
the point of support, and a small displacement given to the 

* Ex. gr. Oughtred, Mathematicall Recreations, p. 24; Ozanam, 1803 edition, 
vol. n, p. 14; 1840 edition, p. 200. 



92 MECHANICAL RECHEATIONS [CH. V 

pencil will raise the centre of gravity of the whole : thus the 
equilibrium is stable. 

Other similar tricks are the suspension of a bucket over 
the edge of a table by a couple of sticks, and the balancing of 
a coin on the edge of a wine-glass by the aid of a couple of 
forks* — the sticks or forks being so placed that the centre of 
gravity of the whole is vertically below the point of support 
and its depth below it a maximum. 

The toy representing a horseman, whose motion continually 
brings him over the edge of a table into a position which seems 
to ensure immediate destruction, is constructed in somewhat 
the same way. A wire has one end fixed to the feet of the 
rider; the wire is curved downwards and backwards, and at 
the other end is fixed a weight. When the horse is placed so 
that his hind legs are near the edge of the table and -his fore- 
feet over the edge, the weight is under his hind feet. Thus 
the whole toy forms a pendulum with a curved instead of a 
straight rod. Hence the farther it swings over the table, the 
higher is the centre of gravity raised, and thus the toy tends 
to return to its original position of equilibrium. 

An elegant modification of the prancing horse was brought 
out at Paris in 1890 in the shape of a toy made of tin and in 
the figure of a manf. The legs are pivoted so as to be movable 
about the thighs, but with a wire check to prevent too long 
a step, and the hands are fastened to the top of a PI -shaped 
wire weighted at its ends. If the figure is placed on a narrow 
sloping plank or strip of wood passing between the legs of the 
n, then owing to the fl -shaped wire any lateral displacement 
of the figure will raise its centre of gravity, and thus for any 
such displacement the equilibrium is stable. Hence, if a slight 
lateral disturbance is given, the figure will oscillate and will 
rest alternately on each foot : when it is supported by one foot 
the other foot under its own weight moves forwards, and thus 
the figure will walk down the plank though with a slight 
reeling motion. 

* Oughtred, p. 30; Ozanam, 1803 edition, vol. ii, p. 12; 1840 edition, 
p. 199. t La Nature^ Paris, March, 1891, 



CH. V] MECHANICAL RECREATIONS 93 

Columbus s Egg. The toy known as Columbus's egg depends 
on the same principle as the magic bottle, though it leads to 
the converse result. The shell of the Qgg is made of tin and 
cannot be opened. Inside it and fastened to its base is a 
hollow truncated tin cone, and there is also a loose marble 
inside the shell. If the Qgg is held properly, the marble runs 
inside the cone and the egg will stand on its base, but so long 
as the marble is outside the cone, the Qgg cannot be made to 
stand on its base. 

Cones running up hill*. Another common experiment, 
which produces the optical effect of a body moving by itself up 
an inclined plane, also depends on the tendency of a body to 
take a position so that its centre of gravity is as low as possible. 
Uusually the experiment is performed as follows. Arrange 
two sticks in the shape of a V, with the apex on a table and 
the two upper ends resting on the top edge of a book placed 
on the table. Take two equal cones fixed base to base, and place 
them with the curved surfaces resting on the sticks near the 
apex of the V, the common axis of the cones being horizontal 
and parallel to the edge of the book. Then, if properly 
arranged, the cones will run up the plane formed by the sticks. 

The explanation is obvious. The centre of gravity of the 
cones moves in the vertical plane midway between the two 
sticks and it occupies a lower position as the points of contact 
on the sticks get farther apart. Hence as the cone rolls up 
the sticks its centre of gravity descends. 

Test of Internal Structure. Here is another simple experi- 
ment of a somewhat different character. Suppose two balls 
constructed of equal size and weight, one of lead and hollow in 
the middle, the other of copper and solid ; and suppose that 
both spheres are gilt so that in weight, appearance, and elasti- 
city they are indistinguishable. How can we tell which of 
them is solid ? The answer is by allowing them to roll down 
a rough inclined plane, side by side. 

Perpetual Motion. The idea of making a machine which 
once set going would continue to go for ever by itself has been 
• Ozanam, 1803 edition, vol. n, p. 49 ; 1840 edition, p. 216. 



94 MECHANICAL RECEEATIONS [CH. V 

the ignis fatuus of self-taught mechanicians in much the same 
way as the quadrature of the circle has been that of self-taught 
geometricians. 

Now the obvious meaning of the third law of motion is 
that a force is only one aspect of a stress, and that whenever a 
force is caused another equal and opposite one is brought also 
into existence — though it may act upon a different body, and 
thus be immaterial for the particular problem considered. The 
law however is capable of another interpretation*, namely, 
that the rate at which an agent does work (that is, its action) 
is equal to the rate at which work is done against it (that is, 
its reaction). If it is allowable to include in the reaction the 
rate at which kinetic energy is being produced, and if work is 
taken to include that done against molecular forces, then it 
follows from this interpretation that the work done by an 
agent on a system is equivalent to the total increase of energy, 
that is, the power of doing work. Hence in an isolated system 
the total amount of energy is constant. If this is granted, 
then since friction and some molecular dissipation of energy 
cannot be wholly prevented, it must be impossible to construct 
in an isolated system a machine capable of perpetual motion. 

I do not propose to describe in detail the various machines 
for producing perpetual motion which have been suggestedf, 
but the machine described below will serve to illustrate one of 
the assumptions commonly made by these inventors. 

The machine to which I refer consists of two concentric 
vertical wheels in the same plane, and mounted on a horizontal 
axle through their centre, G, The space between the wheels is 
divided into compartments by spokes inclined at a constant 
angle to the radii to the points whence they are drawn, and 
each compartment contains a heavy bullet. This will be clear 
from the diagram. Apart from these bullets, the wheels would 
be in equilibrium. Each bullet tends to turn the wheels round 
their axle, and the moment which measures this tendency is 

* Newton's Principles, last paragraph of the Scholium to the Laws of Motion. 
+ Several of them have been described in H. Dirck's Perpetuum Mobile, 
London, 1861, 2nd edition, 1870. 



CH. V] MECHANICAL RECKEATIONS 96 

the product of the weight of the bullet and its distance from 
the vertical through G. 

The idea of the constructors of such machines was that, as 
the bullet in any compartment would roll under gravity to the 
lowest point of the compartment, the bullets on the right-hand 
side of the wheel in the diagram would be farther from the 
vertical through than those on the left. Hence the sum of 




the moments of the weights of the bullets on the right would 
be greater than the sum of the moments of those on the left. 
Thus the wheels would turn continually in the same direction 
as the hands of a watch. The fallacy in the argument is 
obvious. 

Another large group of machines for producing perpetual 
motion depended on the use of a magnet to raise a mass which 
was then allowed to fall under gravity. Thus, if the bob of a 
simple pendulum was made of iron, it was thought that magnets 
fixed near the highest points which were reached by the bob in 
the swing of the pendulum would draw the bob up to the same 
height in each swing and thus give perpetual motion, but the 
inventors omitted to notice that the bob of the pendulum would 
gradually get magnetised. « 

Of course it is only in isolated systems that the total amount 
of energy is constant, and, if a source of external energy can be 
obtained from which energy is continually introduced into the 
system, perpetual motion is, in a sense, possible ; though even 
here materials would ultimately wear out Streams, wind, the 



96 MECHANICAL RECREATIONS [CH. V 

solar heat, and the tides are among the more obvious of such 
sources. 

There was at Paris in the latter half of the eighteenth 
century a clock which was an ingenious illustration of such 
perpetual motion*. The energy which was stored up in it to 
maintain the motion of the pendulum was provided by the 
expansion of a silver rod. This expansion was caused by the 
daily rise of temperature, and by means of a train of levers it 
wound up the clock. There was a disconnecting apparatus, so 
that the contraction due to a fall of temperature produced no 
effect, and there was a similar arrangement to prevent over- 
winding. I believe that a rise of eight or nine degrees 
Fahrenheit was sufficient to wind up the clock for twenty-four 
hours. 

By utilizing the rise and fall of the barometer, James Cox, 
a London jeweller of the eighteenth century, produced, in an 
analogous way, a clock f which ran continuously without 
winding up. 

I have in my possession a watch which produces the 
same effect by somewhat different means. Inside the case is 
a steel weight, and if the watch is carried in a pocket this 
weight rises and falls at every step one takes, somewhat after 
the manner of a pedometer. The weight is raised by the 
action of the person who has it in his pocket in taking a 
step, and in falling it winds up the spring of the watch. 
On the face is a small dial showing the number of hours for 
which the watch is wound up. As soon as the hand of this 
dial points to fifty-six hours, the train of levers which winds 
up the watch disconnects automatically, so as to prevent over- 
winding the spring, and it reconnects again as soon as the 
watch has run down eight hours. The watch is an excellent 
time-keeper, and a walk of about a couple of miles is sufficient 
to wind it up for twenty-four hours. 

* Ozanam, 1803 edition, vol. ii, p. 105 ; 1840 edition, p. 238. 
t A full description of the mechanism will be found in the English Mechanic, 
April 30, 1909, pp. 288—289. 



CH. V] MECHANICAL RECREATIONS 97 

Models. I may add here the observation, which is well 
known to mathematicians, but is a perpetual source of disap- 
pointment to ignorant inventors, that it frequently happens 
that an accurate model of a machine will work satisfactorily 
while the machine itself will not do so. 

One reason for this is as follows. If all the parts of a model 
are magnified in the same proportion, say m, and if thereby a 
line in it is increased in the ratio m : 1, then the areas and 
volumes in it will be increased respectively in the ratios m? : 1 
and m' : 1. For example, if the side of a cube is doubled then 
a face of it will be increased in the ratio 4 : 1 and its volume 
will be increased in the ratio 8:1. 

Now if all the linear dimensions are increased m times 
then some of the forces that act on a machine (such, for 
example, as the weight of part of it) will be increased m^ times, 
while others which depend on area (such as the s.ustaining 
power of a beam) will be increased only m^ times. Hence the 
forces that act on the machine and are brought into play by 
the various parts may be altered in different proportions, and 
thus the machine may be incapable of producing results similar 
to those which can be produced by the model. 

The same argument has been adduced in the case of animal 
life to explain why very large specimens of any particular breed 
or species are usually weak. For example, if the linear dimen- 
sions of a bird were increased n times, the work necessary to 
give the power of flight would have to be increased no less 
than n^ times*. Again, if the linear dimensions of a man 
of height 5 ft. 10 in. were increased by one-seventh his height 
would become 6 ft. 8 in., but his weight would be increased 
in the ratio 512 : 343 {i.e. about half as much again), while 
the cross sections of his legs, which would have to bear this 
weight, would be increased only in the ratio 64 : 49 ; thus 
in some respects he would be less efficient than before. Of 
course the inci eased dimensions, length of limb, or size of 
muscle might be of greater advantage than the relative loss 
of strength ; hence the problem of what are the most efficient 

* Helmholtz, Oesammelte Abhandlungejit Leipzig, 1881, vol. i, p. 165. 
B. K. 7 



98 MECHANICAL RECREATIONS [CH. V 

proportions is not simple, but the above argument will serve 
to illustrate the fact that the working of a machine may not 
be similar to that of a model of it. 

Leaving now these elementary considerations I pass on to 
some other mechanical questions. 

Sailing quicker than the Wind. As a kinematical 
paradox I may allude to the possibility of sailing quicker than 
the wind blows, a fact which strikes many people as curious. 

The explanation* depends on the consideration of the 
velocity of the wind relative to the boat. Perhaps, however, 
a non-mathematician will find the solution simplified if I con- 
sider first the effect of the wind-pressure on the back of the 
sail which drives the boat forward, and second the resistance to 
motion caused by the sail being forced through the air. 

When the wind is blowing against a plane sail the resultant 
pressure of the wind on the sail may be resolved into two 
components, one perpendicular to the sail (but which in general 
is not a function only of the component velocity in that direc- 
tion, though it vanishes when that component vanishes) and 
the other parallel to its plane. The latter of these has no 
effect on the motion of the ship. The component perpen- 
dicular to the sail tends to move the ship in that direction. 
This pressure, normal to the sail, may be resolved again into 
two components, one in the direction of the keel of the boat, 
the other in the direction of the beam of the boat. The 
former component drives the boat forward, the latter to lee- 
ward. It is the object of a boat-builder to construct the boat 
on lines so that the resistance of the water to motion forward 
shall be as small as possible, and the resistance to motion in 
a perpendicular direction (i.e. to leeward) shall be as large as 
possible ; and I will assume for the moment that the former 
of these resistances may be neglected, and that the latter is 
so large as to render motion in that direction impossible. 

Now, as the boat moves forward, the pressure of the air 
on the front of the sail will tend to stop the motion. As 

* Ozanam, 1803 edition, vol. iii, pp. 359, 367 j 1840 edition, pp. 540, 643. 



CH. V] 



MECHANICAL HECKEATIONS 



99 



long as its component normal to the sail is less than the 
pressure of the wind behind the sail and normal to it, the 
resultant of the two will be a force behind the sail and normal 
to it which tends to drive the boat forwards. But as the 
velocity of the boat increases, a time will arrive when the 
pressure of the wind is only just able to balance the resisting 
force which is caused by the sail moving through the air. The 
velocity of the boat will not increase beyond this, and the 
motion will be then what mathematicians describe as "steady." 




In the accompan3dng figure, let BAR represent the keel 
of a boat, B being the bow, and let SAL represent the sail. 
Suppose that the wind is blowing in the direction WA with 
a velocity u; and that this direction makes an angle 6 with 
the keel, i.e. angle WA R= 6. Suppose that the sail is set so 
as to make an aogle a with the keel, i.e. angle BAS= a, and 
therefore angle WAL — -\- a. Suppose finally that v is the 
velocity of the boat in the direction AB. 

I have already shown that the solution of the problem 
depends on the relative directions and velocities of the wind* 
and the boat ; hence to find the result reduce the boat to rest 
by impressing on it a velocity v in the direction BA, The 
resultant velocity of v parallel to BA and of u parallel t-o WA 
will be parallel to SL, if vsina = u sin (^ + a) ; and in this case 
the resultant pressure perpendicular to the sail vacishes. 

Thus, for steady motion we have v sin a = w sin (0 + a). 
Hence, whenever sin (^ + a) > sin a, we have v>u. Suppose, 

7—2 



in the same way as before we have ^; sin a = u sin {B + a), 
or i; sin a = t^ sin (f>, where (f) = angle WAS — tt- -a. Hence 
i; = w sin <^ cosec a. 




Let w be the component velocity of the boat in the teeth 
of the wind, that is, in the direction AW. Then we have 
w = V cos BA IT = 2; cos (a + <^) = 1^ sin <^ cosec jl cos (a + </>). If a 
is constant, this is a maximum when <^ = Jtt — Ja; and, if <^ 
has this value, then ^(; = -^ 2^ (cosec a — 1). This formula shows 
that w is greater than u, if sin a < J. Thus, if the sails 
can be set so that a is less than sin~^J, that is, rather less 
than 19° 29', and if the wind has the direction above assigned, 
then the component velocity of the boat in the face of the 
wind is greater than the velocity of the wind. 

The above theory is curious, but it must be remembered 
that in practice considerable allowance has to be made for 
the fact that no boat for use on water can be constructed in 
which the resistance to motion in the direction of the keel 
can be wholly neglected, or which would not drift slightly to 
leeward if the wind was not dead astern Still this makes less 



CH. V] MECHANICAL RECREATIONS ICl 

difference than might be thouglit by a landsman. In the case 
of boats sailing on smooth ice the assumptions made are sub- 
stantially correct, and the practical results are said to agree 
closely with the theory. 

Boat moved by a Rope. There is a form of boat-racing, 
occasionally used at regattas, which affords a somewhat curious 
illustration of certain mechanical principles. The only thing 
supplied to the crew is a coil of rope, and they have, without 
leaving the boat, to propel it from one point to another as 
rapidly as possible. The motion is given by tying one end of 
the rope to the after thwart, and giving the other end a series 
of violent jerks in a direction parallel to the keel. I am told 
that in still water a pace of two or three miles an hour can be 
thus attained. 

The chief cause for this result seems to be that the friction 
between the boat and the water retards all relative motion, 
but it is not great enough to affect materially motion caused by 
a sufficiently big impulse. Hence the usual movements of the 
crew in the boat do not sensibly move the centre of gravity of 
themselves and the boat, but this does not apply to an impul- 
sive movement, and if the crew in making a jerk move their 
centre of gravity towards the bow n times more rapidly than 
it returns after the jerk, then the boat is impelled forwards 
at least n times more than backwards: hence on the whole 
the motion is forwards. 

Motion of Fluids and Motion in Fluids. The theories 
of motion of fluids and motion in fluids involve considerable 
difficulties. Here I will mention only one or two instances — 
mainly illusbrations of Hauksbee's Law. 

Haukshee's Law. When a fluid is in motion the pressure 
is less than when it is at rest*. Thus, if a current of air is 

• See Besant, Hydromechanics, Cambridge, 1867, art. 149, where however 
it is assumed that the pressure is proportional to the density. Hauksbee was 
the earhest writer who called attention to the problem, but I do not know who 
first explained the phenomenon; some references to it are given by Willis, 
Cambridge Philosophical Transactions, 1830, vol. m, pp. 129 — 140. 



102 MECHANICAL RECREATIONS [CH. V 

moving in a tube, the pressure on the sides of the tube is less 
than when the air is at rest — and the quicker the air moves 
the smaller is the pressure. This fact was noticed by Hauksbee 
nearly two centuries ago. In an elastic perfect fluid in which 
the pressure is proportional to the density, the law connecting 
the pressure, p, and the steady velocity, v, m p— Ha"^, where 
n and a are constants : the establishment of the corresponding 
formula for gases where the pressure is proportional to a power 
of the density presents no difficulty. 

The principle is illustrated by a twopenny toy, on sale in 
most toy-shops, called the pneumatic mystery. This consists 
of a tube, with a cup-shaped end in which rests a wooden ball. 
If the tube is held in a vertical position, with the mouthpiece 
at the upper end and the cup at the lower end, then, if anyone 
blows hard through the tube and places the ball against the 
cup, the ball will remain suspended there. The explanation is 
that the pressure of the air below the ball is so much greater 
than the pressure of the air in the cup that the ball is 
held up. 

The same effect may be produced by fastening to one end of 
a tube a piece of cardboard having a small hole in it. If a piece 
of paper is placed over the hole and the experimenter blows 
through the tube, the paper will not be detached from the card 
but will bend so as to allow the egress of the air. 

An exactly similar experiment, described in many text- 
books on hydromechanics, is made as follows. To one end of a 
straight tube a plane disc is fitted which is capable of sliding 
on wires projecting from the end of the tube. If the disc is 
placed at a small distance from the end, and anyone blows 
steadily into the tube, the disc will be drawn towards the tube 
instead of being blown off the wires, and will oscillate about a 
position near the end of the tube. 

In the same way we may make a tube by placing two books 
on a table with their backs parallel and an inch or so apart and 
laying a sheet of newspaper over them. If anyone blows 
steadily through the tube so formed, the paper will be sucked in 
instead of being blown out. 



CH. V] MECHANICAL RECREATIONS 103 

The following experiment is explicable by the same argu- 
ment. On the top of a vertical axis balance a thin horizontal 
rod. At each end of this rod fasten a small vertical square or 
sail of thin cardboard — the two sails being in the same plane. 
If anyone blows close to one of these squares and in a direction 
parallel to its plane, the square will move towards the side on 
which one is blowing, and the rod with the two sails will rotate 
about the axis. 

The experiments above described can be performed so as 
to illustrate Hauksbee's Law ; but unless care is taken othei 
causes will be also introduced which affect the phenomena : it 
is however unnecessary for my purpose to go into these details. 

Gut on a Tennis-Bali. Racquet and court-tennis players 
know that if a strong cut is given to a ball it can be made 
to rebound off a vertical wall and then (without striking the 
floor or any other wall) return and hit the wall again. This 
affords another illustration of Hauksbee's Law, The effect had 
been noticed by Isaac Newton, who, in his letter to Oldenburg, 
February, 1672, N.S., says " I reBflembered that I had often seen 
**a tennis-ball struck with an oblique racket describe such 
" a curve line. For, a circular as well as a progressive motion 
" being communicated to it by that stroke, its parts on that side 
" where the motions conspire, must press and beat the contiguous 
"air more violently than on the other; and there excite a 
"reluctancy and re-action of the air proportionably greater. 
** And. . .globular bodies," thus acquiring " a circulating motion. . . 
" ought to feel the greater resistance from the ambient aether 
"on that side where the motions conspire, and thence be con- 
" tinually bowed to the other." 

The question was discussed by Magnus in 1837 and by Tait 
in various papers from 1887 to 1896. The explanation* is that 
the cut causes the ball to rotate rapidly about an axis through 

* See Magnus on *Di6 Abweichung der Gescliosse' in the Abhandlungen der 
Akadcmie der Wissenschaften, Berlin, 1852, pp. 1 — 23; Lord Rayleigh, ' On the 
irregular flight of a tennis ball,* Messenger of Mathematics, Cambridge, 1878, 
vol. vn, pp. 14 — 16 ; and P. G, Tait, Transactions Boyal Society, Edinburgh, 
vol. XXVII, 1893; or Collected Scientific Papers, Cambridge, vol. il, 1900, 
pp. 356—387, and references therein. 



104 MECHANICAL RECREATIONS [CH. V 

its centre of figure, and the friction of the surface of the ball on 
the air produces a sort of whirlpool. This rotation is in addition 
to its motion of translation. Suppose the ball to be spherical 
and rotating about an axis through its centre perpendicular to 
the plane of the paper in the direction of the arrow-head, and at 
the same time moving through still air from left to right 
parallel to PQ. Any motion of the ball perpendicular to PQ 
will be produced by the pressure of the air on the surface of the 
ball, and this pressure will, by Hauksbee's Law, be greatest 
where the velocity of the air relative to the ball is least, and vice 
versa. To find the velocity of the air relative to the ball we may 
reduce the centre of the ball to rest, and suppose a stream 
of air to impinge on the surface of the ball moving with a 
velocity equal and opposite to that of the centre of the ball. 



& 



The air is not friction! ess, and therefore the air in contact with 
the surface of the ball will be set in motion by the rotation of 
the ball and will form a sort of whirlpool rotating in the direction 
of the arrow-head in the figure. To find the actual velocity of 
this air relative to the ball we must consider how the motion 
due to the whirlpool is affected by the motion of the stream of 
air parallel to QP. The air at A in the whirlpool is moving 
against the stream of air there, and therefore its velocity is 
retarded : the air at B in the whirlpool is moving in the same 
direction as the stream of air there, and therefore its velocity is 
increased. Hence the relative velocity of the air at A is less 
than at B, and, since the pressure of the air is greatest where 
the velocity is least, the pressure of the air on the surface of the 
ball at A is greater than on that at B. Hence the ball is forced 
by this pressure in the direction from the line PQ, which we may 



CH. V] MECHANICAL RECREATIONS 105 

suppose to represent the section of the vertical wall in a racquet- 
court. In other words, the ball tends to move at right angles to 
the line in which its centre is moving and in the direction 
in which the surface of the front of the ball is being carried by 
the rotation. Sir J. J. Thomson has pointed out that if we 
consider the direction in which the nose (or foremost point) of 
the ball is travelling, we may sum up the results by saying that 
the ball always follows its nose. Lord Rayleigh has shown that 
the line of action of resulting force on the ball is perpendicular 
to the plane containing the direction (m) of motion of the centre 
of the ball and the axis (s) of spin, and its magnitude varies 
directly as the velocity of translation, the velocity of spin, and 
the sine of the angle between the lines m and s. 

In the case of a lawn tennis-ball, the shape of the ball is 
altered by a strong cut, and this introduces additional compli- 
cations. 

Spin on a Cricket-Ball. The curl of a cricket-ball in its 
flight through the air, caused by a spin given by the bowler 
in delivering the ball, is explained by the same reasoning. 

Thus suppose the ball is delivered in a direction lying in a 
vertical plane containing the middle stumps of the two wickets. 
A spin round a horizontal axis parallel to the crease in a 
direction which the bowler's umpire would describe as positive, 
namely, counter clock-wise, will, in consequence of the friction 
of the air, cause it to drop, and therefore decrease the length 
of the pitch. A spin in the opposite direction will cause it to 
rise, and therefore lengthen the pitch. A spin round a vertical 
axis in the positive direction, as viewed from above, will make 
it curl sideways in the air to the left, that is, from leg to off. 
A spin in the opposite direction will make it curl to the right. 
A spin given to the ball round the direction of motion of the 
centre of the ball will not sensibly affect the motion through 
the air, though it would cause the ball, on hitting the ground, 
to break. Of course these various kinds of spin can be combined. 

Flight of Golf -Balls. The same argument explains the 
effect of the spin given to a golf-ball by impact with the club. 
Here the motion takes place for a longer interval of time, and 



106 MECHANICAL RECREATIONS [CH. V 

additional complications are introduced by the fact that the 
velocities of translation and rotation are retarded at different 
rates and that usually there will be some wind blowing in 
a cross direction. But generally we may say that the effect of 
the under-cut given by a normal stroke is to cause the ball 
to rise, and therefore to lengthen the carry. Also, if a wind is 
blowing across the line to the hole from right to left, a drive, if 
the player desires a long carry and has sufficient command over 
his club, should be pulled ; but an approach shot, if it is desired 
that the ball should fall dead, should be sliced because then the 
ball as soon as it meets the wind will tend to fall dead. Con- 
versely, if the wind is blowing across the course from left to 
right, the drive should be sliced if a long carry is desired, and 
an approach shot should be pulled if it is desired that the ball 
should fall dead. 

The questions involving the application of Hauksbee's Law 
are easy as compared with many of the problems in fluid motion. 
The analysis required to attack most of these problems is beyond 
the scope of this book, but one of them may be worth mentioning 
even though no explanation is given. 

The Theory of the Flight of Birds. A mechanical problem of 
great interest is the explanation of the means by which birds are 
enabled to fly for considerable distances with no (perceptible) 
motion of the wings. Albatrosses, to take an instance of special 
difficulty, have been known to follow for some days ships sailing 
at the rate of nine or ten knots, and sometimes for considerable 
periods there is no motion of the wings or body which can 
be detected, while even if the bird moved its wings it is not 
easy to understand how it has the muscular energy to propel 
itself so rapidly and for such a length of time. Of this 
phenomenon various explanations* have been suggested. 
Notable among these are Sir Hiram Maxim's of upward air- 
curreots. Lord Rayleigh's of variations of the wind velocity at 
different heights above the ground, Dr S. P. Langley's of the 

* See G. H. Bryan in the Transactions of the British Association for 1896, 
jrol. Lxvi, pp. 726—728. 



CH. V] MECHANICAL RECREATIONS 107 

incessant occurrence of gusts of wind separated by lulls, and 
Dr Bryan's of vortices in the atmosphere. 

It now seems reasonably certain that the second and third of 
these sources of energy account for at least a portion of the 
observed phenomena. The effect of the third cause may be 
partially explained by noting that the centre of gravity of the 
bird with extended wings is slightly below the aeroplane or wing 
surface, so that the animal forms a sort of parachute. The effect 
of a sudden gust of wind upon such a body is that the aeroplane 
is set in motion more rapidly than the suspended mass, causing 
the structure to heel over so as to receive the wind on the under 
surface of the aeroplane, and this lifts the suspended mass 
giving it an upward velocity. When the wind falls the greater 
inertia of the mass carries it on upwards causing the aeroplane 
to again present its under side to the air; and if while the 
parachute is in this position the wind is still blowing from the 
side, the suspended mass is again lifted. Thus the more the 
bird is blown about, the more it rises in the air ; actually birds 
in flight are carried up by a sudden side gust of wind as we 
should expect from this theory. 

The fact that the bird is in motion tends also to keep it up, 
for it has been recently shown that a horizontal plane under 
the action of gravity falls to the ground more slowly if it is 
travelling through the air with horizontal velocity than it would 
do if allowed to fall vertically, hence the bird's forward motion 
causes it to fall through a smaller height between successive 
gusts of wind than it would do if it were at rest. Moreover it 
has been proved experimentally that the horse-power required 
to support a body in horizontal flight by means of an aeroplane 
is less for high than for low speeds: hence when a side-wind 
(that is, a wind at right angles to the bird's course) strikes the 
bird, the lift is increased in consequence of the bird's forward 
velocity. 

CURIOSA Physica. When I was writing the first edition 
of these Recreations, I put together a chapter, following this 
one, on " Some Physical Questions," dealing with problems such 
as, in the Theory of Sound, the explanation of the fact that in 



108 MECHANICAL RECREATIONS [CH. V 

some of Captain Parry's experiments the report of a cannon, 
when fired, travelled so much more rapidly than the sound 
of the human voice that observers heard the report of the 
cannon when fired before that of the order to fire it* : in the 
Kinetic Theory of Gases, the complications in our universe that 
might be produced by "Maxwell's demon "f : in the Theory of 
Optics, the explanation of the Japanese "magic mirrors/'J 
which reflect the pattern on the back of the mirror, on which 
the light does not fall : to which I might add the theory of the 
" spectrum top," by means of which a white surjp^ce, on which 
some black lines are drawn, can be moved so as to give the 
impression§ that the lines are coloured (red, green, blue, slate, 
or drab), and the curious fact that the colours change with the 
direction of rotation : it has also been recently shown that if two 
trains of waves, whose lengths are in the ratio m — 1 : m + 1, be 
superposed, then every mth wave in the system will be big — 
thus the current opinion that every ninth wave in the open sea 
is bigger than the other waves may receive scientific confirma- 
tion. There is no lack of interesting and curious phenomena 
in physics, and in some branches, notably in electricity and 
magnetism, the difficulty is rather one of selection, but I felt 
that the connection with mathematics was in general either too 
remote or too technical to justify the insertion of such a 
collection in a work on elementary mathematical recreations, 
and therefore I struck out the chapter. I mention the fact now 
partly to express the hope that some physicist will one day give 
us a collection of the kind, partly to suggest these questions to 
those who are interested in such matters. 

* The fact is well authenticated. Mr Earnshaw {Philosophical Tra7isactiom, 
London, 1860, pp. 133 — ]48) explained it by the acceleration of a wave caused by 
the formation of a kind of bore, a view accepted by Clerk Maxwell and most 
physicists, but Sir George Airy thought that the explanation was to be found in 
physiology; see Airy's Sound, second edition, London, 1871, pp. 141, 142. 

t See Theory of Heat, by J. Clerk Maxwell, second edition, London, 1872, 
p. 308. 

X See a memoir by W. E. Ayrton and J. Perry, Proceedings of the Royal 
Society of London, part i, 1879, vol. xxviii, pp. 127—148. 

§ See letters from Mr C. E. Benham and others in Nature, 1894-5; and 
a paper read by Pi of. G. D.Liveing before the Cambridge Philosophical Society, 
November 26, 1894. 



109 



CHAPTER VI. 



CHESS-BOARD RECREATIONS. 



A chess-board and chess-men lend themselves to recreations 
many of which are geometrical. The problems are, however, of 
a distinct type, and sufficiently numerous to deserve a chapter 
to themselves. A few problems which might be included in 
this chapter have been already considered in chapter iv. 

The ordinary chess-board consists of 64 small squares, known 
as cells, arranged as shown below in 8 rows and 8 columns. 
Usually the cells are coloured alternately white and black, or 
white and red. The cells may be defined by the numbers 11, 
12, &c., where the first digit denotes the number of the column, 



18 


28 


38 


48 


58 


68 


78 


88 


17 


27 


37 


47 


57 


67 


77 


87 


16 


26 


36 


46 


56 


66 


76 


86 


15 


25 


35 


45 


55 


65 


75 


85 


14 


24 


34 


44 


54 


64 


74 


84 


13 


23 


33 


43 


53 


63 


73 


83 


12 


22 


32 


42 


52 


62 


72 


82 


11 


21 


31 


41 


51 


61 


71 


81 



and the second digit the number of the row — the two digits 
representing respectively the abscissa and ordinate d!" the mid- 
points of the cells. I use this notation in the following pages. 
A generalized board consists of /i^ cells arranged in n rows and 



110 CHESS-BOARD RECREATIONS [CH. VI 

n columns. Most of the problems which I shall describe can 
be extended to meet the case of a board of v? cells. 

The usual chess-pieces are Kings, Queens, Bishops, Knights, 
and Rooks or Castles; there are also Pawns. I assume that 
the moves of these pieces are known to the reader. 

With the game itself and with chess problems of the usual 
type I do not concern myself. Particular positions of the pieces 
may be subject to mathematical analysis, but in general the 
moves open to a player are so numerous as to make it im- 
possible to see far ahead. Probably this is obvious, but it may 
emphasize how impossible it is to discuss the theory of the 
game effectively if I add that it has been shown that there 
may be as many as 197299 ways of playing the first four moves, 
and nearly 72000 different positions at the end of the first four 
moves (two on each side), of which 16556 arise when the players 
move pawns only*. 

Relative Value of Pieces. The first question to which 
I will address myself is the determination of the relative values 
of the different chess-pieces f. 

If a piece is placed on a cell, the number of cells it com- 
mands depends in general on its position. We may estimate 
the value of the piece by the average number of cells which it 
commands when placed in succession on every cell of the board. 
This is equivalent to saying that the value of a piece may 
be estimated by the chance that if it and a king are put at 
random on the board, the king will be in check : if no other re- 
striction is imposed this is called a simple check. On whatever 
cell the piece is originally placed there will remain 63 other 
cells on which the king may be placed. It is equally probable 
that it may be put on any one of them. Hence the chance 
that it will be in check is 1/63 of the average number of cells 
commanded by the piece. 

♦ VIntermediaire des Mathematiciens, Paris, December, 1903, vol. x, pp. 
305 — 308 : also Royal Engineers Journal, London, August — November, 1889 ; 
or British Association Transactions, 1890, p. 745. 

t H. M. Taylor, Philosophical Magazine ^ March, 1876, series 5, vol. i, 
pp. 221—229. 



CH. Vl] CHESS-UOARD RECREATIONS 111 

A rook put on any cell commands 14 other cells. Wherever 
the rook is placed there will remain 63 cells on which the king 
may be placed, and on which it is equally likely that it 
will be placed. Hence the chance of a simple check is 14/63, 
that is, 2/9. Similarly on a board of n^ cells the chance is 
2 (n - \)l{n' - 1), that is, 2/(n + 1). 

A knight when placed on any of the 4 c6mer cells like 

11 commands 2 cells. When placed on any of the 8 cells like 

12 and 21 it commands 3 cells. When placed on any of the 
4 cells like 22 or any of the 16 boundary cells like 13, 14, 15, 
16, it commands 4 cells. When placed on any of the 16 cells 
like 23, 24, 25, 26, it commands 6 cells. And when placed on 
any of the remaining 16 middle cells it commands 8 cells. 
Hence the average number of cells commanded by a knight put 
on a chess-board is (4x2 + 8x3 + 20 x4 + 16x6 + 16 x 8)/64, 
that is, 336/64. Accordingly if a king and a knight are put on 
the board, the chance that the king will be in simple check 
is 336/64 X 63, that is 1/12. Similarly on a board of n* cells 
the chance is S(n — 2)/?i- (n + 1). 

A bishop when placed on any of the ring of 28 boundary 
cells commands 7 cells. When placed on any ring of the 
20 cells next to the boundary cells, it commands 9 cells. 
When placed on any of the 12 cells forming the next ring, 
it commands 11 cells. When placed on the 4 middle cells 
it commands 13 cells. Hence, if a king and a bishop are put 
on the board the chance that the king will be in simple check 
is (28 X 7 + 20 X 9 + 12 X 11 + 4 X 13)/64 x 63, that is, 5/36. 
Similarly on a board of n^ cells, when n is even, the chance 
is 2 (271 — l)/3n (n + 1). When n is odd the analysis is longer, 
owmg to the fact that in this case the number of white cells 
on the board differs from the number of black cells. I do not 
give the work, which presents no special difficulty. 

A queen when placed on any cell of a board commands all 
the cells which a bishop and a rook when placed on that cell 
would do. Hence, if a king and a queen are put on the board, 
the chance that the king will be in simple check is 2/9 + 5/36, 
that is, 13/36. Similarly on a board of n^ cells, when n is even, 
the chance is 2(5n — l)/37? (n + 1). 



112 CHESS-BOARD KECREATIONS [CU. VI 

On the above assumptions the relative values of the rook, 
knight, bishop, and queen are 16, 6, 10, 26. According to 
Staunton's Chess-Player's Handbook the actual values, estimated 
empirically, are in the ratio of 548, 305, 350, 994 ; according to 
Von Bilguer the ratios are 540, 350, 360, 1000— the value of a 
pawn being taken as 10. 

There is considerable discrepancy between the above results 
as given by theory and practice. It has been, however, sug- 
gested that a better test of the value of a piece would be the 
chance that when it and a king were put at random .on the 
board it would check the king without giving the king the 
opportunity of taking it. This is called a safe check as distin- 
guished from a simple check. 

Applying the same method as above, the chances of a safe 
check work out as follows. For a rook the chance of a safe 
check is (4x12 + 24 x 11 + 36 x 10)/64 x 63, that is, 1/6 ; or 
on a board of n^ cells is 2{n— 2)/n {n + 1). For a knight all 
checks are safe, and therefore the chance of a safe check is 1/12 ; 
or on a board of n^ cells is 8(n — 2)ln^(n + 1). For a bishop 
the chance of a safe check is 364/64 x 63, that is, 13/144; or on a 
board of n^ cells, when n is even, is 2(n — 2) (2n — S)/Sn^ (n + 1). 
For a queen the chance of a safe check is 1036/64 x 63, that is, 
37/144 ; or on a board of n^ cells, is2(n- 2) {5n - S)j3n^ (n + 1), 
when n is even. 

On this view the relative values of the rook, knight, bishop, 
queen are 24, 12, 13, 37 ; while, according to Staunton, experi- 
ence shows that they are approximately 22, 12, 14, 40, and 
aocording to Von Bilguer, 18, 12, 12, 33. 

The same method can be applied to compare the values 
of combinations of pieces. For instance the value of two 
bishops (one restricted to white ceils and the other to black 
cells) and two rooks, estimated by the chance of a simple 
check, are respectively 35/124 and 37/93. Hence on this view 
a queen in general should be more valuable than two bishops 
but less valuable than two rooks. This agrees with experience. 

An analogous problem consists in finding the chance that 
two kings, put at random on the board, will not occupy adjoin- 
ing cells, that is, that neither would (were such a move possible) 



CH. Vl] 



CHESS-BOARD RECREATIONS 



113 



check the other. The chance is 43/48, and therefore the chance 
that they will occupy adjoining cells is 5/48. If three kings 
are put on the board, the chance that no two of them occupy 
adjoining cells is 1061/1488. The corresponding chances* for 
a board of n^ cells are (n - 1) (n - 2) (w* + 3w - 2)/n« (n* - 1) and 
{n - 1) (w - 2) (n* + 3n« - 20/1^ - 30/i + 132)/n» (n' - 1) (n' - 2). 

The Eight Queens PROBLEMf . One of the classical 
problems connected with a chess-board is the determination 
of the number of ways in which eight queens can be placed on 
a chess-board — or more generally, in which n queens can be 
placed on a board of n* cells — so that no queen can take any 
other. This was proposed originally by Franz Nauck in 1850. 

In 1874 Dr S. GuntherJ suggested a method of solution by 
means of determinants. For, if each symbol represents the cor- 
responding cell of the board, the possible solutions for a board 
of n^ cells are given by those terms, if any, of the determinant 



CLi 62 Ci d^ 

A tts 64 C5 

73 A a^ hs 

^4 7b /5« (h 






in which no letter and no suffix appears more than once. 

The reason is obvious. Every term in a determinant con- 
tains one and only one element out of every row and out of 
every column : hence any term will indicate a 'position on the 
board in which the queens cannot take one another by moves 
rook-wise. Again in the above determinant the letters and 
suffixes are so arranged that all the same letters and all the 

♦ VIntermidiaire des MathSmaticiens^ Paris, 1897, vol. iv, p. 6, and 1901, 
▼ol. vui, p. 140. 

t On the history of this problem see W. Ahrens, Mathematitche Unter- 
haltungen und Spiele, Leipzig, 1901, chap. ix. 

X Gmnert's Archiv der Mathematik und Physik, 1874, yd. lvi, pp. 281—292. 



114 CHESS-BOARD RECREATIONS [CH. VI 

same suffixes lie along bishop's paths : hence, if we retain only 
those terms in each of which all the letters and all the suffixes 
are different, they will denote positions in which the queens 
cannot take one another by moves bishop-wise. It is clear that 
the signs of the terms are immaterial. 

In the case of an ordinary chess-board the determinant is of 
the 8th order, and therefore contains 8!, that is, 40320 terms, 
so that it would be out of the question to use this method 
for the usual chess-board of 64 cells or for a board of larger 
size unless some way of picking out the required terms could 
be discovered. 

A way of effecting this was suggested by Dr J. W.L. Glaisher* 
in 1874, and so far as I am aware the theory remains as he left 
it. He showed that if all the solutions of n queens on a board 
of n^ cells were known, then all the solutions of a certain type 
for w -{- 1 queens on a board of (n + 1)^ cells could be deduced, 
and that all the other solutions of n+1 queens on a board of 
{n + iy cells could be obtained without difficulty. The method 
will be sufficiently illustrated by one instance of its application. 

It is easily seen that there are no solutions when n = 2 and 
n—S. If n = 4 there are two terms in the determinant which 
give solutions, namely, b^Csys/Ss and Cgft^eTs- To find the solutions 
when w = 5, Glaisher proceeded thus. In this case, Gunther s 
determinant is 

dl 62 Cj C?4 &6 

I3-2 ^3 h C5 C?6 

73 ^4 ^6 ^6 C? 

K 76 ^6 (h h 

€6 Be 77 A a» 

To obtain those solutions (if any) which involve a^, it is sufficient 
to append a^ to such of the solutions for a board of 16 cells as 
do not involve a. As neither of those given above involves an 
a we thus get two solutions, namely, hiC^y-i^fpii and Cs^zbejidQ- 

* Philosophical Magazine, London, December, 1874, series 4, vol. XLvin, 

pp. 457—407. 



CH. Vl] 



CHESS-BOARD RECREATIONS 



115 



The solutions which involve Oi, e^ and 65 can be written down 
by symmetry. The eight solutions thus obtained are all dis- 
tinct ; we may call them of the first type. 

The above are the only solutions which can involve elements 
in the corner squares of the determinant. Hence the remaining 
solutions are obtainable from the determinant 

62 C3 ^4 

/3a a» h Cb C^a 

7i A «6 &« C7 

^4 7» A ^ ^8 

8, 7; ^8 

If, in this, we take the minor of h^ and in it replace by zero 
every term involving the letter b or the suffix 2 we shall get 
all solutions involving 6,. But in this case the minor at once 
reduces to dta^Siffs' We thus get one solution, namely, b^d^a^iSA' 
The solutions which involve 02, S4, S^, jSs, bs, d^, and d^ can be 
obtained by symmetry. Of these eight solutions it is easily 
seen that only two are distinct ; these may be called solutions 
of the second type. 

Similarly the remaining solutions must be obtained from 
the determinant 

c 
a, 64 Cb 

7» A «6 ^6 C7 

76 A 07 
77 

If, in this, we take the minor of Cj, and in it replace by zero 
every term involving the letter c or the suffix 3, we shall get 
all the solutions which involve c,. But in this case the minor 
vanishes. Hence there is no solution involving c,, and therefore 
by symmetry no solutions which involve 73 , 77, or Oj. Had there 
been any solutions involving the third element in the first or 
last row or column of the determinant we should have described 
them as of the third type. 

8—2 



116 CHESS-BOARD RECREATIONS [CH. VI 

Thus in all there are ten and only ten solutions, namely, 
eight of the first type, two of the second type, and none of the 
third type. 

Similarly, if n = 6, we obtain no solutions of the first type, 
four solutions of the second type, and no solutions of the third 
type ; that is, four solutions in all. If n = 7, we obtain sixteen 
solutions of the first type, twenty-four solutions of the second 
type, no solutions of the third type, and no solutions of the 
fourth type ; that is, forty solutions in all. If n = 8, we obtain 
sixteen solutions of the first type, fifty-six solutions of the second 
type, and twenty solutions of the third type, that is, ninety-two 
solutions in all. 

It will be noticed that all the solutions of one type are not 
always distinct. In general, from any solution seven others can 
be obtained at once. Of these eight solutions, four consist of 
the initial or fundamental solution and the three similar ones 
obtained by turning the board through one, two, or three right 
angles ; the other four are the reflexions of these in a mirror : 
but in any particular case it may happen that the reflexions 
reproduce the originals, or that a rotation through one or two 
right angles makes no diflference. Thus on boards of 4^ 5^ 6^ 
7^ 8^ 9^ lO^ cells there are respectively 1, 2, 1, 6, 12, 46, 92 
fundamental solutions ; while altogether there are respectively 
2, 10, 4, 40, 92, 352, 724 solutions. 

The following collection of fundamental solutions may in- 
terest the reader. Each position on the board of the queens 
is indicated by a number, but as necessarily one queen is on 
each column I can use a simpler notation than that explained 
on page 109. In this case the first digit represents the number 
of the cell occupied by the queen in the first column reckoned 
from one end of the column, the second digit the number in the 
second column, and so on. Thus on a board of 4' cells the 
solution 3142 means that one queen is on the 3rd square of the 
first column, one on the 1st square of the second column, one 
on the 4th square of the third column, and one on the 2nd 
square of the fourth column. If a fundamental solution gives 
rise to only four solutions the number which indicates it is placed 



CH. Vl] 



CHESS-BOARD RECREATIONS 



117 



in curved brackets, ( ) ; if it gives rise to only two solutions the 
number which indicates it is placed in square brackets, [ ] ; the 
other fundamental solutions give rise to eight solutions each. 

On a board of 4' cells there is 1 fundamental solution : 
namely, [3142]. 

On a board of 5* cells there are 2 fundamental solutions: 
namely, 14253, [25314]. It may be noted that the cyclic 
solutions 14253, 25314, 31425, 42531, 53142 give five super- 
posable arrangements by which five white queens, five black 
queens, five red queens, five yellow queens, and five blue queens 
can be put simultaneously on the board so that no queen can 
be taken by any other queen of the same colour. 

On a board of 6' cells there is 1 fundamental solution: 
namely, (246135). The four solutions are superposable. The 
puzzle for this case is sold in the streets of London for a penny, 
a small wooden board being ruled in the manner shown in 
the diagram and having holes drilled in it at the points marked 
by dots. The object is to put six pins into the holes so that 
no two are connected by a straight line. 




On a board of 7' cells there are 6 fundamental solutions : 
namely, 1357246, 3572461, (5724613), 4613572, 3162574, 
(2574136). It may be noted that the solution 1357246 gives 
by cyclic permutations seven superposable arrangements. 

On a board of 8^" cells there are 12 fundamental solutions : 
namely, 25713864, 57138642, 71386425, 82417536, 68241753, 
36824175,64713528,36814752,36815724, 72418536, 26831475, 
(64718253). The arrangement in this order is due to Mr Oram. 
It will be noticed that the 10th, 11th, and 12th solutions some- 
what resemble the 4th, 6th, and 7 th respectively. The 6th 



118 



CHESS-BOARD RECREATIONS 



[CH. VI 



solution is the only one in which no three queens are in a 
straight line. It is impossible* to find eight superposable so- 
lutions ; but we can in five typical ways pick out six solutions 
which can be superposed, and to some of these it is possible to 
add 2 sets of 7 queens, thus filling 62 out of the 64 cells with 
6 sets of 8 queens and 2 sets of 7 queens, no one of which 
can take another of the same set. Here is such a solution : 
16837425, 27368514, 35714286, 41586372, 52473861, 68241753, 
73625140, 04152637. Similar superposition problems can be 
framed for boards of other sizes. 

On a board of 9^* cells there are 46 fundamental solutions, 
and on a board of 10^ cells there are 92 fundamental solutions ; 
these were given by Dr A. Peinf. On a board of 11=» cells 
there are 341 fundamental solutions ; these have been given by 
Dr T. B. SpragueJ. 

On any board empirical solutions may be found with but 
little difficulty, and Mr Derrington has constructed the following 
table of solutions : 



4.1. 



for a board of 4^ cells 



4.1.3.5 

4.6.1.3.5 

4.6.1.8.5.7 

4.6.8.3.1.7.5 

4.1.7.9.6.3.5.8 

4.6.8.10.1.3.5.7.9 

4.6.8.10.1.3.5.7.9.11 

4.6.8.10.12.1.3.5.7.9.11 

4.6.8.10.12.1.3.5.7.9.11.13 

7 . 5 . 3. 1 . 13 . 11 . 6 . 4 . 2 . 14 . 12 . 10 . 8 

.9.7.5.3.1.13.11.6.4.2.14.12.10.8 

4 . 6 . 8 . 10 . 12 . 14 . 16 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 

4 . 6 . 8 . 10 . 12 . 14 . 16 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17 

4 . 6 . 8 . 10 . 12 . 14 . 16 . 18 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17 

4 . 6 . 8 . 10 . 12 . 14 . 16 . 18 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17 . 19 

. 10 . 8 . 6 . 4 . 2 . 20 . 18 . 16 . 14 . 9 . 7 . 5 . 3 . 1 . 19 . 17 . 15 . 13 . 11 

. 12 . 10 . 8 . 6 . 4 . 2 . 20 . 18 . 16 . 14 . 9 . 7 . 5 . 3 . 1 . 19 . 17 . 15 . 13 . 11 



52 
62 
72 

82 
9« 
102 
IP 
122 
132 
142 
152 
162 
172 

182 

192 

202 
212 



♦ See G. T. Bennett, The Messenger of Mathematics, Cambridge, June, 1909, 
vol. XXXIX, pp. 19—21. 

t Aufstellung von n Koniginnen auf einem Schachbrett von n^ Feldern, 
Leipzig, 1889. 

X Proceedings of the Edinburgh Matliematical Society^ vol. xvii, 1898-9, 
pp. 43—68, 



CH. Vl] CHESS-BOARD RECREATIONS 119 

and so on. The rule is obvious except when w is of the form 
6m + 2 or 6m + 3. 

Maximum Pieces Problem*. The Eight Queens Problem 
suggests the somewhat analogous question of finding the 
maximum number of kings — or more generally of pieces of 
one t3rpe — which can be put on a board so that no one can 
take any other, and the number of solutions possible in each 
case. 

In the case of kings the number is 16; for instance, one 
solution is when they are put on the cells 11, 13, 15, 17, 31, 33, 
35, 37, 51, 53, 55, 57, 71, 73, 75, 77. For queens, it is obvious 
that the problem is covered by the analysis already given, and 
the number is 8. For bishops the number is 14, the pieces 
being put on the boundary cells; for instance one solution is 
when they are put on the cells 11, 12, 13, 14, 15, 16, 17, 81, 82, 
83, 84, 85, 86, 87, there are 256 fundamental solutions. For 
knights the number is 32 ; for instance, they can be put on all 
the white or on all the black cells, and there are 2 fundamental 
solutions. For rooks it is obvious that the number is 8, and 
there are in all 8 ! solutions. 

Minimum Pieces Problem*. Another problem of a some- 
what similar character is the determination of the minimum 
number of kings — or more generally of pieces of one type — 
which can be put on a board so as to command or occupy all 
the cells. 

For kings the number is 9; for instance, they can be put on 
the cells 11, 14, 17, 41, 44, 47, 71, 74, 77. For queens the 
number is 5; for instance, they can be put on the cells 18, 35, 
41, 76, 82. For bishops the number is 8; for instance, they can 
be put on the cells 41, 42, 43, 44, 45, 46, 47, 48. For knights 
the number is 12; for instance, they can be put on the cells 
26, 32, 33, 35, 36, 43, 56, 63, 64, 66, 67, and 73— constituting 
four triplets arranged symmetrically. For rooks the number 
is 8, and the solutions are obvious. 

♦ Mr H. E. Dudeney has written on these problems in the Weekly Dispatch. 



120 CHESS-BOARD RECREATIONS [CH. VI 

For queens the problem has been also discussed for a board 
of n' cells where n has various values*. One queen can be 
placed so as to command all the cells when n = 2 or 3, and 
there is only 1 fundamental solution. Two queens are required 
when n = 4 ; and there are 3 fundamental solutions, namely, 
when they are placed on the cells 11 and 33, or on the cells 12 
and 42, or on the cells 22, 23: these give 12 solutions in all. 
Three queens are required when n = 5 ; and there are 37 funda- 
mental solutions, giving 186 solutions in all. Three queens 
are also required when n = 6, but there is only 1 fundamental 
solution, namely, when they are put on the cells 11, 35, and 
53, giving 4 solutions in all. Four queens are required when 
n = 7, one solution is when they are put on the cells 12, 26, 
41, 55. 

Jaenisch proposed also the problem of the determination of 
the minimum number of queens which can be placed on a board 
of n* cells so as to command all the unoccupied cells, subject to 
the restriction that no queen shall attack the cell occupied by 
any other queen. In this case three queens are required when 
71 = 4, for instance, they can be put on the cells 11, 23, 42 ; and 
there are 2 fundamental solutions, giving 16 solutions in all. 
Three queens are required when ti = 5, for instance, they can be 
put on the cells 11, 24, 43, or on the cells 11, 34, 53 ; and there 
are 2 fundamental solutions in all. Four queens are required 
when w = 6, for instance, when they are put on the cells 13, 36, 
41, 64; and there are 17 fundamental solutions. Four queens 
are required when n = 7, and there is only 1 fundamental solu- 
tion, namely, that already mentioned, when they are put on the 
cells 12, 26, 41, 55, which gives 8 solutions in all. Five queens 
are required when n = 8, and there are no less than 91 funda- 
mental solutions ; for instance, one is when they are put on the 
cells 11, 23, 37, 62, 76. 

I leave to any of my readers who may be interested in such 
questions the discussion of the corresponding problems for the 

♦ C. F. de Jaenisch, Applications de VAnalyse MatMmatique au Jeu des 
lilchecs, Petrogrnd, 1862, Appendix, p. 244 ct seq. ; see also L'Intermediaire 
des Mathematiciens, Paris, 1901, vol, vin, p. 88, 



CH. Vl] CHESS-BOAED RECREATIONS 121 

other pieces*, and of the number of possible solutions in each 
case. 

A problem of the same nature would be the determination 
of the minimum number of queens (or other pieces) which 
can be placed on a board so as to protect one another and 
command all the unoccupied cells. For queens the number 
is 5; for instance, they can be put on the cells 24, 34, 44, 
54 and 84. For bishops the number is 10; for instance, they 
can be put on the cells 24, 25, 34, 35, 44, 45, 64, 65, 74, and 75. 
For knights the number is 14; for instance, they can be put on 
the cells 32, 33, 36, 37, 43, 44, 45, 46, 63, 64, 65, 66, 73, and 76: 
the solution is semi-symmetrical. For rooks the number is 8, 
and a solution is obvious. I leave to any who are interested 
in the subject the determination of the number of solutions in 
each case. 

In connexion with this class of problems, I may mention 
two other questions, to which Captain Turton first called my 
attention, of a somewhat analogous character. 

The first of these is to place eight queens on a chess-board 
so as to command the fewest possible squares. Thus, if queens 
are placed on cells 21, 22, 62, 71, 73, 77, 82, 87, eleven cells on 
the board will not be in check; the same number can be 
obtained by other arrangements. Is it possible to place the 
eight queens so as to leave more than eleven cells out of check? 
I have never succeeded in doing so, nor in showing that it is 
impossible to do it. 

The other problem is to place m queens (m being less than 
6) on a chess-board so as to command as many cells as possible. 
For instance, four queens can be placed in several ways on the 
board so as to command 58 cells besides those on which the 
queens stand, thus leaving only 2 cells which are not com- 
manded; for instance, queens may be placed on the cells 35, 41, 

* The problem for knights was discussed in UInterm4diaire des Mathi- 
maticiens, Paris, 1896, vol. m, p. 58; 1BD7, vol. iv, pp. 15—17, 254; 1898, vol. 
V, pp. 87, 230—231. 



122 CHESS-BOARD RECKEATIONS [CH. VI 

76, and 82. Analogous problems with other pieces will suggest 
themselves. 

There are endless similar questions in which combinations 
of pieces are involved. For instance, if queens are put on the 
cells 35, 41, 76, and 82 they command or occupy all but two 
cells, and these two cells may be commanded or occupied by a 
queen, a king, a rook, a bishop, or a pawn. If queens are put 
on the cells 22, 35, 43, and 54 they command or occupy all but 
three cells, and two of these three cells may be commanded by 
a knight which occupies the third of them. 

Re-Entrant Paths on a Chess-Board. Another problem 
connected with the chess-board consists in moving a piece in 
such a manner that it shall move successively on to every pos- 
sible cell once and only once. 

Knight's Re-Entrant Path. I begin by discussing the 
classical problem of a knight's tour. The literature* on this 
subject is so extensive that I make no attempt to give a full 
account of the various methods for solving the problem, and 
I shall content myself by putting together a few notes on some 
of the solutions I have come across, particularly on those due 
to De Moivre, Euler, Vandermonde, WarnsdorfF, and Roget. 

On a board containing an even number of cells the path 
may or may not be re-entrant, but on a board containing an 
odd number of cells it cannot be re-entrant. For, if a knight 
begins on a white cell, its first move must take it to a black 
cell, the next to a white cell, and so on. Hence, if its path 
passes through all the cells, then on a board of an odd number 
of cells the last move must leave it on a cell of the same colour 
as that on which it started, and therefore these cells cannot be 
connected by one move. 

* For a bibliography see A. van der Linda, Qeschichte und Literatur des 
Schachspiels, Berlin, 1874, vol. ii, pp. 101 — 111. On the problem and its 
history see a memoir by P. Volpicelli in Atti della Reale Accademia del Lincei, 
Rome, 1872, vol. xxv, pp. 87 — 162 : also Applications de V Analyse Mathematique 
au Jeu des lichees, by C. F. de Jaenisch, 3 vols., Petrograd, 1862-3; and 
General Parmentier, Association Frangaise pour. Vavancement des Sciences, 1891, 
1892, 1894. 



CH. Vl] 



CHESS-BOARD RECREATIONS 



123 



The earliest solutions of which I have any knowledge are 
those given at the beginning of the eighteenth century by De 
MontmoA and De Moivre*. They apply to the ordinary chess- 
board of 64 cells, and depend on dividing (mentally) the board 
into an inner square containing sixteen cells surrounded by an 
outer ring of cells two deep. If initially the knight is placed 
on a cell in the outer ring, it moves round that ring always in 
the same direction so as to fill it up completely — only going 
into the inner square when absolutely necessary. When the 
outer ring is filled up the order of the moves required for 
filling the remaining cells presents but little difficulty. If 
initially the knight is placed on the inner square the process 
must be reversed. The method can be applied to square and 
rectangular boards of all sizes. It is illustrated sufficiently by 
De Moivre's solution which is given below, where the numbers 



De Moivre*i Solution. 



34 


49 


22 


11 


36 


39 


24 


1 
















21 


10 


35 


50 


23 


12 


37 


40 


30 


21 


6 


15 


28 


19 


48 


33 


62 


67 


38 


25 


2 


13 


7 


16 


29 


20 


5 


14 


9 


20 


51 


54 


63 


60 


41 


26 


22 


31 


8 


35 


18 


27 


32 


47 


58 


61 


56 


53 


14 


3 


9 


36 


17 


26 


13 


4 


19 


8 


65 


52 


59 


64 


27 


42 


32 


23 


2 


11 


34 


25 


46 


31 


6 


17 


44 


29 


4 


15 


1 


10 


33 


24 


3 


12 


7 


18 


45 


30 


5 


16 


43 


28 















Euler's Thirty-tix Cell Solution. 



Indicate the order in which the cells are occupied successively. 
I place by its side a somewhat similar re-entrant solution, due 
to Euler, for a board of 36 cells. If a chess-board is used it 
is convenient to place a counter on each cell as the knight 
leaves it. 

The earliest serious attempt to deal with the subject by 

• They were sent by their authors to Brook Taylor who seems to have 
previously suggested the problem. I do not know where they were first pub- 
lished ; they were quoted by Ozanam and Montuola, see Ozanam, 1803 edition, 
vol. I, p. 178 ; 1840 edition, p. 80, 



124 



CHESS-BOARD RECREATIONS 



[CH. VI 



mathematical analysis was made by Euler* in 1759 : it was 
due to a suggestion made by L. Bertrand of Geneva, who sub- 
sequently (in 1778) issued an account of it. This method is 
applicable to boards of any shape and size, but in general 
the solutions to which it leads are not symmetrical and their 
mutual connexion is not apparent. 

Euler commenced by moving the knight at random over 
the board until it has no move open to it. With care this 
will leave only a few cells not traversed: denote them by 
a, h, .... His method consists in establishing certain rules 
by which these vacant cells can be interpolated into various 
parts of the circuit, and by which the circuit can be made 
re-entrant. 

The following example, mentioned by Legendre as one of 
exceptional difficulty, illustrates the method. Suppose that 



65 


68 


29 


To 


27 


44 


19 


22 




22 


25 


60 


39 


52 


35 


60 


57 


60 


39 


56 


43 


30 


21 


26 


46 


27 


40 


23 


36 


49 


58 


63 


34 


67 


64 


59 


28 


41 


18 


23 


20 


24 


21 


26 


61 


38 


61 


66 


59 


"38 


61 


42 


31 


8 


25 


46 


17 


41 


28 


37 


48 


3 


54 


33 


62 


63 


32 


37 


a 


47 


16 


9 


24 


20 


47 


42 


13 


32 


63 


4 


55 


60 


3 


52 


33 


36 


7 


12 


15 


29 


16 


19 


46 


43 


2 


7 


10 


1 


31 


6 


48 


b 


14 


c 


10 


18 


46 


14 


31 


12 


9 


64 


5 


4 


49 


2 


35 


6 


11 


d 


13 


15 


30 


17 


44 


1 


6 


U 


8 



Figure I. Figure ii. 

Example of Euler'i Method. 

we have formed the route given in figure i above ; namely, 1, 
2, 3, . . . , 59, 60 ; and that there are four cells left untraversed, 
namely, a, h, c, d. 

We begin by making the path 1 to 60 re-entrant. The 
cell 1 commands a cell p, where p is 32, 52, or 2. The cell 60 
commands a cell q, where q is 29, 59, or 51. Then, if any of 
these values of p and q differ by unity, we can make the route 

♦ Memoires de Berlin for 1759, Berlin, 1766, pp. 310—337; or Gommentationes 
Arithmeticae Collectae, Petrograd, 1849, vol. i, pp. 337 — 355. 



CH. Vl] CHESS-BOARD RECREATIONS 126 

re-entrant. This is the case here i£ p = 52, q = 51. Thus the 
cells 1, 2, 3, ..., 51; 60, 59, ..., 52 form a re-entrant route of 60 
moves. Hence,* if we replace the numbers 60, 59, ..., 52 by 52, 
53, . . . , 60, the steps will be numbered consecutively. I recom- 
mend the reader who wishes to follow the subsequent details 
of Euler's argument to construct this square on a piece of paper 
before proceeding further. 

Next, we proceed to add the cells a, b, d to this route. In 
the new diagram of 60 cells formed as above the cell a commands 
the cells there numbered 51, 53, 41, 25, 7, 5, and 3. It is in- 
different which of these we select : suppose we take 51. Then 
we must make 51 the last cell of the route of 60 cells, so that 
we can continue with a, 6, d. Hence, if the reader will add 9 
to every number on the diagram he has constructed, and then 
replace 61, 62, ..., 69 by 1, 2, ..., 9, he will have a route which 
starts from the cell occupied originally by 60, the 60th move is 
on to the cell occupied originally by 51, and the 61st, 62nd, 
63rd moves will be on the cells a, 6, d respectively. 

It remains to introduce the cell c. Since c commands the 
cell now numbered 25, and 63 commands the cell now numbered 
24, this can be effected in the same way as the first route was 
made re-entrant. In fact, the cells numbered 1, 2, ..., 24; 63, 

62, ..., 25, c form a knight's path. Hence we must replace 

63, 62, ..., 25 by the numbers 25, 26, ..., 63, and then we can 
fill up c with 64. We have now a route which covers the 
whole board. 

Lastly, it remains to make this route re-entrant. First, we 
must get the cells 1 and 64 near one another. This can be 
effected thus. Take one of the cells commanded by 1, such as 
28, then 28 commands 1 and 27. Hence the cells 64, 63, ..., 28; 
1, 2, ..., 27 form a route; and this will be represented in the 
diagram if we replace the cells numbered 1, 2, ..., 27 by 27, 
26, ...,1. 

The cell now occupied by 1 commands the cells 26, 38, 54, 
12, 2, 14, 16, 28 ; and the cell occupied by 64 commands the 
cells 13, 43, 63, 55. The cells 13 and 14 are consecutive, and 
therefore the cells 64, 63, ..., 14; 1, 2, .... 13 form a route. 



126 



CHESS-BOARD RECREATIONS 



[CH. VI 



Hence we must replace the numbers 1, 2, ..., 13 by 13, 12, ..., 

1, and we obtain a re-entrant route covering the whole board, 
which is represented in the second of the diagrams given above. 
Euler showed how seven other re-entrant routes can be deduced 
from any given re-entrant route. 

It is not difficult to apply the method so as to form a route 
which begins on one given cell and ends on any other given 
cell. 

Euler next investigated how his method could be modified 
so as to allow of the imposition of additional restrictions. 

An interesting example of this kind is where the first 32 
moves are confined to one-half of the board. One solution 
of this is delineated below. The order of the first 32 moves 



58 


43 


60 


37 


62 


41 


62 


35 




50 


45 


62 


41 


60 


89 


54 


35 


49 


46 


67 


42 


61 


36 


53 


40 


63 


42 


61 


48 


63 


36 


57 


38 


44 


69 


48 


51 


38 


65 


34 


63 


46 


49 


44 


61 


40 


69 


34 


55 


47 


50 


46 


56 


33 


64 


39 


64 


43 


64 


47 


52 


33 


66 


37 


68 


22 


7 


32 


1 


24 


13 


18 


15 


26 


6 


24 


1 


20 


15 


32 


a 


31 


2 


23 


6 


19 


16 


27 


12 


23 


2 


27 


8 


29 


12 


17 


14 


8 


21 


4 


29 


10 


25 


14 


17 


6 


25 


4 


21 


16 


19 


10 


31 


3 


30 


9 


20 


5 


28 


11 


26 


3 


22 


7 


28 


9 


30 


13 


18 



Euler's Half-board Solution. 



Bogefs Half-board Solution, 



can be determined by Euler's method. It is obvious that, if 
to the number of each such move we add 32, we shall have 
a corresponding set of moves from 33 to 64 which would cover 
the other half of the board ; but in general the cell numbered 
33 will not be a knight's move from that numbered 32, nor will 
64 be a knight's move from 1. 

Euler however proceeded to show how the first 32 moves 
might be determined so that, if the half of the board con- 
taining the corresponding moves from 33 to 64 was twisted 
through two right angles, the two routes would become united 
and re-entrant. If x and y are the numbers of a cell reckoned 
from two consecutive sides of the board, we may call the cell 



CH. Vl] CHESS-BOARD RECREATIONS 127 

whose distances are respectively x and y from the opposite 
sides a complementary cell. Thus the cells (a?, y) and (9 — a?, 
9 — y) are complementary, where x and y denote respectively 
the column and row occupied by the cell. Then in Euler's 
solution the numbers in complementary cells differ by 32 : for 
instance, the cell (3, 7) is complementary to the cell (6, 2), the 
one is occupied by 57, the other by 25. 

Roget's method, which is described later, can be also applied 
to give half-board solutions. The result is indicated above. 
The close of Euler's memoir is devoted to showing how the 
method could be applied to crosses and other rectangular 
figures. I may note in particular his elegant re-entrant sym- 
metrical solution for a square of 100 cells. 

The next attempt of any special interest is due to Vander- 
monde*, who reduced the problem to arithmetic. His idea was 
to cover the board by two or more independent routes taken 
at random, and then to connect the routes. He defined the 
position of a cell by a fraction x/y, whose numerator x is the 
number of the cell from one side of the board, and whose 
denominator y is its number from the adjacent side of the 
board; this is equivalent to saying that x and y are the 
co-ordinates of a cell. In a series of firactions denoting a 
knight's path, the differences between the numerators of two 
consecutive fractions can be only one or two, while the corre- 
sponding differences between their denominators must be two 
or one respectively. Also x and y cannot be less than 1 or 
greater than 8. The notation is convenient, but Vander- 
monde applied it merely to obtain a particular solution of 
the problem for a board of 64 cells: the method by which 
he effected this is analogous to that established by Euler, 
but it is applicable only to squares of an even order. The 
route that he arrives at is defined in his notation by the 
following fi-actions: 5/5, 4/3, 2/4, 4/5, 5/3, 7/4, 8/2, 6/1, 7/3, 
8/1, 6/2, 8/3, 7/1, 5/2, 6/4, 8/5, 7/7, 5/8, 6/6, 5/4, 4/6, 2/5, 
1/7, 3/8, 2/6, 1/8, 3/7, 1/6, 2/8, 4/7, 3/5, 1/4, 2/2, 4/1, 3/3, 
1/2, 3/1, 2/3, 1/1, 3/2, 1/3, 2/1, 4/2, 3/4, 1/5, 2/7, 4/8, 3/6, 

• L'Histoire de VAcademie des Sciences for 1771, Paris, 1774, pp. 666—574, 



128 CHESS-BOAUD RECREATIONS [CH. VI 

4/4, 5/6, 7/5, 8/7, 6/8, 7/6, 8/8, 6/7, 8/6, 7/8, 5/7, 6/5, 8/4, 
7/2, 5/1, 6/3. 

The path is re-entrant but unsymmetrical. Had he trans- 
ferred the first three fractions to the end of this series he 
would have obtained two symmetrical circuits of thirty- two 
moves joined unsymmetrically, and might have been enabled 
to advance further in the problem. Vandermonde also con- 
sidered the case of a route in a cube. 

In 1773 Collini* proposed the exclusive use of symmetrical 
routes arranged without reference to the initial cell, but con- 
nected in such a manner as to permit of our starting from 
it. This is the foundation of the modem manner of attacking 
the problem. The method was re-invented in 1825 by Prattf, 
and in 1840 by Koget, and has been subsequently employed 
by various writers. Neither Collini nor Pratt showed skill in 
using this method. The rule given by Roget is described later. 

One of the most ingenious of the solutions of the knight's 
path is that given in 1823 by Warnsdorff J. His rule is that 
the knight must be always moved to one of the cells from 
which it will command the fewest squares not already traversed. 
The solution is not symmetrical and not re-entrant ; moreover 
it is difficult to trace practically. The rule has not been 
proved to be true, but no exception to it is known : apparently 
it applies also to all rectangular boards which can be covered 
completely by a knight. It is somewhat curious that in most 
cases a single false step, except in the last three or four moves, 
will not affect the result. 

Warnsdorff added that when, by the rule, two or more cells 
are open to the knight, it may be moved to either or any of 
them indifferently. This is not so, and with great ingenuity 
two or three cases of failure have been constructed, but it 
would require exceptionally bad luck to happen accidentally 
on such a route. 

• Solution du ProhUme du Cavalier au Jeu des Echecs^ Mannheim, 1778. 
+ Studies of Chess, sixth edition, London, 1825. 

J H. C. WarnsdorfE, Des Rosselsprunges einfachste und allgemeinste LUsung^ 
Schmalkalden, 1823 : see also Jaenisch, vol. n, pp. 56—61, 273—289, 



CH. Vl] CHESS-BOARD RECREATIONS 129 

The above methods have been applied to boards of various 
shapes, especially to boards in the form of rectangles, crosses, 
and circles*. 

All the more recent investigations impose additional restric- 
tions : such as to require that the route shall be re-entrant, or 
more generally that it shall begin and terminate on given cells. 

The simplest solution with which I am acquainted is due to 
De Lavemede, but is more generally associated with the name 
of Roget whose paper in 1840 attracted general notice to it f . 
It divides the whole route into four circuits, which can be 
combined so as to enable us to begin on any cell and termi- 
nate on any other cell of a different colour. Hence, if we like 
to select this last cell at a knight's move from the initial cell, 
we obtain a re-entrant route. On the other hand, the rule 
is applicable only to square boards containing (4!ny cells: for 
example, it could not be used on the board of the French jeu 
des dames, which contains 100 cells. 

Roget began by dividing the board of 64 cells into four 
quarters. Each quarter contains 16 cells, and these 16 cells 
can be arranged in 4 groups, each group consisting of 4 cells 
which form a closed knight's path. All the cells in each such 
path are denoted by the same letter I, e, a, or p, as the case 
may be. The path of 4 cells indicated by the consonants I and 
the path indicated by the consonants p are diamond-shaped: 
the paths indicated respectively by the vowels e and a are 
square-shaped, as may be seen by looking at one of the four 
quarters in figure i below. 

Now all the 16 cells on a complete chess-board which are 
marked with the same letter can be combined into one circuit, 
and wherever the circuit begins we can make it end on any 
other cell in the circuit, provided it is of a different colour 
to the initial cell. If it is indifferent on what cell the 
circuit terminates we may make the circuit re-entrant, and 

* See ex. gr. T. Ciccolini's work Del Cavallo degli Scacchi, Paris, 1836. 

t J. E. T. de LavernMe, M6moire8 de VAcademie Royale du Gard, Nimea, 
1839, pp. 151—179. P. M. Roget, Philosophical Magazine, April, 1840, series 3, 
vol. XVI, pp. 305 — 309 ; see also the Quarterly Journal of Mathematics for 1877, 
vol. XIV, pp. 354—359; and the Leisure Hour, Sept. 13, 1873, pp. 587—590, and 
Dec. 20, 1873, pp. 813—815. 

an. 9 



130 



CHESS-BOARD RECREATIONS 



[CH. VI 



in this case we can make the direction of motion round each 
group (of 4 cells) the same. For example, all the cells 
marked p can be arranged in the circuit indicated by the 
successive numbers 1 to 16 in figure ii below. Similarly all 
the cells marked a can be combined into the circuit indicated 
by the numbers 17 to 32 ; all the I cells into the circuit 33 to 
48 ; and all the e cells into the circuit 49 to 64. Each of the 
circuits indicated above is symmetrical and re-entrant. The 
consonant and the vowel circuits are said to be of opposite 
kinds. 



I 


€ 


a 


P 


I 


e 


a 


P 




34 


61 


32 


16 


38 


53 


18 


3 


a 


P 


I 


e 


a 


P 


I 


e 


31 


14 


35 


52 


17 


2 


39 


64 


e 


I 


P 


a 


e 


I 


P 


a 


50 


33 


16 


29 


56 


37 


4 


19 


P 


a 


e 


I 


P 


a 


e 


I 


13 


30 


49 


36 


1 


20 


65 


40 


I 


e 


a 


P 


I 


e 


a 


P 


48 


63 


28 


9 


44 


67 


22 


5 


a 


P 


I 


e 


a 


P 


I 


e 


27 


12 


45 


64 


21 


8 


41 


68 


e 


I 


P 


a 


e 


I 


P 


a 


62 


47 


10 


25 


60 


43 


6 


23 


P 


a 


e 


I 


P 


a 


e 


I 


11 


26 


61 


46 


7 


24 


59 


42 



Eoget's Solution (i). RogeVs Solution (ii). 

The general problem will be solved if we can combine the 
four circuits into a route which will start from any given cell, 
and terminate on the 64th move on any other given cell of a 
different colour. To effect this Roget gave the two following 
rules. 

First. If the initial cell and the final cell are denoted the 
one by a consonant and the other by a vowel, take alternately 
circuits indicated by consonants and vowels, beginning with 
the circuit of 16 cells indicated by the letter of the initial 
cell and concluding with the circuit indicated by the letter of 
the final cell. 

Second. If the initial cell and the final cell are denoted 
both by consonants or both by vowels, first select a cell, F, in 
bhe same circuit as the final cell, Z, and one move from it, 
next select a cell, X, belonging to one of the opposite circuits 
and one move from F. This is always possible. Then, leaving 



CH. Vl] CHESS-BOARD RECREATIONS 131 

out the cells Z and F, it always will be possible, by the rule 
already given, to travel from the initial cell to the cell X in 
62 moves, and thence to move to the final cell on the 64th 
move. 

In both cases however it must be noticed that the cells in 
each of the first three circuits will have to be taken in such an 
order that the circuit does not terminate on a corner, and it 
may be desirable also that it should not terminate on any of 
the border cells. This will necessitate some caution. As far 
as is consistent with these restrictions it is convenient to make 
these circuits re-entrant, and to take them and every group in 
them in the same direction of rotation. 

As an example, suppose that we are to begin on the cell 
numbered 1 in figure ii above, which is one of those in a 
jo circuit, and to terminate on the cell numbered 64, which is 
one of those in an e circuit. This falls under the first rule : 
hence first we take the 16 cells marked jo, next the 16 cells 
marked a, then the 16 cells marked ?, and lastly the 16 cells 
marked e. One way of effecting this is shown in the diagram. 
Since the cell 64 is a knight's move from the initial cell the 
route is re-entrant. Also each of the four circuits in the 
diagram is symmetrical, re-entrant, and taken in the same 
direction, and the only point where there is any apparent 
breach in the uniformity of the movement is in the passage 
from the cell numbered 32 to that numbered 33. 

A rule for re-entrant routes, similar to that of Roget, has 
been given by various subsequent writers, especially by De 
Polignac* and by Laquieref, who have stated it at much 
greater length. Neither of these authors seems to have been 
aware of Roget's theorems. De Polignac, like Roget, illustrates 
the rule by assigning letters to the various squares in the way 
explained above, and asserts that a similar rule is applicable to 
all even squares. 

• Comptea Rendm, April, 1861 ; and Bulletin de la SoeiiU MathSmatique de 
France, 1881, vol. ix, pp. 17—24. 

t Bulletin de la SocUU Math€matique de France, 1880, vol. vin, pp. 82 — 
102, 132—158. 

9—2 



132 



CUESS-BOARD RECREATIONS 



[CH. VI 



Roget's method can be also applied to two half-boards, as 
indicated in the figure given above on page 126. 

The method which Jaenisch gives as the most fundamental 
is not very different from that of Roget. It leads to eight 
forms, similar to that in the diagram printed below, in which 
the sum of the numbers in every column and every row is 260 ; 
but although symmetrical it is not in my opinion so easy to 
reproduce as that given by Roget. Other solutions, notably 
those by Moon and by Wenzelides, were given in former 
editions of this work The two re-entrant routes printed below, 
each covermg 32 cells, and together covering the board, are 
remarkable as constituting a magic square*. 



63 


22 


15 


40 


1 


42 


59 


18 




15 


20 


17 


IT 


ir 


" 


TT 


34 


14 


39 


64 


21 


60 


17 


2 


43 


18 


37 


14 


21 


60 


35 


12 


63 


37 


62 


23 


16 


41 


4 


19 


58 


25 


16 


19 


44 


5 


62 


33 


56 


24 


13 


38 


61 


20 


57 


44 


3 


38 


45 


26 


59 


22 


55 


4 


11 


11 


36 


25 


52 


29 


46 


5 


56 


27 


24 


39 


6 


43 


10 


57 


54 


26 1 51 


12 


33 


8 


55 


30 


45 


40 


49 


46 


23 


58 


3 


32 


9 


35 


10 


49 


28 


53 


32 


47 


6 


47 


28 


51 


42 


7 


30 


53 


2 


50 


27 


34 


9 


48 


7 


54 


31 


— 


41^ 


48 


29^ 


bd 


1 


8 


Jl 



JaeniseJCs Solution. 



Two Half Board Solutions. 



It is as yet impossible to say how many solutions of the 
problem exist. Legendref mentioned the question, but MindingJ 
was the earliest writer to attempt to answer it. More recent 
investigations have shown that on the one hand the number of 
possible routes is less§ than the number of combinations of 168 
things taken 63 at a time, and on the other hand is greater 
than 31,054144— since this latter number is the number of 
re-entrant paths of a particular type||. 

♦ See A. Billy, Le Probleme du Cavalier des Echecs, Troyes, 1905. 
+ TMorie des Nomhres, Paris, 2nd edition, 1830, vol. ii, p. 165. 
+ Cambridge and Dublin Mathematical Journal, 1852, vol. vn, pp. 147—156 ; 
and Crelle's Journal, 1853, vol. xliv, pp. 73—82. 
§ Jaenisch, vol. n, p. 268. 
11 Bulletin de la Societe Matheinati^ue de France, 1881, vol. ix, pp. 1 — 17. 



CH. Vl] 



CHESS-BOARD RECREATIONS 



133 



Analogous Problems. Similar problems can be constructed 
in which it is required to determine routes by which a piece 
moving according to certain laws {ex. gr. a chess-piece such as 
a king, &c.) can travel from a given cell over a board so as to 
occupy successively all the cells, or certain specified cells, once 
and only once, and terminate its route in a given cell. 
Euler's method can be applied to find routes of this kind : 
for instance, he applied it to find a re-entrant route by which 
a piece that moved two cells forward like a castle and then one 
cell like a bishop would occupy in succession all the black cells 
on the board. 

King's Re- Entrant Path. As one example here is a re- 
entrant tour of a king which moves successively to every cell 



61 


02 


63 


64 


1 


2 


3 


4 


60 


11 


58 


57 


8 


7 


54 


5 


12 


59 


10 


9 


56 


55 


G 


63 


13 


14 


15 


16 


49 


50 


51 


52 


20 


19 


18 


17 


48 


47 


40 


45 


21 


38 


23 


24 


41 


42 


27 


44 


37 


22 


39 


40 


25 


20 


43 


28 


30 


o5 


34 


33 


32 


31 


30 


d 



King^s Magic Tour on a Chess-Board. 

of the board. I give it because the numbers indicating the 
cells successively occupied form a magic square. Of course this 
also gives a solution of a re-entrant route of a queen covering 
the board. 

Rook's Re-Entrant Path. There is no difficulty in con- 
structing re-entrant tours for a rook which moves successively 
to every cell of the board. For instance, if the rook starts from 
the cell 11 it can move successively to the cells 18, 88, 81, 71, 
77, 67, 61, 51, 57, 47, 41, 31, 37, 27, 21, and so back to 11 : 
this is a symmetrical route. Of course this also gives a solution 
of a re-entrant route for a king or a queen covering the board. 



134 CHESS-BOARD RECKEATIONS [CH. VI 

If we start from any of the cells mentioned above, the rook 
takes sixteen moves. If we start from any cell in the middle 
of one of these moves, it will take seventeen moves to cover 
this route, but I believe that in most cases wherever the initial 
cell be chosen sixteen moves will suffice, though in general the 
route will not be symmetrical. On a board of n^ cells it is 
possible to find a route by which a rook can move successively 
from its initial cell to every other cell once and only once. 
Moreover* starting on any cell its path can be made to termi- 
nate, if n be even, on any other cell of a different colour, and, 
if n be odd, on any other cell of the same colour. 

Bishop's Re-Entrant Path. As yet another instance, a bishop 
can traverse all the cells of one colour on the board in seven- 
teen moves if the initial cell is properly chosenf ; for instance, 
starting from the cell 11, it may move successively to the cells 
65, 82, 71, 17, 28, 46, 13, 31, 86, 68, 67, 48, 15, 51, 84, %Q, 88. 
One more move will bring it back to the initial cell. From 
the nature of the case, it must traverse some cells more than 
once. 

Miscellaneous Problems. We may construct numerous such 
problems concerning the determination of routes which cover 
the whole or part of the board subject to certain conditions. 
I append a few others which may tax the ingenuity of those 
not accustomed to such problems. 

Routes on a Chess-Board. One of the simplest is the 
determination of the path taken by a rook, placed in the cell 
11, which moves, one cell at a time, to the cell 88, so that in 
the course of its path it enters every cell once and only once. 
This can be done, though I have seen good mathematicians 
puzzled to effect it. A hasty reader is apt to misunderstand 
the conditions of the problem. 

Another simple problem of this kind is to move a queen from 
the cell 33 to the cell Q6 in fifteen moves entering every cell once 



* L'lnterm^diaire des Mathematicims, Paris, 1901, vol. vm, pp. 163 — 154. 
t H. E. Dudeney, The Tribune, Dec. 3, 1906. 



CH. Vl] CHESS-BOARD RECREATIONS 135 

and only once, and never crossing its own track or entering a 
cell more than once*. 

A somewhat similar, but more difficult, question is the 
determination of the greatest distance which can be travelled 
by a queen starting from its own square in five consecutive 
moves, subject to the condition that it never crosses its own 
track or enters a cell more than oncef. In calculating the 
distance it may be assumed that the paths go through the 
centres of the cells. If the length of the side of a cell is one 
inch, the distance exceeds 3397 inches. 

Another familiar problem can be enunciated as follows. 
Construct a rectangular board of mn cells by ruling m + 1 
vertical lines and n + 1 horizontal lines. It is required to 
know how many routes can be taken from the top left-hand 
corner to the bottom right-hand corner, the motion being along 
the ruled lines and its direction being always either vertically 
downwards or horizontally from left to right. The answer is 
the number of permutations of m + ti things, of which m are 
alike of one kind and n are alike of another kind: this is equal to 
{m + n)\ /mini. Thus on a square board containing 16 cells 
(i.e. one-quarter of a chess-board), where m = n = 4, there are 70 
such routes ; while on a common chess-board, where m = w = 8, 
there are no less than 12870 such routes. A rook, moving ac- 
cording to the same law, can travel from the top left-hand cell 
to the bottom right-hand cell in (m + n — 2)\ / (m — l)\(n — 1)1 
ways. Similar theorems can be enunciated for a parallele- 
piped. 

Another question of this kind is the determination of the 
number of closed routes through mn points arranged in m rows 
and n columns, following the lines of the quadrilateral net- work, 
and passing once and only once through each point J. 

Guarims Problem. One of the oldest European problems 
connected with the chess-board is the following which was 

• H. E. Dudeney, The Tribune, Oct. 3, 1906. 
t Ibid., Oct. 2, 1906. 

X See C. F. Sainte-Marie in U Intermidiaire dcs MatMmatidens, Paris, 
vol. XI, March, 1904, pp. 86—88. 



136 CHESS-BOARD RECREATIONS [CH. VI 

propounded in 1512. It was quoted by Lucas in 1894, but I 
believe has not been published otherwise than in his works and 
the earlier editions of this book. On a board of nine cells, such 
as that drawn below, the two white knights are placed on the 



a G d 
D B 

b A c 



two top comer cells (a, d), and the two black knights on the 
two bottom corner cells (6, c) : the other cells are left vacant. It 
is required to move the knights so that the white knights shall 
occupy the cells h and c, while the black shall occupy the cells 
a and d. The solution is obvious. 

Queens Problem. Another problem consists in placing 
sixteen queens on a board so that no three are in a straight 
line *. One solution is to place them on the cells 15, 16, 25, 26, 
31, 32, 41, 42, 57, 58, 67, 68, 73, 74, 83, 84. It is of course 
assumed that each queen is placed on the middle of its cell. 

Latin Squares. Another problem of the chess-board type 
is the determination of the number x^ of Latin Squares of 
any assigned order n : a Latin Square of the nih. order being 
defined as a square of ri^ cells (in n rows and n columns) in 
which n^ letters consisting of ri "as" n "b's"..., are arranged in 
the cells so that the n letters in each row and each column are 
different. The general theory is difficult f, but it may amuse 
my readers to verify the following results for some of the lower 
values of n : iCa = 2, Xs = 6, a?4 = 576, x^ = 149760. Clearly ccn 
is a multiple of n ! (n—l)\ 

♦ H. E. Dudeney, The Tribune, November 7, 1906. 

t See P. A. MacMahon, Combinatory Analysis^ Cambridge, 1915-16, vol. i, 
pp. 246—263 ; vol. ii, pp. 323—326. 



137 



CHAPTER VII. 



MAGIC SQUARES. 



A Mogic Square consists of a number of integers arranged 
in the form of a square, so that the sum of the numbers in 
every row, in every column, and in each diagonal is the same. 
If the integers are the consecutive numbers from 1 to n' the 
square is said to be of the nth order, and it is easily seen 
that in this case the sum of the numbers in aijy row, column, 
or diagonal is equal to ^r? (n^ + 1) : this number may be 
denoted by N, Unless otherwise stated, I confine my account 
to such magic squares, that is, to squares formed with consecu- 
tive integers from 1 upwards. The same rules however cover 
similar problems with n^ numbers in arithmetical progression. 

Thus the first 16 integers, arranged in either of the forms 
given in figures i and ii below, represent magic squares of the 



16 


3 


2 


13 




15 


10 


3 


6 


5 


10 


11 


8 


4 


5 


16 


9 


9 


6 


7 


12 


14 


11 


2 


7 


4 


15 


14 


1 


1 


8 


13 


12 



Figure i. 



Figure ii. 



fourth order, the sum of the numbers in any row, column, or 
diagonal being 34. Similarly figure iii on page 140, figure vii 
on page 143, figure xii on page 153, and figure xvi on page 
157 represent magic squares of the fifth order; figure xi on 
page 150 represents a magic square of the sixth order; figure 
xviii on page 160 represents a magic square of the seventh 



138 MAGIC SQUARES [CH. VII 

order, and figures xxii and xxiii on pages 163, 164 represent 
magic squares of the eighth order. 

The formation of these squares is an old amusement, and 
in times when mystical philosophical ideas were associated 
with particular numbers it was natural that such arrangements 
should be deemed to possess magical properties. Magic squares 
of an odd order were constructed in India before the Christian 
era according to a law of formation which is explained here- 
after. Their introduction into Europe appears to have been 
due to Moschopulus, who lived at Constantinople in the early 
part of the fifteenth century, and enunciated two methods for 
making such squares. The majority of the medieval astrologers 
and physicians were much impressed by such arrangements. 
In particular the famous Cornelius Agrippa (1486 — 1535) con- 
structed magic squares of the orders 3, 4, 5, 6, 7, 8, 9, which were 
associated respectively with the seven astrological "planets": 
namely, Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and 
the Moon. He taught that a square of one cell, in which unity 
was inserted, represented the unity and eternity of God ; while 
the fact that a square of the second order could not be con- 
structed illustrated the imperfection of the four elements, air, 
earth, fire, and water; and later writers added that it was 
symbolic of original sin. A magic square engraved on a silver 
plate was sometimes prescribed as a charm against the plague, 
and one, namely, that represented in figure i on the last page, 
is drawn in the picture of Melancholy, painted in 1514 by 
Albert Durer : the numbers in the middle cells of the bottom 
row give the date of the work. Such charms are still worn in 
the East. 

The development of the theory was at first due mainly to 
French mathematicians. Bachet gave a rule for the construc- 
tion of any square of an odd order in a form substantially 
equivalent to one of the rules given by Moschopulus. The 
formation of magic squares, especially of even squares, was con- 
sidered by Frenicle and Format. The theory was continued by 
Poignard, De la Hire, Sauveur, D'Ons-en-bray, and Des Ourmes. 
Ozanam included in his work an essay on magic squares which 



CH. VIl] MAGIC SQUARES 139 

was amplified by Montucla. Like most algebraical problems, 
the construction of magic squares attracted the attention of 
Euler, but he did not advance the general theory. In 1837 an 
elaborate work on the subject was compiled by B. Violle, which 
is useful as containing numerous illustrations. I give the 
references in a footnote*. 

I shall confine myself to establishing rules for the con- 
struction of squares subject to no conditions beyond those given 
in the definition. I shall commence by giving rules for the 
construction of a square of an odd order, and then shall proceed 
to similar rules for one of an even order. 

It will be convenient to use the following terms. The 
spaces or small squares occupied by the numbers are called 
cells. The diagonal from the top left-hand cell to the bottom 
right-hand cell is called the leading diagonal or left diagonal. 
The diagonal from the top right-hand cell to the bottom left- 
hand cell is called the right diagonal 

Magic squares of an odd order. I proceed to give three 
methods for constructing odd magic squares, but for simplicity 
I shall apply them to the formation of squares of the fifth 
order ; though exactly similar proofs will apply equally to any 
odd square. 

• For a sketch of the history of the subject and its bibliography see 
S. Giinther's Geschichte der mathematischen Wissenschaften, Leipzig, 1876, 
chapter iv ; and W. Ahrens, Mathematische Unterhaltungen und Spiele, Leipzig, 
1901, chapter xii. The references in the text are to Bachet, Prohllmes plaisans, 
Lyons, 1624, problem xxi, p. 161 : Fr6nicle, Divers Ouvrages de Matheviatique 
par Messieurs de VAcadimie des Sciences^ Paris, 1693, pp. 423 — 483 ; with an 
appendix (pp. 484—507), containing diagrams of all the possible magic squares 
of the fourth order, 880 in number : Fermat, Opera Mathematica, Toulouse, 1679, 
pp. 173—178 ; or Brassinne's Precis, Paris, 1853, pp. 146 — 149 : Poignard, 
Traiti des Quarris Sublimes, Brussels, 1704 : De la Hire, M6moires de VAcadSmie 
des Sciences for 1705, Paris, 1706, part i, pp. 127—171 ; part ii, pp. 364—382 : 
Sauveur, Construction des Quarris Magiques, Paris, 1710 : D '0ns- en-bray, 
MSmoires de VAcadimie des Sciences for 1750, Paris, 1754, pp. 241—271 : Des 
Ourmes, Mimoires de Mathimatiqne et de Physique (French Academy), Paris, 
1763, vol. IV, pp. 196 — 241 : Ozanam and Montucla, RScriations, part i, 
chapter xii : Euler, Commentationes Arithmeticac Collectae, St Petersburg, 1849, 
vol. II, pp. 693 — 602 : Violle, Traiti Complet det Carris Magiques, 3 vols, Paris, 
1837-8. 



140 



MAGIC SQUARES 



[CH. VII 



De la Louhhre*s Method*. If the reader will look at figure iii 
he will see one way in which such a square containing 25 cells 
can be constructed. The middle cell in the top row is occupied 
by 1. The successive numbers are placed in their natural 
order in a diagonal line which slopes upwards to the right, 
except that (i) when the top row is reached the next number 
is written in the bottom row as if it came immediately above 
the top row ; (ii) when the right-hand column is reached, the 
next number is written in the left-hand column, as if it 
immediately succeeded the right-hand column ; and (iii) when 
a cell which has been filled up already, or when the top 
right-hand square is reached, the path of the series drops to 
the row vertically below it and then continues to mount again. 
Probably a glance at the diagram in figure iii will make 
this clear. 



T7 


24 


1 


8 


16 




15 + 2 


20 + 4 


+ 1 


5 + 3 


10+6 


23 


5 


7 


14 


16 


20 + 3 


+ 5 


5f2 


10 + 4 


15 + 1 


4 


6 


13 


20 


22 


+ 4 


5 + 1 


10+3 


15 + 5 


20+2 


10 


12 


19 


21 


3 


5+5 


10 + 2 


15 + 4 


20 + 1 


+ 3 


11 


18 


25 


2 


9 


10 + 1 


15 + 3 


20 + 5 


+ 2 


5+4 



Figure iii. 



Figure iv. 



The reason why such a square is magic can be explained 
best by expressing the numbers in the scale of notation whose 
radix is 5 (or n, if the magic square is of the order n), except 
that 5 is allowed to appear as a unit-digit and is not allowed 
to appear as a unit- digit. The result is shown in figure iv. 
From that figure it will be seen that the method of construc- 
tion ensures that every row and every column shall contain 
one and only one of each of the unit-digits 1, 2, 3, 4, 5, the sum 
of which is 15 ; and also one and only one of each of the radix- 
digits 0, 5, 10, 15, 20, the sum of which is 50. Hence, as 

* De la LoubSre, Du Royaume de Siam (Eng. Trans.), London, 1693, vol. n, 
pp. 227—247. De la Loub^re was the envoy of Louis XIV to Siam in 1687-8, 
and there learnt this method. 



CH. VII] MAGIC SQUARES 141 

far as rows and columns are concerned, the square is magic. 
Moreover if the square is odd, each of the diagonals will 
contain one and only one of each of the unit-digits 1, 2, 3, 4, 5. 
Also the leading diagonal will contain one and only one of the 
radix-digits 0, 5, 10, 15, 20, the sum of which is 50 ; and 
if, as is the case in the square drawn above, the number 10 
is the radix-digit to be added to the unit-digits in the right 
diagonal, then the sum of the radix-digits in that diagonal 
is also 50. Hence the two diagonals also possess the magical 
property. 

And generally if a magic square of an odd order n is 
constructed by De la Loubere's method, every row and every 
column must contain one and only one of each of the unit- 
digits 1, 2, 3, ..., w; and also one and only one of each of the 
radix-digits 0, n, 2n, ,..^n{n — 1). Hence, as far as rows and 
columns are concerned, the square is magic. Moreover each 
diagonal will either contain one and only one of the unit-digits 
or will contain n unit-digits each equal to i (n + 1). It will 
also either contain one and only one of the radix- digits or will 
contain n radix-digits each equal to \n(n—V). Hence the 
two diagonals will also possess the magical property. Thus the 
square will be magic. 

I may notice here that, if we place 1 in any cell and fill 
up the square by De la Loubere's rule, we shall obtain a 
square that is magic in rows and in columns, but it will not 
in general be magic in its diagonals. 

It is evident that other squares can be derived from De la 
Loubere's square by permuting the symbols properly. For 
instance, in figure iv, we may permute the symbols 1, 2, 3, 4, 5 
in 5 ! ways, and we may permute the symbols 0, 5, 15, 20 in 
4 ! ways. Any one of these 5 ! arrangements combined with 
any one of these 4 ! arrangements will give a magic square. 
Hence we can obtain 2880 magic squares of the fifth order of 
this kind, though only 720 of them are really distinct. Other 
squares can however be deduced, for it may be noted that 
from any magic square, whether even or odd, other magic 
squares of the same order can be formed by the mere inter- 



142 MAGIC SQUARES [CH. VII 

change of the row and the column which intersect in a cell on 
a diagonal with the row and the column which intersect in the 
complementary cell of the same diagonal. 

Bachet proposed* a similar rule. In this, we begin by 
placing 1 in the cell above the middle one, and then we write 
the successive numbers in a diagonal line sloping upwards to 
the right, subject to the condition that when the cases (i) and 
(ii) mentioned under De la Loubere's method occur the rules 
there given are followed, but when the case (iii) occurs the path 
of the series, instead of going on to the cell already occupied, 
is continued from one cell to the cell next but one vertically 
above it. If this cell is above the top row the path continues 
from the corresponding cell in one of the bottom two rows 
following the analogy of rule (i) in De la Loubere's method. 
Bachet's method leads ultimately to this arrangement ; except 
that the rules are altered so as to make the line slope down- 
wards. This method also gives 720 magic squares of the fifth 
order. 

In the notation given later (see pp. 157, 158), De la Loubere's 
rule is equivalent to taking steps a = — 1, 6 = 1, and cross-steps 
x—l,y = 0. Bachet's form of it, as here enunciated, is equiva- 
lent to taking steps a = — 1, 6 = 1, and cross-steps a? = — 2, 
y = 0. 

De la Hire's Methodf. I shall now give another rule for 
the formation of odd magic squares. To form an odd magic 
square of the order n by this method, we begin by constructing 
two subsidiary squares, one of the unit-digits, 1, 2, ..., w, and 
the other of multiples of the radix, namely, 0, n, 2n, ..., (n — 1) n. 
We then form the magic square by adding together the numbers 
in the corresponding cells in the two subsidiary squares. 

De la Hire gave several ways of constructing such sub- 
sidiary squares. I select the following method (props, x and 
xiv of his memoir) as being the simplest, but I shall apply it 
to form a square of only the fifth order. It leads to the same 
results as the second of the two rules given by Moschopulus. 

♦ Bachet, Problem xxi, p. 161. 

t Mimoireg de V Academic des Sciencei for 1705, part i, pp. 127—171. 



CH. VIl] 



MAGIC SQUARES 



143 



The first of the subsidiary squares (figure v, below) is 
constructed thus. First, 3 is put in the top left-hand corner, 
and then the numbers 1, 2, 4, 5 are written in the other cells 
of the top line (in any order). Next, the number in each cell 
of the top line is repeated in all the cells which lie in a diagonal 
line sloping downwards to the right (see figure v) according to 
the rule (ii) in De la Loubere's method. The cells filled by the 
same number form a broken diagonal. It follows that every row 
and every column contains one and only one 1, one and only 
one 2, and so on. Hence the sum of the numbers in every row 
and in every column is equal to 15; also, since we placed 3, 
which is the average of these numbers, in the top left-hand 
corner, the sum of the numbers in the left diagonal is 15 ; 





































3 


4 


rr 


5 


2 




15 





20 


6 


10 




18 


^ 


21 


10 


12 


2 


3 


4 


1 


5 





20 


5 


10 


15 


2 


23 


9 


11 


20 


5 


2 


3 


4 


1 


20 


5 


10 


15 





25 


7 


13 


19 


1 


1 


5 


2 


3 


4 


5 


10 


15 





20 


6 


15 


17 


3 


24 


4 


1 


6 


2 


3 


10 


15 





20 


5 


14 


16 


5 


22 


8 



First Subsidiary Square. Second Subsidiary Square. Resulting Magic Square. 
Figure v. Figure vi. Figure vii. 

and, since the right diagonal contains one and only one of 
each of the numbers 1, 2, 3, 4, and 5, the sum of the numbers 
in that diagonal also is 15. 

The second of the subsidiary squares (figure vi) is con- 
structed in a similar way with the numbers 0, 5, 10, 15, and 
20, except that the mean number 10 is placed in the top right- 
hand corner; and the broken diagonals formed of the same 
numbers all slope downwards to the left. It follows that every 
row and every column in figure vi contains one and only one 
0, one and only one 5, and so on ; hence the sum of the 
numbers in every row and every column is equal to 50. Also 
the sum of the numbers in each diagonal is equal to 50. 

If now we add together the numbers in the corresponding 
cells of these two squares, we shall obtain 25 numbers such 



144 MAGIC SQUARES [CH. VIl 

that the sum of the numbers in every row, every column, 
and each diagonal is equal to 16 + 60, that is, to 65. This is 
represented in figure vii. Moreover, no two cells in that figure 
contain the same number. For instance, the numbers 21 to 25 
can occur only in those five cells which in figure vi are occu- 
pied by the number 20, but the corresponding cells in figure. v 
contain respectively the numbers 1, 2, 3, 4, and 5; and thus 
in figure vii each of the numbers from 21 to 25 occurs once 
and only once. De la Hire preferred to have the cells in the 
subsidiary squares which are filled by the same number con- 
nected by a knight's move and not by a bishop's move ; and 
usually his rule is enunciated in that form. 

By permuting the numbers 1, 2, 4, 5 in figure v we get 
4 ! arrangements, each of which combined with that in figure vi 
would give a magic square. Similarly by permuting the numbers 
0, 5, 15, 20 in figure vi we obtain 4! squares, each of which might 
be combined with any of the 4! arrangements deduced from 
figure V. Hence altogether we can obtain in this way 144 magic 
squares of the fifth order. 

There are various other methods by which odd magic 
squares of any order can be constructed, but most or all of them 
depend on the form of n. I content myself here with the two 
methods described above; later, when discussing pandiagonal 
squares, I shall mention another rule for odd magic squares 
whose order is higher than three, which permits us to place 
a selected number in any cell we like. 

Magic squares of an even order. The above methods 
are inapplicable to squares of an even order. I proceed to give 
two methods for constructing any even magic square of an order 
higher than two. 

It will be convenient to use the following terms. Two 
rows which are equidistant, the one fi:om the top, the other 
from the bottom, are said to be complementary. Two columns 
which are equidistant, the one from the left-hand side, the 
other from the right-hand side, are said to be complementary. 
Two cells in the same row, but in complementary columns, are 
said to be horizontally related. Two cells in the same column, 



CH. VIl] MAGIC SQUARES 145 

but in complementary rows, are said to be vertically related,. 
Two cells in complementary rows and columns are said to be 
skewly related ; thus, if the cell h is horizontally related to the 
cell a, and the cell d is vertically related to the cell a, then the 
cells h and d are skewly related ; in such a case if the cell c 
is vertically related to the cell 6, it will be horizontally related 
to the cell d, and the cells a and c are skewly related: the 
cells a, h, c, d constitute an associated group, and if the square 
is divided into four equal quarters, one cell of an associated 
group is in each quarter. 

A horizo.ttal interchange consists in the interchange of the 
numbers in two horizontally related cells. A vertical inter- 
change consists in the interchange of the numbers in two 
vertically related cells. A shew interchange consists in the 
interchange of the numbers in two skewly related cells. A 
cross interchange consists in the change of the numbers in any 
cell and in its horizontally related cell with the numbers in 
the cells skewly related to them ; hence, it is equivalent to two 
vertical interchanges and two horizontal interchanges. 

First Method''^. This method is the simplest with which I 
am acquainted. Begin by filling the cells of the square with 
the numbers 1, 2, ...,n^ui their natural order commencing (say) 
with the top left-hand corner, writing the numbers in each row 
from left to right, and taking the rows in succession from the 
top. I will commence by proving that a certain number of 
horizontal and vertical interchanges in such a square must make 
it magic, and will then give a rule by which the cells whose 
numbers are to be interchanged can be at once picked out. 

First we may notice that the sum of the numbers in each 
diagonal is equal to N, where N = n{n^+l)l2\ hence the 
diagonals are already magic, and will remain so if the numbers 
therein are not altered. 

Next, consider the rows. The sum of the numbers in the 

• It seems to have been first enunciated in 1889 by W. Firth, but later was 
independently discovered by various writers : see the Messenger of Mathematics, 
Cambridge, September, 1893, vol. xxin, pp. 65 — 69, and the Monist, Chicago, 
1912, vol. xxn, pp. 53 — 81. I leave my account as originally written, though 
perhaps the procedure used by C. Planck in the latter paper is somewhat simpler. 

B, IL 10 



146 MAGIC SQUARES [CH. VII 

a;th row from the top is N - n^ (n — 2.'P + l)/2. The sum of the 
numbers in the complementary row, that is, the arth row from 
the bottom, is N' + n^(n—2x+ l)/2. Also the number in any 
cell in the a;th row is less than the number in the cell vertically 
related to it hy n (n — 2x -^ 1). Hence, if in these two rows we 
make ri/2 interchanges of the numbers which are situated in 
vertically related cells, then we increase the sum of the numbers 
in the «;th row hy nxn(n — 2x-\- 1)/2, and therefore make that 
row magic ; while we decrease the sum of the numbers in the 
complementary row by the same number, and therefore make 
that row magic. Hence, if in every pair of complementary 
rows we make n/2 interchanges of the numbers situated in 
vertically related cells, the square will be made magic in rows. 
But, in order that the diagonals may remain magic, either we 
must leave both the diagonal numbers in any row unaltered, or 
we must change both of them with those in the cells vertically 
related to them. 

The square is now magic in diagonals and in rows, and it 
remains to make it magic in columns. Taking the original 
arrangement of the numbers (in their natural order) we might 
have made the square magic in columns in a similar way to 
that in which we made it magic in rows. The sum of the 
numbers originally in the yth. column from the left-hand side is 
N — n (n — 27/ -\- 1)/2. The sum of the numbers originally in the 
complementary column, that is, the yth. column from the right- 
hand side, 13 N + n(n — 2y -{- 1)/2. Also the number originally 
in any cell in the yth. column was less than the number in the 
cell horizontally related to ithy n — 2y + 1. Hence, if in these 
two columns we had made n/2 interchanges of the numbers 
situated in horizontally related cells, we should have made the 
sum of the numbers in each column equal to N. If we had done 
this in succession for every pair of complementary columns, we 
should have made the square magic in columns. But, as before, 
in order that the diagonals might remain magic, either we must 
have left both the diagonal numbers in any column unaltered, 
or we must have changed both of them with those in the cells 
horizontally related to them. 



CH. VIl] 



MAGIC SQUARES 



147 



It remains to show that the vei'tical and horizontal inter- 
changes, which have been considered in the last two paragraphs, 
can be made independently, that is, that we can make these 
interchanges of the numbers in complementary columns in such 
a manner as will not affect the numbers already interchanged in 
complementary rows. This will require that in every column 
there shall be exactly n/2 interchanges of the numbers in 
vertically related cells, and that in every row there shall be 
exactly n/2 interchanges of the numbers in horizontally related 
cells. I proceed to show how we can always ensure this, if n is 
greater than 2. I continue to suppose that the cells are initially 
filled with the numbers 1, 2, ..., n^ in their natural order, and 
that we work from that arrangement. 

A doubly-even square is one where n is of the form 4m. If 
the square is divided into four equal quarters, the first quarter 
will contain 2?n columns and 2ni rows. In each of these columns 
take m cells so arranged that there are also m cells in each row, 
and change the numbers in these 2m* cells and the 6m^ cells 
associated with them by a cross interchange. The result is 
equivalent to 2m interchanges in every row and in every 
column, and therefore renders the square magic. 

One way of selecting the 27n^ cells in the first quarter is to 
divide the whole square into sixteen subsidiary squares each 



rt 


b 


b 


a 


b 


a 


a 


b 


h 


a 


a 


b 


a 


b 


b 


a 



Figure viii. 

containing m* cells, which we may represent by the diagram 
above, and then we may take either the cells in the a squares 
or those in the h squares ; thus, if every number in the eight 
a squares is interchanged with the number skewly related to 
it the resulting square is magic. A magic square of the eighth 
order, constnicted in this way, is shown in figure xxiii on page 
164. 

10—2 



148 MAGIC SQUARES [CH. VII 

Another way of selecting the 2m* cells in the first quarter 
would be to take the first m cells in the first column, the cells 
2 to m+1 in the second column, and so on, the cells m+1 to 2m 
in the (m + l)th column, the cells m + 2 to 2m and the first cell 
in the (m + 2)th column, and so on, and finally the 2mth cell 
and the cells 1 to m — 1 in the 2mth column. 

A singly -even square is one where n is of the form 2 (2m + 1). 
If the square is divided into four equal quarters, the first quarter 
will contain 2m + 1 columns and 2m + 1 rows. In each of these 
columns take m cells so arranged that there are also m cells in 
each row: as, for instance, by taking the first m cells in the 
first column, the cells 2 to m + 1 in the second column, and so 
on, the cells m + 2 to 2m 4- 1 in the (m + 2)th column, the cells 
m + 3 to 2m + 1 and the first cell in the (m + 3)th column, and 
so on, and finally the (2m + l)th cell and the cells 1 to m — 1 in 
the (2m+l)th column. Next change the numbers in these 
m (2m + 1) cells and the 3m {2m + 1) cells associated with them 
by cross interchanges. The result is equivalent to 2m inter- 
changes in every row and in every column. In order to make 
the square magic we must have 7i/2, that is, 2m + 1, such inter- 
changes in every row and in every column, that is, we must 
have one more interchange in every row and in every column. 
This presents no difficulty; for instance, in the arrangement 
indicated above the numbers in the (2m -f- l)th cell of the first 
column, in the first cell of the second column, in the second cell 
of the third column, and so on, to the 2mth cell in the (2m + l)th 
column may be interchanged with the numbers in their vertically 
related cells ; this will make all the rows magic. Next, the 
numbers in the 2mth cell of the first column, in the (2m + l)th 
cell of the second column, in the first cell of the third column, in 
the second cell of the fourth column, and so on, to the (2m--l)th 
cell of the (2m -\- l)th column may be interchanged with those 
in the cells horizontally related to them; and this will make 
the columns magic without afi'ecting the magical properties of 
the rows. 

It will be observed that we have implicitly assumed that m 
is not zero, that is, that n is greater than 2; also it would seem 



CH. VIl] MAGIC SQUARES 149 

that, if m = 1 and therefore n=6, then the numbers in the 
diagonal cells must be included in those to which the cross 
interchange is applied, but, if n is greater than 6, this is not 
necessary, though it may be convenient. 

The construction of odd magic squares and of doubly-even 
magic squares is very easy. But though the rule given above 
for singly-even squares is not difficult, it is tedious of applica- 
tion. It is unfortunate that no more obvious rule — such, for 
instance, as one for bordering a doubly-even square — can be 
suggested for writing down instantly and without thought 
singly-even magic squares. 

De la Hire's Method*. I now proceed to give another way, 
due to De la Hire, of constructing any even magic square of an 
order higher than two. 

In the same manner as in his rule for making odd magic 
squares, we begin by constructing two subsidiary squares, one 
of the unit-digits 1, 2, 3, ...,n, and the other of the radix- 
digits 0, 71, 2n, ..., (n — l)n. We then form the magic square 
by adding together the numbers in the corresponding cells in 
the two subsidiary squares. Following the analogy of the 
notation used above, two numbers which are equidistant from 
the ends of the series 1, 2, 3, ..., n are said to be comple- 
mentary. Similarly numbers which are equidistant from the 
ends of the series 0, n, 2n, ,.., (n — l)n are said to be comple- 
mentary. 

For simplicity I shall apply this method to construct a 
magic square of only the sixth order, though an exactly similar 
method will apply to any even square of an order higher than 
the second. 

The first of the subsidiary squares (figure ix) is constructed 

as follows. First, the cells in the leading diagonal are filled 

with the numbers 1, 2, 3, 4, 5, 6 placed in any order whatever 

that puts complementary numbers in complementary positions 

* The rule is due to De la Hire (part 2 of his memoir) and is given by 
Montucla in his edition of Ozanam's work : I have used the modified enunci< 
ation of it inserted in Labosne's edition of Bachet's Problkmes, as it saves the 
introduction of a third subsidiary square. I do not know to whom the modifi- 
cation is due. 



150 



MAGIC SQUARES 



[CH. VII 



{ex. gr. in the order 2, 6, 3, 4, 1, 5, or in their natural order 
1, 2, 3, 4, 5, 6). Second, the cells vertically related to these are 
filled respectively with the same numbers. Third, each of the 
remaining cells in the first vertical column is filled either with 
the same number as that already in two of them or with the 
complementary number {ex. gr, in figure ix with a "1" or a "6") 
in any way, provided that there are an equal number of each 
of these numbers in the column, and subject also to proviso 
(ii) mentioned in the next paragraph. Fourth, the cells hori- 
zontally related to those in the first column are filled with 
the complementary numbers. Fifth, the remaining cells in 
the second and third columns are filled in an analogous way 



1 


5 


4 


3 


2 


6 







30 


30 





30 







1 


35 


34 


3 


32 


6 


6 


2 


4 


3 


5 


1 


24 


6 


24 


24 


G 


6 


30 


8 


28 


27 


11 


7 


6 


5 


3 


4 


2 


1 


18 


18 


12 


12 


12 


18 


24 


23 


15 


IG 


14 


19 


1 


5 


3 


f 


2 


6 


12 


12 


18 


18 


18 


12 


13 


17 


21 


22 


20 


18 


6 


2 


3 


4 


5 


1 


G 


24 


G 


G 


24 


24 


12 


26 


9 


10 


29 


25 


1 


2 


4 


3 


5 


6 


30 








30 





30 


31 


2 


4 


33 


5 


36 



First Subsidiary Square, 
Figure ix. 



Second Subsidiary Square. 
Figure x. 



Resulting Magic Square, 
Figure xi. 



to that in which those in the first column were filled : and then 
the cells horizontally related to them are filled with the comple- 
mentary numbers. The square so formed is necessarily magic 
in rows, columns, and diagonals. 

The second of the subsidiary squares (figure x) is con- 
structed as follows. First, the cells in the leading diagonal 
are filled with the numbers 0, 6, 12, 18, 24, 30 placed in any 
order whatever that puts complementary numbers in comple- 
mentary positions. Second, the cells horizontally related to 
them are filled respectively with the same numbers. Third, 
each of the remaining cells in the first horizontal row is filled 
either with the same number as that already in two of them 
or with the complementary number {ex. gr. in figure x with a 
"0" or a "30") in any way, provided (i) that there are an equal 



CH. VIl] MAGIC SQUARES 151 

number of each of these numbers in the row, and (ii) that if 
any cell in the first row of figure ix and its vertically related 
cell are filled with complementary numbers, then the corre- 
sponding cell in the first row of figure x and its horizontally 
relat*ed cell must be occupied by the same number*. Fourth, 
the cells vertically related to those in the first row are filled 
with the complementary numbers. Fifth, the remaining cells 
in the second and the third rows are filled in an analogous way 
to that in which those in the first row were filled: and then the 
cells vertically related to them are filled with the complementary 
numbers. The square so formed is necessarily magic in rows, 
columns, and diagonals. 

It remains to show that proviso (ii) in the third step de- 
scribed in the last paragraph can be satisfied always. In a 
doubly-even square, that is, one in which n is divisible by 4, we 
need not have any complementary numbers in vertically related 
cells in the first subsidiary square unless we please, but even if 
we like to insert them they will not interfere with the satisfac- 
tion of this proviso. In the case of a singly-even square, that 
is, one in which n is divisible by 2, but not by 4, we cannot 
satisfy the proviso if any horizontal row in the first square has 
all its vertically related squares, other than the two squares in 
the diagonals, filled with complementary numbers. Thus in 
the case of a singly-even square it will be necessary in con- 
structing the first square to take care in the third step that in 
every row at least one cell which is not in a diagonal shall have 
its vertically related cell filled with the same number as itself: 
this is always possible if n is greater than 2. 

The required magic square will be constructed if in each cell 
we place the sum of the numbers in the corresponding cells of 
the subsidiary squares, figures ix and x. The result of this is 
given in figure xi. The square is evidently magic. Also every 
number firom 1 to 36 occurs once and only once, for the numbers 
from 1 to 6 and from 31 to 36 can occur only in the top or the 
bottom rows, and the method of construction ensures that the 

* The insertion of this step evades the necessity of constructing (as Montucla 
did) a third subsidiary square. 



152 MAGIC SQUARES [CH. VII 

same number cannot occur twice. Similarly the numbers from 
7 to 12 and from 25 to 30 occupy two other rows, and no 
number can occur twice; and so on. The square in figure i on 
page 137 may be constructed by the above rules; and the 
reader will have no difficulty in applying them to any other 
even square. 

Other Methods for Constructing any Magic Square. 
The above methods appear to me to be the simplest which have 
been proposed. There are however two other methods^ of less 
generality, to which I will allude briefly in passing. Both 
depend on the principle that, if every number in a magic square 
is multiplied by some constant, and a constant is added to the 
product, the square will remain magic. 

The first method applies only to such squares as can be 
divided into smaller magic squares of some order higher than 
two. It depends on the fact that, if we know how to construct 
magic squares of the mth and nth orders, we can construct one 
of the mnth order. For example, a square of 81 cells may be 
considered as composed of 9 smaller squares each containing 
9 cells, and by filling the cells in each of these small squares in 
the same relative order and taking the small squares themselves 
in the same order, the square can be constructed easily. Such 
squares are called Gomposite Magic Squares. 

The second method, which was introduced by Fr^nicle, con- 
sists in surrounding a magic square with a Border, Thus in 
figure xii on page 153 the inner square is magic, and it is 
surrounded with a border in such a way that the whole square 
is also magic. In this manner from the magic square of the 
3rd order we can build up successively squares of the orders 
5, 7, 9, &c., that is, any odd magic square. Similarly from the 
magic square of the 4th order we can build up successively any 
higher even magic square. 

If we construct a magic square of the first n^ numbers by 
bordering a magic square of (n — 2y numbers, the usual process 
is to reserve for the 4(?i — 1) numbers in the border the first 
2 (w - 1) natural numbers and the last 2(n — l) numbers. Now 
the sum of the numbers in each line of a square of the order 



CH, VIl] 



BIAQIC SQUARES 



153 



(n - 2) is J (^ - 2) [(n - 2f + 1}, and the average is J{(n-2)H 1}. 
Similarly the average number in a square of the nth order is 
J (n" + 1). The difference of these is 2 (n — 1). We begin then 
by taking any magic square of the order (n - 2), and we add to 
every number in it 2(n — 1); this makes the average number 

The numbers reserved for the border occur in pairs, n' and 
1, n' — 1 and 2, n'* — 2 and 3, &c., such that the average of each 
pair is ^ (?i' + 1), and they must be bordered on the square so 
that these numbers are opposite to one another. Thus the 
bordered square will be necessarily magic, provided that the 
sum of the numbers in two adjacent sides of the external 
border is correct. The arrangement of the numbers in the 
borders will be somewhat facilitated if the number w' 4- 1 - jp 
(which has to be placed opposite to the number p) is denoted 
by p, but it is not worth while going into further details here. 



1 


2 


Id 


20 


23 


18 


16 


9 


14 


8 


21 


11 


13 


15 


5 


22 


12 


17 


10 


4 


8 


24 


7 


6 


25 



Bordered Magic Square, 
Figure xii. 

Since a doubly- even square can be formed with ease, this 
method would provide a simple way of constructing a singly- 
even square if definite rules for bordering a square could be 
laid down. I am however unable to formulate precise instruc- 
tions for this purpose, though it is possible to give directions 
which require only slight and easy empirical work. It will 
sufficiently illustrate the general method if I explain how the 
square in figure xii is constructed. A magic square of the third 
order is formed by De la Loubere's rule, and to every number 
in it 8 is added : the result is the inner square in figure xii. 
The numbers not used are 25 and 1, 24 and 2, 23 and 3, 22 
and 4, 21 and 5, 20 and 6, 19 and 7, 18 and 8. The sum of 



154 



MAGIC SQUARES 



[CH. Vll 



each pair is 26, and obviously they must be placed at opposite 
ends of any line. 

With a little patience a magic square of any order can be 
thus built up, border upon border, and of course it will have 
the property that, if each border is successively stripped off, 
the remaining square will still be magic. This is a method of 
construction constantly adopted by self-taught mathematicians. 

I may add here (figure xiii) the following general solution 
of a magic square of the fourth order in which any numbers (not 
necessarily consecutive) are used*. There are eight indepen- 
dent quantities. 



V~v 


Y+v + y 


X+x-y 


Z-x 




32768 


4 


2 


4096 


Z + v-z 


X 


Y 


V-v + z 


16 


512 


1024 


128 


Y-x + z 


V 


Z 


X+x-z 


256 


32 


64 


2048 


X+x 


Z-v-y 


V-x + y 


Y+v 


8 


16384 


8192 


1 



Figure xiii. 



Figure xiv. 



I may also mention that Montucla suggested the construc- 
tion of squares whose cells are occupied by numbers such that 
the product of the numbers in each row, column, and diagonal 
is constant. The formation of such figures is immediately 
deducible from that of magic squares, for if the consecutive 
numbers in a magic square are replaced by consecutive powers 
of any number m the products of the numbers in every line 
will be magic. This is obvious, for if the numbers in any line 
of a square are a, a\ a", &c., such that Sa is constant for every 
line in the square, then Um^ is also constant. For instance, 
figure xiv represents such a magic figure in which the product 
of the numbers in each line is 1,073,741,824; it is constructed 
from the magic square given in figure i on page 137. 

Magic Stars. Some elegant magic constructions on star- 
shaped figures (pentagons, hexagons, &c.) may be noticed in 
passing, though I will not go into details. One instance will 
suffice. Suppose a re-entrant octagon is constructed by the 

* E. Bergholt, Nature, London, vol. lxxxiu, May 26, 1910, pp. 368—369. 



CH. VIl] 



MAQIC SQUARES 



155 



intersecting sides of two equal concentric squares. It is required 
to place the first 16 natural numbers on the comers and points 
of intersection of the sides so that the sum of the numbers on 
the corner of each square and the sum of the numbers on every 




side of each square is equal to 34. Eighteen fundamental solu- 
tions exist. One of these is given above *. 

There are magic circles, rectangles, crosses, diamonds, and 
other figures : also magic cubes, cylinders, and spheres. The 
theory of the construction of such figures is of no value, and 
I cannot spare the space to describe rules for forming them. 

In the above sketch, two questions remain unsolved. 
One is the determination of a definite rule for bordering a 
square; such a rule might lead to a simpler method than 
that given above for forming oddly-even squares. The other 
is the determination of the number of magic squares of the 
fifth (or any higher) order. There is, in effect, only one magic 
square of the third order though by reflexions and rotations 
it can be presented in 9 forms. There are 880 magic squares 
of the fourth order, but by reflexions and rotations these 
can be presented in 7040 forms. De la Hire showed that, 

* Communicated to me by Mr R. Strachey. 



156 MAGIC SQUARES [CH. VII 

apart from mere reflexions and rotations, there were 57600 
magic squares of the fifth order which could be formed by the 
methods he enumerated. Taking account of other methods, it 
would seem that the total number of magic squares of the fifth 
order is very large, perhaps exceeding half a million. Notwith- 
standing these two unsolved problems we may fairly say that 
the theory of the construction of magic squares as defined 
above has been worked out in sufficient detail — though not 
exhaustively, since methods other than those given above may 
be expounded. Accordingly attention has of late been chiefly 
directed to the construction of squares which, in addition to 
being magic, satisfy other conditions. I shall term such 
squares h)rper-magic. 

Hyper-Magic Squares. Of hyper-magic squares, I will 
deal only with the theory of Pan-Diagonal and of Symmetrical 
Squares, though I will describe without going into details what 
are meant by Doubly and Trebly Magic Squares. 

Pandiagonal Squares. One of the earliest additional 
conditions to be suggested was that the square should be magic 
along the broken diagonals as well as along the two ordinary 
diagonals*. Such squares are called Pandiagonal. They are 
also known as Nasik, or perfect, or diabolic squares. 

For instance, a magic pandiagonal square of the fourth 
order is represented in figure ii on page 137. In it the sum 
of the numbers in each row, column, and in the two diagonals 
is 34, as also is the sum of the numbers in the six broken 
diagonals formed by the numbers 15, 9, 2, 8, the numbers 
10, 4, 7, 13, the numbers 3, 6, 14, 12, the numbers 6, 4, 11, 13, 
the numbers 3, 9, 14, 8, and the numbers 10, 16, 7, 1. 

It follows from the definition that if a pandiagonal square 
be cut into two pieces along a line between any two rows or 

* Squares of this type were mentioned by De la Hire, Sauveur, and Euler. 
Attention was again called to them by A. H. Frost in the Quarterly Journal of 
Mathematics, London, 1878, vol. xv, pp. 34 — 49, and subsequently their properties 
have been discussed by several writers. Besides Frost's papers I have made 
considerable use of a paper by E. McClintock in the American Journal of MathC' 
matics, vol. XIX, 1897, pp. 99—120. 



CH. VI l] 



MAGIC SQUARES 



167 



any two columns, and the two pieces be interchanged, the new 
square so formed will be also pandiagonally magic. Hence it is 
obvious that by one vertical and one horizontal transposition 
of this kind any number can be made to occupy any specified 
cell. 

Pandiagonal magic squares of an odd order can be con- 
structed by a rule somewhat analogous to that given by De la 
Loubere, and described above. I proceed to give an outline of 
the method. 

If we write the numbers in the scale of notation whose 
radix is n, with the understanding that the unit-digits run from 
1 to n, it is evident, as in the corresponding explanation of why 
De la Loub^re's rule gives a magic square, that all we have to 
do is to ensure that each row, column, and diagonal (whether 
broken or not) shall contain one and only one of each of the 
unit-digits, as also one and only one of each of the radix-digits. 



7 


20 


3 


11 


24 




5 + 2 


15 + 5 


0+3 


10 fl 


20 + 4 


13 


21 


9 


17 


5 


10 + 3 


20+1 


5 + 4 


15 + 2 


+ 5 


19 


2 


15 


23 


6 


15+4 


042 


10+5 


20 + 3 


5 + 1 


25 


8 


16 


4 


12 


20 + 5 


5 + 3 


15+1 


+ 4 


10 + 2 


-LJ 


14 


22 


10 


18 


+ 1 


10 + 4 


20 f 2 


5.. 5 


15 + 3 



Figure xvL 



Figure xvii. 



This is seen to be the case in the square of the fifth order 
delineated above, figures xvi and xvii. 

Let us suppose that we write the numbers consecutively, 
and proceed fi:om cell to cell by steps, using the term step 
{a, h) to denote going a cells to the right and b cells up. Thus 
a step (a, b) will take us from any cell to the ath column to the 
right of it, to the 6th row above it, to the (b + a)th diagonal 
above it sloping down to the right, and to the (6 — a)th 
diagonal above it sloping down to the left. In all cases we 
have the convention, as in De la Loubk*e's rule, that the move- 
ments along lines are taken cyclically; thus a step w + a is 
equivalent to a step a. Of course, also, if a means going a cells 



158 MAGIC SQUARES [CU. VII 

to the right, then — a will mean going a cells to the left ; thus 
if the 6th upper line is outside the square we take it as 
equivalent to the (n — 6)th lower line. 

It is clear that a and h cannot be zero, and that if a and h 
are prime to n (that is, if each has no divisor other than unity 
which also divides n) we can make n — 1 steps from any cell 
from which we start, before we come to a cell already occupied. 
Thus the first n numbers form a path which will give a different 
unit-digit in every row, column, and in one set of n diagonals ; 
of the other diagonals, n — 1 are empty, and one contains every 
unit-digit — thus they are constructed on magical lines. We 
must take some other step (^, k) from the cell n to get to an 
unoccupied cell in which we place the number n-l-1. Con- 
tinuing the process with n — 1 more steps (a, h) we get another 
series of n numbers in various cells. If h and k are properly 
selected this second series will not interfere with the first series, 
and the rows, columns, and diagonals, as thus built up, will 
continue to be constructed on magical lines provided h and 
k are chosen so that the same unit-digit does not appear 
more than once in any row, column, and diagonal. We will 
suppose that this can be done, and that another cross-step 
(h, k) of the same form as before enables us to continue filling 
in the numbers in compliance with the conditions, and that 
this process can be continued until the square is filled. If 
this is possible, the whole process will consist of n series of 
n steps, each series consisting of n — 1 uniform steps (a, h) 
followed by one cross-step Qi, k). The numbers inscribed after 
the n cross-steps will be n + 1, 2n + l, Sn + 1, ..., and these 
will be themselves connected by uniform steps (u, v), where 
u=^(n — l)a + k = k— a, mod. n, and v=(n — l)b-\-k=k — b. 

I proceed to investigate the conditions that a, 6, h, and k 
must satisfy in order that the square can be constructed as 
above described with uniform steps (a, b) and (h, k). We 
notice at once that in order to secure the magic property in 
the rows and columns, we must have a and b prime to n ; and 
to secure it in the diagonals, we must have a and b unequal 
and b-\- a and 6 — a prime to n. The leading numbers of the 



CH. VII] MAGIC SQUARES 159 

n sequences of n numbers, namely 1, n-\-l, 2n-\-l, ..., are con- 
nected by steps (u, v), where u = h — a and v = k — b. Hence, if 
these are to fit in their places, we must also have u and v 
unequal, and w, v, w + v, and u — v prime to n. Also a, b, u, and 
V cannot be zero. Lastly the cross-steps {h, k) must be so 
chosen that in no case shall a cross-step lead to a cell already 
occupied. This would happen, and therefore the rule would 
fail, if p steps (a, b) from any cell and q steps (u, v) from it, 
where p and q are each less than n, should lead to the same cell. 
Thus, to the modulus n, we cannot have pa = qa = q (h — a), and 
at the same time pb = qv = q(k'' b). 

It is impossible to satisfy these conditions if n is equal to 3 
or to a multiple of 3. For a and b are to be unequal, not zero, 
and less than n, and a -t- 6 is to be less than n and prime to n. 
Thus we cannot construct a pandiagonal square of the third 
order. 

Next I will show that, if n is not a multiple of 3, these 
conditions are satisfied when a = l, 6 = 2, /i=0, /j= — 1, and 
therefore that in this case these values provide a particular 
solution of the general problem. It is at once obvious that 
in this case a and b are unequal, not zero, and prime to n, 
that b-\- a and b — a are prime to n, and that the correspond- 
ing relations for u and v are true. The remaining condition 
for the validity of a rule based on these particular steps is 
that it shall be impossible to find integral values of p and q 
each less than n, which will simultaneously make p = — q, and 
2p = — 2q. This condition is satisfied. Hence, any odd pan- 
diagonal square of an order which is not a multiple of 3 can be 
constructed by this rule. Thus, to form a pandiagonal square 
of the fifth order we may put 1 in any cell; proceed by four 
successive steps, like a knight's move, of one cell to the right 
and two cells up, writing consecutively numbers 2, 3, 4, 5 in 
each cell, until we come to a cell already occupied ; then take 
one step, like a rook's move, one cell down, and so on until the 
square is filled. This is illustrated by the square delineated 
in figure xvi. 

Further discussion of the general case depends on whether 
or not n is prime ; here I will confine myself to the simpler 



160 



MAGIC SQUARES 



[CU. VII 



alternative, and assume that n is prime: this will sufficiently 
illustrate the theory. From the above relations it follows that 
we cannot have pqa (k — b) = pqb (h — a), that is, j^q (ak — bh) ~ 0. 
Therefore ak - bh cannot be a multiple of n, that is, it must be 
prime to n. If this condition is fulfilled, as well as the other 
conditions given above, each cross-step {h, k) can be made in 
due sequence, and the square can be constructed. The result 
that ah — hh is prime to n shows that the cross-step (A, k) must 
be chosen so as to take us to an unoccupied ce'^ not in the 
same row, column, or diagonal (broken or not) as the initial 
number. By noting this fact we can in general place any two 
given numbers in two assigned cells. 

There are some advantages in having the cross-steps uniform 
with the other steps, since, as we shall see later, the square can 
then be written in a form symmetrical about the centre. This 
will be effected if we take h= — b,k = a. If w is prime our 
conditions are then satisfied if b be any number from 2 to 
(n — 1)/2, if a be positive and less than b, and if a^ -f- b^ be prime 
to n. We can, if we prefer, take h = b, k = — a; but it is not 
possible to take h = a and k = ^b, or h = — a and k = b, since 
they make u = or v = 0. 

For instance, if we use a knight's move, we may take a= 1, 
6 = 2. The square of the seventh order given below (figure xviii) 



35 


23 


T? 


13 


1 


45 


40 


4 


48 


36 


31 


26 


21 


9 


22 


17 


12 


7 


44 


39 


34 


47 


42 


30 


25 


20 


8 


3 


16 


11 


6 


43 


38 


33 


28 


41 


29 


24 


19 


14 


2 


46 


10 


5 


49 


37 


32 


27 


15 



A Pandiagonal Symmetrical Square, 
Figure xviii. 

is constructed by this rule. But in the case of a square of the 
fifth order we cannot use a knight's move, since, if a = 1, and 
6 = 2, we have a^ + ¥ = 5. Hence the use of a knight's move 
is not applicable when n is 5, or a multiple of 5. 



CH. Vll] 



MAQIC SQUARES 



161 



The construction of singly-even pandiagonal squares (that is, 
those whose order is ^mi + 2) is impossible, but that of doubly- 
even squares (that is, those whose order is 4m) is possible. 

Here is one way of constructing a doubly-even square. 
Suppose the order of the square is 4r>i, and as before let us 
write the number in a cell in the scale 4m, that is, as 4mjp -f- r, 
so that 'p and r are the radix and unit-digits, with the conven- 
tion that r cannot be zero. Place Pi,p2,pB, ..., p^m in order 
in the cells in the bottom row. Proceeding from pi by steps 
(2m, 1) fill up 2m cells with it. And proceed similarly with 
the other radix-digits. Next place ri,r2, ..., r^m in order in 
the cells in the first column. Proceeding from r^ by steps 
(1, 2m) fill up 2m cells with it. And proceed similarly with 
the other unit-digits. Then if we take for rj, r^, ..., r^, the 
values 1, 2, ..., 2m, 4m, 4m — 1, ..., 2m-Hl, and for j)i,2>a> .••,P4m, 
the values 0, 1, ..., 2m — 1, 4m — 1, ..., 2m, the square will be 
pandiagonally magic. I leave the demonstration to my readers. 
The resulting square in the case when m = l, n = 4, and the p 
and r subsidiary squares are shown below. This is the square 
represented in figure ii on page 137. 



3 


2 


7 


T 




3 


7 


T 


2 




12 + 3 


8 + 2 


+ 3 


4 + 2 





1 


3 


2 


4 


1 


4 


1 


+ 4 


4 + 1 


12 + 4 


8 + 1 


3 


2 





1 


2 


3 


2 


3 


12 + 2 


8 + 3 


+ 2 


4+3 





1 


3 


2 


1 


4 


1 


4 


+ 1 


4 + 4 


12 + 1 


8 + 4 



Subiidiary p Square. Subsidiary r Square. 
Figure xix. Figure xx. 



Resulting Square, 
Figure xxi. 



The rows, columns, and all diagonals of pandiagonal squares 
possess the magic property. So also do a group of any n numbers 
connected cyclically by steps (c, d) provided the first two 
numbers of the group are such that when divided by n they have 
either different quotients or different remainders. Such groups 
include rows, columns, and diagonals as particular cases. Thus 
in the square delineated in figure ii on page 137 the numbers 
1, 7, 10, 16 form a magic group whose sum is 34, connected 
B. E. U 



162 MAGIC SQUARES [CH. VII 

cyclically by steps (1, 3). Again in the square delineated in 
figure xviii on page 160, 10, 30, 1, 28, 48, 19, 39 form a magic 
group connected cyclically by steps (2, 3). 

Symmetrical Squares. It has been suggested that we 
might impose on the construction of a magic square of the 
order n the condition that the sum of any two numbers in 
cells geometrically symmetrical to the centre {ex. gr. 22 and 
28 in figure xviii) shall be constant and equal to w* + 1. Such 
squares are called Symmetrical. 

The construction of odd symmetrical squares of the order n, 
when n is prime to 3 and 5, involves no difficulty. We can 
begin by placing the mean number in the middle cell and work 
from that, either in both directions or forwards, making the 
number 1 follow after n^ ; we can also effect the same result by 
constructing any pandiagonal square of the order n and then 
transposing a certain number of rows and columns. If the rule 
given above on page 160, where a — 1, b = 2, h = 2, k = — l,he 
followed, this will lead to placing the number 1 in the (n+3)/2th 
cell of the top row : see, for instance, figure xviii. 

Such a square must be symmetrical, for if we begin with 
the middle number (n^+ l)/2, which I will denote by m, in the 
middle cell, and work from it forwards with the numbers m 4- 1, 
m+2, ..., and backwards with the numbers m— 1, m — 2, ..., 
the pairs of cells filled by the numbers m + 1 and m — 1, m + 2 
and m — 2, &c., are necessarily situated symmetrically to the 
middle cell and the sum of each pair is 2m. I believe this was 
first pointed out by McClintock. 

The construction of doubly-even symmetrical pandiagonal 
squares is also possible, but the analysis is too lengthy for me 
to find room for it here. 

In a symmetrical square any n such pairs of numbers 
together with the number in the middle cell will form a 
magic group. For instance in figure xviii, the group 32, 18, 
36, 14, 47, 3, and 25 is magic. So also is the group 47, 3, 35, 
15, 13, 37, and 25. Thus in a symmetrical pandiagonal square, 
even of a low order, there are hundreds of magic groups of 
n numbers whose sum is constant. 



CH. VIl] 



MAGIC SQUARES 



163 



Doubly-Magic Squares. In another species of hyper- magic 
squares the problem is to construct a magic square of the nth 
order in such a way that if the number in each cell is replaced 
by its mth power the resulting square shall also be magic. 
Here for example (see figure xxii) is a magic square* of the 
eighth order, the sum of the numbers in each line being equal 



5 


31 


35 


60 


67 


34 


8 


30 


19 


9 


53 


46 


47 


56 


18 


12 


16 


22 


42 


39 


52 


61 


27 


1 


63 


37 


25 


24 


3 


14 


44 


50 


26 


4 


64 


49 


38 


43 


13 


23 


41 


51 


15 


2 


21 


28 


62 


40 


54 


48 


20 


11 


10 


17 


55 


45 


36 


58 


6 


29 


32 


7 


33 


59 



A Doubly-Magic Square, 
Figure xxii. 

to 260, 80 constructed that if the number in each cell is re- 
placed by its square the resulting square is also magic (the sum 
of the numbers in each line being equal to 11180). 

Trebly-Magic Squares. The construction of squares which 
shall be magic for the original numbers, for their squares, and 
for their cubes has also been studied. I know of no square 
of this kind which is of a lower order than 128. 

Magic Pencils. Hitherto I have concerned myself with 
numbers arranged in lines. By reciprocating the figures com- 
posed of the points on which the numbers are placed we obtain 
a collection of lines forming pencils, and, if these lines be 
numbered to correspond with the points, the pencils will be 
magic f. Thus, in a magic square of the nth order, we arrange 
n^ consecutive numbers to form 2n-\-2 lines, each containing 

* See M. Coccoz in Ulllustrati&n, May 29, 1897. The subject has been 
studied by Messieurs G. Tarry, B. Portier, M. Coccoz and A. Eilly. More than 
200 such squares huve been given by the last-named in liis J^tude sur let 
Triangles et let Carres Magiques aux deux premiers degris, Troyes, 1901. 

t See Magic Reciprocals by G. Frankenstein, Cincinnati, 1876. 

11—2 



164 



MAGIC SQUAIIES 



[CH. VII 



n numbers so that the sum of the numbers in each line is the 
same. Reciprocally we can arrange n* lines, numbered con- 
secutively to form 27^4-2 pencils, each containing n lines, so 
that in each pencil the sum of the numbers designating the 
lines is the same. 

For instance, figure xxiii represents a magic square of 64 



1 


2 


G2 


CI 


GO 


59 


7 


8 


9 


10 


54 


53 


52 


51 


15 


16 


48 


47 


19 


20 


21 


22 


42 


41 


40 


39 


27 


28 


29 


30 


34 


33 


32 


31 


35 


36 


37 


38 


26 


25 


24 


23 


43 


44 


45 


46 


18 


17 


49 


60 


14 


13 


12 


11 


55 


56 


57 


58 


6 


5 


4 


3 


63 


64 



Figure xxiii. 

consecutive numbers arranged to form 18 lines, each of 8 
numbers. Reciprocally, figure xxiv represents 64 lines arranged 
to form 18 pencils, each of 8 lines. The method of construc- 
tion is fairly obvious. The eight-rayed pencil, vertex 0, is 
cut by two parallels perpendicular to the axis of the pencil, 
and all the points of intersection are joined cross-wise. This 
gives 8 pencils, with vertices A,B,... H\% pencils, with vertices 
A\ ... H'\ one pencil with its vertex at 0; and one pencil with 
its vertex on the axis of the last-named pencil. 

The sum of the numbers in each of the 1^ lines in figure xxiii 
is the same. To make figure xxiv correspond to this we must 
number the lines in the pencil A from left to right, 1, 9, ..., 57, 
following the order of the numbers in the first column of the 
square: the lines in pencil B must be numbered similarly to 
correspond to the numbers in the second column of the square, 
and so on. To prevent confusion in the figure I have not in- 
serted the numbers, but it will be seen that the method of 
construction ensures that the sum of the 8 numbers which 
designate the lines in each of these 18 pencils is the same. 



CH. Vll] 



MAGIC SQUARES 



165 



We can proceed a step further, if the resulting figure is 
cut by two other parallel lines perpendicular to the axis, and 
if all the points of their intersection with the cross-joins be 
joined cross- wise, these new cross-joins will intersect on the 




axis of the original pencil or on lines perpendicular to it. The 
whole figure will now give 8' lines, arranged in 244 pencils 
each of 8 rays, and will be the reciprocal of a magic cube of 
the 8th order. If we reciprocate back again we obtain a 
representation in a plane of a magic cube. 



166 MAGIC SQUARES [CH. VII 

Magic Square Puzzles. Many empirical problems, closely 
related to magic squares, will suggest themselves; but most 
of them are more correctly described as ingenious puzzles 
than as mathematical recreations. The following will serve 
as specimens. 

Magic Card Square*. The first of these is the familiar 
problem of placing the sixteen court cards (taken out of a 
pack) in the form of a square so that no row, no column, and 
neither of the diagonals shall contain more than one card of 
each suit and one card of each rank. The solution presents 
no difficulty, and is indicated in figure xxvi below. There 
are 72 fundamental solutions, each of which by reflexions 
and reversals produces 7 others. 

Euler's Officers Problemf. A similar problem, proposed 
by Euler in 1779, consists in arranging, if it be possible, thirty- 
six officers taken from six regiments — the officers being in 
six groups, each consisting of six officers of equal rank, one 
drawn from each regiment ; say officers of rank a, b, c, d, e, /, 
drawn from the 1st, 2nd, 3rd, 4th, 5th, and 6th regiments — 
in a solid square formation of six by six, so that each row and 
each file shall contain one and only one officer of each rank and 
one and only one officer from each regiment. The problem is 
insoluble. 

Extension of Euler^s Problem. More generally we may 
investigate the arrangement on a chess-board, containing n* 
cells, of n^ counters (the counters being divided into n groups, 
each group consisting of n counters of the same colour and 
numbered consecutively 1, 2, ..., n), so that each row and each 
column shall contain no two counters of the same colour or 
marked with the same number. Such arrangements are termed 
Eulerian Squares. 

* Ozanam, 1723 edition, vol. iv, p. 434. 

t Euler's Conimentationes Arithmeticae, Petrograd, 1849, vol. n, pp. 302— 
361. See also a paper by G. Tarry in the Comptes rendus of the French Associa- 
tion for the Advancement of Science, Paris, 1900, vol. ir, pp. 170—203 ; and 
various notes in U Intermediaire des Mathematiciens, Paris, vol. iii, 1896, 
pp. 17, 90; vol. V, 1898, pp. 83, 176, 252; vol. vi, 1899, p. 251 ; vol. vii', 1900, 
pp. 14, 311. 



CH. VI l] 



MAGIC SQUARES 



167 



For instance, if n=3, with three red counters Oj, a^, a^, 
three white counters bj, 62, 63, and three black counters Cj, Ca, C3, 
we can satisfy the conditions by arranging them as in figure xxv 
below. If w = 4, then with counters Ui, a,, (h, a^; 61, 62, 63, 
64; Ci, Ca, c C4; c^i, c^j, rf„ (£4, we can arrange them as in 
figure xxvi below. A solution when n = 5 is indicated in 
figure xxvii. 



«1 


h 


^3 


^3 


Cl 


^2 


C, 


«3 


^ 



«1 


&3 


^3 


^4 


<^4 


4, 


«2 


61 


d. 


Ci 


64 


«3 


&a 


a^ 


^ 


<^S 



«1 


&2 


<^8 


^4 


fj 


h 


^1 


d. 


<'3 


^4 


^4 


<i, 


<', 


^•i 


^3 


^^3 


U 


«0 


*1 


^'•i 


^"2 


«3 


^ 


Cj 


rf, 



Figure xxv. Figure xxvi. Figure xxvii. 

The problem is soluble if n is odd ; it is insoluble if n is 
of the form 2(2m+l). If solutions when w=a and when 
n = 6 are known, a solution when n= ah can be written down 
at once. The theory is closely connected with that of magic 
squares and need not be here discussed further. 

Reversible Magic Squares. The digits 0, 1, 2, 6, and 8, 
when turned upside down, can be read as 0, 1, 7, 9, and 8. 
This property has been utilized in constructing magic squares 



// 


ZZ 


62 


29 


69 


22 


IZ 


Zl 


2t 


61 


19 


12 


Z2 


19 


Zl 


6Z 



Reversible Magic Square. 
Figure xxviii. 

of non-consecutive numbers, which remain magic when the 
paper on which they are written is turned upside down. Here, 
for instance, figure xxviii*, is such a square in which only the 
digits 1, 2, 6 and their reversals are used. 

• See The Tribune, April 29, 1907. 



168 



MAGIC SQUARES 



[CH. VII 



Magic Domino Sqimres. Analogous problems can be made 
with dominoes. An ordinary set of dominoes, ranging from 
double zero to double six, contains 28 dominoes. Each 
domino is a rectangle formed by fixing two small square 



blocks together side by side 









• • • m\ 

• • » • 



CH 



of these 56 blocks, eight are 



• • • • 



1« • 
•—1. 



C3LE 






ET 



Magic Domino Square, 
Figure xxix. 



blank, on each of eight of them is on^ pip, on each of another 
eight of them are two pips, and so on. It is required to 
arrange the dominoes so that the 56 blocks form a square of 
7 by 7 bordered by one line of 7 blank squares and so that 
the sum of the pips in each row, each column, and in the 
two diagonals of the square is equal to 24. A solution* is 
given above. 

If we select certain dominoes out of the set and reject thg 
others we can use them to make various magic puzzles. As 
instance, I give on the next page magic squares of this kind due 
to Mr Escott and Mr Dudeney. Numerous squares of this kind 
can be formed. 

♦ See UIllustrati(m, July 10, 1897. 



CH. VIl] 



MAGIC SQUARES 



169 



• ••• •• ••• 

• ct* •• ••• 

• • • • • 

• ••••• • 

• • • 



• • • • • 

• • • 

• • • • • 

• • • 

• • • • • __^ 

• • • • • 

- • • • 

• • • • • 

• • • • • 

• • • 
• • • • • 

• J. • • 

• • • • • 

• • • 

• • • • • 



Magic Domino Squares, 
Figure xxx. 

Magic Coin Squares*. There are somewhat similar ques- 
tions concerned with coins. Here is one applicable to a square 
of the third order divided into nine cells, as in figure xxv 
above. If a five-shilling piece is placed in the middle cell c^ 
and a florin in the cell below it, namely, in as, it is required to 
place the fewest possible current English coins in the remaining 
seven cells so that in each cell there is at least one coin, so 
that the total value of the coins in every cell is different, and 
so that the sum of the values of the coins in each row, column, 
and diagonal is fifteen shillings : it will be found that thirteen 
additional coins will suffice. Similar problems can be proposed 
with postage stamps. 



ADDENDUM. 

Note. Page 169. Magic Coin Squares. Taking the notation of 
iBgure xxv we must put a double-florin and a sixpence in cell aj, two 
double-florins in cell 62, a half-crown in cell C3, a florin and a shilling 
in cell 63, a crown and a florin in cell 02, a crown and a half-crown in 
cell cj, a crown and a sixpence in cell 61. 



• See T}ie Strand Magazine^ London, December, 1896, pp. 720, 721. 



170 



CHAPTER VIII. 

TJNICXJRSAL PROBLEMS. 

I propose to consider in this chapter some problems which 
arise out of the theory of unicursal curves. I shall commence 
with Euler's Problem and Theorems, and shall apply <!he results 
briefly to the theories of Mazes and Geometi'ical Trees. The 
reciprocal unicursal problem of the Hamilton Game will be 
discussed in the latter half of the chapter. 

Euler's Problem. Euler's problem has its origin in a 
memoir* presented by him in 1736 to the St Petersburg 
Academy, in which he solved a question then under discussion 
as to whether it was possible from any point in the town of 
Konigsberg to take a walk in such a way as to cross every 
bridge in it once and only once and return to the starting point. 

The town is built near the mouth of the river Pregel, 
which there takes the form indicated below and includes the 
island of Kneiphof. In the eighteenth century there were 
(and according to Baedeker there are still) seven bridges in 
the positions shown in the diagram, and it is easily seen that 
with such an arrangement the problem is insoluble. Euler 
however did not confine himself to the case of Konigsberg, but 
discussed the general problem of any number of islands con- 
nected in any way by bridges. It is evident that the question 

* ' Solutio prohlematis ad Geometriam situs pertinentis,* Commentarii 
Academiae Scientiarum PetropoUtanae for 1736, Petrograd, 1741, vol. viii, 
pp. 128—140. This has been translated into French by M. Oh. Henry; see 
Lucas, vol. I, part 2, pp. 21—33. 



CH. VIll] 



UNICUKSAL PROBLEMS 



171 



will not be affected if we suppose the islands to diminish to 
points and the bridges to lengthen out. In this way we 




ultimately obtain a geometrical figure or network. In the 
Konigsberg problem this figure is of the shape indicated below, 
the areas being represented by the points Ay B, C, Z), and the 
bridges being represented by the lines /, m, n, p, q, r, «. 




Euler's problem consists therefore in finding whether a 
given geometrical figure can be described by a point moving 
so as to traverse every line in it once and only once. A more 
general question is to determine how many strokes are neces- 
sary to describe such a figure so that no line is traversed twice: 
this is covered by the rules hereafter given. The figure may 
be either in three or in two dimensions, and it may be repre- 
sented by lines, straight, curved, or tortuous, joining a number 
of given points, or a model may be constructed by taking a 
number of rods or pieces of string furnished at each end with 
a hook so as to allow of any number of them being connected 
together at one point. 

The theory of such figures is included as a particular case 



172 UNICURSAL PROBLEMS [CH. VIII 

in the propositions proved by Listing in his Topologie*. I 
shall, however, adopt here the methods of Euler, and I shall 
begin by giving some definitions, as it will enable me to put 
the argument in a more concise form. 

A node (or isle) is a point to or from which lines are 
drawn. A branch (or bridge or path) is a line connecting two 
consecutive nodes. An end (or hook) is the point at each 
termination of a branch. The order of a node is the number 
of branches which meet at it. A node to which only one 
branch is drawn is a free node or a free end. A node at which 
an even number of branches meet is an even node : evidently 
the presence of a node of the second order is immaterial. A 
node at which an odd number of branches meet is an odd node. 
A figure is closed if it has no free end : such a figure is often 
called a closed network. 

A route consists of a number of branches taken in con- 
secutive order and so that no branch is traversed twice. A 
closed route terminates at a point from which it started. 
A figure is described unicursally when the whole of it is 
traversed in one route. 

The following are Euler's results, (i) In a closed net- 
work the number of odd nodes is even, (ii) A figure which 
has no odd node can be described unicursally, in a re-entrant 
route, by a moving point which starts fi:om any point on it. 
(iii) A figure which has two and only two odd nodes can be 
described unicursally by a moving point which starts from one 
of the odd nodes and finishes at the other, (iv) A figure 
which has more than two odd nodes cannot be described com- 
pletely in one route; to which Listing added the corollary 
that a figure which has 2n odd nodes, and no more, can be 
described completely in n separate routes. I now proceed to 
prove these theorems. 

* Die Studien, Gottingen, 1847, part x. See also Tait on 'Listing's Topologie,' 
Philosophical Magazine, London, January, 1884, series 5, vol. xvii, pp. 30 — 46; 
and Collected Scientific Papers, Cambridge, vol. n, 1900, pp. 85—98. The 
problem was discussed by J. C. Wilson in his Traversing of Oeometrical 
Figures, Oxford, 1905, 



Cfl. Vlll] UNICURSAL PROBLEMS 173 

First. The number of odd nodes in a closed network is 
even. 

Suppose the number of branches to be h. Therefore the 
number of hooks is 26. Let kn be the number of nodes of 
the nth order. Since a node of the nth order is one at which 
n branches meet, there are n hooks there. Also since the figure 
is closed, n cannot be less than 2. 

/. 2k^ + Ski + 4A:4 + ... + nkn + ... = 26. 
Hence Sk^ + 6ka+ ,,. is even. 

.*. kz-\- ks-\-.., is even. 

Second. A figure which has no odd node can he described 
vmcursally in a re-entrant route. 

Since the route is to be re-entrant it will make no difference 
where it commences. Suppose that we start from a node A. 
Every time our route takes us through a node we use up one 
hook in entering it and one in leaving it. There are no odd 
nodes, therefore the number of hooks at every node is even: 
hence, if we reach any node except A, we shall always find 
a hook which will take us into a branch previously untraversed. 
Hence the route will take us finally to the node A irom. which 
we started. If there are more than two hooks at ^, we can 
continue the route over one of the branches from A previously 
untraversed, but in the same way as before we shall finally 
come back to A. 

It remains to show that we can arrange our route so as 
to make it cover all the branches. Suppose each branch of 
the network to be represented by a string with a hook at each 
end, and that at each node all the hooks there are fastened 
together. The number of hooks at each node is even, and if 
they are unfastened they can be re-coupled together in pairs, 
the arrangement of the pairs being immaterial. The whole 
network will then form one or more closed curves, since now 
each node consists merely of two ends hooked together. 

If this random coupling gives us one single curve then the 
proposition is proved; for starting at any point we shall go 
along every branch and come back to the initial point. But 



174 UNICUKSAL PROBLEMS [CH. VIII 

if this random coupling produces anjrwhere an isolated loop, L, 
then where it touches some other loop, M, say at the node P, 
unfasten the four hooks there (viz. two of the loop L and two 
of the loop M) and re-couple them in any other order: then 
the loop L will become a part of the loop M. In this way, 
by altering the couplings, we can transform gradually all the 
separate loops into parts of only one loop. 

For example, take the case of three isles, A, B, C, each 
connected with both the others by two bridges. The most 
unfavourable way of re-coupling the ends a.t A, B, G would be 
to make ABA, AG A, and BGB separate loops. The loops 
ABA and AG A are separate and touch at A] hence we should 
re-couple the hooks at J. so as to combine ABA and AG A into 

A 




one loop A BAG A. Similarly, by re-arranging the couplings 
of the four hooks at B, we can combine the loop BGB with 
A BAG A and thus make only one loop. 

I infer from Euler's language that he had attempted to 
solve the problem of giving a practical rule which would 
enable one to describe such a figure unicursally without 
knowledge of its form, but that in this he was unsuccessful. 
He however added that any geometrical figure can be de- 
scribed completely in a single route provided each part of it 
is described twice and only twice, for, if we suppose that every 
branch is duplicated, there will be no odd nodes and the figure 
is unicursal. In this case any figure can be described com- 
pletely without knowing its form : rules to effect this are 
given below. 

Third. A figure which has two and only two odd nodes can 
be described unicursally by a point which starts from one of the 
odd nodes and finishes at the other odd node. 



CH. VIIl] UNICURSAL PROBLEMS 175 

This at once reduces to the second theorem. Let A and Z 
be the two odd nodes. First, suppose that Z is not a free 
end. We can, of course, take a route from A to Z; if we 
imagine the branches in this route to be eliminated, it will 
remove one hook from A and make it even, will remove two 
hooks from every node intermediate between A and Z and 
therefore leave each of them even, and will remove one hook 
from Z and therefore will make it even. All the remaining 
network is now even: hence, by Euler's second proposition, 
it can be described unicursally, and, if the route begins at Z, 
it will end at Z. Hence, if these two routes are taken in 
succession, the whole figure will be described unicursally, be- 
ginning at A and ending at Z. Second, if -^ is a free end, 
then we must travel from Z to some node, F, at which more 
than two branches meet. Then a route from A to Y which 
covers the whole figure exclusive of the path from Y to Z can 
be determined as before and must be finished by travelling 
from Y to Z. 

Fourth. A figure having 2n odd nodes, and no more, can 
be described completely in n separate routes, n being a positive 
number. 

If any route starts at an odd node, and if it is continued 
until it reaches a node where no fresh path is open to it, this 
latter node must be an odd one. For every time we enter an 
even node there is necessarily a way out of it; and similarly 
every time we go through an odd node we use up one hook in 
entering and one hook in leaving, but whenever we reach it 
as the end of our route we use only one hook. If this route 
is suppressed there will remain a figure with 2/1 — 2 odd nodes. 
Hence n such routes will leave one or more networks with 
only even nodes. But each of these must have some node 
common to one of the routes already taken and therefore 
can be described as a part of that route. Hence the com- 
plete passage will require n and not more than n routes. It 
follows, as stated by Euler, that, if there are more than two 
odd nodes, the figure cannot be traversed completely in one 
route. 



176 



UNICURSAL PROBLEMS 



[cm. VIII 



The Konigsberg bridges lead to a network with four odd 
nodes; hence, by Euler's fourth proposition, it cannot be 
described unicursally in a single journey, though it can be 
traversed completely in two separate routes. 

The first and second diagrams figured below contain only 
even nodes, and therefore each of them can be described uni- 
cursally. The first of these is a regular re-entrant pentagon ; 
the second is the so-called sign-manual of Mohammed, said to 
have been originally traced in the sand by the point of his 
scimetar without taking it off the ground or retracing any part 
of the figure — which, as it contains only even nodes, is possible. 
The third diagram is taken from Tait's article: it contains 
only two odd nodes, and therefore can be described unicursally 
if we start from one of them, and finish at the other. 








The re-entrant pentagon, figured above, has some interest 
from having been used by the Pythagoreans as a sign — known 
as the triple triangle or pentagram star — by which they could 
recognize one another. It was considered symbolical of health, 
and probably the angles were denoted by the letters of the word 
vyieta, the diphthong ec being replaced by a 6. lamblichus, who 
is our authority for this, tells us that a certain Pythagorean, when 
travelling, fell ill at a roadside inn where he had put up for 
the night ; he was poor and sick, but the landlord, who was a 
kind-hearted fellow, nursed him carefully and spared no trouble 
or expense to relieve his pains. However, in spite of all efforts, 
the student got worse. Feeling that he was dying and unable 
to make the landlord any pecuniary recompense, he asked for 
a board on which he inscribed the pentagram star; this he 
gave to his host, begging him to hang it up outside so that 
all passers by might see it, and assuring him that the result 



CH. VIII I UNICURSAL PROBLEMS 177 

would recompense him for his charity. The scholar died and 
was honourably buried, and the board was duly exposed. After 
a considerable time had elapsed, a traveller one day riding by 
saw the sacred symbol ; dismounting, he entered the inn, and 
after hearing the story, handsomely remunerated the landlord. 
Such is the anecdote, which if not true is at least well found. 

As another example of a unicursal diagram I may mention 
the geometrical figure formed by taking a (2n + l)gon and 
joining every angular point to every other angular point. The 
edges of an octahedron also form a unicursal figure. On the 
other hand a chess-board, divided as usual by straight lines 
into 64 cells, has 28 odd nodes: hence it would require 
14 separate pen-strokes to trace out all the boundaries 
without going over any more than once. Again, the diagram 
on page 117 has 20 odd nodes and therefore would require 
10 separate pen-strokes to trace it out. 

It is well known that a curve which has as many nodes as 
is consistent with its degree is unicursal. 

I turn next to discuss in how many ways we can describe a 
unicursal figure, all of whose nodes are even*. 

Let us consider first how the problem is affected by a path 
which starts from a node A of order 2n and returns to it, 
forming a closed loop L. If this loop were suppressed we 
should have a figure with all its nodes even, the node A 
being now of the order 2(n— 1). Suppose the original figure 
can be described in N ways, and the reduced figure in N' ways. 
Then each of these N' routes passes (n— 1) times through Ay 
and in any of these passages we could describe the loop L in 
either sense as a part of the path. Hence N=2(n — 1) N\ 

Similarly if the node A on the original figure is of the order 
2{n + l), and there are I independent closed loops which start 
fi'om and return to ^, we shall have 

N=2^n(n+l){n-{-2)...{n + l^l)N\ 
where N' is the number of routes by which the figure obtained 
by suppressing these I loops can be described. 

* See G. Tarry, Association Frangaise pour VAvancement des Sciences, 1886, 
pp. 49—53. 

B. B. 12 



178 UNICURSAL PROBLEMS [CH. VIII 

By the use of these results, we may reduce any unicursal 
figure to one in which there are no closed loops of the kind 
above described. Let us suppose that in this reduced figure 
there are k nodes. We can suppress one of these nodes, say A, 
provided we replace the figure by two or more separate figures 
each of which has not more than k — \ nodes. For suppose 
that the node A is of the order 2n. Then the 2n paths which 
meet at A may be coupled in n pairs in 1 .3. 5 ... (2ri— 1) 
ways and each pair will constitute either a path through -4, 
or (in the special case where both members of the pair abut on 
another node B) a loop from A, This path or loop will form 
a portion of the route through A in which this pair of paths 
are concerned. Hence the number of ways of describing the 
original figure is equal to the sum of the number of ways of 
describing 1.3.5... {2n — 1) separate simpler figures. 

It will be seen that the process consists in successively 
suppressing node after node. Applying this process continually 
we finally reduce the figure to a number of figures without 
loops and in each of which there are only two nodes. If in one 
of these figures these nodes are each of the order 2/i it is easily 
seen that it can be described in 2 x (2?i — 1)! ways. 

We know that a figure with only two odd nodes, A and J5, 
is unicursal if we start at A (or B) and finish at B (or A), 
Hence the number of ways in which it can be described uni- 
cursally will be the same as the number required to describe 
the figure obtained from it by joining A and B. For if we 
start at A it is obvious that at the B end of each of the routes 
which cover the figure we can proceed along BA to the node 
A whence we started. 

This theory has been applied by Monsieur Tarry* to deter- 
mine the number of ways in which a set of dominoes, running 
up to even numbers, can be arranged. This example will serve 
to illustrate the general method. 

A domino consists of a small rectangular slab, twice as 
long as it is broad, whose face is divided into two squares, 

* See the second edition of the French Translation of this work, Paris, 1908, 
Tol. II, pp. 253—263 ; see also Lucas, vol. iv, pp. 145—150. 



CH. VIIl] UNICURSAL PROBLEMS 179 

which are either blank or marked with 1, 2, 3... dots. An 
ordinary set contains 28 dominoes marked 6-6, 6-5, 6-4, 6-3, 
6-2, 6-1, 6-0, 5-5. 5-4, 5-3, 5-2, 5-1, 5-0, 4-4, 4-3, 4-2, 4-1, 
4_0, 3-3, 3-2, 3-1, 3-0, 2-2, 2-1, 2-0, 1-1, 1-0, and 0-0. 
Dominoes are used in various games in most, if not all, of 
which the pieces are played so as to make a line such that 
consecutive squares of adjacent dominoes are marked alike. 
Thus if 6-3 is on the table the only dominoes which can be 
placed next to the 6 end are 6-6, 6-5, 6-4, 6-2, 6-1, or 6-0. 
Similarly the dominoes 3-5, 3-4, 3-3, 3-2, 3-1, or 3-0, can 
be placed next to the 3 end. Assuming that the doubles 
are played in due course, it is easy to see that such a set of 
dominoes will form a closed circuit*. We want to determine 
the number of ways in which such a line or circuit can be formed. 
Let us begin by considering the case of a set of 15 dominoes 
marked up to double-four. Of these 15 pieces, 5 are doubles. 
The remaining 10 dominoes may be represented by the sides 
and diagonals of a regular pentagon 01, 02, &c. The intersec- 
tions of the diagonals do not enter into the representation, 




and accordingly are to be neglected. Omitting these from our 
consideration, the figure formed by the sides and diagonals of 
the pentagon has five even nodes, and therefore is unicursal. 
Any unicursal route (ex. gr. 0-1, 1-3, 3-0, 0-2, 2-3, 3-4, 4-1, 
1-2, 2-4, 4-0) gives one way of arranging these 10 dominoes. 
Suppose there are a such routes. In any such route we may 
put each of the five doubles in any one of two positions {ex. gr. 

* Hence if we remove one domino, say 5-4, we know that the line formed by 
the rest of the dominoes must end on one side in a 5 and on the other in a 4. 

12—2 



180 



UNICUUSAL PROBLEMS 



[CH. VIIl 



in the route given above the double-two can be put between 
0-2 and 2-3 or between 1-2 and 2-4). Hence the total 
number of unicursal arrangements of the 15 dominoes is 2"a. 
If we arrange the dominoes in a straight line, then as we 
may begin with any of the 15 dominoes, the total number of 
arrangements is 15 . 2" . a. 

We have next to find the number of unicursal routes of 
the pentagon delineated above in figure A. At the node 
there are four paths which may be coupled in three pairs. If 
1 and 2 are coupled, as also 3 and 4, we get figure B. 
If 1 and 3 are coupled, as also 2 and 4, we get figure C, 
If 1 and 4 are coupled, as also 2 and 3, we get figure D. 




Figure B. Figure G. Figure D. 

Let us denote the number of ways of describing figure B by b, 
of describing figure C by c, and so on. The efi'ect of suppressing 
the node in the pentagon A is to give us three quadrilaterals, 
B, C, D. And, in the above notation, we have a = b -\- c-{-d. 

Take any one of these quadrilaterals, for instance D. We 
can suppress the node 1 in it by coupling the four paths which 
meet there in pairs. If we couple 1 2 with the upper of the 
paths 14s, as also 1 3 with the lower of the paths 1 4, we get 





Figure E. Figure F. 

the figure E. If we couple 1 2 with the lower of the paths 1 4, 
as also 1 3 with the upper of the paths 1 4, we again get the 
figure E. If we couple 1 2 and 1 3, as also the two paths 1 4, 
we get the figure F. Then as above, rf = 2e-f-/ Similarly 
6 = 2e +/, and c = 2e -f/. Hence a = 6 + c + oJ = 6e + 3/ 



CH. Vlll] UNICURSAL PROBLEMS 181 

We proceed to consider each of the reduced figures E and 
F. First take E, and in it let us suppress the node 4. For 
simplicity of description, denote the two paths 2 by )S and 
P'y and the two paths 4 3 by 7 and 7'. Then we can couple ^ 
and 7, as also /S' and 7', or we can couple fi and 7', as also ^' 
and 7 : each of these couplings gives the figure G. Or we can 
couple /3 and /3', as also 7 and 7': this gives the figure H. Thus 
e — ^g-\rh. Each of the figures Q and H has only two nodes. 
Hence by the formulae given above, we have ^ = 2.3.2=12, 
and A = 2.2.2 = 8. Therefore e = 2^+^ = 32. Next take 
the figure F, This has a loop at 4. If we suppress this 




Figure O. Figure H. Figure J. 

loop we get the figure J, and /= 2j. But the figure J, if 
we couple the two lines which meet at 4, is equivalent 
to the figure G. Thus /= 2j =2g = 24. Introducing these 
results we have a = 6e-4-3/= 192 + 72 = 264. And therefore 
i\r= 15. 2». a =126720. This gives the number of possible 
arrangements in line of a set of 15 dominoes. In this solution 
we have treated an arrangement from right to left as distinct 
firom one which goes from left to right: if these are treated 
as identical we must divide the result by 2. The number of 
arrangements in a closed ring is 2'a, that is 8448. 

We have seen that this number of unicursal routes for a 
pentagon and its diagonals is 264. Similarly the number for 
a heptagon is A = 129976320. Hence the number of possible 
arrangements in line of the usual set of 28 dominoes, marked 
up to double-six, is 28.3'.^, which is equal to 7959229931520. 
The number of unicursal routes covering a polygon of nine 
sides is 7i = 2^' . 3^^ 5^ . 711 . 40787. Hence the number of 
possible arrangements in line of a set of 45 dominoes marked 
up to double-eight is 48. 4°. n*. 

* These numerical conclusions have also been obtained by algebraical 
analysis : see M. Reiss, Annali di Matematica, Milan, 1871, vol. v, pp. 63—120. 



182 UNICURSAL PROBLEMS [CH. VIII 

Mazes. Eveiyone has read of the labyrinth of Minos in 
Crete and of Rosamund's Bower. A few modem mazes exist 
here and there — notably one, a very poor specimen of its 
kind, at Hampton Court — and in one of these, or at any 
rate on a drawing of one, most people have at some time 
threaded their way to the interior. I proceed now to consider 
the manner in which any such construction may be completely 
traversed even by one who is ignorant of its plan. 

The theory of the description of mazes is included in 
Euler's theorems given above. The paths in the maze are 
what previously we have termed branches, and the places 
where two or more paths meet are nodes. The entrance to 
the maze, the end of a blind alley, and the centre of the maze 
are free ends and therefore odd nodes. 

If the only odd nodes are the entrance to the maze and the 
centre of it — which will necessitate the absence of all blind 
alleys — the maze can be described unicursally. This follows 
from Euler's third proposition. Again, no matter how many 
odd nodes there may be in a maze, we can always find a 
route which will take us from the entrance to the centre 
without retracing our steps, though such a route will take us 
through only a part of the maze. But in neither of the cases 
mentioned in this paragraph can the route be determined 
without a plan of the maze. 

A plan is not necessary, however, if we make use of Euler's 
suggestion, and suppose that every path in the maze is dupli- 
cated. In this case we can give definite rules for the complete 
description of the whole of any maze, even if we are entirely 
ignorant of its plan. Of course to walk twice over every path 
in a labyrinth is not the shortest way of arriving at the centre, 
but, if it is performed correctly, the whole maze is traversed, 
the arrival at the centre at some point in the coarse of the 
route is certain, and it is impossible to lose one's way. 

I need hardly explain why the complete description of 
such a duplicated maze is possible, for now every node is even, 
and hence, by Euler's second proposition, if we begin at the 
entrance we can traverse the whole maze; in so doing we 



CH. VIIl] UNICURSAL PROBLEMS 183 

shall at some point arrive at the centre, and finally shall 
emerge at the point from which we started. This description 
will require us to go over every path in the maze twice, and 
as a matter of fact the two passages along any path will be 
always made in opposite directions. 

If a maze is traced on paper, the way to the centre is 
generally obvious, but in an actual labyrinth it is not so easy 
to find the correct route unless the plan is known. In order 
to make sure of describing a maze without knowing its plan it 
is necessary to have some means of marking the paths which 
we traverse and the direction in which we have traversed them 
— for example, by drawing an arrow at the entrance and end 
of every path traversed, or better perhaps by marking the 
wall on the right-hand side, in which case a path may not be 
entered when there is a mark on each side of it. 

Of the various practical rules for threading a maze those 
enunciated by M. Tr^maux seem to be the simplest*. These 
I proceed to explain. For brevity I shall describe a path or a 
node as old or new according as it has been traversed once 
before or not at all. Then the rules are (i) whenever you come 
to a new node, take any path you like ; (ii) whenever you come 
by a new path to an old node or to the closed end of a blind 
alley, turn back along the path by which you have just come ; 
(iii) whenever you come by an old path to an old node, take a 
new path, if there is one, but if not, an old path ; (iv) of course 
a path traversed twice must not be entered. I should add that 
on emerging at any node then, of the various routes which are 
permitted by these rules, it will be convenient always to select 
that which lies next to one's right hand, or always that which 
lies next to one's left hand. 

Few if any mazes of the type I have been considering 
(namely, a series of interlacing paths through which some 
route can be obtained leading to a space or building at the 
centre of the maze) existed in classical or medieval times. 
One class of what the ancients called mazes or labjrrinths seems 
to have comprised any complicated building with numerous 
• Lucas, vol. I, part iii, p. 47 et seq. 



184 



UNICURSAL PROBLEMS 



[CH. VIII 



vaults and passages*. Such a building might be termed a 
labyrinth, but it is not what is now usually understood by the 
word. The above rules would enable anyone to traverse the 
whole of any structure of this kind. I do not know if there 
are any accounts or descriptions of Rosamund's Bower other 
than those by Drayton, Bromton, and Knyghton: in the 
opinion of some, these imply that the bower was merely a 
house, the passages in which were confusing and ill-arranged. 

Another class of ancient mazes consisted of a tortuous path 
confined to a small area of ground and leading to a tree or 
shrine in the centre f. This is a maze in which there is no 
chance of taking a wrong turning; but, as the whole area 
can be occupied by the windings of one path, the distance 
to be traversed from the entrance to the centre may be 
considerable, even though the piece of ground covered by the 
maze is but small. 




Figure i. 



Figure ii. 



The traditional form of the labyrinth constructed for the 
Minotaur is a specimen of this class. It was delineated on 
the reverses of the coins of Cnossus, specimens of which are 
not uncommon ; one form of it is indicated in the accompanying 
diagram (figure i). The design really is the same as that 

* For instance, see the descriptions of the labyrinth at Lake Moeris given 
by Herodotus, bk. ii, c. 148; Strabo, bk. xvii, o. 1, art. 37; Diodorus, bk. i, 
CO. 61, 66; and Pliny, Hist. Nat., bk. xxxvi, c. 13, arts. 84 — 89. On these and 
other references see A. Wiedemann, Herodots zweites Buck, Leipzig, 1890, 
p. 522 et seq. See also Virgil, Aeneid, bk. v, c. v, 588; Ovid, Met., bk. viii, c. 5, 
159; Strabo, bk. viii, c. 6. 

t On ancient and medieval labyrinths — ^particularly of this kind — see an 
article by Mr E. Trollope in The Archaeological Journal, 1858, vol. xv, pp. 21 6— 
235, from which much of the historical information given above is derived. 



CH. VII l] UNICURSAL PROBLKMS 186 

drawn in fi^re ii, as can be easily seen by bending round a 
circle the rectangular figure there given. 

Mr Inwards has suggested* that this design on the coins 
of Cnossus may be a survival from that on a token given by 
the priests as a clue to the right path in the labyrinth there. 
Taking the circular form of the design shown above he sup- 
posed each circular wall to be replaced by two equidistant 
walls separated by a path, and thus obtained a maze to which 
the original design would serve as the key. The route thus 
indicated may be at once obtained by noticing that when a 
node is reached (i.e. a point where there is a choice of paths) 
the path to be taken is that which is next but one to that 
by which the node was approached. This maze may be also 
threaded by the simple rule of always following the wall on 
the right-hand side or always that on the left-hand side. 
The labyrinth may be somewhat improved by erecting a few 
additional barriers, without affecting the applicability of the 
above rules, but it cannot be made really difficult. This 
makes a pretty toy, but though the conjecture on which it is 
founded is ingenious it has no historical justification. Another 
suggestion is that the curved line on the reverse of the coins 
indicated the form of the rope held by those taking part in 
some rhythmic dance ; while others consider that the form was 
gradually evolved fi-om the widely prevalent svastika. 

Copies of the maze of Cnossus were frequently engraved on 
Greek and Roman gems; similar but more elaborate designs 
are found in numerous Roman mosaic pavements f. A copy 
of the Cretan labyrinth was embroidered on many of the state 
robes of the later Emperors, and, apparently thence, was 
copied on to the walls and floors of various churches J:. At 
a later time in Italy and in France these mural and pavement 
decorations were developed into scrolls of great complexity, 
but consisting, as far as I know, always of a single line. Some 
of the best specimens now extant are on the walls of the 

* Knowledge, London, October, 1892. 

t See ex. gr. Breton's Pompeia, p. 303. 

X Ozanam, Graphia aureae urbis Romae, pp. 92, 178. 



186 UNICURSAL PROBLEMS [CH. Vlll 

cathedrals at Lucca, Aix in Provence, and Poitiers; and on 
the floors of the churches of Santa Maria in Trastevere at 
Rome, San Vitale at Ravenna, Notre Dame at St Omer, and 
the cathedral at Chartres. It is possible that they were used 
to represent the journey through life as a kind of pilgrim's 
progress. 

In England these mazes were usually, perhaps always, cut 
in the turf adjacent to some religious house or hermitage: and 
there are some slight reasons for thinking that, when traversed 
as a religious exercise, a pater or ave had to be repeated at 
every turning. After the Renaissance, such labyrinths were 
frequently termed Troy-Towns or Julian's Bowers. Some of 
the best specimens, which are still extant, or were so until 
recently, are those at Rockliff Marshes, Cumberland ; Asenby, 
Yorkshire; Alkborough, Lincolnshire; Wing, Rutlandshire; 
Boughton-Green, Northamptonshire; Comberton, Cambridge- 
shire; Saffron Walden, Essex; and Chilcombe, near Winchester. 




Maze at Hampton Court, 

The modern maze seems to have been introduced — probably 
from Italy — during the Renaissance, and many of the palaces 
and large houses built in England during the Tudor and the 
Stuart periods had labyrinths attached to them. Those 
adjoining the royal palaces at South wark, Greenwich, and 
Hampton Court were well known from their vicinity to 
the capital. The last of these was designed by London and 
Wise in 1690, for William III, who had a fancy for such 
conceits: a plan of it is given in various guide-books. For 
the majority of the sight-seers who enter, it is sufficiently 



CH. VIIl] 



UNICURSAL PROBLEMS 



187 



elaborate; but it is an indifferent construction, for it can be 
described completely by always following the hedge on one 
side (either the right hand or the left hand), and no node is 
of an order higher than three. 

Unless at some point the route to the centre forks and 
subsequently the two forks reunite, forming a loop in which 
the centre of the maze is situated, the centre can be reached 
by the rule just given, namely, by following the wall on one 
side — either on the right hand or on the left hand. No 
labyrinth is worthy of the name of a puzzle which can be 
threaded in this way. Assuming that the path forks as 
described above, the more numerous the nodes and the higher 
their order the more difficult will be the maze, and the 
difficulty might be increased considerably by using bridges and 
tunnels so as to construct a labyrinth in three dimensions. 
In an ordinary garden and on a small piece of ground, often 
of an inconvenient shape, it is not easy to make a maze which 
fulfils these conditions. Here is a plan of one which I put up 




E^mPoD 



in my own garden on a plot of ground which would not allow 
of more than 36 by 23 paths, but it will be noticed that none 
of the nodes are of a high order. 



188 UNICUllSAL PROBLEMS [CH. VIII 

Geometrical Trees. Euler's original investigations were 
confined to a closed network. In the problem of the maze it 
was assumed that there might be any number of blind alleys 
in it, the ends of which formed free nodes. We may now 
progress one step further, and suppose that the network or 
closed part of the figure diminishes to a point. This last 
arrangement is known as a tree. The number of unicursal 
descriptions necessary to completely describe a tree is called 
the base of the ramification. 

We can illustrate the possible form of these trees by rods, 
having a hook at each end. Starting with one such rod, we 
can attach at either end one or more similar rods. Again, 
on any free hook we can attach one or more similar rods, 
and so on. Every free hook, and also every point where two 
or more rods meet, are what hitherto we have called nodes. 
The rods are what hitherto we have termed branches or paths. 

The theory of trees — which already plays a somewhat 
important part in certain branches of modern analysis, and 
possibly may contain the key to certain chemical and biological 
theories — originated in a memoir by Cayley*, written in 
1856. The discussion of the theory has been analytical rather 
than geometrical. I content myself with noting the following 
results. 

The number of trees with n given nodes is n"~^ If An is 
the number of trees with n branches, and B^ the number of 
trees with n free branches which are bifurcations at least, 
then 

i\-x)-^{l'-x'')-^^{l-o(f)-^> = 1 + « + 2B.,x'' + 2^3^- -f .... 

♦ Philosophical Magazine^ March, 1857, series 4, vol. xm, pp. 172 — 176 ; or 
Collected Works, Cambridge, 1890, vol. in, no. 203, pp. 242—246 : see also the 
paper on double partitions, Philosophical Magazine, November, 1860, series 4, 
vol. XX, pp. 337 — 341. On the number of trees with a given number of nodes, 
see the Quarterly Journal of Mathematics, London, 1889, vol. xxiii, pp. 376—378. 
The connection with chemistry was first pointed out in Cayley*s paper on 
isomers, Philosophical Magazine, June, 1874, series 4, vol. xlvii, pp. 444 — 447, 
and was treated more fully iti his report on trees to the British Association in 
1875, Reports, pp. 257—305. 



CH. VIIl] 



UNICURSAL PROBLEMS 



189 



Using these formulae we can find successively the values of 

AiyA^y ..., and B^, B^y The values of An when n = 2, 3, 4, 

5, 6, 7, are 2, 4, 9, 20, 48, 115; and of ^n are 1, 2, 5, 12, 33, 
90 

I turn next to consider some problems where it is desired 
to find a route which will pass once and only once through 
each node of a given geometrical figure. This is the reciprocal 
of the problem treated in the first part of this chapter, and is 
a far more difficult question. I am not aware that the general 
theory has been considered by mathematicians, though two 
special cases — namely, the Hamiltonian (or Icosian) Oaine and 
the Knight's Path on a Chess-Board — have been treated in 
some detail. 

The Hamiltonian Game. The Hamiltonian Game consists 
in the determination of a route along the edges of a regular 
dodecahedron which will pass once and only once through 
every angular point. Sir William Hamilton*, who invented 
this game — if game is the right term for it — denoted the 
twenty angular points on the solid by letters which stand for 
various towns. The thirty edges constitute the only possible 
paths. The inconvenience of using a solid is considerable, 
and the dodecahedron may be represented conveniently in 




perspective by a flat board marked as shown in the first of 
the annexed diagrams. The second and third diagrams will 
answer our purpose equally well and are easier to draw. 

* See Quarterly Journal of Mathematics, London, 1862, vol. v, p. 305 ; or 
Philosophical Magazine^ January, 1884, series 6, vol. xvii, p. 42; also Lucas, 
vol. II, part vii. 



190 UNICURSAL PROBLEMS [CH. VlII 

The first problem is to go "all round the world," that is, 
starting from any town, to go to every other town once and 
only once and to return to the initial town ; the order of the 
n towns to be first visited being assigned, where n is not 
greater than five. 

Hamilton's rule for effecting this was given at the meeting 
in 1857 of the British Association at Dublin. At each 
angular point there are three and only three edges. Hence, 
if we approach a point by one edge, the only routes open to 
us are one to the right, denoted by r, and one to the left, 
denoted by I. It will be found that the operations indicated 
on opposite sides of the following equalities are equivalent, 

lrH = rlr, rl^r=lrl, lrH = r^, rl^r = lK 

Also the operation ^"^ or r* brings us back to the initial point: 
we may represent this by the equations 

Z6 = l, r» = l. 

To solve the problem for a figure having twenty angular 
points we must deduce a relation involving twenty successive 
operations, the total effect of which is equal to unity. By 
repeated use of the relation l^^rl^r we see that 

1 = ^6= W = (rl'r) I* = {rl'Y = {r (rl'r) l\^ 

= {rH^rl]^ = {r^ (rPr) Irl^ = {r^Mrl]\ 

Therefore {r'Z»(^0? = l (i), 

and similarly {I'r' (IrfY == 1 (ii). 

Hence on a dodecahedron either of the operations 

rrrlllrlrlrrrlllrlrl ... (i), 

lllrrrlr I r I llrrrlrlr,,, (ii), 

indicates a route which takes the traveller through every town. 
The arrangement is cyclical, and the route can be commenced 
at any point in the series of operations by transferring the 
proper number of letters from one end to the other. The 
point at which we begin is determined by the order of certain 
towns which is given initially. 



CH. VIIl] UNICURSAL PROBLEMS 191 

Thus, suppose that we are told that we start from F and 
then successively go to B, A, U, and T, and we want to find 
a route from T through all the remaining towns which will 
end at F. If we think of ourselves as coming into F from 
G, the path FB would be indicated by I, but if we think of 
ourselves as coming into F from E, the path FB would be 
indicated by r. The path from 5 to ^ is indicated by I, 
and so on. Hence our first paths are indicated either by lllr 
or hy r I Ir, The latter operation does not occur either in (i) 
or in (ii), and therefore does not fall within our solutions. The 
former operation may be regarded either as the 1st, 2nd, 3rd, 
and 4th steps of (ii), or as the 4th, 5th, 6th, and 7th steps 
of (i). Each of these leads to a route which satisfies the 
problem. These routes are 

FBA VTPONGDEJKLMQRSHGF, 
and FBAVT8RKLMQPGNGDEJHGF, 

It is convenient to make a mark or to put down a counter 
at each corner as soon as it is reached, and this will prevent 
our passing through the same town twice. 

A similar game may be played with other solids provided 
that at each angular point three and only three edges meet. 
Of such solids a tetrahedron and a cube are the simplest 
instances, but the reader can make for himself any number of 
plane figures representing such solids similar to those drawn 
on page 189. Some of these were indicated by Hamilton. 
In all such cases we must obtain from the formulae analogous 
to those given above cyclical relations like (i) or (ii) there 
given. The solution will then follow the lines indicated above. 
This method may be used to form a rule for describing any 
maze in which no node is of an order higher than three. 

For solids having angular points where more than three 
edges meet — such as the octahedron where at each angular 
point four edges meet, or the icosahedron where at each 
angular point five edges meet — we should at each point have 
more than two routes open to us; hence (unless we suppress 
some of the edges) the symbolical notation would have to be 



192 UNIGURSAL PROBLEMS [CH. VIII 

extended before it could be applied to the^e solids. I offer 
the suggestion to anyone who is desirous of inventing a new 
game. 

Another and a very elegant solution of the Hamiltonian 
dodecahedron problem has been given by M. Hermary. It 
consists in unfolding the dodecahedron into its twelve penta- 
gons, each of which is attached to the preceding one by only 
one of its sides; but the solution is geometrical, and not 
directly applicable to more complicated solids. 

Hamilton suggested as another problem to start from any 
town, to go to certain specified towns in an assigned order, 
then to go to every other town once and only once, and to end 
the journey at some given town. He also suggested the con- 
sideration of the way in which a certain number of towns 
should be blocked so that there was no passage through them, 
in order to produce certain effects. These problems have not, 
so far as I know, been subjected to mathematical analysis. 

The problem of the knight's path on a chess-board is some- 
what similar in character to the Hamiltonian game. This I 
have already discussed in chapter VI. 



193 



CHAPTER IX. 

KIKKMAN*S SCHOOL-GIRLS PROBLEM. 

The Fifteen School-Girls Problem — first enunciated by 
T. P. Kirkman, and commonly known as Kirkmans Problem 
— consists in arranging fifteen things in different sets of 
triplets. It is usually presented in the form that a school- 
mistress was in the habit of taking her girls for a daily walk. 
The girls were fifteen in number, and were arranged in five 
rows of three each so that each girl might have two companions. 
The problem is to dispose them so that for seven consecutive 
days no girl will walk with any of her school-fellows in any 
triplet more than once. 

In the general problem, here discussed, we require to 
arrange n girls, where n is an odd multiple of 3, in triplets to 
walk out for y days, where y = (n — l)/2, so that no girl will 
walk with any of her school-fellows in any triplet more than once. 

The theory of the formation of all such possible triplets in 
the case of nine girls is comparatively easy, but the general 
theory involves considerable difficulties. Before describing any 
methods of solution, I will give briefly the leading facts in the 
history of the problem. For this and much of the material of 
this chapter I am indebted to 0. Eckenstein. Detailed refer- 
ences to the authorities mentioned are given in the bibliography 
mentioned in the footnote*. 

* The problem was first published in the Lady's and Gentleman's Diary for 
1850, p. 48, and has been the subject of numerous memoirs. A bibliography 
of the problem by 0. Eckenstein appeared in the Messenger of Mathematics, 
Cambridge, July, 1911, vol. xli, pp. 33—36. 

B. & 13 



194 KIRKMAN*S SCHOOL-GIRLS PROBLEM [CH. IX 

The question was propounded in 1850, and in the same year 
solutions were given for the cases when n = 9, 15, and 27 ; but 
the methods used were largely empirical. 

The first writer to subject it to mathematical analysis was 
K. R. Anstice who, in 1852 and 1853, described a method for 
solving all cases of the form 12m + 3 when 6m 4-1 is prime. 
He gave solutions for the cases when n=15, 27, 39. Sub- 
stantially, his process, in a somewhat simplified form, is covered 
by that given below under the heading Analytical Methods. 

The next important advance in the theory was due to 
B. Peirce who, in 1860, gave cyclical methods for solving all 
cases of the form 12m + 3 and 24m + 9. But the processes 
used were complicated and partly empirical. 

In 1871 A. H. Frost published a simple method applicable 
to the original problem when w = 15 and to all cases when n is 
of the form 2^"* — 1. It has been applied to find solutions 
when 71 = 15 and w = 63. 

In 1883 E. Marsden and A. Bray gave three-step cyclical 
solutions for 21 girls. These were interesting because Kirkman 
had expressed the opinion that this case was insoluble. 

Another solution when n = 21, by T. H. Gill, was given in 
the fourth edition of this book in 1905. His method though 
empirical appears to be applicable to all cases, but for high 
values of n it involves so much preliminary work by trial and 
error as to be of little value. 

A question on the subject which I propounded in the 
Educational Times in 1906, attracted the attention of L. A. 
Legros, H. E. Dudeney and 0. Eckenstein, and I received from 
them a series of interesting and novel solutions. As illustra- 
tions of the processes used, Dudeney published new solutions 
for w = 27, 83, 51, 57, 69, 75, 87, 93, 111 ; and Eckenstein for 
w = 27, 33, 39, 45, 51, 57, 69, 75, 93, 99, 111, 123, 135. 

I now proceed to describe some of the methods applicable 
to the problem. We can use cycles and combinations of them. 
I confine my discussion to processes where the steps of the 
cycles do not exceed three symbols at a time. It will be con- 
venient to begin with the easier methods, where however a 



CH. ix] kirkman's school-girls problem 196 

certain amount of arrangement has to be made empirically, and 
then to go on to the consideration of the more general method. 

One-Step Cycles. As illustrating solutions by one-step 
cyclical permutations I will first describe Legros's method. 
Solutions obtained by it can be represented by diagrams, and 
their use facilitates the necessary arrangements. It is always 
applicable when n is of the form 24/n + 3, and seems to be 
also applicable when n is of the form 24m -H 9. Somewhat 
similar methods were used by Dudeney, save that he made 
no use of geometrical constructions. 

We have n = 2y-\-l=^ 24m -f-3 or n=2y-{-l = 24m 4- 9. 
We may denote one girl by k, and the others by the numbers 
1, 2, 3, ... 21/. Place k at the centre of a circle, and the 
numbers 1, 2, 3, ... 2y at equidistant intervals on the circum- 
ference. Thus the centre of the circle and each point on its 
circumference will indicate a particular girl. A solution in 
which the centre of the circle is used to denote one girl is 
termed a central solution. 

The companions of k are to be different on each day. If we 
suppose that on the first day they are 1 and y+1, on the 
second 2 and y + 2, and so on, then the diameters through k 
will give for each day a triplet in which k appears. On each 
day we have to find 2 (y — l)/3 other triplets satisfying the 
conditions of the problem. Every triplet formed from the 
remaining 2y — 2 girls will be represented by an inscribed 
triangle joining the points representing these girls. The sides 
of the triangles are the chords joining these 2y — 2 points. 
These chords may be represented symbolically by [1], [2], 
[3], ... [y— 1]; these numbers being proportional to the smaller 
arcs subtended. I will denote the sides of a triangle so repre- 
sented by the letters p, q, r, and I will use the term triad or 
grouping to denote any group of p, q, r which determines the 
dimensions of an inscribed triangle. I shall place the numbers 
of a triad in square brackets. If p, q, r are proportional to 
the smaller arcs subtended, it is clear that \i p -\- q is less 
than y, we have p -\-q = r\ and li p -{- q is greater than y we 
have p-\- q + r=^2y. If we like to use arcs larger than the 

13—2 



196 KUIKMAN's school-girls problem [CH. IX 

semi-circumference we may confine ourselves to the relation 
p-\'q = r. In the geometrical methods described below, we 
usually first determine the dimensions of the triangles to be 
used in the solution, and then find how they are to be arranged 
in the circle. 

If {y — l)/3 scalene triangles, whose sides are p, q, r, can be 
inscribed in the circle so that to each triangle corresponds an 
equal complementary triangle having its equal sides parallel to 
those of the first and with its vertices at free points, then the 
system of 2 (y — l)/3 triangles with the corresponding diameter 
will give an arrangement for one day. If the system be per- 
muted cyclically y — 1 times we get arrangements for the other 
y — 1 days. No two girls will walk together twice, for each 
chord occupies a different position after each permutation, and 
as all the chords forming the (y — 1)/3 triangles are unequal the 
same combination cannot occur twice. Since the triangles are 
placed in complementary pairs, one being y points in front of 
the other, it follows that after 2/ — 1 permutations we shall 
come to a position like the initial one, and' the cycle will be 
completed. If the circle be drawn and the triangles cut out 
to scale, the arrangement of the triangles is facilitated. The 
method will be better understood if I apply it to one or two of 
the simpler cases. 

The first case is that of three girls, a, 6, c, walking out for 
one day, that is, n = 3, m = 0, y = 1. This involves no discussion, 
the solution being (a. b. c). 

The next case is that of nine girls walking out for four days, 
that is, n = 9, m = 0, y = 4. The first triplet on the first day 
is (1. k. 5). There are six other girls represented by the points 
2, 3, 4, 6, 7, 8. These points can be joined so as to form 
triangles, and each triangle will represent a triplet. We want 
to find one such triangle, with unequal sides, with its vertices 
at three of these points, and such that the triangle formed by 
the other three points will have its sides equal and parallel to 
the sides of the first triangle. 

The sides of a triangle are p, q, r. The only possible values 
are 1, 2, 8, and they satisfy the condition p-{-q = r. If a 



CH. ix] kirkman's school-giri^ problem 197 

triangle of this shape is placed with its vertices at the points 
3, 4, 6, we can construct a complementary equal triangle, four 
points further on, having 7, 8, 2 for its vertices. All the points in 
the figure are now joined, and form the three triplets for the first 
day, namely (k. 1. 5), (3. 4. 6), (7. 8. 2). It is only necessary 
to rotate the figure one step at a time in order to obtain the 
triplets for the remaining three days. Another similar solution 
is obtained from the diameter (1. k. 5), and the triangles (2. 3. 8), 
(6. 7. 4). It is the reflection of the former solution. 




Figure i. 

The next case to which the method is applicable is when 
n = 27, m= 1, y = 13. Proceeding as before, the 27 girls must 
be arranged with one of them, ky at the centre and the other 26 
on the circumference of a circle. The diameter (1. k. 14) gives 
the first triplet on the first day. To obtain the other triplets 
we have to find four dissimilar triangles which satisfy the con- 
ditions mentioned above. The chords used as sides of these 
triangles may be of the lengths represented symbolically by 
[1], [2], ... [12]. We have to group these lengths so that 
p-^q = r or p-\-q + r=2y; if the first condition can be satis- 
fied it is the easier to use, as the numbers are smaller. In this 
instance the triads [3, 8, 11], [5, 7, 12], [2, 4, 6], [1, 9, 10] 
will be readily found. Now if four triangles with their sides 
of these lengths can be arranged in a system so that all the 
vertices fall on the ends of different diameters (exclusive of the 
ends of the diameter 1, k, 14), it follows that the opposite ends of 
those diameters can be joined by chords giving a series of equal 
triangles, symmetrically placed, each having its sides parallel to 
those of a triangle of the first system. The following arrangement 



198 kirkman's school-girls problem [CH. IX 

of triangles satisfies the conditions: (4. 11. 25), (5. 8. 23), 
(6. 7. 16), (9. 13. 15). The complementary system is (17. 24. 12), 
(18. 21. 10), (19. 20. 3), (22. 26. 2). These triplets with 
(k. 1. 14) give an arrangement for the first day; and, by 
rotating the system cyclically, the arrangements for the re- 
maining 12 days can be found immediately. 

I proceed to give one solution of tbis type for every remaining case where n 
is less than 100. From the result the triads or groupings used can be obtained. 
It is sufficient in each case to give an arrangement on the first day, since the 
arrangements on the following days are at once obtainable by cyclical per- 
mutations. 

I take first the three cases, 33, 57, 81, where n is of the form 24m+9. 
In these cases the arrangements on the other days are obtained by one-step 
cyclical permutations. 

For 33 girls, a solution is given by the system of triplets (2. 11. 16), 
(4. 6. 10), (5. 13. 30), (7. 8. 19), (9. 28. 31), and the complementary system 
(18. 27. 32), (20 22. 26), (21. 29. 14), (23. 24. 3), (25. 12. 15). These 10 triplets, 
together with that represented by {k. 1. 17), will give an arrangement for the 
first day. 

For 67 girls, a possible arrangement of triplets is (18. 13. 50), (20. 11. 28), 
(21. 52. 3), (8. 10. 61), (4. 25. 26), (2. 6. 12), (7. 19. 33), (27. 43. 16), (37, 14. 17). 
These, with the 9 complementary triplets, and the diameter triplet (1. k. 29), 
give an arrangement for the first day. 

For 81 girls an arrangement for the first day consists of the diameter triplet 
(1. k. 41), the 13 triplets (3. 35. 42), (4. 10. 29), (5. 28. 66), (6. 26. 39), (7. 15. 17), 
(8. 11. 32), (13. 27. 49), (14. 19. 30), (20. 37. 38), (21. 25. 52), (24. 36. 62), 
(18. 33. 63), (31. 40. 74), and the 13 complementary triplets. 

I take next the three cases, 51, 75, 99, where n is of the form 24m + 3. In 
these cases the arrangements on the other days are obtained either by one-step 
or by two-step cyclical permutations 

For 51 girls, an arrangement for the first day consists of the diameter 
triplet {k. 1. 26), the 8 triplets (2. 9. 36), (4. 7. 25), (6. 10. 19), (8. 14. 22), 
(12.17.45), (13.24.48), (15.16.46), (18.28.30), and the 8 complementary 
triplets^ 

For 75 girls, an arrangement for the first day consists of the diameter triplet 
{k. 1. 38), the 12 triplets (2. 44. 55), (4. 11. 19), (5. 50. 66), (6. 52. 57), 
(8. 46. 58), (10. 59. 65), (12. 60. 64), (14, 24. 68), (16 25. 72), (17. 3. 74), 
(33. 34. 73), (30. 32. 63), and the 12 complementary triplets. 

For 99 girls an arrangement for the first day consists of the diameter 
triplet (1. k. 50), the 16 triplets (2. 17. 47), (3. 9. 68), (4. 44. 82), (5. 12. 75), 
(6. 32. 42), (7. 23. 97), (8. 21. 30), (15. 20. 76), (16. 35. 85), (18. 45. 62), 
(22. 40. 6B), (25. 37. 92), (28. 29. 80), (34. 38. 59), (39„ 41. 73), (46. 49. 60), 
and the 16 complementary triplets. 

It is also possible to obtain, for numbers of the form 24:7n + 3, solutions 
which are uniquely two-step, but in these the complementary triangles are not 



CH. ix] kirkman's school-girls problem 199 

placed symmetrically to each other. I give 27 girls as an instance, using the 
same triads as in the solution of this case given above. The triplets for the 
first day are {k. 1. 14), (2. 12. 3), (21. 5. 22), (20. 24. 26), (11. 15. 17), 
(8. 16. 19), (25. 7. 10), (6. 18. 13), and (23. 9. 4). From this the arrangements 
on the other days can be obtained by a two-step (but not by a one-step) cyclical 
permutation. 

It is unnecessary to give more examples, or to enter on* the 
question of how from one solution others can be deduced, or 
how many solutions of each case can be obtained in this way. 
The types of the possible triangles are found analytically, but 
their geometrical arrangement is empirical. The defect of this 
method is that it may not be possible to arrange a given 
grouping. Thus when n = 27, we easily obtain 24 different 
groupings, but two of them cannot be arranged geometrically 
to give solutions ; and whether any particular grouping will 
give a solution can, in many cases, be determined only by long 
and troublesome empirical work. The same objection applies 
to the two-step and three-step methods which are described 
below. 

Two-Step Cycles. The method used by Legros was extended 
by Eckenstein to cases where n is of the form 12m + 3. When 
n is of this form and m is odd we cannot get sets of comple- 
mentary triangles as is required in Legros's method ; hence, to 
apply a similar method, we have to find 2 (y — 1)/3 different 
dissimilar inscribed triangles having no vertex in common and 
satisfying the condition p-}-q = r or p -{■ q ■{• r — 2y. These 
solutions are also central. Since there are 2y points on the 
circumference of the circle the permutations, if they are to be 
cyclical, must go in steps of two numbers at a time. In 
Legros's method we represented one triplet by a diameter. But 
obviously it will answer our purpose equally well to represent 
it by a triangle with k as vertex and two radii as sides, one 
drawn to an even number and the other to an odd number: 
in fact this will include the diameter as a particular case. 

I begin by considering the case where we use the diameter 
(1. k. y) to represent one triplet on the first day. Here the 
chords used for sides of the triangles representing the other 
triplets must be of lengths [1], [2], ... [y-l\ Also each given 



200 



KIRKMAN S SCHOOL-GIRLS PROBLEM 



[CH. IX 



length must appear twice, and the two equal lines so repre- 
sented must start one from an even number and the other 
from an odd number, so as to avoid the same combination of 
points occurring again when the system is rotated cyclically. 
Of course a vertex cannot be at the point 1 or 3/, as these 
points will be required for the diameter triplet (1. k. y). 

These remarks will be clearer if we apply them to a definite 
example. I take as an instance the case of 15 girls. As before 
we represent 14 of them by equidistant points numbered 1, 2, 
3, ... 14 on the circumference of a circle, and one by a point k 
at its centre. Take as one triplet the diameter (1. k. 8). Then 
the sides p, q, r may have any of the values [1], [2], [3], [4], [5], 
[6], and each value must be used twice. On examination it will 
be found that there are only two possible groupings, namely 
[1, 1, 2], [2, 4, 6], [3, 3, 6], [5, 5, 4], and [1, 2, 3], [1, 4, 5], 
[3, 5, 6], [2, 4, 6]. One of the solutions to which the first set of 
groupings leads is defined by the diameter (1. k. 8) and the four 
triplets (9. 10. 11), (4. 6. 14), (2. 5. 13), (3. 7. 12); see figure ii, 
below: of the four triangles used three are isosceles. The 




Figure ii. 



Figure iii. 



second set leads to solutions defined by the triplets {k. 1. 8), 
(4. 5. 7), (13. 14. 9), (3. 6. 11), (10. 12. 2), or by the triplets 
(k. 1. 8), (6. 7. 9), (3. 4. 13), (5. 11. 14), (10. 12. 2): in these 
solutions all the triangles used are scalene. If any one of 
these three sets of triplets is rotated cyclically two steps at 



CH. ix] kirkman's school-girls problem 201 

a time, we get a solution of the problem for the seven days 
required. Each of these solutions hy reflection and inversion 
gives rise to three others. 

Next, if we take (k. 1. 2) for one triplet on the first day we 
shall have the points 3, 4,... 14 for the vertices of the four 
triangles denoting the other triplets on that day. The sides 
must be of the lengths [1], [2], ... [7], of which [2], ... [6] must 
be used not more than twice and [1], [7] must be used 
only once. The [1] used must start from an even number, 
for otherwise the chord denoted by it would, when the system 
was rotated, occupy the position joining the points 1 and 2, 
which has been already used. The only possible groupings are 
[2, 4, G], [2, 3, 5], [3, 4, 7], [1, 5, 6] ; or [2, 4, G], [2, 5, 7], [1, 4, 5], 
[3, 3, 6] ; or [2, 4, 6], [2, 5, 7], [1, 3, 4], [3, 5, 6]. Each of these 
groupings gives rise to various solutions. For instance the first 
grouping gives a set of triplets (k. 1. 2), (3. 7. 10), (4. 5. 13), 
(6. 9. 11), (8. 12. 14). From this by a cyclical two-step per- 
mutation we get a solution. This solution is represented in 
figure iii. If we take (k. 1. 4) or (k. 1. 6) as one triplet on the 
first day, we get other sets of solutions. 

Solutions involving the triplets (k. 1. 2), (k. 1. 4), (k. 1. 6), 
{k. 1. 8), and other analogous solutions, can be obtained from 
the solutions illustrated in the above diagrams by re-arranging 
the symbols denoting the girls. For instance, if in figure iii, 
where all the triangles used are scalene, we replace the numbers 
2, 8, 3, 13, 4, 6, 5, 11, 7, 9, 10, 14 by 8, 2, 13, 3, 6, 4, 11, 5, 9, 
7, 14, 10, we get the solution (k. 1. 8), (4. 5. 7), etc., given above. 
Again, if in figure iii we replace the symbols 1, 2, 3, 4, 5, 6, 
7, 8, 9, 10, 11, 12, 13, 14, k by 12, 2, 10, 6, 1, 14, 5, 13, 9, 
15, 4, 3, 11, 8, 7, we obtain a solution equivalent to that given 
by T. H. Gill and printed in the fourth edition of this book. In 
this the arrangement on the first day is (1. 6. 11), (2. 7. 12), 
(3. 8. 13), (4. 9. 14), (5. 10. 15). The arrangements on the other 
days are obtained as before by rotating the system so de- 
lineated round 7 as a centre two steps at a time. Gill's 
arrangement is thus presented in its canonical form as a 
central two-step cyclical solution. 



202 KIRK man's school-girls problem [ch. IX 

I proceed to give one solution of this type for every remaining case where 
n is less than 100. In each case I give an arrangement on the first day; the 
arrangements for the other days can be got from it by a two-step cyclical 
permutation of the numbers. 

In the case of 27 girls, one arrangement on the first day is {k. 1. 14), 
(19. 21. 20), (3. 9. 6), (13. 23. 18), (5. 17. 24), (7. 25. 16), (11. 15. 26), (10. 4. 2), 
(12. 22. 8). 

In the case of 39 girls, one arrangement for the first day is {k. 1. 20), 
(35. 37. 36), (7. 13. 10), (19. 29. 24), (11. 25. 18), (3. 21. 12), (17. 33. 6), 
(15. 27. 2), (23. 31. 8), (5. 9. 26), (4. 16. 32), (14. 34. 38), (22. 28. 30). 

In the case of 51 girls, one arrangement for the first day is {k. 1. 26), 
(21. 23. 22), (11. 17. 14), (35. 45. 40), (25. 39. 32), (15. 33. 24), (13. 41. 2), 
(5. 31. 18), (19. 49. 34), (27. 43. 10), (9. 47. 28), (29. 37. 8), (3. 7. 30), 
(4. 20. 46), (6. 36. 42), (12. 16. 44), (38. 48. 50). 

In the case of 63 girls, one arrangement for the first day is {k. 1. 32), 
(67. 59. 58), (23. 29. 26), (37. 47. 42), (5. 53. 60), (17. 61. 8), (3. 43. 54), 
(7. 33. 20), (9. 39. 24), (27. 55. 10), (21. 45. 2), (11. 31. 52), (35. 51. 12), 
(13. 25. 50), (41. 49. 14), (15. 19. 48), (16. 18. 36), (6. 34. 38), (46. 56. 62), 
(22. 30. 44), (4. 28. 40). 

In the case of 75 girls, one arrangement for the first day is {k. 1. 38), 
(23. 25. 24), (3. 9. 6), (29. 39. 34), (7. 21. 14), (51. 69. 60), (33. 55. 44), 
(11. 59. 72), (35. 65. 60), (37. 71. 54), (27. 63. 8), (15. 57. 36), (13. 41. 64), 
(43. 67. 18), (19. 73. 46), (31. 47. 2), (5. 17. 48), (53. 61. 20), (45. 49. 10), 
(30. 32. 52), (22. 26. 66), (56. 62. 70), (12. 28. 74), (16. 40. 58), (4. 42. 68). 

In the case of 87 girls, one arrangement for the first day is {k. 1. 44), 
(61. 63. 62), (73. 79. 76), (35. 45. 40), (11. 83. 4), (25. 43. 34), (59. 81. 70), 
(7. 33. 20), (41. 71. 56), (23. 75. 6), (27. 65. 46), (13. 57. 78), (5. 51. 28), 
(3. 39. 64), (15. 47. 74), (9. 67. 38), (31. 55. 86), (19. 85. 52), (21. 37. 72), 
(17. 29. 66), (69. 77. 30), (49. 53. 8), (10. 12. 24), (22. 26. 84), (18. 54. 60), 
(2. 50. 80), (32. 42. 58), (14. 36. 82), (16. 48. 68). 

In the case of 99 girls, one arrangement for the first day is (k. 1. 50), 
(47. 49. 48), (53. 59. 66), (55. 65. 60), (57. 71. 64), (23. 41. 32), (17. 39. 28), 
(63. 89. 76), (5. 35. 20), (3. 67. 84), (9. 69. 88), (29. 85. 8), (27. 79. 4), 
(25. 73. 98), (33. 77. 6), (21. 61. 90), (45. 81. 14), (51. 83. 18), (15. 43. 78), 
(13. 37. 74), (11. 31. 70), (75. 91. 34), (7. 19. 62), (87. 95. 42), (93. 97. 46), 
(58. 94. 96), (40. 44. 66), (10. 16. 26), (30. 38. 82), (68. 80. 2), (22. 36. 92), 
(24. 54. 72), (12. 52. 86). 

This method may be also represented as a one-step cycle. 
For if we denote the girls by a point k at the centre of the 
circle, and points a^, b^ a^, K, a^, 63, ... placed in that order 
on the circumference, we can re-write the solutions in the 
suffix notation, and then the cyclical permutation of the 
numbers denoting the suffixes is by one step at a time. 

The one-step and two-step methods described above cover 



CH. IX] kirkman's school-girls problem 203 

all cases except those where n is of the form 24m + 21. These 
I have failed to bring under analogous rules, but we can solve 
them by recourse to the three-step cycles next described. 

Three-Step Cycles. The fact that certain cases are soluble 
by one-step cycles, and others by two-step cycles, suggests 
the use of three-step cycles, and the fact that n is a multiple 
of 3 points to the same conclusion. On the other band, if 
we denote the n girls by 1, 2, 3, ... n, and make a cyclical 
permutation of three steps at a time (or if we denote 
the girls by a,, b^ Ci, aj, h^, Ca, ..., and make a cyclical 
permutation of the suffixes one step at a time), we cannot 
get arrangements for more than n/S days. Hence there will 
remain (n — l)/2 - n/3 days, that is, (n — 3)/6 days, for which 
we have to find other arrangements. In fact, however, we 
can arrange the work so that in addition to the cyclical 
arrangements for w/3 days we can find (n — 3)/6 single triplets 
from each of which by a cyclical permutation of the numbers 
or suffixes an arrangement for one of these remaining days can 
be obtained ; other methods are also sometimes available. 

For instance take the case of 21 girls. An arrangement 
for the first day is (1. 4. 10), (2. 5. 11), (3. 6. 12), (7. 14. 18), 
(8. 15. 16), (9. 13. 17), (19. 20. 21). From this by cyclical 
permutations of the numbers three steps at a time, we can 
get arrangements for 7 days in all. The arrangement for the 
8th day can be got from the triplet (1. 6. 11) by a three-step 
cyclical permutation of the numbers in it. Similarly the 
arrangement for the 9th day can be got from the triplet 
(2. 4. 12), and that for the 10th day from the triplet (3. 5. 10), 
by three-step cyclical permutations. 

This method was first used by A. Bray in 1883, and was 
subsequently developed by Dudeney and Eckenstein. It gives 
a solution for every value of n except 15, but it is not so easy 
to use as the methods already described, partly because the 
solution is in two parts, and partly because the treatment 
varies according as n is of the form 18m + 3, or 18m + 9, or 
18m + 15. Most of the difficulties in using it arise in the case 
when 71 is of the form 18m + 15. 



204 KIUKMAN's school-girls problem [CH. IX 

The geometrical representation is sufficiently obvious. In 
the methods used by Legros and Eckenstein, previously de- 
scribed, the girls were represented by 2y equidistant points 
on the circumference of a circle and a point at its centre. 
It is evident that we may with equal propriety represent all 
the girls by symbols placed at equidistant intervals round 
the circumference of a circle : such solutions are termed non- 
central. The symbols may be 1, 2, 3, ... w, or letters «!, 2>i, Cj, 
tta, h^, Ca, .... Any triplet will be represented by a triangle 
whose sides are chords of the circle. The arrangement on any 
day is to include all the girls, and therefore the triangles re- 
presenting the triplets on that day are n/3 in number, and as 
each girl appears in only one triplet no two triangles can have 
a common vertex. 

The complete three-step solution will require the determina- 
tion of a system of {n— l)/2 inscribed triangles. In the first 
part of the solution n/3 of these triangles must be selected to 
form an arrangement for the first day, so that by rotating this 
arrangement three steps at a time we obtain triplets for njo days 
in all. In the second part of the solution we must assure ourselves 
that the remaining {n — 3)/6 triangles are such that from each of 
them, by a cyclical permutation of three steps at a time, an 
arrangement for one of the remaining {n — 3)/6 days is obtainable. 

As before we begin by tabulating the possible differences 
[1], [2], [3], ... \{n — l)/2], whose values denote the lengths of 
the sides p, q, r of the possible triangles, also, we have either 
p-{-q^r or p-hq + r — n. From these values of p, q, r are 
formed triads, and in these triads each difference must be 
used three times and only three times. Triangles of these 
types must be then formed and placed in the circle so that 
the side denoting any assigned difference p must start once 
from a number of the form 3m, once from a number of the 
form 3m + 1, and once from a number of the form .Sm-f2. 
Also an isosceles triangle, one of whose sides is a multiple of 
three, cannot be used: thus in any particular triad a 3, 6, 9, ... 
cannot appear more than once. Save in some exceptional 
cases of high values of », every triangle, one of whose 



CH. ix] kirkman's school-girls problem 205 

side^ is a multiple of 3, must be used in the first part 
of the solution. In the whole arrangement every possible 
difference will occur n times, and, since any two assigned 
numbers can occ\ir together only once, each difference when 
added to a number must start each time from a different 
number. I will not go into further details as to how these 
triangles are determined, but I think the above rules will be 
clear if I apply them to one or two easy examples. 

For 9 girls, the possible differences are [1], [2], [3], [4-], 
each of which must be used three times in the construction of 
four triangles the lengths of whose sides p, q, r are such that 
|j-f^ = r orjp+g-|-r=9. One possible set of triads formed 
from these numbers is [1, 2, 3], [1, 2, 3], [2, 3, 4], and [1, 4, 4]. 
Every triangle with a side of the length [3] must appear in the 
first part of the solution ; thus the triplets used in the first part 
of the solution must be obtained from the first three of these 
triads. Hence we obtain as an arrangement for the first day 
the triplets (1. 3. 9), (2. 4. 7), (5. 6. 8). From this, three-step 
cyclical permutations give arrangements for other two days. 
The remaining triad [1, 4, 4] leads to a triplet (1. 2. 6) which, 
by a three-step cyclical permutation, gives an arrangement for 
the remaining day. 

If we use the suffix notation, an arrangement for the first 
day is Oi^a^s, J1C363, Cxa^c^, From this, by simple cyclical per- 
mutations of the suffixes, we get arrangements for the second 
and third days. Lastly, the triplet ajftjCi gives, by cyclical 
permutation of the suffixes, the arrangement for the fourth day, 
namely, aitjCi, a2h2Ciy a^b^Cs' 

For 15 girls, the three-step process is inapplicable. The 
explanation of this is that two triads are required in the 
second part of the solution, and in neither of them may a 3 
appear. The triads are to be formed from the differences 
[1], [2], ... [7], each of which is to be used three times, and the 
condition that in any particular triad only one 3 or one 6 may 
appear necessitates that six of the triads shall involve a 3 or 
a 6. Hence only one triad will be available fur the second part 
of the solution. 



206 kirkman's school-girls problem [CH. IX 

I proceed to give one solution of this type for every remaining case where n 
is less than 100. I give some in the numerical, others in the suffix notation. 
The results will supply an indication of the process used. 

First, I consider those cases where n is of the form 18m + S. In these cases 
it is always possible to find 2m triads each repeated thrice, and one equilateral 
triad, and to use the equilateral and m triads in the first part of the solution, 
the 3 triplets representing any one of these m triads being placed in the circle 
at equal intervals from each other in the first day's arrangement. From this, 
three-step or one-step cyclical permutations give arrangements for 6ni-l-l days 
in all. In the second part of the solution each of the m remaining triads 
is used thrice ; it suffices to give the first triplet on each day, since from it the 
other triplets on that day are obtained by a three-step cyclical permutation. 

For 21 girls an arrangement for the first day, for the first part of the solution, 
is (1. 4. 10), (8. 11. 17), (15. 18. 3), (2. 6. 7), (9. 13. 14), (16. 20. 21), (5. 12. 19). 
From this, three- step or one-step cyclical permutations give arrangements for 
7 days in all. The first triplets used in the second part of the solution are 
(1. 3. 11), (2. 4. 12), (3. 5. 13). Each of these three triplets gives by a three- 
step cyclical permutation an arrangement for one of the remaining 3 days. 

For 39 girls, arrangements for 13 days can be obtained from the following 
arrangement of triplets for the first day: (1. 4. 13), (14. 17. 26), (27. 30. 39), 
(2. 8. 23), (16. 21. 36), (28. 34. 10), (7. 11. 12), (20. 24. 25), (33. 37. 38), (3. 5. 19), 
(16. 18. 32), (29. 31. 6), (9. 22. 35). From each of the 6 triplets (1. 8. 18), 
(2. 9. 19), (3. 10. 20), (1. 9. 20), (2. 10. 21), (3. 11. 22), an arrangement for one 
of the remaining 6 days is obtainable. 

For 57 girls, arrangements for 19 days can be obtained from the following 
arrangement of triplets for the first day: (1. 4. 25), (20. 23. 44), (39. 42. 6), 
(2. 8. 17), (2L 27. 36), (40. 46. 55), (3. 15. 33), (22. 34. 52), (41. 53. 14), 
(18. 19. 26), (37. 38. 45), (56. 57. 7), (13. 30. 35), (32. 49. 54), (51. 11. 16), 
(9. 29. 43), (28. 48. 5), (47. 10. 24), (12. 31. 60). From each of the 9 triplets 
(1. 3. 14), (2. 4. 15), (3. 5. 16), (1. 5. 30), (2. 6. 31), (3. 7. 32), (1. 11. 27), 
(2. 12. 28), (3. 13. 29), an arrangement for one of the remaining 9 days is 
obtainable. 

For 75 g-irls, arrangements for 25 days can be obtained from the following 
arrangement of triplets for the first day : (1. 4. 10), (26. 29. 35), (51. 54. 60), 
(3. 15. 36), (28. 40. 61), (53. 65. 11), (2. 17. 41), (27. 42. 66), (52. 67. 16), 
(13. 31. 58), (38. 56. 8), (63. 6. 33), (14. 21. 22), (39. 46. 47), (64. 71. 72), 
(7. 18. 20), (32. 43. 45), (57. 68. 70), (5. 9. 37), (30. 34. 62), (55. 59. 12), 
(19. 24. 50), (44. 49. 75), (69. 74. 25), (23. 48. 73). From each of the 12 triplets 
(1. 11. 30), (2. 12. 31), (3. 13. 32), (1. 15. 35), (2. 16. 36), (3. 17. 37), (1. 17. 39), 
(2. 18. 40), (3. 19. 41), (1. 18. 41), (2. 19. 42), (3. 20. 43), an arrangement for 
one of the remaining 12 days is obtainable. 

For 93 girls, arrangements for 31 days can be obtained from the following 
arrangement of triplets for the first day: (1. 76. 79), (32. 14. 17), (63. 45. 48), 
(13. 25. 55), (44. 56. 86), (75. 87. 24), (29. 50. 74), (60. 81. 12), (91. 19. 43), 
(20. 26. 59), (51. 57. 90), (82. 88. 28), (3. 30. 39), (34. 61. 70), (65. 92. 8), 
(27. 64. 71), (58. 2. 9), (89. 33. 40), (35. 36. 52), (66. 67. 83), (4. 5. 21), 
(11. 15. 37), (42. 46. 68), (73. 77. 6), (16. 18. 41), (47. 49. 72), (78. 80. 10), 



CH. ix] kirkman's school-girls problem 207 

(23. 31. 69), (54. 62. 7), (85. 93. 38), (22. 63. 84). From each of the 16 triplets 
(1. 14. 42), (2. 16. 43), (3. 16. 44), (1. 33. 44), (2. 34. 45), (3. 35. 46), (1. 11. 30), 
(2. 12. 31), (3. 13. 32), (1. 6. 41), (2. 7. 42), (3. 8. 43), (1. 15. 35), (2. 16. 36), 
(3. 17. 37), an arrangement for one of the remaining 15 days is obtainable by a 
three-step cyclical permutation. 

Before leaving the subject of numbers of this type I give two other solutions 
of the case when n = 21, one to illustrate the use of the suffix notation, and the 
other a cyclical solution which is uniquely three-step. 

If we employ the suffix notation, the suffixes, with the type here used, are 
somewhat trying to read. Accordingly hereafter I shall write al, a2, ... , instead 
of Oj, Oa, .... In the case of 21 girls, an arrangement for the first day is 
(al. a2. a4), (61. 62. 64), (cl. c2. c4), (a3. 66. c5), (63. c6. a5), (c3. a6. 65), 
{al. 67. c7). From this, by one-step cyclical permutations of the suffixes, we 
get arrangements for the 2nd, 3rd, 4th, 5th, 6th and 7th days. The arrange- 
ment for the 8th day can be obtained from the triplet (al. 62. c4) by permuting 
the suffixes cyclically one step at a time. Similarly the arrangement for the 
9th day can be obtained from the triplet (61. c2. a4) and that for the lOfch day 
from the triplet (cl. a2. 64). Thus with seven suffixes we keep 7 for each sjonbol 
in one triplet, and every other triplet depends on one or other of only two 
arrangements, namely, (1. 2. 4), or (3. 6. 5). If the solution be written out 
at length the principle of the method used will be clear. 

Cyclical solutions which are uniquely three-step can also be obtained for 
numbers of the form 18/n + 3; in them the same triads can be used as 
before, but they are not placed at equal intervals in the circle. I give 21 girls 
as instance. The arrangements on the first 7 days can be obtained from the 
arrangement (1. 4. 10), (2. 20. 14), (15. 18. 3), (16. 17. 21), (8. 9. 13), (6. 7. 11), 
(5. 12. 19) by a three-step (but not by a one-step) cyclical permutation. From 
each of the triplets (1. 3. 11), (2. 4. 12), (3. 5. 13), an arrangement for one of 
the remaining 3 days is obtainable. 

Next, I consider those cases where n is of the form 18m -i- 9. Here, regular 
solutions in the suffix notation can be obtained in all cases except in that 
of 27 girls, but if the same solutions are expressed in the numerical notation, 
the triads are irregular. Accordingly, except when n=27, it is better to use the 
suffix notation. I will deal with the case when n=27 after considering the 
cases when n=45, 63, 81, 99. 

For 45 girls, an arrangement for the first day consists of the 5 triplets 
(al. al2. al3), (a2. a9. all), (a5. alO. 615), (a4. 63. c7), (a8. 66. cl4), and the 
10 analogous triplets, namely, (61. il2. 613), (cl. cl2. cl3), (62. 69. 611), 
(c2. c9. ell), (65. 610. cl5), (c5. clO. al5), (64. c3. a7), (c4. a3. 67), (68. c6. al4), 
(c8. a6. 614). From these, by one-step cyclical permutations of the suffixes, the 
arrangements for 15 days can be got. Each of the 2 triplets (c4. 63. a7), 
(c8.66.al4), the 4 analogous triplets, namely, (a4.c3.67), (64.a3.c7), (a8.c6,614), 
(68. a6. cl4), and the triplet (al. 61. cl), gives, by a one-step cyclical permuta- 
tion of the suffixes, an arrangement for one of the remaining 7 days. 

For 63 girls, an arrangement for tlie first day consists of the 7 triplets 
(al. alO. a9), (a5. a8. a3), (a4. al9. al5), (a7. al4. 621), (a20. 611. cl2), 
(a 16. 613. cl8), (al7. 62. c6), and the 14 analogous triplets. From these, by 



208 kirkman's school-girls problem [CH. IX 

one-step cyclical permutations of the suffixes, the arrangements for 21 days can 
be got. Each of the 10 triplets, consisting of the 3 triplets (c20. 611. al2), 
(cl6. blB, al8), {cl7. 62. a6), the 6 analogous triplets, and the triplet (al. 61. cl), 
gives, by a one-step cyclical permutation of the suffixes, an arrangement for 
one of the remaining 10 days. 

For 81 girls, an arrangement for the first day consists of the 9 triplets 
(a5. al. aS), (a3. alO. ali), (all. a21. a26), (a4. al2. a25), (a9. alS. 627), 
(a22. 620. cl9), (a24. 617. cl3), (al6. 66. cl), (a23. 615. c2), and the 18 analogous 
triplets. From these, by cyclical permutations of the suffixes, the arrangements 
for 27 days can be got. Each of the 13 triplets consisting of the 4 triplets 
(c22. 620. al9), {c24. 617. al3), (cl6. 66. al), (c23. 615. a2), the 8 analogous 
triplets, and the triplet {al. 61. cl), gives, by a cyclical permutation of the 
suffixes, an arrangement for one of the remaining 13 days. 

For 99 girls, an arrangement for the first day consists of the 11 triplets 
(al. a3. alO), (a2. a6. a20), (a4. al2. a7), (a8. a24. al4), (al6. alo. a28), 
(all. a22. 633), (a32. 630. c23), (a31. 627. cl3), (a29. 621. c26), (a25. 69. cl9), 
(al7. 618. c5), and the 22 analogous trij)lets. From these, by cyclical permuta- 
tions of the suffixes, the arrangements for 33 days can be got. Each of the 16 
triplets consisting of the 5 triplets (c32. 630. a23), (c31. 627. al3), (c29. 621. a26), 
(c25. 69. al9), (cl7. 618. a5), the 10 analogous triplets, and the triplet 
(al. 61. cl), gives, by a cyclical permutation of the suffixes, an arrangement for 
one of the remaining 16 days. 

For 27 girls, an arrangement of triplets for the first day is (1. 7. 10), (14. 17. 8), 
(27. 6. 9), (13. 20. 25), (11. 23. 18), (12. 16. 24), (21. 2. 4),''(15. 22. 26), (19. 3. 5). 
From this, three-step cyclical permutations give arrangements for 9 days in all. 
The first triplets on the remaining 4 days are (1. 2. 3), (1. 6. 20), (1. 11. 15), 
(1. 17. 27), from each of vs^hich, by a three-step cyclical permutation, an arrange- 
ment for one of those days is obtainable. 

Regular solutions in the numerical notation can also be obtained for all 
values of n, except 9, where n is of the form 18w + 9. I give 27 girls as 
an instance. The first day's arrangement is (1. 2. 4), (10. 11. 13), (19. 20. 22), 
(8. 15. 23), (17. 24. 5), (26. 6. 14), (3. 25. 9), (12. 7. 18), (21. 16. 27); from this, 
arrangements for 9 days in all are obtained by one-step cyclical permutations. 
Each of the three triplets (1. 5. 15), (2. 6. 16), (3. 7. 17) gives an arrangement 
for one day by a three-step cyclical permutation. Finally the triplet (1. 10. 19), 
represented by an equilateral triangle, gives the arrangement on the last day by 
a one-step cyclical permutation. 

Lastly, I consider those cases where n is of the form 18m + 15. As before, 
the solution is divided into two parts. In the first part, we obtain an arrange- 
ment of triplets for the first day, from which arrangements for 6m + 5 days are 
obtained by three-step cyclical permutations. In the second part, we obtain the 
first triplet on each of the remaining 3m -f 2 days, from which the other triplets 
on that day are obtained by three-step cyclical permutations. 

For 33 girls, an arrangement in the first part is (1. 13. 19), (23. 11. 5), 
(3. 16. 21), (2. 4. 7), (12. 14. 17), (28. 30. 33), (6. 16. 25), (10. 20. 29), (8. 18. 27), 
(24. 31. 32), (9. 22. 26). The triplets in the second part are (25. 32. 33), 
(26. 33. 1), (10. 23. 27), (11. 24. 28), (1. 12. 23). 



CH. JX] KIRKMAN's school-girls PROBLE\f 209 

For 51 girls, an arrangement in the first part is {17. 34. 51), (49. 16. 10), 
(11. 44. 50), (15. 33. 27), (19. 4. 46), (2. 38. 29), (21. 36. 45), (22. 25. 24), (5. 8. 7), 
(39. 42. 41), (43. 13. 20), (26. 47. 3), (9. 30. 37), (1. 14. 6), (35. 48. 40), (18. 31. 23), 
(28. 32. 12). The triplets in the second part are (29. 33. 13), (30. 34. 14), 
(1. 26. 12), (2. 27. 13), (3. 28. 14), (1. 11. 30), (2. 12. 31), (3. 13. 32). 

For 69 girls, an arrangement in the first part ia (23. 46. 69), (31. 34. 43), 
(11. 8. 68), (54. 57. 66), (58. 52. 28), (35. 29. 5), (45. 61. 6), (16. 1. 49), 
(62. 47. 26), (39. 24. 3), (4. 55. 42), (60. 32. 19), (27. 9. 65), (40. 67. 18), 
(17. 44. 64), (63. 21. 41), (10. 14. 15), (33. 37. 38), (56. 60. 61), (25. 53. 36), 
(2. 30. 13), (48. 7. 59), (22. 20. 12). The triplets in the second part are 
(23. 21. 13), (24. 22. 14), (1. 8. 33), (2. 9. 34), (3. 10. 35), (1. 17. 36), (2. 18. 37), 
(3. 19. 38), (1. 41. 15), (2. 42. 16), (3. 43. 17). 

For 87 girls, an arrangement in the first part is (29. 58. 87), (76. 82. 73), 
(47. 53. 44), (15. 9. 18), (70. 1. 13), (41. 59. 71), (12. 30. 42), (4. 67. 31), 
(2. 26. 62), (60. 84. 33), (40. 79. 25), (11. 50. 83), (69. 21. 54), (43. 64. 63), 
(14. 35. 34), (72. 6. 5), (61. 16. 77), (32. 74. 48), (3. 45. 19), (28. 68. 66), 
(37. 86. 39), (10. 8. 57), (46. 80. 36), (22. 65. 75), (7. 17. 51), (85. 23. 78), 
(52. 20. 27), (49. 66. 81), (65. 38. 24). The triplets in the second part are 
(56. 39. 25), (57. 40. 26), (1. 5. 42), (2. 6. 43), (3. 7. 44), (1. 29. 6), (2. 30. 7), 
(3. 31. 8), (1. 9. 20), (2. 10. 21), (3. 11. 22), (1. 14. 36), (2. 15. 37), (3. 16. 38). 

Before proceeding to the consideration of other methods I should add that 
it is also possible to obtain irregular solutions of cases where n is of any 
of these three forms. As an instance I give a three-step solution of 33 girls. 
A possible arrangement in the first part is (1. 4. 10), (14. 23. 26), (9. 15. 30), 
(3. 28. 33), (2. 6. 8), (11. 18. 27), (13. 24. 25), (5. IG. 20), (7. 17. 22), (21. 29. 31), 
(12. 19. 32). The triplets in the second part are (1. 2. 21), (1. 8. 30), (1. 3. 26), 
(1. 15. 20), (1. 17. 18). I describe this solution as irregular, since all, save one, 
of the triads used are different. 

The Focal Method. Another method of attacking the 
problem, comparatively easy to use in practice, is applicable 
when n is of the form 24m + 3p, where ^ = 65^ + 3. It is due 
to Eckenstein. Here it is convenient to use a geometrical 
representation by denoting 24m + 2p girls by equidistant 
numbered points on the circumference of a circle, and the 
remaining p girls by lettered points placed inside the circle; 
these p points are termed foci. The solution is in two parts. 
In the first part, we obtain an order from which the arrange- 
ments for 12m -\-p days are deducible by a two-step cycle 
of the numbers : in none of these triplets does more than 
one focus appear. In the second part, we find the arrange- 
ments for the remaining 3^' + 1 days ; here the foci and the 
numbered points are treated separately, the former being 
an. 14 



210 kirkman's school-girls problem [ch. IX 

arranged by any of the methods used for solving the case of 
6g' + 3 girls, while of the latter a typical triplet is used on each 
of those days, from which the remaining triplets on that day 
are obtained by cyclical permutations. 

This method covers all cases except when n=15, 21, 39; 
and solutions by it for all values of n less than 200 have been 
written out. Sets of all the triplets required can be definitely 
determined. One way of doing this is by finding the primitive 
roots of the prime factors of 4m + 23^ + 1, though in the simpler 
cases the triplets can be written down empirically without 
much trouble. An advantage of this method is that solutions 
of several cases are obtained by the same work. Suppose that 
we have arranged suitable triangles in a circle, having on 
its circumference 12m + p or 3c equidistant points, and let y 
be the greatest integer satisfying the indeterminate equation 
2a; + 4?/ -f- 1 = c, where x — or a? = 1, and a the highest multiple 
of 6 included in x-\-y, then solutions of not less than y + 1 — a 
cases can be deduced. Thus from a 27 circle arrangement 
where c = 9, 3/ = 2, a; = 0, a = 0, we can by this method deduce 
three solutions, namely when n = 57, 69, 81 ; from a 39 circle 
arrangement where c=13, 3/ = 3, x = 0, a = 0, we can deduce 
four solutions, namely when n = S\, 93, 105, 117. 

I have no space to describe the method fully, but I will give 
solutions for two cases, namely for 33 girls {n = 33, ?n = 1, p = 3) 
where there are 3 foci, and for 51 girls {n = 51, m = 1, p = 9) 
where there are 9 foci. 

For 33 girls we have 3 foci which we may denote by a, h, c, 
and 30 points which we may denote by the numbers 1 to 30 
placed at equidistant intervals on the circumference of a circle. 
Then if the arrangement on the first day is (a. 5. 10), (6. 20. 25), 
(c. 15. 30). (1. 2. 14), (16. 17. 29), (4 23. 20), (19. 8. 11), (9. 7. 3), 
(24. 22. 18), (6. 27. 13), (21. 12. 28), a two-step cyclical per- 
mutation of the numbers gives arrangements for 15 days; 
that on the second day being (a. 7. 12), (6. 22. 27), &c. The 
arrangement on the 16th day is (a. b. c), (1. 11. 21), (2. 12. 22), 
(3. 13. 23), . . (10 20. 30). 

For 51 girls we have 9 foci which we may denote by a, b, c, 



CH. ix] kirkman's school-girls problem 211 

dy e,f, g, h,j, and 42 points denoted by 1, 2, ... 42, placed at 
equidistant intervals on the circumference of a circle. Then if 
the arrangement on the first day is (a. 5. 6), (6. 26, 27), (c. 3. 10), 
(d. 24. 31), (e. 19. 34), (/. 40. 13), (g. 39. 16), (h. 18. 37), (j. 21. 42), 
(9. 11. 22), (30. 32. 1), (35. 41. 2), (14. 20. 23), (17. 29. 12), 
(38. 8. 33), (7. 15. 25), (28. 36. 4), a two-step cyclical per- 
mutation of the numbers gives arrangements for 21 days. 
Next, arrange the 9 foci in triplets by any of the methods 
already given so as to obtain arrangements for 4 days. From 
the numbers 1 to 42 we can obtain four typical triplets 
not already used, namely (1. 5. 21), (2. 6. 22), (3. 7. 23), 
(14. 28. 42). From each of these triplets we can, by a three- 
step cyclical permutation, obtain an arrangement of the 42 girls 
for one day, thus getting arrangements for 4 days in all. 
Combining these results of letters and numbers we obtain 
arrangements for the 4 days. Thus an arrangement for the 
first day would be (a. c.j), (b. d. g), {e.f. h\ (1. 5. 21), (4. 8. 24), 
(7. 11. 27), (10. 14. 30), (13. 17. S3), (16. 20. 36), (19. 23. 39), 
(22. 26. 42), (25. 29. 3), (28. 32. 6), (31. 35. 9), (34. 38. 12), 
(37. 41. 15), (40. 2. 18). For the second day the corresponding 
arrangement would be {d. f. c), (e. g. a), (A. j. b), (2. 6. 22), 
(5. 9. 25). &c. 

Analytical Methods. The methods described above, under 
the headings One-Step, Two-Step and Three-Step Cycles, involve 
some empirical work. It is true that with a little practice 
it is not difficult to obtain solutions by them when w is a low 
number, but the higher the value of n the more troublesome 
is the process and the more uncertain its success. A general 
arithmetical process has, however, been given by which it is 
claimed that some solutions for any value of n can be always 
obtained. Most of the solutions given earlier in this chapter 
can be obtained in this way. 

The essential feature of the method is the arrangement of 
the numbers by which the girls are represented in an order 
such that definite rules can be laid down for grouping them in 
pairs and triplets so that the differences of the numbers in each 
pair or triplet either are all different or are repeated as often as 

14—2 



212 KIRKMAN's school-girls problem [cH, IX 

may be required. The process depends on finding primitive roots 
of the prime factors of whatever number is taken as the base 
of the solution. When (n — 1)/2 is prime, of the form 6m + 1, 
and is taken as base, the order is obtainable at once, and the 
rules for grouping the numbers are easy of application ; owing 
to considerations of space I here confine myself to such instances, 
but similar though somewhat longer methods are applicable 
to all cases. 

I use the geometrical representation already explained. 
We have n—2y-i-l, and i/ is a prime of the form 6u + 1. In 
forming the triplets we either proceed directly by arranging all 
the points in threes, or we arrange some of them in pairs and 
make the selection of the third point dependent on those of the 
two first chosen, leaving only a few triplets to be obtained 
otherwise. In the former case we have to arrange the numbers 
in triplets so that each difference will appear twice, and so that 
no two differences will appear together more than once. In 
the latter case we have to arrange the numbers so that the 
differences between the numbers in each pair comprise con- 
secutive integers from 1 upwards and are all different. In both 
cases, we commence by finding a primitive root of y, say oo. The 
residues to the modulus y of the 6u successive powers of x form 
a series of numbers, el, e2, eS,..., comprising all the integers from 
1 to 6u, and when taken in the order of the successive powers, 
they can be arranged in the manner required by definite rules. 

I will apply the method to the case of 27 girls from which 
the general theory, in the restricted case where y is a prime 
of the form 6u + l, will be sufficiently clear. In this case 
we have n=27, 2/= 13, u = 2y and x = 2. I take 13 as the 
base of the analysis. I will begin by pairing the points, and 
this being so, it is convenient to represent the girls by a point 
k at the centre of a circle and points al, 61, a2, 62, ... at 
equidistant intervals on the circumference. We reserve k, al3, 
613 for one triplet, and we have to arrange the other 24 points 
so as to form 8 triangles of certain types. The residues are in 
the order 2, 4, 8, 3, 6, 12, 11, 9, 5, 10, 7, 1, and these may be 
taken as the suffixes of the remaining ' a 's and ' 6 s. 



CH. ix] kirkman's school-girls problem 213 

First arrange these residues in pairs so that every difference 
between the numbers in a pair occurs once. One rule, by which 
this can be effected, is to divide the residues into two equal 
sections and pair the numbers in the two sections. This gives 
(2. 11), (4. 9), (8. 5), (3. 10), (6. 7), (12. 1) as possible pairs. 
Another such rule is to divide the residue into six equal 
sections, and pair the numbers in the first and second sections, 
those in the third and fourth sections, and those in the fifth 
and sixth sections. This gives (2. 8), (4. 3), (6. 11), (12. 9), 
(5. 7), (10. 1) as possible pairs. Either arrangement can be 
used, but the first set of pairs leads only to scalene triangles. 
In none of the pairs of the latter set does the sum of the 
numbers in a pair add up to 13, and since this may allow the 
formation of isosceles as well as of scalene triangles, and thus 
increase the variety of the resulting solutions, I will use the 
latter set of pairs. We use these basic pairs as suffixes of the 
* a *s, and each pair thus determines two points of one of the 
triangles required. We have now used up all the *a*s. The 
third point associated with each of these six pairs of points 
must be a ' 6/ and the remaining six * b 's must be such that 
they can be arranged in suitable triplets. 

Next, then, we must arrange the 6u residues el, e2, eS, ... in 
possible triplets. To do this arrange them cyclically in triplets, 
for instance, as shown in the first column of the left half of the 
annexed table. We write in the second column the differences 
between the first and second numbers in each triplet, in the 
third column the differences between the second and third 
numbers in each triplet, and in the fourth column the differences 
between the third and first numbers in each triplet. If any 
of these differences d is greater than Su we may replace it 
by the complementary number y — d: that this is permissible 
is obvious from the geometrical representation. By shifting 
cyclically the symbols in any vertical line in the first column we 
change these differences. We can, however, in this way always 
displace the second and third vertical lines in the first column 
so that the numbers in the second, third, and fourth columns 
include the numbers 1 to 3u twice over. This can be effected 



214 



kirkman's school-girls problem 



[CH. IX 



thus. If any term in the residue series is greater than Su re- 
place it by its complementary number y — e. In this way, from 
the residue series, we get a derivative series dl, (Z2, dS, ... such 
that any Su consecutive terms comprise all the integers from 
1 to 3m. The first half of this series may be divided into three 
equal divisions thus: (1) dl, c?4, d*7y ... ; (2) d2, do, dS, ...; 
(3) dS, d6, d9, .... If the displacement is such that the first 
numbers in the second, third, and fourth columns are contained 
in different divisions, each difference must occur twice, and it 
will give a possible solution. Other possible regular arrange- 
ments give other solutions. Applying this to our case we have 
the residue series, 2, 4, 8, 3, 6, 12, 11, 9, 5, 10, 7, 1. The 
derivative series is 2, 4, 5, 3, 6, 1, 2, 4, 5, 3, 6, 1. The three 
divisions are (i) 2, 3 ; (ii) 4, 6 ; (iii) 5, 1. The cyclical arrange- 
ment we started with and the consequent differences are shown 
in the left half of the accompanying table. A cyclical change 



2. 4. 8 


2 


4 


6 


2. 6. 5 


4 


1 


3 


3. 6. 12 


3 


6 


4 


3. 9. 1 


6 


5 


2 


11. 9. 5 


2 


4 


6 


11. 7. 8 


4 


1 


3 


10. 7. 1 


3 


6 


4 


10. 4. 12 


6 


5 


2 



as described above of the vertical lines of the symbols in the 
first column gives the arrangement in the right half of the 
table. Here each difference occurs twice and accordingly this 
gives a possible arrangement of the triplets, namely, 2, 6, 5 ; 
3, 9, 1 ; 11, 7, 8; 10, 4, 12. These are the suffixes of the *6's. 
We have now to use six of these '6's in connection with the 
basic ' a 's already determined, keeping the other six ' b 's for 
the remaining two triangles. 

For instance we may obtain a scalene solution by taking as 
a suffix of the * h ' associated with any pair of * a 's, a number 
equal to the sum of the suffixes of the ' a 's. We thus get as a 
solution (a2. aS. 610), (a4. aS. 67), (a6. all. 64), (al2. a9. 68), 
(a5. a1, 612), (alO. al. 611), (62. 66. 6 5), (63. 6 9. 61), 
{k a 13. 613). Or we might take as a suffix of the '6 ' associated 
with any pair of ' a's, the number midway between the suffixes 



CH. ix] kirkman's school-girls problem 215 

of the * a 8 on the 13 circle. This gives the following solution, 
in which the first six triangles are isosceles: (a2. a8. 65), 
(a4>. aS. 610), (ae. all. 62), (al2. a9. 64), (a5. a1. 66), 
(alO. al. 612), (63. 69. 61), (611. 67. 68), (k. al3. 613). 

In the case of 27 girls, we may equally well represent the 
points by k at the centre of the circle and 26 equidistant points 
1, 2, ... 26 on the circumference. The points previously denoted 
by a and 6 with the suffix h are now denoted by the numbers 
2^—1 and 2h. Hence the basic pairs (2. 8), (4. 3), ...become 
(3. 15), (7. 5), (11. 21), (23. 17), (9. 13), (19. 1), and the corre- 
sponding scalene arrangement for the first day is (3. 15. 20), 
(7. 5. 14), ... (k. 25. 26). From this by a two-step cyclical 
permutation of the numbers, an arrangement for 13 days can 
be got. 

The case of 27 girls can also be treated by the direct 
formation of triplets. The triplets must be such that each 
difference is represented twice, but so that the groups of 
differences are different. There are analytical rules for forming 
such triplets somewhat analogous to those I have given for 
forming basic pairs, but their exposition would be lengthy, 
and I will not discuss them here. One set which will answer 
our purpose is (1. 12. 5), (2. 3. 10). (4. 6. 9), (8. 11. 7), giving 
respectively the differences [2, 6, 4], [1, 6, 5], [2, 3, 5], [3, 4, 1]. 
Now every difference c? in a 13 circle will correspond to d 
or 13 — ci in a 26 circle, and every residue e in a 13 circle 
will correspond to e or IS + e in a 26 circle. Further, a 
triplet in the 26 circle must either have three even differences, 
or one even and two odd differences. Hence from the above 
sets we can get the following arrangement for the first day, 
(1.25.5) and (14.12.18) with differences [2,6,4], (15.3.10) 
and (2. 16. 23) with differences [12, 7, 5], (17. 6. 9) and (4. 19. 22) 
with differences [11, 3, 8], (21. 11. 20) and (8. 24. 7) with 
differences [10, 9, 1], and {k. 13. 26). From this by either a 
one-step or a two-step cyclical permutation of the numbers, an 
arrangement for 13 days can be got. I will not go into further 
details about the deduction of other similar solutions. A similar 
method is always applicable when n is of the form 24?/i + 3. 



216 



kirkman's school-girls problem 



[CH. IX. 



The process by pairing when y is a prime of the form 6i^+l 
is extremely rapid. For instance, in the case of 16 girls we 
have n — 15, 3/ = 7, ^t = 1, a; = 5. The order of the residues is 
6, 4, 6, 2, 3, 1. By our rule we can at once arrange basic pairs 
(5. 4), (6. 2), (3. 1). From these pairs we can obtain numerous 
solutions. Thus using scalene triangles as above explained, we 
get as an arrangement for the first day (a 5, a4, 62), {aQ. a2. 61), 
(a3. al. 64), (63. 65. 66), (k a1. 67), from which by a one-step 
cyclical permutation of the numbers, arrangements for the seven 
days can be obtained. Using the basic pairs as bases of 
isosceles triangles, we get as an arrangement for the first day 
(a5. a4. 61), (aQ. a2. 64), (a3. al. 62), (63. 65. 66), {h al. 67). 

Again, take the case of 39 girls. Here we have n = 39, 
2/= 19, w=3, a;=3. The order of the residues is 3, 9, 8; 5, 15, 7; 

2, 6, 18 ; 16, 10, 11 ; 14, 4, 12 ; 17, 13, 1. The basic pairs are 
(3. 5), (9. 15), (8. 7), (2. 16), &c. These are the suffixes of the 
*a's. The possible triplets which determine what '6's are to 
be associated with these, and what '6's are to be left 
for the remaining three triangles, can be determined as 
follows : From the residue series we obtain the derivative series 

3, 9, 8, 5, 4, 7, 2, 6, 1, &c. The divisions are (i) 3, 5, 2; (ii) 9, 4, 6; 
(iii) 8, 7, 1. A cyclical arrangement like that given above 
leads to the result in the left half of the annexed table which 
does not satisfy our condition. A cyclical displacement of the 
symbols in the vertical lines in the first column leads to the 
arrangement given in the right half of the table, and shows 



3. 9. 8 


6 


2 


5 


3. 15. 18 


7 


3 


4 


5. 15. 7 


9 


8 


2 


5. 6. 11 


1 


5 


6 


2. 6. 18 


4 


7 


3 


2. 10. 12 


8 


2 


9 


16. 10. 11 


6 


1 


5 


16. 4. 1 


7 


3 


4 


14. 4. 12 


9 


8 


2 


14. 13. 8 


1 


5 


6 


17. 13. 1 


6 


7 


3 


17. 9. 7 


8 


2 


9 



that (3. 15. 18), (5. 6. 11), &c. are possible triplets. From 
these results numerous solutions can be deduced in the same 
way as above. For instance one solution is (a 3. a 5. 64), 



CH. ix] kirkman's school-girls problem 217 

(a9. aU. 612), (aH. a7. 617), (a2. al6. 6 9), (a6. alO. 68), 
(al8. all. 65), (al4. a] 7. 66), (a 4. al3. 618), (al2. al. 61G), 
(63. 610. 61), (62. 613. 67), (614. 615. 611), (k al9, 619). 

In the case of 39 girls we may also extend the method 
used above by \vhich for 27 girls we obtained the solution 
(1. 25. 5), (14. 12. 18), .... We thus get a solution for 39 girls 
as follows: (1.25.18), (14.38.31), (27.12.5); (15.16.10), 
(28. 29. 23), (2. 3. 36); (17. 19. 35), (30. 32. 9), (4. 6. 22); 
(21. 24. 33), (34. 37. 7), (8. 11. 20); (13. 26. 39). From this 
the arrangements for the first 13 days are obtained either by 
a one-step or a three-step cyclical permutation of the numbers. 
The single triplets from each of which an arrangement for one 
of the other six days is obtainable are (1. 5. 15), (2. 6. 16), 
(3. 7. 17); (1. 9. 20), (2. 10. 21), (3. 11. 22). From each of 
these the arrangement for one day is obtainable by a three-step 
cyclical permutation of the numbers. 

These examples of the use of the Focal and Analytical 
Methods are given only by way of illustration, but they will 
serve to suggest the applications to other cases. When the 
number taken as base is composite, the formations of the series 
used in the Analytical Method may be troublesome, but the 
principle of the method is not afifected, though want of space 
forbids my going into further details. Eckenstein, to whom 
the development of this method is mainly due, can, with the 
aid of a table of primitive roots and sets of numbers written on 
cards, within half an hour obtain a solution for any case in 
which n is less than 500, and can within one hour obtain a 
solution for any case in which n lies between 500 and 900. 

Number of Solutions, The problems of 9 and of 15 girls 
have been subjected to an exhaustive examination. It has 
been shown that all solutions of the problem of 9 girls are 
reducible to one type, and that the number of independent 
solutions is 840, where, however, any arrangement on Monday, 
Tuesday, Wednesday and Thursday, and the same arrangement 
on (say) Monday, Tuesday, Thursday and Wednesday are regarded 
as identical. [If they are regarded as different the number of 
possible independent solutions is 20,160.] The total number of 



218 KIRKMAN'S school-girls problem [cH. IX 

possible arrangements of the girls in triplets for four days is 
(280)Y4 ! ; hence the probability of obtaining a solution by a 
chance arrangement is about 1 in 300,000. 

All solutions of the problem of 15 girls are reducible to one 
of eleven types, distinguished by the number of cycles required 
for expressing them. The number of independent solutions is 
said to be 65 x (13 !), but I do not vouch for the correctness of the 
result. The total number of ways in which the girls can walk 
out for a week in triplets is (455)' ; so the probability that any 
chance way satisfies the condition of the problem is very small. 

Harisons Theorem. If we know solutions for Kirkman s 
Problem for 3^ girls and for 3m girls we can find a solution 
for Zlm girls. The particular case of this when ^ = 1 was 
established by Walecki and given in the earlier editions of 
this work. Harison's proof of the more general theorem is 
as follows: — 

If the school-girls be denoted by the consecutive numbers 
from 1 to Zlm and the numbers be divided into 3^ sets, each of 
m consecutive numbers, each of these sets can, by the method 
for the 3Z problem, be divided in (3^ — 1)/2 collections of groups 
of three sets, so that every set shall be included once in the 
same group with every other set. 

In one of these collections, each group of three sets (in- 
volving 3m numbers) can, by the method for the 3m problem, 
be arranged in triplets for (3m — l)/2 days, so as to have each 
number in each of the sets composing the group included once 
in the same triplet with every other number in the set to which 
it belongs and with every number in the other two sets in the 
group. This will give arrangements of all the numbers for that 
number of days. 

In the remaining (3Z — 3)/2 collections, each group can be 
arranged in triplets for m days, so as to have each number in 
each of the sets composing the group included once in the 
same triplet with each number of the other two sets. In the 
first arrangement in each collection, the first triplet in each 
group is composed of the first number of each set. In the 
second arrangement, the first triplet is composed of the first 



CH. ix] kirkman's school-girls problem 219 

number of the first set, the second number of the second set, 
and the last number of the third set. In every other arrange- 
ment the first triplet is formed from the first triplet in the next 
proceeding arrangement, by adding unity to the number of the 
second set, subtracting unity from the number of the third set, 
and leaving the number of the first set unchanged. In every 
arrangement the second and all subsequent triplets are formed 
cyclically from the next preceding triplet. 

The arrangements thus made of the numbers in all the 
groups of all the collections will give arrangements of all the 
numbers for (Sim — 1)/2 days, and will provide a solution of 
the problem. 

Harison's Theorem provides solutions, alternative to those 
given above, for the cases when n = 27, 45, 63, 75, 81, 99. 
Also we can, by it, from the solutions already given for all 
values of n less than 100, at once deduce solutions for the 
cases when n= 105, 117, 135, 147, 153, 165, 171, 189, 195, &c. 

Extension to n^ Girls. Peirce suggested the corresponding 
problem of arranging 'n? girls in n groups, each group containing 
n girls, on 71 + 1 days so that no two girls will be together in 
a group on more than one day. We may conveniently represent 
the girls by a point k at the centre of a circle and n"^ — 1 equi- 
distant points, numbered 1, 2, 3, ... , on the circumference. 

When n — 2, we may arrange initially the 4 points in two 
pairs, one pair consisting of k and one of the points, say, 3, 
and the other pair of the remaining points (1. 2). These two 
pairs give the arrangement for the first day. From them, 
the solution for the other days is obtained by one- step cyclical 
permutations. 

When n = 3, we may arrange initially the 9 points in three 
triplets, namely, k and the ends of a diameter {k. 4. 8); a 
triangle (1. 2. 7) ; and the similar triangle (5. 6. 3) obtained by 
a four-step cyclical permutation. These three triplets give an 
arrangement for the first day. From them the solutions for 
the other days are obtained by one-step cyclical permutations. 

When n = 4, we may arrange initially the 16 points in four 
quartets, namely, k and three equidistant points, (A;. 5. 10. 15) ; 



220 KIRKMAN's school-girls problem [CH. IX 

a quadrilateral (1. 2. 4. 8) ; and the two similar quadrilaterals 
(6. 7. 9. 13) and (11. 12. 14. 3) obtained by five-step cyclical 
permutations. These four quartets give an arrangement for 
the first day. From them the solutions for the other days are 
obtained by one-step cyclical permutations. 

When n = 5, we may arrange initially the 25 points in five 
quintets, namely, k and four equidistant points, {k. 6. 12. 18. 24); 
a pentagon (2. 3. 5. 13. 22); and the three similar pentagons 
(8. 9. 11. 19. 4), (14. 15. 17. 1. 10), (20. 21. 23. 7. 16) obtained 
by six-step cyclical permutations. These five quintets give 
an arrangement for the first day. From them the solutions for 
the other days are obtained by one-step cyclical permutations. 
There is a second solution (k. 6. 12. 18. 24), (1. 2. 15. 17. 22), &c. 

Hitherto the case when n = 6 has bafiled all attempts to 
find a solution. 

When n = *7, we may initially arrange the 49 points in 
seven groups, namely {k. 8. 16. 24. 32. 40. 48), a group 
(1. 2. 5. 11. 31. 36. 38) and five similar groups obtained 
by successive additions of 8 to these numbers. There are 
three other solutions: in these the second group is either 
(2. 3. 17. 28. 38. 45. 47), or (3. 4. 6. 18. 23. 41. , 45), or 
(3. 4. 14. 17. 26. 45. 47). 

When n = 8 there are three solutions, due to Eckenstein. 
If the first group is (k. 9. 18. 27. 36. 45. 54. 63), the second 
group is either (1.2.4.8.16.21.32.42), or (2.3.16.22.24.50.55.62), 
or (3. 4. 7. 19. 24. 26. 32. 56) : the other groups in each solution 
being obtained by successive addition of 9 to the numbers in 
the second group. 

When n is composite no general method of attacking the 



M, /c2, 

al, «2, 

&1, &2, 


kn. 

an. 

K 


M, al, 62, 
k2, a2, 64, 
kd, aS, 66, 


c3, 


c6, 

c9, 






kn, an, hn. 


fn 






Arrangement on 


First Day 


Arrangement on 


I Second Day 



I 



CH. ix] kirkman's school-girls problem 221 

problem has been discovered, though solutions for various par- 
ticular cases have been given. But when n is prime, we can 
proceed thus. Denote the n^ girls by the suffixed letters shown 
in the left half of the above table. Take this as giving the 
arrangement on the first day. Then on the second day we may 
take as an arrangement that shown in the right half of the 
table. From the arrangement on the second day, the arrange- 
ments for the other days are obtained by one-step cyclical 
permutations of the suffixes of a, b, &c.; the suffixes of k being 
unaltered. 

Kirkmans Problem in Quartets. The problem of arranging 
4mi girls, where m is of the form 3?i + 1, in quartets to walk out 
for (4m — l)/3 days, so that no girl will walk with any of her 
school-fellows in any quartet more than once has been attacked. 
Methods similar to those given above are applicable, and solu- 
tions for all cases where m does not exceed 31, have been written 
out. Analogous methods seem to be applicable to corresponding 
problems about quintets, sextets, &c. 

Bridge Problem. Another analogous question is where we deal 
with arrangements in pairs instead of triplets. One problem of 
this kind is to arrange 4m members of a bridge club for 4m — 1 
rubbers so that (i) no two members shall play together as 
partners more than once, and (ii) each member shall meet every 
other member as opponent twice. The general theory has been 
discussed by E. H. Moore and 0. Eckenstein. A typical method 
for obtaining cyclic solutions is as follows. Denote the members 
by a point k at the centre of a circle and by 4m — 1 equidistant 
points, numbered 1, 2, 3, . . ., on the circumference. We can join 
the points 2, 3, 4, ... by chords, and these chords with (k. 1) 
give possible partners at the m tables in the first rubber. 
A one-step cyclical permutation of the numbers will give the 
arrangements for the other rubbers if, in the initial arrange- 
ment, (i) the lengths of the chords representing every pair of 
partners are unequal and thus appear only once, and (ii) the 
lengths of the chords representing every pair of opponents 
appear only twice. Since the chords representing pairs of 
partners are unequal, their lengths are uniquely determined, but 



222 kirkman's school-girls problem [ch. ix 

the selection of the chords is partly empirical. In the following 
examples for m = 2, 3, 4, 1 give an arrangement of the card 
tables for the first rubber: the arrangements for the subsequent 
rubbers being thence obtained by one-step cyclical permuta- 
tions of the numbers. If m = 2, such an initial arrangement 
is {k. 1 against 5. 6) and (2. 4 against 3. 7). If m = 3, one such 
initial arrangement is {k. 1 against 5. 6), (2. 11 against 3. 9) 
and (4. 8 against 7. 10). If m = 4, one such initial arrange- 
ment is {k. 1 against 6. 11), (2. 3 against 5. 9), (4. 12 against 
13. 15), and (7. 10 against 8. 14). There are also solutions by 
other methods. 

Sylvesters Corollary. To the original theorem J. J. Sylvester 
added the corollary that the school of 15 girls could walk out in 
triplets on 91 days until every possible triplet had walked abreast 
once, and he published a solution in 1861. 

The generalized problem of finding the greatest number of 
ways in which x girls walking in rows of a abreast can be 
arranged so that every possible combination of h of them may 
walk abreast once and only once has been solved for various 
cases. Suppose that this greatest number of ways is y. It is 
obvious that, if all the x girls are to walk out each day in rows 
of a abreast, then x must be an exact multiple of a and the 
number of rows formed each day is xja. If such an arrangement 
can be made for z days, then we have a solution of the problem 
to arrange x girls to walk out in rows of a abreast for z days so 
that they all go out each day and so that every possible com- 
bination of h girls may walk together once and only once. 

An example where the solution is obvious is if x = 2n, 
a — 2, 6 = 2, in which case y = n (2n — 1), z = 2n— 1. If we 
take the case ic = 15, a = 3, h = 2, we find 3/ = 35; and it 
happens that these 35 rows can be divided into 7 sets, each 
of which contains all the symbols ; hence z = 7. More generally, 
ifa? = 5x3"*, a = 3, 6 = 2, we find y = S(x-l)l2x,z=-{x-l)/2. 
It will be noticed that in the solutions of the original fifteen 
school-girls problem and of Walecki's extension of it given above 
every possible pair of girls walk together once ; hence we might 
infer that in these cases we could determine z as well as y. 



CH. ix] kirkman's school-girls problem 223 

The results of the last paragraph were given by Kirkman 
in 1850. In the same memoir he also proved that, if a; = 9, 
a=3, b = S, then y=84, ^ = 28; and if a;=15, a = 3, 6 = 3, 
y=455, 2" = 91, but some of the extensions he gave are not 
correct. He showed, however, how 9 girls can be arranged to 
walk out 28 times (say 4 times a day for a week) so that in any 
day the same pair never are together more than once and so 
that at the end of the week each girl has been associated with 
every possible pair of her school-fellows. Three years later 
he attacked the problem when ar = 2", a = 4, 6=3, but his 
analysis is open to objection. In 1867 S. Bills showed that 
if 37=15, a =3, 6 = 2, then 3/ = 35: and the method used 
will give the value of y, if ^ = 2^-1, a =3, 6 = 2. Shortly 
afterwards W. Lea showed that if a?= 16, a =4, 6 = 3, then 
y= 140. These writers did not confine their discussion to cases 
where x is an exact multiple of a. 

Steiner's Prohlem. I should add that Kirkman's problem, but 
in a somewhat more general form, was proposed independently 
by J. Steiner in 1852, and, as enunciated by him, is known as 
Steiner's Combinatorische Aufgahe. In substance Steiner sought 
to find for what values of n it is possible to arrange n things in 
triplets, so that every pair of things is contained in one and only 
one triplet: any triplet forming part of such an arrangement 
is called a triad. Also, if n is a number for which such an 
arrangement can be formed, he asked whether there are other 
arrangements that cannot be obtained from it by permutations 
of the things. Other problems proposed by him are as follows. 
When such an arrangement of triads has been made, is it 
possible to arrange the n things in sets of four, called tetrads, 
so that no one of the triads is contained in any tetrad, but 
every triplet which is not a triad is contained in one and only 
one teUad ? And generally when an arrangement in yfc-ads 
has been made, is it possible to arrange the things in sets of 
(k -\- l)-ads so that no A-ad, where h :^ k, is obtained in a 
{k + l)-ad, and so that every k-\et that is not or does not 
contain an A-ad is contained in one and only one (k + l)-ad ? 



224 



CHAPTER X. 

MISCELLANEOUS PROBLEMS. 

I propose to discuss in this chapter the mathematical theory 
of a few common mathematical amusements and games. I 
might have dealt with them in the first four chapters, but, since 
most of them involve mixed geometry and algebra, it is rather 
more convenient to deal with them apart from the problems 
and puzzles which have been described already; the arrange- 
ment is, however, based on convenience rather than on any 
logical distinction. 

The majority of the questions here enumerated have no 
connection one with another, and I jot them down ahnost at 
random. 

I shall discuss in succession the Fifteen Puzzle^ the Tower 
of Hanoi, Chinese Rings, and some miscellaneous Problems 
connected with a Pack of Cards, 

The Fifteen Puzzle*. Some years ago the so-called 
Fifteen Puzzle was on sale in all toy- shops. It consists of a 
shallow wooden box — one side being marked as the top — in the 
form of a square, and contains fifteen square blocks or counters 
numbered 1, 2, 3, ... up to 15. The box will hold just sixteen 
such counters, and, as it contains only fifteen, they can be 
moved about in the box relatively to one another. Initially 
they are put in the box in any order, but leaving the sixteenth 

* There are two articles on the subject in the American Journal of 
Mathematics, 1879, vol. ii, by Professors Woolsey Johnson and Storey ; but 
the whole theory is deducible immediately from the proposition I give in 
the text. 



CH. X] 



MISCELLANEOUS PROBLEMS 



225 



cell or small square empty; the puzzle is to move them so 
that finally they occupy the position shown in the fii'st of the 
annexed figures. 

n Top of Box C 



Li 1 2 1 


3 


sl 

.;,l 


r 1 

5 ^ , 6 1 


9 


10 

■ „.. 1 

14 


15i 



A Bottom of Box B 




!!■■ 



SL^r^^ra 



1011 9ii12i11;i 






We may represent the various stages in the game by sup- 
posing that the blank space, occupying the sixteenth cell, is 
moved over the board, ending finally where it started. 

The route pursued by the blank space may consist partly of 
tracks followed and again retraced, which have no effect on the 
arrangement, and partly of closed paths travelled round, which 
necessarily are cyclical permutations of an odd number of 
counters. No other motion is possible. 

Now a cyclical permutation of n letters is equivalent to 
n—1 simple interchanges ; accordingly an odd cyclical permu- 
tation is equivalent to an even number of simple interchanges. 
Hence, if we move the counters so as to bring the blank space 
back into the sixteenth cell, the new order must differ from 
the initial order by an even number of simple interchanges. If 
therefore the order we want to get can be obtained from this 
initial order only by an odd number of interchanges, the 
problem is incapable of solution ; if it can be obtained by an 
even number, the problem is possible. 

Thus the order in the second of the diagrams given 
above is deducible fi-om that in the first diagram by six 
interchanges ; namely, by interchanging the counters 1 and 2, 

15 



B. B. 



226 MISCELLANEOUS PROBLEMS [CH. X 

3 and 4, 5 and 6, 7 and 8, 9 and 10, 11 and 12. Hence the one 
can be deduced from the other by moving the counters about 
in the box. 

If however in the second diagram the order of the last 
three counters had been 13, 15, 14, then it would have required 
seven interchanges of counters to bring them into the order 
given in the first diagram. Hence in this case the problem 
would be insoluble. 

The easiest way of finding the number of simple inter- 
changes necessary in order to obtain one given arrangement 
from another is to make the transformation by a series of cycles. 
For example, suppose that we take the counters in the box in 
any definite order, such as taking the successive rows from left 
to right, and suppose the original order and the final order to 
be respectively 

1, 13, 2, 3, 5, 7, 12, 8, 15, 6, 9, 4, 11, 10, 14, 
and 11, 2, 3, 4, 5, 6, 7, 1, 9, 10, 13, 12, 8, 14, 15. 

We can deduce the second order from the first by 12 simple 
interchanges. The simplest way of seeing this is to arrange the 
process in three separate cycles as follows : — 



1, 11, 8; 


13, 2, 3, 4, 12, 7, 6, 10, 14, 15, 9; 


5. 


11, 8, 1; 


2, 3, 4, 12, 7, 6, 10, 14, 15, 9, 13 ; 


5. 



Thus, if in the first row of figures 11 is substituted for 1, then 
8 for 11, then 1 for 8, we have made a cyclical interchange of 
3 numbers, which is equivalent to 2 simple interchanges (namely, 
interchanging 1 and 11, and then 1 and 8). Thus the whole 
process is equivalent to one cyclical interchange of 3 numbers, 
another of 11 numbers, and another of 1 number. Hence it is 
equivalent to (2 + 10 + 0) simple interchanges. This is an even 
number, and thus one of these orders can be deduced from the 
other by moving the counters about in the box. 

It is obvious that, if the initial order is the same as the 
required order except that the last three counters are in the 
order 15, 14, 13, it would require one interchange to put them 
in the order 13, 14, 15 ; hence the problem is insoluble. 

If however the box is turned through a right angle, so as 



CM. X] MISCELLANEOUS PROBLEMS 227 

to make AD the top, this rotation will be equivalent to 13 
simple interchanges. For, if we keep the sixteenth square 
always blank, then such a rotation would change any order 
such as 

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 
to 13, 9, 5, 1, 14, 10, 6, 2, 15, 11, 7, 3, 12, 8, 4, 
which is equivalent to 13 simple interchanges. Hence it will 
change the arrangement from one where a solution is impossible 
to one where it is possible, and vice versa. 

Again, even if the initial order is one which makes a 
solution impossible, yet if the first cell and not the last is left 
blank it will be possible to arrange the fifteen counters in their 
natural order. For, if we represent the blank cell by 6, this 
will be equivalent to changing the order 

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 6, 
to 6, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15: 
this is a cyclical interchange of 16 things and therefore is 
equivalent to 15 simple interchanges. Hence it will change 
the arrangement from one where a solution is impossible to 
one where it is possible, and vice versa. 

So, too, if it were permissible to turn the 6 and the 9 upside 
down, thus changing them to 9 and 6 respectively, this would be 
equivalent to one simple interchange, and therefore would change 
an arrangement where a solution is impossible to one where 
it is possible. 

It is evident that the above principles are applicable equally 
to a rectangular box containing mn cells or spaces and mn — 1 
counters which are numbered. Of course m may be equal to n. 
If such a box is turned through a right angle, and m and n are 
both even, it will be equivalent to mn — 3 simple interchanges — 
and thus will change an impossible position to a possible one, 
and vice versa — but unless both m and n are even the rotation 
is equivalent to only an even number of interchanges. Similarly, 
if either m or w is even, and it is impossible to solve the problem 
when the last cell is left blank, then it will be possible to solve 
it by leaving the first cell blank. 

15-2 



228 MISCELLANEOUS PROBLEMS [CH. X 

The problem may be made more difficult by limiting the 
possible movements by fixing bars inside the box which will 
prevent the movement of a counter transverse to their directions. 
We can conceive also of a similar cubical puzzle, but we could 
not work it practically except by sections. 

The Tower of Hanoi. I may mention next the ingenious 
puzzle known as the Tower of Hanoi. It was brought out in 
1883 by M. Glaus (Lucas). 

It consists of three pegs fastened to a stand, and of eight 
circular discs of wood or cardboard each of which has a hole in 
the middle through which a peg can be passed. These discs 
are of different radii, and initially they are placed all on one 
peg, so that the biggest is at the bottom, and the radii of the 
successive discs decrease as we ascend : thus the smallest disc 
is at the top. This arrangement is called the Tower. The 
problem is to shift the discs from one peg to another in such 
a way that a disc shall never rest on one smaller than itself, 
and finally to transfer the tower {i.e. all the discs in their proper 
order) fi:om the peg on which they initially rested to one of the 
other pegs. 

The method of effecting this is as follows, (i) If initially 
there are n discs on the peg A, the first operation is to transfer 
gradually the top n—1 discs from the peg A to the peg B, 
leaving the peg G vacant : suppose that this requires x separate 
transfers. (ii) Next, move the bottom disc to the peg G. 
(iii) Then, reversing the first process, transfer gradually the 
w — 1 discs from B to (7, which will necessitate x transfers. 
Hence, if it requires x transfers of simple discs to move a tower 
oi n — 1 discs, then it will require 2x-\-l separate transfers of 
single discs to move a tower of n discs. Now with 2 discs it 
requires 3 transfers, i.e. 2^—1 transfers ; hence with 3 discs the 
number of transfers required will be 2 (2^ - 1) + 1, that is, 2» - 1. 
Proceeding in this way we see that with a tower of n discs it 
will require 2^—1 transfers of single discs to effect the complete 
transfer. Thus the eight discs of the puzzle will require 255 
single transfers. It will be noticed that every alternate move 



CH. X] MISCELLANEOUS PROBLEMS 229 

consists of a transfer of the smallest disc from one peg to another, 
the pegs being taken in cyclical order : further if the discs be 
numbered consecutively 1, 2, 3, ... beginning with the smallest, 
all those with odd numbers rotate in one direction, and all those 
with even numbers in the other direction. 

De Parville gave an account of the origin of the toy which 
is a sufficiently pretty conceit to deserve repetition*. In the 
great temple at Benares, says he, beneath the dome which 
marks the centre of the world, rests a brass plate in which are 
fixed three diamond needles, each a cubit high and as thick 
as the body of a bee. On one of these needles, at the creation, 
God placed sixty-four discs of pure gold, the largest disc resting 
on the brass plate, and the others getting smaller and smaller 
up to the top one. This is the Tower of Bramah. Day and 
night unceasingly the priests transfer the discs from one diamond 
needle to another according to the fixed and immutable 
laws of Bramah, which require that the priest on duty must 
not move more than one disc at a time and that he must place 
this disc on a needle so that there is no smaller disc below it. 
When the sixty-four discs shall have been thus transferred 
from the needle on which at the creation God placed them to 
one of the other needles, tower, temple, and Brahmins alike 
will crumble into dust, and with a thunderclap the world 
will vanish. Would that other writers were in the habit of 
inventing equally interesting origins for the puzzles they 
produce ! 

The number of separate transfers of single discs which the 
Brahmins must make to effect the transfer of the tower is 
2«-l, that is, is 18,446744,073709,551615: a number which, 
even if the priests never made a mistake, would require many 
thousands of millions of years to carry out. 

Chinese Rings f. A somewhat more elaborate toy, known 
as Chinese Rings, which is on sale in most English toy-shops, 

♦ La Nature, Paris, 1884, part i, pp. 285—286. 

t It was described by Cardan in 1650 in his De Subtilitate^ bk. xv, 
paragraph 2, ed. Sponius, vol. ni, p. 587 ; by Wallis in his Algebra, Latin edition, 
1693, Opera, vol. n, chap, cxi, pp. 472—478 ; and allusion is made to it also 
in Ozanam's Mcriationn, 1723 edition, vol. iv, p. 439. 



230 MISCELLANEOUS PROBLEMS [CH. X 

is represented in the accompanying figure. It consists of a 
number of rings hung upon a bar in such a manner that the 
ring at one end (say A) can be taken off or put on the bar 
at pleasure; but any other ring can be taken off or put on 
only when the one next to it towards A is on, and all the 
rest towards A are off the bar. The order of the rings cannot 
be changed. 

Only one ring can be taken off or put on at a time. [In 
the toy, as usually sold, the first two rings form an exception 
to the rule. Both these can be taken off or put on together. 



C^t 




To simplify the discussion I shall assume at first that only one 
ring is taken off or put on at a time.] I proceed to show that, 
if there are n rings, then in order to disconnect them from the 
bar, it will be necessary to take a ring off or to put a ring on 
either J(2'*+i— 1) times or J (2"+^ — 2) times according as n is 
odd or even. 

Let the taking a ring off the bar or putting a; ring on the 
bar be called a step. It is usual to number the rings from the 
fi:ee end A. Let us suppose that we commence with the first 
m rings off the bar and all the rest on the bar; and suppose 
that then it requires x—1 steps to take off the next ring, 
that is, it requires x—1 additional steps to arrange the rings 
so that the first m + 1 of them are off the bar and all the 
rest are on it. Before taking these steps we can take off 



CH. X] MISCELLANEOUS PROBLEMS 231 

the (m + 2)th ring and thus it will require x steps from our 
initial position to remove the (m 4- l)th and (m + 2)th rings. 

Suppose that these x steps have been made and that thus 
the first m + 2 rings are off the bar and the rest on it, and 
let us find how many additional steps are now necessary to 
take off the (m + 3)th and (m + 4)th rings. To take these off 
we begin by taking off the (m + 4)th ring : this requires 1 step. 
Before we can take off the (m + 3)th ring we must arrange the 
rings so that the (m + 2)th ring is on and the first m -f 1 rings 
are off: to effect this, (i) we must get the (m + l)th ring on 
and the first m rings off, which requires x—1 steps, (ii) then 
we must put on the (m + 2)th ring, which requires 1 step, 
(iii) and lastly we must take the (m + l)th ring off, which re- 
quires x — 1 steps: these movements require in all j2 (a; — 1) + 1} 
steps. Next we can take the (m + 3)th ring off, which requires 
1 step ; this leaves us with the first m + 1 rings off, the 
(m + 2)th on, the (m + 3)th off and all the rest on. Finally to 
take off the (m + 2)th ring, (i) we get the (m+ l)th ring on 
and the first m rings off, which requires x—1 steps, (ii) we take 
off the (m + 2)th ring, which requires 1 step, (iii) we take the 
(m + l)th ring off, which requires x — 1 steps : these movements 
require {2 (a; - 1) + 1) steps. 

Therefore, if when the first m rings are off it requires x 
steps to take off the (m + l)th and (m + 2)th rings, then the 
number of additional steps required to take off the (m + 3)th 
and (m+4)th rings is 1 + [2 (a;- 1) + 1) + 1 + (2 (a7-l) + 1), 
that is, is 4a;. 

To find the whole number of steps necessary to take off an 
odd number of rings we proceed as follows. ^ 

To take off the first ring requires 1 step ; 
,'. to take off the first 3 rings requires 4 additional steps ; 

• f; 4,3 

• • » » *' » )> ^ » » 

In this way we see that the number of steps required to take 
off the first 2n + 1 rings is 1 + 4 -f- 4^ + . .. + 4", which is equal 
to J(22^+»-l). 

To find the number of steps necessary to take off an even 
number of rings we proceed in a similar manner. 



232 MISCELLANEOUS PROBLEMS [CH. X 

To take off the first 2 rings requires 2 steps ; 
/, to take off the first 4 rings requires 2x4 additional steps ; 

In this way we see that the number of steps required to take 
off the first 2n rings is 2 + (2 x 4) + (2 x 4^) + ... + (2 x 4**-i), 
which is equal to i (22'*+i - 2). 

If we take off or put on the first two rings in one step 
instead of two separate steps, these results become respectively 
22^ and 22^-^-1. 

I give the above analysis because it is the direct solution 
of a problem attacked unsuccessfully by Cardan in 1550 and by 
Wallis in 1693, and which at one time attracted some attention. 

I proceed next to give another solution, more elegant 
though rather artificial. This, which is due to Monsieur Gros*, 
depends on a convention by which any position of the rings 
is denoted by a certain number expressed in the binary scale of 
notation in such a way that a step is indicated by the addition 
or subtraction of unity. 

Let the rings be indicated by circles : if a ring is on the 
bar, it is represented by a circle drawn above the bar ; if the 
ring is off the bar, it is represented by a circle below the bar. 
Thus figure i below represents a set of seven rings of which the 
first two are off the bar, the next three are on it, the sixth is off 
it, and the seventh is on it. 

Denote the rings which are on the bar by the digits 1 or 
alternately, reckoning from left to right, and denote a ring 
which is off the bar by the digit assigned to that ring on the 
bar which is nearest to it on the left of it, or by a if there is 
no ring to the left of it. 

. Thus the three positions indicated below are denoted re- 
spectively by the numbers written below them. The position 
represented in figure ii is obtained from that in figure i by 
putting the first ring on to the bar, while the position repre- 
sented in figure iii is obtained from that in figure i by taking 
the fourth ring off the bar. 

* TMorie du Baguenodier, by L. Gros, Lyons, 1872. I take the account of 
this from Lucas, vol. i, part 7. 



CH. X] MISCELLANEOUS PROBLEMS 233 

It follows that every position of the rings is denoted by a 
number expressed in the binary scale: moreover, since in going 
from left to right every ring on the bar gives a variation (that 
is, 1 to or to 1) and every ring ofif the bar gives a continu- 
ation, the effect of a step by which a ring is taken off or put on 
the bar is either to subtract unity from this number or to add 
unity to it. For example, the number denoting the position of 
the rings in figure ii is obtained from the number denoting that 
in figure i by adding unity to it. Similarly the number de- 
noting the position of the rings in figure iii is obtained from 
the number denoting that in figure i by subtracting unity 
from it. 

I P OOP lo p o o p [o P o 

o o o I o o I o o o p 

1101000 1101001 1100111 

Figure i. Figure ii. Figure iii. 

The position when all the seven rings are off the bar is 
denoted by the number 0000000 : when all of them are on 
the bar by the number 1010101. Hence to change from one 
position to the other requires a number of steps equal to the 
difference between these two numbers in the binary scale. The 
first of these numbers is 0: the second is equal to 2*' + 2* -f 2' + 1, 
that is, to 85. Therefore 85 steps are required. In a similar 
way we may show that to put on a set of 2/1 + 1 rings requires 
(1 -I- 2«4- ... +22«) steps, that is, ^{2^+^- 1) steps; and to put 
on a set of 2n rings requires (24-2' + ...+ 2^~^) steps, that is, 
j(23«+i_2) steps. 

I append a table indicating the steps necessary to take off 
the first four rings from a set of five rings. The diagrams in 
the middle column show the successive position of the rings 
after each step. The number following each diagram indicates 
that position, each number being obtained from the one above 
it by the addition of unity. The steps which are bracketed to- 
gether can be made in one movement, and, if thus effected, the 
whole process is completed in 7 movements instead of 10 steps: 
this is in accordance with the formula given above. 



234 MISCELLANEOUS PROBLEMS [CH. X 

Gros asserted that it is possible to take from 64 to 
80 steps a minute, which in my experience is a rather high 
estimate. If we accept the lower of these numbers, it would 
be possible to take off 10 rings in less than 8 minutes ; to take 
off 25 rings would require more than 582 days, each of ten 
hours work ; and to take off 60 rings would necessitate no less 
than 768614,336404,564650 steps, and would require nearly 
55000,000000 years work — assuming of course that no mistakes 
were made. 

Initial position [.^ Q ^ ^ ^ 10101 

After 1st step ^ ^ ^ ^ ^ 10110 \ 

„ 2nd „ r ^ ^ , ^ 10111 1 

3rd „ [^-7T-2 r—rr 11000 



1° 


o 


o 


o 


o 


1 
1°. 


o 


o 




o 


1 
IP 


o 


o 


o 




1 

1° 




o 


o 


o 


1 

1° 


o 


o 


© 


o 

o 


1 

o 


o 


o 


o 
o 


, 2. 


li^.. 


o 


-9._ 


o 




r 

1^ 


o 




o 


o 


1 


o 


o 


o 


o 
o 


lo 


o 


o 




o 




o 


o 


o 




1 


o 


o 


o 


o 



4th „ r o o noon 

5th „ 1 ° _ ° ° ° 11010 ) 



6th „ 1° o ° o "Oil 

7th „ l2_^ 2 11100 

loo o 

8th „ |.o__ o o 11101 

9th « I" r r o "^ 11110) 

10th „ 1^ n . . n mil J 



Problems connected with a Pack of Cards. An ordinary 
pack of playing cards can be used to illustrate many questions 
depending on simple properties of numbers, or involving the 
relative position of the cards. In problems of this kind, the 
principle of solution generally consists in re-arranging the pack 
in a particular manner so as to bring the card into some definite 
position. Any such re-arrangement is a species of shuffling. 

I shall treat in succession of problems connected with 
Shuffling a Pack, Arrangements by Rows and Columns, the 
Determination of a Pair out of in{n + l) Pairs, Gergonne's 
Pile Problem, and the game known as the Mouse Trap, 



CH. X] MISCELLANEOUS PROBLEMS 235 

Shuffling a Pack. Any system of shuffling a pack of 
cards, if carried out consistently, leads to an arrangement which 
can be calculated; but tricks that depend on it generally require 
considerable technical skill. 

Suppose for instance that a pack of n cards is shuffled, as 
is not unusual, by placing the second card on the first, the third 
below these, the fourth above them, and so on. The theory of 
this system of shuffling is due to Monge*. The following are 
some of the results and are not difficult to prove directly. 

If the pack contains 6^ + 4 cards, the (2^ + 2)th card will 
occupy the same position in the shuffled pack. For instance, if 
a complete pack of 52 cards is shuffled as described above, the 
18th card will remain the 18th card. 

Again, if a pack of lOp + 2 cards is shuffled in this way, the 
(2j3 + l)th and the (6p + 2)th cards will interchange places. For 
instance, if an ^carte pack of 32 cards is shuffled as described 
above, the 7th and the 20th cards will change places. 

More generally, one shuffle of a pack of 2p cards will move 
the card which was in the a?oth place to the o^jth place, where , 
^1 = i (2p + a^o + 1) if Xq is odd, and x^ = ^ {2p — Xq-\-2) if x^ is 
even, from which the above results can be deduced. By repeated 
applications of the above formulae we can show that the effect 
of m such shuffles is to move the card which was initially in the 
a^oth place to the aj^th place, where 

2"»+ia?^ = (4p + 1) (2"»-i ± 2"*-'» ± ... ± 2 ± 1) ± 1x, + 2"» ± 1, 
the sign + representing an ambiguity of sign. 

Again, in any pack of n cards after a certain number of 
shufflings, not greater than w, the cards will return to their 
primitive order. This will always be the case as soon as the 
original top card occupies that position again. To determine 

* Monge's investigations are printed in the Memoires de VAcadimie de$ 
Sciences, Paris, 1773, pp. 390—412. Among those who have studied the subject 
afresh I may in particular mention V. Bouniakowski, Bulletin physico-matMma- 
tique de St PStersbourg, 1857, vol. xv, pp. 202—205, summarised in the Nouvelles 
annalei de mathimatiques, 1858, Bulletin, pp. 66 — 67 ; T. de St Laurent, Mimoires 
de VAcadimie de Gard, 1865 ; L. Tanner, Educational Times Reprints, 1880, 
vol. xxxni, pp. 73 — 75 ; M. J. Bourget, Liouville's Journal, 1882, pp. 413 — 434 ; 
and H. F. Baker, Transactions of the British Association for 1910, pp. 626—528. 



236 MISCELLANEOUS PROBLEMS [CH. X 

the number of shuffles required for a pack of 2p cards, it is 
sufficient to put a?^ = aj^ and find the smallest value of m which 
satisfies the resulting equation for all values of Xq from 1 to 2p. 
It follows that, if m is the least number which makes 4"* — 1 
divisible by 4jo+l, then m shuffles will be required if either 
2*" + 1 or 2"* — 1 is divisible by 4p + 1, otherwise 2m shuffles will 
be required. The number for a pack of 2p+l cards is the same 
as that for a pack of 2p cards. With an ^cart6 pack of 32 cards, 
six shuffles are sufficient; with a pack of 2** cards, n + 1 shuffles 
are sufficient ; with a full pack of 52 cards, twelve shuffles are 
sufficient; with a pack of 13 cards ten shuffles are sufficient ; 
while with a pack of 50 cards fifty shuffles are required ; and 
so on. 

W. H. H. Hudson* has also shown that, whatever is the 
law of shuffling, yet if it is repeated again and again on a pack 
of n cards, the cards will ultimately fall into their initial posi- 
tions after a number of shufflings not greater than the greatest 
possible L.C.M. of all numbers whose sum is n. 

For suppose that any particular position is occupied affcer 
the 1st, 2nd, ..., pth. shuffles by the cards A^, A^, ..., Ap re- 
spectively, and that initially the position is occupied by the 
card Aq, Suppose further that after the pth shuffle A^ returns 
to its initial position, therefore AQ = Ap. Then at the second 
shuffling xia succeeds ^1 by the same law by which Ai succeeded 
Aq at the first; hence it follows that previous to the second 
shuffling A 2 must have been in the place occupied by A^ pre- 
vious to the first. Thus the cards which after the successive 
shuffles take the place initially occupied by A^ are J-g, A^, ..., 
Ap, Ai', that is, after the pth shuffle A^ has returned to the 
place initially occupied by it: and so for all the other cards 

Aij -O-g, ,.., XJ-p^-y. 

Hence the cards Ai, A^, ..., Ap form a cycle of p cards, one 
or other of which is always in one or other of p positions in the 
pack, and which go through all their changes in p shufflings. 
Let the number n of the pack be divided into p, q^ ?•, . . . such 
cycles, whose sum is n; then the L.C.M. of p, q, r, ... ia the 

* Educational Times Reprints^ London, 1865, voL ii, p. 105. 



CH. X] 



MISCELLANEOUS PROBLEMS 



237 



utmost number of shufflings necessary before all the cards will 
be brought back to their original places. In the case of a 
pack of 52 cards, the greatest L.c.M. of numbers whose sum 
is 52 is 18U180. 

Arrangements by Rows and Columns. A not uncommon 
trick, which rests on a species of shuffling, depends on the 
obvious fact that if n" cards are arranged in the form of a 
square of n rows, each containing n cards, then any card will 
be defined if the row and the column in which it lies are 
mentioned. 

This information is generally elicited by first asking in 
which row the selected card lies, and noting the extreme left- 
hand card of that row. The cards in each column are then 
taken up, face upwards, one at a time beginning with the 
lowest card of each column and taking the columns in their 
order from right to left — each card taken up being placed on 
the top of those previously taken up. The cards are then 
dealt out again in rows, from left to right, beginning with the 
top left-hand corner, and a question is put as to which row 
contains the card. The selected card will be that card in the 
row mentioned which is in the same vertical column as the 
card which was originally noted. 

The trick is improved by allowing the pack to be cut as 
often as is liked before the cards are re-dealt, and then giving 
one cut at the end so as to make the top card in the pack 
one of those originally in the top row. For instance, take the 



0000 
0000 
0000 
0000 



2 e 



000H 
00 
00B 
0000 



Figure L 



Figure ii. 



238 MISCELLANEOUS PROBLEMS [CH. X 

case of 16 cards. The first and second arrangements may be 
represented by figures i and ii. Suppose we are told that in 
figure i the card is in the third row, it must be either 9, 10, 
11, 12: hence, if we know in which row of figure ii it lies, it is 
determined. If we allow the pack to be cut between the deals, 
we must secure somehow that the top card is either 1, 2, 3, 
or 4, since that will leave the cards in each row of figure ii 
unaltered though the positions of the rows will be changed. 

Determination of a selected pair of cards out of 
in(n + l) GIVEN PAIRS*. Another common triok is to throw 
twenty cards on to a table in ten couples, and ask someone to 
select one couple. The cards are then taken up, and dealt out 
in a certain manner into four rows each containing five cards. 
If the rows which contain the given cards are indicated, the 
cards selected are known at once. 

This depends on the fact that the number of homogeneous 
products of two dimensions which can be formed out of four 
things is 10. Hence the homogeneous products of two dimen- 
sions formed out of four things can be used to define ten things 

Suppose that ten pairs of cards are placed on a table and 
someone selects one couple. Take up the cards in their 



00000 
00000 
00000 
000H 



couples. Then the first two cards form the first couple, the 
next two the second couple, and so on. Deal them out in 
four rows each containing five cards according to the scheme 
shown above. 

♦ Bachet, problem xvii, avertissement, p. 146 et seq. 



CH. X] MISCELLANEOUS PROBLEMS 

The fii-st couple (1 and 2) are in the first row. Of the next 
couple (3 and 4), put one in the first row and one in the 
second. Of the next couple (5 and 6), put one in the first row 
and one in the third, and so on, as indicated in the diagram. 
After filling up the first row proceed similarly with the second 
row, and so on. 

Enquire in which rows the two selected cards appear. If 
only one line, the mth, is mentioned as containing the cards, 
then the required pair of cards are the mth and (m+ l)th cards 
in that line. These occupy the clue squares of that line. 
Next, if two lines are mentioned, then proceed as follows. 
Let the two lines be the pth and the 5th and suppose q>p» 
Then that one of the required cards which is in the qih. line 
will be the {q—p)i\i card which is below the first of the clue 
squares in the pih. line. The other of the required cards is in 
the pth line and is the {q —p)ih. card to the right of the second 
of the clue squares. 

Bachet's rule, in the form in which I have given it, is 
applicable to a pack of n(n + l) cards divided into couples, 
and dealt in n rows each containing n + 1 cards ; for there are 
\n{n + \) such couples, also there are ^n(n + l) homogeneous 
products of two dimensions which can be formed out of n 
things. Bachet gave the diagrams for the cases of 20, 30, and 
42 cards : these the reader will have no difficulty in constructing 
for himself, and I have enunciated the rule for 20 cards in a 
form which covers all the cases. 

I have seen the same trick performed by means of a 
sentence and not by numbers. If we take the case of ten 
couples, then after collecting the pairs the cards must be dealt 
in four rows each containing five cards, in the order indicated 
by the sentence Maids dedit nomen Gods, This sentence must 
be imagined as written on the table, each word forming one 
line. The first card is dealt on the M. The next card (which 
is the pair of the first) is placed on the second m in the sentence, 
that is, third in the third row. The third card is placed on 
the a. The fourth card (which is the pair of the third) is 
placed on the second a, that is, fourth in the first row. Each 



240 MISCELLANEOUS PROBLEMS [CH. X 

of the next two cards is placed on a t, and so on. Enquire 
in which rows the two selected cards appear. If two rows are 
mentioned, the two cards are on the letters common to the 
words that make these rows. If only one row is mentioned, 
the cards are on the two letters common to that row. 

The reason is obvious : let us denote each of the first pair 
by an a, and similarly each of any of the other pairs by an 
e, i, 0, c, d, m, n, s, or t respectively. Now the sentence Matas 
dedit nomen Gods contains four words each of five letters ; ten 
letters are used, and each letter is repeated only twice. Hence, 
if two of the words are mentioned, they will have one letter in 
common, or, if one word is mentioned, it will have two like 
letters. 

To perform the same trick with any other number of cards 
we should require a different sentence. 

The number of homogeneous products of three dimensions 
which can be formed out of four things is 20, and of these the 
number consisting of products in which three things are alike 
and those in which three things are different is 8. This leads 
to a trick with 8 trios of things, which is similar to that last 
given — the cards being arranged in the order indicated by the 
sentence Lanata levete livini novoto. 

I believe that these arrangements by sentences are well- 
known, but I am not aware who invented them. 

Gergonne's Pile Problem. Before discussing Gergonne's 
theorem I will describe the familiar three pile problem, the 
theory of which is included in his results. 

The Three Pile Problem*, This trick is usually performed 
as follows. Take 27 cards and deal them into three piles, face 
upwards. By " dealing " is to be understood that the top card 
is placed as the bottom card of the first pile, the second card in 
the pack as the bottom card of the second pile, the third card 
as the bottom card of the third pile, the fourth card on the top 
of the first one, and so on : moreover I assume that throughout 

* The trick is mentioned by Bachet, problem xvui, p. 143, but his analysis 
of it is insufficient. 



CH. X] MISCELLANEOUS PROBLEMS 241 

the problem the cards are held in the hand face upwards. The 
result can be modified to cover any other way of dealing. 

Request a spectator to note a card, and remember in which 
pile it is. After finishing the deal, ask in which pile the card 
is. Take up the three piles, placing that pile between the 
other two. Deal again as before, and repeat the question as 
to which pile contains the given card. Take up the three 
piles again, placing the pile which now contains the selected 
card between the other two. Deal again as before, but in 
dealing note the middle card of each pile. Ask again for the 
third time in which pile the card lies, and you will know that 
the card was the one which you noted as being the middle 
card of that pile. The trick can be finished then in any way 
that you like. The usual method — but a very clumsy one — is 
to take up the three piles once more, placing the named pile 
between the other two as before, when the selected card will bf 
the middle one in the pack, that is, if 27 cards are used it wil\ 
be the 14th card. 

The trick is often performed with 15 cards or with 21 cards, 
in either of which cases the same rule holds. 

Gergonne^s Generalization. The general theory for a pack 
of m"* cards was given by M. Gergonne*. Suppose the pack 
is arranged in m piles, each containing m"^^ cards, and that, 
after the first deal, the pile indicated as containing the selected 
card is taken up ath ; after the second deal, is taken up 6th ; 
and so on, and finally after the mth deal, the pile containing 
the card is taken up A;th. Then when the cards are collected 
after the mth deal the selected card will be /ith from the top 
where 

if m is even, n = km^~^ — jvi^~'^ -{•... -\-bm — a+1, 
if m is odd, n = km"^~^ — jiri"^^ + ... —bm-\-a. 

For example, if a pack of 256 cards {i.e. m = 4) was given, 
and anyone selected a card out of it, the card could be de- 
termined by making four successive deals into four piles of 
64 cards each, and after each deal asking in which pile the 

• Gergonue's Annalea de MatMmatiquet, Nismes, 1813-4, vol. rr, pp. 276 — 
283. 

B. R. 16 



242 MISCELLANEOUS PROBLEMS [CH. X 

selected card lay. The reason is that after the first deal you 
know it is one of sixty-four cards. In the next deal these 
sixty-four cards are distributed equally over the four piles, and 
therefore, if you know in which pile it is, you will know that it 
is one of sixteen cards. After the third deal you know it is 
one of four cards. After the fourth deal you know which card 
it is. 

Moreover, if the pack of 256 cards is used, it is immaterial 
in what order the pile containing the selected card is taken up 
after a deal. For, if after the first deal it is taken up ath, 
after the second 6th, after the third cth, and after the fourth 
c^th, the card will be the (6M - 16c + 46 - a + l)th from the 
top of the pack, and thus will be known. We need not take 
up the cards after the fourth deal, for the same argument will 
show that it is the (64 — 16c + 46 — a-f l)th in the pile then 
indicated as containing it. Thus if a = 3, 6 = 4, c = 1, d = 2, 
it will be the 62nd card in the pile indicated after the fourth 
deal as containing it and will be the 126th card in the pack 
as then collected. 

In exactly the same way a pack of twenty-seven cards may 
be used, and three successive deals, each into three piles of 
nine cards, will suffice to determine the card. If after the 
deals the pile indicated as containing the given card is taken 
up ath, 6th, and cth respectively, then the card will be the 
(9c — 36-1- a)th in the pack or will be the (9 - 36 + a)th card in 
the pile indicated after the third deal as containing it. 

The method of proof will be illustrated sufficiently by 
considering the usual case of a pack of twenty-seven cards, for 
which m = 3, which are dealt into three piles each of nine cards. 

Suppose that, after the first deal, the pile containing the 
selected card is taken up ath : then (i) at the top of the pack 
there are a — 1 piles each containing nine cards ; (ii) next there 
are 9 cards, of which one is the selected card ; and (iii) lastly 
there are the remaining cards of the pack. The cards are dealt 
out now for the second time : in each pile the bottom 3 (a — 1) 
cards will be taken from (i), the next 3 cards from (ii), and the 
remaining 9 - 3a cards from (iii). 



CH. X] MISCELLANEOUS PROBLEMS 243 

Suppose that the pile now indicated as containing the 
selected card is taken up 6th: then (i) at the top of the 
pack are 9(6—1) cards; (ii) next are 9 — 3a cards; (iii) next 
are 3 cards, of which one is the selected card ; and (iv) lastly 
are the remaining cards of the pack. The cards are dealt out 
now for the third time: in each pile the bottom 3(6—1) cards 
will be taken from (i), the next 3 — a cards will be taken from 
(ii), the next card will be one of the three cards in (iii), and the 
remaining 8 — 36 + a cards are from (iv). 

Hence, after this deal, as soon as the pile is indicated, it 
is known that the card is the (9 — 36 + a)th from the top 
of that pile. If the process is continued by taking up this 
pile as cth, then the selected card will come out in the place 
9 (c — 1) + (8 — 36 + a) + 1 from the top, that is, will come out 
as the (9c — 36 + a)th card. 

Since, after the third deal, the position of the card in the 
pile then indicated is known, it is easy to notice the card, in 
which case the trick can be finished in some way more effective 
than dealing again. 

If we put the pile indicated always in the middle of the 
pack we have a — 2, 6=2, c = 2, hence n = 9c - 36 + a = 14, 
which is the form in which the trick is usually presented, as 
was explained above on page 241. 

I have shown that if a, 6, c are known, then n is determined. 
We may modify the rule so as to make the selected card come 
out in any assigned position, say the 7ith. In this case we 
have to find values of a, 6, c which will satisfy the equation 
71= 9c — 36 + a, where a, 6, c can have only the values 1, 2, 
or 3. 

Hence, if we divide ti by 3 and the remainder is 1 or 2, this 
remainder will be a; but, if the remainder is 0, we must decrease 
the quotient by unity so that the remainder is 3, and this 
remainder will be a. In other words a is the smallest positive 
number (exclusive of zero) which must be subtracted from n 
to make the difference a multiple of 3. 

Next let p be this multiple, i.e. p is the next lowest integer 
to 72/3 : then Sp = 9c— 36, therefore p = 3c — 6. Hence 6 is 

16—2 



244 MISCELLANEOUS PROBLEMS [CH. X 

the smallest positive number (exclusive of zero) which must 
be added to p to make the sum a multiple of 3, and c is that 
multiple. 

A couple of illustrations will make this clear. Suppose 
we wish the card to come out 22nd from the top, therefore 
22 = 9c — 36 + a. The smallest number which must be sub- 
tracted from 22 to leave a multiple of 3 is 1, therefore a = 1. 
Hence 22 = 9c -36 + 1, therefore 7 = 3c - 6. The smallest 
number which must be added to 7 to make a multiple of 3 is 2, 
therefore 6=2. Hence 7 = 3c — 2, therefore c = 3. Thus a = 1, 
6 = 2,c = 3. 

Again, suppose the card is to come out 21st. Hence 
21 = 9c - 36 + a. Therefore a is the smallest number which 
subtracted from 21 makes a multiple of 3, therefore a = 3. 
Hence 6 = 3c - 6. Therefore 6 is the smallest number which 
added to 6 makes a multiple of 3, therefore 6 = 3. Hence 
9 = 3c, therefore c = 3. Thus a = 3, 6 = 3, c = 3. 

If any difficulty is experienced in this work, we can proceed 
thus. Let a=x + l,h = S — y, c = z-\-l; then x, y, z may have 
only the values 0, 1, or 2. In this case Gergonne's equation 
takes the form 9^ + 3^/ + a; = ri — 1. Hence, if n — 1 is expressed 
in the ternary scale of notation, x, y, z will be determined, and 
therefore a, 6, c will be known. 

The rule in the case of a pack of wF- cards is exactly similar. 
We want to make the card come out in a given place. Hence, 
in Gergonne's formula, we are given n and we have to find 
a, 6, . . . , A;. We can effect this by dividing n continually by m, 
with the convention that the remainders are to be alternately 
positive and negative and that their numerical values are to be 
not greater than m or less than unity. 

An analogous theorem with a pack of Im cards can be con- 
structed. C. T. Hudson and L. E. Dickson* have discussed the 
general case where such a pack is dealt n times, each time into 
I piles of m cards ; and they have shown how the piles must be 

* Educational Times Reprints, 1868, vol. ix, pp. 89 — 91 ; and Bulletin of 
the American Mathematical Society, New York, April, 1895, vol. i, pp. 184 — 
186. 



CH. X] MISCELLANEOUS PROBLEMS 245 

taken up in order that after the nth. deal the selected card may 
be rth from the top. 

The principle will be sufficiently illustrated by one example 
treated in a manner analogous to the cases already discussed. 
For instance, suppose that an 6carte pack of 32 cards is dealt 
into four piles each of 8 cards, and that the pile which contains 
some selected card is picked up ath. Suppose that on dealing 
again into four piles, one pile is indicated as containing the 
selected card, the selected card cannot be one of the bottom 
2 (a — 1) cards, or of the top 8 — 2a cards, but must be one of 
the intermediate 2 cards, and the trick can be finished in any 
way, as for instance by the common conjuring ambiguity of 
asking someone to choose one of them, leaving it doubtful 
whether the one he takes is to be rejected or retained. 

The Mouse Trap. Treize. I will conclude this chapter 
with the bare mention of another game of cards, known as 
the Mouse Trap, the discussion of which involves some rather 
difficult algebraic analysis. 

It is played as follows. A set of cards, marked with the 
numbers 1, 2, 3, ..., n, is dealt in any order, face upwards, in 
the form of a circle. The player begins at any card and counts 
round the circle always in the same direction. If the A;th card 
has the number k on it — which event is called a hit — the 
player takes up the card and begins counting afresh. According 
to Cayley, the player wins if he thus takes up all the cards, and 
the cards win if at any time the player counts up to n without 
being able to take up a card. 

For example, if a pack of only four cards is used and these 
cards come in the order 3214, then the player would obtain 
the second card 2 as a hit, next he would obtain 1 as a hit, 
but if he went on for ever he would not obtain another hit. 
On the other hand, if the cards in the pack were initially in 
the order 1423, the player would obtain successively all four 
cards in the order 1, 2, 3, 4. 

The problem may be stated as the determination of what 
hits and how many hits can be made with a given number of 



246 MISCELLANEOUS PROBLEMS [CH. X 

cards; and what permutations will give a certain number of 
hits in a certain order. 

Cay ley* showed that there are 9 arrangements of a pack 
of four cards in which no hit will be made, 7 arrangements in 
which only one hit will be made, 3 arrangements in which only 
two hits will be made, and 5 arrangements in which four hits 
will be made. 

Prof. Steen"f* has investigated the general theory for a pack 
of n cards. He has shown how to determine the number of 
arrangements in which x is the first hit [Arts. 3 — 6]; the 
number of arrangements in which 1 is the first hit and x is the 
second hit [Art. 6] ; and the number of arrangements in which 
2 is the first hit and x the second hit [Arts. 7 — 8] ; but beyond 
this point the theory has not been carried. It is obvious that, 
if there are n — 1 hits, the nth. hit will necessarily follow. 

The French game of treize is very similar. It is played 
with a full pack of fifty-two cards (knave, queen, and king 
counting as 11, 12, and 13 respectively). The dealer calls out 
1, 2, 3, ..., 13, as he deals the 1st, 2nd, 3rd, ..., 13th cards 
respectively. At the beginning of a deal the dealer offers to 
lay or take certain odds that he will make a hit in the thirteen 
cards next dealt. 

* Quarterly Journal of Mathematics, 1878, vol. xv, pp. 8 — 10. 
t Ibid., vol. XV, pp. 230—241. 



247 



PAET n. 

* No man of science should think it a waste of time to learn 
something of tJie history of his own subject ; nor is the investigation 
qf laborious methods now fallen into disuse, or of errors once cormnonly 
accepted, the least valuable of mental disciplines.*^ 

" The most worthless book of a bygone day is a record woi'thy of 
preservation. Like a telescopic star, its obscurity may render it 
VAiavailahle for most purposes ; but it serves, in hands which know 
how to use it, to determine the places qf more important bodies." 

(De Mobqan.) 



248 



CHAPTER XL 

CALCULA.TING PRODIGIES. 

At rare intervals there have appeared lads who possess 
extraordinary powers of mental calculation*. In a few seconds 
they gave the answers to questions connected with the multipli- 
cation of numbers and the extraction of roots of numbers, which 
an expert mathematician could obtain only in a longer time 
and with the aid of pen and paper. Nor were their powers 
always limited to such simple problems. More difficult questions, 
dealing for instance with factors, compound interest, annuities, 
the civil and ecclesiastical calendars, and the solution of equa- 
tions, were solved by some of them with facility as soon as the 
meaning of what was wanted had been grasped. In most cases 
these lads were illiterate, and usually their rules of working 
were of their own invention. 

The performances were so remarkable that some observers 
held that these prodigies possessed powers differing in kind 
from those of their contemporaries. For such a view there is no 
foundation. Any lad with an excellent memory and a natural 
turn for arithmetic can, if he continuously gives his undivided 
attention to the consideration of numbers, and indulges in 
constant practice, attain great proficiency in mental arithmetic, 
and of course the performances of those that are specially 
gifted are exceptionally astonishing. 

* Most of the facts about calculating prodigies have been collected by 

E. W. Scripture, American Journal of PsTjchology, 1891, vol. iv, pp. 1 — 59; by 

F. D. Mitchell, Ibid., 1907, vol. xviii, pp. 61—143; and G. E. Miiller, Zur 
Analyse der Geddchtnistdtigkeit und des Vorstellungsverlaufes, Leipzig, 1911. 
I have used these papers freely, and in some cases where authorities are quoted 
of which I have no first-hand information have relied exclusively on them. 
These articles should be consulted for bibliographical notes on the numerous 
original authorities. 



CH. Xl] CALCULATING PRODIGIES 249 

In this chapter I propose to describe briefly the doings of the 
more famous calculating prodigies. It will be seen that their 
performances were of much the same general character, though 
carried to different extents, hence in the later cases it will 
be enough to indicate briefly peculiarities of the particular 
calculators. 

I confine myself to self-taught calculators, and thus exclude 
the consideration of a few public performers who by practice, 
arithmetical devices, and the tricks of the showman have 
simulated like powers. I also concern myself only with those 
who showed the power in youth. As far as I know the only 
self-taught mathematician of advanced years whom I thus 
exclude is John Wallis, 1616 — 1703, the Savilian Professor at 
Oxford, who in middle-life developed, for his own amusement, 
his powers in mental arithmetic. As an illustration of his 
achievements, I note that on 22 December 1669 he, when in 
bed, occupied himself in finding (mentally) the integral part of 
the square root of 3 x 10^ ; and several hours afterwards wrote 
down the result from memory. This fact having attracted 
notice, two months later he was challenged to extract the 
square root of a number of fifty-three digits; this he per- 
formed mentally, and a month later he dictated the answer 
which he had not meantime committed to writing. Such efforts 
of calculation and memory are typical of calculating prodigies. 

One of the earliest of these prodigies, of whom we have 
records,' was Jedediah Buxton, who was born in or about 1707 
at Elmton, Derbyshire. Although a son of the village school- 
master, his education was neglected, and he never learnt to 
write or cipher. With the exception of his power of dealing 
with large numbers, his mental faculties were of a low order : 
he had no ambition, and remained throughout his life a farm- 
labourer, nor did his exceptional skill with figures bring him 
any material advantage other than that of occasionally receiving 
small sums of money from those who induced him to exhibit 
his peculiar gift. He does not seem to have given public 
exhibitions. He died in 1772. 

He had no recollection as to when or how he was first 



250 CALCULATING PRODIGIES [CH. XI 

attracted by mental calculations, and of his performances in early 
life we have no reliable details. Mere numbers however seem 
always to have had a strange fascination for him. If the size 
of an object was stated, he began at once to compute how many 
inches or hair-breadths it contained; if a period of time was 
mentioned, he calculated the number of minutes in it ; if 
he heard a sermon, he thought only of the number of words 
or syllables in it. No doubt his powers in these matters in- 
creased by incessant practice, but his ideas were childish, and 
do not seem to have gone beyond pride in being able to state 
accurately the results of such calculations. He was slow witted, 
and took far longer to answer arithmetical questions than most 
of these prodigies. The only practical accomplishment to which 
his powers led him was the ability to estimate by inspection 
the acreage of a field of irregular shape. 

His fame gradually spread through Derbyshire. Among 
many questions put to him by local visitors were the follow- 
ing, which fairly indicate his powers when a young man: — 
How many acres are there in a rectangular field 351 yards 
long and 261 wide; answered in 11 minutes. How many cubic 
yards of earth must be removed in order to make a pond 
426 feet long, 263 feet wide, and 2J feet deep ; answered in 
15 minutes. If sound travels 1142 feet in one second, how 
long will it take to travel 5 miles; answered in 15 minutes. 
Such questions involve no difficulties of principle. 

Here are a few of the harder problems solved by Buxton 
when his powers were fully developed. He calculated to what 
sum a farthing would amount if doubled 140 times : the answer 
is a number of pounds sterling which requires thirty-nine digits 
to represent it with 25. 8d. over. He was then asked to multiply 
this number of thirty-nine digits by itself : to this he gave the 
answer two and a half months later, and he said he had carried 
on the calculation at intervals during that period. In 1751 he 
calculated how many cubic inches there are in a right-angled 
block of stone 23,145,789 yards long, 5,642,732 yards wide, and 
54,965 yards thick ; how many grains of corn would be required 
to fill a cube whose volume is 202,680,000,360 cubic miles; and 



CH. XI] CALCULATING PRODIGIES 251 

how many hairs one inch long would be required to fill the 
same space — the dimensions of a grain and a hair being given. 
These problems involve high numbers, but are not intrinsically 
difficult, though they could not be solved mentally unless the 
calculator had a phenomenally good memory. In each case he 
gave the correct answer, though only after considerable effort. 
In 1753 he was asked to give the dimensions of a cubical 
cornbin, which holds exactly one quarter of malt. He recog- 
nized that to answer this required a process equivalent to the 
extraction of a cube root, which was a novel idea to him, 
but in an hour he said that the edge of the cube would be 
between 25^ and 26 inches, which is correct : it has been sug- 
gested that he got this answer by trying various numbers. 

Accounts of his performances were published, and his repu- 
tation reached London, which he visited in 1754. During his 
stay there he was examined by various members of the Royal 
Society, who were satisfied as to the genuineness of his perform- 
ances. Some of his acquaintances took him to Drury Lane 
Theatre to see Garrick, being curious to see how a play would 
impress his imagination. He was entirely unaffected by the 
scene, but on coming out informed his hosts of the exact 
number of words uttered by the various actors, and of the 
number of steps taken by others in their dances. 

It was only in rare cases that he was able to explain his 
methods of work, but enough is known of them to enable us to 
say that they were clumsy. He described the process by which 
he arrived at the product of 456 and 378 : shortly it was as 
follows : — If we denote the former of these numbers by a, he 
proceeded first to find 5a = (say) b ; then to find 20h — (say) c ; 
and then to find 3c = (say) d. He next formed 156 = (say) e, 
which he added to d. Lastly he formed 3a which, added to 
the sum last obtained, gave the result. This is equivalent 
to saying that he used the multiplier 378 in the form 
(5 X 20 X 3) -f- (5 X 15) 4- 3. Mitchell suggests that this may 
mean that Buxton counted by multiples of 60 and of 15, and 
thus reduced the multiplication to addition. It may be so, for 
it is difficult to suppose that he did not realize that successive 



252 CALCULATING PRODIGIES [CH. XI 

multiplications by 5 and 20 are equivalent to a multiplication 
by 100, of which the result can be at once obtained. Of 
billions, trillions, &c., he had never heard, and in order to 
represent the high numbers required in some of the questions 
proposed to him he invented a notation of his own, calling 10^^ 
a tribe and 10^* a cramp. 

As in the case of all these calculators, his memory was 
exceptionally good, and in time he got to know a large number 
of facts (such as the products of certain constantly recurring 
numbers, the number of minutes in a year, and the number 
of hair-breadths in a mile) which greatly facilitated his calcu- 
lations. A curious and perhaps unique feature in his case was 
that he could stop in the middle of a piece of mental calculation, 
take up other subjects, and after an interval, sometimes of 
weeks, could resume the consideration of the problem. He 
could answer simple questions when two or more were proposed 
simultaneously. 

Another eighteenth-century prodigy was Thomas FulleVy a 
negro, born in 1710 in Africa. He was captured there in 1724, 
and exported as a slave to Virginia, U.S.A., where he lived till 
his death in 1790. Like Buxton, Fuller never learnt to read 
or write, and his abilities were confined to mental arithmetic. 
He could multiply together two numbers, if each contained 
not more than nine digits, could state the number of seconds 
in a given period of time, the number of grains of com in a 
given mass, and so on — in short, answer the stock problems com- 
monly proposed to these prodigies, as long as they involved only 
multiplications and the solutions of problems by rule of three. 
Although more rapid than Buxton, he was a slow worker as 
compared with some of those whose doings are described below. 

I mention next the case of two mathematicians of note who 
showed similar aptitude in early years. The first of these was 
Andre Marie Ampere, 1775 — 1836, who, when a child some four 
years old, was accustomed to perform long mental calculations, 
which he effected by means of rules learnt from playing with 
arrangements of pebbles. But though always expert at mental 
arithmetic, and endowed with a phenomenal memory for figures, 



CH. Xl] CALCULATING PRODIGIES 253 

he did not specially cultivate this arithmetical power. It is 
more difficult to say whether Carl Friedrich Gauss, VJ*ll — 1855, 
should be reckoned among these calculating prodigies. He 
had, when three years old, taught himself some arithmetical 
processes, and astonished his father by correcting him in his 
calculations of certain payments for overtime ; perhaps, however, 
this is only evidence of the early age at which his consummate 
abilities began to develop. Another remarkable case is that 
of Richard Whately, 1787 — 1863, afterwards Archbishop of 
Dublin. When he was between five or six years old he showed 
considerable skill in mental arithmetic: it disappeared in about 
three years. I soon, said he, "got to do the most difficult 
sums, always in my head, for I knew nothing of figures beyond 
numeration, nor had I any names for the different processes 
I employed. But I believe my sums were chiefly in multi- 
plication, division, and the rule of three...! did these sums 
much quicker than any one could upon paper, and I never 
remember committing the smallest error. I was engaged either 
in calculating or in castle-building. . .morning, noon, and night. . . 
When I went to school, at which time the passion was worn off, 
I was a perfect dunce at ciphering, and so have continued ever 
since." The archbishop's arithmetical powers were, however, 
greater in after-life than he here allows. 

The performances of Zerah Colburn in London, in 1812, 
were more remarkable. Colburn*, bom in 1804, at Cabut, 
Vermont, U.S.A., was the son of a small farmer. While still 
less than six years old he showed extraordinary powers of mental 
calculation, which were displayed in a tour in America. Two 
years later he was brought to England, where he was repeatedly 
examined by competent observers. He could instantly give 
the product of two numbers each of four digits, but hesitated 
if both numbers exceeded 10,000. Among questions asked 
him at this time were to raise 8 to the 16th power; in a few 
seconds he gave the answer 281,474,976,710,656, which is 
correct. He was next ask^d to raise the numbers 2, 3,... 9 to 

• To the authorities mentioned by E. W. Scripture and F. D. Mitchell 
should be added The Annual Register, London, 1812, p. 507 et seq. 



254 CALCULATING PRODIGIES [CH. XI 

the 10th power : and he gave the answers so rapidly that the 
gentleman who was taking them down was obliged to ask him 
to repeat them more slowly ; but he worked less quickly when 
asked to raise numbers of two digits like 37 or 59 to high 
powers. He gave instantaneously the square roots and cube 
roots (when they were integers) of high numbers, e.g., the square 
root of 106,929 and the cube root of 268,336,125, such integral 
roots can, however, be obtained easily by various methods. 
More remarkable are his answers to questions on the factors of 
numbers. Asked for the factors of 247,483 he replied 941 and 
263; asked for the factors of 171,395 he gave 5, 7, 59, and 83; 
asked for the factors of 36,083 he said there were none. He, 
however, found it difficult to answer questions about the factors 
of numbers higher than 1,000,000. His power of factorizing high 
numbers was exceptional and depended largely on the method 
of two-digit terminals described below. Like all these public 
performers he had to face buffoons who tried to make fun of him, 
but he was generally equal to them. Asked on one such occasion 
how many black beans were required to make three white ones, 
he is said to have at once replied "three, if you skin them" — 
this, however, has much the appearance of a pre-arranged show. 
It was clear to observers that the child operated by certain 
rules, and during his calculations his lips moved as if he was 
expressing the process in words. Of his honesty there seems 
to have been no doubt. In a few cases he was able to explain 
the method of operation. Asked for the square of 4,395 he 
hesitated, but on the question being repeated he gave the 
correct answer, namely 19,395,025. Questioned as to the cause 
of his hesitation, he said he did not like to multiply four 
figures by four figures, but said he, " I found out another way ; 
I multiplied 293 by 293 and then multiplied this product twice 
by the number 15." On another occasion when asked for the 
product of 21,734 by 543 he immediately replied 11,801,562 ; 
and on being questioned explained that he had arrived at this 
by multiplying 65,202 by 181. These remarks suggest that 
whenever convenient he factorized the numbers with which he 
was dealing. 



CH. Xl] CALCULATING PRODIGIES 255 

In 1814 he was taken to Paris, but amid the political turmoil 
of the time his exhibitions fell flat. His English and American 
friends however raised money for his education, and he was 
sent in succession to the Lyc^e Napoleon in Paris and West- 
minster School in London. With education his calculating 
powers fell off, and he lost the frankness which when a boy had 
charmed observers. His subsequent career was diversified and 
not altogether successful. He commenced with the stage, then 
tried schoolmastering, then became an itinerant preacher in 
America, and finally a " professor " of languages. He wrote his 
own biography which contains an account of the methods he 
used. He died in 1840. 

Contemporary with Colburn we find another instance of a 
self-taught boy, George Parker Bidder, who possessed quite 
exceptional powers of this kind. He is perhaps the most 
interesting of these prodigies because he subsequently received 
a liberal education, retained his calculating powers, and in later 
life analyzed and explained the methods he had invented and 
used. 

Bidder was born in 1806 at Moreton Hampstead, Devon- 
shire, where his father was a stone-mason. At the age of six he 
was taught to count up to 100, but though sent to the village 
school learnt little there, and at the beginning of his career was 
ignorant of the meaning of arithmetical terms and of numerical 
symbols. Equipped solely with this knowledge of counting 
he taught himself the results of addition, subtraction, and 
multiplication of numbers (less than 100) by arranging and re- 
arranging marbles, buttons, and shot in patterns. In after-life 
he attached great importance to such concrete representations, 
and believed that his arithmetical powers were strengthened 
by the fact that at that time he knew nothing about the 
symbols for numbers. When seven years old he heard a dispute 
between two of his neighbours about the price of something 
which was being sold by the pound, and to their astonishment 
remarked that they were both wrong, mentioning the correct 
price. After this exhibition the villagers delighted in trying to 
pose him with arithmetical problems. 



256 CALCULATING PRODIGIES [CH. XI 

His reputation increased and, before he was nine years old, 
his father found it profitable to take him about the country 
to exhibit his powers. A couple of distinguished Cambridge 
graduates (Thomas Jephson, then tutor of St John's, and 
John Herschel) saw him in 1817, and were so impressed by 
his general intelligence that they raised a fund for his educa- 
tion, and induced his father to give up the r61e of showman ; 
but after a few months Bidder senior repented of his abandon- 
ment of money so easily earned, insisted on his son's return, 
and began again to make an exhibition of the boy's powers. 
In 1818, in the course of a tour young Bidder was pitted 
against Colburn and on the whole proved the abler calculator. 
Finally the father and son came to Edinburgh, where some 
members of that University intervened and persuaded his 
father to leave the lad in their care to be educated. Bidder 
remained with them, and in due course graduated at Edinburgh, 
shortly afterwards entering the profession of civil engineering 
in which he rose to high distinction. He died in 1878. 

With practice Bidder's powers steadily developed. His 
earlier performances seem to have been of the same type as 
those of Buxton and Colburn which have been already 
described. In addition to answering questions on products of 
numbers and the number of specified units in given quantities, 
he was, after 1819, ready in finding square roots, cube roots, 
&c. of high numbers, it being assumed that the root is an 
integer, and later explained his method which is easy of appli- 
cation : this method is the same as that used by Colburn. By 
this time he was able also to give immediate solutions of easy 
problems on compound interest and annuities which seemed to 
his contemporaries the most astonishing of all his feats. In 
factorizing numbers he was less successful than Colburn and 
was generally unable to deal at sight with numbers higher than 
10,000. As in the case of Colburn, attempts to be witty at 
his expense were often made, but he could hold his own. Asked 
at one of his performances in London in 1818, how many bulls' 
tails were wanted to reach to the moon, he immediately 
answered one, if it is long enough. 



CH. Xl] CALCULATiNG PRODIGIES 257 

Here are some typical questions put to and answered by 
him in his exhiljitions during the years 1815 to 1819 — they are 
taken from authenticated lists which comprise some hundreds 
of such problems: few, if any, are inherently difficult. His 
rapidity of work was remarkable, but the time limits given 
were taken by unskilled observers and can be regarded as only 
approximately correct. Of course all the calculations were 
mental without the aid of books, pencil, or paper. In 1815, 
being then nine years old, he was asked : — If the moon be distant 
from the earth 123,256 miles, and sound travels at the rate of 

4 miles a minute, how long would it be before the inhabitants 
of the moon could hear of the battle of Waterloo: answer, 
21 days, 9 hours, 34 minutes, given in less than one minute. 
In 1816, being then ten years old, just learning to write, but 
unable to form figures, he answered questions such as the 
following: — What is the interest on £11,111 for 11,111 days at 

5 per cent, a year: answer, £16,911. 11*., given in one minute. 
How many hogsheads of cider can be made from a million of 
apples, if 30 apples make one quart: answer, 132 hogsheads, 
17 gallons, 1 quart, and 10 apples over, given in 35 seconds. If 
a coach-wheel is 5 feet 10 inches in circumference, how many 
times will it revolve in running 800,000,000 miles: answer, 
724,114,285,704 times and 20 inches remaining, given in 50 
seconds. What is the square root of 119,550,669,121 : answer 
345,761, given in 30 seconds. In 1817, being then eleven 
years old, he was asked: — How long would it take to fill a 
reservoir whose volume is one cubic mile if there flowed into 
it from a river 120 gallons of water a minute: answered in 
2 minutes. Assuming that light travels from the sun to the 
earth in 8 minutes, and that the sun is 98,000,000 miles off, if 
light takes 6 years 4 months travelling from the nearest fixed 
star to the earth, what is the distance of that star, reckoning 
365 days 6 hours to each year and 28 days to each month — 
asked by Sir William Herschel: answer, 40,633,740,000,000 
miles. In 1818, at one of his performances, he was asked: — If the 
pendulum of a clock vibrates the distance of 9 j inches in a second 
of time, how many inches will it vibrate in 7 years 14 days 

B. E. 17 



258 CALCULATING PRODIGIES [CH. XI 

2 hours 1 minute 56 seconds, each year containing 365 days 
5 hours 48 minutes 55 seconds: answer, 2,165,625,744| inches, 
given in less than a minute. If I have 42 watches for sale and 

1 sell the first for a farthing, and double the price for every 
succeeding watch I sell, what will be the price of the last watch : 
answer, £2,290,649,224. 10s. 8d, If the diameter of a penny 
piece is If inches, and if the world is girdled with a ring of 
pence put side by side, what is their value sterling, supposing 
the distance to be 360 degrees, and a degree to contain 69*5 
miles: answer, £4,803,340, given in one minute. Find two 
numbers, whose difference is 12, and whose product, multiplied 
by their sum, is equal to 14,560 : answer, 14 and 26. In 1819, 
when fourteen years old, he was asked : — Find a number whose 
cube less 19 multiplied by its cube shall be equal to the cube 
of 6 : answer, 3, given instantly. What will it cost to make 
a road for 21 miles 5 furlongs 37 poles 4 yards, at the rate 
of £123. 145. 6d. a mile : answer, £2688. 135. 9f c?., given in 

2 minutes. If you are now 14 years old and you live 50 years 
longer and spend half-a-crown a day, how many farthings will 
you spend in your life : answer, 2,805,120, given in 15 seconds. 
Mr Moor contracted to illuminate the city of London with 
22,965,321 lamps, the expense of trimming and lighting was 
7 farthings a lamp, the oil consumed was |ths of a pint for 
every three lamps, and the oil cost Ss. 7 Jd a gallon ; he gained 
16 J per cent, on his outlay: how many gallons of oil were 
consumed, what was the cost to him, and what was the amount 
of the contract : answer, he used 212,641 gallons of oil, the cost 
was £205,996. 16s. l^d., and the amount of the contract was 
£239,986. 13^. 2d. If the distance of the earth from the moon 
be 29,531,531 J yards, what is the weight of a thread which 
will extend that distance, supposing 7|^ yards of it weigh J^th 
part of a drachm : answer, 8 cwt. 1 qr. 13 lbs. 9 oz. 1 dr. and 
i|ths of a drachm. 

It should be noted that Bidder did not visualize a number 
like 984 in symbols, but thought of it in a concrete way as so 
many units which could be arranged in 24 groups of 41 each. 
It should also be observed that he, like Inaudi whom I mention 



CH. Xl] CALCULATING PRODIGIES 259 

later, relied largely on the auditory sense to enable him to recollect 
numbers. " For my own part," he wrote, in later life, " though 
much accustomed to see sums and quantities expressed by the 
usual symbols, yet if I endeavour to get any number of figures 
that are represented on paper fixed in my memory, it takes me 
a much longer time and a very great deal more exertion than 
when they are expressed or enumerated verbally." For instance 
suppose a question put to find the product of two numbers each 
of nine digits, if they were " read to me, I should not require 
this to be done more than once ; but if they were represented 
in the usual way, and put into my hands, it would probably take 
me four times to peruse them before it would be in my power 
to repeat them, and after all they would not be impressed so 
vividly on my imagination." 

Bidder retained his power of rapid mental calculation to the 
end of his life, and as a constant parliamentary witness in 
matters connected with engineering it proved a valuable 
accomplishment. Just before his death an illustration of his 
powers was given to a friend who talking of then recent dis- 
coveries remarked that if 36,918 waves of red light which only 
occupy one inch are required to give the impression of red, and 
if light travels at 190,000 miles a second, how immense must be 
the number of waves which must strike the eye in one second 
to give the impression of red. " You need not work it out," 
said Bidder, "the number will be 444,433,651,200,000." 

Other members of the Bidder family have also shown 
exceptional powers of a similar kind as well as extraordinary 
memories. Of Bidder's elder brothers, one became an actuary, 
and on his books being burnt in a fire he rewrote them in six 
months from memory but, it is said, died of consequent brain 
fever; another was a Plymouth Brother and knew the whole 
Bible by heart, being able to give chapter and verse for any 
text quoted. Bidder's eldest son, a lawyer of eminence, was 
able to multiply together two numbers each of fifteen digits. 
Neither in accuracy nor rapidity was he equal to his father, 
but then he never steadily and continuously devoted himself 
bo developing his abilities in this direction. He remarked 

17—2 



260 CALCULATING PRODIGIES [CH. XI 

that in his mental arithmetic he worked with pictures of the 
figures, and said "If I perform a sum mentally it always 
proceeds in a visible form in my mind ; indeed I can conceive 
no other way possible of doing mental arithmetic": this it 
will be noticed is opposed to his father's method. Two of 
his children, one son and one daughter representing a third 
generation, have inherited analogous powers. 

I mention next the names of Henri Mondeux, and Vito 
Mangiamele. Both were born in 1826 in humble circumstances, 
were sheep-herds, and became when children, noticeable for 
feats in calculation which deservedly procured for them local 
fame. In 1839 and 1840 respectively they were brought to 
Paris where their powers were displayed in public, and tested 
by Arago, Cauchy, and others. Mondeux's performances were 
the more striking. One question put to him was to solve the 
equation a;^4-84 = 87ic: to this he at once gave the answer 3 
and 4, but did not detect the third root, namely, — 7. Another 
question asked was to find solutions of the indeterminate 
equation a:^ — y^ = lSS: to this he replied immediately 66 
and 67 ; asked for a simpler solution he said after an instant 
6 and 13. I do not however propose to discuss their feats in 
detail, for there was at least a suspicion that these lads were not 
frank, and that those who were exploiting them had taught 
them rules which enabled them to simulate powers they did not 
really possess. Finally both returned to farm work, and ceased 
to interest the scientific world. If Mondeux was self-taught we 
must credit him with a discovery of some algebraic theorems 
which would entitle him to rank as a mathematical genius, but 
in that case it is inconceivable that he never did anything more, 
and that his powers appeared to be limited to the particular 
problems solved by him. 

Johann Martin Zacharias Base, whom I next mention, is 
a far more interesting example of these calculating prodigies. 
Dase was born in 1824 at Hamburg. He had a fair education, 
and was afforded every opportunity to develop his powers, but 
save in matters connected with reckoning and numbers he 
made little progress and struck all observers as dull. Of 



CH. Xl] CALCULATING PRODIGIES 261 

geometry and any language but German he remained ignorant 
to the end of his days. He was trustworthy and filled various 
small official posts in Germany. He gave exhibitions of his 
calculating powers in Germany, Austria, and England. He died 
in 1861. 

When exhibiting in Vienna in 1840, he made the acquaint- 
ance of Strasznicky who urged him to apply his powers to 
scientific purposes. This Dase gladly agreed to do, and so 
became acquainted with Gauss, Schumacher, Petersen, and 
Encke. To his contributions to science I allude later. In 
mental arithmetic the only problems to which I find allusions 
are straightforward examples like the following: — Multiply 
79,532,853 by 93,758,479 : asked by Schumacher, answered in 
54 seconds. In answer to a similar request to find the product 
of two numbers each of twenty digits he took 6 minutes ; to 
find the product of two numbers each of forty digits he took 
40 minutes; to find the product of two numbers each of a 
hundred digits he took 8 hours 45 minutes. Gauss thought 
that perhaps on paper the last of these problems could be 
solved in half this time by a skilled computator. Dase once 
extracted the square root of a number of a hundred digits in 
52 minutes. These feats far surpass all other records of the 
kind, the only calculations comparable to them being Buxton's 
squaring of a number of thirty-nine digits, and Wallis' extrac- 
tion of the square root of a number of fifty-three digits. Dase's 
mental work however was not always accurate, and once (in 
1845) he gave incorrect answers to every question put to him, 
but on that occasion he had a headache, and there is nothing 
astonishing in his failure. 

Like all these calculating prodigies he had a wonderful 
memory, and an hour or two after a performance could repeat 
all the numbers mentioned in it. He had also the peculiar 
gift of being able after a single glance to state the number (up 
to about 30) of sheep in a flock, of books in a case, and so on ; 
and of visualizing and recollecting a large number of objects. 
For instance, after a second s look at some dominoes he gave 
the sum (117) of their points; asked how many letters were in 



262 CALCULATING PRODIGIES [CH. XI 

a certain line of print chosen at random in a quarto page he 
instantly gave the correct number (63); shown twelve digits 
he had in half a second memorized them and their positions so 
as to be able to name instantly the particular digit occupying 
any assigned place. It is to be regretted that we do not know 
more of these performances. Those who are acquainted with 
the delightful autobiography of Robert-Houdin will recollect 
how he cultivated a similar power, and how valuable he found 
it in the exercise of his art. 

Dase's calculations, when also allowed the use of paper and 
pencil, were almost incredibly rapid, and invariably accurate. 
When he was sixteen years old Strasznicky taught him the use 
of the familiar formula 7r/4 = tan-^ (^) + tan-^ (i) + tan-^ (4), and 
asked him thence to calculate tt. In two months he carried 
the approximation to 205 places of decimals, of which 200 
are correct*. Dase's next achievement, was to calculate the 
natural logarithms of the first 1,005,000 numbers to 7 places of 
decimals; he did this in his off-time from 1844 to 1847, when 
occupied by the Prussian survey. During the next two years 
he compiled in his spare time a hyperbolic table which was 
published by the Austrian Government in 1857. Later he 
offered to make tables of the factors of all numbers from 
7,000 000 to 10,000,000 and, on the recommendation of Gauss, 
the Hamburg Academy of Sciences agreed to assist him so that 
he might have leisure for the purpose, but he lived only long 
enough to finish about half the work. 

Truman Henry Safford, born in 1836 at Royal ton, Vermont, 
U.S.A., was another calculating prodigy. He was of a some- 
what different type for he received a good education, graduated 
in due course at Harvard, and ultimately took up astronomy in 
which subject he held a professional post. I gather that though 
always a rapid calculator, he gradually lost the exceptional 
powers shown in his youth. He died in 1901. 

Safford never exhibited his calculating powers in public, and 
I know of them only through the accounts quoted by Scripture 

* The result was published in Grelle's Journal, 1844, vol. xxvii, p. 198 : on 
closer approximations and easier formulae, see below chapter xiii. 



CH. Xl] CALCULATING PRODIGIES 263 

and Mitchell, but they seem to have been t3^ical of these calcu- 
lators. In 1842, he amused and astonished his family by mental 
calculations. In 1846, when ten years old, he was examined, 
and here are some of the questions then put to him : — Extract 
the cube root of a certain number of seven digits; answered 
instantly. What number is that which being divided by the 
product of its digits, the quotient is three, and if 18 be added 
the digits will be inverted : answer 24, given in about a minute. 
What is the surface of a regular pyramid whose slant height is 
17 feet, and the base a pentagon of which each side is 33*5 feet : 
answer 3354*5558 square feet, given in two minutes. Asked to 
square a number of eighteen digits he gave the answer in a 
minute or less, but the question was made the more easy as the 
number consisted of the digits 365 repeated six times. Like 
Colburn he factorized high numbers with ease. In such 
examples his processes were empirical, he selected (he could 
not tell how) likely factors and tested the matter in a few 
seconds by actual division. 

There are to-day four calculators of some note. These are 
Ugo Zamehone, an Italian, born in 1867 ; Pericles Biamandi, a 
Greek, bom in 1868; Carl Ruckle^ a German; and Jacques 
Inaudi, bom in 1867. The three first mentioned are of the 
normal type and I do not propose to describe their perform- 
ances, but Inaudi's performances merit a fuller treatment. 

Jacques Inaudi* was born in 1867 at Onorato in Italy. He 
was employed in early years as a sheep-herd, and spent the long 
idle hours in which he had no active duties in pondering on 
numbers, but used for them no concrete representations such as 
pebbles. His calculating powers first attracted notice about 
1873. Shortly afterwards his elder brother sought his fortune 
as an organ grinder in Provence, and young Inaudi, accom- 
panying him, came into a wider world, and earned a few coppers 
for himself by street exhibitions of his powers. His ability 
was exploited by showmen, and thus in 1880 he visited Paris 

* See Charcot and Darboux, Memoires de Vlnstitut, Comptes Revdus, 1892, 
vol. cxiv, pp. 275, 528, 678 ; and Binet, R€vue des deux Moiides, 1892, vol. cxi, 
pp. 905—924. 



264 CALCULATING PRODIGIES [CH. XI 

where he gave exhibitions: in these he impressed all observers as 
being modest, frank, and straightforward. He was then ignorant 
of reading and writing : these arts he subsequently acquired. 

His earlier performances were not specially remarkable as 
compared with those of similar calculating prodigies, but with 
continual practice he improved. Thus at Lyons in 1873 he 
could multiply together almost instantaneously two numbers of 
three digits. In 1874 he was able to multiply a number of six 
digits by another number of six digits. Nine years later he 
could work rapidly with numbers of nine or ten digits. Still 
later, in Paris, asked by Darboux to cube 27, he gave the 
answer in 10 seconds. In 13 seconds he calculated how many 
seconds are contained in 18 years 7 months 21 days 3 hours : 
and he gave immediately the square root of one-sixth of the 
difference between the square of 4801 and unity. He also 
calculated with ease the amount of wheat due according to 
the traditional story to Sessa who, for inventing chess, was 
to receive 1 grain on the first cell of a chess-board, 2 on the 
second, 4 on the third, and so on in geometrical progression. 

He can find the integral roots of equations and integral 
solutions of problems, but proceeds only by trial and error. 
His most remarkable feat is the expression of numbers less 
than 10^ in the form of a sum of four squares, which he can 
usually do in a minute or two ; this power is peculiar to him. 
Such problems have been repeatedly solved at private perform- 
ances, but the mental strain caused by them is considerable. 

A performance before the general public rarely lasts more 
than 12 minutes, and is a much simpler affair. A normal 
programme includes the subtraction of one number of twenty- 
one digits from another number of twenty-one digits : the 
addition of five numbers each of six digits : the multiplying 
of a number of four digits by another number of four digits : 
the extraction of the cube root of a number of nine digits, and 
of the fifth root of a number of twelve digits : the determina- 
tion of the number of seconds in a period of time, and the day 
of the week on which a given date falls. Of course the 
questions are put by members of the audience. To a pro- 



CH. Xl] CALCULATING PRODIGIES 265 

fessional calculator these problems are not particularly difficult. 
As each number is announced, Inaudi repeats it slowly to his 
assistant, who writes it on a blackboard, and then slowly reads 
it aloud to make sure that it is right. Inaudi >^then repeats 
the number once more. By this time he has generally solved 
the problem, but if he wants longer time he makes a few 
remarks of a general character, which he is able to do with- 
out interfering with his mental calculations. Throughout the 
exhibition he faces the audience : the fact that he never even 
glances at the blackboard adds to the eflfect. 

It is probable that the majority of calculating prodigies rely 
on the speech muscles as well as on the eye and the ear to help 
them to recollect the figures with which they are dealing. It 
was formerly believed that they all visualized the numbers 
proposed to them, and certainly some have done so. Inaudi 
however trusts mainly to the ear and to articulation. Bidder 
also relied partly on the ear, and when he visualized a number 
it was not as a collection of digits but as a concrete collection 
of units divisible, if the number was composite, into definite 
groups. Ruckle relies mainly on visualizing the numbers. So 
it would seem that there are different types of the memories 
of calculators. Inaudi can reproduce mentally the sound of 
the repetition of the digits of the number in his own voice, and 
is confused, rather than helped, if the numbers are shown to 
him in writing. The articulation of the digits of the number 
also seems necessary to enable him fully to exhibit his powers, 
and he is accustomed to repeat the numbers aloud before 
beginning to work on them — the sequence of sounds being 
important. A number of twenty-four digits being read to 
him, in 59 seconds he memorized the sound of it, so that he 
could give the sequence of digits forwards or backwards from 
selected points — a feat which Mondeux had taken 5 minutes 
to perform. Numbers of about a hundred digits were similarly 
memorized by Inaudi in 12 minutes, by Diamandi in 25 minutes, 
and by Ruckle in under 5 minutes. This power is confined to 
numbers, and calculators cannot usually recollect a long sequence 
of letters. Numbei-s are ever before Inaudi : he thinks of little 



266 CALCULATING PRODIGIES [CH. XI 

else, he dreams of them, and sometimes even solves problems 
in his sleep. His memory is excellent for numbers, but normal 
or subnormal for other things. At the end of a seance he can 
repeat the questions which have been put to him and his 
answers, involving hundreds of digits in all. Nor is his memory 
in such matters limited to a few hours. Once eight days after 
he had been given a question on a number of twenty-two 
digits, he was unexpectedly asked about it, and at once 
repeated the number. He has been repeatedly examined, and 
we know more of his work than of any of his predecessors, with 
the possible exception of Bidder. 

Most of these calculating prodigies find it difficult or im- 
possible to explain their methods. But we have a few analyses 
by competent observers of the processes used : notably one by 
Bidder on his own work ; another by Colbum on his work ; and 
others by Mtiller and Darboux on the work of Ruckle and 
Inaudi respectively. That by Bidder is the most complete, and 
the others are on much the same general lines. 

Bidder's account of the processes he had discovered and 
used is contained in a lecture* given by him in 1856 to the 
Institution of Civil Engineers. Before describing these pro- 
cesses there are two remarks of a general character which 
should, I think, be borne in mind when reading his statement. 
In the first place he gives his methods in their perfected form, 
and not necessarily in that which he used in boyhood : more- 
over it is probable that in practice he employed devices to 
shorten the work which he did not set out in his lecture. In 
the second place it is certain, in spite of his belief to the 
contrary, that he, like most of these prodigies, had an excep- 
tionally good memory, which was strengthened by incessant 
practice. One example will suffice. In 1816, at a performance, 
a number was read to him backwards : he at once gave it in its 
normal form. An hour later he was asked if he remembered 

* Institution of Civil Engineers, Proceedings, London, 1856, vol. xv, 
pp. 251 — 280. An early draft of the lecture is extant in MS. ; the variations 
made in it are interesting, as showing the history of his mental development, 
but are not sufficiently important to need detailed notice here. 



CH. Xl] CALCULATING PRODIGIES 267 

it : he immediately repeated it correctly. The number was : — 
2,563,721,987,653,461,598,746,231,905,607,541,128,975,231. 

Of the four fundamental processes, addition and subtraction 
present no difficulty and are of little interest. The only point 
to which it seems worth calling attention is that Bidder, in 
adding three or more numbers together, always added them one 
at a time, as is illustrated in the examples given below. Rapid 
mental arithmetic depended, in his opinion, on the arrangement 
of the work whenever possible, in such a way that only one fact 
had to be dealt with at a time. This is also noticeable in 
Inaudi's work. 

The multiplication of one number by another was, naturally 
enough, the earliest problem Bidder came across, and by the time 
he was six years old he had taught himself the multiplication 
table up to 10 times 10. He soon had practice in harder sums, 
for, being a favourite of the village blacksmith, and constantly 
in the smithy, it became customary for the men sitting round 
the forge-fire to ask him multiplication sums. From products 
of numbers of two digits, which he would give without any 
appreciable pause for thought, he rose to numbers of three 
and then of four digits. Halfpence rewarded his efforts, and 
by the time he was eight years old, he could multiply together 
two numbers each of six digits. In one case he even multiplied 
together two numbers each of twelve digits, but, he says, 
"it required much time," and "was a great and distressing 
effort." 

The method that he used is, in principle, the same as that 
explained in the usual text-books, except that he added his 
results as he went on. Thus to multiply 397 by 173 he pro- 



ceeded as follows : — 








We have 


100 X 397 = 39700, 






to this must be added 


70 X 300 = 21000 


making 


60,700, 


>j » >> >> >> 


70 X 90 = 6300 


ii 


67,000, 


» » » }> j> 


70 X 7 = 490 


» 


67,490, 


}) »» » » » 


3 X 300 = 900 


>» 


68,390, 


)> » » » >j 


3 X 90 = 270 


it 


68,660, 


» » n a »> 


3 X 7 = 21 


f* 


68,681. 



CALCULATING PRODIGIES [CH. XI 

We shall underrate his rapidity if we allow as much as 
a second for each of these steps, but even if we take this low 
standard of his speed of working, he would have given the 
answer in 7 seconds. By this method he never had at one time 
more than two numbers to add together, and the factors are 
arranged so that each of them has only one significant digit : 
this is the common practice of mental calculators. It will also 
be observed that here, as always, Bidder worked from left to 
right: this, though not usually taught in our schools, is the 
natural and most convenient way. In effect he formed the 
product of (100 + 70 + 3) and (300 + 90 + 7), or (a + 6 + c) and 
(d+e +/) in the form ad -\- ae . . . -\- ef. 

The result of a multiplication like that given above was 
attained so rapidly as to seem instantaneous, and practically gave 
him the use of a multiplication table up to 1000 by 1000. On. 
this basis, when dealing with much larger numbers, for instance, 
when multiplying 965,446,371 by 843,409,133, he worked by 
numbers forming groups of 3 digits, proceeding as if 965, 446, 
&c., were digits in a scale whose radix was 1000 : in middle 
life he would solve a problem like this in about 6 minutes. 
Such difficulty as he experienced in these multiplications seems 
to have been rather in recalling the result of the previous step 
than in making the actual multiplications. 

Inaudi also multiplies in this way, but he is content if one of 
the factors has only one significant digit: he also sometimes makes 
use of negative quantities : for instance he thinks of 27 x 729 as 
27 (730 - 1) ; so, too, he thinks of 25 x 841 in the form 84100/4: 
and in squaring numbers he is accustomed to think of the 
number in the form a + b, choosing a and b of convenient forms, 
and then to calculate the result in the form a^ + 2ah + b\ 

In multiplying concrete data by a number Bidder worked 
on similar lines to those explained above in the multiplication 
of two numbers. Thus to multiply £14. 15*. 6fd by 787 he 
proceeded thus: 

We have £(787) (14) = £11018. Os. Od. 

to which we add (787) (15) shillings =£590. 5s. O^^. making £11608. 5s. Od. 
to which we add (787) (27) farthings = £22. 2s. 8|o?. making £11630. 7s.8^d, 



CH. Xl] CALCULATING PRODIGIES 269 

Division was performed by Bidder much as taught in school- 
books, except that his power of multiplying large numbers at 
sight enabled him to guess intelligently and so save unnecessary 
work. This also is Inaudi's method. A division sum with a 
remainder presents more difficulty. Bidder was better skilled 
in dealing with such questions than most of these prodigies, 
but even in his prime he never solved such problems with the 
same rapidity as those with no remainder. In public perform- 
ances difficult questions on division are generally precluded by 
the rules of the game. 

If, in a division sum, Bidder knew that there was no 
remainder he often proceeded by a system of two-digit 
terminals. Thus, for example, in dividing (say) 25,696 by 
176, he first argued that the answer must be a number of 
three digits, and obviously the left-hand digit must be 1. 
Next he had noticed that there are only 4 numbers of two 
digits (namely, 21, 46, 71, 96) which when multiplied by 76 
give a number which ends in 96. Hence the answer must 
be 121, or 146, or 171, or 196; and experience enabled him 
to say without calculation that 121 was too small and 171 
too large. Hence the answer must be 146. If he felt 
any hesitation he mentally multiplied 146 by 176 (which he 
said he could do "instantaneously") and thus checked the 
result. It is noticeable that when Bidder, Colbum, and some 
other calculating prodigies knew the last two digits of a product 
of two numbers they also knew, perhaps subconsciously, that 
the last two digits of the separate numbers were necessarily of 
certain forms. The theory of these two-digit arrangements has 
been discussed by Mitchell. 

Frequently also in division, Bidder used what I will call a 
digital process, which a priori would seem far more laborious 
than the normal method, though in his hands the method was 
extraordinarily rapid : this method was, I think, peculiar to him. 
I define the digital of a number as the digit obtained by finding 
the sum of the digits of the original number, the sum of the 
digits of this number, and so on, until the sum is less than 10. 
Obviously the digital of a number is the same as the digital of 



270 CALCULATING PRODIGIES [CH. XI 

the product of the digitals of its factors. Let us apply this in 
Bidder's way to see if 71 is an exact divisor of 23,141. The 
digital of 23,141 is 2. The digital of 71 is 8. Hence if 71 is 
a factor the digital of the other factor must be 7, since 7 times 
8 is the only multiple of 8 whose digital is 2. Now the only 
number which multiplied by 71 will give 41 as terminal digits 
is 71. And since the other factor must be one of three digits 
and its digital must be 7, this factor (if any) must be 871. 
But a cursory glance shows that 871 is too large. Hence 71 
is not a factor of 23,141. Bidder found this process far more 
rapid than testing the matter by dividing by 71. As another 
example let us see if 73 is a factor of 23,141. The digital of 
23,141 is 2 ; the digital of 73 is 1 ; hence the digital of the 
other factor (if any) must be 2. But since the last two digits 
of the number are 41, the last two digits of this factor (if any) 
must be 17. And since this factor is a number of three digits 
and its digital is 2, such a factor, if it exists, must be 317. 
This on testing (by multiplying it by 73) is found to be a 
factor. 

When he began to exhibit his powers in public, questions 
concerning weights and measures were, of course, constantly 
proposed to him. In solving these he knew by heart many 
facts which frequently entered into such problems, such as the 
number of seconds in a year, the number of ounces in a ton, 
the number of square inches in an acre, the number of pence 
in a hundred pounds, the elementary rules about the civil and 
ecclesiastical calendars, and so on. A collection of such data is 
part of the equipment of all calculating prodigies. 

In his exhibitions Bidder was often asked questions con- 
cerning square roots and cube roots, and at a later period 
higher roots. That he could at once give the answer excited 
unqualified astonishment in an uncritical audience ; if, however, 
the answer is integral, this is a mere sleight of art which 
anyone can acquire. Without setting out the rules at length, 
a few examples will illustrate his method. 

He was asked to find the square root of 337,561. It is 
obvious that the root is a number of three digits. Since the 



CH. Xl] CALCULATING PRODIGIES 271 

given number lies between 500^ or 250,000 and eOO'' or 360,000, 
the left-hand digit of the root must be a 5. Reflection had 
shown him that the only numbers of two digits, whose squares 
end in 61 are 19, 31, 69, 81, and he was familiar with this fact. 
Hence the answer was 519, or 531, or 569, or 581. But he 
argued that as 581 was nearly in the same ratio to 500 and 
600 as 337,561 was to 250,000 and 360,000, the answer must 
be 581, a result which he verified by direct multiplication in a 
couple of seconds. Similarly in extracting the square root of 
442,225, he saw at once that the left-hand digit of the answer 
was 6, and since the number ended in 225 the last two digits 
of the answer were 15 or 35, or 65 or 85. The position of 
442,225 between (600)^ and (700)^ indicates that 65 should be 
taken. Thus the answer is 665, which he verified, before 
announcing it. Other calculators have worked out similar 
rules for the extraction of roots. 

For exact cube roots the process is more rapid. For 
example, asked to extract the cube root of 188,132,517, Bidder 
saw at once that the answer was a number of three digits, and 
since 5» = 125 and 6' = 216, the left-hand digit was 5. The only 
number of two digits whose cube ends in 17 is 73. Hence the 
answer is 573. Similarly the cube root of 180,362,125 must be 
a number of three digits, of which the left-hand digit is a 5, and 
the two right-hand digits were either 65 or 85. To see which 
of these was required he mentally cubed 560, and seeing it was 
near the given number, assumed that 565 was the required 
answer, which he verified by cubing it. In general a cube root 
that ends in a 5 is a trifle more difficult to detect at sight by 
this method than one that ends in some other digit, but since 
5^ must be a factor of such numbers we can divide by that and 
apply the process to the resulting number. Thus the above 
number 180,362,125 is equal to 5=^ x 1,442,897 of which the 
cube root is at once found to be 5 (113), that is, 565. 

For still higher exact roots the process is even simpler, and 
for fifth roots it is almost absurdly easy, since the last digit of 
the number is always the same as the last digit of the root. 
Thus if the number proposed is less than 10^^ the answer 



272 CALCULATING PRODIGIES [CH. XI 

consists of a number of two digits. Knowing the fifth powers 
to 10, 20, ...90 we have, in order to know the first digit of the 
answer, only to see between which of these powers the number 
proposed lies, and the last digit being obvious we can give the 
answer instantly. If the number is higher, but less than 10^', 
the answer is a number of three digits, of which the middle 
digit can be found almost as rapidly as the others. This is 
rather a trick than a matter of mental calculation. 

In his later exhibitions, Bidder was sometimes asked to 
extract roots, correct to the nearest integer, the exact root 
involving a fraction. If he suspected this he tested it by 
"casting out the nines," and if satisfied that the answer was 
not an integer proceeded tentatively as best he could. Such 
a question, if the answer is a number of three or more digits, 
is a severe tax on the powers of a mental calculator, and is 
usually disallowed in public exhibitions. 

Colburn's remarkable feats in factorizing numbers led to 
similar questions being put to Bidder, and gradually he evolved 
some rules, but in this branch of mental arithmetic I do not 
think he ever became proficient. Of course a factor which is a 
power of 2 or of 5 can be obtained at once, and powers of 3 can 
be obtained almost as rapidly. For factors near the square root 
of a number he always tried the usual method of expressing the 
number in the form a^ — 6^ in which case the factors are obvious. 
For other factors he tried the digital method already described. 

Bidder was successful in giving almost instantaneously the 
answers to questions about compound interest and annuities: 
this was peculiar to him, but his method is quite simple, and 
may be illustrated by his determination of the compound 
interest on £100 at 5 per cent, for 14 years. He argued that 
the simple interest amounted to £(14) (5), i.e. to £70. At the 
end of the first year the capital was increased by £5, the annual 
interest on this was 5^. or one crown, and this ran for 13 years, 
at the end of the second year another £5 was due, and the 5s. 
interest on this ran for 12 years. Continuing this argument he 
had to add to the £70 a sum of (13 + 12 + ... + 1) crowns, i.e. 
(13/2) (14) (5) shillings, i.e. £22. los. Od., which, added to the 



CH. Xl] CALCULATING PRODIGIES 273 

£70 before mentioned, made £92. 155. Od. Next the 5*. due at 
the end of the second year (as interest on the £5 due at the end 
of the first year) produced in the same way an annual interest 
of Sd. All these three-pences amount to (12/3) (13/2) (14) (3) 
pence, i.e. £4. lis. Od. which, added to the previous sum of 
£92. 155. Od., made £97. 6s. Od. To this we have similarly 
to add (11/4) (12/3) (13/2) (14) (3/20) pence, i.e. 12s. 6d., which 
made a total of £97. I85. 6d. To this again we have to add 
(10/5) (11/4) (12/3) (13/2) (14) (3/400) pence, i.e. Is. 3d, which 
made a total of £97. 19^. 9d. To this again we have to add 
(9/6) (10/5) (11/4) (12/3) (13/2) (14) (3/8000) pence, i.e. Id., 
which made a total of £97. 195. 10c?. The remaining sum to 
be added cannot amount to a farthing, so he at once gave the 
answer as £97. 195. 10c?. The work in this particular example 
did in fact occupy him less than one minute — a much shorter 
time than most mathematicians would take to work it by aid 
of a table of logarithms. It will be noticed that in the course 
of his analysis he summed various series. 

In the ordinary notation, the sum at compound interest 
amounts to £(1-05)" x 100. If we denote £100 by P and '05 
by r, this is equal to P(l+r)" or P (1 + 14r + 91r»+ ...), 
which, as r is small, is rapidly convergent. Bidder in effect 
arrived by reasoning at the successive terms of the series, and 
rejected the later terms as soon as they were sufficiently small. 

In the course of this lecture Bidder remarked that if his 
ability to recollect results had been equal to his other intel- 
lectual powers he could easily have calculated logarithms. 
A few weeks later he attacked this problem, and devised a 
mental method of obtaining the values of logarithms to seven 
or eight places of decimals. He asked a friend to test his 
accuracy, and in answer to questions gave successively the 
logarithms of 71, 97, 659, 877, 1297, 8963, 9973, 115249, 
175349, 290011, 350107, 229847, 369353, to eight places of 
decimals ; taking from thirty seconds to four minutes to make 
the various calculations. All these numbers are primes. The 
greater part of the answers were correct, but in a few cases 
there was an error, though generally of only one digit : such 
B. a. 18 



274 CALCULATING PRODIGIES [CH. XI 

mistakes were at once corrected on his being told that his 
result was wrong. This remarkable performance took place 
when Bidder was over 50. 

His method of calculating logarithms is set out in a paper* 
by W. Pole. It was, of course, only necessary for him to deal 
with prime numbers, and Bidder began by memorizing the 
logarithms of all primes less than 100. For a prime higher 
than this he took a composite number as near it as he could, 
and calculated the approximate addition which would have to 
be added to the logarithm : his rules for effecting this addition 
are set out by Pole, and, ingenious though they are, need not 
detain us here. They rest on the theorems that, to the 
number of places of decimals quoted, if log n is p, then 
log (n + n/W^) is p + log I'Ol, i.e. is^ + 0-0043214, log (n + n/W) 
is j9+ 0-00043407, log (n-fn/10^) is ^+0*0000434, log (n+n/W) 
isp + 0-0000043, and so on. 

The last two methods, dealing with compound interest and 
logarithms, are peculiar to Bidder, and show real mathematical 
skill. For the other problems mentioned his methods are much 
the same in principle as those used by other calculators, though 
details vary. Bidder, however, has set them out so clearly that 
I need not discuss further the methods generally used. 

A curious question has been raised as to whether a law for 
the rapidity of the mental work of these prodigies can be found. 
Personally I do not think we have sufficient data to enable us 
to draw any conclusion, but I mention briefly the opinions of 
others. We shall do well to confine ourselves to the simplest 
case, that of the multiplication of a number of n digits by 
another number of n digits. Bidder stated that in solving 
such a problem he believed that the strain on his mind (which 
he assumed to be proportional to the time taken in answering 
the question) varied as n'^, but in fact it seems in his case 
according to this time test to have varied approximately as w'. 
In 1855 he worked at least half as quickly again as in 1819, but 
the law of rapidity for different values of n is said to have been 

* Institution of Civil Engineers, Proceedings, London, 1890 — 1S91, vol. cm, 
p. 250. 



CH. Xl] CALCULATING PRODIGIES 275 

about the same. In Base's case, if the time occupied is pro- 
portional to n^, we must have x less than 3. From this, some 
have inferred that probably Base's methods were different in 
character to those used by Bidder, and it is suggested that the 
results tend to imply that Base visualized recorded numerals, 
working in much the same way as with pencil and paper, while 
Bidder made no use of symbols, and recorded successive results 
verbally in a sort of cinematograph way; but it would seem 
that we shall need more detailed observations before we can 
frame a theory on this subject. 

The cases of calculating prodigies here mentioned, and as 
far as I know the few others of which records exist, do not 
differ in kind. In most of them the calculators were unedu- 
cated and self-taught. Blessed with excellent memories for 
numbers, self-confident, stimulated by the astonishment their 
performances excited, the odd coppers thus put in their pockets 
and the praise of their neighbours, they pondered incessantly 
on numbers and their properties ; discovered (or in a few cases 
were taught) the fundamental arithmetical processes, applied 
them to problems of ever increasing difficulty, and soon acquired 
a stock of information which shortened their work. Probably 
constant practice and undivided devotion to mental calculation 
are essential to the maintenance of the power, and this may 
explain why a general education has so often proved destructive 
to it. The performances of these calculators are remarkable, 
but, in the light of Bidder's analysis, are not more than might 
be expected occasionally from lads of exceptional abilities. 



18—2 



276 



CHAPTER XII. 

ARITHMETICAL CALCULATING MACHINES. 

The advantage of employing mechanical aids to calculation 
has been recognized from very early times. For instance, the 
abacus — common in Europe until the Renaissance and still 
employed in China — has long been widely used for ordinary 
arithmetical calculation ; slips for facilitating multiplication and 
division, by which an operator escapes the necessity of knowing 
the multiplication table, were well known in the East before 
Napier introduced his "rods," and are familiar to students of 
the history of arithmetic; while slide-rules for arithmetical 
work, introduced a year or two after the publication of the 
earliest logarithmic tables, are in general use to-day in western 
Europe and America. Of late, however, more elaborate calcu- 
lating machines have come into vogue in banks, offices, and 
shops*. 

Calculating machines may be classified as addition machines ; 
machines directly fitted for multiplication and division as well 
as addition and subtraction ; difference machines ; and machines 
for performing other analytical calculations such as solving 
equations and evaluating integrals. Of these I propose to 
consider only those constructed for executing elementary 
arithmetical processes, and of these latter I shall not further 
allude to the abacus whose form and use are described in many 

* On modern Calculating Machines, see an article by F. J. W. Whipple 
printed in the Handbook to the Napier Tercentenary Exhibition, Edinburgh, 
1914, pp. 69—135. On the use of the Abacus see C. G. Knott, Ibid., pp. 136— 
154 ; and F. P. Barnard, TJie Casting Counter, Oxford, 1916, pp. 264—319. 



CH. XIl] ARITHMETICAL CALCULATING MACHINES 277 

text-books, or to the modem machines where plates are moved 
by hand over one another. Moreover I concern myself only 
with general principles and do not attempt to describe the 
mechanical devices by which the desired results are secured. 

Addition Machines, in order to be eflBcient, must in general 
contain arrangements for showing the primary number on which 
the operation is to be performed, and if possible also the 
secondary number to be added to it ; for adding to each digit 
of the primary number the requisite number of units shown by 
the corresponding digit of the secondary number; for auto- 
matically " carrying " whenever that operation is required ; and 
for rapidly " clearing " the machine so that any other addition 
can be started on it afresh. The motive power is commonly 
supplied by turning a handle or pressing a key, and the arrange- 
ments of wheels and levers must be such that the final result 
can be obtained mechanically. 

In the majority of modern machines these requirements are 
met thus : There are a series of wheels, each wheel having 
twenty faces marked successively with the digits 0, 1, 2, ... 9, 
0, 1, ... 9. The wheels for the primary number are mounted on 
a single horizontal axis, and usually are covered by a frame with 
a slit or window cut in it through which one and only one face 
of each wheel can be seen. On turning any wheel through one 
twentieth of a complete revolution, called a step, the visible 
figure will be changed into that immediately above or below it. 
If two neighbouring wheels are geared together it is easy to 
see how the addition of unity to the digit 9 on any particular 
wheel which changes the 9 into may also be used to rotate 
the wheel to the left of it through one digit, thus " carrying" 1, 
a process familiar to everyone in the mechanism of a gas meter, 
a speedometer, or watch with a second hand. A similar set of 
wheels is provided for the secondary number. The wheels are 
usually constructed with twenty faces rather than ten faces in 
order that half a revolution of the wheel may suffice to show 
all the right-hand digits of the various additions and the second 
half of the revolution be available for "carrying." 

Suppose for instance that we want to add to the primary 



278 ARITHMETICAL CALCULATING MACHINES [CH. XII 

number 978 the secondary number 94. The machine is set so 
that the primary number appears through the window. In the 
earliest machines the operator first added 4 to the units by 
rotating the unit wheef through 4 steps, which made the 
number read 982, and then rotated the second wheel through 
9 steps which gave the total result of 1072 ; originally these 
13 steps were all done separately and by hand, and nothing 
appeared on the machine to show that 94 was the sum which 
had been added. In such a form the machine was of no 
practical use. In modern machines the secondary number 94 is 
set on a secondary set of wheels and is shown on a second 
window placed below the first window. Each wheel in the 
secondary set is connected with the corresponding wheel in the 
primary set. The whole 13 steps are then made by a single 
mechanical movement, such as turning a handle attached to 
the axis on which the primary wheels are set, and the result 
is shown on the face of the machine. 

We can in various ways make a single half revolution of the 
axis give the unit digit of the sum of the corresponding digits 
of the primary and secondary numbers. One method is by 
the use of a "rocking segment" — the amplitude of the rock 
corresponding to each digit in the secondary number being 
determined by the key used to indicate that digit in the 
secondary window ; this is used in Burroughs machine and in 
many Cash Registers. Another way is to use a "stepped 
reckoner " or a " rack " in which the gearing connecting the 
corresponding digits in the primary and secondary numbers 
varies according to the digit set in the secondary number ; this 
is used in machines of the Thomas type, such as the Arithmo- 
meter and Saxonia. A third method is to make the setting of 
the digit in the secondary number push out in the gearing wheel 
a number of cogs corresponding to that digit ; this is easy to 
effect and is used in the Brunsviga machine. By any of these 
methods the additions of all the corresponding digits in the 
primary and secondary numbers are made simultaneously. 
We have also to provide for the requisite " carryings," and 
these necessarily are done successively from right to left. 



CH. XIl] ARITHMETICAL CALCULATING MACHINES 279 

These are effected by another half-turn of the handle. The 
whole of these additions and carryings are thus performed 
smoothly by a single revolution of the handle. In the above 
description the primary number is replaced by the required 
sum. Practically it is often convenient to see on the machine 
the primary and secondary numbers as well as their sum, and 
clearly this can be easily effected without complicating the 
mechanism. 

I have talked about applying motive power by turning a 
handle. I ought to add that in some machines it is obtained 
otherwise by the force exerted in pressing the knobs or keys 
used to record the digits in the secondary number ; the former 
method is simpler and mechanically preferable, but it is alleged 
that the latter method is slightly more rapid. Originally all 
the wheels were geared in train. Consequently when unity was 
added to a number like 9999, where there were four carryings, 
considerable force had to be used to drive the machine, and a 
heavy strain was put on the mechanism. This defect was 
obviated by a way, invented by Roth, about 1840, of placing the 
wheels on the axis, and in machines worked by a handle the 
carrying is done by the second half-turn of the axis. In modem 
machines the expenditure of but little force is necessary. 

It is important that the operations should be rapid, and 
accordingly the velocity of rotation of the wheels is great. All 
modem machines contain devices for stopping the wheels 
abruptly without straining the mechanism. 

Various schemes have been proposed for clearing the 
machine by pressing a lever followed by a single turn of the 
handle, and some arrangement of the kind is necessary in 
practice. It is also necessary to be able to throw a single wheel 
out of gear so as to correct a mistake in setting a figure if one 
has been made. These mechanical problems have been solved 
in many ways. 

There is no difficulty in extending the scope of the machine 
so as to enable numerous additions to be made at once. The 
mechanism by which the total sum is produced is called a 
Totaliser, 



280 ARITHMETICAL CALCULATING MACHINES [CH. XII 

An addition machine can always be used to effect subtrac- 
tion, multiplication, division, and commercial operations which 
involve them, since all these processes can be reduced to 
successive additions. It is not suited to operations in which 
we should usually employ logarithms or slide-rules, and there is 
no advantage in evolving complicated processes by which it can 
be used in such calculation. 

The adaption of addition machines to direct Subtraction 
requires a device for reversing the motion of the secondary 
wheels. If the motive power is supplied by turning a handle 
this can be easily done. When the machine is driven by pressing 
keys the motion is always direct and it is usual to evade the 
difficulty by substituting for subtraction the addition of a 
number made up of the complementary digits. For instance, 
if we wish to subtract 723 from any number we can obtain the 
result by adding the complementary number 277 and then 
subtracting unity from the thousand digits. Preferably this 
is done by prefixing to the complementary number as many 
" 9's " as the scope of the machine allows, in which case it is 
unnecessary to subtract anything from the visible result of the 
addition. Thus, if we are working with a machine which shows 
only six digits and we wish to subtract 723 from 8147 we can 
obtain the result by adding to it 999277, and the visible result 
will read 007424, the machine only showing six digits. Machines 
are made which on pressing a lever or using another row of 
keys set the complementary number directly, thus making the 
process of finding it entirely automatic. 

Multiplication consists essentially in repeated additions. In 
the majority of arithmetical machines it is effected in this way, 
namely, by adding the number to itself the requisite number of 
times. For multipliers not exceeding 9 or 10 this suffices, but 
for higher numbers the process is not convenient. In order to 
multiply by a number like 72 we want to introduce subsidiary 
mechanism by which we can first multiply by 70 by the use of a 
lever moving the primary number one place to the left, followed 
by 7 turns of the handle and then multiply by 2 by making 
2 turns of the handle, from which the final result would be 



CH. XIl] ARITHMETICAL CALCULATING MACHINES 281 

obtainable. The problem of effecting this, without constant 
attention to the machine, has (I believe) so far baffled in- 
ventors, but various arrangements exist by which the result 
can be secured by some manipulation after a few revolutions 
of the handle. 

The result of Division can always be obtained by successive 
subtraction, and for low divisors we may use this method. For 
divisors exceeding 9 or 10 this would be a tedious process and 
practically useless. If the divisors are large the older machines 
are rarely of use : if however they are employed the method is 
analogous to ordinary long division, and requires the operator to 
obtain the successive digits of the quotient by inspection. 
Unless he is very careful he will at some point probably make 
his quotient too large, and then have to retrace his steps which 
may not be easy. For this reason it is not uncommon to use 
a table of reciprocals, and thus substitute multiplication for 
division. 

Recently, however, machines have been invented by which 
multiplication and division can be performed directly. One of 
these is Steiger's Millionaire Machine*. In this if we want to 
multiply 4 by 7, a marker is set say to 4, and a pointer to 7, and 
the product 28 is recorded after a single turn of the handle. 
During this turn there are two distinct operations ; at the end 
of the first half-turn the 2 appears in the right place in the 
product and the product-carriage moves one place to the left ; in 
the second half-turn the 8 appears to the right of the 2. This 
effect is secured by controlling the amplitude of the motion of 
racks which move under pinions similar to those used with the 
stepped reckoner. Corresponding to each multiplier there is a 
tongue-plate which forms a multiplication table. For example, 
the " 7 " tongue-plate has nine pairs of tongues, the lengths of 
which correspond in length to so many cogs on the racks, 0, 7 ; 
1,4; 2,1; 2,8; 3,5; etc. When 4 is multiplied by 7 the 
fourth rack is pushed by a short tongue on the seventh tongue- 
piece through two teeth, then the tongue-piece is itself displaced 
laterally, whilst the rack returns to its original position, and 
* See Whipple, pp. 104—122. 



282 ARITHMETICAL CALCULATING MACHINES [CH. XII 

finally a longer tongue pushes the same rack through eight 
teeth. Another machine of this kind is Hamann's Mercedes- 
Euklid Arithmometer*. This is far too elaborate for me to 
attempt to describe, but it is compact, strong, noiseless, and so 
ingenious as to render it almost impossible for an operator to 
make a mistake in using it without having the error shown in 
the windows. In this, division is effected by a series of approxi- 
mations which automatically get closer and closer. 

To sum up the matter we may say that the problem of 
performing addition and subtraction mechanically has been 
completely solved, but further improvements will have to be 
introduced before multiplication and division can be performed 
as expeditiously and simply. 

Of late years typewriters have been introduced which are 
also addition machines. They can be used to type columns of 
figures and they have attached to them a totaliser which, when 
switched into use, types the total of the sums already printed. 
These machines can be arranged to suit any system of moneys, 
weights, or measures, and are now largely used in banks, 
insurance offices, and businesses. If a machine is in order there 
is no risk of error, but if it gets out of order the fact that it is 
regarded as absolutely reliable may cause trouble. I beHeve 
that recently at the headquarters of a large bank, where some 
thirty or more of these machines were used, a cog in one 
machine broke off without the fracture being detected. The 
mishap was not found until early the next morning, and mean- 
time all the additions given by that machine were inaccurate. 
It was impossible to say definitely what documents had been 
typed on this particular machine, and some weeks elapsed 
before all the errors occasioned by this unlucky breakage had 
been tracked and corrected. 

Pascal has the credit of having constructed, about 1642, the 
earliest addition machine of the type here described ; in it the 
addition in each place of figures was performed separately by 
moving a wheel by hand, and subtraction was effected by adding 
the numerical complements. In 1666 Samuel Morland adapted 
* See Whipple, pp. 104—122. 



CH. Xll] ARITHMETICAL CALCULATING MACHINES 283 

Pascal's machine for the addition of sums of money. Leibnitz 
suggested in 1671, and later in 1694 constructed, a machine 
which may be considered the predecessor of modern machines 
of the Thomas type. These machines of the seventeenth century 
were, however, little more than scientific toys. In the eighteenth 
century various technical improvements were introduced which 
brought the instruments into the field of practical usefulness 
though in fact they were as yet only rarely employed either by 
men of science or men of business. The machines described by 
Babbage during the first half of the nineteenth century were 
of a far more elaborate character and were designed to make 
and print tables of all kinds. During the closing years of that 
century the normal machines were greatly improved, mostly by 
German and American inventors, and many of the instruments 
now on the market are marvels of mechanical ingenuity. 



284 



CHAPTER XIII. 
THREE CLASSICAL GEOMETRICAL PROBLEMS. 

Among the more interesting geometrical problems of anti- 
quity are three questions which attracted the special attention 
of the early Greek mathematicians. Our knowledge of geometry 
is derived from Greek sources, and thus these questions have 
attained a classical position in the history of the subject. The 
three questions to which I refer are (i) the duplication of a 
cube, that is, the determination of the side of a cube whose 
volume is double that of a given cube ; (ii) the trisection of an 
angle; and (iii) the squaring of a circle, that is, the deter- 
mination of a square whose area is equal to that of a given 
circle — each problem to be solved by a geometrical construction 
involving the use of straight lines and circles only, that is, by 
Euclidean geometry. 

This limitation to the use of straight lines and circles implies 
that the only instruments available in Euclidean geometry are 
compasses and rulers. But the compasses must be capable of 
opening as wide as is desired, and the ruler must be of un- 
limited length. Further the ruler must not be gi'aduated, for 
if there were two fixed marks on it we can obtain constructions 
equivalent to those obtained by the use of the conic sections. 

With the Euclidean restriction all three problems are in- 
soluble*. To duplicate a cube the length of whose side is a, 

* See F. C. Klein, Vortrdge iiber ausgewdhlte Fragen der Elementargeometrie, 
Leipzig, 1895; and F. G. Texeira, Sur les Problemes celebres de la Geometrie 
Elementaire non resolubles avec la Regie et le Compos, Coimbra, 1915. It is said 
that the earliest rigorous proof that the problejus were insoluble by Euclidean 
geometry was given by P. L. Wantzell in 1837. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 286 

we have to find a line of length a?, such that a? = 2a'. Again, 
to trisect a given angle, we may proceed to find the sine of the 
angle, say a, then, if x is the sine of an angle equal to one-third 
of the given angle, we have ^s? — Zx — a. Thus the first and 
second problems, when considered analytically, require the solu- 
tion of a cubic equation ; and since a construction by means of 
circles (whose equations are of the form ar* + y^ -}- aa? + % + c = 0) 
and straight lines (whose equations are of the form aa;+ /S^/ 4- 7= 0) 
cannot be equivalent to the solution of a cubic equation, it is 
inferred that the problems are insoluble if in our constructions 
we are restricted to the use of circles and straight lines. If the 
use of the conic sections is permitted, both of these questions 
can be solved in many ways. The third problem is different in 
character, but under the same restrictions it also is insoluble. 

I propose to give some of the constructions which have 
been proposed for solving the first two of these problems. To 
save space I shall not draw the necessary diagrams, and in 
most cases I shall not add the proofs: the latter present but 
little difficulty. I shall conclude with some historical notes on 
approximate solutions of the quadrature of the circle. 

The Duplication of the Cube*, 

The problem of the duplication of the cube was known in 
ancient times as the Delian problem, in consequence of a 
legend that the Delians had consulted Plato on the subject. 
In one form of the story, which is related by Philoponusf , it 
is asserted that the Athenians in 430 B.C., when suffering fi'om 
the plague of eruptive typhoid fever, consulted the oracle at 
Delos as to how they could stop it. Apollo replied that they 
must double the size of his altar which was in the form of a 
cube. To the unlearned suppliants nothing seemed more easy, 
and a new altar was constructed either having each of its edges 

• See Histona Problematis de Cubi Duplicatione by N. T. Reimer, Gottingen, 
1798; and Historia Prohlematis Cubi Duplieandi by C. H. Biering, Copenhagen, 
1844 : also Das Delische Problem, by A. Sturm, Linz, 1895-7. Some notes on 
the subject are given in my History of Mathematics. 

t Philoponiu ad Aristotelis Analytica Posteriora, bk. i, chap. vii. 



286 THREE GEOMETRICAL PROBLEMS [CH. XIII 

double that of the old one (from which it followed that the 
volume was increased eight-fold) or by placing a similar cube 
altar next to the old one. Whereupon, according to the legend, 
the indignant god made the pestilence worse than before, and 
informed a fresh deputation that it was useless to trifle with 
him, as his new altar must be a cube and have a volume exactly 
double that of his old one. Suspecting a mystery the Athenians 
applied to Plato, who referred them to the geometricians. The 
insertion of Plato's name is an obvious anachronism. Eratos- 
thenes* relates a somewhat similar story, but with Minos as 
the propounder of the problem. 

In an Arab work, the Greek legend was distorted into the 
following extraordinarily impossible piece of history, which I 
cite as a curiosity of its kind. "Now in the days of Plato," 
says the writer, " a plague broke out among the children of 
Israel. Then came a voice from heaven to one of their prophets, 
saying, ' Let the size of the cubic altar be doubled, and the 
plague will cease ' ; so the people made another altar like unto 
the former, and laid the same by its side. Nevertheless the 
pestilence continued to increase. And again the voice spake 
unto the prophet, saying, ' They have made a second altar like 
unto the former, and laid it by its side, but that does not pro- 
duce the duplication of the cube.' Then applied they to Plato, 
the Grecian sage, who spake to them, saying, ' Ye have been 
neglectful of the science of geometry, and therefore hath God 
chastised you, since geometry is the most* sublime of all the 
sciences.' Now, the duplication of a cube depends on a rare 
problem in geometry, namely.../' And then follows the solu- 
tion of Apollonius, which is given later. 

If a is the length of the side of the given cube and x that 
of the required cube, we have a^ = 2a^, that is, x : a= ^2 : 1. 
It is probable that the Greeks were aware that the latter ratio 
is incommensurable, in other words, that no two integers can 
be found whose ratio is the same as that of v^2 : 1, but it did 
not therefore follow that they could not find the ratio by 

• Archimedis Opera cum Eutocii Commentariis, ed. Torelli, Oxford, 1792, 
p. 144; ed. Heiberg, Leipzig, 1880-1, vol. iii, pp. 104—107. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 287 

geometry: in fact, the side and diagonal of a square are 
instances of lines whose numerical measures are incommen- 
surable. 

I proceed now to give some of the geometrical constructions 
which have been proposed for the duplication of the cube*. 
With one exception, I confine myself to those which can be 
effected by the aid of the conic sections. 

Hippocrates f (circ. 420 B.C.) was perhaps the earliest mathe- 
matician who made any progress towards solving the problem. 
He did not give a geometrical construction, but he reduced the 
question to that of finding two means between one straight 
line (a), and another twice as long (2a). If these means are x 
and 2/, we have a-, x^x \ y — y \1a, from which it follows that 
a^ = 2a'. It is in this form that the problem is always presented 
now. Formerly any process of solution by finding these means 
was called a mesolabum. 

One of the first solutions of the problem was that given by 
Archytasj in or about the year 400 B.C. His construction is 
equivalent to the following. On the diameter OA of the base 
of a right circular cylinder describe S semicircle whose plane is 
perpendicular to the base of the cylinder. Let the plane con- 
taining this semicircle rotate round the generator through 0, 
then the surface traced out by the semicircle will cut the 
cylinder in a tortuous curve. This curve will itself be cut by 
a right cone, whose axis is OA and semi-vertical angle is (say) 
60°, in a point P, such that the projection of OF on the base of 
the cylinder will be to the radius of the cylinder in the ratio of 
the side of the required cube to that of the given cube. Of 
course the proof given by Archytas is geometrical ; and it is 
interesting to note that in it he shows himself familiar with the 
results of the propositions Euc. ill, 18, iii, 35, and xi, 19. To 

• On the application to this problem of the traditional Greek methods of 
analysis by Hero and Philo (leading to the solution by the use of Apollonius's 
circle), by Nicomedes (leading to the solution by the use of the conchoid), and 
by Pappus (leading to the solution by the use of the cissoid), see Geometrical 
Analysis by J. Leslie, Edinburgh, second edition, 1811, pp. 247 — 250, 463. 

t Proclus, ed. Friedlein, pp. 212—213. 

J Archimedis Opera^ ed. Torelli, p. 143 ; ed. Heiberg, vol. m, pp. 98—103, 



288 THREE GEOMETRICAL PROBLEMS [CH. XIII 

show analytically that the construction is correct, take OA as 
the axis of a?, and the generator of the cylinder drawn through 
as axis of z, then with the usual notation, in polar co-ordinates, 
if a is the radius of the cylinder, we have for the equation 
of the surface described by the semicircle r = 2a sin ^ ; for that 
of the cylinder r sin ^ =*= 2a cos <f> ; and for that of the cone 
sin 6 cos </) = i- These three surfaces cut in a point such that 
sin' B = \, and therefore {r sin Oy = 2a^ Hence the volume of 
the cube whose side is r sin ^ is twice that of the cube whose 
side is a. 

The construction attributed to Plato* (circ. 360 B.C.) de- 
pends on the theorem that, if CAB and DAB are two right- 
angled triangles, having one side, AB, common, their other 
sides, AD and BG, parallel, and their hypothenuses, AG and 
BD, at right angles, then if these hypothenuses cut in P, we 
have PG:PB = PB:PA=PA: PD. Hence, if such a figure 
can be constructed having PD = 2P(7, the problem will be 
solved. It is easy to make an instrument by which the figure 
can be drawn. 

The next writer whose name is connected with the problem 
is Menaechmusf, who in or about 340 B.C. gave two solutions 
of it. 

In the first of these he pointed out that two parabolas 
having a common vertex, axes at right angles, and such that 
the latus rectum of the one is double that of the other, will 
intersect in another point whose abscissa (or ordinate) will 
give a solution. If we use analysis this is obvious; for, if 
the equations of the parabolas are 3/^ = 2ax and a^ = ay, they 
intersect in a point whose abscissa is given by a;' = 2a^ It is 
probable that this method was suggested by the form in which 
Hippocrates had cast the problem : namely, to find 00 and y so 
that a-, x — x \ y = y :2a, whence we have a^—ay and y"^ = 2ax. 

The second solution given by Menaechmus was as follows. 
Describe a parabola of latus rectum I. Next describe a rect- 
angular hyperbola, the length of whose real axis is 4^, and 

* Archimedis Opera, ed. Torelli, p. 136; ed. Heiberg, voL iii, pp. 66—71. 
t Ihid,, ed. Torelli, pp. 141—113 ; ed. Heiberg, vol. iii, pp. 92—99. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 289 

having for its asymptotes the tangent at the vertex of the 
parabola and the axis of the parabola. Then the ordinate and 
the abscissa of the point of intersection of these curves are the 
mean proportionals between I and 21. This is at once obvious 
by analysis. The curves are x^ = ly and (cy = 2l\ These cut in 
a point determined by x^ — 21^ and y^ = 4^'. Hence 
I : X = X : y = y : 21. 

The solution of ApoUonius*, which was given about 220 B.C., 
was as follows. The problem is to find two mean proportionals 
between two given lines. Construct a rectangle OADB, of 
which the adjacent sides OA and OB are respectively equal to 
the two given lines. Bisect ABm. C. With G as centre describe 
a circle cutting OA produced in a and cutting OB produced in 
b, so that aDb shall be a straight line. If this circle can be so 
described, it will follow that OA : Bb = Bh : Aa = Aa : OB, that 
is, Bb and Aa are the two mean proportionals between OA 
and OB. It is impossible to construct the circle by Euclidean 
geometry, but ApoUonius gave a mechanical way of describing it. 

The only other construction of antiquity to which I will 
refer is that given by Diodes and Sporusf. It is as follows. 
Take two sides of a rectangle OA, OB, equal to the two lines 
between which the means are sought. Suppose OA to be the 
greater. With centre and radius OA describe a circle. Let 
OB produced cut the circumference in C and let A produced 
cut it in D. Find a point E on BG so that if DE cuts AB 
produced in F and cuts the circumference in G, then FE = EG. 
If E can be found, then OE is the first of the means between 
OA and OB. Diodes invented the cissoid in order to determine 
Ey but it can be found equally conveniently by the aid of 
conies. 

In more modern times several other solutions have been 
suggested. I may allude in passing to three given by HuygensJ, 

* Archimedis Opera, ed. Torelli, p. 137 ; ed. Heiberg, vol. in, pp. 76 — 79. 
The solution is given in my History of Mathematics, London, 1901, p. 84. 

t Ibid., ed. Torelli, pp. 138, 139, 141 ; ed. Heiberg, vol. m, pp. 78 -8i, 
90-93. 

X Opera Varia, Leyden, 1724, pp. 393—396. 

B. E. 19 



290 



THREE GEOMETRICAL PROBLEMS [CH. XIII 



but I will enunciate only those proposed respectively by Vieta, 
Descartes, Gregory of St Vincent, and Newton. 

Vieta's construction is as follows*. Describe a circle, centre 
0, whose radius is equal to half the length of the larger of the 
two given lines. In it draw a chord AB equal to the smaller 
of the two given lines. Produce -45 to ^ so that BE = AB. 
Through A draw a line AF parallel to OE. Through draw a 
line i)0(7i^(T, cutting the circumference in D and (7, cutting Ai^ 
in F, and cutting BA produced in G, so that GF=OA. If this 
line can be drawn then AB : GO =GC : GA = GA : CD. 

Descartes pointed outf that the curves 

x^ = ay and x^ + y^= ay + bx 
cut in a point {x, y) such that a -. x = x -. y = y :h. Of course 
this is equivalent to the first solution given by Menaechmus, 
but Descartes preferred to use a circle rather than a second 
conic. 

Gregory's construction was given in the form of the following 
theorem J. The hyperbola drawn through the point of inter- 
section of two sides of a rectangle so as to have the two other 
sides for its asymptotes meets the circle circumscribing the 
rectangle in a point whose distances from the asymptotes are 
the mean proportionals between two adjacent sides of the rect- 
angle. This is the geometrical expression of the proposition 
that the curves xy = ah and a^ + y^ = ay •{■ bx cut in a point 
(x, y) such that a : x=^x : y — y :h. 

One of the constructions proposed by Newton is as follows §. 
Let OA be the greater of two given lines. Bisect OA in B. 
With centre and radius OB describe a circle. Take a point 
G on the circumference so that BG is equal to the other of the 
two given lines. From draw QBE cutting A G produced in 
jD, and BG produced in E, so that the intercept DE= OB. Then 

* Opera Mathematical ed. Schooten, Leyden, 1646, prop, v, pp. 242—243. 

t Geometria, bk. ni, ed. Schooten, Amsterdam, 1659, p. 91. 

X Gregory of St Vincent, Opus Geometricum Quadraturae Girculi, Antwerp, 
1647, bk, VI, prop. 138, p. 602. 

§ Arithmetica Universalis, Ralphson's (second) edition, 1728, p. 242 ; see 
also pp. 213, 245. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 291 

BG:OD=OD:GE=GE:OA. Hence OD and CE are two 
mean proportionals between any two lines BG and OA, 

The Trisection of an Angle*. 

The trisection of an angle is the second of these classical 
problems, but tradition has not enshrined its origin in romance. 
The following two constructions are among the oldest and best 
known of those which have been suggested; they are quoted 
by Pappus f, but I do not know to whom they were due 
originally. 

The first of them is as follows. Let A OB be the given 
angle. From any point P in OB draw PM perpendicular to 
OA. Through P draw PR parallel to OA. On MP take a point 
Q so that if OQ is produced to cut PR in R then QR = 2. OP. 
If this construction can be made, then AOR = j^AOB. The 
solution depends on determining the position of R. This was 
effected by a construction which may be expressed analytically 
thus. Let the given angle be tan"^ (^/a). Construct the hjrper- 
bola xy = «6, and the circle (x - ay + (y - 6)2 = 4 (a^ -f 6^. Of 
the points where they cut, let x be the abscissa which is greatest, 
then PR = x— a, and tan~^ (6/a;) = J tan~^ (^/^)- 

The second construction is as follows. Let A OB be the 
given angle. Take OB = OA, and with centre and radius OA 
describe a circle. Produce AO indefinitely and take a point C 
on it external to the circle so that if GB cuts the circumference 
in D then GB shall be equal to OA. Draw OE parallel to GDB. 
Then, if this construction can be made, A OE = ^A OB. The 
ancients determined the position of the point G by the aid of 
the conchoid : it could be also found by the use of the conic 
sections. 

I proceed to give a few other solutions, confining myself to 
those effected by the aid of conies. 

* On the bibliography of the subject see the supplements to Ulntermidiaire 
des MatMmaticiens, Paris, May and June, 1904. 

t Pappus, Mathematicae Collectiones, bk. iv, props. 32, 33 (ed. CommanJino, 
Bonn, 1670, pp. 97 — 99). On the application to this problem of the traditional 
Greek methods of analysis see Geometrical Analysis^ by J. Leslie, Edinburgh, 
second edition, 1811, pp. 245—247. 

19—2 



292 THREE GEOMETRICAL PROBLEMS [CH. XIII 

Among other constructions given by Pappus* I may quote 
the following. Describe a hyperbola whose eccentricity is two. 
Let its centre be G and its vertices A and A'. Produce GA' 
to S so that A' 8 = GA'. On AS describe a segment of a 
circle to contain the given angle. Let the orthogonal bisector 
of AS cut this segment in 0. With centre and radius OA or 
OS describe a circle. Let this circle cut the branch of the 
hyperbola through A' in P. Then SOP = ^SOA. 

In modem times one of the earliest of the solutions by a 
direct use of conies was suggested by Descartes, who effected 
it by the intersection of a circle and a parabola. His con- 
struction! is equivalent to finding the points of intersection, 
other than the origin, of the parabola y'^—\x and the circle 
g&j^yi-. 13,^ ^ 4^2/ = 0. The ordinates of these points are given 
by the equation 43/' =Sy — a. The smaller positive root is the 
sine of one-third of the angle whose sine is a. The demonstra- 
tion is ingenious. 

One of the solutions proposed by Newton is practically 
equivalent to the third one which is quoted above from Pappus. 
It is as followsj. Let A be the vertex of one branch of a 
hyperbola whose eccentricity is two, and let S be the focus of 
the other branch. On ^iSf describe the segment of a circle con- 
taining an angle equal to the supplement of the given angle. 
Let this circle cut the S branch of the hyperbola in P. Then 
FAS will be equal to one- third of the given angle. 

The following elegant solution is due to Clairaut§. Let 
A OB be the given angle. Take OA = OB, and with centre 
and radius OA describe a circle. Join AB, and trisect it in 
H, K, so that AH=:^ HK = KB. Bisect the angle A OB by OG 
cutting AB in L. Then AH= 2 . HL. With focus A, vertex 
H, and directrix OG, describe a hyperbola. Let the branch of 

* Pappus, Mathematicae CollectioneSf bk. rv, prop. 34, pp. 99 — 104, 

t Geometria, bk. iii, ed. Schooten, Amsterdam, 1659, p. 91. 

X Arithmetica Universalis, problem xlii, Ralphson's (second) edition, London, 
1728, p. 148; see also pp. 243—245. 

§ I believe that this was first given by Clairaut, but I have mislaid my 
reference. The construction occurs as an example in the Geometry of Conies, 
by C. Taylor, Cambridge, 1881, No. 308, p. 126. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 293 

this hyperbola which passes through H cut the circle in P. 
Draw PM perpendicular to OC and produce it to cut the circle 
in Q. Then by the focus and directrix property we have 
AP : PM = AH :HL=^2:1, .', AP = 2.PM=PQ. Hence, by 
symmetry, AP = PQ = QR. .-. AOP = POQ = QOR 

I may conclude by giving the solution which Chasles* 
regards as the most fundamental. It is equivalent to the 
following proposition. If OA and OB are the bounding radii 
of a circular arc AB, then a rectangular hyperbola having OA 
for a diameter and passing through the point of intersection of 
OB with the tangent to the circle at A will pass through one 
of the two points of trisection of the arc. 

Several instruments have been constructed by which 
mechanical solutions of the problem can be obtained. 

The Quadrature of the Circle f. 

The object of the third of the classical problems was the 
determination of a side of a square whose area should be equal 
to that of a given circle. 

The investigation, previous to the last two hundred years, 
of this question was fruitful in discoveries of allied theorems, 
but in more recent times it has been abandoned by those who 
are able to realize what is required. The history of this subject 
has been treated by competent writers in such detail that I shall 
content myself with a very brief allusion to it. 

Archimedes showed J (what possibly was known before) that 
the problem is equivalent to finding the area of a right-angled 

• Traitg des sections coniques, Paris, 1865, art. 37, p. 36, 
t See Montucla's Histoire des Recherches sur la Quadrature du Gerele, 
edited by P. L. Lacroix, Paris, 1881 ; also various articles by A. De Morgan, 
and especially his Budget of Paradoxes, London, 1872. A popular sketch of 
the subject has been compiled by H. Schubert, Die Quadratur des Zirkels, 
Hamburg, 1889 ; and since the publication of the earlier editions of these 
Recreations Prof. F. Rudio of Zurich has given an analysis of the arguments of 
Archimedes, Huygens, Lambert, and Legendre on the subject, with an intro- 
duction on the history of the problem, Leipzig, 1892. 

t Archimedis Opera, KiJ/tXou fiiTfyrjan, prop, i, ed. Torelli, pp. 203—205 ; ed. 
Heiberg, vol. i, pp. 258—261, vol. iii, pp. 209—277. 



294 THREE GEOMETRICAL PROBLEMS [CH. XIII 

triangle whose sides are equal respectively to the perimeter of 
the circle and the radius of the circle. Half the ratio of these 
lines is a number, usually denoted by tt. 

That this number is incommensurable had been long sus- 
pected, and has been now demonstrated. The earliest analytical 
proof of it was given by Lambert* in 1761 ; in 1803 Legendref 
extended the proof to show that tt'* was also incommensurable ; 
and recently LindemannJ has shown that tt cannot be the root 
of a rational algebraical equation. 

An earlier attempt by James Gregory to give a geometrical 
demonstration of this is worthy of notice. Gregory proved § 
that the ratio of the area of any arbitrary sector to that of 
the inscribed or circumscribed polygons is not expressible by 
a finite number of algebraical terms. Hence he inferred that 
the quadrature was impossible. This was accepted by Montucla, 
but it is not conclusive, for it is conceivable that some particular 
sector might be squared, and this particular sector might be the 
whole circle. 

In connection with Gregory's proposition above cited, I may 
add that Newton || proved that in any closed oval an arbitrary 
sector bounded by the curve and two radii cannot be expressed 
in terms of the co-ordinates of the extremities of the arc by a 
finite number of algebraical terms. The argument is condensed 
and difficult to follow: the same reasoning would show that 
a closed oval curve cannot be represented by an algebraical 
equation in polar co-ordinates. From this proposition no 

♦ Mimoires de VAcademie de Berlin for 1761, Berlin, 1768, pp. 265 — 
322. 

t Legendre's Geometry^ Brewster's translation, Edinburgh, 1824, pp. 239 — 
245. 

X Ueber die Zabl x, Mathematische Annalen, Leipzig, 1882, vol, xx, rp. 213 — 
225. The proof leads to the conclusion that, if a; is a root of a rational 
integral algebraical equation, then e* cannot be rational : hence, if iri was the 
root of such an equation, e^* could not be rational ; but e"* is equal to - 1, and 
therefore is rational ; hence vi cannot be the root of such an algebraical 
equation, and therefore neither can tt. 

§ Vera Circuli et Hyperbolae Quadratura, Padua, 1668 : this is reprinted in 
Huygens's Opera Varia, Leyden, 1724, pp. 405 — 462. 

il Principia^ bk. i, section vi, lemma xxviii. 



CH. Xlll] THREE GEOMETKICAL PROBLEMS 2d6 

conclusion as to the quadrature of the circle is to be drawn, 
nor did Newton draw any. In the earlier editions of this work 
I expressed an opinion that the result presupposed a particular 
definition of the word oval, but on more careful reflection I think 
that the conclusion is valid without restriction. 

With the aid of the quadratrix, or the conchoid, or the 
cissoid, the quadrature of the circle is easy, but the construction 
of those curves assumes a knowledge of the value of tt, and thus 
the question is begged. 

I need hardly add that, if tt represented merely the ratio 
of the circumference of a circle to its diameter, the deter- 
mination of its numerical value would have but slight interest. 
It is however a mere accident that tt is defined usually in 
that way, and it really represents a certain number which 
would enter into analysis from whatever side the subject was 
approached. 

I recollect a distinguished professor explaining how diflferent 
would be the ordinary life of a race of beings born, as easily 
they might be, so that the fundamental processes of arithmetic, 
algebra and geometry were different to those which seem to us 
so evident, but, he added, it is impossible to conceive of a 
universe in which e and ir should not exist. 

I have quoted elsewhere an anecdote, which perhaps will 
bear repetition, that illustrates how little the usual definition 
of TT suggests its properties. De Morgan was explaining to an 
actuary what was the chance that a certain proportion of some 
group of people would at the end of a given time be alive ; and 
quoted the actuarial formula, involving tt, which, in answer to 
a question, he explained stood for the ratio of the circumference 
of a circle to its diameter. His acquaintance, who had so far 
listened to the explanation with interest, interrupted him and 
exclaimed, " My dear friend, that must be a delusion, what can 
a circle have to do with the number of people alive at the end 
of a given time ? " In reality the fact that the ratio of the 
length of the circumference of a circle to its diameter is the 
number denoted by tt does not afford the best analytical defini- 
tion of TT, and is only one of its properties. 



296 



THREE GEOMETRICAL PROBLEMS [CH. XIII 



The use of a single symbol to denote this number 3-14159... 
seems to have been introduced about the beginning of the 
eighteenth century. William Jones* in 1706 represented it by tt; 
a few years laterf John Bernoulli denoted it by c; Euler in 
1734 used p, and in 1736 used c; Christian Goldback in 1742 
used tt; and after the publication of Euler s Analysis the 
symbol tt was generally employed. 

The numerical value of tt can be determined by either of 
two methods with as close an approximation to the truth as 
is desired. 

The first of these methods is geometrical. It consists in 
calculating the perimeters of polygons inscribed in and circum- 
scribed about a circle, and assuming that the circumference of 
the circle is intermediate between these perimetersj. The ap- 
proximation would be closer if the areas and not the perimeters 
were employed. The second and modern method rests on the 
determination of converging infinite series for tt. 

We may say that the 7r-calculators who used the first 
method regarded tt as equivalent to a geometrical ratio, but 
those who adopted the modern method treated it as the symbol 
for a certain number which enters into numerous branches of 
mathematical analysis. 

It may be interesting if I add here a list of some of the 
approximations to the value of tt given by various writers§. 
This will indicate incidentally those who have studied the sub- 
ject to the best advantage. 

* Synopsis Palmariorum Matheseos, London, 1706, pp. 243, 263 et seq. 

t See notes by G. Enestrom in the Bibliotheca Mathematical Stockholm, 
1839, vol. in, p. 28; Ibid., 1890, vol. iv, p. 22. 

X The history of this method has been written by K. E. I. Selander, Historik 
djver Ludolphska Talet, Upsala, 1868. 

§ For the methods used in classical times and the results obtained, see the 
notices of their authors in M. Cantor's Geschichte der Mathematik, Leipzig, 
vol. I, 1880. For medieval and modern approximations, see the article by 
A. De Morgan on the Quadrature of the Circle in vol. xix of the Penny 
Cyclopaedia^ London, 1841; with the additions given by B. de Haan in the 
Verhandelingen of Amsterdam, 1868, vol. iv, p. 22 : the conclusions were 
tabulated, corrected, and extended by Dr J. W. L. Glaisher in the Messenger of 
Mathematics, Cambridge, 1873, vol. n, pp. 119—128; and I6td., 1874, vol. m, 
pp. 27—46. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS • 297 

The ancient Egyptians* took 256/81 as the value of tt, this 
is equal to 31605... ; but the rougher approximation of 3 was 
used by the Babylonians f and by the Jews J. It is not unlikely 
that these numbers were obtained empirically. 

We come next to a long roll of Greek mathematicians who 
attacked the problem. Whether the researches of the members 
of the Ionian School, the Pythagoreans, Anaxagoras, Hippias, 
Antipho, and Bryso led to numerical approximations for the 
value of TT is doubtful, and their investigations need not 
detain us. The quadrature of certain lunes by Hippocrates of 
Chios is ingenious and correct, but a value of tt cannot be 
thence deduced; and it seems likely that the later members 
of the Athenian School concentrated their efforts on other 
questions. 

It is probable that Euclid §, the illustrious founder of the 
Alexandrian School, was aware that tt was greater than 3 and 
less than 4, but he did not state the result explicitly. 

The mathematical treatment of the subject began with 
Archimedes, who proved that tt is less than 3| and greater 
than 3fJ, that is, it lies between 31428... and 31408.... He 
established|| this by inscribing in a circle and circumscribing 
about it regular polygons of 96 sides, then determining by 
geometry the perimeters of these polygons, and finally 
assuming that the circumference of the circle was inter- 
mediate between these perimeters : this leads to a result from 
which he deduced the limits given above. This method 
is equivalent to using the proposition sin ^ < ^ < tan 6^ where 
6 — 71/96: the values of sin^ and tan ^ were deduced by 
Archimedes from those of sin Jtt and tan Jtt by repeated 
bisections of the angle. With a polygon of n sides this 

* Ein mathematische* Handhuch der alten Aegypter {i.e. the Ehind papyrus), 
by A. Eisenlohr, Leipzig, 1877, arts. 100—109, 117, 124. 

t Oppert, Journal Asiatique, August, 1872, and October, 1874. 

J 1 Kings, ch. 7, ver. 23 ; 2 Chronicles, ch. 4, yer. 2. 

§ These results can be deduced from Euo. iv, 15, and iv, 8: see also book xii, 
prop. 16. 

II ArchimedU Opera, K6k\ov fihprjan, prop, iir, ed. Torelli, Oxford, 1792, 
pp. 205—216; ed. Heiberg, Leipzig, 1880, vol. i, pp. 263—271. 



298 THREE GEOMETRICAL PROBLEMS [CH. XllI 

process gives a value of ir correct to at least the integral part 
of (2 log n — 1*19) places of decimals. The result given by 
Archimedes is correct to two places of decimals. His analysis 
leads to the conclusion that the perimeters of these polygons 
for a circle whose diameter is 4970 feet would lie between 
15610 feet and 15620 feet— actually it is about 15613 feet 
9 inches. 

ApoUonius discussed these results, but his criticisms have 
been lost. 

Hero of Alexandria gave* the value 3, but he quoted f the 
result 22/7 : possibly the former number was intended only for 
rough approximations. 

The only other Greek approximation that I need mention 
is that given by Ptolemy J, who asserted that tt = 3° 8' 80". 
This is equivalent to taking tt = 3 + ^% + ^fg^ = 3^^^ = 3141 G. 

The Roman surveyors seem to have used 3, or sometimes 4, 
for rough calculations. For closer approximations they often 
employed 3J instead of 3f , since the fractions then introduced 
are more convenient in duodecimal arithmetic. On the other 
hand Gerbert§ recommended the use of 22/7. 

Before coming to the medieval and modern European mathe- 
maticians it may be convenient to note the results arrived at in 
India and the East. 

Baudhayana|| took 49/16 as the value of tt. 

Arya-BhatalF, circ. 530, gave 62832/20000, which is equal 
to 3-1416. He showed that, if a is the side of a regular 
polygon of n sides inscribed in a circle of unit diameter, and if 
h is the side of a regular inscribed polygon of In sides, then 
62=^_J(1— a^)^. From the side of an inscribed hexagon, he 
found successively the sides of polygons of 12, 24, 48, 96, 192, 

♦ Mensurae, ed. Hultsch, Berlin, 1864, p. 188. 

+ Geometria, ed. Hultsch, Berlin, 1864, pp. 115, 136. 

X Almagest, bk. vi, chap. 7 ; ed. Halma, vol. i, p. 421. 

§ (Euvres de Gerbert, ed. Olleris, Clermont, 1867, p. 453. 

II The Sulvasutras by G. Thibaut, Asiatic Society of Bengal, 1875, arts. 
26—28. 

U Legons de calcul d'Aryahhata, by L. Bodet in the Journal Adatique, 1879, 
Beries 7, vol. xm, pp. 10, 21. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 299 

and 384 sides. The perimfeter of the last is given as equal 
to V98694, from which his result was obtained by approxi- 
mation. 

Brahmagupta*, circ. 650, gave ^/T0, which is equal to 
31622.... He is said to have obtained this value by inscribing 
in a circle of unit diameter regular polygons of 12, 24, 48, and 
96 sides, and calculating successively their perimeters, which 
he found to be V9^, \/9^, V9^, \/9^ respectively; and to 
have assumed bhat as the number of sides is increased indefinitely 
the perimeter would approximate to VlO. 

Bhaskara, circ. 1150, gave two approximations. Onef — pos- 
sibly copied from Arya-Bhata, but said to have been calculated 
afresh by Archimedes's method from the perimeters of regular 
polygons of 384 sides— is 3927/1250, which is equal to 31416: 
the otherj is 754/240, which is equal to 31416, but it is un- 
certain whether this was not given only as an approximate 
value. 

Among the Arabs the values 22/7, VTO, and 62832/20000 
were given by Alkarismig, circ. 830; and no doubt were derived 
from Indian sources. Ho described the first as an approximate 
value, the second as used by geometricians, and the third as 
used by astronomers. 

In Chinese works the values 3, 22/7, 157/50 are said to 
occur: probably the last two results were copied from the 
Arabs. The Japanese || approximations were closer. 

Returning to European mathematicians, we have the following 
successive approximations to the value of tt: many of those prior 
to the eighteenth century having been calculated originally 
with the view of demonstrating the incorrectness of some 
alleged quadrature. 

* Algebra. ..from Brahmegupta and Bhascara, trans, by H. T. Colebrooke, 
London, 1817, chap, xii, art. 40, p. 308. 

t Ibid., p. 87. 

t Ibid., p. 95. 

§ The Algebra of Mohammed ben Musa, ed. by F. Rosen, London, 1831, 
pp. 71—72. 

II On Japanese approximations and the methods used, see P. Harzer, 
Transactions of the British Association for 1905, p. 325. 



300 THREE GEOMETRICAL PROBLEMS [CH. XIII 

Leonardo of Pisa*, in the thirteenth century, gave for tt tho 
value 1440/458J, which is equal to 3-1418.... In the fifteenth 
century, Purbachf gave or quoted the value 62832/20000, 
which is equal to 3*1410; Cusa believed that the accurate 
value was | (\/3 + \/6), which is equal to 3-1423...; and, in 
1464, RegiomontanusJ is said to have given a value equal to 
3-14243. 

Vieta§, in 1579, showed that ir was greater than 
31415926535/10^^ and less than 31415926537/10^°. This was 
deduced from the perimeters of the inscribed and circumscribed 
polygons of 6 X 2^^ sides, obtained by repeated use of the formula 
2 sin^ J^ = 1 — cos ^. He also gave|| a result equivalent to the 
formula 

2=n/2\^±V2) V{2 + V(2 + V2)l 
TT 2 2 2 

The father of Adrian MetiusIF, in 1585, gave 355/113, 
which is equal to 3*14159292..., and is correct to six places 
of decimals. This was a curious and lucky guess, for all that 
he proved was that ir was intermediate between 377/120 and 
333/106, whereon he jumped to the conclusion that he would 
obtain the true fractional value by taking the mean of the 
numerators and the mean of the denominators of these fractions. 

In 1593 Adrian Romanus** calculated the perimeter of the 
inscribed regular polygon of 1073,741824 {i.e. 2^) sides, from 
which he determined the value of ir correct to 15 places of 
decimals. 

* Boncompagni's Scritti di Leonardo, vol. ii {Practica Geometriae), Rome, 
1862, p. 90. 

t Appendix to the De Triangulis of Regiomontanus, Basle, 1541, p. 131. 

X In his correspondence with Cardinal Nicholas de Cusa, De Quadratura 
Circuit, Nuremberg, 1533, wherein he proved that the cardinal's result was 
wrong. I cannot quote the exact reference, but the figures are given by com- 
petent writers and I have no doubt are correct. 

§ Canon Mathematicus seu ad Triangula, Paris, 1679, pp. 56, 66 : probably 
this work was printed for private circulation only, it is very rare. 

II Vietae Opera, ed. Schooten, Leyden, 1646, p. 400. 

H Arithmeticae libri duo et Geometriae, by A. Metius, Leyden, 1626, pp. 88 — 
89. [Probably issued originally in 1611.] 

•* Ideae Mathematicae, Antwerp, 1593: a rare work, which I have never been 
able to consult. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 301 

L. van Cenlen devoted no inconsiderable part of his life to 
the subject. In 1596* he gave the result to 20 places of deci- 
mals: this was calculated by finding the perimeters of the 
inscribed and circumscribed regular polygons of 60 x 2^ sides, 
obtained by the repeated use of a theorem of his discovery 
equivalent to the formula 1 — cos^ =2 sin^^^. I possess a 
finely executed engraving of him of this date, with the result 
printed round a circle which is below his portrait. He died in 
1610, and by his directions the result to 35 places of decimals 
(which was as far as he had calculated it) was engraved on his 
tombstone f in St Peter's Church, Leyden. His posthumous 
arithmetic:!: contains the result to 32 places ; this was obtained 
by calculating the perimeter of a polygon, the number of whose 
sides is 2^\ i.e. 4,611686,018427,387904 Van Ceulen also com- 
piled a table of the perimeters of various regular polygons. 

Willebrord Snell§, in 1621, obtained firom a polygon of 2^ 
sides an approximation to 34 places of decimals. This is less 
than the numbers given by van Ceulen, but Snell's method 
was so superior that he obtained his 34 places by the use of a 
polygon from which van Ceulen had obtained only 14 (or 
perhaps 16) places. Similarly, Snell obtained from a hexagon 
an approximation as correct as that for which Archimedes had 
required a polygon of 96 sides, while from a polygon of 96 sides 
he determined the value of tt correct to seven decimal places 
instead of the two pieces obtained by Archimedes. The reason 
is that Archimedes, having calculated the lengths of the sides 
of inscribed and circumscribed regular polygons of n sides, 
assumed that the length of l/nth of the perimeter of tne circle 
was intermediate between them; whereas Snell constructed 

♦ Vanden Girckel, Delf, 1596, foL 14, p. 1 ; or De Girculo, Leyden, 1619, p. 3. 

t The in8criptioii is quoted by Prof, de Haan in the Messenger of Mathematics, 
1874, vol. m, p. 25. 

X De Arithmetische en Oeometrische Fondamenten, Leyden, 1615, p. 163 ; or 
p. 144 of the Latin translation by W. Snell, published at Leyden in 1616 under 
the title Fundamenta Arithmetica et Geometrica. This was reissued, together 
with a Latin translation of the Vanden Circkel, in 1619, under the title De 
Circulo; in which see pp. 3, 29 — 32, 92. 

§ Cyclometricus, Leyden, 1C21, p. 65. 



302 THREE GEOMETRICAL PROBLEMS [CH. XIII 

from the sides of these polygons two other lines which gave 
closer limits for the corresponding arc. His method depends 
on the theorem 3 sin ^/(2 + cos ^) < ^ < (2 sin J ^ + tan | 6), by 
the aid of which a polygon of n sides gives a value of tt correct 
to at least the integral part of (4 log n — -2305) places of decimals, 
which is more than twice the number given by the older rule. 
Snell's proof of his theorem is incorrect, though the result is true. 

Snell also added a table* of the perimeters of all regular 
inscribed and circumscribed polygons, the number of whose 
sides is 10 x 2" where n is not greater than 19 and not less than 
3. Most of these were quoted from van Ceulen, but some were 
recalculated. This list has proved useful in refuting circle- 
squares. A similar list was given by James Gregory f. 

In 1630 Grienbergerj, by the aid of Snell's theorem, carried 
the approximation to 39 places of decimals. He was the last 
mathematician who adopted the classical method of finding the 
perimeters of inscribed and circumscribed polygons. Closer 
approximations serve no useful purpose. Proofs of the theorems 
used by Snell and other calculators in applying this method 
were given by Huygens in a work§ which may be taken as 
closing the history of this method. 

In 1656 Wallisll proved that 

TT^ 2.2.4.4.6.6... 
2~1.3.3.5.5.7.7...' 

and quoted a proposition given a few years earlier by Viscount 
Brouncker to the effect that 

4 ^■*"2+ 2 + 2 + ..., 

* It is quoted by Montucla, ed. 1.831, p. 70. 

t Vera Circuli et Hyperbolae Quadratura, prop. 29, quoted by Huygens, 
Opera Varia, Leyden, 1724, p. 447. 

X Elementa Trigonometrical Rome, 1630, end of preface. 

§ Be Circula Magnitudine Inventa, 1654; Opera Varia, pp. 351 — 387. The 
proofs are given in G. Pirie's Geometrical Methods of Approximating to the Value 
of IT, London, 1877, pp. 21—23. 

II Arithmetica Injinitorum, Oxford, 1656, prop. 191. An analysis of the 
investigation by Wallis was given by Cayley, Quarterly Journal of Mathematics^ 
1889, vol. xxiii, pp. 165—169. 



CH. XIIl] THREE GEOMETRICAL PROBLEMS 303 

but neither of these theorems was used to any large extent for 
calculation. 

Subsequent calculators have relied on converging infinite 
series, a method that was hardly practicable prior to the in- 
vention of the calculus, though Descartes* had indicated a 
geometrical process which was equivalent to the use of such 
a series. The employment of infinite series was proposed by 
James Gregory f, who established the theorem that 

^ = tan^-Jtan»^4-itan»^-..., 
the result being true only if 6 lies between — Jtt and Jtt. 

The first mathematician to make use of Gregory's series 
for obtaining an approximation to the value of ir was Abraham 
Sharp J, who, in 1699, on the suggestion of Halley, determined 
it to 72 places of decimals (71 correct). He obtained this 
value by putting B—^irm Gregorys series. 

Machin§, earlier than 1706, gave the result to 100 places 
(all correct). He calculated it by the formula 
i7r = 4tan-4-tan-i^. 

De Lagnyll, in 1719, gave the result to 127 places of 
decimals (112 correct), calculating it by putting = ^ir in 
Gregory's series. 

Hutton1[,in 1776, and Euler**,in 1779, suggested the use of 

• See Euler's paper in the Novi Commentarii Academiae Scientiarum, 
Petrograd, 1763, vol. vm, pp. 157—168. 

t See the letter to Collins, dated Feb. 15, 1671, printed in the Commercium 
EpistoHcum, London, 1712, p. 25, and in the Macclesfield Collection, Corre- 
spondejice of Scientific Men of the Seventeenth Century^ Oxford, 1841, vol. ii. 
p. 216. 

I See Life of A. Sharp by W. Cudworth, London, 1889, p. 170. Sharp's 
work is given in one of the preliminary discourses (p. 53 et seq.) prefixed to 
H. Sherwin's Mathematical Tables. The tables were issued at London in 1705 : 
probably the discourses were issued at the same time, though the earliest copies 
I have seen were printed in 1717. 

§ W.Jones's Synopns Palmariorum, London, 1706, p. 243; and Maseres, 
Scriptores Logarithmici, London, 1796, vol. iii, pp. vii — ix, 156 — 1G4. 

II Uistoire de V Academic for 1719, Paris, 1721, p. 144. 

IT Philosophical Transactions, 177(3, vol. lxvi, pp. 476 — 492. 
** Nova Acta Academiae Scientiarum Petropolitanae for 1793, Petrograd, 
1798, vol. XI, pp. 133—149 : the memoir was read in 1779. 



304 THREE GEOMETRICAL PROBLEMS [CH. XIII 

the formula Jtt = tan~^ J + tan~^ J or Jtt = 5 tan~^ 1 + 2 tan~* y^^, 
but neither carried the approximation as far as had been done 
previously. 

Vega, in 1789*, gave the value of ir to 143 places of 
decimals (126 correct); and, in I794f, to 140 places (136 
correct). 

Towards the end of the eighteenth century F. X. von Zach 
saw in the Radcliffe Library, Oxford, a manuscript by an 
unknown author which gives the value of tt to 154 places of 
decimals (152 correct). 

In 1837, the result of a calculation of ir to 154 places of 
decimals (152 correct) was published J. 

In 1841 Rutherford§ calculated it to 208 places of decimals 
(152 correct), using the formula ^77= 4 tan~^ J— tan~^ T^^j+tan"^ ■^. 

In 1844 Dase || calculated it to 205 places of decimals (200 
correct), using the formula Jtt = tan~^ -J 4- tan~^ ^ + tan~^ J. 

In 1847 Clausen IT carried the approximation to 250 places 
of decimals (248 correct), calculating it independently by the 
formulae Jtt = 2 tan~^ J 4- tan"^ f and Jtt = 4 tan~^ | — tan-^ ^. 

In 1853 Rutherford** carried his former approximation to 
440 places of decimals (all correct), and William Shanks pro- 
longed the approximation to 530 places. In the same year 
Shanks published an approximation to 607 places ff : and in 
1873 he carried the approximation to 707 places of decimalsJJ. 
These were calculated from Machin's formula. 

In 1853 Richter, presumably in ignorance of what had been 

* Nova Acta Academiae Scientiarum Petropolitanae for 1790, Petrograd, 
1795, vol. IX, p. 41. 

t Thesaurus Logarithrmrum{logarithmisch-trigonometrischer Tafeln), Leipzig, 
1794, p. 633. 

X J. F. Callet's Tables, etc.. Precis Elementaire, Paris, tirage, 1837. Tirage, 
1894, p. 96. 

§ Philosophical Transactions, 1841, p. 283. 

li Crelle^s Journal, 1844, vol. xxvii, p. 198. 

IT Schumacher, Astronomische Nachrichten, vol. xxv, col. 207. 

** Proceedings of the Royal Society, Jan. 20, 1853, vol. vi, pp. 273 — 275. 

+t Contributions to Mathematics, W. Shanks, London, 1853, pp. 86 — 87. 

XX Proceedings of the Royal Society, 1872-3, vol. xxi, p. 318; 1873-4, vol 
XXII, p. 45. 



CH. XII l] THREE GEOMETRICAL PROBLEMS 305 

done in England, found the value of ir to 333 places* of 
decimals (330 correct) ; in 1854 he carried the approximation 
to 400 places -f ; and in 1855 carried it to 500 places|. 

Of the series and formulae by which these approximations 
have been calculated, those used by Machin and Dase are perhaps 
the easiest to employ. Other series which converge rapidly 
are the following: 



^ = 1 11 1.3 1 
6~2'^2'3.2='"*"2.4*5.2="^*' 



and 



i = 22 tan- 1 + 2 tan- ^^g - 5 tan- ^ - 10 tan- ^ ; 

the latter of these is due to Mr Escott§. 

As to those writers who believe that they have squared the 
circle their number is legion and, in most cases, their ignorance 
profound, but their attempts are not worth discussing here. 
" Only prove to me that it is impossible," said one of them, 
" and I will set about it immediately " ; and doubtless the 
statement that the problem is insoluble has attracted much 
attention to it. 

Among the geometrical ways of approximating to the truth 
the following is one of the simplest. Inscribe in the given 
circle a square, and to three times the diameter of the circle 
add a fifth of a side of the square, the result will differ from 
the circumference of the circle by less than one-seventeen- 
thousandth part of it. 

An approximate value of tt has been obtained experimentally 
by the theory of probability. On a plane a number of equi- 
distant parallel straight lines, distance apart a, are ruled ; and 
a stick of length Z, which is less than a, is dropped on the 
plane. The probability that it will fall so as to lie across one of 
the lines is 2Z/7ra. If the experiment is repeated many hundreds 

* GrUnerVs Arehiv, vol. xxi, p. 119. 

t Ibid., vol. XXIII, p. 476: the approximation" given in vol. xxn, p. 473, is 
correct only to 330 places. 

X Ibid., vol. XXV, p. 472; and Elbinger Anzeigen, No. 85. 

§ L'Intermediaire det Mathimaticiem, Paris, Dec. 1896, vol. iii, p. 276. 

B. R. 20 



306 THREE GEOMETRICAL PROBLEMS [CH. XIII 

of times, the ratio of the number of favourable cases to the 
whole number of experiments will be very nearly equal to 
this fraction: hence the value of ir can be found. In 1855 
Mr A. Smith* of Aberdeen made 3204 trials, and deduced 
7r = 31553. A pupil of Prof De Morgan*, from 600 trials, 
deduced TT = 3-137. In 1864 Captain Foxf made 1120 trials 
with some additional precautions, and obtained as the mean 
value 7r = 3-1419. 

Other similar methods of approximating to the value of ir 
have been indicated. For instance, it is known that if two 
numbers are written down at random, the probability that 
they will be prime to each other is 6/7r^ Thus, in one case J 
where each of 50 students wrote down 5 pairs of numbers at 
random, 154 of the pairs were found to consist of numbers prime 
to each other. This gives 6/7r'^ = 154/250, from which we get 
7r = 312. 

* A. De Morgan, Budget of Paradoxes, London, 1872, pp. 171, 172 [quoted 
from an article by De Morgan published in 1861]. 

t Messenger of Mathematics, Cambridge, 1873, vol. ii, pp. 113, 114. 

X Note on ir by E. Chartres, Philosophical Magazine, London, series 6, 
vol. XXXIX, March, 1904, p. 315. 



307 



CHAPTER XIV. 

THE PARALLEL POSTULATE. 

In the last chapter I considered three classical problems. 
Another geometrical question, perhaps of greater interest, is 
concerned with whether the sum of the angles of a plane 
triangle is exactly equal to two right angles. This is a propo- 
sition which in ordinary textbooks on elementary geometry is 
enunciated — and properly so — as if it were undoubtedly true. 
In one sense this theorem, like the problems discussed in 
the last chapter, or like the algebraic solution of the general 
equation of the fifth degree, is insoluble; but the efforts to 
prove it afford materials for an interesting chapter in the 
history of mathematics, since many of the demonstrations 
formerly proposed are fallacious. The fact however that in 
the reasoning there are pitfalls, logical as well as mathe- 
matical, adds to the interest of the discussion, and the 
treacherous nature of the path makes its safe passage the 
more interesting*. 

* The subject has been discussed by numerous writers. A good account of 
it to the end of the 18th century is given in the notes to J. Playfair's Elements 
of Geometry, Edinburgh, 1st edition, 1813, and in the Appendix to Geometry 
without Axioms by T. P. Thompson, London, 4th edition, 1833. For other and 
more recent researches, see H. Schotten, Planim^trischen Unterrichts, vol. ii, 
Leipzig, 1893 ; F. Engel and P. Stackel, Die Theorie der Parallellinien, Leipzig, 
1895, 1899; J. Richard, La Philosophie des Mathematiques, Paris, 1903; 
J. W. Withers, Euclid's Parallel Postulate, Chicago, 1905; M. Simon, Ueher 
die Entwicklu7ig der Elementar-geometrie, Leipzig, 1906. Some of the solutions 
offered have no interest, and are evidently fallacious. Hence I make no 
attempt to treat the subject exhaustively, but I mention Che more plausible 
efforts. 

20-2 



808 THE PARALLEL POSTULATE [CH. XIV 

Earliest Proof. Tholes. We know from Geminus that this 
proposition was one of the first general results discovered by 
the Greeks*. From the extant notices the following has been 
suggested, with considerable probability, as indicating the 
manner in which it was proved ; at any rate this demonstration 
involves nothing with which Thales, the traditional founder of 
the science of abstract geometry, was not acquainted, and it 
has been conjectured that it is in fact due to him. According 
to this view, it was, in the first place, stated (or more likely 
assumed) that in a rectangle the angles were right angles and 
the opposite sides equal. Hence the sum of the four angles is 
equal to four right angles. Next, by drawing a diagonal of 
a rectangle, it will be seen that any right-angled triangle can 
be placed in juxtaposition with an equal and similar triangle 
in such a way as to make up a rectangle: this step in the 
argument may have been suggested by the tiles used in paving 
floors. Hence the sum of the angles of a right-angled triangle 
is equal to two right angles. Lastly, any triangle ABC can 
be divided into two right-angled triangles by drawing a per- 
pendicular AD from the biggest angle A to the opposite side 
BG. The sum of the angles of the triangle ABD is equal to 
two right angles. Hence the sum of the angles B and BAD is 
equal to one right angle. Similarly the sum of the angles G 
and GAD is equal to one right angle. Hence the sum of the 
angles B, G, BAD, and GAD, that is, the sum of the angles of 
the original triangle ABG, is equal to two right angles. 

The only criticism I would make on this proof is that it 
rests frankly on the assumption that we can construct a rect- 
angle, and that the opposite sides of the rectangle are equal. 
This, unless we refer to direct observation or measurement, 
involves an assumption about parallel lines, which is equivalent 
to that made in Euclid's postulate. 

Pascal's Proof. Another proof, also resting immediately 
on experiment, to which I may here refer, was discovered 
by Pascal in the seventeenth century. It is interesting 

♦ G. J. Allman, Greek Geometry Jram Thalea to Euclid, Dublin, 1889, 
ohap. i. 



CH. XIV] THE PARALLEL POSTULATE 309 

from its history. Pascal was a delicate and precocious boy, 
and in order to ensure his not being over-worked his father 
directed that his education should at first be only linguistic 
and literary, and should not include any mathematics. Natur- 
ally this excited the boy's curiosity, and one day, being about 
twelve years old, he asked in what geometry consisted. His 
tutor replied that it was the science of constructing exact 
figures and determining the relations between their parts. 
Pascal, stimulated no doubt by the injunction against reading 
it, gave up his playtime to the new amusement, and in a few 
weeks had discovered for himself several properties of recti- 
linear figures, and in particular the proposition in question. 

His proof is said * to have consisted in taking a triangular 
piece of paper and turning over the angular points to meet at 
the foot of the perpendicular drawn from the biggest angle to 
the opposite side. The conclusion is obvious from a figure, for 




if the paper be creased so that A is turned over to D, as also 
B and G, we get B = FDB, A=FDE, and G = EDG: hence 
A+B-hG = EDF + FDB + EDG = tt. But we can only prove 
these relations on the assumption that when the paper is folded 
over BF and ^i?' will lie along DF, and thus that BF=FA=^ FD, 
and similarly that GE = EA = ED ; this assumption involves 
properties of parallel lines. A similar proof can be obtained 
by turning over the angular points to meet at the centre of 
the inscribed circle, and according to some accounts this was 
the method used by Pascal. I may add in passing that his 
father, struck by this evidence of Pascal's geometrical ability, 

* I believe that this rests merely on tradition. 



310 THE PARALLEL POSTULATE [CH. XIV 

gave him a copy of Euclid's Elements, and allowed him to take 
up the subject for which evidently he had a natural aptitude. 

Pythagorean and Euclidean Proof. Leaving the above 
demonstrations which rest on observation and experiment, I 
proceed to the classical proof given by Euclid*. This was 
taken from the Pythagoreans f, and is generally attributed to 
Pythagoras himself. The proof rests on properties of parallel 
lines, and the Pythagoreans must have prefaced it by some 
statement of those properties, but there is now no record as 
to how they treated parallels. 

Euclid's treatment of parallels is well known. There is no 
doubt that he put, at the beginning of his Geometry, certain 
definitions, axioms, and postulates ; but in the earliest manu- 
scripts, according to Peyxard, the assumption about parallels 
was not stated there, but was placed in the demonstration of his 
proposition 29 as a fact conformable to experience, which had 
to be assumed for the validity of the argument. If this be so, 
this exceptional treatment seems to indicate that, in Euclid's 
opinion, the assumption was of a different character to the 
other postulates, and the difficulty was faced frankly without 
any attempt to conceal it under a vague phraseology. Unluckily 
the postulate is often printed in modern school books as an 
axiom or a self-evident statement. This misplacement may 
have been due in the first instance to Theon of Alexandria 
who, about 370 A.D., lectured on Euclid's Geometry. Our 
modem texts of Euclid are mainly based on Theon's lectures, 
and it is only comparatively recently that the commentaries on 
Euclid's teaching have been subjected to critical discussion. 

At any rate Euclid, either at the commencement of his 
work or more likely in the course of his demonstration, boldly 
assumed that if a straight line meets two other straight lines 
so as to make the sum of the two interior angles on one side 
of it less than two right angles, then these straight lines if 
continually produced will meet upon that side on which these 

* Euclid's Elements, book i, prop. 32. 

t Eudemus is our authority for this ; see Proclus, ed. G. Friedlein, Leipzig, 
1873, p. 379. 



CH. XIV] THE PARALLEL POSTULATE 311 

angles are situated. Accepting this or some similar assumption, 
the demonstration is rigorous, and was given by him as follows. 




Take any triangle ABC. Produce the side BA to any distance 
AH, and through A draw a line AK parallel to BG. On the 
assumption that his postulate is true, Euclid showed {Euc, I. 29) 
that the angle ABC must be equal to the angle HAK, and the 
angle AGB to the angle KAG. Hence the sum of the three 
angles of the triangle ABG must be equal to the sum of the 
angles HAK, KAG, and GAB, that is, to two right angles. 

Euclid's postulate and this theorem mutually involve the 
one the other: if we can prove his postulate this theorem 
is true, if otherwise we can prove this theorem, then his 
postulate is true*. Hence the question with which I com- 
menced the chapter (namely whether the sum of the angles 
of a triangle is, and can be shown to be, equal to two right 
angles) comes in effect to asking whether Euclid's postulate is 
true and can be proved to be true. 

Features of the Problem. The postulate, as enunciated 
by Euclid, has the semblance of a proposition. For many 
centuries mathematicians believed that it could be directly 
deduced from the fundamental principles of geometry, and they 
devoted much labour to trying to prove it. The more notable 
of these attempts I propose to describe, but I may anticipate 
matters by saying that about a hundred years ago it was shown 

* The demonstration is given by J. Richard, La Fhilosophie dea Mathema- 
Itjuet, Paris, 1903, pp. 81—84, 



312 THE PARALLEL POSTULATE [CH. XIV 

that this postulate, or any of its equivalent forms, cannot be 
proved. Thus in every one of the proposed demonstrations 
there is either a fallacy, or some assumption similar to that 
made by Euclid. 

In order to be able to appreciate the criticisms on some of 
these attempts it will be convenient to preface the discussion 
by saying that the postulate and its conclusions do in fact 
involve considerations of the nature of the space considered. 
For example, we say that we can draw a line parallel to a 
given line, and that however far the lines are produced they 
will not meet. This is not at variance with what we observe, 
but we have never got to infinity to see what does happen 
there. Hence, though it is conformable to our experience we 
cannot say that it is actually true. In fact, it is not certain 
that the statement is absolutely true of the space we know. 
An example will show this. If small intelligent beings lived 
on a strictly circumscribed portion of the surface of a sphere, 
and evolved a geometry of figures drawn on that surface, 
they might form a body of propositions similar to those given 
by Euclid, and resting on the same axioms and assumptions. 
All their assumptions, except this postulate and the axiom 
about the impossibility of two straight lines enclosing a space, 
would be correct. But if the sphere were large enough, and 
they were confined to a comparatively small part of its surface, 
they would not be able to find out that this postulate about 
parallels was incorrect. Accordingly it would be not unreason- 
able that they should believe it to be true, though in fact 
it would be false. 

What is here said of the surface of a sphere is by way of 
illustration, but it indicates the possibility of the existence 
of surfaces such that consistent systems of geometries, closely 
resembling the Euclidean geometry, might be constructed 
dealing with figures drawn thereon. This question is men- 
tioned again later. The above remarks suffice, however, to 
show that the postulate involves properties of the space in 
which the figures are constructed. It follows that the best 
way of stating the postulate will be that which is directly 



CH. XI V] THE PARALLEL POSTULATE olS 

characteristic of what we may call plane space, as opposed to 
other kinds of space. Euclid's statement answers this purpose, 
and it is remarkable that he should have thus gone to the root 
of the matter. By implication he admitted that the statement 
could not be demonstrated, and he frankly met the difficulty 
by telling his hearers that though he could not prove it, they 
must grant him the postulate as a foundation for his reasoning. 

Attempted Demonstrations of the Postulate. I proceed now 
to describe a few of these attempts to prove the postulate or the 
proposition. 

Ptolemy's Proof of the Postulate. One of the earliest of 
these efforts to prove the postulate was due to Ptolemy, the 
astronomer, in the second century after Christ. It is as follows*. 
Let the straight line EFGH meet the two straight lines AB and 
CD so as to make the sum of the angles BFG and FGD equal to 
two right angles. It is required to prove that AB and CD are 
parallel. If possible let them not be parallel, then they will 
meet when produced say at M (or N). But the angle AFG 
is the supplement of BFG and is therefore equal to FGD. 
Similarly the angle FGG is equal to BFG. Hence the sum 
of the angles AFG and FGG is equal to two right angles, and 
therefore the lines BA and DC, if produced, will meet at M 
(or N). But two straight lines cannot enclose a space, therefore 
AB and CD cannot meet when produced, that is they are 
parallel. 




Conversely, li AB and CD be parallel, then ^Fand CG are 
not less parallel than FB and GD ; and therefore whatever be 
the sum of the angles AFG and FGGy such also must be the 

• Proclus, ed. G. Friedlein, Leipzig, 1873, pp. 362—868. 



314j the parallel postulate [CH. XIV 

sum of the angles FOB and 'BFQ, But the sum of the four 
angles is equal to four right angles, and therefore the sum of 
the angles BFQ and FGD must be equal to two right angles. 

This proof is not valid. Apart from all considerations about 
the nature of space, no reason is given why the sums of the 
angles on either side of the secant should be assumed to be 
equal. The whole question turns on whether the straight lines 
would not meet, even though the sum of the angles on one side 
is a little more than two right angles, and on the other a little 
less. It is conceivable that parallels might open out as they are 
prolonged, and thus that a straight line inclined at a small 
angle to one of them should never overtake the other, but chase 
it unsuccessfully through infinite space, just as a curve pursues 
its asymptote and never catches it. 

Procluss Proof of the Postulate. Proclus, after criticising 
Ptolemy's demonstration, gave a proof of his own, but in the 
course of it he assumed that if two intersecting straight lines 
be produced far enough the distance between a point on one of 
them and the other line can be made greater than any assigned 
finite length, and that if two parallel straight lines be produced 
indefinitely the perpendicular from a point on one of them to 
the other remains finite. On these assumptions the postulate 
can be proved. But just as we cannot assume that two con- 
verging lines (ex. gr. a curve and its asymptote) will ultimately 
meet, so we must not assume that the distance between two 
diverging lines will be ultimately infinite. 

Wallis's Proof of the Postulate. I will give next a demons tra- 
tion offered by J. Wallis, Savilian Professor of Geometry, in a 
lecture delivered at Oxford on July 11, 1663. The substance of 




CH. XIV] 



THE PARALLEL POSTULATE 



315 



his argument may be put thus*. It is desired to prove that if 
two lines AB, CD meet a transversal HACK, so that the sum 
of the angles BAG, AGD is less than two right angles, then AB 
and CD must (if produced) meet. One of the angles BAC, AGD 
must be acute ; suppose it is BA C. He first showed that, in 
this case, from any point B in AB we can draw a line BE 
which will cut AC in E, so that the sum of the angles BEG, 
EGD is equal to two right angles; hjgnce the angle BE A is 
equal to the angle DC A. Then if we take the triangle BAE 
(drawn on ^^ as base and with B as vertex) and construct 
a similar triangle on -4 C as base, he proved that its vertex 
must be at a finite distance firom AC, must lie on AB produced, 
and must lie on CD (produced if necessary). Hence AB and 
CD when produced must meet. 

The proof is ingenious, but it rests on the assumption that 
it is possible to construct a triangle on any specified scale 
similar to a given triangle. This cannot be considered axiomatic 
and in fact is not true of spherical triangles. The assumption, 
however, is made explicitly, and it can be used instead of 
Euclid's postulate if it be thought desirable. 

Bertrand's Proof of the Postulate. The following is another 
interesting demonstration. It was originally given by Bertrand 




B K 

of Geneva f. Suppose AX and BY are two lines which meet 
a third line HABK so that XAB + YBA < ir. It is required 
to show that AX and BY must cut. For simplicity, I draw 
the figure so that XAB — 7r/2 and therefore YBA < 7r/2, 

• J. Wallis, Opera, Oxford, 1693, vol. ii, pp. 674—673. 
t I do not know where or when it was first published. It was given by 
S. F. Lacroix in his Elements de Geometrie, Paris, 1802, p. 23. 



316 THE PARALLEL POSTULATE [CH. XIV 

but this does not affect the argument. Produce AB indefi- 
nitely in both directions to H and K. Draw BZ perpen- 
dicular to AB, and denote the angle YBZ by a. Then 
the area between BY and BZ is the fraction o/tt of the 
space round B above UK, that is, a/ir of the area of the 
plane above HK. Also the area between AX and BZ is the 
fraction AB/HK of the plane above HK. Now, however small 
a may be, g/tt is a (infinite finite fraction, but AB/HK is 
indefinitely small. Hence the area between BY and BZ is 
greater than the area between AX and BZ. But as long 
as -BF does not cut AX the area between BY and BZ is less 
than that between AX and BZ. Hence BY must cut AX. 

The objection to this demonstration is that it depends upon 
a comparison of infinite areas. But we have no test by which 
we can compare such areas, and to consider the order of 
infinities involves questions outside the region of elementary 
geometry. There is also a more fundamental difficulty: the 
argument assumes that space is infinite, but it is possible that 
it may be boundless and finite, as, for instance, is the surface of 
a sphere. 

Play/air's Earlier Proof of the Postulate. I will mention 
next an attempt to prove this postulate by assuming that two 
lines which cut cannot be both parallel to another line, that is, 
that through a point one and only one straight line can be 
drawn parallel to a given straight line. It has been said that 
this assumption is not axiomatic, for a reason similar to that 
given above in my criticism of Ptolemy's Proof, but to most 
readers it seems simpler than Euclid's postulate, and as its 
meaning is easily grasped, some mathematicians prefer it to 
Euclid's postulate. Like the latter it is characteristic of the 
space considered. If this assumption is made, it is easy 
to show* that a transversal meeting two parallel straight 
lines makes the alternate angles equal, from which the other 
conclusions of Euclid follow. 

• Elements of Geometry, by J. Playfair, Edinburgh, 1st edition, 1813, book i. 
prop 29. The book is in the same form as Euclid's Elements except for the 
substitution of this postulate for that given by Euclid. 



CH. XIV] THE PARALLEL POSTULATE 317 

Personally I agree with those who consider Playfiiir's 
assertion as axiomatic, that is, as being a part of our conception 
of plane space as derived from experience. This is not in- 
consistent with admitting that mathematicians can conceive 
a more general view of space (i.e. non-Euclidean space) and 
that to them the space of our experience is only, to the highest 
degree of approximation, Euclidean space*. But many writers 
do not accept Playfair's assertion as axiomatic. On such an 
issue no argument is possible. 

Attempted Direct Demonstrations of the Proposition. The 
difficulties connected with the subject of parallelism led to 
various attempts to prove the proposition directly and thence 
to deduce some property of parallelism equivalent to Euclid's 
postulate. Substantially this was the method used by Thales 
and Pascal. I will mention one or two of these attempts. 

Flay/airs Rotational Proof of the Proposition. First I will 
describe an attempt, given by Playfair f in 1813. His argument 
is as follows. An angle is measured by the amount of turning 

By ^ 




H 

of a vector. Let ABC be any triangle. Suppose we have 
a rod AL placed along AB with one end a.t A. If we rotate 
it clockwise round ^ as a pivot through the angle BAG it 
will move from AL to AK. It will make no difference if we 
now slide the rod along AK ao that the end moves from A 

* See A. Cayley, British Association Report, London, 1883, p. 9. 

+ See the notes appended to J. Playfair's Elements of Geometry, p. 432 in 
the fifth edition. Playfair finds the sum of the exterior angles and thence 
deduces the Bum of the interior angles, but the method is the same as that 
given above. 



318 THE PARALLEL POSTULATE [CH. XIV 

to G. If we now turn the rod, in the same direction as before, 
round (7 as a pivot through the angle AGB it will move from 
GK to GH, It will make no difference if we now slide it back 
along GB so that the end moves from G to B. If we now turn 
the rod, again in the same direction, round j5 as a pivot through 
the angle GBA it will move from BH to BA» We can then 
again slide the rod along BA so that the end B moves to A, 
when the rod will lie along AU, Thus the rotation, always in 
the same direction, successively through the three angles of the 
triangle produces exactly the same effect as a rotation through 
two right angles. 

The demonstration is incorrect. In fact it is assumed that 
if the angle ABG in the figure above on page 311 is equal to 
HAK, then BG and AK will be parallel. The fallacy can be 
seen at once by applying the argument to the case of a spherical 
triangle or to one whose sides are circular arcs all convex — or 
all concave — to its median point. 

Legendre's First Proof of the Proposition. Legendre devoted 
special attention to the problem and offered various demonstra- 
tions of it. I give three of them. In one, which appears in the 
earlier editions of his Geometry, he tried to show that the sum 
of the angles of a triangle could not be greater than two right 
angles and could not be less than two right angles, and that 
therefore it must be equal to two right angles. His demonstra- 
tion assumes that if any number of equal triangles are placed 
in juxtaposition along a line it is possible to draw a triangle 
enclosing them all: the same assumption was made by T. P. 
Thompson. But unless we assume that space is infinite this 
is not justified: the insufficiency of the argument is clearly 
brought out by applying it to spherical triangles. 

Legendre s Analytical Proof of the Proposition. I think 
however that the following is the most ingenious of the proofs 
given by Legendre*. A triangle ABG is completely deter- 
mined by one side and two angles, say, a, B, G. Given these, 
the triangle can be constructed, and therefore the angle A de- 
termined. Now if the unit of length be changed the measure 
♦ Elements de Geometrie, Paris, 12th edition, p. 281. 



CH. XI V^] 



THE PARALLEL POSTULATE 



319 



of a will be changed but the triangle, and therefore A, will 
not be altered. Hence A cannot depend on the value of a; 
accordingly it must depend only on B and 0, 




Now take a right-angled triangle DEF, of which D is the 
right angle. Draw DG perpendicular to EF. The angle EDG 
in the triangle EDG is calculated from the other two angles of 
that triangle, namely E and a right angle, in the same way 
as the angle F in the triangle DEF is calculated from the 
other two angles of that triangle, namely E and a right angle. 
Hence F is equal to EDG. Similarly E is equal to GDF. 
Therefore the sum of the angles F and E is equal to the sum of 
EDG and GDF, and therefore is a right angle. Hence the sum 
of the angles F, E, and D of the triangle DEF is equal to two 
right angles. Thus the result is proved for a right-angled 
triangle, and it will follow for any other triangle in the same 
way as in Thales's proof. 

J. Leslie criticised this proof on the ground that in the 
corresponding theorem of Spherical Trigonometry, we know 
that the expression for the value of the angle A involves alR, 
where R is the radius of the sphere, and it is conceivable that 
in plane geometry there might be a length R' (the reciprocal of 
the space constant) which entered in a similar way in the 
problem : hence A might involve a and yet not change with the 
unit of measurement. To this it was replied that the point of 
Legendre's argument was that the discussion related only to 
plane geometry : this might, no doubt, be considered as the 
special case of spherical geometry in which R was infinite ; if 
so, any term in the expression for A which involved a/R 



320 THE PARALLEL POSTULATE [CH. XIV 

disappeared, and thus his reasoning was valid ; and to introduce 
an unknown quantity R' was contrary to all canons of reasoning. 

Legendre's Latest Proof of the Proposition. At the end of 
his life, 1833, Legendre showed* that if we could construct one 
triangle the sum of whose angles was equal to two right angles, 
then the sum of the angles of every triangle would be equal to 
two right angles. All attempts to obtain direct proofs that such 
a triangle existed failed. He showed, however, that if the sum 
of the angles of a triangle is not equal to two right angles then 
linear magnitudes can be determined by angular measurements. 
Assuming that this latter result is impossible, the proposition is 
proved. The second part of the argument is in effect a translation 
into geometry of the analytical proof given above. 

Lagranges Memoir. Legendre's great contemporary, La- 
grange, believed at one time that he had found a solution 
of the problem. It rested on establishing plane geometry by 
a generalization from geometry on a spherical surface. He 
commenced to read to the Institute a paper on the subject, 
but had hardly begun when he stopped abruptly, put his 
memoir in his pocket and saying "Gentlemen, I must think 
further about this," left the room. We do not know what his 
argument was, but doubtless some flaw in it flashed on him as 
he commenced his paper f. 

Other Parallel Postulates, Euclid's postulate is in accord- 
ance with experience, and like the axioms and other postulates 
it rests ultimately on the results of observation, but his 
statement of the property in question is not easy, and it 
requires some thought before the point is grasped. For this 
reason many attempts have been made to put it in other forms 
which are more likely to be readily granted by an ordinary 
reader. I enumerate two or three of these. 

It will be noticed that the demonstration offered by Wallis 
and the earlier one given by Playfair rest on alternative 
postulates about parallels. That assumed by Wallis is sufficient, 

♦ MSmoires de VInstitut de France, Paris, 1833, voL xii, pp. 367—410. The 
paper also contains an account of Legendre's earlier investigations. 
t A. de Morgan, Budget of Paradoxes, London, 1872, p. 173. 



CH. XIV] THE PARALLISL POSTULATE 321 

but is not axiomatic, as may be seen by its incorrectness when 
applied to spherical triangles. It was adopted by Carnot, 
Laplace, and J. Delboeuf. Playfair's axiom answers the purpose 
as well as Euclid's : this form was also used by Ludlam. Thales's 
assumption that a rectangle exists also suffices. This was 
assumed by Clairaut. 

It has been suggested that Euclid's postulate might be re- 
placed by assuming that, if at a point ^ in a given line AB 
a line AX he drawn perpendicular to it, and at another point 
B in AB a line BY he drawn, making with it an acute angle, 
then AX and BY will cut. But essentially this is only Euclid's 
form expressed diagrammatically. 

Another alternative form which has been suggested is to 
the effect that through every point within an angle a line can 
be drawn intersecting both sides (substantially the view of 
Lorentz and Legendre). This also is sufficient, but the appli- 
cation is less easy than that of Euclid's postulate. 

It has also been proposed that we may reasonably assume 
that the distance between two parallel lines is always the same 
(Dilrer, T. Simpson, R. Simpson), or that a line which is every- 
where equidistant from a given straight line in the same plane 
is itself straight (Clavius). Neither of these forms is satisfactory. 
The conception of distance involves measurement, and this in 
turn involves a theory of incommensurable magnitudes. Thus 
before we can rest the theory on such a postulate, other as- 
sumptions have to be made, and the resulting discussion is 
neither simple nor clear. 

Legendre suggested that it was sufficient to assume that 
the lesser of two homogeneous magnitudes if multiplied by a 
sufficiently large number would exceed the greater of them. 
But to make use of this he had to introduce the assumptions 
and principles of the infinitesimal calculus, and this can hardly 
be regarded as permissible in elementary geometry. 

A different kind of postulate was suggested by Dodgson, 
who proposed* to replace Euclid's postulate by assuming that 
2** times the area of an equilateral four-sided figure inscribed 
* C. L. Dodgson, Curiosa Mathematiea, London, 1890, p. 35. 

B. R. 21 



322 THE PARALLEL POSTULATE [CH. XIV 

in a circle is gi'eater than the area of any one of the segments 
of the circle which lies outside it, where n is any positive 
integer. Granting that we can inscribe such a figure in a 
circle, this assumption seems obviously true. But a comparison 
by the eye of the area of a rectilineal figure with an incom- 
mensurable area bounded by a curve and a straight line is 
contrary to all the traditions of classical geometry and to what 
is usually regarded as permissible in elementary geometry. 

Definitions of Parallels, Other writers have tried to turn 
the difficulty by altering the definitions of parallel lines*. One 
of the best known suggestions made with this object defines 
parallel lines as lines which have the same direction, by which 
is meant lines which make the same angle with a line cutting 
them (Varignon, Bezout, Lacroix). The phrasings of the proposed 
definition vary slightly. There is no objection to this if the 
cutting line is fixed, but then it does not avoid the necessity 
of our having to assume some postulate. If, however, as is 
usual, the definition is taken to mean that parallel lines make 
equal angles with every secant, it involves an unwarrantable 
assumption. In fact it would seem that the term direction 
cannot be defined without predicating a theory or properties 
of parallels f. 

Another suggested definition which has met with some 
favour is to the effect that parallel lines are lines which neither 
recede from nor approach each other, that is, lines whose 
distance apart is always the same (Wolf, Boscovich, Bonnycastle). 
This definition is really equivalent to the postulates laid down 
by Diirer and Glavius, and to give a definition which involves 
a disputed assumption is worse than frankly postulating the 
assumption. The definition, however, agrees with the popular 
view, but if we take a straight line, and erect at every point 
a perpendicular of given length, we have no right to assume 
that the locus of the extremities of these transverses will be 
a straight line, and even less right to assume that it is a 

♦ See J. Playfair, Elements of Geometry, Edinburgh, 1813 ; notes to book i, 
prop. 29. 

t W. Killing, Grundlagen der Geometrie, Paderborn, 1898. 



CH. XIV] THE PARALLEL POSTULATE 323 

straight line perpendicular to them; and, unless we assume 
that the distance between the parallel lines is measured by a 
transverse perpendicular to both of them, we cannot use the 
definition to much purpose. D'Alembert avoided these diffi- 
culties by saying that one line is parallel to another if it 
contains two points on the same side of and equidistant from the 
other ; but this by itself is of no use, unless we assume Euclid's 
postulate or some similar property of parallels. 

These definitions, if they are to be useful, involve assump- 
tions. They slur over the real difficulty, and are less 
satisfactory than a frank statement of what is assumed. 

Non- Euclidean Systems*, It had long been well known that 
the postulate and proposition were not true in the corresponding 
geometry on a spherical surface — in fact the sum of the angles of 
a spherical triangle always exceeds two right angles — and since 
there were such difficulties in establishing the Euclidean postu- 
late in plane space, mathematicians began, rather more than a 
century ago, to consider whether that postulate was true either 
necessarily or in fact. It required courage, even genius, to make 
such a conjecture, for though on the one hand the postulate 
could not be proved, there was on the other no reason to doubt 
its correctness, and no conclusion inconsistent with observation 
had been deduced from it, while at first sight nothing seemed 
to justify the assumption that it was not true. 

Saccheri and Lambert raised this question in the eight- 
eenth century, but their investigations, though intelligent, 
were incomplete and attracted little attention. Gauss went 
further, as appears fi-om his correspondence in 1829 and 1831, 
but even before then he had shoAvn that the proposition and 
postulate could be proved to be true if it were admitted that a 
triangle could be drawn with an area greater than a given area; 
this, however, he rightly regarded as non-axiomatic. Later he 
discussed some of the properties of hyperbolic geometry. He 
did not publish his results, and they did not afifect the treat- 
ment of the problem by other writers. 

* See R. Bonola, La Geometria Non- Euclidea, Bologna, 1906 ; and D. M. Y. 
Sommerville, Bibliography of Non-Euclidean Qeometry, St Andrews, 1911. 

21—2 



324 THE PAKALLEL POSTULATE [CH. XIV 

The credit of first showing that the postulate is not neces- 
sarily true is due to Lobatschewsky and the Bolyais. They 
boldly assumed that the postulate was not true and that 
through a point a number of straight lines can be drawn 
parallel to a given straight line. On this assumption, they 
deduced a consistent body of propositions, which is termed 
hyperbolic geometry. 

These investigations attracted but slight notice. The writers 
were almost unknown. N. I. Lobatschewsky, 1793 — 1856, was 
professor at Kasan, and his works were written in Russian. 
Wolfgang Bolyai, 1776 — 1856, was an eccentric, simple, rough- 
clad teacher in Transylvania. He now lies buried at his request 
under an apple tree, commemorating the three apples which, 
he said, had so profoundly affected the history of the human 
race — those of Eve and Paris, which had made earth a hell, and 
that of Newton which had raised earth again into the company 
of the heavenly bodies. His son John, 1802 — 1860, had 
excellent mathematical abilities, and worked out the principles 
of the new geometry, but he spent his life soldiering, and to 
him mathematics was only a recreation. Probably he valued 
his reputation as a musician far above his mathematical abili- 
ties. He was noted for his fiery temper ; in one of his quarrels 
he accepted the challenge of thirteen officers of a regiment on 
condition that after each duel he might play to each of them 
a piece on his violin. He is said to have vanquished them all, 
and been, in consequence, retired fi-om the army. 

The subject, however, was "in the air," and attracted the 
attention of G. F. B. Riemann. Riemann was one of the most 
brilliant German mathematicians of the nineteenth century and, 
though short-lived, his writings have profoundly affected the 
development of the subject. His paper on the hypotheses on 
which geometry is founded was read in 1854. He showed that 
a consistent system of geometry of two dimensions can be con- 
structed in which all straight lines are of a finite length. This 
science, now known as elliptic geometry, is characterised by the 
fact that through a point no straight line can be drawn which 
if produced far enough will not meet every other line. The 



Cir. XIV] THE PARALLEL POSTULATE 325 

resulting geometry ma}'^ be compared with the geometry of 
figures drawn on the surface of a sphere ; in it, space, though 
boundless, is finite. The discussion of Riemann's paper led to 
the discovery of the earlier researches of Lobatschewsky and the 
Bolyais. The subject has since been studied by several mathe- 
maticians of repute, notably by E. Beltrami and F. C. Klein. 

Here then we have three geometries — Elliptic, Euclidean 
(or Parabolic or Homaloidal) and Hyperbolic — each consistent 
on its own hypotheses, distinguished from one another according 
9S no straight line, or only one straight line, or a pencil of 
straight lines can be drawn through a point parallel to a given 
straight line. 

In the parabolic and hyperbolic systems straight lines are 
infinitely long : in the elliptic they are finite. In the hyperbolic 
system there are no similar figures of unequal size; the area 
of a triangle can be deduced from the sum of its angles, which 
is always less than two right angles ; there is a finite maximum 
to the area of a triangle; and its angles can be made as small as 
we like by making its sides sufficiently long. In the elliptic 
system all straight lines, if produced, are of the same finite length; 
any two lines intersect ; and the sum of the angles of a triangle 
is always greater than two right angles. In the elliptic system 
it is possible to get from one point to a point on the other side 
of a plane without passing through the plane ; thus a watch- 
dial moving face upwards continuously forward in a plane in 
a straight line in the direction from the mark VI to the mark 
XII will finally appear to a stationary observer with its face 
downwards ; and if originally the mark III was to the right of 
the observer it will finally be on his left-hand. 

In spite of these and other peculiarities of hyperbolic and 
elliptic geometries, it is impossible to prove by observation that 
one of them is not true of the space in which we live. For in 
measurements in each of these geometries we must have a unit 
of distance; and if we live in a space whose properties are 
those of either of these geometries, and such that the greatest 
distances with which we are acquainted (e.(^. the distances of 
the fixed stars) are immensely smaller than any unit natural 



326 THE PARALLEL POSTULATE [CH. XIV 

to the system, then it may be impossible for our observations 
to detect the discrepancies between these three geometries. It 
might indeed be possible for us by observations of the paral- 
laxes of stars to prove that the parabolic system and either the 
hyperbolic or the elliptic system were false, but never can it be 
proved by measurements that the Euclidean geometry is true. 
Similar difficulties might arise in connection with excessively 
minute quantities. In short, though the results of Euclidean 
geometry are more exact than present experiments can verify 
for finite things, such as those with which we have to deal, yet 
for much larger things or much smaller things, or for parts of 
space at present inaccessible to us, they may not.be true. Even, 
however, if our space is only approximately Euclidean, the 
propositions of ordinary geometry are none the less true of 
Euclidean space, though that may not be the space of our 
experience. 

I mention later, in Chapter XX, some other problems con- 
nected with different kinds of space. 



827 



CHAPTER XV. 

INSOLUBTLTTY OP THE ALGEBRAIC QUTNTIC. 

Another of the famous problems in the history of mathe- 
matics, which long proved an ignis fatuus to mathematicians, is 
the solution of the general algebraic equation of the fifth degree. 
By a solution of an algebraic equation we mean the expression, 
by a finite number of radicals and rational functions, of a root 
of it in terms of its coefficients. The solution of an algebraic 
quadratic equation presents but little difficulty. In the sixteenth 
century, solutions of the general cubic and quarfcic equations 
were obtained. It did not seem unnatural to suppose that by 
similar analytic methods the solution of quintic equations, as also 
those of a higher order, might be efifected. This is now known 
to be impossible, and I propose to give a brief sketch of the 
reason why it is so. The proof rests on the fact that the 
equations x^ — 1 and a?" = 1 have no common complex root. 

Quadratic, cubic and quartic equations can be solved by 
various methods ; but, in effect, all of them reduce the solution 
of the particular equation to the solution of one, called a 
resolvant, of a lower order. These methods fail when applied 
to equations of an order higher than four. Lagrange was the 
earliest \vriter to ask whether this was necessarily the case. 
In 1770 and 1771 he published a critical examination of the 
known solutions of quadratic, cubic and quartic equations, and 
showed that such solutions were possible only because a function 
of the roots of such equations could be formed which had a 
smaller number of possible values than the order of the equation, 
and this function was such that its value could be determined. 



828 INSOLUBILITY OF THE ALGEBRAIC QUINTIC [CH. XV 

For instance, take the quadratic equation x^-\- aa)-{-b = 0. 
It has two roots x^ and x^. We can reduce the solution to 
that of a simple equation because we can form a function of 
Wi and iPg which has only one value, and this value can be deter- 
mined. Such a value is y = (xi - x^y, since (xi — x^Y = (x^ — x^y. 
Moreover we have y = (x^ — x^y = (x^ + a^j)^ — ^fX^x^ = a^ — 46. 
Thus y is known. Hence the roots of the quadratic are given by 

Xi -\- X2 = — d, 
x,-X2 = ^/y. 

Similarly, in the cubic equation ar* + ax' -\-hx + c = 0, if 
yz=(^00i-\- (0X2 + co^XsY, where &> is one of the complex cube roots 
of unity, then y is a function of the roots which, when x^ X2 x^ 
are interchanged, has only two values, namely {xi -\- (0X2 + (o^x^y 
which may be also written in the form (xz + (0X3 + co^Xj^)^ or 
(xs + coxi + co^x^y, and (x^ + (oxs + (o'^x^^ which may be also 
written in the form {x^ + cax^ H- oy^xy or {x^ + aix^ + co^x^y. Let y 
and z denote these two values, that is, y = (xi-\- (DX2 + (o^x^^, and 
z = {x^-{- (DX^ + 0)2^2^. Then 3/ + = - 2^^ + 9a6 - 27c = A, say, 
and yz — (a^ — Sby = B, say. Thus y and z are the roots of 
t^— At -¥3 = 0, an equation which can be solved. And the 
roots of the cubic are determined by 

Xi -T X2 T" X^ ^= Ct, 

Xi + (0X2 + co^Xs = ^y, 

cci + (0^X2 + 0)^3 = ^/z. 
Again, in the quartic equation a^ + aa^ + ho(? + cx-\-d = 0, if 
y—{x^ — Xi-\-Xi — x^y, then y is a function of the roots which has 
only three values, namely, {x^— X2+ x^ — x^"^ or {x2 — x^-\-x^ — Xzy, 
(xi — x^-h X4 — X2y or (xs '-Xi-\- x^ — XiY, and {x^ — Xi-\- X2 — x-^y 
or (Xi — Xj^-\-X3 — X2y. If these values be denoted by y, z, and u, 
we have y-{-z-{-u=: Sa^ —8b = A, say, yz + zu + uy= 3a* — IQa^h 
-f 166- + 166c - 64x1 = B, say, and yzu = (a^ - 4a6 + 86)^ = C, say. 
Hence y, z, u are the roots of the equation t^ — At^ + Bt+C=^0. 
And the roots of the quartic are determined by 
Xi + X2 + Xa + Xi = '-a, 

Xi-X2-\-Xs-Xi = Vy, 

Xi-X2-X3 + x^ = siz, 
Xx-\- x^ — Xi — x^= Vw. 



CH. XV] INSOLUBILITY OF THE ALGEBRAIC QUINTIC 329 

Lagrange showed that for an equation of the nth. degree, an 
analogous function y of the roots could be formed which had 
only n — 1 values, and which led to a resolvant of the degree 
n — 1, but that the coefficients of this latter equation could not 
be obtained without the previous solution of an equation of 
the degree (n — 2) !. Hence the form assumed for y did not 
provide a solution of a quintic or of an equation of a higher 
degree. But though he suspected that the general quintic 
and higher equations could not be solved algebraically, he failed 
to prove it. The subject was next taken up by P. Ruffini, 
1798 — 1806, but his analysis lacked precision. 

The earliest rigorous demonstration that quintic and higher 
equations cannot be solved in general terms was given by 
N. H. Abel in 1824, and published in Crelles Journal in 1826. 
The result was interesting, not only in itself, but as definitely 
limiting a field of investigation which had attracted many 
workers. Abel's proof was simplified by E. Galois* in 1831, 
and is now accessible in various text-books. Essentially the 
argument is as follows. 

Let Xi, x^y Xi, ... Xn be the roots of the equation 
f (x) = x"" + ax^-^ + ...-{- k = 0. 
Any one of these roots, aii, is a function (which will generally 
involve radicals) of the coefficients a, 6, .... These coefficients 
are symmetrical functions of the roots, namely, a = Xxj, 
b^'ZxiXi, .... If these values of a, 6, ... be substituted in 
the expression for x^ we get, on simplification, an identity. 
If we interchange Xi and x.^, or any pair of roots, this will 
remain an identity. Similarly it will remain an identity, if 
we permute cyclically an odd number of roots, since such a 
permutation is equivalent to an interchange of a number of 
pairs of roots. For instance, if Xi and x^ are the roots of the 
quadratic equation a^ + cw? + 6 = 0, we have 

= i [{x^ + x^) + VK^i + ^y - 4a7ia?a}] 
• See LiomnlU's Jaumalf Paris, 1846, vol. xi, pp. 417—433. 



830 INSOLUBILITY OF THE ALOEBUAIC QUINTIC [CH. XV 

This is an identity, and if we interchange x^ and x^ it will 
remain an identity. 

Now suppose that we have a solution of the equation, that 
is, an expression for a;, in terras of the coefficients, involving 
only a finite number of radicals and rational functions. This 
expression may be a sum of a number of quantities. To fix 
our ideas let us suppose that in one of these quantities we 
come first, in the order of operations, to a radical, say, the ^^th 
root of H, where we may without loss of generality take p as 
prime. Of course H is rational: it involves a, b, c, ..., and 
therefore is a symmetrical function of iCi, a^a, •••• Thepth root 
of H also will be rational, but it will not be symmetrical : let us 
denote it by </)(fl7i, a^a, ... ccn). For instance, in the quadratic 
equation II=a^ — 4:h = (x^ + x^y — 4>XiX2, which is a rational and 
symmetrical function of Xi and X2. But ^H = </> = a?! — iCg, which 
is not symmetrical, though it is rational. 

In the general case </> is rational but not symmetrical. It 
involves the two roots, Xi and x^, and therefore it must change 
in value if they are interchanged. Further, since the values 
of <j) are determined by (f)P = H, and H does not vary when the 
roots are interchanged, one of the values of </> must be deducible 
from the other by multiplying it by «, where w is a ^^th root 
(other than unity) of unity, that is, 

^(iCa, ^1, ^3 ...) = «»^(^i'^2, ^z .••)• 

For instance, in the quadratic equation x^+ax + b = 0, we 
observe that, if in <^(a,i, x^), that is, in Xi — x^, we interchange 
Xi and 572, we necessarily get 4>(x2, Xj) — — <j)(xi, x^) since the 
two values of <^ are roots of (j)'^ = H, where // is invariable. 
Hence if one is ± *JH, the other must be + V^ 
The relation thus reached, 

<l)(X2,Xi,Xs,,.,) = G)(p{x^, /Ta, Xs, . . .), 

is an identity ; hence if we interchange Xi and x.2, we have 

<^(a7i, a?2, Xs, ...) = a)(p{x2, x^ a?,, ...). 

Hence eo^ = 1. And as « ^ 1, we have eo = - 1. Since q)^=1, 
we have p = 2: this shows that in the expression ior a root of 



CH. XV] INSOLUBILITY OF THE ALGEBRAIC QUINTIC 331 

an equation the first radical which occurs in the order of opera- 
tions must be of the second degree. 

In the case of a quadratic equation this conchides the dis- 
cussion, for there are only two roots which can be interchanged. 
We may note that if it were possible to take the value &> = 1, it 
would at once give x^ — x^y which, in the general case, is clearly 
impossible. 

We proceed to the case of a cubic or higher equation. We 
will first suppose that in the expression for Xi we substitute the 
above value of </>, and combine it and similar square roots with 
the various rational functions of the coefficients, a, 6, .... As 
long as we only introduce such square roots we obtain a function 
of the roots, say iT, susceptible of taking only two values, and 
therefore invariable when three (or any odd number) of the roots 
are permuted cyclically. This cannot lead to the determination 
of three or more roots. Hence we must, in this process of 
reduction and simplification, arrive, in the expression for x^, 
at a radical, say the gth root of if, of a higher order than 
a square root. In this expression K will be invariable 
when three (or any odd number) of the roots are permuted 
cyclically. 

We can express the gth root of ^ as a rational function of 
the roots '^{xi,a;^, ... Xn), and, from the nature of the case, yjr 
takes different values when three roots are permuted cyclically. 
The values of yfr are roots of i/r? = K. Accordingly, following 
the same argument as that given above, we have 

where « is a gth root (other than unity) of unity. If we 
permute x^, x^^ Xg cyclically, we have 

>Jr(a7„a?i, X2, a?4, ...) = ayyirixo, x^, a?i, aj^, ...), 
and i/r (a^i, x^, Xi, x^, . . .) = (oyjr (xs, x^, x^, x^, ...). 

Hence ©" = 1. We have previously shown that the first radical 
involved in the general expression for a root must be a square 
root, and now we see that the next radical must be a cube root. 
We observe that the solution usually given of a cubic confirms 



332 INSOLUBILITY OF THE ALGEBRAIC QUINTIC [OR. XV 

our analysis. The solution of the cubic af^ -{■ bx •}- c = i) is 
generally written as 

x={- c/2 4- V^l* + {- c/2 - ^u\K 
where u = c-/4f + 6727. In each term of the expression for x 
the first surd which occurs in the order of operations is a square 
root and the next surd a cube root. 

In the case of a cubic equation there are only three roots, 
and we cannot continue the process further. So also we cannot 
proceed further in the case of a quartic equation, for as we want 
to permute an odd number of roots cyclically we cannot permute 
more than three. 

We proceed to the case of an equation of the fifth or higher 
degree. We have already shown that in this case if, when we 
substitute in the expression for x^ the value of <p and similar 
square roots, we arrive at a radical j^K for which the equivalent 
rational function yjr takes different values when an odd number 
of the roots are permuted cyclically, it follows that if we per- 
mute three roots cyclically we get w^ = 1, where © is a root 
(other than unity) of co^ = 1. Hence q = S. Also if we permute 
five roots cyclically, we obtain by a similar argument co^ = 1. 
Thus q—b. These equations for w and values of q are incon- 
sistent. In fact the argument shows that the first surd which 
has to be calculated in the general expression for x^ is a square 
root, and the next surd is at the same time a complex cube 
root of unity and a complex fifth root of unity. This is 
impossible. Hence an equation of the fifth or higher degree 
cannot be solved by a finite number of radicals and rational 
functions of the coefiicients. 

It may be added that just as we can express the root of a 
cubic equation in terms of trigonometrical functions, so we can 
express the root of a quintic or sextic equation in terms of 
elliptic or hyperelliptic functions. But such functions lie 
outside the field of algebra. 



a33 



CHAPTER XVL 
mersenne's numbers. 

One of the unsolved riddles of higher arithmetic, to which 
I alluded in the second chapter, is the discovery of the method 
by which Mersenne or his contemporaries determined values 
of p which make a number of the form 2^ — 1 a prime. It is 
convenient to describe such primes as Mersenne's Numbers, 
a name which I believe I introduced. In this chapter, for 
shortness, I use N to denote a number of the form 2^ — 1. In 
a memoir in the Messenger of Mathematics in 1891 I gave 
a brief sketch of the history of the problem. I here repeat the 
facts in somewhat more detail, and add some notes on methods 
used in attacking the problem. 

Mersenne's enunciation of the results associated with his 
name is in the preface to his Cogitata*, The passage is as 
follows : 

Vbi fuerit operae pretium aduertere xxvm numeros a Petro Bungo pro 
perfectis exhibitos, capite xxvm, libri de Numeris, non esse omnes Perfectos, 

quippe 20 sunt imperfecti, adeovt [adeunt?] solos octo perfectos habeat qui 

sunt h regione tabulae Buogi, 1, 2, 3, 4, 8, 10, 12, et 29: quique soli perfect! 
sunt, vt qui Bungum habuerint, errori medicinam faciant. 

Porr6 numeri perfecti adeo rari sunt, vt vndecim dumtaxat potuerint hac- 
ienus inueniri : hoc est, alii tres a Bongianis differentes : neque enim vllus est 
alius perfectus ab illis octo, nisi superes exponentem numerum 62, progressionis 
duplae ab 1 incipientis. Nonus enim perfectus est potestas exponentis 68 
minus 1. Decimus, potestas exponentis 128, minus 1. Vndecimus denique, 
potestas 258, minus 1, hoc est potestas 257, vnitate decurtata, raultiplicata per 
potestatem 256. 

* Cogitata Phy$ico-MatJiematica, Paris, 1644, Praefatio Generalis, article 19. 



334 MEUSENNES NUMBEKS [CH. XVI 

Qui vndecim alios repererit, nouerit se analysim omnem, quae fuerit bacte- 
nu8, superasse : memineritque interea nullum esse perfectum k 17000 potestate 
ad 32000; & nullum potestatum interuallum tantum assignari posse, quin detur 
illud absque perfectis. Verbi gratia, si fuerit exponens 1050000, nullus erit 
numerus proRressionis duplae vsque ad 2090000, qui perfectis numeris seruiat, 
hoc est qui minor vnitate, primus exiatat. 

Vnde clarum est quam rari sint perfecti nuraeri, & qu^m merito viris per- 
fectis comparentur ; esseque vnam ex maximis totius Matheseoa diflScultatibus, 
praescriptam numerorum perfectorum multitudinum exhibere ; quemadmodum 
& agnoscere num dati numeri 15, aut 20 caracteribus constantes, sint primi 
necne, ciim nequidem saeculum integrum huic examini, quocumque modo 
hactenus cognito, sufficiat. 

It is evident that, if p is not a prime, then N is composite, 
and two or more of its factors can be written down by 
inspection. Hence we may confine ourselves to prime values 
of p. Mersenne, in effect, asserted that the only values of p, 
not greater than 257, which make N a prime, are 1, 2, 3, 5, 7, 
13, 17, 19, 31, 67, 127, 257 : to these numbers 89 and 107 must 
be added. I gave reasons, some years ago, for thinking that 67 
here is a misprint for 61, and I assume this is so. With these 
corrections we have no reason to doubt the truth of the state- 
ment, but it has not been definitely established. 

There are 56 primes not greater than 257. The deter- 
mination of the prime or composite character of JSf for the 
9 cases when p is less than 20 presents no difficulty: in only 
one of them is N composite. For 2 of the remaining 47 cases 
(namely, when p=2S and 37) the decomposition of N had 
been given by Fermat. For 9 of them (namely, when p = 29, 
43, 73, 83, 131, 179, 191, 239, 251) the factors of JSf were 
given by Euler. He also proved that N was prime w^hen 
p = 31. Reuschle gave the factors of JSf when p = 47, and Plana 
the factors when p = 41. Landry and Le Lasseur discovered the 
factors in 9 cases, namely, when p = 53, 59, 79, 97, 113, 151, 211, 
223, and 233. Seelhoff showed that N was prime when ^ = 61, 
Cunningham gave the factors when ^ = 71, 163, 173, and 197, 
Cole the factors when p = 67, Woodall the factors when ^ = 181, 
and Powers proved that iV was prime when ^=89 and 107. It has 
been asserted that the prime character of iV when p=127 has 
been established, but the proof has not been published or verified. 



CH. xvi] mersenne's numbers 335 

Thus there are 15 values of p for which Mersenne's state- 
ment still awaits verification. These arc 101, 103, 109, 127, 
137, 139, 149, 157, 167, 193, 199, 227, 229, 241, 257. For 
these values N is (according to Mersenne) prime when p — 127 
and 257, and is composite for the other values. If w^e admit 
that the character of N is known when p = 127, the number of 
cases yet to be verified is reduced to 14. 

To put the matter in another way. According to Mersenne's 
statement (corrected by the substitution of 61 for 67 and with the 
addition of 89 and 107 to his list) 42 of the 56 primes less than 258 
makeiVcomposite and the remaining 14 primes make iV prime. In 
29 out of the 42 cases in which iVis said to be composite we know 
its factors, and in 14 cases the statement is still unverified. In 
12 out of the 14 cases in which it is said that N is prime the 
statement has been verified, and in 2 cases it is still unverified. 

From the wording of the last clause in the above quotation 
it has been conjectured that the result had been communicated 
to Mersenne, and that he published it without being aware of 
how it was proved. In itself this seems probable. He was 
a good mathematician, but not an exceptional genius. It would 
be strange if he established a proposition which has baffled 
Euler, Lagrange, Legendre, Gauss, Jacobi, and other mathe- 
maticians of the first rank ; but if the proposition is due to 
Fermat, with whom Mersenne was in constant correspondence, 
the case is altered, and not only is the absence of a demon- 
stration explained, but we cannot be sure that we have attacked 
the problem on the best lines. 

The known results as to the prime or composite character 
of N, and in the latter case its smallest factor, are given in 
the table on the next page. The cases that remain as yet 
unverified are marked with an asterisk. 

Before describing the methods used for attacking the 
problem it will be convenient to state in more detail when 
and by whom these results were established. 

The factors (if any) of such values of iV as are less than 
a million can be verified easily: they have been known for 
a long time, and I need not allude to them in detail. 



S36 



MERSENNES NUMBERS 



[CH. XVI 





TABLE OF MERSENNE'S NUMBERS. 




p 


Value of N=2P-1 


Character 


Discoverer 


1 


1 


prime 




2 


8 


prime 




8 


7 


prime 




5 


31 


prime 




7 


127 


prime 




11 


2047 = 23x89 


composite 




18 


8191 


prime 




17 


131071 


prime 




19 


524287 


prime 




23 


8388607 = 47x178481 


composite 


Fermat 


29 


636870911 = 233 x 1103 x 2089 


composite 


Euler 


31 


2147483647 


prime 


Euler 


37 


137438953471 = 223 x 616318177 


composite 


Fermat 


41 


2199023255551 = 13367 x 164511353 


composite 


Plana 


43 


8796093022207 = 431 x 9719 x 2099863 


composite 


Euler 


47 


2351 X 4513 X 13264529 


composite 


Eeuschle 


53 


6361 X 69431 x 20394401 


composite 


Landry 


59 


179951 X 3203431780337 


composite 


Landry 


61 


2305843009213693951 


prime 


Seelhoff 


67 


= 0(193707721) 


composite 


Cole 


71 


= (228479) 


composite 


Cunningham 


73 


= (439) 


composite 


Euler 


79 


= (2687) 


composite 


Le Lasseur 


83 


= 0(167) 


composite 


Euler 


89 


618970019642690137449562111 


prime 


Powers 


97 


= (11447) 


composite 


Le Lasseur 


LOl 


2535301200456458802993406410751 


♦ 


• 


LOB 


10141204801825835211973625643007 


* 


• 


LOT 


162259276829213363391578010288127 


prime 


Powers 


L09 


649037107316853453566312041162511 


* 


* 


L13 


= (3391) 


composite 


Le Lasseur 


L27 


170141183460469231731687303715884105727 


* 


» 


L31 
L37 


= (263) 


composite 

* 

* 

* 
composite 

* 
composite 

composite 


Euler 

• 


.39 




* 


149 




* 


L51 

L57 


'=0(18121) 


Le Lasseur 

It- 


L63 
167 


=0'(i5b287)' 


Cunningham 
« 


L73 


=0(730753) 


Cunningham 


L79 


= (359) 


composite 


Euler 


L81 


= 0(43441) 


composite 


Woodall 


L91 
193 


= (383) 


composite 

composite 

* 
composite 


Euler 


197 
L99 


•-o'(7487) 


Cunningham 

* 


ill 


i"()"('i5193) ' 


Le Lasseur 


523 
J27 


asO (18287) 


composite 
» 

composite 


Le Lasseur 

* 


529 




♦ 


533 


'=0(1399) 


Le Tiasseur 


539 
>4.1 


= (479) 


composite 


Euler 


51 


30(503) 


composite 


Euler 


57 




« 


• 



CH. xvi] mersenne's numbers 337 

The factors of N when p = 11, 23, and 87 had been indicated 
by Fermat*, some four years prior to the publication of 
Mersenne's work, in a letter dated October 18, 1640. The 
passage is as follows: 

En la progression double, si d'un nombre quarr6, g^n^ralement parlant, vous 
tiez 2 ou 8 ou 32 &c., les nombres premiers moindres de I'unit^ qu'un multiple 
du quaternaire, qui mesureront le reste, feront I'effet requis. Comme de 26, 
qui est un quarr6, 6tez 2 ; le reste 23 mesurera la 11® puissance - 1 ; 6tez 2 de 
49, le reste 47 mesurera la 23* puisaance - 1. Otez 2 de 225, le reste 223 mesu- 
rera la 37* puissance - 1, &c. 

Factors of iV" when p = 29, 43, and 73 were given by Eulerf 
in 1732 or 1733. The fact that iVis composite for the values 
p = 83, 131, 179, 191, 239, and 251 follows from a proposition 
enunciated, at the same time, by Euler to the effect that, 
if 4/1 + 3 and Sn + 7 are primes, then 2*"+» - 1 eh (mod. 8n + 7). 
This was proved by Lagrange J in his classical memoir of 1775. 
The proposition also covers the cases of _p = ll and ^=23. 
This is the only general theorem on the subject which has been 
yet established. 

The fact that N is prime when p = Sl was proved by Euler § 
in 1771. Fermat had asserted, in the letter mentioned above, 
that the only possible prime factors of 2^ + 1, when p was 
prime, were of the form np -{-1, where n is an integer. This 
was proved by Euler || in 1748, who added that, since 2^ + 1 is 
odd, every factor of it must be odd, and therefore if p is odd 
n must be even. But Up is a given number we can define n 
much more closely, and Euler showed that, if ^ = 31, the prime 
factors (if any) of N were necessarily primes of the form 248n + 1 
or 248?! + 63 ; also they must be less than V-^j that is, than 

• Oeuvres de Fermat, Paris, vol. ii, 1894, p. 210; or Opera Mathematical 
Toulouse, 1679, p. 164; or Brassinne's Precis, Paris, 1853, p. 144. 

t Commentarii Academiae Scientiarum Petropolitanae, 1738, vol. vi, p. 105; 
or Commentationes Arithmeticae Gollectae, vol. i, p. 2. 

X Nouveaux Mimoires de VAcadimie des Sciences de Berlin, 1775, pp. 323 — 
856. 

§ Histoire de VAcadimie des Sciences for 1772, Berlin, 1774, p. 36. See also 
Legendre, Thdorie des Nombres, third edition, Paris, 1830, vol. i, pp. 222—229. 

U Novi Commentarii Academiae Scientiarum Petropolitanae, vol. i, p. 20; or 
Commentationes Arithmeticae Collectae, Petrograd, 1849, vol. i, pp. 55, 56. 

B. R. 22 



388 mersemne's numbers [ch. xvi 

4G339. Hence it is necessary to try only forty divisors to see 
if N is prime or composite. 

The factors when p = 4<*7; the factor 1433 when p=l79, 
and the factor 1913 when p = 239, were given by Reuschle 
in 1856*. 

The factors of N when p=41 were given by Plana f in 1859. 
He showed that the prime factors (if any) are primes of the 
form 328n + 1 or 328n + 247, and lie between 1231 and ^N, 
that is, 1048573. Hence it is necessary to try only 513 
divisors to see if N is composite: the seventeenth of these 
divisors gives the required factors. This is the same method 
of attacking the problem which was used by Euler in 1771, 
but it would be laborious to employ it for values of p greater 
than 41. Plana J added the forms of the prime divisors of N, if 
p is not greater than 101. 

That iVis prime when p = 127 seems to have been verified 
by Lucas § in 1876 and 1877. The demonstration has not been 
published. 

The discovery of factors of iV for the values p = 5S and 59 
is due apparently to F. Landry, who established theorems on 
the factors (if any) of numbers of certain forms. He seems to 
have communicated his results to Lucas, who quoted them in 
the memoir cited below ||. 

Factors of JSf when p = 79 and 113 were given first by 
Le Lasseur, and were quoted by Lucas in the same 
memoir 1 1. 

A factor of N when p = 233 was discovered later by Le 
Lasseur, and was quoted by Lucas in 1882^. 

* C. G. Eeuschle, Neue Zahlentheoretische Tahellen, Stuttgart, 1856, pp. 21, 
22, 42-53. 

t G. A. A. Plana, Memorie della Beale Accadeviia delle Scienze di Torino, 
Series 2, vol. xx, 1863, p. 130. 

t lUd., p. 137. 

§ SuT la TMorie des Nomhres Premiers, Turin, 1876, p. 11; and Becherches 
eur les Outrages de Leonard de Pise, Rome, 1877, p. 26, quoted by A. J. C. 
Cunningham, Proceedings of the London Mathematical Society, Nov. 14, 1895, 
vol. xxvn, p. 54. 

II American Journal of Mathematics, 1878, vol. i, pp. 234 — 238. 

% Recreations, 1882-3, vol. i, p. 241. 



CH. xvi] mersenne's numbers 839 

Factors of iV when p = 97, 151, iill, and 223 were deter- 
mined subsequently by Le Lasseur, and were quoted by Lucas * 
in 1883. 

That N is prime when p = 61 had been conjectured by 
Landry and in 1886 a demonstration was offered by Seelhofff. 
His demonstration is open to criticism, but the fact has been 
verified by others |, and may be accepted as proved. 

Cunningham showed in 1895 § that 7487 is a factor 
of N when p = 197: in 1908 1| that 150287 is a factor of N 
when p = 16S; in 19091F that 228479 is a factor of N when 
p=7l; and in 1912** that 730753 is a factor of i\r when p=11S. 
The factors of -^ when j9 = 71 were discussed independently by 
Ramesam in 1912tf. 

That ^is not prime when p=67 seems to have been verified 
by Lucas|| in 1876 and 1877. The composite nature of iV, 
when p = 67, was confirmed by E. Fauquembergue§§, and was 
also implied by Lucas in 1891. The factors were given by 
Cole||||inl903. 

A factor of N when p=lSl was discovered by Woodall in 

i9iiirir. 

♦ Bicr^ations, 1882-3, vol. n, p. 230. 

t P. H. H. SeelhofE, Zeitschrift fUr Mathematik und Physik, 1886, vol. xxxi, 
p. 178. 

X See Weber-Wellstein, Encyclopaedie der Elementar-Matliematik, p. 48; 
and F. N. Cole, Bulletin of the American Matliematical Society, December, 
1903, p. 136. 

§ A. J. C. Cunningham, Proceedings of the London Mathematical Society^ 
March 14, 1895, vol. xxvi, p. 261. 

II Ibid., April 30, 1908, vol. vi (2nd series), p. xxii. 

H U Inter mediaire des Mathimaticiens, Paris, 1909, vol. xvi, p. 252. 

** Proceedings of the London Matliematical Society, April 11, 1912, vol. xi, 
p. xxiv. 

tt Nature, London, March 28, 1912, vol. lxxxix, p. 87. 

XX Sur la Theoric des Nombres Premiers, Turin, 1876, p. 11, quoted by A. J. C. 
Cunningham, Proceedings of the London Mathematical Society, Nov. 14, 1895, 
vol. xxvn, p. 54, and Reclierches sur les Ouvrages de Leonard de Pise, Rome, 
1877, p. 26. 

§§ L' Intermediaire de» Mathematiciens, Paris, Sept. 1894, vol. i, p. 148. 

nil F. N. Cole, "On the Factoring of Large Numbers," Bulletin of the 
American Matliematical Society, December, 1903, pp. 134 — 137. 

nil H. J. Woodall, Nature, London, July 20, 1911, vol. lxxxvii, p. 78. 

22 2 



.340 MERSENNE's numbers [CH. XVI 

That N is prime when p = 89 and p = 107 was proved by 
Powers in 1911 and 1914 respectively*. 

Bickmore in the memoir -f- cited below showed that 5737 
is another factor of iV" if p = 239. Cunningham has also shown 
that 55871 is another factor of N if p= 151, and that 54217 is 
another factor of N if p = 251. 

I turn next to consider the methods by which these results 
can be obtained. It is impossible to believe that the statement 
made by Mersenne rested on an empirical conjecture, but the 
puzzle as to how it was discovered is still, after more than 
250 years, unsolved. 

I cannot offer any solution of the riddle. But it may be 
interesting to indicate some ways which have been used in 
attacking the problem. The object is to find a prime divisor 
q (other than N and 1) of a number N when N is of the form 
2P — 1 and p is a prime. 

I may observe that Lucas J showed that if we find the residue 
(mod N) of each term of the series 4, 14, 194, ... w^, constructed 
according to the law Un+i = iCn — % then N is prime if the first 
residue which is zero lies between the {p— l)/2th and the p\\i 
residues. If an earlier residue is zero the theorem does not 
help us, but if none of the p residues is zero, N is composite. 
The application of the theorem to high numbers is so laborious 
that for the cases still unverified we are driven to seek other 
methods. 

It can be easily shown that the prime divisor q must be of 
the form 2pt + 1. Also q must be of one of the forms 8i + 1 : 
for N is of the form 2A^ — B'\ where A is even and B odd, 
hence § any factor of it must be of the form 2a^ — b\ that is, of 
the form Si ± 1, and 2 must be a quadratic residue of q. 
The theory of residues is, however, of but little use in finding 

* B. E. Powers, American Mathematical Monthly , November, 1911, vol. xviii, 
pp. 195 — 197. Proceedings of the London Mathematical Society, 11 June 1919, 
series 2, vol. xiii, p. xxxix. 

t C. E. Bickmore, Messenger of Mathematics, Cambridge, 1895, vol. xxv, p. 19. 

X American Journal of Matheviatics, 1878, vol. i, p. 316. 

§ Legendre, Theorie des Nomhres, third edition, Paris, 1830, vol. i, § 143. 
In the case of Mersenne's numbers, JS = 6 = 1. 



CH. xvi] mersenne's numbers 341 

factors of the cases that still await solution, though the possi- 
bility some day of finding a complete series of solutions by 
properties of residues must not be neglected*. Our present 
knowledge of the means of factorizing N has been summed up 
in the statement f that a prime factor of the form 2pt-\- 1 can 
be found directly by rules due to Legendre, Gauss, and Jacobi, 
when ^= 1, 3, 4, 8, or 12 ; and that a factor of the form 2ptif+l 
can be found indirectly by a method due to Bickmore when 
< = 1, 3, 4, 8, or 12, and *' is an odd integer greater than 3. 
But this only indicates how little has yet been done towards 
finding a general solution of the problem. 

First There is the simple but crude method of trying all 
possible prime divisors q which are of the form 2pt + 1 as well 
as of one of the forms 8i ±1. 

The chief known results for the smaller factors may be 
summarized by saying that a prime of this form, when i is odd, 
will divide N when «=1, if p=ll, 23, 83, 131, 179, 191, 239, 
or 251; when i = 3, ifp = 37, 73, or 233; when ^=5, if p = 43; 
when « = 15, if jo = 113; when i = 17, if ^ = 79; when <=19, 
if ;) = 29, or 197; when <=25, if ^ = 47; when ^ = 41, if 
p = 22S] when « = 59, if p = 97; when « = 163, if p = 4il; when 
^ = 461, if p = 163; when « = 1525, if p = 59; when « = 1609, if 
p — 11. Similarly for even values of t, a prime of this form will 
divide N when < = 4, if p = ll, 29, 179, or 239; when ^ = 8, 
if _p = ll; when t=12, if ;) = 239; when ^ = 36, if ^ = 29, 
or 211; when « = 60, if p = 5S, or 151; when ^=120, if 
p^lHl; when « = 2112, if ^=173; and when «= 1445580, if 
p = 67. 

Of the 29 cases in which we know that the statement of 
the composite character of N is correct all save 7 can be easily 
verified by trial in this way. For, neglecting all values of t not 

* For methods of finding the residue indices of 2 see Bickmore, Messenger of 
Mathematics, Cambridge, 1895, vol. xxv, pp. 15—21; see also A. J.- C. Cunning- 
ham on 2 as a 16-io residue, Proceedings of the London Mathematical Society, 
1895-6, vol. xxvn, pp. 85—122 ; and on Haupt-exponents of 2, Quarterly Journal 
of Pure and Applied Mathematics, Cambridge, 1906, vol. xxxvii, pp. 122 — 145. 

t Transactions of the British Association for the Adoancement of Science 
(Ipswich Meeting), 1895, p. 614. 



342 meksenjse's numbers [ch. xvi 

exceeding, say, 60 which make q either composite or not of one 
of the forms 8^ ± 1, we have in each case to try only a com- 
paratively small number of divisors. Of the 7 other cases in 
which Mersenne's statement of the composite character of N 
has been verified, one verification (p = 41) is due to Plana, who 
frankly confessed that the result was reached " par un heureux 
hasard"; and a second is due to Landry (^ = 59), who did not 
explain how he obtained the factors. The third is due to Cole 
{p = 67), who established it by the use of quadratic residues of 
N, three others (^ = 71, 163, and 173) are due to Cuimingham, 
and one is due to Woodall. The last five verifications involved 
laborious numerical work, and it is possible that the results 
would have been obtained as easily by trial of prime divisors' of 
the form 2pt + 1. 

Of the 12 cases in which we know that the statement of the 
prime character of N is correct all save three (namely, when 
p=61, jp = 89 and ^=107) may be verified by trial in this way, 
for the number of possible factors is not large. 

Thus practically we may say that simple empirical trials 
would at once lead us to all the conclusions known except in 
the cases of ^ = 41 due to Plana, of jo=59 due to Landry, of 
p = Ql due to Seelhoff, of ^=67 due to Cole, of ^5=71, 163, 
and 173 due to Cunningham, of ^=181 due to Woodall, and of 
j9=89 and 107 due to Powers. In fact, save for these ten 
results the conclusions of all mathematicians to date could be 
obtained by anyone by a few hours' arithmetical work. 

As p increases the number of factors to be tried increases 
so fast that, if p is large, it would be practically impossible 
to apply the test to obtain large factors. This is an important 
point, for Colonel Cunningham has stated that in the cases 
siill awaiting verification there are no factors less than 1,000,000. 
Hence, we may take it as reasonably certain that this cannot 
have been the method by which the result was originally 
obtained; nor, as here enunciated, is it likely to give many 
factors not yet known. Of course it is possible there may be 
ways by which the number of possible values of t might be 
further limited, and if we could find them we might thus 



CH. xvi] mersenne's numbers 343 

diminish the number of possible factors to be tried, but it will 
be observed that the values of N which still await verification 
are very large, for instance, when |) = 257, N contains no less 
than 78 digits. 

It is hardly necessary to add that if q is known and is not 
very large we can determine whether or not it is a factor of N 
without the labour of performing the division. 

For instance, if we want to verify that q = 13367 is a 
factor of N when p = 41, we proceed thus. Take the power 
of 2 nearest to q or to its square-root. We have, to the 
modulus q, 

2" =16384 = 3017 = 7x431, 

.-. 2^8 =49 (-1377) --638, 

.-. 2'" =-319, 

.-. 2^^+" = (3017) (-319)=!, 

.-. 2« =1. 

Second. We can proceed by reducing the problem to the 
solution of an indeterminate equation. 

It is clear that we can obtain a factor of N if we can express 
it as the difference of the squares, or more generally of the nth 
powers, of two integers u and v. Further, if we can express 
a multiple of N, say mN, in this form, we can find a factor of 
mN and (with certain obvious limitations as to the value of m) 
this will lead to a factor of N. It may be also added that if m 
can be found so that N/m is expressible as a continued fraction 
of a certain form, a certain continuant* defined by the form of 
the continued fraction is a factor of N. 

Since N can always be expressed as the difference of two 
squares, this method seems a natural one by which to attack 
the problem. If we put 

iV = n» + a = (?i + 6)2 - (62 + 2hn - a\ 

we can make use of the known forms of u and v, and thus 

* See J. G. Birch in the Messenger of MaUienuitics , Cambridge, 1902, vol. 
xxu, pp. 52-50. 



344 MERSENNES NUMBERS [CH. XVI 

. obtain an indeterminate equation between two variables x and 
y of the I'orm 

where H and K are numbers which can be easily calculated. 
Integral values of x and y where y < K will determine values 
of u and v, and thus give factors of N, 

We can also attack the problem by indeterminate equations 
in another way. For the factors must be of the form 2pt + 1 
and %ps + 1, hence 

(2j)^ + 1) (8^5 + 1) = iV 

= 2^-1 

= 2(2^-1-1) + !, 
.'. 4s + f + 8;js^ = (2^-i-l)/p 
= (say) a + 8^/3. 

Hence 45 + ^ = a + 8pa;, and st — ^ — x, 

where x")^ ^ and t is odd. These results again lead to an 
indeterminate equation. 

But, in both cases, unless p is small, the resulting equations 
are intractable. 

Third. A not uncommon method of attacking problems 
such as this, dealing with the factorization of large numbers, 
is through the theory of quadratic forms*. At best this is 
a difficult method to use, and we have no reason to think that 
it would have been employed by a mathematician of the 
seventeenth century I here content myself with alluding 
to it. 

Fourth, There is yet another way in which the problem 
might be attacked, "the problem will be solved if we can find 
an odd prime q so that to it as modulus 2P^y = z, and 2y = z, 
where y and z may have any values we like to choose. If 
such values of q, y, and z can be found, we have 2y(2P— 1) = 0. 
Therefore 2^ = 1, that is, g is a divisor of iV. 

* For a sketch of this see G. B. Mathews, Theory of Numbers, part 1, Cam- 
bridge, 1891, pp. 261—271. See also F. N. Cole's paper, " On the Factoring of 
Large Numbers," Bulletin of the American Mathematical Society, December, 1903, 
pp. 134 — 137 ; and Quadratic Partitions by A. J, C. Cunningham, London, 1904. 



CH. xvi] mersenne's numbers 845 

For example, to the modulus 23, we have 
2«=3, 
2'« = 3^ 
Also 2=* = 32. 

Therefore 2i« - 2= = 0, 

.-. 2^1-1 =0. 
Without going further into the matter we may say that the 
a pHori determination of the values of q, y, and z introduces 
us to an almost untrodden field. It is just possible (though 
I should suppose unlikely) that the key to the riddle is to be 
found on methods of finding g, y, z, to satisfy the above con- 
ditions. For instance, if we could say what was the remainder 
when 2^ was divided by a prime q of the form Ipt + 1, and 
if the remainders were the same when x^u and x — v, then to 
the modulus q we should have 2" = 2", and therefore 2"~' b 1. 

It should however be noted that Jacobi's Canon Arithmeticus 
and the similar canon drawn up by Cunningham would, if 
carried far enough, enable us to solve tt^ problem by this 
method. Cunningham's Canon gives the solution of the con- 
gruence 2^^ = 12 for all prime moduli less than 1000, but it is 
of no use in determining factors of N larger than 1000. It 
is however possible that if R or q have certain forms an 
extended canon of this kind might be constructed, and thus 
lead to a solution of the problem 

Fifth. It is noteworthy that the odd values of p specified 
by Mersenne are primes of one of the four forms 2^ ± 1 or 2? ± 3, 
but it is not the case that all primes of these forms make 
N a prime, for instance, N is composite if ^ = 2^ + 3 = 11 or 
ifp = 2'^-3 = 29. 

This fact has suggested to more than one mathematician 
the possibility that some test as to the prime or composite 
character of N when p is of one of these forms may be dis- 
coverable. Of course this is merely a conjecture. There is 
however this to say for it, that we know that Fermab* had paid 
attention to numbers of this form. 

* For instance, see above, pp. 39, 40. 



346 mersenne's numbers [ch. xvi 

Sixth. The number N when expressed in the binary scale 
consists of 1 repeated p times. This has suggested whether 
the work connected with the determination of factors of N 
might not with advantage be expressed in the binary scale. 
A method based on the use of properties of this scale has been 
indicated by G. de Longchamps*, but as there given it would 
be unlikely to lead to the discovery of large divisors. I am, 
however, inclined to think that greater advantages would be 
gained by working in a scale whose radix was 4p or may-be 
8p — the resulting numbers being then expressed by a reasonably 
small number of digits. In fact when expressed in the latter 
scale in only one out of the cases in which the factors of N are 
known does the smallest factor contain more than two digits. 

Seventh. I have reserved to the last the description of the 
method which seems to me to be the most hopeful. 

We know by Fermat's Theorem that if a; + 1 is a prime 
then 2^ — 1 is divisible hy x-\-l. Hence if 2pt-k-l is a prime 
we have, to the modulus 2pt 4- 1, 

Hence, a divisor of 2^ — 1 will be known, if we can find a 
value of t such that '2.pt + 1 is prime and the second factor 
is prime to it. 

This method may be used to establish Euler's Theorem of 
1732. For if we put ^ = 1, and if 2j) + 1 is a prime, we have, 
to the modulus 2p 4- 1, 

(2^-1) (2^ 4- 1)^0. 

Hence 2^ = 1 if 2^ + 1 is prime to 2p + 1. This is the case 
if _p = 4m + 3. Hence 2j? + 1 is a factor of iV^ if ^ = 11, 23, 
83, 131, 179, 191, 239, and 251, for in these cases 2^ + 1 
is prime. 

The problem of Mersenne's Numbers is a particular case 
of the determination of the factors of a" — 1. This has been 

* Coviptes Rendus de VAcadeiide des Sciences, Paris, 1877, vol. lxxxv, 
pp. 950-052. 



CH. XVl] MERSENNES NUMBERS 347 

the subject of investigations by many mathematicians : an 
outline of their conclusions has been given by Bickmore*. 
I ought also to add a reference to the general method 
suggested by Lawrence f for the factorization of any high 
number: it is possible that Fermat used some method 
analogous to this. 

Finally, I should add that machinest have been devised for 
investigating whether a number is prime, but I do not know 
that any have been constructed suitable for numbers as large 
as those involved in the numbers in question. 

* Messenger of Mathematics, Cambridge, 1895-6, vol. xxv, pp. 1 — 44; also 
1896-7, vol. XXVI, pp. 1 — 38 ; see also a note by Mr E. B. Escott in the Messenger, 
1903-4, vol. xxxm, p. 49. 

t F. W. Lawrence, Messenger of Mathematics, Cambridge, 1894-5, vol. xxiv, 
pp. 100 — 109 ; Quarterly Journal of Mathematics, Cambridge, 1896, vol. xxvni, 
pp. 285 — 311 ; and Transactions of the London Mathematical Society, May 13, 
1897, vol. XXVIII, pp. 465—475. 

X F. W. Lawrence, Quarterly Journal of Mathematics, Cambridge, 1896, 
already quoted, pp. 310 — 311 ; see also C. A. Laisant, Comptes Rendns, Association 
Fian<;ais your I'Avancement dcs Sciences, 1891 (Marseilles), vol. xx, pp. 165—168. 



34)8 



CHAPTER XVII. 
STRING FIGURES. 

An amusement of considerable antiquity consists in the 
production of figures, known as Gat's-Cradles, by twisting or 
weaving on the hands an endless loop of string, say, from six 
to seven feet long. The formation of these figures is a 
fascinating recreation with an interesting history. It cannot, 
with accuracy, be described as mathematical, but as I de- 
liberately gave this book a title which might allow me a free 
hand to write on what I liked, I propose to devote a chapter 
to an essay on certain string figures*. The subject is ex- 
tensive. I propose however merely to describe the production 
of a few of the more common forms, and do not concern myself 
with their ethnographical aspects. Should, as I hope, some of 
my readers find the results interesting, they may serve as an 
introduction to innumerable other forms which, with a little 
ingenuity, can be constructed on similar lines. 

First we must note that there are two main types of the 
string figures known as Cat's-Cradles. In one, termed the 
European or Asiatic Variety, common in England and parts 
of Europe and Asia, there are two players one of whom, at 
each move, takes the string from the other. In this, the more 

* For my knowledge of the subject I am mainly indebted to Dr A. 0. 
Haddon, of Cambridge ; to String Figures by C. F. Jayne, New York, 1906 ; 
and to articles by W. I. Pocock and others in Folk-Lore, and the Journal of the 
Anthropological Society. Since writing this chapter I have come across another 
book on the subject by K. Haddon, London, 1911 : it contains descriptions of 
fifty Figures and a dozen String Tricks. 



CH. XVIl] STRING FIGURES 349 

usual forms produced are supposed to suggest the creations of 
civilized man, such as cradles, trays, dishes, candles, &c. In 
the other, termed the Oceanic Variety, common among the 
aborigines of Oceania, Africa, Australasia and America, there 
is generally (but not always) only one player. In this, the 
more usual forms are supposed to represent, or be connected 
witJi, natural objects, such as the sun and moon, lightning, 
clouds, 'animals, &c., and on the whole these are more varied 
and interesting than those of the European type. There are 
a large number of known forms of each species, and it is easy 
to produce additional forms as yet undescribed. We can pass 
from a figure of the European type to one of the Oceanic type 
and vice versa, but it is believed that this transformation is an 
invention of recent date and has no place in the history of the 
game. 

To describe the construction of these figures we need an 
accurate terminology. The following terms, introduced by 
Rivers and Haddon*, are now commonly used. The part of 
a string which lies across the palm of the hand is described 
as palmar, the part lying across the back of the hand as 
dorsal. The part of a string passed over a thumb, finger or 
fingers is a loop. A loop is described or distinguished by the 
projection — such as the thumb, index, middle-finger, ring-finger, 
or little-finger of either the right or the left hand — over which 
it passes. The part of the loop on the thumb side of a 
loop is termed radial, the part on the little-finger side is 
called ulnar] thus each loop is composed of a radial string 
and an ulnar string. If, as is not uncommon, the figure is 
held by some one, with his hands held apart, palm facing 
palm and the fingers pointing upwards, then the radial string 
of any loop is that nearest him, and the ulnar string is that 
farthest from him ; in this case we may use the terms far and 
near instead of ulnar and radial. If there are two or more 
loops on one finger (or other object), the one nearest the root 
of the finger is termed proximal, the one nearest the tip or 

♦ Torres Straits Siring Figures, by W. H. B. Bivers and A. C. Haddon, Man, 
London, 1902, pp. 147, 148. 



350 STRING FIGURES [CH. XVII 

free end of the finger is termed distal. If the position of the 
hands is unambiguous, it is often as clear to speak of taking up 
a string from above or below it as to say that we take it from 
the distal or proximal side. 

The following descriptions are I believe sufficient to enable 
anyone to construct the figures, and I do not attempt to make 
them more precise. They are long, but this is only because of 
the difficulty of explaining the movements in print, and the 
figures are produced much more easily than might be inferred 
from the elaborate descriptions. Unless the contrary is stated 
all of them are made with a piece of string, some seven feet 
long, whose ends are tied together. In the descriptions I 
assume that after every movement the hands are separated so 
as to draw the strings tight. In the diagrams the string is 
represented by two parallel lines: this enables us to indicate 
whether one string appears in front of or behind another. 
Unless otherwise stated the diagi-ams show the figures as seen 
by the operator. 

I recommend any one desirous of making the Figures and 
not already acquainted with the subject to commence with the 
Oceanic Varieties described on pp. 357 to 371, where only one 
operator is required. 

Cafs-Cradles, European Varieties. I begin by discussing the 
form of Cat's-Cradle known in so many English nurseries. It is 
typical of the European Variety. It is played by two persons, 
P and Q, each of whom in turn takes the string off the fingers 
of the other. In the following descriptions, the terms near and 
far, or radial and ulnar, refer to the player from whom the string 
is being taken. In most of the forms two or more parallel 
horizontal strings are stretched between the hands of the player 
who is holding the string, and the other strings cross one another 
forming an even number of crosses. When there are more than 
two such crosses, two will usually be at the ends of the figure 
near the hands, and two at the sides nearer the middle of the 
figure. Parallel strings, if taken up, are usually taken up by 
hooking each little-finger in one of them. A cross, if taken up, 



CH. XVIl] 



STRING Jj^IGURES 



351 



is usually taken by inserting the thumb and index-finger of one 
hand in opposite angles of the cross, holding the cross with the 
thumb and finger, and then turning the plane containing the 
thumb and finger through one or two right angles. When one 
player takes the strings from the other, it is assumed that he 
draws his hands apart so as to keep the string stretched. 

The initial figure is termed the Cradle; from this we can 
produce Snuff ei-- Trays. From Snuffer-Trays we can obtain 
forms known as a Pound- of- Candles, Cafs-Eye, and Trellis- 
Bridge. From each of these forms again we can proceed in 
various ways. I will describe first the figures produced when 
the string is taken off so as to lead successively from the 
Cradle to Snuffer-Trays, Cat's- Eye, and Fish-in-a-Dish. This 
is the normal sequence. 

The Cradle — see figure i — is formed by six loops, three on 
each hand : there are two horizontal strings, one near and the 
other far, and two pairs of strings which cross one another, 
each cross being over one of the horizontal strings. 




Figure i. The Cradle. 

The Cradle is produced thus. First. One of the operators, P, loops the 
string over the four fingers of each hand, which are held upright, palm towards 
palm, the near or radial string lying between the thumbs and index-fingers, and 
the far or ulnar string beyond the little fingers. Second. P puts a second loop 
on the right hand by bending it over outside the radial string and up into the 
loop. There are now two dorsal strings and one palmar string on the right 
hand. Third. P puts a similar loop on the left hand by bending it over 
outside the radial string and up into the space between the hands. Lastly. 
P with the back of the right middle-finger takes up from the proximal side {i.e. 
from below) the left palmar string. And then similarly with the back of the 
left middle-finger takes up from the proximal side that part of the right palmar 



352 STRING FIGURES [CH. XVII 

string which lies across the base of the right middle-finger. The hands are 
now drawn apart so as to make the strings taut. 

The figure known as Snuffer-Trays — see figure ii — k formed 
by six loops, three on each hand. Four of the strings cross 
diamond-wise in the middle so as to form two side crosses and 
two end crosses, the other two strings are straight, one being 
near and the other far. All the strings lie in a horizontal 
plane. This figure is also known as Soldier's Bed, the Church 
Window, and the Fish-Fond. 




Figure ii. Snuffer-Trays. 

Snuffer -Trays can be got from the Cradle thus. I suppose that the Cradle 
is held by one operator, P, and that the other operator, Q, faces P. First. 
Q inserts the thumb and index-finger of his left hand from his far side in those 
angles of the cross of the Cradle farthest from him which lie towards P's hands. 
Similarly Q inserts the thumb and index-finger of his right hand from his near 
side in those angles of the cross nearest to him which lie towards P's hands, 
both thumbs being towards P's right hand, and his index-fingers towards P's 
left hand. Second. Q takes hold of each cross by the tips of the thumb and 
finger, then pulls each cross outwards {i.e. away from the centre of the figure) 
over and beyond the corresponding horizontal string, then down, and then 
round the corresponding horizontal strings. Third. Q turns the thumbs and 
fingers upwards through a right angle, thus passing the cross up between the 
two horizontal strings. By this motion the thumb and index-finger of each 
hand (still holding the crossed strings) are brought against the horizontal 
strings. Lastly. Q, having pushed his fingers up, releases the crosses by 
separating his index-fingers from his thumbs, and drawing his hands apart 
removes the string from P's hands. The diagram represents the figure as 
seen by P. 

Cat's-Eye — see figure iii — also is formed by six loops, three 
on each hand. There are two near (or radial) thumb strings, 
two far (or ulnar) index strings, while the radial thumb and 
ulnar index strings form a diamond-shaped lozenge in the 
middle of the figure, giving rise to two side crosses and two 
end crosses. All the strings lie in a horizontal plane. 



CH. XVIl] 



STRING FIGURES 



868 




Figure iii. CaVs-Eye. 

Cat*8-Eye can be got from Snnffei-Trays thus. I suppose that Snuffer- 
Trays is held by Q. First. P inserts the thumb and index-finger of each 
hand from below into those angles of the two side crosses of Snufifer-Trays 
which lie towards Q's hands, both thumbs being towards Q's left hand. 
Second. P takes hold of each cross with the thumbs and fingers and pulls it 
downwards, then he separates his hands thus bringing each cross below its 
corresponding horizontal string, and continuing the motion he passes the cross 
outside, round, and then above the horizontal string. Third. P turns each 
thumb and finger inwards {i.e. towards the centre of the figure) between the two 
horizontal strings through two right angles taking the horizontal strings with 
them. Lastly. P pushes his fingers down, separates the index-fingers from the 
thumbs, and then, drawing his hands apart, removes the figure from Q'b hands. 

The figure now is in a horizontal plane ; but for clearness I have drawn it 
as seen by Q, when P lifts his hands up. 

Fish-in-a-Dish — see figure iv — is composed of a central 
lozenge (the dish) on which rest lengthwise two strings (the 
fish). There are two loops on each hand. 




Figure iv. Fish-in-a-Dish, 



B. R. 



23 



354 S'lUlNO FIGURES [CH. XVIT 

Fith-in-a-Dish is produced from Cat's-Eye thus. I suppose that Cat's-Eye 
is held by P in the normal position, his fingers pointing down, and the figure 
lying in a horizontal plane. First. Q puts the thumb and index-finger of each 
hand from above into those angles of the two side crosses which lie towards 
P's hands, both thumbs being towards P's right hand. Second. Q turns each 
hand inwards {i.e. towards the centre of the figure) through two right angles, 
and as he does so, catches the sides of the central diamond on the thumb and 
index-fingers. At the end of the motion the thumbs and fingers will be 
pointing upwards. Lastly. Q draws his hands apart and thus takes the 
figure off P's hands. The diagram represents the figure as seen by Q. 

The same result is produced if Q inserts his thumbs and index-fingers from 
below into the two side crosses, and turns his hands inwards through two right 
angles. 

There are a few other standard forms which may be 
mentioned. In the first place instead of proceeding directly 
from Snuffer-Trays to Cat's-Eye, we can obtain the latter 
figure through three intermediate forms known as a Pound- 
of-Gandles, the Hammock, and Lattice-Work. The figure last 
named is the same as Snuffer-Trays, though it is held some- 
what differently, and from it we can obtain Cat's-Eye by similar 
movements to those described above when forming it from 
Snuffer-Trays, though it will be held somewhat differently. 

A Pound-of -Candles is formed by six parallel strings held 
in a horizontal plane. It is, in fact, only an extension sideways 
of the figure obtained in the course of the formation of the 
Cradle before the palmar strings are taken up ; and, if desired, 
it can be obtained in that way. 

A Found-of -Candles is produced from Snuffer-Trays (assumed to be held by 
Q) thus. First. P inserts the thumb and index-finger of each hand from 
above into those angles of the two side crosses which lie towards Q's hands. 
Second. P takes hold of each cross, lifts it, and takes it above, and then 
round the corresponding horizontal string. Third. P turns each hand up- 
wards between the horizontal string through two right angles ; this brings each 
horizontal string on to the thumbs and index-finger of the hand corresponding 
to it. Lastly. P separates his thumbs and index-fingers, and drawing his 
hands apart he takes the figure off Q's hands. This corresponds exactly with 
the process above described by which Cat's-Eye is obtained from Snuffer-Trays, 
except that the side crosses are taken from above instead of from below. 

The Hammock is an inverted Cradle and can be obtained 
directly from it without any intermediate forms. The Hammock 
is also known as Bahys-Got and the Manger. 

The Hammock can be obtained from a Pound-of-Candles (assumed to be held 
by P) thus. First. Q, holding his right-hand palm upwards, hooks with his 



CH. XVIl] STRING FIGURES 355 

right little-finger the ulnar thumb string from above, and then pulls it up and 
over the three index strings. Second. Q passes his left hand, held palm up- 
wards, from above through the loop on the right little-finger, and with the little- 
finger of the left hand he hooks the radial index string, and pulls it up and over 
the pair of thumb strings. Third. Q places the thumb and index-finger of the 
right hand from the far side of P below the pair of far index strings, and similarly 
he places the thumb and index of the left hand from the near side of P below the 
pair of near thumb strings. Fourth. Q turns his hands inwards and upwards 
through two right angles ; by this motion the thumbs and indices will take up 
these two pairs of strings. Lastly. Q separates his index-fingers from his 
thumbs, and drawing his hands apart removes the figure from P's hands. 

Lattice-Work. The form known as Lattice- Work is the 
same as Snuffer-Trays, except that it is held with the fingers 
pointing downwards. 

Lnttice-Work can be produced from the Hammock (assumed to be held 
^y Q) l>y the same process as that described above for the production of 
Snufifer-Trays from the Cradle, namely by P inserting the thumb and index- 
finger. of each hand from the outside in the crosses; then pulling each cross 
outwards, then up and over the corresponding horizontal string ; and finally 
turning each hand downwards through a right angle. Also, since it is the 
same as Snufifer-Trays, though held differently, it can be produced from 
Snuffer-Trays by P putting his thumbs and fingers pointing downwards in 
the place of those of Q which are pointing upwards. 

Other standard forms are Trellis-Bridge, Douhle-Gromns, 
Suspension-Bridge, Tiidents, and See-Saw. For the formation 
of these I give only a brief outline of the necessary steps. 

Trellis-Bridge can be produced from Snuffer-Trays by the second player 
hooking his little-fingers from above in the two parallel strings, one in one 
string and one in the other, pulling the loops so formed over and down to 
opposite sides, and taking up the side crosses from above and turning the hands 
inwards through two right angles. 

Double-Crowns can be got from Cat's-Eye by the second player putting the 
thumb and index-finger of each hand through the far and near angles of the 
end crosses, either from above or below, and making the usual turning movement 
inwards through two right angles. 

Suspension-Bridge can be got from Fish-in-a-Dish by the second player 
hooking his little-fingers in the two parallel strings that form the dish, one 
in one string and one in the other, pulling the loops so formed up, then taking 
up the side crosses from above, and turning the hands inwards towards the 
centre of the figure through two right angles. Suspension-Bridge can be also 
obtained from Double- Crowns. 

Tridents is produced from Suspension-Bridge by the second player releasing 
the left-hand thumb and index-finger, and then drawing his hands apart. 

23—2 



356 STRING FIGURES [CH. XVII 

See-Saw is an arrangement of the string which each player in turn draws 
out. It is said that in such figures children draw the string backwards and 
forwards to the chant of a doggerel line See Saw Johnnie Maw, See San Johnnie 
Man (Jayne, p. xiii). One form of it can be obtained from the Cradle, which 
I will suppose is held by P, thus. Q takes one of the straight strings with the 
index-finger of one hand and the other straight string with the index-finger of 
the other hand ; P then slips his hands out of the loops round them but retains 
the middle-fingers in the loops on them. If now P separates his hands the 
loops held by Q diminish, and vice versa. Another variety is obtained by 
P taking the ulnar base string from below in his mouth and then withdrawing 
his hands from the loops while retaining his middle-fingers in their respective 
loops : in this way we obtain a sawing figure of three loops. 

The forms above described may be arranged in sequence 
in tabular form, as shown below. It will be noticed that 
parallel strings when taken up may be pulled up or down, 
and they may be crossed or not. So too in the horizontal 
figures there are often a pair of side crosses and a pair of 
end crosses, and either pair may be taken from above or 
from below, and in many cases may be crossed or not as is 
desired. Frequently also one player Q can insert two fingers 
in a pair of crosses and if P leaves go Q may, by drawing 
his hands apart, produce a new figure. By combining these 
motions we can obtain various forms, and can secure sequences 

Cat's- Cradle 

I 

I ^ 71 

Snuffer-Trays See-Saw 

, 1 . . 

Cat's-Eye Pound-of- Trellis-Bridge 

I Candles 



Fish-in-a- Double- Manger 

Dish Crowns 



(Numerous Cat's-Eye 

Forms) 



of them. The movements and forms described above will how- 
ever illustrate the process sufficiently. 



CH. XVIl] STRING FIGURES 357 

Cat\s-Cradles, Oceanic Varieties. I will next enumerate a 
few forms of the Oceanic Variety. One or two specimens of this 
type are known in England, but they may be recent importa- 
tions, perhaps by sailors, and not indigenous. There are so many 
examples of this kind, that it is difficult to select specimens, but 
those I have chosen will serve to illustrate the methods. In 
many cases the figures can be formed in more ways than one. 

I again emphasize the fact that the figures are produced 
much more easily than might be inferred from the lengthy 
descriptions given. This is so partly because I have tried to 
mention every detail of the process, and partly because I con- 
stantly describe similar movements with the two hands as if 
they were made consecutively, whereas in practice to produce 
the figures effectively the movements should be simultaneous. 
I may add further that frequently two or more of the move- 
ments mentioned can be combined in one, and when practicable 
this is desirable. Also usually the more rapidly the movements 
are made the better is the presentation. By rotating the 
wrists, considerable play is given to the figure, and the move- 
ments are facilitated. Where two loops are on one finger, it is 
generally advisable to place one on the tip and the other on the 
base of the finger so as to keep them distinct. 

Once a figure has been constructed or the rule given for 
making it understood, the brief description of the method 
(which in many cases I insert after the exposition of the rule) 
will suffice for the reproduction of the figure. 

Openings A and B. The greater number of Oceanic figures 
begin with the same initial steps known as Opening A or 
Opening B. These steps I proceed to describe. 

Opening A. In Opening A, the operator commences by placing the string 
on the left hand as follows. The hands are held with the fingers pointing up- 
wards, and palm facing palm. The tips of the left thumb and little-finger are 
put together, and a loop of the string put over them. On opening the hand it 
will be found that the string from the far (or ulnar) side passes round the back of 
the little-finger, then between the ring and little-finger, then across the palm, 
then between the index and thumb, and then round the back of the thumb to the 
near (or radial) side of the hand. The string is then taken up similarly by tlie 
right hand. The hands are now drawn apart. This is called the " first position." 



358 



STRING FIGURES 



[CH. XVII 



Next with the back of the index of the right hand take up from the proximal 
side {i.e. from below) the left palnmr striug, and return : in these descriptions 
the word return is used to mean a return to the position occupied at the 
beginning of the movement by the finger or fingers concerned. Then, 
similarly, with the back of the index of the left hand take up from the proximal 
side that part of the right palmar string which lies across the base of the right 
index, and return. The figure now consists of six loops on the thumb, index, 
and little-finger of each hand. The resulting figure, in a horizontal plane, is 
shown in the diagram, looking down at it from above. 




Opening A. 



Opening B. Opening B is obtained as above, save that, in the second part 
of the Opening, the right palmar string is taken up by the left index before 
the left palmar string is taken up by the right index. In most of the figures 
described below it is immaterial whether we begin with Opening A or Opening B. 

Movement T. There is also another movement which is 
made in the construction of many of the figures and which 
may be described once for all. 

This movement is when we have on a finger two loops, one proximal and 
the other distal, and the proximal loop is pulled up over the distal loop, then 
over the tip of the finger, and then dropped on the palmar side. I term this the 
Movement T. 

A Door. The first example I will give is a Door — see 
figure V — which comes from the Apache Red Indians. It 
affords a good introduction to the Oceanic Varieties, for it is 
one of the easiest figures to construct, as the movements are 
simple and involve no skill in manipulation. The rubbing the 
hands together in the final movement has nothing to do with 
the formation of the figure, though it adds to its effective 
presentation. The diagram represents the final figure held in 
a horizontal plane. 



CH. XVI I ] 



STKING FIGURES 



359 




Figure v. A Boor. 

The Door is prodaced thus (Jayne, pp. 12—15). First. Take up the string 
in the form of Opening A. Second. With the right thumb and index lift the 
left index loop off that finger, put it over the left hand, and drop it on the left 
wrist. Make a similar movement with the other hand. Third. With the dorsal 
tip of the right thumb take up the near right little-finger string, and return. 
Make a similar movement with the other hand. Fourth. With the dorsad tip 
of the right little-finger take up the far right thumb string, and return. Make a 
similar movement with the other hand. Fifth. Keeping all the loops in position 
on both hands, with the left hand grasp tightly all the strings where they cross 
in the centre of the figure, and pass this bunch of strings from left to right between 
the right thumb and index {i.e. from the palmar side to the back of the hand), and 
let them lie on the back of the hand between the thumb and finger. Next with 
the left thumb and index take hold of the two loops already on the right thumb, 
and draw them over the tip of the right thumb. Then, continuing to hold 
these two loops, let the bunch of strings still lying between the right index and 
thumb slip over the right thumb to the palmar side, and after this replace 
these loops on the right thumb. Make a similar set of movements with the 
other hand. Sixth. With the right thumb and index lift the left wrist loop 
from the back of the left wrist up over the tips of the left thumb and fingers, 
and let it fall on the palmar side. Make a similar movement with the other 
hand. Finally. Bub the palms of the hands together, separate the hands, and 
the Door will appear. 

More briefly thus. Opening A. Index strings over wrists. Each thumb 
over one and takes up one. Each little-finger takes up one. Thumb loops 
over groups of strings. Wrist loops over hands. Extend, 

Climbing a Tree. I select this as another easy example 
starting from Opening A. The tree — see figure vi — consists of 
two straight strings, which slope slightly towards one another: 
if the figure here delineated is turned through a right angle, 



360 STRING FIGURES [CH. XVIt 

the straight lines representing the tree will be upright, and 
this is the way in which the figure is nornially produced. Up 
this tree, some loops, which represent a boy, are made to ascend. 
The production is very simple, but it is interesting because the 
design, and nothing more, was obtained from the Blacks of 
Queensland, and this is a conjectural restoration, by Pocock, of 
the way it was produced. 




Figure vi. Boy Climhing a Tree. 

The Boy Climhing a Tree may be produced thus. First. Take up the string 
in the form of Opening A. Second. Pass the little-fingers over four strings 
(namely, all the strings except the near or radial thumb string), then with the 
backs of the little-fingers take up the near thumb string, and return. Third. 
Make movement T on the little-finger loops. Fourth. Bend the right index over 
that part of the right palmar string which lies along the base of the finger, and 
press the tip of the finger on the palm. Make a similar movement with the 
left index. Fifth. Holding the strings loosely, release the thumbs and pull 
the index loops over the knuckles so that they hang on the cross strings. 
Finally. Put the far (or ulnar) little-finger string under one foot or on any 
fixed object. Then release the little-fingers, and pull steadily with the index- 
fingers on the strings they hold. This will cause the loop round the two straight 
strings to rise. The two strings starting from the foot represent the tree, and 
the loops, which ascend on them, as the fingers pull, represent the boy climbing 
up the tree. If in the fourth movement we put the middle-fingers instead of 
the index-fingers through the index loops, we shall slightly modify the result. 

More briefly thus. Opening A. Each little-finger over 4, and take up 
one. T to little-finger loops. Each index on palm. Release thumbs. Index 
loops over knuckles. Far little-finger string under foot. Release little-fingers, 
and pull. • 

Throwing the Spear is a New Guinea figure. It is peculiar 
in that the design can be transferred or thrown from one hand 
to the other as often as is desired. The construction is very 
simple. 

Throwing the Spear was obtained by Haddon. It is made thus (Jayne, 
pp. 131—132). First. Take up the string in the form of Opening A. Second. 



CH. XVJl] 



STRING FIGURES 



361 



Take the loop off the left index, put it on the right index, and pass it down the 
finger over the right index loop to the base of the finger. Third. Take off 
from the right index the original right index loop, release it, and extend the 
hands. This is the spear, the handle being held by the left hand. 

More briefly thns. Opening A. Left index loop off finger, and then over 
right index loop to base of right index. Release original right index loop, and 
pulL 

To throw the spear from one hand to the other, proceed thus. Put the 
left index between the left ulnar thumb string and the left radial little-finger 
string, then push it up from below into the right index loop. Then opening the 
right hand, drawing it sharply to the right, and at the same time taking the 
right index out of its loop, the spear is transferred to the other hand. The 
process can be indefinitely repeated with one hand or the other. 

Diamonds. Numerous lattice-work forms have been col- 
lected in which diamonds or lozenge-shaped figures are strung 
in a row, or in two or more rows, between two parallel strings. 
T describe a few of these. 

THple Diamonds. This is an interesting figure — see 
figure vii — and lends itself to a catch described hereafter. 
It comes from the Natik natives in the Caroline Islands, but 
possibly is of negro origin. It is not symmetrical. 




Figure vii. Triple Diamonds. 

The formation of Triple Diamonds is described by Mrs Jayne (pp. 142 — 146), 
and may be effected as follows. First. Take up the string in the form of 
Opening A. Second. Take the right hand out of all the loops, and let the 
string hang straight down from the left hand, which is held upright with the 
fingers pointing upwards. Third. Put the tips of the right thumb and little- 
finger together, and insert them from the right side into the left index loop. 



362 



STRING FIGUREf- 



[CH. XVII 



Next, separate the right thumb and little-finger, take the loop oft the left index, 
and draw the hands apart. Fourth. Put the right index-finger under the left 
palmar string, and draw the loop out on the back of the finger. Fifth. Bend 
the right thumb over one string (viz. the near or radial right index string), take 
up from below on the back of the thumb the far or ulnar right index string, and 
return. Sixth. Bend the left thumb away from you over one string (viz. the 
far or ulnar thumb string) and take up from below on its back the near or radial 
little-finger string, and return. Seventh. With the back of the tip of the right 
index-finger pick up from below the near right index string, and return. Eighth. 
With the back of the tip of the left index pick up from below the far left 
thumb string (not the string passing across the palm), and return. These 
strings on the index-fingers should be kept well up at the tips by pressing 
the middle-fingers against them, and the radial left thumb string should now 
cross between the left thumb and index ; if it be not in this position it should be 
shifted there. Ninth. Make movement T on the thumb loops of each hand. 
Lastly. Eelease the loops from the little-fingers, and extend the figure between 
the thumbs and the tips of the index-fingers : it is usual but not necessary, at 
the same time, to rotate the hands to face outwards. 

More briefly thus. Opening A. Eight hand out. Eight little-finger and 
thumb through index loop. Eight index takes up palmar string. Each thumb 
over one and takes up one. Eight index picks up near right index string and 
left index picks up far left thumb string. T to thumb loops. Eelease little- 
fingers and extend. 

Quadruple Diamonds. This design — see figure viii — was 
given me by a friend who was taught it when a boy in 
Lancashire. It is the same as one described by Mrs Jayne 
(pp. 24 — 27), which was derived by her from the Osage Red 
Indians. 




Figure viii. Quadruple Diamonds. 

Quadruple Diamonds can be produced thus. First. Take up the string 
in the form of Opening A. Second. Eelease the thumbs. Third. Pass the 
right thumb away from you under all the strings, and take up from below with 
the back of the thumb the far right little-finger string, and return. Make a 
similar movement with the other hand. Fourth. Pass each thumb away from 



CH. XVIl] STRING FIGURES 363 

you over the near index string, and take up from below with the back of the 
thumb tlie corresponding far index string, and return. Fifth. Release the 
little-fingers. Sixth. Pass each little-finger towards you over one string (viz. 
the near index string) and take up from below on the back of the little-finger 
the corresponding far thumb string, and return. Seventh. Release the thumbs. 
Eighth. Pass each thumb away from you over the two index strings and take up 
from below, with the back of the thumb, the corresponding far little-finger string, 
and return. Ninth. With the right thumb and index pick up the left near index 
string, close to the left index and above the left palmar string, and put it over the 
tip of the left thumb. Next make movement T on the left thumb loops. Make a 
similar movement with the other hand. Finally. Insert each index into the small 
triangle near it whose sides are formed by the corresponding palmar string and 
its immediate continuation. Then, rotate the right hand counter-clockwise 
and the left hand clockwise. In making this movement the little-finger loops 
and the proximal index loops will fall off, and for the production of the figure 
it is essential they should do so. At the end of the movement the palms of the 
hands should be facing outwards and away from you, the thumbs lowest and 
pointing away from you, and the index-fingers pointing upwards. On separating 
the hands the Diamonds will appear. It is a matter of indifference whether the 
top string is taken on the middle-fingers or on the index-fingers. If the fourth 
step be taken before the third we get a form of Double Diamonds. 

More briefly thus. Opening A. Let go thumbs. Each thumb under 3 and 
takes up one. Each thumb over one and picks up one. Release little-fingers. 
Each little-finger over one and picks up one. Release thumbs. Each thumb 
over 2 and picks up one. Near index strings on tips of thumbs. T to loops on 
thumbs. Index-fingers in triangle. Release little-fingers, and extend. 

Multiple Diamonds. In the figures above described the 
diamonds are in a row. I give an instance — see figure ix — de- 
rived from the Natik natives, where the diamonds are in three 
rows. The reader will be easily able to introduce modifications 
in the process of the formation of Multiple Diamonds which will 
lead to other figures which he can thus invent for himself. 




Figure ix. Multiple Diamnridt. 
Mnltiple Diamonds may be formed thus (Jayne, pp. 150— 15G). First. Take 
up the string in the form of Opening A. Second. With the teeth draw the far 
little-finger string towards you over all the strings. Then, bending the lefc 



364 



STRING FIGURES 



[CH. XVII 



index over the left string of the loop held by the teeth, pick up from below 
on the back of the index the right string of the loop, .ind return. Next bend 
the right index over to the left, and pick up from below on its back the left 
string of the loop, and return. Release the loop held by the teeth. Third. 
Release the thumbs. Fourth. Put each thumb away from you, under both 
index loops, and pick up on its back the near little-finger string, and return. 
Fifth. Pass each thumb up over the lower near index string, and put its 
tip from below into the upper index loop. Sixth. Make movement T on the 
thumb loops. Seventh. Withdraw each index from the loop which passes 
around both thumb and index. Fdghth. Transfer the thumb loops to the 
index-fingers by putting each index from below into the thumb loop, and 
withdrawing the thumb. Ninth. Repeat the fourth movement, namely, put 
each thumb away from you under both index loops, and pick up on its back 
the near little-finger string, and return. Tenth. Repeat the fifth movement, 
namely, pass each thumb up over the lower near index string, and put its tip 
from below into the upper index loop. Eleventh. Make movement T on the 
thumb loops. Twelfth. Bend each middle-fiuger over the upper far index 
string, and take up from below on the back of the finger the lower near 
index string {i.e. the one passing from index to index), and return. Lastly. 
Keeping the middle and index-fingers close together, release the loops from the 
little-fingers, and extend the figure keeping the palms turned away from you. 

Many Stains. A somewhat similar figure — see figure x — 
is made by the Navaho Mexican Indians, and by the Oregon 
Indians. The Oregon method is much the more artistic, since 
the movements are carried on by both hands simultaneously 




Figure x. Many Stan, 



and symmetrically, and the one hand is not used to arrange the 
strings on the other hand. But I give the Navaho method partly 
because it is easier to perform and partly because, by slightly 
varying the movements, it gives other interesting figures. 



CH. XVIl] 



STRING FIGURES 



365 



Many Stars. The formation of this figure has been described by Haddon 
{American Anthropologist^ vol. v, 1903, p. 222) and Mrs Jayne (pp. 48 — 53). It 
is produced thus. First. Take up the string in the form of Opening A. 
Second. Pass each thumb away from you over three strings (viz. the far thumb 
and both index strings) and pick up from below on its back the near little- 
finger string, and return. Third. Bend each middle-finger down towards you 
over two strings (viz. both the index strings) and take up from below on its back 
the far thumb string, and return. Fourth, Release the thumbs. Fifth. Pass 
each thumb away from you over one string (viz. the near index string), under 
the remaining five strings, and pick up on its back the far little-finger string, 
and return. Sixth. Release the little- fingers. Seventh. Take the far string of 
the right middle-finger loop, pass it under the near string of that loop, and 
then, taking it over the other strings, put it over the tips of the right thumb 
and index, so as to be the distal loop on them. Release the right middle- 
finger. Make a similar movement with the other hand. Eighth. Make 
movement T on the loops on the thumbs and index-fingers. There is now 
on each hand a string passing from the thumb to the index, and on each 
of these strings are two loops, one nearer you than the other. Ninth, Bend 
each thumb away from you over the upper string of these nearer loops. 
Lastly. Rotate the hands so that the palms face away from you, the fingers 
point up, and the thumbs are stretched as far from the hands as possible. 

More briefly thus. Opening A. Each thumb over 3 and picks up one. Each 
middle-finger over 2 and picks up one. Release thumbs. Each thumb over 
one, under 5, and picks up one. Release little-fingers. Take up near string of 
each middle-finger loop, turn it over, and transfer it to tips of corresponding 
thumb and index-finger. T to loops on thumbs and index-fingers. Place 
thumbs on upper strings of near loops. Rotate, and extend. 

Owls. Certain figures, called Owls, can be produced like 
Many Stars save for the interpolation or the alteration of one 
movement. Their resemblance to Owls is slight, but they 




Figure xi. An Owl. 



366 



STRING FIGUKES 



[CH. XVII 



may be taken without much straining to represent Bats. One 
instance — see figure xi — will suffice. 

The example of an Owl which I select is produced thus (Jayne, pp. 54—55). 
Immediately after taking up the string in the form of Opening A, give a twist 
to the index loops by bending each index down between the far index string and 
the near little-finger string and, keeping the loop on it, bringing it towards you 
up between the near index string and the far thumb string. Continue with the 
second and subsequent movements described in Many Stars. 

In another example (Jayne, pp. 55 — 56) all the movements are the same as 
in Many Stars save that in the fifth movement the far little-finger string is 
drawn from above, instead of from below, through the thumb loops. 

Single Stars. Other figures, which we may call Single 
Stars or Diamonds, can be produced like Many Stars save for 
the alteration or omission of one movement. One instance — 
see figure xii — will suffice. 




Figure xii. North Star. 



The exanaple of the Single Stars which I select is termed the North Star, 
and is produced thus (Jayne, p. 65). Replace the second, third, and fourth 
movements in Many Stars by the following. Bend each middle-finger towards 
you over the index loop, and take up from below on the back of the finger the 
far thumb string. Release the thumbs, and return the middle-fingers. The 
effect of this is to transfer the thumb loops to the middle-fingers. This is 
followed by the fifth and subsequent movements described in Many Stars. 
This figure may at our option be regarded as a single or double diamond. 

W. W. Another elegant design, forming two interlaced W's, 
can be produced somewhat similarly. 

This figure is made in the same way as North Star, except that after 
transferring the thumb loops to the middle- fingers, a twist is given to each 
middle-finger loop by bending each middle-finger down on the far side of the far 
middle-finger string and (keeping the loop on it) bringing it towards you up 
between the near middle-finger string and the far index string. 



CII. XVIl] STRING FIGURES 

The reader who has followed me in my descriptions of the 
movements for producing Many Stars will find it easy to make 
other modifications which lead to other figures. 

The Setting Sun is derived from the Torres Straits natives. 
This figure, if well done, is effective, but the process is some- 
what long. The method and result illustrate another type of 
string figure. 

The Setting Sun is produced thus (Rivers and Haddon, p. 150; Jayne, 
pp. 21 — 24). First. Take up the string in the form of Opening A. Second, 
Pass the little-fingers over four strings (viz. the radial or near little-finger string, 
the index loops, and the ulnar or far thumb string), insert them into the thumb- 
loops from above, take up with the backs of the little-fingers the near thumb 
string, and return. Third. Release the thumbs. Fourth. Pass the thumbs 
under the index loops, take up from below the two near strings of the little- 
finger loops and return, passing under the index loops. Fifth. Release the 
little-fingers. Sixth. Pass the little-fingers over the index loops, and take up 
from below the two far strings of the thumb loops and return. This arrange- 
ment is known as the Lem Opening. 

I continue, from the Lem Opening, the movements for the production of the 
Setting Sun. Seventh. Transfer the loop on the left index to the right index ; 
and then transfer the loop originally on the right index to the left index, 
by taking it over the original left index loop. Eighth. Pass the right 
middle-finger from the distal side {i.e. from above) through the right index loop 
and take up from the proximal side (i.e. from below) the two far thumb 
strings. Return the middle-finger through the index loops. Make similar 
movements with the other hand. Ninth. Release the thumbs and index- 
fingers. Tenth. Pass the thumbs from below into the middle-finger loops, 
and then transfer the middle-finger loops to the thumbs. Extend the figure 
with the thumbs towards the body. There will now be in the middle of the 
figure a triangle whose apex is towards your body, whose base is formed by the 
two far little-finger strings, and whose sides are formed by the mid-parts of the 
two near thumb strings. On either side of this triangle is a small four-sided 
figure. Eleventh. Insert the index-fingers from above into these quadrilaterals, 
and with the backs of the index-fingers take up the strings forming the sides 
of the triangle, and return. Twelfth. Put each middle-finger from above 
through the index loop, and take up from below the two far thumb strings, 
and return through the index loop. Thirteenth. Release the thumbs and 
index-fingers, put the index-fingers into the middle-finger loops to make them 
wider ; and with the thumbs manipulate the figure so as to make an approxi- 
mate semicircle (the sun on the horizon) with four diverging loops (the rays). 
Finally. Release the loops on the index and middle-fingers, separate the 
hands, and the semicircle will slowly disappear representing the setting of 
the son. 



368 STRING FIGURES [CH. XVII 

The Head Hunters is another and more difficult example, 
in which the Lem Opening is used. It too is derived from 
the Torres Straits, and is interesting because it is a graphical 
illustration of a story. 

The Head Hunters are produced thus (Rivers and Haddon, p. 150; Jayne, 
pp. 16—20). First. Make the Lem Opening, which involves six movements. 
Continue thus. Seventh. Insert the index-fingers from below into the central 
triangle and take up on their backs the near thumb strings. Eighth. Loop 
the lowest or proximal index string of each hand over the two upper or distal 
strings and over the tip of the index on to its palmar aspect. Ninth, Release 
the thumbs. Tenth. Take the index loops off the right hand, twist them tightly 
three or four times, and let the twist drop. Similarly form a twist out of the 
loops on the left index. 

This is the figure. If now the little-finger loops are drawn slowly apart, 
the two index loops will approach each other and become entangled. One 
represents a Murray man, and the other a Dauar man. They " fight, fight, 
fight," and, if worked skilfully, one loop, the victor, eventually remains, while a 
kink in the string represents all that is left of the other loop. The victorious 
loop can now be drawn to one hand along the two strings, sweeping the kink in 
front of it: it represents the victor carrying off the head of his opponent. 
Sometimes, if the two index loops are twisted exactly alike, they both break up, 
representing a duel fatal to both parties. In the hands of the Murray man 
who showed the figure to Dr Haddon, the result of the fight always led to the 
defeat of the Dauar warrior. 

It is not easy to make the figure so as to secure a good fight. For the benefit 
of any who wish to predict the result I may add that if, in the first position, 
there be a knot in the right palmar string the left loop will be usually Tictorious 
over the right loop, and vice versa. 

The Parrot Cage. As another instance I give the Parrot 
Cage. This is a string figure made by Negroes on the Gold 
Coast in West Africa. The construction and design are not 
interesting in themselves, but the method used is somewhat 
different to those employed in the foregoing examples, and for 
this reason I insert it. 

The Parrot Cage is made thus (Yoruba Figures by J. Parkinson, Journal of 
the Anthropological Institute, London, 1906, vol. xxxvi, p. 136). First. Take 
up the string in the form of Opening A. Second. Transfer the little-finger 
loops of each hand to the ring-fingers of that hand, the index loops to the middle- 
fingers, and the thumb loops to the index-fingers. By an obvious modification of 
Opening A, the string can be taken up initially in this form. Third. Lace the 
dorsal loops thus. Turn the back of the right hand upwards, pull the dorsal 
string of the ring-finger loop a little way out, and through it pass the dorsal 
string of the middle-finger loop. Similarly pass the dorsal string of the index loop 



CH. XVIl] STRING FIGURES 369 

through the middle-finger loop, and then put the index loop on the thumb. Pull 
all the strings taut. Repeat the same movements with the other hand. Fourth. 
Lace the palmar loops thus. Holding the right hand with the fingers pointing 
upwards, take up with the dorsal tip of the right thumb the nearest string 
which passes from hand to hand, pass the string already on the thumb over the 
string so taken up and then over the tip of the thumb on to the far and palmar 
side of the thumb. Repeat the same process successively with the far index 
string, with the near middle-finger string, with the far proximal middle-finger 
string, with the remaining far middle- finger string, and with the far ring-finger 
string. Repeat all these movements with the left hand. Finally. Transfer 
the thumb loops to the little-fingers, and extend the figure. This is the cage. 

See-Saw. I described above a couple of See-Saw figures 
made from the European Opening. Such figures can be also 
produced from Opening A. 

A See- Saw arrangement can be made by two players, P and Q, thus 
(Haddon quoted by Jayne, p. xiii). First. The string is taken up by P, 
with his hands pointing upwards and palm facing palm, in the form of 
Opening A. Second. Q hooks his right index from above in the straight 
thumb string and pulls it away from P. Third. Q, passing his hand from P 
over the other strings, hooks his left index in the straight little-finger string, 
and pulls it towards P. Finally. P releases all but the index loops. The 
sawing movement can then be made. 

Lightning. I proceed next to give two examples of figures 
which do not start from Opening A : both are easy to produce. 
The first instance I select, known as Lightning — see figure xiii — 
is derived from the North American Red Indians. The final 
movement should be performed sharply, so that the zig-zag 
lightning may flash out suddenly. 




Figure xiii. Zigzag Lightning. 
Lightning is produced thus (Jayne, pp. 216—219). First. Hold the string 
in one place between the tips of the thumb and index-finger of the right hand 
and in another place between the tips of the thumb and index-finger of the left 
hand, so that a piece passes between the hands and the rest hangs down in a 
loop. With the piece between the hands make a ring, hanging down, by putting 
the right-hand string away from you over the left-hand string. Next, insert 

B. B. 24 



370 STRING FIGURES [CH. XVII 

the index-fingers towards you in the ring and put the thumbs away from you 
into the long hanging loop. Separate the hands, and turn the index-fingers 
upward and outward with the palms of the hands facing away from you. 
Then, turn the hands so that the palms are almost facing you, and the thumbs 
and the palms come toward you and point upward. You now have a long 
crossed loop on each thumb and a single cross in the centre of the figure. 
This is the Navaho Opening. 

I continue from the Navaho Opening the movements for the production 
of Lightning. Second. Pass each thumb away from you over the radial or near 
index string and take up from below with the back of the thumb the far index 
string, and return the thumb to its former position. Third. Pass each middle- 
finger toward you over the near index string, and take up from below on 
the back of the finger the far thumb string and return the middle-finger 
to its original position. Fourth. Bend each ring-finger toward you over the 
far middle-finger string, take up from below with the back of the finger 
the near index string, and return the ring-finger to its position. Fifth. Pass 
each little-finger over the far ring-finger string, take up from below on 
the back of the finger the far middle-finger string, and return the little- 
finger to its position. You now have two twisted strings passing between 
the two little-fingers, two strings laced round the other fingers, and two loose 
strings (which may represent thunder clouds) passing over the thumbs. Sixth. 
Holding the hands with the fingers pointing upwards, put the tips of the 
thumbs from below (or if it is easier, from above) into the small spaces between 
the little-fingers and the twisted strings on them. Finally. With the thumbs 
raise (or depress, as the case may be) the near ring-finger string, and separate 
the hands so as to make the little- finger strings taut, turn the hands outwards 
so that the palms are away from you, and at the same time throw or jerk the 
thumb loops off the thumbs so that they hang away from you over the tightly 
drawn strings between the little-fingers. These movements will cause the 
strings between the little-fingers to untwist, making the lightning spring into 
view. The diagram represents the figure when the thumb in the sixth movement 
is inserted from below in the loop. 

More briefly thus. Form Navaho Ring. Each thumb over 2 and takes up 
one. Each middle-finger over one and takes up one. Each ring-finger over 
one and takes up one. Each little-finger over one and takes up one. Each 
thumb released, placed under (or over) the near little-finger string, and pressed 
up (or down) while hands are rotated. 

A Butterfly. As yet another of figures of this kind I will 
give the formation of that known as a Butterfly — see figure xiv. 
It is also derived £rom the North American Red Indians. 

The Butterfly is produced as follows (Jayne, pp. 219 — 221). Begin with the 
Navaho Opening, that is, make the first movements as when forming Lightning. 
Second. Twist each index loop by rotating each index down toward you and up 
again. In all make (say) three or four such twists on each index loop. Third. 
Put the right thumb from below into the right index loop, and, without removing 



CH. XVIl] 



STRING FIGURES 



371 



the index, sepwrate the thumb from the index. Make movement T on the thumb 
loops. Repeat the movement with the other hand. Fourth. Bring the hands 
close together with the index and thumb of the one hand pointing toward the 
index and thumb of the other hand ; then hang the right index loop /3 on the tip 
of the left index, and the right thumb loop 5 on the tip of the left thumb. Thus 




Figure xiv. A Butterfly. 
on the left index there are two loops, a and /3 ; on the left thumb there are two 
loops, 7 and 5 ; and the right hand is free. Fifth. Put the tips of the right 
index and thumb against the left thumb knuckle. Take up with the right index 
from the proximal side the loop 5 and take up with the right thumb from the 
distal side the loop 7. With the right thumb and index grasp the loops a and 
/3 where they lie on the top of the left index. Remove the left hand. Then, 
holding the right hand up, from the left, put the left index into the loop /S, 
and the left thumb mto the loop a. Finally. Placing the hands with 
the thumbs up and the fingers pointing away from you, draw the hands slowly 
apart, and when the strings have partially rolled up in the middle of the figure 
pull down with the middle, the ring, and the little-fingers of each hand the far 
index string and the near thumb string. The butterfly will now appear ; its 
wings being held up by the strings extended between the widely separated 
thumbs and index-fingers, and its proboscis appearing on the strings held 
down by the other fingers. 

More briefly thus. Form Navaho Ring. Twist index loops. Thumbs into 
index loops and T to thumb loops. Take up figure afresh with thumbs and 
index-fingers, and pull. 

There are numerous other figures which can be formed from 
an endless loop of string. For examples I refer the reader to 
Mrs Jayne's fascinating volume. 

String Tricks. Of string tricks there are two classes. One 
comprises tricks in which the hand of one player is unexpectedly 
caught by the other player pulling one string or certain strings 
in the figure, whereas it is left free if other strings are pulled. 

24—2 



372 STRING FIGURES [CH. XVII 

The other comprises tricks where the string is released from oi 
is made to take some position which prima facie is impossible. 
Aborigines are usually very proud of their ability to perform 
such feats, and Mrs Jayne acutely remarks that it is " delightful 
to witness their pleasure when they are successful, and their 
gratification at the observer's astonishment, which it will amply 
pay him to make very evident." 

The Lizard Twist. I give a couple of examples of Catch- 
Tricks. The first I select is one brought by Rivers and 
Haddon from the Torres Straits. If a loop of string hangs 
from (say) the left hand and if the right hand is twisted once 
round one of the strings of the loop, the right hand or wrist 
will be caught when the string is pulled by the left hand. If 
the right hand is twisted first round one string of the loop and 
then round the other string, it will in general be caught more 
firmly. The Lizard movement is a way of taking the twist on 
the second string so as to undo the effect of the first twist. 

There is no difficulty in taking up the string so that the hand is caught. 
To take up the string so that the hand is not caught, proceed thus (Rivers and 
Haddon, Man, 1902, p. 152 ; Jayne, pp. 337—339). First. Hold the string by the 
left hand, held rather high, the string hanging down on the right and left sides 
in a loop. Second. Put the right hand away from you through the loop. Turn 
the right hand round the right pendant string clockwise ; this will be done by 
pointing the fingers to the right, then towards you, and then upwards. Third. 
Keeping the fingers pointing upwards, move the right hand to the left between 
your body and the pendant strings, then clockwise beyond the left pendant 
string, then away from you, then to the right, and finally towards you thiough 
the loop. Lastly. Draw the hand down and to the right, and it will come 
free from the noose round the wrist. 

The Caroline Catch. The other example of a Catch-Trick 
which I propose to describe is derived from natives in the 
Caroline Isles. First, form the figure described above (p. 361) 
as Triple Diamonds. Next a second person puts his hand 
through the middle-lozenge shaped space in this figure. Then 
his wrist will be caught in a loop if the strings be dropped 
from the right hand and the left-hand strings pulled to the 
left. On the other hand his wrist will not be caught if the 
strings be dropped from the left hand, and the right-hand 
strings pulled to the right. 



CH. XVll] STRING FIGURES 373 

Of the second class of string tricks there are numerous 
examples familiar to English schoolboys — presumably well- 
known over large parts of the world, and I conjecture of 
considerable antiquity. In many old journals and books written 
for boys these are described, but such descriptions are often 
vague. Recently W. I. Pocock and Mrs Ja3me have described 
some of them in accurate language. I give as typical examples : 
Threading the Needle ; the Yam Thief, otherwise known as the 
Mouse Trick; the Halter; the Fly on the Nose; the Hand- 
Cuff; and the Elusive Loop. Of these the first, second, and 
fifth, are well-known. I owe my knowledge of the others to 
Mrs Jayne's book*. 

Threading the Needle. This is a familiar trick. The effect 
is that a small loop (representing the eye of the needle) held 
by the left thumb and index is threaded by a string held by 
the right hand throughout the motion. The trick is best 
worked with a single piece of string, but a doubled string will 
answer the purpose. 

Threading the Needle is performed thus (Jayne, pp. 354 — 355). First. 
Take hold of a piece of string, ABC, some three or four feet long, at a point B 
about 8 inches from one end A. Hold this piece AB in the left palm and 
hold the left hand so that the thumb points to the right. Second. Holding 
the other end, C, of the string with the right hand, wind the rest of the string 
BG, beginning with the B end, round the left thumb, above the thumb when 
moving towards the body and under the thumb when moving away from the 
body. Leave about 6 inches at the end G unwound. Third. Out of this 
6 inches, make a small loop by carrying the end of G held by the right hand 
to the far side of the rest of the loop. "With the tips of the left thumb and 
index-finger hold this loop where the strings cross so that it stands erect. 
Fourth. Pick up with the right thumb and index-finger the end A of the piece 
of string AB which is in the left palm and open the left palm. Make this piece 
of string AB taut, and carry the end of it A to the right close under and between 
the left thumb and index. If this is properly done the piece AB will pass near 
B between the far and near portions of the cross of the loops, and then will be 
caught by the left thumb and index : thus between the thumb and index there 
are now three pieces of string. Fifth. Make passes with the end A as if you 

• The second is described by Rivers and Haddon in Man, London, 
October, 1902, pp. 141, 153. All, except the Hand-Cuff, are described in 
C. F. Jayne's String Figures, New York, 1906, chapter viii. See, also, 
W. I. Pocock, Folk-Lore, London, 1906, pp. 349—373. 



374» STRING FIGURES [CH. XVII 

were trying to thread it through the erect loop held by the left hand. Lastly. 
Pass the right hand sharply to the left over and beyond the left hand. This 
will carry the piece AB beyond the two strings of the loop. Hence the loop 
which is still held up by the left thumb and index appears to have been threaded 
by the right-hand string, but in reality the part of the string which hangs from 
the right hand is drawn between the left thumb and index up into the loop. 

The Yam Thief or Mouse Trick. This is also a familiar 

trick, and is interesting as having stories connected with its 

performance. 

The Yam Thief or Mouse Trick is effected thus (Jayne, pp. 340 — 343). 
First. Hold the left hand with the palm facing you, the thumb upright, 
and the fingers pointing to the right. With the right hand, loop the string 
over the left thumb, cross the strings, and let one string hang down over 
the palm and the other over the back of the left hand. Second. Pass the 
right index from below under (i.e. on the proximal side of) the pendant 
palmar string and then between the left thumb and index, and with the 
palmar tip of the right index loop up a piece of the string hanging on the 
back of the left hand. Pull this loop back between the left thumb and index 
and on the upper (or distal) side of the left palmar string. Then with the right 
index give the loop one twist clockwise, and put it over the left index. Pull the 
two pendant strings in order to hold tight the loops on the thumb and index. 
Third. In the same way pass the right index from below under the pendant 
palmar string and then between the left index and middle-fingers, and with the 
palmar tip of the right index loop up a piece of the pendant dorsal string. 
Pull it back between the left index and the middle-fingers and on the upper side 
of the left palmar string. With the right index give the loop one twist clock- 
wise, and put it over the left middle-finger. Fourth. In the same way pick up 
a loop of the pendant dorsal string, and put it on the left ring-finger. Fifth. 
In the same way pick up a loop of the pendant dorsal string, and put it on the 
left little-finger. Sixth. Take off the left thumb loop, or slip it to the tip 
of the thumb and hold it between the left thumb and index. The pendant 
dorsal string on the left hand can be pulled to show that the loops are still on 
the fingers. Finally. Pull the left pendant palmar string and all the string 
will come away. 

In one version of the story the thumb loop represents the owner of a yam 
or cabbage patch. He is supposed to be asleep. The loops successively taken 
up from the dorsal string represent the yams or cabbages dug up by a thief 
from the patch, and secured by him in bundles on the fingers. The loop 
coming off the thumb represents the owner waking, and going out to see what is 
the matter. He walks down the dorsal side, sees the yams, pulls at the dorsal 
string, is satisfied that his yams are still on the ground, and returns to catch 
the thief. Meanwhile the thief walks down the palmar side of the hand, and as 
the owner returns from the dorsal side, the left palmar string is pulled and the 
thief disappears with the stolen yams. 

Cheating the Halter. This is a trick of the Philippinoes. 



CH. XVIl] STRING FIGURES 375 

A halter is put round the neck, but by a movement, which in 
effect reverses the turn on the neck, the string, when pulled, 
comes off. 

The Halter Trick is performed thus (Jayne, pp. 339—340). First. Put 
your head through a loop of string, and let the test of the loop hang down 
in front of you. Second. Pass the right string round the nock from the left 
side, draw the loop tight, and let it hang down in front of you. Third. Put 
the hanging loop on the hands, and form Opening A. Fourth. Pass the index 
loops over the head, and remove the hands and fingers from the other loops ; 
a loop now hangs down in front of you. Lastly. If this loop, or either string 
of it, be pulled all the strings will come off the neck. 

We can vary the presentation by twisting the left string round the neck 
from the right side. In this case we must use Opening B. 

The Fly on the Nose, In this trick the string seems to 
come away although looped on to the nose. 

The Fly on the Nose is performed thus (Jayne, pp. 348 — 349). First. 
Hold the string at some point with the thumb and index-finger of one 
hand ; and take hold of the string at a place, some 9 or 10 inches off, 
with the thumb and index of the other hand. Second. Make a small ring 
hanging down by passing the right hand to the left and on the near side of 
the string held by the left hand. There is now a long loop and a small 
ring, both hanging down, the right string of the ring being the left string 
of the loop. Third. Hold the place where the strings cross between the 
teeth, the string originally held by the right hand being on the lower side. 
Hold the strings or one string of the long loop with the left hand. Fourth. 
Place the right index from the far side through the ring, taking the ring string 
up to the root of the fingers. Close the right fist, and carry the fist (holding 
the ring) round in a circle, first back to the far side, then to the right, and 
so on round the right string of the loop, to its original position. Fifth. Open 
the index, keeping the rest of the fist shut, and put the tip of the index on the 
tip of the nose. Finally. Let go with the mouth and pull the left string of the 
loop. The whole string will then come away. 

Of course the same result can be obtained if the ring is made by passing the 
right hand to the left on the far side of the string held by the left hand, so that 
when the cross is held in the mouth the string originally held by the right hand 
is uppermost. But in this case the ring should be taken by the left index, which 
is carried round the left string of the loop. 

The Hand-Guff Trick. This is a very ancient string trick. 
Two players, P and Q, are connected thus. One end of a piece 
of string is tied round P's right wrist, the other is tied round 
his left wrist. Another piece of string is passed through the 
space bounded by the string tied to P's wrist, his body, and his 



376 STRING FIGURES [CH. XVII 

arms. The ends of this piece are then tied one on Qs right 
wrist, and the other on Q's left wrist. The players desire to 
free themselves from the entanglement. 

The Hand-Cuff Trick. Either player can free himgelf thus. First. P takes 
up a small loop L near the* middle of the string tied to his wrists, and pulling 
the loop to one of Q's wrists on the palmar side of it, pauses it, from the elbow 
to the finger side, between Q's wrist and the loop on that wrist. Next, P draws 
this loop L sufificiently far through until he is able to pass it, first over Q's 
hand, and then under the wrist loop on the outer side of Q's wrist, passing this 
time from the finger to the elbow side of Q. When in this position P can pull 
his own string clear of Q on the outer side of Q's arm. 

The Elusive Loop. This is a trick in which a loop is offered 
to some one, and then unexpectedly disappears. Almost any 
of the forms in which the string is so arranged that if pulled it 
runs off the fingers — and there are many examples of this kind, 
ex. gr. the Yam Thief — lends itself to this presentation. I give 
an instance derived from the Torres Straits where the loop is 
supposed to represent a Yam. 

The offer of the Elusive Yam is performed thus (Jayne, pp. 352 — 354). First. 
Take up the string in the first position. Second. Pass each index away from 
you over the little-finger string and to the far side of it, draw the string towards 
you in the bend of the index, and then turn the index up towards you in 
its usual position, thus twisting the string round the tip of the finger. Third. 
Pass each thumb away from you under the far index string, pick up from below 
on the back of the thumb the near index string which crosses the palm 
obliquely, and return the thumb under the near thumb string to its original 
position. Fourth. Pass each little-finger towards you over the far index 
string, and pick up from below on the back of the little -finger the near string 
which passes directly from hand to hand, and return the little-finger. Fifth. 
Pass each thumb away from you, and pick up from below the near string 
of the figure, and return the thumb. Lastly. Kelease the loop from the 
left index, and hold it erect between the left index and thumb. This loop 
represents a Yam. 

One boy, who is supposed to be hungry, says Have you any food for me ? 
Thereupon another boy, who has made the figure, offers the loop or Yam to 
the first boy saying. Take it if you can. On this the first boy grabs at the 
Yam, while the second boy pulls the right-hand strings. If the former is quick 
enough he gets the Yam, but if not, it disappears, and all the strings with it. 
The same trick can be repeated with the right hand. 

More briefly thus. First position. Index-fingers, take up twist on far 
string. Thumbs, under one, pick up palmar near index string, and return 
under two. Little fingers, pick up near string. Thumbs, pick up near string. 
Release loop from left index. 



CH. XVIl] STRING FIGURES 377 

There also are some string tricks which must be familiar 
to most of my readers. I give as well-known examples the 
Button-Hole Trick and the Loop Trick. These require no skill 
and present no difficulty. It is with some hesitation that I 
describe them, but age gives them a certain claim. I have no 
idea when or by whom they were invented. The devices used 
are so obvious that the tricks will hardly bear repetition. 

The Button-Hole Trick. In this a loop of string is passed 
through a button-hole or ring, and on each side of the hole a 
finger or thumb is put into the loop on that side. The object 
is to free the loop from that hole. It may be done in several 
ways which however do not differ in principle. I give two 
methods. 

The Button-Hole Trick may be done thus. Firtt. Pass a loop of string 
through a button-hole or key-ring. Second. Hold the thumbs upright, and 
insert them from below in the two loops one on each side of the button-hole. 
Move the hands a little out from the body and towards each other. Third. Hook 
the right little-finger into the right-hand string of the left thumb loop, and pull 
it across to the right hand. Fourth. Pass the left hand above the right little- 
finger loop, hook the left little-finger into the left hand string of the right thumb 
loop and pull it across to the left hand. Fifth. Drop the right thumb loop and 
put the right thumb into the right little-finger loop. Finally. Withdraw both 
little-fingers, and separate the hands ; the string will then come off the button- 
hole. In fact the effect of the movements described is to get both thumbs in 
one loop. 

This trick may be also performed by two people, P and Q, by the following 
movements, the description of which I take from Pocock's paper in Folk- 
LorCy 1906, p. 355. First. Pass a loop of the string through a ring. Second. 
P, holding his thumbs upright, inserts them from below in the two loops, 
one on each side of the ring. The string nearest him will be the radial, that 
farthest from him the ulnar string. Third. Q, who is facing P, puts his 
left index from below into the loop on P's right thumb. Q then takes up on 
the back of his left index-finger the ulnar string at some point, say H, of it 
between the ring and P's right thumb. At the same time Q, with the inside of 
his left little-finger, hooks the same string at some point between H and P's 
right thumb and draws it towards himself. Thus there is now one loop on 
the back of Q's left index-finger and another ou the inside of his left little- 
finger. Fourth. Q places the tip of his left index-finger on the tip of P's right 
thumb and transfers the loop on that finger to P's thumb. At the same time 
Q draws the little-finger loop well away towards P's left side. Fifth. Q pushes 
his left index from above into the loop near P's left thumb but on the other side 
of the ring, and with the back of the finger he picks up tlic radial string. 
Sixth. Q transfers this index loop to P's right thumb, by touching the tips as 



378 STRING FIGURES [CH. XVII 

before. Seventh. Q grasps the ring with his right hand, and at the same time 
drops the little-finger loop. Finally. P separates his hands and the ring 
comes away from the string. 

The Loop Trick is merely an illustration of how easily an 
unobservant person can be deceived. It requires two persons, 
F and Q, and is performed thus. 

The Loop Trick. A string is looped on the index-fingers held upright of 
a player P : thus there are two parallel strings, a near and a far one. The loop 
is taken up by another player, Q, thus. First. Q presses his left index on both 
strings about half-way between P's hands and holds them firmly down. 
Second. P moves his right index over the strings until the tip meets the tip of 
his left index, and if he likes shifts the right index loop from the right finger to 
the base of the left finger. Lastly. Q slips his left index off the distal string, 
and at the same time pulls the left index, which now rests on only one string, 
and of course the loop comes away. 

This is the common presentation of the trick, but W. I. Pocock remarks 
that it is somewhat less easy to detect the method used if the strings be 
struck sharply down with the right hand at the instant when the left index 
is pulled. 

The Waistcoat Puzzle. There is one other trick of this type 
which was, I believe, published for the first time in this book 
in 1888. It is applicable to a man wearing a coat and waist- 
coat of the usual pattern. The problem is to take off the 
waistcoat, which may be unbuttoned, without pulling it over 
the head and without taking off the coat. 

Those of my readers who are conversant with the history of 
white magic will recollect that Pinetti, the celebrated conjurer 
of the eighteenth century, in his performance at Versailles 
before the French court in 1782, relieved some of the courtiers 
of their shirts without disturbing the rest of their attire. It 
was his most striking trick, and not a very seemly one. In 
fact he threw a shawl over the person operated on, and then 
pulled the back of the shirt over his head ; an obvious method 
which I have barred in the waistcoat trick. The victim and 
the spectators were uncritical, and Pinetti's technical skill was 
sufficient to conceal the modus operandi from them. 

The Waistcoat Puzzle can be done thus. First. Take the left corner (or 
lappel) of the coat and push it through the left armhole of the waistcoat, 
from outside to inside. Second. Put the left hand and arm through the same 



CH. XVIl] STRING FIGURES 379 

armhole. The effect of this is to leave the left armhole of the waistcoat at the 
back of the neck. Third. Take the right lappel of the coat and put it through 
the left armhole of the waistcoat. Fourth. Put the right hand and arm 
through the same armhole. Finally. Pass the waistcoat down the right sleeve 
of the coat. 

I should have liked to add another section to this chapter 
on knots and lashings. Some references to the mathematics of 
the subject will be found in papers by Listing, Tait, Boddicker *, 
but its presentation in a popular form is far from easy, and this 
chapter has already run to dimensions which forbid any ex- 
tension of it. 

* J. B. Listing, Vorstudien zur Topologie, Die Studien, Gottingen, 1847, 
Part X ; 0. Boddicker, Erineiterung der Gauss' schen Theorie der Verschlinrfwigen, 
Stuttgart, 1876 ; and P. G. Tait, Collected Scientific Papers, Cambridge, vol. i, 
ISyS, pp. 273—347. The more common forms of knots and lashings and their 
special uses are described in most naval, engineering, and scouting manuals. 



3b0 



CHAPTER XVIII. 
ASTROLOGY. 

Astrologers professed to be able to foretell the future, and 
within certain limits to control it. I propose to give in this 
chapter a concise account of the rules they used for this purpose*. 

I have not attempted to discuss the astrology of periods 
earlier than the middle ages, for the technical laws of the 
ancient astrology are not known with accuracy. At the same 
time there is no doubt that, as far back as we have any definite 
historical information, the art was practised in the East ; that 
thence it was transplanted to Egypt, Greece, and Rome ; and 
that the medieval astrology was founded on it. It is probable 
that the rules did not differ materially from those described in 
this chapter f, and it may be added that the more intelligent 
thinkers of the old world recognized that the art had no valid 
pretences to accuracy. I may note also that the history of the 
development of the art ceases with the general acceptance of 
the Copernican theory, after which the practice of astrology 
rapidly became a mere cloak for imposture. 

* I have relied mainly on the Manual of Astrology by Eapliael — whose real 
name was E. C. Smith — London, 1828, to which the references to Raphael 
hereafter given apply ; and on Cardan's writings, especially his commentary on 
Ptolemy's work and his Geniturarum Exempla. I am indebted also for various 
references and gossip to Whewell's History of the Inductive Sciences ; to 
various works by Raphael, published in London between 1825 and 1832 ; and 
to a pamphlet by M. Uhlemann, entitled Grundziige der Astronomic und Astro- 
logic, Leipzig, 1857. 

t On the influences attributed to the planets, see The Dialogue of Bardesan 
(m Fate, translated by W. Cureton in the Spicilegium Syrfacum, London, 1855. 



CH. XVIIl] ASTROLOGY 881 

All the rules of the medieval astrology — to which I confine 
myself — are based on the Ptolemaic astronomy, and originate 
in the Tetrahihlos* which is said, it may be fjilsely, to have 
been written by Ptolemy himself. The art was developed by 
numerous subsequent writers, especially by Albohazenf, and 
Firmicus. The last of these collected the works of most of 
his predecessors in a volume J, which remained a standard 
authority until the close of the sixteenth century. 

I may begin by reminding the reader that though there 
was a fairly general agreement as to the methods of procedure 
and interpretation — which alone I attempt to describe — yet 
there was no such thing as a fixed code of rules or a standard 
text-book. It is therefore difficult to reduce the rules to any 
precise and definite form, and almost impossible, within the 
limits of a chapter, to give detailed references. At the same 
time the practice of the elements of the art was tolerably well 
established and uniform, and I feel no doubt that my account, 
80 far as it goes, is substantially correct. 

There were two distinct problems with which astrologers 
concerned themselves. One was the determination in general 
outline of the life and fortunes of an enquirer: this was known 
as natal astrology, and was effected by the erection of a scheme 
of nativity. The other was the means of answering any specific 
question about the individual : this was known as horary 
astrology. Both depended on the casting or erecting of a 
horoscope. The person for whom it was erected was known as 
the native, 

A horoscope was ca^t according to the following rules §. The 
space between two concentric and similarly situated squares 
«was divided into twelve spaces, as shown in the annexed dia- 
gram. These twelve spaces were known technically as houses ; 
they were numbered consecutively 1, 2, ..., 12 (see figure); and 

♦ There is an English translation by J. Wilson, London [n.d.] ; and a French 
translation is given in Raima's edition of Ptolemy's works, 
t De Judiciis Astrorum, ed. Liechtenstein, Basle, 1571. 
X Astronomicorum, eight books, Venice, 1499. 
§ Raphael, pp. 91—109. 



382 



ASTROLOGY 



[CH. 



XVIIT 



were described as the first house, the second house, and so on. 
The dividing lines were termed cus'ps: the line between the 
houses 12 and 1 was called the cusp of the first house, the line 
between the houses 1 and 2 was called the cusp of the second 
house, and so on, finally the line between the houses 11 and 12 




was called the cusp of the twelfth house. Each house had also 
a name of its own — thus the first house was called the ascendant 
house, the eighth house was called the house of death, and so 
on — but as these names are immaterial for my purpose I shall 
not define them. 

Next, the positions which the various astrological signs 
and planets had occupied at some definite time and place (for 
instance, the time and place of birth of the native, if hi? 
nativity was being cast) were marked on the celestial sphere. 
This sphere was divided into twelve equal spaces by great 
circles drawn through the zenith, the angle between any two 
consecutive circles being 30°. The first circle was drawn 
through the East point, and the space between it and the next 
circle towards the North corresponded to the first house, and 
sometimes was called the first house. The next space, proceed- 
ing from East to North, corresponded to the second house, and 



CH. XVIIl] ASTROLOGY 383 

80 on. Thus each of the twelve spaces between these circles 
corresponded to one of the twelve houses, and each of the circles 
to one of the cusps. 

In delineating* a horoscope, it was usual to begin by in- 
serting the zodiacal signs. A zodiacal sign extends over 30°, 
and was marked on the cusp which passed through it : by its 
side was written a number indicating the distance to which its 
injfluence extended in the earlier of the two houses divided by 
the cusp. Next the positions of the planets in these signs were 
calculated, and each planet was marked in its proper house and 
near the cusp belonging to the zodiacal sign in which the planet 
was then situated : it was followed by a number indicating its 
right ascension measured from the beginning of the sign. The 
name of the native and the date for which the horoscope was 
cast were inserted usually in the central square. The diagram 
near the end of this chapter is a fatjsimile of the horoscope of 
Edward VI as cast by Cardan and will serve as an illustration 
of the above remarks. 

We are now in a position to explain how a horoscope was 
read or interpreted. Each house was associated with certain 
definite questions and subjects, and the presence or absence in 
that house of the various signs and planets gave the answer to 
these questions or information on these subjects. 

These questions cover nearly every point on which informa- 
tion would be likely to be sought. They may be classified 
roughly as follows. For the answer, so far as it concerns the 
native, to all questions connected with his life and health, look 
in house 1 ; for questions connected with his wealth, refer to 
house 2 ; for his kindred and communications to him, refer to 
3 ; for his parents and inheritances, refer to 4 ; for his children 
and amusements, refer to 5 ; for his servants and illnesses, refer 
to 6 ; for his marriage and amours, refer to 7 ; for his death, 
refer to 8 ; for his learning, religion, and travels, refer to 9 ; for 
his trade and reputation, refer to 10; for his friends, refer to 
11 ; and finally for questions connected with his enemies, refer 
to house 12. 

* Raphael, pp. 118—131. 



384 ASTROLOGY [CH. XVIII 

I proceed to describe briefly the influences of the planets, 
and shall then mention those of the zodiacal signs; I should 
note however that in practice the signs were in many respects 
considered to be more influential than the planets. 

The astrological " planets " were seven in number, and in- 
cluded the Sun and the Moon. They were Saturn or the Great 
Infortune, Jupiter or the Great Fortune, Mars or the Lesser 
Infortune, the Sun, Venus or the Lesser Fortune, Mercury, and 
the Moon : the above order being that of their apparent times 
of rotation round the earth. 

Each of them had a double signification. In the first place 
it impressed certain characteristics, such as good fortune, feeble- 
ness, &c., on the dealings of the native with the subjects con- 
nected with the house in which it was located; and in the 
second place it imported certain objects into the* house which 
would affect the dealings of ♦the native with the subjects of that 
house. 

To describe the exact influence of each planet in each house 
would involve a long explanation, but the general effect of 
their presence may be indicated roughly as follows*. The 
presence of Saturn is malignant : that of Jupiter is propitious : 
that of Mars is on the whole injurious: that oC the Sun indicates 
respectability and moderate success: that of Yenus is rather 
favourable : that of Mercury implies rapid practical action : and 
lastly the presence of the Moon merely faintly reflects the 
influence of the planet nearest her, and suggests rapid changes 
and fickleness. Besides the planets, the Moon's nodes and 
some of the more prominent fixed stars f also had certain 
influences. 

These vague terms may be illustrated by taking a few 
simple cases. 

For example, in casting a nativity, the life, health, and 
general career of the native were determined by the first or 
ascendant house, whence comes the expression that a man's 
fortune is in the ascendant. Now the most favourable planet 

* Eaphael, pp. 70—90, pp. 204—209. 
t Raphael, pp. 129—131. 



CH. XVIIl] ASTROLOGY 385 

was Jupiter. Therefore, if at the instant of birth Jupiter was 
in the first house, the native might expect a long, happy, 
healthy life ; and being born under Jupiter he would have a 
"jovial " disposition. On the other hand,. Saturn was the most 
unlucky of all the planets, and was as potent as malignant. If 
at the instant of birth he was in the first house, his potency 
might give the native a long life, but it would be associated 
with an angry and unhappy temper, a spirit covetous, revengeful, 
stern, and unloveable, though constant in friendship no less than 
in hate, which was what astrologers meant by a " saturnine " 
character. Similarly a native bom under Mercury, that is, with 
Mercury in the first house, would be of a mercurial nature, 
while anyone bom under Mars would have a martial bent. 

Moreover it was the prevalent opinion that a jovial person 
would have his horoscope affected by Jupiter, even if that 
planet had not been in the ascendant at the time of birth. 
Thus the horoscope of an adult depended to some extent on his 
character and previous life. It is hardly necessary to point 
out how easily this doctrine enabled an astrologer to make the 
prediction of the heavens agree with facts that were known 
or probable. 

In the same way the other houses are affected. For in- 
stance, no astrologer, who believed in the art, would have 
wished to start on a long journey when Saturn was in the 
ninth house or house of travels ; and if, at the instant of birth, 
Saturn was in that house, the native would always incur con- 
siderable risk on his journeys. 

Moreover every planet was affected to some extent by its 
aspect (conjunction, opposition, or quadrature) to every other 
planet according to elaborate rules* which depended on their 
positions and directions of motion : in particular the angular 
distance between the Sun and the Moon — sometimes known 
as the " part of fortune " — was regarded as specially important, 
and this distance affected the whole horoscope. In general, 
conjunction was favourable, quadrature unfavourable, and oppo- 
sition ambiguous. 

♦ Raphael, pp. 132—170. 

B. a. 25 



386 ASTROLOGY [CH. XVIII 

Each planet not only influenced the subjects in the house 
in which it was situated, but also imported certain objects into 
the house. Thus Saturn was associated with grandparents, 
paupers, beggars, labourers, sextons, and gravediggers. If, for 
example, he was present in the fourth house, the native might 
look for a legacy from some such person; if he was present 
in the twelfth house, the native must be careful of the con- 
sequences of the enmity of any such person ; and so on. 

Similarly Jupiter was associated generally with lawyers, 
priests, scholars, and clothiers ; but, if he was conjoined with a 
malignant planet, he represented knaves, cheats, and drunkards. 
Mars indicated soldiers (or, if in a watery sign, sailors on ships 
of war), masons, doctors, smiths, carpenters, cooks, and tailors ; 
but, if afflicted with Mercury or the Moon, he denoted the 
presence of thieves. The Sun implied the action of kings, 
goldsmiths, and coiners ; but, if afflicted by a malignant planet, 
he denoted false pretenders. Venus imported musicians, em- 
broiderers, and purveyors of all luxuries; but, if afflicted, 
prostitutes and bullies. Mercury imported astrologers, philo- 
sophers, mathematicians, statesmen, merchants, travellers, men 
of intellect, and cultured workmen ; but, if afflicted, he signified 
the presence of pettifoggers, attorneys, thieves, messengers, 
footmen, and servants. Lastly, the presence of the Moon 
introduced sailors and those engaged in inferior offices. 

I come now to the influence and position of the zodiacal 
signs. So far as the first house was concerned, the sign of the 
zodiac which was there present was even more important than 
the planet or planets, for it was one of the most important 
indications of the duration of life. 

Each sign was connected with certain parts of the body — 
ex. gr. Aries influenced the head, neck and shoulders — and that 
part of the body was affected according to the house in which 
the sign was. Further each sign was associated with certain 
countries and connected the subjects of the house in which the 
sign was situated with those countries: ecc. gr. Aries was 
associated especially with events in England, France, Syria, 
Verona, Naples, &c. 



CH. XVIIl] ASTROLOGY 387 

The sign in the first house determined also the character 
and appearance of the native*. Thus the character of a native 
born under Aries (m) was passionate; under Taurus (/) was 
dull and cruel; under Gemini (m) was active and ingenious; 
under Cancer (/) was weak and yielding ; under Leo (m) was 
generous, resolute, and ambitious ; under Virgo (/) was sordid 
and mean ; under Libra (m) was amorous and pleasant ; under 
Scorpio (/) was cold and reserved ; under Sagittarius (m) was 
generous, active, and jolly ; under Capricorn (/) was weak and 
narrow ; under Aquarius (m) was honest and steady ; and under 
Pisces (/) was phlegmatic and effeminate. 

Moreover the signs were regarded as alternately masculine 
and feminine, as indicated above by the letters m or / placed 
after each sign. A masculine sign is fortunate, and all planets 
situated in the same house have their good influence rendered 
thereby more potent and their unfavourable influence mitigated. 
But all feminine signs are unfortunate, their direct eflect is evil, 
and they tend to nullify all the good influence of any planet 
which they afflict (i.e. with which they are connected), and to 
increase all its evil influences, while they also import an element 
of fickleness into the house and often turn good influences into 
malignant ones. The precise effect of each sign was different 
on every planet. 

I think the above account is sufficient to enable the reader 
to form a general idea of the manner in which a horoscope was 
cast and interpreted, and I do not propose to enter into further 
details. This is the less necessary as the rules — especially as 
to the relative importance to be assigned to various planets 
when their influence was conflicting — were so vague that astro- 
logers had little difficulty in finding in the horoscope of a client 
any fact about his life of which they had information or any 
trait of character which they expected him to possess. 

That this vagueness was utilized by quacks is notorious, 
but no doubt many an astrologer in all honesty availed himself 
of it, whether consciously or unconsciously. It must be re- 
membered also that the rules were laid down at a time when 
* Raphael, pp. 61—69. 

25—2 



388 ASTROLOGY [CH. XVIII 

men were unacquainted with exact sciences, with the possible 
exception of mathematics, and further that, if astrology had 
been reduced to a series of inelastic rules applicable to all horo- 
scopes, the number of failures to predict the future correctly 
would have rapidly led to a recognition of the folly of the art. 
As it was, the failures were frequent and conspicuous enough 
to shake the faith of most thoughtful men. Moreover it was a 
matter of common remark that astrologers showed no greater 
foresight in meeting the difficulties of life than their neigh- 
bours, while they were neither richer, wiser, nor happier for 
their supposed knowledge. But though such observations were 
justified by reason they were often forgotten in times of diffi- 
culty and danger. A prediction of the future and the promise 
of definite advice as to the best course of action, revealed by the 
heavenly bodies themselves, appealed to the strongest desires of 
all men, and it was with reluctance that the futility of the 
advice was gradually recognized. 

The objections to the scheme had been stated clearly by 
several classical writers. Cicero* pointed out that not one of 
the futures foretold for Pompey, Crassus, and Caesar had been 
verified by their subsequent lives, and added that the planets, 
being almost infinitely distant, cannot be supposed to affect 
us. He also alluded to the fact, which was especially pressed by 
Pliny f, that the horoscopes of twins are practically identical 
though their careers are often very different, or as Pliny put it, 
every hour in every part of the world are born lords and slaves, 
kings and beggars. 

In answer to the latter obvious criticism astrologers replied 
by quoting the anecdote of Publius Nigidius Figulus, a cele- 
brated Roman astrologer of the time of Julius Caesar. It is 
said that when an opponent of the art urged as an objection 
the different fates of persons born in two successive instants, 
Nigidius bade him make two contiguous marks on a potter's 
wheel, which was revolving rapidly near them. On stopping 
the wheel, the two marks were found to be far removed firom 

*■ Cicero, Be Divivatione, n, 42. 

•f- Pliny, Historia Naturalist vii, 49 ; xxix, 1. 



CH. XVIIl] ASTROLOGY 889 

each other. Nigidius received the name of Figulus, the 
potter, in remembrance of this story, but his argument, says 
St Augustine*, who gives us the narrative, was as fragile as 
the ware which the wheel manufactured. 

On the other hand Seneca and Tacitus may be cited as 
being on the whole favourable to the claims of astrology, 
though both recognized that it was mixed up with knavery and 
fraud. An instance of successful prediction which is given 
by the latter of these writers f may be used more correctly as 
an illustration of how the ordinary professors of the art varied 
their predictions to suit their clients and themselves. The 
story deals with the first introduction of the astrologer Thra- 
syllus to the emperor Tiberius. Those who were brought to 
Tiberius on any important matter were admitted to an inter- 
view in an apartment situated on a lofty cliff in the island 
of Capreae. They reached this place by a narrow path over- 
hanging the sea, accompanied by a single freedman of great 
bodily strength ; and on their return, if the emperor had con- 
ceived any doubts of their trustworthiness, a single blow buried 
the secret and its victim in the ocean. After Thrasyllus had, 
in this retreat, stated the results of his art as they concerned 
the emperor, the latter asked the astrologer whether he had 
calculated how long he himself had to live. The astrologer 
examined the aspect of the stars, and while he did this showed, 
as the narrative states, hesitation, alarm, increasing terror, and 
at last declared that the present hour was for him critical, 
perhaps fatal. Tiberius embraced him, and told him he was 
right in supposing he had been in danger but that he should 
escape it ; and made him thenceforth a confidential counsellor. 
But Thrasyllus would have been but a soiTy astrologer had he 
not foreseen such a question and prepared an answer which he 
thought fitted to the character of his patron. 

A somewhat similar story is told J of Louis XI of France. 

• St Augustine, De Civitate Dei, bk. v, chap, iii ; Opera Omnia^ ed. Migne, 
vol. VII, p. 143. 

t Amialea, vi, 22 : quoted by Whewell, History of the Inductive Sciences, 
vol. I, p. 313. 

+ Personal Characteristics from French History, by Baron F. Rothschild, 



390 ASTROLOGY [CH. XVIII 

He sent for a famous astrologer whose death he was meditating 
and asked him to show his skill by foretelling his own future. 
The astrologer replied that his fate was uncertain, but it was 
so inseparably interwoven with that of his questioner that the 
latter would survive him but by a few hours, whereon the 
superstitious monarch not only dismissed him uninjured, but 
took steps to secure his subsequent safety. The same anecdote 
is also related of a Scotch student who, being captured by 
Algerian pirates, predicted to the Sultan that their fates were 
so involved that he should predecease the Sultan by only a few 
weeks. This may have been good enough for a barbarian, but 
with most civilized monarchs probably it would be less effectual, 
as certainly it is less artistic, than the answer of Thrasyllus. 

I may conclude by mentioning a few notable cases of 
horoscopy. 

Among the most successful instances of horoscopy enumerated 
by Raphael* is one by W. Lilly, given in his Monarchy or No 
Monarchy, published in 1651, in which he predicted a plague 
in London so terrible that the number of deaths should exceed 
the number of coffins and graves, to be followed by " an exorbi- 
tant fire." The prediction was amply verified in 1665 and 
1666. In fact Lilly's success was embarrassing, for the 
Committee of the House of Commons, which sat to investigate 
the causes of the fire and ultimately attributed it to the papists, 
thought that he must have known more about it than he chose 
to declare, and on October 25, 1666, summoned him before them: 
Lilly proved himself a match for his questioners. 

An even more curious instance of a lucky hit is told of 
Flamsteedf, the first astronomer royal. It is said that an old 
lady who had lost some property wearied Flamsteed by her 
perpetual requests that he would use his observatory to discover 



London, 1896, p. 10. The story was introduced by Sir Walter Seott in Quentin 
Durward, chap. xv. 

♦ Manual of Astrology, p. 37. 

t The story, though in a slightly different setting, is given in The London 
Chronicle, Dec. 3, 1771, and it is there stated that Flamsteed attributed the 
result to the direct action of the deviL 



CH. XVIIl] ASTROLOGY S91 

her property for her. At last, tired out with her importunities, 
he determined to show her the folly of her demand by making 
a prediction, and, after she had found it false, to explain 
again to her that nothing else could be expected. Accordingly 
he drew circles and squares round a point that represented her 
house and filled them with all sorts of mystical symbols. 
Suddenly striking his stick into the ground he said, " Dig there 
and you will find it." The old lady dug in the spot thus 
indicated, and found her property ; and it may be conjectured 
that she believed in astrology for the rest of her life. 

Perhaps the belief that the royal observatory was built for 
such purposes may still be held, for De Morgan, writing in 
1850, says that "persons still send to Greenwich to have their 
fortunes told, and in one case a young gentleman wrote to know 
who his wife was to be, and what fee he was to remit." 

It is easier to give instances of success in horoscopy than of 
failure. Not only are all ambiguous predictions esteemed to 
be successful, but it is notorious that prophecies which have 
been verified by the subsequent course of events are remembered 
and quoted, while the far more numerous instances in which the 
prophecies have been falsified are forgotten or passed over in 
silence. 

As exceptionally well-authenticated instances of failures 
I may mention the twelve cases collected by Cardan in his 
Geniturarum Exempla. These are good examples because 
Cardan was not only the most eminent astrologer of his time, 
but was a man of science, and perhaps it is not too much to 
say was accustomed to accurate habits of thought; moreover, 
I believe he was honest in his belief in astrology. To English 
readers the most interesting of these is the horoscope ot 
Edward VI of England, the more so as Cardan has left a full 
account of the affair, and has entered into the reasons of his 
failure to predict Edward's death. 

To show how Cardan came to be mixed up in the transaction 
I should explain that in 1552 Cardan went to Scotland to 
prescribe for John Hamilton, the archbishop of St Andrews, who 
was ill with asthma and dropsy and about whose treatment the 



392 ASTROLOGY [CH. XVIII 

physicians had disagreed*. On his return through London, 
Cardan stopped with Sir John Cheke, the Professor of Greek at 
Cambridge, who was tutor to the young king. Six months 
previously, Edward had been attacked by measles and small-pox 
which had made his health even weaker than before. The 
king's guardians were especially anxious to know how long he 
would live, and they asked Cardan to cast Edward's nativity 
with particular reference to that point. 

The Italian was granted an audience in October, of which 
he wrote a full account in his diary, quoted in the Genitnrarum 
Exempla. The king, says hef, was "of a stature somewhat 
below the middle height, pale faced, with grey eyes, a grave 
aspect, decorous, and handsome. He was rather of a bad habit 
of body than a sufferer from fixed diseases, and had a somewhat 
projecting shoulder-blade." But, he continues, he was a boy of 
most extraordinary wit and promise. He was then but fifteen 
years old and he was already skilled in music and dialectics, a 
master of Latin, English, French, and fairly proficient in Greek, 
Italian, and Spanish. He " filled with the highest expectation 
every good and learned man, on account of his ingenuity and 
suavity of manners.... When a royal gravity was called for, you 
would think that it was an old man you saw, but he was bland 
and companionable as became his years. He played upon the 
lyre, took concern for public affairs, was liberal of mind, and in 
these respects emulated his father, who, while he studied to be 
[too] good, managed to seem bad." And in another place]: he 

* Luckily they left voluminous reports on the case and the proper treatment 
for it. The only point on which there was a general agreement was that the 
phlegm, instead of being expectorated, collected in his Grace's brains, and that 
thereby the operations of the intellect were impeded. Cardan was celebrated 
for his success in lung diseases, and his remedies were fairly successful in 
curing the asthma. His fee was 500 crowns for travelling expenses from Pavia, 
10 crowns a day, and the right to see other patients ; the archbishop actually 
gave him 2300 crowns in money and numerous presents in kind ; his fees from 
other persons during the same time must have amounted to about an equal sum 
(see Cardan's De Libris Propriis, ed. 1557, pp. 159—175; Consilia Medica, 
Opera, vol. ix, pp. 124—148 ; De Vita Propria, ed. 1557, pp. 138, 193 et seq.). 

t I quote from Morley's translation, vol. n, p. 135 et seq^. 

X Be Rerum Varietate, p. 285, 



CH. XVIIl] 



ASTROLOGY 



393 



describes him as '* that boy of wondrous hopes." At the close 
of the interview Cardan begged leave to dedicate to Edward a 
work on which he was then engaged. Asked the subject of the 
work, Cardan replied that he began by showing the cause of 
comets. The subsequent conversation, if it is reported correctly, 
shows good sense on the part of the young king. 




I have reproduced above a facsimile of Cardan's original 
drawing of Edward's horoscope. The horoscope was cast and 
read with unusual care. I need not quote the minute details 
given about Edward's character and subsequent career, but 
obviously the predictions were founded on the impressions 
derived from the above-mentioned interview. The conclusion 
about his length of life was that he would certainly live past 
middle age, though after the age of 55 years 3 months and 
17 days various diseases would fall to his lot*. 

In the following July the king died, and Cardan felt it 
necessary for his reputation to explain the cause of his error. 
The title of his dissertation is Quae post consider avi de eodemf. 
In effect his explanation is that a weak nativity can never be 

* Oeniturarum Excmpla, p. 19. 
t Ibid., p. 23. 



394 ASTROLOGY [CH. XVIII 

predicted from a single horoscope, and that to have ensured 
success he must have cast the nativity of every one with whom 
Edward had come intimately into contact; and, failing the 
necessary information to do so, the horoscope could be regarded 
only as a probable prediction. 

This was the argument usually offered to account for non- 
success. A better defence would have been the one urged by 
Raphael* and by Sou they f that there might be other planets 
unknown to the astrologer which had influenced the horoscope, 
but I do not think that medieval astrologers assigned this 
reason for failure. 

I have not alluded to the various adjuncts of the art, but 
astrologers so frequently claimed the power to be able to raise 
spirits that perhaps I may be pardoned for remarking that I 
believe some of the more important and elaborate of these 
deceptions were effected not infrequently by means of mirrors 
and lenses or perhaps by the use of a magic lantern, the pictures 
being sometimes thrown on to a fixed surface or a mirror and 
at other times on to a cloud of smoke which caused the images 
to move and finally disappear in a fantastic way capable of 
many explanations |. 

I would conclude by repeating again that though the 
practice of astrology was so often connected with impudent 
quackery, yet one ought not to forget that most physicians 
and men of science in medieval Europe were astrologers or 
believers in the art. These observers did not consider that its 
rules were definitely established, and they laboriously collected 
much of the astronomical evidence that was to crush it. Thus, 
though there never was a time when astrology was not practised 
by knaves, there was a period of intellectual development when 
it was honestly accepted as a difficult but a real science. 

* The Familiar Astrologer, London, 1832, p. 248. 
t The Doctor, chap. xcii. 

* See ex. gr. the life of Cellini, chap, xiii, Roscoe's translation, pp. 144— 
146. See also Sir David Brewster's Letters on Natural Magic, 



395 



CHAPTER XIX. 

CRYPTOGKAPHS AND CIPHERS. 

The art of constructing cryptographs or ciphers — intelligible 
to those who know the key and unintelligible to others — has 
been studied for centuries. Their usefulness on certain occasions, 
especially in time of war, is obvious, while their right interpre- 
tation may be a matter of great importance to those from whom 
the key is concealed. But the romance connected with the 
subject, the not uncommon desire to discover a secret, and the 
implied challenge to the ingenuity of all from whom it is hidden, 
have attracted to the subject the attention of many to whom 
its utility is a matter of indifference. 

Among the best known of the older authorities on the 
subject are J. Tritheim of Spanheim, G. Porta of Naples, 
G. Cardan, J. F. Niceron, J. Wilkins, and E. A. Poe. More 
modem writers are J. E. Bailey in the Encyclopaedia Britan- 
nica, E. B. von Wostrowitz of Vienna, 1881, F. Delastelle of 
Paris, 1902, and J. L. Kluber of Tubingen, 1809. My know- 
ledge, however, is largely the result of casual reading, and I 
prefer to discuss the subject as it has presented itself to me, 
with no attempt to make it historically complete. 

Most writers use the words cryptograph and cipher as 
synonymous. I employ them, however, with different mean- 
ings, which I proceed to define. 

A cr3rptograph may be defined as a manner of writing in 
which the letters or symbols employed are used in their normal 
sense, but are so arranged that the communication is intelligible 



896 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

only to those possessing the key: the word is also sometimes 
used to denote the communication made. A simple example 
is a communication in which every word is spelt backwards. 
Thus: 

ymene deveileh ot eh gniriter troper noitisop no ssorc daor. 

A cipher may be defined as a manner of writing by characters 
arbitrarily invented or by an arbitrary use of letters, words, or 
characters in other than their ordinary sense, intelligible only 
to those possessing the key: the word is also sometimes used 
to denote the communication made. A simple example is when 
each letter is replaced by the one that immediately follows it in 
the natural order of the alphabet, a being replaced by h, h by c, 
and so on, and finally 2 by a. In this cipher the above message 
would read: 

fofnz cfmjfwfe up cf sfujsjoh sfqpsu qptjujpo po dsptt spbe. 

In both cryptographs and ciphers the essential feature is 
that the communication may be freely given to all the world 
though it is unintelligible save to those who possess the key. 
The key must not be accessible to anyone, and if possible it 
should be known only to those using the cryptograph or 
cipher. The art of constructing a cryptograph lies in the 
concealment of the proper order of the essential letters or 
words : the art of constructing a cipher lies in concealing what 
letters or words are represented by the symbols used. 

In an actual communication cipher symbols may be arranged 
cryptographically, and thus further hinder a reading of the 
message. Thus the message given above might be put in a 
cryptographic cipher as 

znfofefwfjmfc pufc hojsjufs uspqfs opjujtpq op ttpsd ehps. 

If the message were written in a foreign language it would 
further diminish the chance of it being read by a strafnger 
through whose hands it passed. But I may confine myself to 
messages in English, and for the present to simple cryptographs 
and ciphers. 

A communication in cryptograph or cipher must be in 



CH. XIXj CRYPTOGRAPHS AND CIPHERS 397 

writing or in some permanent form. Thus to make small 
muscular movements — such, ex. gr., as talking on the fingers, 
or breathing long and short in the Morse dot and dash system, 
or making use of pre-arranged signs by a fan or stick, or 
flashing signals by light— do not here concern us. 

The mere fact that the message is concealed or secretly 
conveyed does not make it a cryptograph or cipher. The 
majority of stories dealing with secret communications are 
concerned with the artfulness with which the message is con- 
cealed or conveyed and have nothing to do with cryptographs 
or ciphers. Many of the ancient instances of secret communi- 
cation are of this type. Illustrations are to be found in 
messages conveyed by pigeons, or wrapped round arrows shot 
over the head of a foe, or written on the paper wrapping of a 
cigarette, or by the use of ink which becomes visible only when 
the recipient treats the paper on which it is written by some 
chemical or physical process. 

Again, a communication in a foreign language or in any 
recognized notation like shorthand is not an instance of a 
cipher. A letter in Chinese or Polish or Russian might be 
often used for conveying a secret message from one part of 
England to another, but it fails to fulfil our test that if 
published to all the world it would be concealed, unless sub- 
mitted to some special investigation. On the other hand, in 
practice, foreign languages or systems of shorthand which are 
but little known may serve to conceal a communication better 
than an easy cipher, for in the last case the key may be 
found with but little trouble, while in the other cases, though 
the key may be accessible, it is probable that there are only a 
few who know where to look for it. 

Gryj>tograj)hs. I proceed to enumerate some of the better 
known types of cryptographs. There are at least three distinct 
types. The types are not exclusive, and any particular crypto- 
graph may comprise the distinctive feature of two or all the 
types. 

First Type of Cryptographs (Transposition Type). A crypto- 
graph of the first type is one in which the successive letters 



398 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

or words of the message are re-arranged in some pre-determined 
manner. 

One of the most obvious cryptographs of this type is to 
write each word or the message itself backwards. Here is an 
instance in which the whole message is written backwards: 

tsop yh tnes tnemeergafo seniltuo smret ruo tpecca yeht. 

It is unnecessary to indicate the division into words by 
leaving spaces between them, and we might introduce capitals 
or make a pretence of other words, as thus : 

Ts opyhtne sine meer gafos eniltu Osmret ruot peccaye ht 

A recipient who was thus mis-led would be very careless. 
Preferably, according to modern practice, we should write 
the message in groups of five letters each: the advantage of 
such a division being that the number of such groups can be 
also communicated, and the casual omission of letters thus 
detected. 

Systems of this kind which depend on altering the places of 

letters or lines in some pre-arranged manner have always been 

common. One example is where the letters which make up 

the communication are written vertically up or down. Thus 

the message: The pestilence continues to increase, might be 

eiotnlit written in 8 columns as shown in the margin, 

sntioeth and then sent in five letter groups as eiotn 

acsncns e litsn, etc. If before reading off the message 

ereuecep. the 8 columns were interchanged according 

to some prearranged scheme the cryptograph would be greatly 

improved, and this is said to be a method used in the German 

Army. This cryptograph might be further obscured by writing 

the 32 letters according to the Route Method described below. 

Another method is to write successively the 1st, 18th, 35th 
letters of the original message, and then the 2nd, 19th, 36th 
letters, and so on. If, however, we know the clue number, say c, 
it is easy enough to read the communication. For if it divides 
into the number of letters n times with a remainder r it suffices 
to re-write the message in lines putting n + 1 letters in each 
of the first r lines, and n letters in each of the last c — r lines, 



cu. xix] 



CRYPl'OGRAPHS AND CIPHERS 



399 



and then the communication can be read by reading the columns 
downwards. For instance, if the following communication, 
containing 270 letters, were received: Ahtze ipqhg esoae ouazs 
esewa eqtmu sfdtb enzce sjteo ttqiz yczht zjioa rhqet tjrfe sftnz 
mroom ohyea rziaq neorn hreot lennk aerwi zesju asjod ezwjz 
zszjb rritt jnfjl weuzr oqyfo htqay eizsl eopji dihal oalhp epicrh 
eanaz srvli imosi adygt pekij scerq vvjqj qajqn yjint kaehs hhsnb 
goaot qetqe uuesa yqurn tpehq stzam ztqrj, and the clue number 
were 17 we should put 16 letters in each of the first 15 lines 
and IS letters in each of the last 2 lines. The communication 
could then be discovered by reading the columns downwards : 
the letters j, q and z marking the ends of words. 

A better cryptograph of this kind may be made by arranging 
the letters cyclically, and agreeing that the communication is to 
be made by selected letters, as, for instance, every seventh, second, 
seventh, second, and so on. Thus if the communication were 
Ammunition too low to allow of a sortie, which consists of 32 
letters, the successive significant letters would come in the 
order 7, 9, 16, 18, 25, 27, 2, 4, 13, 15, 24, 28, 5, 8, 20, 22, 1, 6, 
21, 26, 11, 14, 32, 10, 31, 12, 17, 23, 3, 29, 30, 19— the numbers 
being selected as in the decimation problem given above at 
the end of chapter I, and being struck out from the 32 cycle as 
soon as they are determined. The above communication would 
then read Ttrio oalmo laoon msueo awotn lioti fw. This is a 
good method, but it is troublesome to use, and for that reason 
is not to be recommended. 

In another cryptograph of this type, known as the Route 
Method, the words are left unaltered, but are re-arranged in a 



11 


8 


13 


2 


15 


4 


w 


X 


1 


10 


17 


6 


y 


z 


9 


12 7 


14 


3 


16 


5 



pre-determined manner. Thus, to take a very simple example, 
the words might be written in tabular form in the order shown 



400 



CRYPTOGRAPHS AND CIPHERS ^ [CH. XTX 



in the diagram, certain spaces being filled with dummy words 
X, 2/, 2^, ..., and the message being sent in the order 11, 8, 13, 2, 
15, 4, w, X, 1,.... This method was used successfully by the 
Federals in the American Civil War, 18G1 — 1865, equivalents 
for proper names being used. It is easy to work, but the 
key would soon be discovered by modem experts. 

A double cryptograph is said to have been used by the 
Nihilists in Russia from 1890 — 1900. Such double transpo- 
sitions are always awkward, and mistakes, which would make 
the message unintelligible, may be easily introduced, but if 
time is of little importance, and the message is unlikely to fall 
into the hands of any but ordinary officials, the concealment is 
fairly effective, though a trained specialist who had several 
messages in it could work out the key. 

Second Type of Cryptographs. A cryptograph of the second 
type is one in which the message is expressed in ordinary 
writing, but in it are introduced a number of dummies or non- 
significant letters or digits thus concealing which of the letters 
are relevant. 

One way of picking out those letters which are relevant 
is by the use of a perforated card of the shape of (say) a 
sheet of note-paper, which when put over such a sheet permits 
only such letters as are on certain portions of it to be visible. 
Such a card is known as a grille. An example of a grille with 
four openings is figured below. A communication made in this 



A 










B 

























































































way may be easily concealed irom anyone who does not possess 
a card of the same pattern. If the recipient possesses such 



CH. XIX] 



CRYITOGRAPHS AND CIPHERS 



401 



a card he htos only to apply it in order to read the message. 
This method was used by Richelieu. 

The use of the grille may be rendered less easy to detect 
if it be used successively in different positions, for instance, 
with the edges AB and CD successively put along the top of 
the paper containing the message. Below, for instance, is a 
message which, with the aid of the grille figured above, is at 
once intelligible. On applying the grille to it with the line 
AB along the top HK we get the first half of the communica- 
tion, namely, 1000 rifles se. On applying the grille with the 



981 


264 


070 


523 


479 


100 


NTT 


OKI 


SON 


SON 


AHY 


DTG 


BFS 


PUM 


OLT 


KFE 


LJO 


EGX 
WLA 


AEU 


QJT 


EGO 


FLE 


HVE 


FML 


AES 


REM 


REM 


ODA 
TIM 


SSE 


YZZ 


EPD 


QJC 


EKS 


OEF 



line CD along the top HK we get the rest of the message, 
namely, nt to L to-day. The other spaces in the paper are 
filled with non-significant letters or numerals in any way we 
please. Of course any one using such a grille would not divide 
the sheet of paper on which the communication was written 
into cells, but in the figure I have done so in order to render 
the illustration clearer. 

We can avoid the awkward expedient of having to use a 
perforated card, which may fall into undesired hands, by 
introducing a certain pre-arranged number of dummies or 
non-significant letters or symbols between those which make 
up the message. For instance, we might arrange that (say) 
only every alternate second and third letter shall be relevant. 
Thus the first, third, sixth, eighth, eleventh, &c., letters are those 
that make up the message. Such a communication would be 
two and a half times as long as the message, and this might 

B. R. 26 



402 



CRYPTOGRAPHS AND CIPHERS 



[CH. XIX 



be a great disadvantage if time in sending the message was of 
importance. 

Another method, essentially the same as the grille method, 
is to arrange that every nth word shall give the message, the 
other words being non -significant, though of course inserted as 
far as possible so as to make the complete communication run 
as a whole. But the difficulty of composing a document of this 
kind and its great length render it unsuitable for any purpose 
except an occasional communication composed at leisure and 
sent in writing. This method is said to have been used by 
the Earl of Argyle when plotting against James II. 

Third Type of Cryptographs. A kind of secret writing 
which may perhaps be considered to constitute a third type of 
cryptograph is a communication on paper which is legible only 
when the paper is folded in a particular way. An example is 
a message written across the edges of a strip of paper wrapped 
spiral-wise round a stick called a scytale. When the paper 
is unwound and taken off the stick the letters appear broken, 
and may seem to consist of arbitrary signs, but by wrapping 
the paper round a similar stick the message can be again 
read. This system is said to have been used by the Lace- 
demonians. The concealment can never have been effectual 
against an intelligent reader who got possession of the paper. 
As another illustration take the appended communication which 




e ..,. 'A f A 



3.1 



.^l 



I iigSWS85S5wi»SSSS8»gB«tag»gg»«»^^ag8ga^-"g'ig»5afi'Bgggg?S"^HB! 



I i 



is said to have been given to the Young Pretender during his 
wanderings after Culloden. If it be creased along the lines 
BB and OC (CO being along the second line of the second score), 
and then folded over, with B inside, so that the crease G lies 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 403 

over the line A (which is the second line of the first score) thus 
leaving only the top and bottom of the piece of paper visible, 
it will be found to read Conceal yourself, your foes look for you. 
I have seen what purports to be the original, but of the truth 
of the anecdote I know nothing, and the desirability qf con- 
cealing himself must have been so patent that it was kardly 
necessary to communicate it by a cryptograph. 

Ciphers. I proceed next to some of the more common types 
of ciphers. It is immaterial whether we employ special char- 
acters to denote the various letters; or whether we use the 
letters in a non-natural sense, such as the letter z for a, the 
letter y for 6, and so on. In the former case it is desirable to 
use symbols, for instance, musical notes, which are not likely to 
attract special notice. Geometrical figures have also been used 
for the same purpose. It is not even necessary to employ 
written signs. Natural objects have often been used, as in a 
necklace of beads, or a bouquet of flowers, where the different 
shaped or coloured beads or different flowers stand for different 
letters or words. An even more subtle form of disguising the 
cipher is to make the different distances between consecutive 
knots or beads indicate the different letters. Of all such 
systems we may say that a careful scrutiny shows that different 
symbols are being used, and as soon as the various symbols are 
distinguished one from the other no additional complication 
is introduced, while for practical purposes they give more 
trouble to the sender and the recipient than those written 
in symbols in current use. Accordingly I confine myself 
to ciphers written by the use of the current letters and 
numerals. There are four types of ciphers. 

First Type of Ciphers. Simple Substitution Alphabets. A 
cipher of the first type is one in which the same letter or word 
is always represented by the same symbol, and this symbol 
always represents the same letter or word. 

Perhaps the simplest illustration of a cipher of this type 
is to employ one language, written as far as practical in the 
alphabet of another language. It is said that during the 
Indian Mutiny messages in English, but written in Greek 

26—2 



404 CRYPTOGRAPHS AND CIPHERS [cH. XIX 

characters, were used freely, and successfully baffled the in- 
genuity of the enemy, into whose hands they fell. 

A common cipher of this type is made by using the actual 
letters of the alphabet, but in a non-natural sense as indicating 
other letters. Thus we may use each letter to represent the one 
immediately following it in the natural order of the alphabet — 
the letters being supposed to be cyclically arranged — a standing 
for h wherever it occurs, h standing for c, and so on, and finally 
z standing for a: this scheme is said to have been used by the 
Carthaginians and Romans. 

More generally we may write the letters of the alphabet in 
a line, and under them re-write the letters in any order we like. 
For instance 

ah c d efg hi j hi m n opqrstuvwxyz 
olkmazsqxeufy rthcwhvnidgjp 

In such a scheme, we must in our communication replace a 
by 0, b by l, etc. The recipient will prepare a key by re- 
arranging the letters in the second line in their natural order 
and placing under them the corresponding letter in the first 
line. Then whenever a comes in the message he receives 
he will replace it by an e; similarly he will replace b by s, 
and so on. 

A cipher of this kind is not uncommonly used in military 
signalling, the order of the letters being given by the use of 
a key word. Ciphers of this class were employed by the British 
forces in the Sudan and South African campaigns. If, for 
instance, Pretoria is chosen as the key word, we write the letters 
in this order, striking out any which occur more than once, 
and continue with the unused letters of the alphabet in their 
natural order, writing the whole in two liaes thus : 

p r et iab c dfg h 
z y xwvus qnml k j 

Then in using the cipher p is replaced by z and vice versa, 
r by y, and so on. A long message in such a cipher would be 
easily discoverable, but it is rapidly composed by the sender 
and read by the receiver, and for some purposes may be useful, 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 405 

especially if the discovery of the purport of the message is, 
after a few hours, immaterial. 

The key to ciphers of this type may usually be found by 
using tables of the normal frequency with which letters may be 
expected to occur. Such tables, and other characteristic features 
of the English, French, German, Italian, Dutch, Latin, and Greek 
languages, were given by D. A. Conrad in 1742*. His results 
have since been revised, and extended to Russian, Spanish, and 
other tongues. In English the percentage scale of frequency of 
the letters is approximately as follows: — e, 12 0; t, 9*4; a, 7*8 
0, 7-5; i 7-4; n, 73; s, 6'S; r, 59; h, 57; d, 39; I, 36; w, 3*0 
c, 2-8; m, 2-7; / 2*5; p, 1'9; g, IS; y, I'S; 6, 1*7; w, 1*7; v, I'l 
k, 0-6; j, 0-3; q, 03; x, 0-3; z, 02. The order of frequency for 
combinations of two letters is th, he, in, an, on, re, ti, er, it, nt, 
es, to, st; of three letters is the, ion, &c., &c.; of four letters is 
tion, that, &c., &c. ; and of double letters is tt, ss, &c., &c. Other 
peculiarities, such as that h, I, m, n, v, and y, when at the begin- 
ning of a word, must be followed by a vowel, that q must be 
followed by u and another vowel, have been classified and are 
important. I need not go here into further details. Unless, 
however, the message runs to 400 words or more, we cannot 
reasonably expect to find the scale of frequency the same as 
in Conrad's Table. 

In ciphers of this class it is especially important to avoid 
showing the division into words, for a long word may easily 
betray the secret. For instance, if the decipherer has reason 
to suspect that the message related to something connected 
with Birmingham, and he found that a particular word of ten 
letters had its second and fifth letters alike, as also its fourth 
and tenth letters, he would naturally see how the key would 
work if the word represented Birmingham, and on this hypo- 
thesis would at once know the letters represented by eight 
symbols. With reasonable luck this should suffice to enable 
him to tell if the hypothesis was tenable. To avoid this risk it 

• Gentleman^s Magazine, 1742, vol. xn, pp. 133—135, 185—186, 241—242, 
473—475. See also the Collected Works of E. A. Foe in 4 volumes, vol. i, p. 30 
et 8cg. 



406 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

is usual to send the cipher in groups of five letters, and, before 
putting it into cipher, to separate the words in the message by 
letters like j, q, x, z. 

Ciphers of this type suggest themselves naturally to those 
approaching the subject for the first time, and are commonly 
made by merely shifting the letters a certain number of places 
forward. If this is done we may decrease the risk of detection 
by altering the amount of shifting at short (and preferably 
irregular) intervals. Thus it may be agreed that if initially we 
shift every letter one place forward then whenever we come to 
the letter (say) n we shall shift every letter one more place 
forward. In this way the cipher changes continually, and is 
essentially changed to one of the third class ; but even with 
this improvement it is probable that an expert would decode a 
fairly long message without much difficulty. 

We can have ciphers for numerals as well as for letters: 
such ciphers are common in many shops. Any word or sentence 
containing ten different letters will answer the purpose. Thus, 
an old tradesman of my acquaintance used the excellent precept 
Be just Man — the first letter representing 1, the second 2, 
and so on. In this cipher the price 10/6 would be marked hnjt 
This is an instance of a cipher of the first type. 

Second Type of Ciphers. A cipher of the second type is one 
in which the same letter or word is, in some or all cases, repre- 
sented by more than one symbol, and this symbol always repre- 
sents the same letter or word. Such ciphers were uncommon 
before the Renaissance, but the fact that to those who held the 
key they were not more difficult to write or read than ciphers of 
the first type, while the key was not so easily discovered, led to 
their common adoption in the seventeenth century. 

A simple instance of such a cipher is given by the use of 
numerals to denote the letters of the alphabet. Thus a may 
be represented by 11 or by 37 or by 63, 6 by 12 or by 38 or 
by 64, and so on, and finally z by 36 or by 62 or by 88, while 
we can use 89 or 90 to signify the end of a word and the 
numbers 91 to 99 to denote words or sentences which con- 
stantly occur. Of course in practice no one would employ the 



CH. XIX] 



CRYPTOGRAPHS AND CIPHERS 



407 



numbers in an order like this, which suggests their meaning, 
but it will servo to illustrate the principle. 

The cipher can be improved by introducing after every (say) 
eleventh digit a non-significant digit. If this is done the 
recipient of the message must erase every twelfth digit before 
he begins to read the message. With this addition the difficulty 
of discovering the key is considerably increased. 

The same principle is sometimes applied with letters instead 
of numbers. For instance, if we take a word (say) of n letters, 
preferably all different, and construct a table as shown below 
of n" cells, each cell is defined by two letters of the key word. 
Thus, if we choose the word smoldng-cap we shall have 100 





8 


M 





K 


I 


N 


G 


M 


^ 


p 


s 

~M 


T 

G 

"c 

A 
P 


a 
k 
u 
e 
o 

y 

i 

8 


b 
1 

V 

f 

P 

z 

j 

t 


c 
m 

z 

8 

q 

a 
k 
u 


d 
n 

z 

~v 

r 
b 

T 

V 


e 



y 

i 

8 


m 
w 


{ 

P 
z 

J 
t 
d 
n 

z 


g 

q 

a 
'k 

XI 

e 



7 


h 
z 


i 

8 


m 
w 

g 

q 


i 

±_ 

d 
n 

z 

IT 

r 



cells, and each cell is determined uniquely by the two letters 
denoting its row and column. If we fill these cells in order 
with the letters of the alphabet we shall have a system similar 
to that explained above, where a will be denoted by ss or og 
or no, and so for the other letters. The last 22 cells may be 
used to denote the first 22 letters of the alphabet, or better, 
three or four of them may be used after the end of a word to 
show that it is ended, and the rest may be used to denote words 
or sentences which are likely to occur frequently. The statement 
in cipher however is twice as long as when it is in clear.' 

Like the similar cipher with numbers this can be improved 



408 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

by introducing after every mth letter any single letter which it 
is agreed shall be non-significant. To decipher a communication 
so written it is necessary to know the clue word and the clue 
number. 

Here for instance is a communication written in the above 
cipher with the clue word smoking-cap, and with 7 as the clue 
number: ngmks igrio icpss amcks cakqi gnass nxmig poasu 
iamno cmpam inscn ogcpn cisyi kskam sssgn nncae kknoo mkhsc 
pcmsc hgpng siaws sgigg ndiic a. In this sentence the letters 
denoting the 79th, 80th, 81st, and 82nd cells have been used to 
denote the end of a word, and no use has been made of the last 
18 cells. 

Another cipher of this type is made as follows. The sender 
and recipient of the message furnish themselves with identical 
copies of some book. In the cipher only numerals are used, 
and these numerals indicate the locality of the letters in 
the book. For example, the first letter in the communication 
might be indicated by 79-8-5, meaning that it is the 5th letter 
in the 8th line of the 79th page. But though secrecy might 
be secured, it would be very tedious to prepare or decode a 
message, and the method is not as safe as some of those de- 
scribed below. 

Another cipher of this type is obtained by the sender 
and receiver agreeing on some common book of reference and 
further on a number which, if desired, may be communicated 
as part of the message. To employ this method the page of 
the book indicated by the given number must be used. The 
first letter in it is taken to signify a, the next h, and so on — 
any letter which occurs a second time or more frequently 
being neglected. It may be also arranged that after n letters 
of the message have been ciphered, the next n letters shall be 
written in a similar cipher taken from the pih. following page 
of the book, and so on. Thus the possession of the code-book 
would be of little use to anyone who did not also know the 
numbers employed. It is so easy to conceal the clue number 
that with ordinary prudence it would be almost impossible for 
an unauthorized person to discover a message sent in this cipher. 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 409 

The clue number may be communicated indirectly in many 
ways. For instance, it may be arranged that the number to 
be used shall be the number sent, plus (say) g, or that the 
number to be used shall be an agreed multiple of the number 
actually sent. 

Third Type of Ciphers. Complex Shifting Alphabets. A 
cipher of the third type is one in which the same symbol 
represents sometimes one letter or word and sometimes 
another, 

A simple example, known as Gronf eld's Method, is the employ- 
ment of pre-arranged numbers in shifting forward the letters 
that make the communication. For instance, if we agree on 
the key number 6814, then the first letter in the communica- 
tion is replaced by the sixth letter which follows it in the 
natural order of the alphabet: for instance, if it were an 
a it would be replaced by g. The next letter is replaced by 
the eighth letter which follows it in the natural order of the 
alphabet: for instance, if it were an a it would be replaced by i. 
The next letter is replaced by the first after it; the next by 
the fourth after it; the next by the sixth; and so on to the 
end of the message. Of course to read the message the reci- 
pient would reverse the process. If the letters of the alphabet 
are written at uniform intervals along a ruler, and another ruler 
similarly marked with the digits is made to slide along it, the 
letter corresponding to the shifting of any given number of places 
can be read at once. Here is such a message : — Gisvg vumya 
vijnp vgzsi yhpjp woiy. Such ciphers are easy to make and 
read by those who have the key. But in recent years their 
construction has been subjected to critical analysis, and experts 
now can generally obtain the key number if the message contains 
80 or 100 words; an example of the way by which this is done is 
given below. It would be undesirable to allow the division into 
words to appear in the message, and either the words must be run 
on continuously, or preferably the less common letters j, q, z 
may be used to mark the division of words and the message 
then written in five letter groups. 

It is most important to conceal the number of digits in the 



410 CRYPTOGRAPHS AND ClPliEliS [CH. XIX 

key number. The difficulty of discovering the key number is 
increased if after every (say) with letter (or word) a non-significant 
letter is inserted. I suggest this as an improvement in the cipher. 

Here for instance is a communication written in this cipher 
with the clue numbers 4276 and 7: atj^zn hvaxu xhiep xafwg 
hzniy prpsi khdkz yyghq prgez uytlk obldi febzm xlpog quyit 
cmgxk ckuex vsqka ziagg sigay tnvvs styvu aslyw gjuzm csfct 
qhpwj vaepf xhihw pxiul txlav vtqzo xwkvt uvvfh cqbxn pvism 
phzmq tuwxj ykeev Itif. The recipient would begin by striking 
out every eighth letter. He would then shift back every letter 
4, 2, 7, 6, 4, 2, &c., places respectively, and in reading it, would 
leave out the letters j, q, and z as only marking the ends of words. 

With these modifications, this is an excellent cipher, and it has 
the additional merit of not materially lengthening the message. 
It can be rendered still more difficult by arranging that either 
or both the clue numbers shall be changed according to some 
definite scheme, and it may be further agreed that they shall 
change automatically every day or week. 

A similar system, now known as the St Cyr Method, was 
proposed by Wilkins *. He took a key word, such as prudentia, 
and constructed as many alphabets as there were letters in 
it, each alphabet being arranged cyclically and beginning 
respectively with the letters p, r, u, d, e, n, t, i, and a. He thus 
got a table like the following, giving nine possible letters 
which might stand for any letter of the alphabet. Using this 
we may vary the cipher in successive words or letters of the 
communication. Thus the message The prisoners have mutinied 
and seized the railway station would, according as the cipher 
changes in successive words or letters, read as Hwt fhziedvhi 
bupy pxwmqmhg erh ervmrq max zirteig station or as Hyy 
svvlwnthm lehx uuhzgmiq tvd gvcciq mqe frcoanr atpkci^r. 

The name by which the method is known is derived from 
the fact that it was taught at St Cyr under Napoleon. This 
system is said to have been widely employed by both armies in 
the Franco-German war in 1870 — 1871. The construction of 
military ciphers must be so simple that messages can be rapidly 
♦ Mercury, by J. Wilkins, London, 1641, pp. 59, 60, 



CH. XIX] 



CRYFIOGRAPHS AND CIPHERS 



411 



enciphered and decoded by non-experts: the St Cyr code fulfils 
this requirement. 



a 


b 


c 


d 


T 


f 


g 


h 


i 


k 


1 m 


n 





P 


q 


r 


8 


t 


u 


~ 


w 


T 


y 


z 


P 


q 


r 


8 


t 


u 


V 


w 


z 


y 


z 


a 


b 


c 


d 


e 


£ 


g 


h 


i 


k 


■i 


m 


n 





r 


8 


t 


U 


V 


w 


X 


y 


z 


a 


b 


c 


d 


e 


f 


g 


h 


i 


k 


' 


m 


n 





P 


q 


11 


V 


w 


X 


y 


z 


a 


b 


c 


d 


e 


f 


g 


li 


i 


k 


1 


m 


n 


o 


P 


q 


r 


s 


t 


d 


e 


f 


g 


h 


i 


k 


1 


m 


n 





P 


q 


r 


8; 


t 


u 


V 


w 


X 


y 


z 


•a 


b 





e 


f 


g' 


h 


i 


k 


1 


m 


u 





P 


q 


r 


8 


t 


a 


V 


w 


X 


y 


z 


a 


b 


c 


d 


n 


o 


P 


q 


r 


8 


t 


u 


V 


w 


X 


y 


z 


a 


b 


c 


d 


6 


f 


g 


h 


i 


k 


1 


m 


t 


u 


V 


w 


X 


y 


z 


a 


b 





d 


e 


f 


g 


h 


i 


k 


1 


m 


n 





P 


q 


r 


' s 


i 


k 


1 


m 


n 





P 


q 


r 


8 


t 


u 


V 


w 


X 


y 


z 


a 


b 


c 


d 


e 


f 


g 


h 


a 


b 


c 


d 


e 


f 


g 


h 


i 


k 


1 


in 


n 





P 


q 


r 


s 


t 


u 


V 


w 


X 


y 


z 



The St Cyr scheme is essentially the same as Gronfeld's. 
For instance, in the St Cyr system the key word gibe leads to 
the same result as the key number 6814 by Gronfeld's method. 
One advantage of the St Cyr system over that advocated by 
Gronfeld is that key words are more easily recollected than key 
numbers. Another advantage comes from the fact that the 
employment of words is equivalent to using 26 digits instead of 
10 : thus the key word kinr/s is equivalent to a number whose 
digits, from left to right, are 11, 9, 14, 7, 19. Messages in this 
cipher of any considerable length can be read by the same 
rules as are used to discover the key in Gronfeld's code. 
To hamper a decipherer I recommend the introduction, as 
in Gronfeld's method, of a non-significant letter after every 
with letter. 

The Beaufort Cipher, introduced in the British Navy by 
Admiral Beaufort in 1857, is of the St Cyr type. Its inventor 
thought it insoluble, but French writers have shown that no 
special difficulties occur in the discovery of the key word in the 
solution, though the analysis is tedious. 

A far better system of this kind is the Playfair Cipher. 
In this 25 cells arranged in the form of a square are filled by 
the letters of a key word such as Manchester (striking out any 



412 



CKYPTOGRAPHS AND CIPHERS 



[CH. XIX 



letter which occurs more than once) and the remaining letters 
of the alphabet, thus : — 



m 


a 


n 


c 


h 


e 


8 


t 


r 


b 


d 


f 


9 


i 


J 


k 


I 





P 


u 


V 


W 


X 


y 


z 



Only 25 cells are available, so k has been used for both h and q. 
The message is then divided into pairs of consecutive letters, 
but to prevent any pair consisting of the same two letters, a 
dummy letter like z is, when necessary, introduced. If both 
letters of a pair appear in the same vertical (or horizontal) line 
of the square, each of them is replaced by the letter in the 
square immediately to its right (or below it) — the letters in 
every line being treated as in cyclical order. If the letters in 
a pair do not appear in the same line in the square they must 
necessarily be at opposite angles of some rectangle, and they are 
replaced by those at the other angles of the rectangle, each by 
that which is in the same horizontal line. Thus the message 
will meet you at noon would first be written wi Iz Im ez et yo 
ua til oz on ; then be put in cipher as yf uiv ka bv sr xp Ih gt ux 
pc; and finally be sent as yfuwk abvsr xplhg tuxpc. It is curious 
that this cipher is not used more extensively, for the discovery of 
the key is difficult, even to specialists. 

Fourth Type of Ciphers. A cipher of the fourth type is one 
in which each letter is always represented by the same symbol, 
but more than one letter may be represented by the same 
symbol. Such ciphers were not uncommon at the beginning of 
the nineteenth century, and were usually framed by means of 
a key sentence containing about as many letters as there are 
letters in the alphabet. 



CH. XI X] CRYPTOGRAPHS AND CIPHERS 413 

Thus if the key phrase is The fox jumped over the garden 
gate, we write under it the letters of the alphabet in their 
usual sequence as shown below: 

The fox jumped over the garden gate. 

ah c d ef g h i j k I inn op qrs t u vw x y z ah c. 

Then we write the message replacing a hy t ov a, h hy h 
or t, c hy e, d by/, and so on. Here is such a message. 
M foemho nea ge eoo jmdhohg avf teg ev urne afrmeo. But it will 
be observed that in the cipher a may represent a ox % d may 
represent I ov w, e may represent c or k or a or s or x, g may 
represent t or z, h may represent 6 or r, o may represent e or 
m, r may represent p or v, and t may represent a or 6 or q. 
And the recipient, in deciphering it, must judge as best he can 
what is the right meaning to be assigned to these letters when 
they appear. 

An instance of a cipher of the fourth type is aflforded by 
a note sent by the Duchesse de Berri to her adherents in 
Paris, in which she employed the key phrase 

I e g u V e rn e me n t provisoire, 
ah cdefghijklmn opqrstuvxy. 

Hence in putting her message into cipher she replaced a by ^, 
6 by e, c by g, and so on. She forgot however to supply the 
key to the recipients of the message, but her friend Berryer 
had little difficulty in reading it by the aid of the rules I 
have indicated, and thence deduced the key phrase she had 
employed. 

Desiderata in Cryptographs and Ciphers. Having men- 
tioned various classes of cryptographs and ciphers, I may add 
that the shorter a message in cryptograph, the more easily it is 
read. On the other hand, the longer a message in cipher, the 
easier it is to get the key. In choosing a cipher for practical 
purposes, which will usually imply that it can be telegraphed 
or telephoned, we should seek for one in which only current 
letters, symbols, or words are employed ; such that its use does 
not unduly lengthen the message; such that the key to it can 
be reproduced at will and need not be kept in a form which 



414 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

might betray the secret to an unauthorized person; such that 
the key to it changes or can be changed at short intervals; 
and such that it is not ambiguous. Many ciphers of the 
second and third types fulfil these conditions; in particular 
the Gronfeld or St Cyr Method, or the Playfair Cipher, may be 
noted. A cipher written cryptographically, or a cryptograph 
written in cipher, or a cipher again enciphered by another 
process, is almost insoluble even by experts, unless accidents 
reveal something in the construction, but it is troublesome to 
make, and such elaborate processes are suited only for the study, 
where the time spent in making them up and deciphering them 
is not of much consequence. 

Cipher Machines. The use of instruments giving a cipher, 
which is or can be varied constantly and automatically, has 
been often recommended*. The possession of the key of the 
instrument as well as a knowledge of the clue word is necessary 
to enable anyone to read a message, but the risk of some instru- 
ment, when set, falling into unauthorized hands must be taken 
into account. Since equally good ciphers can be constructed 
without the use of mechanical devices I do not think their 
employment can be recommended. 

On the Solution of Cryptographs and Ciphers. Much in- 
genuity has been shown in devising means for reading messages 
written in cryptograph or cipher. It is a fascinating pursuit, 
but I can find space for only a few remarks about it. 

In such problems we must begin by deciding whether 
the message is a cryptograph or a cipher. If it is a combi- 
nation of both, the problem is one of extreme difficulty, and is 
likely to baffle anyone but a specialist, but such combinations 
are unusual, and most secret messages belong to one class or 
the other. 

If the scale of fi-equency of the letters agrees generally with 
Conrad's Table, presumably the message is in cryptograph, 

* See, for instance, the descriptions of those devised by Sir Charles Wheat- 
stone, given in his Scientific Papers, London, 1879, pp. 342—347; and by 
Capt. Bazeries in Comptes Rendus, Association Frangais pour V Avancemmt des 
Sciences, vol. xx (Marseilles), 1891, p. 160 et »eq. 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 415 

though we must allow for the possibility that dummy letters, 
like j, k, X, and z, have been introduced either to separate words 
or deliberately to confuse those not in the secret. A short 
sentence of this kind may be read by an amateur, but only an 
expert is likely to discover the key to a long and well con- 
structed cryptographic message. 

If the message is long enough, say about 80 words, and 
the scale of frequency of particular letters differs markedly 
from Conrad's scale, there is a presumption that the message is 
in cipher. If the numbers of the two scales agree generally, 
probably a simple substitution alphabet has been used, i.e. it is 
a cipher of the first type, and generally the discovery of the key 
is easy. If it is not a cipher of this type, we must next try to 
find whether it is of any of the other recognised types. The 
majority of other ciphers are included in Gronfeld's number 
(or the St Cyr word) system, and here I will confine myself to 
a discussion of how such ciphers may be read. 

The discovery of a key to a cipher of this kind is best 
illustrated by a particular case. I will apply the method to 
the message cisvg vumya vijnp vgzsi yhpjp woiy. This is an 
example of a Gronfeld's cipher with no additional complications 
introduced, but the message is short, and it so happens that the 
letters used are not in the normal scale of frequency ; yet it can 
be read with ease and certainty. 

The first thing is to try to find the number of digits in the 
key number. Now we notice that the pair of letters vg occurs 
twice, with an interval of 12. If in each case these represent 
the same pair of letters in the original message, the number of 
digits in the Jkey number must be 12 or a divisor of 12. Again 
the pair of letters ij occurs twice, with an interval of 8, and 
this suggests that the number of digits in the key number is 8 
or a divisor of 8. Accordingly we conjecture that the key 
number is one of either 2 or 4 digits: this conclusion is 
strengthened by noting the intervals between the recurrences 
of the same letters throughout the message. We may put 2 
on one side till after we have tried 4, for anyone using Gron- 
feld's method would be unlikely to employ a key number less 



416 CRYPTOGRAPHS AND CIPHKliS [cH. XIX 

than 100. Accordingly we first try 4, and if that fails try 2. 
Had no clue of this kind been obtained from the recurrence of 
a pair of letters, we should have had to try successively making 
the key numbers comprise 2, 3, 4, 5, . . . digits, but here (and in 
most messages) a cursory examination suggests the number of 
digits in the key number. We commence then by assuming 
provisionally that the key number has 4 digits. Accordingly 
we must now re-write our message in columns, each of 4 letters, 
giving altogether 4 lines, thus : 

g y 3 g y p y 

i V a n z b w 
s u V p s p 
V mi V i j i 

If the Gronfeld method was used, the letters in each of these 
lines were obtained from the corresponding letters in the original 
message by a simple substitution alphabet. Had the message 
been long we could probably obtain this alphabet at once by 
Conrad's Table. Here, however, the message is so short that the 
Table is not likely to help us decisively, and we must expect 
to be obliged to try several shifts of the alphabet in each line. 

In the first line y occurs three times, and g twice. Accord- 
ing to Conrad's Table, the most common letters in English are 
e, t, a, 0, i, n, s, r, h. Probably y stands for one of these and g 
for another. If y is made to stand successively for each of 
these, it is equivalent to putting every letter 6 places back- 
ward, where 6 is successively 20, 5, 24, 10, 16, 11, 6, 7, 17. 
Similarly, making g stand successively for e, t, a, o, i, n, s, r, h, 
we have 6 equal to 2, 13, 6, 18, 24, 19, 14, 15, 25. Altogether 
this gives us 16 systems for the representation of the first line. 
We might write these out on 16 slips, and provisionally reject any 
slip in which many unusual letters appear, but obviously, the 
most probable hypothesis is that where y stands for s, and g 
for a, both of which changes give ^ = 6, or that where y stands 
for a, and g for i, both of which changes give ^ = 24: these 
give for the first line either w, a, s, d, a, s, j, s, or e, i, a, I, 
if a, r, a. 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 417 

In the second line no letter occurs more than once, so we 
get no clue from Conrad's Table. This could not happen if the 
message were of any considerable length. 

In the third line 'p occurs twice, and s twice. Hence, as 
before, we must make p and s successively stand for the letters 
e, t, a, 0, 1, n, s, r, h. These give respectively 6 = 11, 22, 15, 1, 
7, 2, 23, 24, 8, and 6 = 14, 25, 18, 4, 10, 5, 0, 1, 11. Altogether 
this gives us 16 systems for the representation of this line. 
Obviously the most probable hypothesis is that where ^=11, 
p= e, s = h, or that where 6=1^ p = o, and s = r : these give for 
the third line either h, j, k, e, h, e, d, or r, t, u, o, r, o, n. 

In the fourth line i occurs three times. As before, make i 
stand successively for e, t, a, o, i, n, s, r, h. Of these the first, 
where ^ = 4, is the most probable. The slip corresponding to 
this is r, i, e, r, e, /, e. 

Now try combinations of these slips each in its proper line 
until, when we read the message in columns, we get the begin- 
ning of a word; if words appear in more than one column it is 
almost certain that we are right. We begin by taking the 
five slips which are indicated as being specially probable. The 
slip in the first line derived from ^ = 6, the slip in the third 
line derived from 6—1, and the slip derived from ^ = 4 in the 
fourth line give w . rra . tis . ued . ora . res , ofj . nes, and of course 
the solution is obvious. The key number was 6814, and the 
message is deciphered by using 6814 backwards. The corre- 
sponding St Cyr key word is gibe. The message was Warrant 
issued for arrest of Jones. 

If the combination of the slips is troublesome we can some- 
times get assistance by choosing those combinations which make 
the recurring pairs of letters (here vg and ij) represent pairs 
which occur in Conrad's Table. Also the occurrence of double 
letters in the cipher will often settle what combinations of slips 
are possible. 

It may be said that this is a tedious operation. Of course 
it is. Deciphering is bound to be troublesome, but a great 
deal of the work can be done by unskilled clerks working under 
the direction of experts. The longer the message, the fewer the 

B. a. 27 



418 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

slips we have to try, and had the above message been three 
times as long, we could have solved the problem with half the 
trouble. The above example was not complicated by employing 
dummy letters or artificial alphabets: their use increases the 
difficulty of the decipherer, but if the message is a long one, 
the difficulties are not insuperable. Specialists, especially if 
working in combination, are said to select the right methods 
with almost uncanny quickness. 

This chapter has already run to such a length that I cannot 
find space to describe more than one or two ciphers that appear 
in history. 

It is said that Julius Caesar in making secret memoranda 
was accustomed to move every letter four places forward, writing 
d for a, e for h, &c. This would be a very easy instance of a 
cipher of the first type, but it may have been effective at that 
time. His nephew Augustus sometimes used a similar cipher, 
in which each letter was moved forward one place*. 

Bacon proposed a cipher in which each letter was denoted 
by a group of five letters consisting of A and B only. Since 
there are 32 such groups, he had 6 symbols to spare, which 
he could use to separate words or to which he could assign 
special meanings. A message in this cipher would be five 
times as long as the original message. This may be compared 
with the far superior system of the five (or four) digit code- 
book system in use at the present time. 

In the Morse code employed in telegraphy, as in the 
Baconian system, only two signs are used, commonly a dot 
or a short mark or a motion to the left, and a dash or a 
long mark or a motion to the right. The Morse Alphabet is 

as follows: a ( ), h ( • • ), c ( — ), d { ),&{-), 

/( ).9{ ),h{----),i(--),i{ ),k{ ), 

l{.— .),m{ ),n(—),o( ),p{ ),2( ), 

»•(•—)- «(■••).«(-). «(— ). «( ). «' ( ). ^( ). 

* Of some of Caesar's correspondence, Suetonius says (cap. 56) si quis 
investigare et perscqui velit, quartam elenuntorum literam, id est, d pro a, et 
perinde reliquas covimutet. And of Augustus he says (cap. 88) quoties autemper 
notas scribit, b pro a, c pro &, ac deinceps eadem ratione, sequentes literas ponit; 
pro X autem duplex a. 



CH. xix] chyptographs and ciphers 419 

y ( ), z ( ). Since there are 30 possible per- 
mutations of two signs taken not more than four together, 

this leaves four signals unemployed, ( ), ( ), 

( ), ( ), which might have been utilized for special 

signals. In telegraphy there are also recognized signs or 
combinations for numerals, for the ends of words and messages, 
and for various calls between the sender and the recipient of 
a message. 

Charles I used ciphers freely in important correspondence 
— the majority being of the second type. He was foolish 
enough to take a cabinet, containing many confidential notes 
in cipher, with him to the field of Naseby, where they fell into 
the hands of Fairfax*. In these papers each letter was repre- 
sented by a number. Clues were provided by the King who 
had written over the number the letter which it represented. 
Thus in two letters written in 1643, a is represented by 17 or 
18, h by 13, c by 11 or 12, ci by 5, e by 7 or 8 or 9 or 10, 
/ by 15 or 16, g by 21, h by 31 or 32, i by 27 or 28, k by 25, 
I by 23 or 24, m by 42 or 44, n by 39 or 40 or 41, o by 35 or 
36 or 37 or 38, p by 33 or 34, r by 50 or 51 or 52, s by 47 or 48, 
t by 45 or 46, u by 62 or 63, w by 58, and y by 74 or 77. 
Numbers of three digits were used to represent particular 
people or places. Thus 148 stood for Francey 189 for the King, 
260 for the Queen, 354 for Prince Rupert, and so on. Further, 
there were a few special symbols, thus k\ stood for of, n\ for 
to, and /I for is. The numbers 2 to 4 and 65 to 72 were non- 
significant, and were to be struck out or neglected by the 
recipient of the message. Each symbol is separated from that 
which follows it by a full-stop. 

A similar, though less elaborate, system was used by the 
French in the Peninsular War. An excellent illustration of 
the inherent defects of this method is to be found in the 
writings of the late Sir Charles Wheatstone. A paper in 
cipher, every page of which was initialed by Charles I, 
and countersigned by Lord Digby, was purchased some years 

* First Eejaort of the Royal Commution on Historical Manuscripts, 1870, 
pp. 2, 4. 

27—2 



420 CRYPTOGRAPHS AND CIPHERS [CH. XiX 

ago by the British Museum. It was believed to be a state 
paper of importance. It consists of a series of numbers with- 
out any clue to their meaning, or any indication of a division 
between words. The task of reading it was rendered the more 
difficult by the supposition, which proved incorrect, that the 
document was in English ; but notwithstanding this, Sir Charles 
Wheatstone discovered the key *. In this cipher a was repre- 
sented by any of the numbers 12, 13, 14, 15, 16, or 17, h by 18, 
19, and so on, while some 65 special words were represented 
by particular numbers: in all about 150 different symbols were 
used. 

The famous diary of Samuel Pepys is commonly said to 
have been written in cipher, but in reality it is written in 
shorthand according to a system invented by T. Sheltonf. 
It is however somewhat difficult to read, for the vowels are 
usually omitted, and Pepys used some arbitrary signs for 
terminations, particles, and certain words — so far turning it 
into a cipher. Further, in certain places, where the matter is 
such that it can hardly be expressed with decency, he changed 
from English to a foreign language, or inserted non-significant 
letters. Shelton's system had been forgotten when attention 
was first attracted to the diary. Accordingly we may say that, 
to those who first tried to read it, it was written in cipher, but 
Pepy's contemporaries would have properly described it as being 
written in shorthand, though with a few modifications of his 
own invention. 

A system of shorthand specially invented for the purpose 
is a true cipher. Such a system in which the letters were 
represented by four strokes varying in length and position was 
employed by Charles I. Another such system in which each 
letter is represented either by a dot or by a line of constant 
length was used by the Earl of Glamorgan, better knovvn by his 
subsequent title of Marquess of Worcester, in 1645; each of these 

* The document, its translation, and the key used, are given in Wheatstone's 
Scientific Papers, London, 1879, pp. 321—341. 

t Tachy-graphy, by T. Shelton. The earliest edition I have seen is dated 
1641. A somewhat simOar system by W. Cartwright was issued by J, Rich 
under the title Semographief London, 1644. 



CH. XIX] CRYPTOGRAPHS AND CIPHERS 421 

was a cipher of the first type and had the defects inherent in 
almost every cipher of this kind: in fact Glamorgan's letter 
was deciphered, and the system was discovered by H. Dircks*. 
Obsolete systems of shorthand f may be thus used as ciphers. 

It is always difficult to read a very short message in cipher, 
since necessarily the clues are few in number. When the 
Chevalier de Rohan was sent to the Bastille, on suspicion of 
treason, there was no evidence against him except what might 
be extracted from Monsieur Latruaumont. The latter died 
without making any admission. De Rohan's friends had ar- 
ranged with him to communicate the result of Latruaumont's 
examination, and accordingly in sending him some fresh body 
linen they wrote on one of the shirts Mg dulhxcclgu ghj yxuj, 
Im ct ulgc alj. For twenty-four hours de Rohan pored over 
the message, but, failing to read it, he admitted his guilt, and 
was executed November 27, 1674. The cipher is a simple one 
of the first type, but the communication is so short that unless 
the key were known it would not be easy to read it. Had 
de Rohan suspected that the second word was prisonnier, it 
would have given him 7 out of the 12 letters used, and as the 
first and third words suggest the symbols used for I and ty he 
could hardly have failed to read the message. 

Marie Antoinette used what was in effect a St Cyr cipher, 
consisting of 11 substitution alphabets employed in succession. 
The first alphabet was n.o,^^ z, a, 6, Z, m; the next, 

0, p, q, m, n; the next p, q, r, n, o; and so on. An 

expert would easily read a message in this cipher. 

One of the systems in use to-day is the five digit code- 
book cipher, to which I have already alluded. In this, a code 
dictionary is prepared in which every word likely to be used 
is printed, and the words are numbered consecutively 00000, 

00001, ... up, if necessary, to 99999. Thus each word is 

• Life of the Marquis of Worcester by H. Dircks, London, 1865. Worcester's 
system of shorthand was described by him in his Century of Inventiom, London, 
1663, sections 3, 4, 5. 

••• Various systems, including those used in clflsaical and medieval times, are 
described in the History of Shorthand by T. Anderson, Lon^lon, 1882. 



422 CRYPTOGRAPHS AND CIPHERS [CH. XIX 

represented by a number of five digits, and there are 10'' such 
numbers available. The message is first written down in words. 
Below that it is written in numbers, each word being replaced 
by the number corresponding to it. To each of these numbers 
is added some definite pre-arranged clue number — the words in 
the dictionary being assumed to be arranged cyclically, so that 
if the resulting number exceeds 10^ it is denoted onl}^ by the 
excess above 10^ The resulting numbers are sent as a message. 
On receipt of the message it is divided into consecutive groups 
of five numbers, each group representing a word. From each 
number is subtracted the pre-arranged clue number, and then 
the message can be read off by the code dictionary. If and 
when such a message is published, the construction of the 
sentences is usually altered before publication, so that the key 
may not be discoverable by anyone in possession of the code- 
book or who has seen the cipher message. This is a rule 
applicable to all cryptographs and ciphers. 

This is a cipher with 10^ symbols, and as each symbol 
consists of five digits, a message of n words is denoted by 5n 
digits, and probably is not longer than the message when 
written in the ordinary way. Since however the number of 
words required is less than 10^ the spare numbers may be 
used to represent collocations of words which constantly occur, 
and if so the cipher message may be slightly shortened. 

If the clue number is the same all through the message it 
would be possible by not more than 10^ trials to discover the 
message. This is not a serious risk, but, slight though it is, it 
can be avoided if the clue number is varied; the clue number 
might, for instance, be 781 for the next three words, 791 for the 
next five words, 801 for the next seven words, and so on. 
Further it may be arranged that the clue numbers shall be 
changed every day; thus on the seventh day of the month they 
might be 781, 791, &c., and on the eighth day 881, 891, &c., 
and so on. 

This cipher can however be further improved by inserting 
at some step, say after each mth digit, an unmeaning digit. 
For example, if, in the original message written in numbers, we 



CH. Xix] CRYPTOGRAPHS AND CIPHERS 423 

insert a 9 after every seven digits we shall get a collection of 
words (each represented by five digits), most of which would 
have no connection with the original message, and probably 
the number of digits used in the message itself would no longer 
be a multiple of 5. Of course the receiver has only to reverse 
the process in order to read the message. 

It is however unnecessary to use five symbols for each word. 
For if we make a similar code with the twenty-six letters of 
the alphabet instead of the ten digits, four letters for each word 
or phrase would give us 26*, that is, 456976 possible variations. 
Thus the message would be shorter and the power of the code 
increased. Further, if we like to use the ten digits and the 
twenty-six letters of the alphabet — all of which are easily 
telegraphed — we could, by only using three symbols, obtain 36', 
that is, 46656 possible words, which would be sufficient for all 
practical purposes. 

This code, at any rate with these modifications, is unde- 
cipherable by strangers, but it has the disadvantages that those 
who use it must always have the code dictionary available, and 
that it takes a considerable time to code or decode a com- 
munication. For practical purposes its use would be confined 
to communications which could be deciphered at leisure in an 
office. 



NOTE. Mr C. H. Harrison has pointed out that an objection to the Play- 
fair system described above on pages 411 — 412 is that if a particular word is 
repeated its central letters can appear in the cipher in only two forms. If this 
word is a long one and it is guessed correctly the key-word can generally be 
found. 

Mr Harrison has devised a slide-rule cipher of the complex shifting substitu- 
tion type but such that the recurrence of the shifts is non-cyclical. I have not 
space to describe it here, but it seems safer than most of those mentioned in 
this chapter, and its use, in an oflBce with intelligent clerks, would present no 
difficulty. I am not however convinced that it would be easier or safer to use 
than a simple cryptographic cipher. (See p. 396.) 



4^24 



CHAPTER XX. 

HYPER-SPACE*. 

I propose to devote the remaining pages to the considera- 
tion, from the point of view of a mathematician, of certain 
properties of space, time, and matter, and to a sketch of some 
hypotheses as to their nature. Philosophers tell us that space, 
time, and matter are the "categories under which physical 
phenomena are concerned." They cannot be defined, and such 

* On the possibility of the existence of space of more than three dimensions 
see C. H. Hinton, Scientific Romances, London, 1886, a most interesting work, 
from which I have derived much assistance in compiling the earlier part of this 
chapter; his later work, The Fourth Dimension, London, 1904, may be also 
consulted. See also G. F. Eodwell, Nature, May 1, 1873, vol. viii, pp. 8, 9 ; 
and E. A. Abbott, Flatland, London, 1884. 

On Non-Euclidean geometry, see chapter xiv above. The theory is due 
primarily to N. L Lobatschewsky, Geometrische Untersuchungen zur Theorie dcr 
Parallellinien, Berlin, 1840 (originally given in a lecture in 1826) ; to 0. F. Gauss 
{ex. gr. letters to Schumacher, May 17, 1831, July 12, 1831, and Nov. 28, 1846, 
printed in Gauss's collected works); and to J. Bolyai, Appendix to the first 
volume of his father's Tentamen, Maros-Vasarkely, 1832; though the subject 
had been discussed by J. Saccheri as long ago as 1733 : its development was 
mainly the work of G. F. B. Kiemann, Ueber die Hypothesen welche dcr Geo- 
metrie zu Grunde liegen, written in 1854, Gottinger Abhandlungen, 1866-7, vol. 
xin, pp. 131 — 152 (translated in Nature, May 1 and 8, 1873, vol. vin, pp. 14 — 17, 
36—37); H. L. F. von Helmholtz, Gottinger Nachrichten, June 3, 1868, pp. 193 
— 221 ; and E.Beltrami, Saggio di Interpretazione della Geomctria non-Euclidea, 
Naples, 1868, and the Annali di Matematica, series 2, vol. n, pp. 232 — 255 : see 
an article by von Helmholtz in the Academy, Feb. 12, 1870, vol. i, pp. 128 — 
131. In recent years the theory has been treated by several mathematicians. 

On hyper-space, see V. Schlegel, Enseignement Mathematique, Paris, vol. n, 
1900 ; and D. M. Y. Sommerville, Bibliography of Non-Euclidean Geometry, 
St Andrews, 1911. 



CH. XX] HYPER-SPACE 425 

explanations of them as have been offered involve difficulties of 
the highest order and are far from simplifying our conceptions 
of them. I shall not discuss the metaphysical theories that 
profess to account for the origin of our conceptions of them, 
for these theories rest on assertions which are incapable of 
definite proof— a foundation which does not commend itself 
to a scientific student. The means of measuring space, time, 
and mass, and the investigation of their properties fall within 
the domain of mathematics. 

I devote this chapter to considerations connected with space, 
leaving the subjects of time and mass to the following two 
chapters. 

I confine my remarks to two speculations which recently 
have attracted considerable attention. These are (i) the possi- 
bility of the existence of space of more than three dimensions, 
and (ii) the possibility of kinds of geometry, especially of two 
dimensions, other than those which are treated in the usual 
text-books: some aspects of the latter question have been 
already considered in chapter xiii. These problems are related. 
The term hyper-space was used originally of space of more 
than three dimensions, but now it is often employed to denote 
also any non-Euclidean space. I attach the wider meaning to 
it, and it is in that sense that this chapter is on the subject of 
hyper-space. 

In regard to the first of these questions, the conception of 
a world of more than three dimensions is facilitated by the fact 
that there is no difficulty in imagining a world confined to only 
two dimensions — which we may take for simplicity to. be a 
plane, though equally well it might be a spherical or other 
surface. We may picture the inhabitants of flatland as moving 
either on the surface of a plane or between two parallel and 
adjacent planes. They could move in any direction along the 
plane, but they could not move perpendicularly to it, and would 
have no consciousness that such a motion was possible. We 
may suppose them to have no thickness, in which case they 
would be mere geometrical abstractions ; or, preferably, we may 



426 HYPER-SPACE [CH. XX 

think of them as having a small but uniform thickness, in which 
case they would be realities. 

Several writers have amused themselves by expounding and 
illustrating the conditions of life in such a world. To take 
a very simple instance* a knot is impossible in flatland, a simple 
alteration which alone would make some difference in the 
experience of the inhabitants as compared with our own. 

If an inhabitant of flatland was able to move in three 
dimensions, he would be credited with supernatural powers by 
those who were unable so to move; for he could appear or 
disappear at will, could (so far as they could tell) create matter 
or destroy it, and would be free from so many constraints to 
which the other inhabitants were subject that his actions would 
be inexplicable to them. 

We may go one step lower, and conceive of a world of one 
dimension — like a long tube — in which the inhabitants could 
move only forwards and backwards. In such a universe there 
would be lines of varying lengths, but there could be no 
geometrical figures. To those who are familiar with space of 
higher dimensions, life in line-land ^vould seem somewhat dull. 
It is commonly said that an inhabitant could know only two 
other individuals; namely, his neighbours, one on each side. 
If the tube in which he lived was itself of only one dimension, 
this is true ; but we can conceive an arrangement of tubes in 
two or three dimensions, where an occupant would be conscious 
of motion in only one dimension, and yet which would permit 
of more variety in the number of his acquaintances and con- 
ditions of existence. 

Our conscious life is in three dimensions, and naturally the 
idea occurs whether there may not be a fourth dimension. No 
inhabitant of flatland could realize what life in three dimensions 

* It is obvious that a knot cannot be tied in space of two dimensions. As 
long ago as 1876, F. C. Klein showed that knots cannot exist in space of four 
dimensions ; see Mathematische Annalen^ Leipzig, 1876, vol. ix, p. 478. It is 
not easy to give a definition of a knot in hyper- space, but, taking it in its 
ordinary sense, it would seem that it is only in space of three dimensions that 
knots can be tied in strings : see D. M. Y. Sommerville, Messenger of Mathe- 
matics, N.S., vol. xxxvi, 1907, pp. 139 — 144. 



CH. XX] HYPER-SPACE 427 

would mean, though, if he evolved an analytical geometry 
applicable to the world in which he lived, he might be able to 
extend it so as to obtain results true of that world in three 
dimensions which would be to him unknown and inconceivable. 
Similarly we cannot realize what life in four dimensions is like, 
though we can use analytical geometry to obtain results true of 
that world, or even of worlds of higher dimensions. Moreover 
the analogy of our position to the inhabitants of flatland enables 
us to form some idea of how inhabitants of space of four 
dimensions would regard us. 

Just as the inhabitants of flatland might be conceived as 
being either mere geometrical abstractions, or real and of a 
uniform thickness in the third dimension, so, if there is a fourth 
dimension, we may be regarded either as having no thickness 
in that dimension, in which event we are mere (geometrical) 
abstractions — as indeed idealist philosophers have asserted to 
be the case — or as having a uniform thickness in that dimension, 
in which event we are living in four dimensions although we 
are not conscious of it. In the latter case it is reasonable to 
suppose that the thickness in the fourth dimension of bodies in 
our world is small and possibly constant; it has been conjectured 
also that it is comparable with the other dimensions of the 
molecules of matter, and if so it is possible that the constitution 
of matter and its fundamental properties may supply experi- 
mental data which will give a physical basis for proving or 
disproving the existence of this fourth dimension. 

If we could look down on the inhabitants of flatland we 
could see their anatomy and what was happening inside them. 
Similarly an inhabitant of four-dimensional space could see 
inside us. 

An inhabitant of flatland could get out of a room, such as 
a rectangle, only through some opening, but, if for a moment 
he could step into three dimensions, he could reappear on the 
other side of any boundaries placed to retain him. Similarly, 
if we came across persons who could move out of a closed 
prison-cell without going through any of the openings in it, 
there might be some reason for thinking that they did it by 



428 HYPER-SPACE [CH. XX 

passing first in the direction of the fourth dimension and then 
back again into our space. This however is unknown. 

Again, if a finite solid was passed slowly through flatland, 
the inhabitants would be conscious only of that part of it which 
was in their plane. Thus they would see the shape of the 
object gradually change and ultimately vanish. In the same 
way, if a body of four dimensions was passed through our space, 
we should be conscious of it only as a solid body, namely, the 
section of the body by our space, whose form and appearance 
gradually changed and perhaps ultimately vanished. It has 
been suggested that the birth, growth, life, and death of animals 
may be explained thus as the passage of finite four- dimensional 
bodies through our three-dimensional space. I believe that this 
idea is due to Hinton. 

The same argument is applicable to all material bodies. 
The impenetrability and inertia of matter are necessary conse- 
quences ; the conservation of energy follows, provided that the 
velocity with which the bodies move in the fourth dimension is 
properly chosen: but the indestructibility of matter rests on 
the assumption that the body does not pass completely through 
our space. I omit the details connected with change of density 
as the size of the section by our space varies. 

We cannot prove the existence of space of four dimensions, 
but it is interesting tp enquire whether it is probable that such 
space actually exists. To discuss this, first let us consider how 
an inhabitant of flatland might find arguments to support the 
view that space of three dimensions existed, and then let us see 
whether analogous arguments apply to our world. I commence 
with considerations based on geometry and then proceed to 
those founded on physics. 

Inhabitants of flatland would find that they could have two 
triangles of which the elements were equal, element to element, 
and yet which could not be superposed. We know that the 
explanation of this fact is that, in order to superpose them, one 
of the triangles would have to be turned over so that its under- 
surface came on to the upper side, but of course such a movement 
would be to them inconceivable. Possibly however they might 



CH. XX] HYPER-SPACE 429 

have suspected it by noticing that inhabitants of one-dimensional 
space might experience a similar difficulty in comparing the 
equality of two lines, ABC and CB'A\ each defined by a set of 
three points. We may suppose that the lines are equal and 
such that corresponding points in them could be superposed by 
rotation round G — a movement inconceivable to the inhabitants 
— but an inhabitant of such a world in moving along from A to 
A' would not arrive at the corresponding points in the two 
lines in the same relative order, and thus might hesitate to 
believe that they were equal. Hence inhabitants of flatland 
might infer by analogy that by turning one of the triangles over 
through three-dimensional space they could make them coincide. 

We have a somewhat similar difficulty in our geometry. 
We can construct triangles in three dimensions — such as two 
spherical triangles — whose elements are equal respectively one 
to the other, but which cannot be superposed. Similarly we 
may have two helices whose elements are equal respectively, 
one having a right-handed twist and the other a left-handed 
twist, but it is impossible to make one fill exactly the same 
parts of space as the other does. Again, we may conceive of 
two solids, such as a right hand and a left hand, which are 
exactly similar and equal but of which one cannot be made to 
occupy exactly the same position in space as the other does. 
These are difficulties similar to those which would be experi- 
enced by the inhabitants of flatland in comparing triangles; 
and it may be conjectured that in the same way as such 
difficulties in the geometry of an inhabitant in space of one 
dimension are explicable by temporarily moving the figure into 
space of two dimensions by means of a rotation round a point, 
and as such difficulties in the geometry of flatland are explicable 
by temporarily moving the figure into space of three dimensions 
by means of a rotation round a line, so such difficulties in our 
geometry would disappear if we could temporarily move our 
figures into space of four dimensions by means of a rotation 
round a plane — a movement which of course is inconceivable to us. 

Next wo may enquire whether the hypothesis of our exist- 
ence in a space of four dimensions affords an explanation of 



430 HYPKR-SPACE [CH. XX 

any difficulties or apparent inconsistencies in our physical 
science*. The current conception of the luminiferous ether, 
the explanation of gravity, and the fact that there are only a 
finite number of kinds of matter, all the atoms of each kind 
being similar, present such difficulties and inconsistencies. To 
see whether the hypothesis of a four-dimensional space gives 
any aid to their elucidation, we shall do best to consider first 
the analogous problems in two dimensions. 

We live on a solid body, which is nearly spherical, and 
which moves round the sun under an attraction directed to it. 
To realize a corresponding life in flatland we must suppose that 
the inhabitants live on the rim of a (planetary) disc which 
rotates round another (solar) disc under an attraction directed 
towards it. We may suppose that the planetary world thus 
formed rests on a smooth plane, or other surface of constant 
curvature ; but the pressure on this plane and even its existence 
would be unknown to the inhabitants, though they would be 
conscious of their attraction to the centre of the disc on which 
they lived. Of course they would be also aware of the bodies, 
solid, liquid, or gaseous, which were on its rim, or on such points 
of its interior as they could reach. 

Every particle of matter in such a world would rest on this 
plane medium. Hence, if any particle was set vibrating, it 
would give up a part of its motion to the supporting plane. 
The vibrations thus caused in the plane would spread out in 
all directions, and the plane would communicate vibrations to 
any other particles resting on it. Thus any form of energy 
caused by vibrations, such as light, radiant heat, electricity, 
and possibly attraction, could be transmitted from one point to 
another without the presence of any intervening medium which 
the inhabitants could detect. 

If the particles were supported on a uniform elastic plane 
film, the intensity of the disturbance at any other point would 
vary inversely as the distance of the point from the source of 

* See a note by myself in the Messenger of Mathematics, Cambridge, 1891, 
vol. XXI, pp. 20 — 24, from which the above argument is extracted. The question 
has been treated by Hinton on similar lines. 



Cn. XX] HYPER-SPACE 431 

disturbance; if on a uniform elastic solid medium, it would 
vary inversely as the square of that distance. But, if the 
supporting medium was vibrating, then, wherever a particle 
rested on it, some of the energy in the plane would be given 
up to that particle, and thus the vibrations of the intervening 
medium would be hindered when it was associated with 
matter. 

If the inhabitants of this two-dimensional world were 
sufficiently intelligent to reason about the manner in which 
energy was transmitted they would be landed in a difficulty. 
Possibly they might be unable to explain gravitation between 
two particles — and therefore between the solar disc and their 
disc — except by supposing vibrations in a rigid medium between 
the two particles or discs. Again, they might be able to detect 
that radiant light and heat, such as the solar light and heat, 
were transmitted by vibrations transverse to the direction from 
which they came, though they could realize only such vibrations 
al were in their plEine, and they might determine experimentally 
that in order to transmit such vibrations a medium of great 
rigidity (which we may call ether) was necessary. Yet in both 
the above cases they would have also distinct evidence that 
there was no medium capable of resisting motion in the space 
around them, or between their disc and the solar disc. The 
explanation of these conflicting results lies in the fact that their 
universe was supported by a plane, of which they were necessarily 
unconscious, and that this rigid elastic plane was the ether 
which transmitted the vibrations. 

Now suppose that the bodies in our universe have a uniform 
thickness in the fourth dimension, and that in that direction 
our universe rests on a homogeneous elastic body whose thick- 
ness in that direction is small and constant. The transmission 
of force and radiant energy, without the intervention of an 
intervening medium, may be explained by the vibrations of 
the supporting space, even though the vibrations are not them- 
selves in the fourth dimension. Also we should find, as in 
fact we do, that the vibrations of the luminiferous ether are 
hindered when it is associated with matter. I have assumed 



432 HYPER-SPACE [CH. XX 

that the thickness of the supporting space is small and uniform, 
because then the intensity of the energy transmitted from a 
source to any point would vary inversely as the square of the 
distance, as is the case ; vi^hereas if the supporting space was a 
body of four dimensions, the law would be that of the inverse 
cube of the distance. 

The application of this hypothesis to the third difficulty 
mentioned above — namely, to show why there are in our universe 
only a finite number of kinds of atoms, all the atoms of each 
kind having in common a number of sharply defined properties — 
will be given later*. 

Thus the assumption of the existence of a four-dimensional 
homogeneous elastic body on which our three-dimensional 
universe rests, affords an explanation of some difficulties in 
our physical science. 

It may be thought that it is hopeless to try to realize a 
figure in four dimensions. Nevertheless attempts have been 
made to see what the sections of such a figure would look 
like. 

If the boundary of a solid is {x, y, z) = 0, we can obtain 
some idea of its form by taking a series of plane sections by 
planes parallel to ^ = 0, and mentally superposing them. In four 
dimensions the boundary of a body would be ^ (x, y, z, w) = 0, 
and attempts have been made to realize the form of such a 
body by making models of a series of solids in three dimensions 
formed by sections parallel to w = Q. Again, we can represent 
a solid in perspective by taking sections by three co-ordinate 
planes. In the case of a four-dimensional body the section 
by each of the four co-ordinate solids will be a solid, and 
attempts have been made by drawing these to get an idea of 
the form of the body. Of course a four-dimensional body will 
be bounded by solids. 

The possible forms of regular bodies in four dimensions, 
analogous to polyhedrons in space of three dimensions, have 
been discussed by Stringhamf". 

* See below, p. 475. 

t American Journal oj Mathematics ^ 1880, vol. iii, pp. 1 — 14. 



CII. XX] HYPER-SPACE 433 

I now turn to the second of the two problems mentioned 
at the beginning of the chapter: namely, the possibility of 
there being kinds of geometry other than those which are 
treated in the usual elementary text-books. This subject is so 
technical that in a book of this nature I can do little more than 
give a sketch of the argument on which the idea is based. 

The Euclidean system of geometry, with which alone most 
people are acquainted, rests on a number of independent axioms 
and postulates. Those which are necessary for Euclid's geometry 
have, within recent years, been investigated and scheduled. 
They include not only those explicitly given by him, but some 
others which he unconsciously used. If these are varied, or 
other axioms are assumed, we get a different series of propo- 
sitions, and any consistent body of such propositions constitutes 
a system of geometry. Hence there is no limit to the number 
of possible non-Euclidean geometries that can be constructed. 

Among Euclid's axioms and postulates is one on parallel 
lines, which is usually stated in the form that if a straight 
line meets two straight lines, so as to make the sum of the two 
interior angles on the same side of it less than two right angles, 
then these straight lines being continually produced will at 
length meet upon that side on which are the angles whose sum 
is less than two right angles. Expressed in this form the axiom 
is far from obvious, and from early times numerous attempts 
have been made to prove it. All such attempts failed, and it is 
now known that the axiom cannot be deduced from the other 
axioms assumed by Euclid. I have already discussed this 
question in chapter xiii, and I do not propose to add here 
anything more. The conclusion was that three consistent 
systems of geometry could be constructed, termed respectively 
hyperbolic, parabolic or Euclidean, and elliptic. These are dis- 
tinguished from one another according as no straight line (that 
is, a geodetic line), or only one straight line, or a pencil of 
straight lines can be drawn through a point parallel to a given 
straight line. 

To work out a body of propositions relating to figures on a 
surface (that is, a two-dimensional space) analogous to that given 

B. R. 28 



434 HYPER-SPACE [CH. XX 

by Euclid relating to figures drawn on a plane, it is necessary 
that it should be possible at any point on the surface to con- 
struct a figure congruent to a given figure ; this is equivalent to 
saying that if we take up a triangle drawn anywhere on the 
surface and move it to another part of the surface, it will lie 
flat on the surface there. This is so only if the measure of 
curvature at every point of the surface is constant. Such 
surfaces of constant curvature are spherical surfaces, where the 
product is positive; plane surfaces, where it is zero; pseudo- 
spherical surfaces, where it is negative. A tractroid, that is, a 
figure produced by the revolution of a tractrix about its asymp- 
tote, is an example of a pseudo-spherical surface ; it is saddle- 
shaped at every point. Hence on spheres, planes, and tractroids 
we can construct these systems of geometry. And these 
systems are respectively examples of hyperbolic, Euclidean, and 
elliptic geometries. 

Moreover if any surface is bent without dilation or contrac- 
tion, the measure of curvature remains unaltered. Thus from 
these three surfaces we can form others on which congruent 
figures, and therefore consistent systems of geometry, can be 
constructed. For instance, a plane can be rolled into a cone or 
cylinder, and the system of geometry on a conical or cylindrical 
surface will be similar to that on a plane. Similarly a hemi- 
sphere can be rolled up into a sort of spindle, and the system of 
geometry on such a spindle will be similar to that on a sphere. 
In fact there are three kinds of surfaces of constant positive 
curvature, which are respectively spherical, spindle-shaped, and 
bolster-shaped, and on each of these a system of hyperbolic 
geometry can be constructed. So too there are three kinds of 
surfaces of constant negative curvature. 

Throughout this discussion I have tacitly assumed that the 
measure of distance employed remains the same wherever it is 
employed. If this is not so, we may evolve in plane space 
non-Euclidean geometries which are not inconsistent with ex- 
perience. Suppose, to take one example, that a foot-rule shrunk 
as it was moved away from some point of the plane — as it 
might do by a fall of temperature. Then a distance, which we 



CH. XX] HYPER-SPACE 435 

should describe as finite, might wlien measured by this rule 
appear to be infinite, since repeated applications of the ever- 
Bhortening rule would not cover it. Thus the boundary of what 
we should describe as a finite area round the point would to 
those who were confined to the area of this foot-rule appear to 
be infinitely distant from the point. If the law of shrinking be 
properly chosen, the geometry of figures in this area would be 
hyperbolic. The length of the foot-rule might also alter in 
such a way as to lead to an elliptic geometry*. 

Thus the conception of h3rperbolic, Euclidean, and elliptic 
geometries can be reached from the theory of the measure of 
distances as well as from the theory of parallels and of con- 
gruent figures. This view has led to further discussion of 
the essential characteristics of space by F. C. Klein, S. Lie, 
D. Hilbert, M. L. Gerard, A. N. Whitehead, and others. 

The above remarks refer only to space of two dimensions. 
Naturally there arises the question whether there are different 
kinds of non-Euclidean space of three or more dimensions. 
Riemann showed that there are three kinds of non-Euclidean 
space of three dimensions having properties analogous to the 
three kinds of non-Euclidean space of two dimensions already 
discussed. These are differentiated by the test whether at 
every point no geodetical surface, or one geodetical surface, or 
a fasciculus of geodetical surfaces can be drawn parallel to a 
given surface : a geodetical surface being defined as such that 
every geodetic line joining two points on it lies wholly on the 
surface. It may be added that each of the three systems of 
geometry of two dimensions described above may be deduced 
as properties of a surface in each of these three kinds of 
non-Euclidean space of three dimensions. 

It is evident that the properties of non-Euclidean space of 
three dimensions are deducible only by the aid of mathematics, 
and cannot be illustrated materially, for in order to realize or 
construct surfaces in non-Euclidean space of two dimensions we 
think of or use models in space of three dimensions ; similarly 

• See A. Cayley, Collected Mathematical Papers, Cambridge, 1896, vol. xi, 
p. 435 et seq. 

28—2 



436 HYPER-SPACE [CH. XX 

the only way in which we could construct models illustrating 
non-Euclidean space of three dimensions would be by utilizing 
space of four dimensions. 

We may proceed yet further and conceive of non-Euclidean 
geometries of more than three dimensions, but this remains, as 
yet, an unworked field. 

Returning to the former question of non-Euclidean geome- 
tries of two dimensions, I wish again to emphasize the fact that, 
if the axioms enunciated in the usual books on elementary 
geometry are replaced by others, it is possible to construct other 
consistent systems of geometry. For instance, just as one kind of 
non-Euclidean geometry has been constructed by assuming that 
Euclid's parallel postulate is not true, so D. Hilbert and M. Dehn 
of Gottingen have elaborated another kind, known as non- 
Archimedian geometry. Archimedes had assumed as axiomatic 
that if A and B are magnitudes of the same kind and order, 
it is possible to find a multiple of A which is greater than B, 
which implied that the geometrical magnitudes considered are 
continuous. If this be denied, Hilbert and Dehn showed* that 
it is still possible to construct consistent systems of geometry 
closely analogous to that given by Euclid. Assuming that in 
these a pencil of straight lines can be drawn through a point 
parallel to a given straight line, then in one form, known as the 
non-Legendrian system, the angle-sum of a triangle is greater 
than two right angles, while in another form, termed the semi- 
Euclidean system, the sum-angle is equal to two right angles. 
I do not however concern myself here further with these systems, 
for the methods and results appeal only to the professional 
mathematician. On the other hand the elliptic, parabolic, and 
hyperbolic systems described in chapter xiv have a special 
interest, from the somewhat sensational fact that they lead to 
no results necessarily inconsistent with the properties, as far as 
we can observe them, of the space in which we live ; we are not 
at present acquainted with any other systems which are con- 
sistent with our experience. We may, however, fairly say that 
of these systems the Euclidean is the simplest. 

* M. Dehn, Mathematischen Annalen^ Leipzig, 1900, vol. lvii, pp. 404 — 439. 



CH. XX] HYPER-SPACE 437 

If we go a step further and ask what is meant by saying 
that a geometry is true or false we land ourselves in an inter- 
minable academic dispute. Some philosophers hold that certain 
axioms are necessarily true independent of all experience, or at 
any rate are necessarily true as far as our experience extends. 
Others agree with Poincare, that the selection of a geometry is 
really a matter of convenience, and that that geometry is the 
best which enables us to state the known physical laws in the 
simplest form ; or, more generally, that it is desirable to choose 
axioms and to define quantities so as to permit the expression 
in as simple a way as possible of all observed laws and facts in 
nature. But for practical purposes the conclusion is immaterial, 
and at any rate the discussion belongs to metaphysics rather 
than mathematics. 



4.38 



CHAPTER XXL 
TIME AND ITS MEASUREMENT. 

The problems connected with time are totally different in 
character from those concerning space which I discussed in the 
last chapter. I there stated that the life of people living in 
space of one dimension would be uninteresting, and that probably 
they would find it impossible to realize life in space of higher 
dimensions. In questions connected with time we find ourselves 
in a somewhat similar position. Mentally, we can realize a past 
and a future — thus going backwards and forwards — actually we 
go only forwards. Hence time is analogous to space of one 
dimension. Were our time of two dimensions, the conditions 
of our life would be infinitely varied, but we can form no con- 
ception of what such a phrase means, and I do not think that 
any attempts have been made to work it out. 

The idea of time, when we examine it carefully, involves 
many difficulties. For instance, we speak of an instant of time 
as if it were absolutely definite. If so we could represent it by 
a point on a line, and the idea of simultaneity would be simple, 
for two events could be regarded as simultaneous when their 
representative points were coincident. But in reality sensations 
have an appreciable duration, even though it be very small. 
This duration may be represented by an interval on a line, and 
it would seem reasonable to say that two events are simultaneous 
when their representative intervals have a common part ; hence 
two events which are simultaneous with the same event are 
not necessarily simultaneous with one another. Here, however, 
I exclude these quasi-metaphysical questions, and concern myself 



CH. XXl] TIME AND ITS MEASUREMENT 439 

mainly with questions concerning the measurement of time, and 
I shall treat these rather from a historical than from a physical 
point of view. 

In order to measure anything we must have an unalterable 
unit of the same kind, and we must be able to determine how 
often that unit is contained in the quantity to be measured. 
Hence only those things can be measured which are capable of 
addition to things of the same kind. 

Thus to measure a length we may take a foot-rule, and by 
applying it to the given length as often as is necessary, we shall 
find how many feet the length contains. But in comparing 
lengths we assume as the result of experience that the length 
of the foot-rule is constant, or rather that any alteration in it 
can be determined; and, if this assumption was denied, we 
could not prove it, though, if numerous repetitions of the 
experiment under varying conditions always gave the same 
result, probably we should feel no doubt as to the correctness 
of our method. 

It is evident that the measurement of time is a more 
difficult matter. We cannot keep a unit by us in the same way 
as we can keep a foot-rule ; nor can we repeat the measurement 
over and over again, for time once passed is gone for ever. 
Hence we cannot appeal directly to our sensations to justify our 
measurement. Thus, if we say that a certain duration is four 
hours, it is only by a process of reasoning that we can show that 
each of the hours is of the same duration. 

The establishment of a scientific unit for measuring dura- 
tions has been a long and slow affair. The process seems to 
have been as follows. Originally man observed that certain 
natural phenomena recurred after the interval of a day, say 
from sunrise to sunrise. Experience — for example, the amount 
of work that could be done in it — showed that the length of 
every day was about the same, and, assuming that this was 
accurately so, man had a unit by which he could measure 
durations. The present subdivision of a day into hours, minutes, 
and seconds is artificial, and apparently is derived from the 
Babylonians. 



440 TIME AND ITS MEASUREMENT [CH. XXI 

Similarly a month and a year are natural units of time 
though it is not easy to determine precisely their beginnings 
and endings. 

So long as men were concerned merely with durations which 
were exact multiples of these units or which needed only a 
rough estimate, this did very well ; but as soon as they tried to 
compare the different units or to estimate durations measured 
by part of a unit they found difficulties. In particular it cannot 
have been long before it was noticed that the duration of the 
same day differed in different places, and that even at the same 
place different days differed in duration at different times of the 
year, and thus that the duration of a day was not an invariable 
unit. 

The question then arises as to whether we can find a fixed 
unit by which a duration can be measured, and whether we 
have any assurance that the seconds and minutes used to-day 
for that purpose are all of equal duration. To answer this we 
must see how a mathematician would define a unit of time. 
Probably he would say that experience leads us to believe that, 
if a rigid body is set moving in a straight line without any 
external force acting on it, it will go on moving in that line ; 
and those times are taken to be equal in which it passes over 
equal spaces : similarly, if it is set rotating about a principal axis 
passing through its centre of mass, those times are taken to be 
equal in which it turns through equal angles. Our experiences are 
consistent with this statement, and that is as high an authority 
as a mathematician hopes to get. 

The spaces and the angles can be measured, and thus dura- 
tions can be compared. Now the earth may be taken roughly 
as a rigid body rotating about a principal axis passing through 
its centre of mass, and subject to no external forces affecting 
its rotation : hence the time it takes to turn through four right 
angles, i.e. through 360°, is always the same; this is called 
a sidereal day: the time to turn through one twenty-fourth 
part of 360°, i.e. through 15°, is an hour: the time to turn 
through one-sixtieth part of 15°, i,e. through 15', is a minute : 
and so ow. 



CH. XXl] TIME AND ITS JIEASUREMENT 441 

If, by the progress of astronomical research, we find that 
there are external forces affecting the rotation of the earth, 
mathematics would have to be invoked to find what the time 
of rotation would be if those forces ceased to act, and this 
would give us a correction to be applied to the unit chosen. 
In the same way we may say that although an increase of 
temperature affects the length of a foot-rule, yet its change of 
length can be determined, and thus applied as a correction to 
the foot-rule when it is used as the unit of length. As a matter 
of fact there is reason to think that the earth takes about one 
sixty-sixth of a second longer to turn through four right angles 
now than it did 2500 years ago, and thus the duration of a 
second is just a trifle longer to-day than was the case when the 
Romans were laying the foundations of the power of their city. 

The sidereal day can be determined only by refined astro- 
nomical observations and is not a unit suitable for ordinary 
purposes. The relations of civil life depend mainly on the 
sun, and he is our natural time-keeper. The true solar day 
is the time occupied by the earth in making one revolution on 
its axis relative to the sun ; it is true noon when the sun is on 
the meridian. Owing to the motion of the sun relative to the 
earth, the true solar day is about four minutes longer than 
a sidereal day. 

The true solar day is not however always of the same 
duration. This is inconvenient if we measure time by clocks 
(as now for nearly two centuries has been usual in Western 
Europe) and not by sun-dials, and therefore we take the average 
duration of the true solar day as the measure of a day : this is 
called the mean solar day. Moreover to define the noon of 
a mean solar day we suppose a point to move uniformly round 
the ecliptic coinciding with the sun at each apse, and further 
we suppose a fictitious sun, called the mean sun, to move in the 
celestial equator so that its distance from the first point of Aries 
is the same as that of this point : it is mean noon when this 
mean sun is on the meridian. The mean solar day is divided 
into hours, minutes, and seconds ; and these are the usual units 
of time in civil lil'e. 



442 TIME AND ITS MEASUREMENT [CH. XXI 

The time indicated by our clocks and watches is mean solar 
time; that marked on ordinary sun-dials is true solar time. 
The difference between them is the equation of time : this may 
amount at some periods of the year to a little more than a 
quarter of an hour. In England we take the Greenwich 
meridian as our origin for longitudes, and instead of local mean 
solar time we take Greenwich mean solar time as the civil 
standard. 

Of course mean time is a comparatively recent invention. 
The French were the last civilized nation to abandon the use 
of true time : this was in 1816. 

Formerly there was no common agreement as to when the 
day began. In parts of ancient Greece and in Japan the 
interval from sunrise to sunset v,^as divided into twelve hours, and 
that from sunset to sunrise into twelve hours. The Jews, Chinese, 
Athenians, and, for a long time, the Italians, divided their day into 
twenty-four hours, beginning at the hour of sunset, which of course 
varies every day : this method is said to have been used as late as 
the latter half of the nineteenth century in certain villages near 
Naples, except that the day began half-an-hour after sunset — 
the clocks being re-set once a week. Similarly the Babylonians, 
Assyrians, Persians, and until recently the modern Greeks and 
the inhabitants of the Balearic Islands counted the twenty-four 
hours of the day from sunrise. Until 1750, the inhabitants of 
Basle reckoned the twenty-four hours from our 11.0 p.m. The 
ancient Egyptians and Ptolemy counted the twenty-four hours 
from noon: this is the practice of modern astronomers. In 
Western Europe the day is taken to begin at midnight — as 
was first suggested by Hipparchus — and is divided into two 
equal periods of twelve hours each. 

The week of seven days is an artificial unit of time. It had 
its origin in the East, and was introduced into the West probably 
during the second century by the Koman emperors, and, except 
during the French Revolution, has been subsequently in general 
■use among civilized races. The names of the days are derived 
from the seven astrological planets, arranged, as was customary, 
in the order of their apparent times of rotation round the earth, 



CH. XXl] TIME AND ITS MEASUREMENT 443 

namely, Saturn, Jupiter, Mars, the Sun, Venus, Mercury, and 
the Moon. The twenty-four hours of the day were dedicated 
successively to these planets : and the day was consecrated to 
the planet of the first hour. 

Thus if the first hour was dedicated to Saturn, the second 
would be dedicated to Jupiter, and so on ; but the day would be 
Saturn's day. The twenty-fourth hour of Saturn's day would 
be dedicated to Mars, thus the first hour of the next day would 
belong to the Sun ; and the day would be Sun's day. Similarly 
the next day would be Moon's day ; the next, !RIars's day ; the 
next, Mercury's day; the next, Jupiter's day; and the next, 
Venus's day. 

The astronomical month is a natural unit of time depending 
on the motion of the moon, and containing about 29J days. 
The months of the calendar have been evolved gi-adually as 
convenient divisions of time, and their history is given in 
numerous astronomies. In the original Julian arrangement 
the months in a leap year contained alternately 31 and 30 days, 
while in other years February had 29 days. This was altered 
by Augustus in order that his month should not be inferior to 
one named after his uncle. 

The solar tropical year is another natural unit of time. 
According to a recent determination, it contains 365*242216 
days, that is, 365"*- 5*"- 48°^ 47** '4624. Civilized races usually 
number the passing years consecutively from some fixed date. 
The Romans reckoned from the traditional date of the foundation 
of their city. In the sixth century of our era it was suggested 
that the birth of Christ was a more fitting epoch from which to 
reckon dates, but it was not until the ninth century that this 
suggestion was generally adopted. 

The Egyptians knew that the year contained between 365 
and 366 days, but the Romans did not profit by this information, 
for Numa is said to have reckoned 355 days as constituting 
a year — extra months being occasionally intercalated, so that 
the seasons might recur at about the same period of the year. 

In 46 B.C. Julius Caesar decreed that thenceforth the year 
should contain 365 days, except that in every fourth or leap 



444) TIME AND ITS MEASUREMENT [CH. XXI 

year one additional day should be introduced. He ordered this 
rule to come into force on January 1, 45 B.C. The change was 
made on the advice of Sosigenes of Alexandria. 

It must be remembered that the year 1 a.d. follows imme- 
diately 1 B.C., that is, there is no year 0, and thus 45 B.C. would 
be a leap year. All historical dates are given now as if the 
Julian calendar was reckoned backwards as well as forwards 
from that year*. As a matter of fact, owing to a mistake in 
the original decree, the Romans, during the first 36 years after 
45 B.C., intercalated the extra day every third year, thus pro- 
ducing an error of 3 days. This was remedied by Augustus, 
who directed that no intercalation of an extra day should be 
made in any of the twelve years A.u.c. 746 to 757 inclusive, 
but that the intercalation should be again made in the year 
A.U.C. 761 (that is, 8 a.d.) and every succeeding fourth year. 

The Julian calendar made the year, on an average, con- 
tain 365*25 days. The actual value is, very approximately, 
365'242216 days. Hence the Julian year is too long by about 
11 J minutes: this produces an error of nearly one day in 128 
years. If the extra day in every thirty-second leap year had 
been omitted — as was suggested by some unknown Persian 
astronomer — the error would have been less than one day in 
100,000 years. It may be added that Sosigenes was aware that 
his rule made the year slightly too long. 

The error in the Julian calendar of rather more than eleven 
minutes a year gradually accumulated, until in the sixteenth 
century the seasons arrived some ten days earlier than they 
should have done. In 1582 Gregory XIII corrected this by 
omitting ten days from that year, which therefore contained 
only 355 days. At the same time he decreed that thenceforth 
every year which was a multiple of a century should be or not 
be a leap year according as the multiple was or was not divisible 
by four. 

The fundamental idea of the reform was due to Lilius, who 
died before it was carried into effect. The work of framing 
the new calendar was entrusted to Clavius, who explained the 
* Herschel, Astronomy^ London, 11th ed. 1871, arts. 916 — 919. 



CH. XXl] TIME AND ITS MEASUREMENT 445 

principles and necessary rules in a prolix but accurate work* of 
over 700 folio pages. The plan adopted was due to a suggestion 
of Pitatus made in 1552 or perhaps 1537 : the alternative and 
more accurate proposal of Stoffler, made in 1518, to omit one 
day in every 134 years, being rejected by Lilius and Clavius for 
reasons which arc not known. 

Clavius believed the year to contain 3652425432 days, but 
he framed his calendar so that a year, on the average, contained 
365-2425 days, which he thought to be wrong by one day in 
3323 years : in reality it is a trifle more accurate than this, the 
error amounting to one day in about 3600 years. 

The change was unpopular, but Ricciolif tells us that, as 
those miracles which take place on fixed dates — ex. gr. the 
liquefaction of the blood of S. Januarius — occurred according 
to the new calendar, the papal decree was presumed to have a 
divine sanction — Deo ipso huic correctioni Gregorianae sub- 
scribente — and was accepted as a necessary evil. 

In England a bill to carry out the same reform was intro- 
duced in 1584, but was withdrawn after being read a second 
time; and the change was not finally effected till 1752, when 
eleven days were omitted from that year. In Roman Catholic 
countries the new style was adopted in 1582. In the German 
Lutheran States it was made in 1700. In England, as I have 
said above, it was introduced in 1752; and in Ireland it was 
made in 1782. It is well known that the Greek Church still 
adheres to the Julian calendar. 

The Mohammedan year contains 12 lunar months, or 354J 
days, and thus has no connection with the seasons. 

The Gregorian change in the calendar was introduced in 
order to keep Easter at the right time of year. The date of 
Easter depends on that of the vernal equinox, and as the Julian 
calendar made the year of an average length of 365*25 days 
instead of 365*242216 days, the vernal equinox came earlier and 
earlier in the year, and in 1582 had regreded to within about 
ten days of February. 

• Romani Calendarii a Greg, XIII Restituti Explicatio, Kome, 1603. 
t Chronologia Reformata^ Bonn, IGG9, vol. ii, p. 20G. 



446 TIME AND ITS MEASUREMENT [CH. XXI 

The rule for determining Easter is as follows*. In 325 the 
Nicene Council decreed that the Roman practice should be 
followed ; and after 463 (or perhaps, 530) the Roman practice 
required that Easter-day should be the first Sunday after the 
full moon which occurs on or next following the vernal equinox — 
full moon being assumed to occur on the fourteenth day from 
the day of the preceding new moon (though as a matter of fact 
it occurs on an average after an interval of rather more than 
14 J days), and the vernal equinox being assumed to fall on 
March 21 (though as a matter of fact it sometimes falls on 
March 22). 

This rule and these assumptions were retained by Gregory 
on the ground that it was inexpedient to alter a rule with 
which so many traditions were associated ; but, in order to save 
disputes as to the exact instant of the occurrence of the new 
moon, a mean sun and a mean moon defined by Clavius were 
used in applying the rule. One consequence of using this 
mean sun and mean moon and giving an artificial definition 
of full moon is that it may happen, as it did in 1818 and 1845, 
that the actual full moon occurs on Easter Sunday. In the 
British Act, 24 Geo. II. cap. 23, the explanatory clause which 
defines full moon is omitted, but practically full moon has 
been interpreted to mean the Roman ecclesiastical full moon; 
hence the Anglican and Roman rules are the same. Until 
1774 the German Lutheran States employed the actual sun 
and moon. Had full moon been taken to mean the fifteenth 
day of the moon, as is the case in the civil calendar, then the 
rule might be given in the form that Easter-day is the Sunday 
on or next after the calendar full moon which occurs next after 
March 21. 

Assuming that the Gregorian calendar and tradition are 
used, there still remains one point in this definition of Easter 
which might lead to different nations keeping the feast at 
different times. This arises from the fact that local time is 
introduced. For instance the difference of local time between 

* De Morgan, Companion to the Almanac^ London, 1845, pp. 1 — 36 j Ibid.^ 
1846, pp. 1—10. 



CH. XXl] TIME AND ITS MEASUREMENT 447. 

Rome and London is about 50 minutes. Thus the instant of 
the first full moon next after the vernal equinox might occur in 
Rome on a Sunday morning, say at 12.30 a.m., while in England 
it would still be Saturday evening, 11.40 p.m., in which case 
our Easter would be one week earlier than at Rome. Clavius 
foresaw the difficulty, and the Roman Communion all over the 
world keep Easter on that day of the month which is determined 
by the use of the rule at Rome. But presumably the British 
Parliament intended time to be determined by the Greenwich 
meridian, and if so the Anglican and Roman dates for Easter 
might differ by a week ; whether such a case has ever arisen or 
been discussed I do not know, and I leave to ecclesiastics to say 
how it should be settled. 

The usual method of calculating the date on which Easter- 
day falls in any particular year is involved, and possibly the 
following simple rule* may be unknown to some of my readers. 

Let m and n be numbers as defined below, (i) Divide the 
number of the year by 4, 7, 19 ; and let the remainders be 
a, b, c respectively, (ii) Divide 19c + m by 30, and let d be the 
remainder, (iii) Divide 2a + 46 + 6c? + n by 7, and let e be 
the remainder, (iv) Then the Easter full moon occurs d days 
after March 21 ; and Easter-day is the {22 + d -h e)th. of March 
or the (d-\-e — 9)th day of April, except that if the calculation 
gives cZ = 29 and e= 6 (as happens in 1981) then Easter-day is 
on April 19 and not on April 26, and if the calculation gives 
d = 28, e = 6, and also c> 10 (as happens in 1954) then Easter- 
da;y is on April 18 and not on April 25, that is, in these two 
cases Easter falls one week earlier than the date given by the 
rule. These two exceptional cases cannot occur in the Julian 
calendar, and in the Gregorian calendar they occur only very 
rarely. It remains to state the values of m and n for the par- 
ticular period. In the Julian calendar we have m = 15, ?i = 6. 
In the Gregorian calendar we have, from 1582 to 1699 in- 
clusive, m = 22, n = 2; from 1700 to 1799, m=23, ?i = .3; 
from 1800 to 1899, m = 23, n = 4 ; from 1900 to 2099, m = 24, 

* It is due to Gauss ; his proof is given in Zach'a Monatliche Correspondenz^ 
August, 1800, vol. u, pp. 221—230. 



.448 TIME AND ITS MEASUREMENT [CH. XXI 

n = 5; from 2100 to 2199, m-24, n = 6; from 2200 to 2299, 
7n = 25, n = 0; from 2300 to 2399, m=26, n = l; and from 
2400 to 2499, m=25, 71=1. Thus for the year 1908 we 
have m = 24, n — 5; hence a = 0, 6 = 4, c = 8; d = 26; and 
e = 2: therefore Easter Sunday was on April 19. After the 
year 4200 the form of the rule will have to be slightly 
modified. 

The dominical letter and the golden number of the ecclesi- 
astical calendar can be at once determined from the values of 
b and c. The epact, that is, the moon's age at the beginning 
of the year, can be also easily calculated from the above 
data in any particular case ; the general formula was given 
by Delambre, but its value is required so rarely by any but 
professional astronomers and almanac-makers that it is un- 
necessary to quote it here. 

We can evade the necessity of having to recollect the 
values of m and n by noticing that, if iV is the given year, 
and if [N/x] denotes the integral part of the quotient when 
N is divided by x, then m is the remainder when 15 + f is 
divided by 30, and n is the remainder when 6 + t; is divided 
by 7 : where, in the Julian calendar, f = 0, and ^ = ; and, in 
the Gregorian calendar, f = (i\^/100}- {iV'/400} - {iV/300}, and 
i7 = {A^/100}-{iV/400}-2. 

If we use these values of m and n, and if we put for 
a, h, c their values, namely, a = iV - 4 (A^/4}, h = N-7 {iV/7}, 
c = iV"— 19 {i^/19), the rule given on the last page takes the 
following form. Divide 19iV- {iV'/19} + 15 + f by 30, and let 
the remainder be d. Next divide 6(iV+(^ + l)- {iV74} +97 
by 7, and let the remainder be e. Then Easter full moon 
is on the dth. day after March 21, and Easter-day is on the 
{22 + d + e)th of March or the (d + e-9)th of April as the 
case may be ; except that if the calculation gives c^ = 29, and 
e = 6, or if it gives d = 2S, e = 6, and c > 10, then Easter-day 
is on the {d + e — 16)th of April. 

Thus, if i\r= 1920, we divide 

19 (1920) _ 101 -}- 15 + (19 - 4 - 6) by 30, 



CH. XXl] TIME AND ITS MEASUREMENT 449 

which gives cZ = 13, and then we proceed to divide 

6(1920 + 13 + l)-480 + (19-4-2) by 7, 
which gives e = : therefore Easter-day will be on April 4. 

The above rules cover all the cases worked out with so much 
labour by Clavius and others*. 

I may add here a rule, quoted by Zeller, for determining the 
day of the week corresponding to any given date. Suppose that 
the pth day of the qth month of the year N anno domini is the 
rth day of the week, reckoned from the preceding Saturday. 
Then r is the remainder when 

1) + 2^ + (3 (5 + l)/5) +i\^+ (iV^/41 -17 

is divided by 7 ; provided January and February are reckoned 
respectively as the 13th and 14th months of the preceding year. 

For instance, Columbus first landed in the New World on 
October 12, 1492. Here p = 12, ^ = 10, iV'=1492, rj^Q. If 
we divide 12 + 20 + 6 4- 1492 -f 373 by 7 we get r = 6 ; hence it 
was on a Friday. Again, Charles I was executed on January 30, 
1649 N.S. Here p = 30, ^=13, N^ 1648, 77 = 0, and we find 
r = 3; hence it was on a Tuesday. As another example, the 
battle of Waterloo was fought on June 18, 1815. Here /) = 18, 
g = 6, iV= 1815, 7} = 12, and we find r = 1 ; hence it took place 
on a Sunday. 

Various rules have been given for obtaining these results 
with less arithmetical calculation, but they depend on the 
construction of tables which must be consulted in all cases. 
One rule of this kind is given in Whitakers Almanac, 
Lightning calculators use such rules and commit the tables 
to memory. The same results can be also got by mechanical 
contrivances. The best instrument of this kind with which 

• Most of the above-mentioned facts about the calendar are taken from 
Delambre's Aitronomie, Paris, 1814, vol. iii, chap, xxxviii ; and his Histoire de 
Vastronomie moderne, Paris, 1821, vol. i, chap, i: see also A. De Morgan, The 
Book of Almanacs, London, 1851; S. Butcher, The Ecclesiastical Calendar , 
Dublin, 1877; and C. Zeller, Acta Mathematical Stockholm, 1887, vol. ix, 
pp. 131 — 136: on the chronological details see J. L. Ideler, Lehrbitch der 
Chronologie, Berlin, 1831. 

B. R. 29 



450 TIME AND ITS MEASUREMENT [CH. XXI 

I am acquainted is one called the World's Calendar, invented 
by J. P. Wiles, and issued in London in 1906. 

I proceed now to give a short account of some of the means 
of measuring time which were formerly in use. 

Of devices for measuring time, the earliest of which we have 
any positive knowledge are the styles or gnomons erected in 
Egypt and Asia Minor. These were sticks placed vertically in 
a horizontal piece of ground, and surrounded by three concentric 
circles, such that every two hours the end of the shadow of the 
stick passed from one circle to another. Some of these have 
been found at Pompeii and Tusculum. 

The sun-dial is not very different in principle. It consists 
of a rod or style fixed on a plate or dial; usually, but not 
necessarily, the style is placed so as to be parallel to the axis 
of the earth. The shadow of the style cast on the plate by the 
* sun falls on lines engraved there which are marked with the 
corresponding hours. 

The earliest sun-dial, of which 1 have read, is that made 
by Berosus in 540 B.C. One was erected by Meton at Athens 
in 433 B.C. The first sun-dial at Kome was constructed by 
Papirius Cursor in 306 B.C. Portable sun-dials, with a compass 
fixed in the face, have been long common in the East as well 
as in Europe. Other portable instruments of a similar kind 
were in use in medieval Europe, notably the sun-rings, hereafter 
described, and the sun-cylinders *. 

I believe it is not generally known that a sun-dial can 
be so constructed that the shadow will, for a short time 
near sunrise and sunset, move backwards on the dialf. This 
was discovered by Nonez. The explanation is as follows. 
Every day the sun appears to describe a circle round the 
pole, and the line joining the point of the style to the sun 
describes a right cone whose axis points to the pole. The 
section of this cone by the dial is the curve described by the 

* Thus Chaucer in the Shipman's Tale, "by my chilindre it is prime of day," 
and Lydgate in the Siege of Thebes, "by my chilyndre I gan anon to see... that 
it drew to nine." 

t Ozanam, 1803 edition, vol. m, p. 321 ; 1840 edition, p. 529. 



CH. XXl] TIME AND ITS MEASUREMENT 451 

extremity of the shadow, and is a conic. In our latitude the 
sun is above the horizon for only part of the twenty-four 
hours, and therefore the extremity of the shadow of the style 
describes only a part of this conic. Let QQ' be the arc 
described by the extremity of the shadow of the style from 
sunrise at Q to sunset at Q\ and let S be the point of the style 
and F the foot of the style, i.e. the point where the style meets 
the plane of the dial. Suppose that the dial is placed so that 
the tangents drawn from F to the conic QQ' are real, and that 
P and P'y the points of contact of these tangents, lie on the 
arc QQ'. If these two conditions are fulfilled, then the shadow 
will regrede through the angle QFP as its extremity moves 
from Q to P, it will advance through the angle PFF as its 
extremity moves from P to P', and it will regrede through the 
angle PFQ as its extremity moves from P' to Q'. 

If the sun's apparent diurnal path crosses the horizon — 
as always happens in temperate and tropical latitudes — and 
if the plane of the dial is horizontal, the arc QQ' will consist 
of the whole of one branch of a hyperbola, and the above con- 
ditions will be satisfied if P is within the space bounded by this 
branch of the hyperbola and its asymptotes. As a particular 
case, in a place of latitude 12° N. on a day when the sun is in 
the northern tropic (of Cancer) the shadow on a dial whose 
face is horizontal and style vertical will move backwards for 
about two hours between sunrise and noon. 

If, in the case of a given sun-dial placed in a certain 
position, the conditions are not satisfied, it will be possible to 
satisfy them by tilting the sun-dial through an angle properly 
chosen. This was the rationalistic explanation, offered by the 
French encyclopaedists, of the miracle recorded in connection 
with Isaiah and Hezekiah*. Suppose, for instance, that the 
style is perpendicular to the face of the dial. Draw the celestial 
sphere. Suppose that the sun rises at M and culminates at iV, 
and let Z be a point between M and N on the sun's diurnal 
path. Draw a great circle to touch the sun's diurnal path 
MLN at L, let this great circle cut the celestial meridian in A 
* 2 Kiugs, chap, xx, vv. 9—11. 

29-2 



452 TIME AND ITS MEASUKEMENT [CH. XXI 

and A', and of the arcs AL, A'L suppose that AL is the less 
and therefore is less than a quadrant. If the style is pointed to 
A, then, while the sun is approaching L, the shadow will 
regrede, and after the sun passes L the shadow will advance. 
Thus if the dial is placed so that a style which is normal to it 
cuts the meridian midway between the equator and the tropic, 
then between sunrise and noon on the longest day the shadow 
will move backwards through an angle 

sin~^ (cos ft) sec .| £») — cot~^ {sin w cos {I — ico) (cos^ I — sin^ ft)) ~ ^}, 
where I is the latitude of the place and co is the obliquity of the 
ecliptic. 

The above remarks refer to the sun-dials in ordinary use. 
In 1892 General Oliver brought out in London a dial with a 
solid style, the section of the style being a certain curve whose 
form was determined empirically by the value of the equation 
of time as compared with the sun's declination*. The shadow 
of the style on the dial gives the local mean time, though of 
course in order to set the dial correctly at any place the latitude 
of the place must be known : the dial may be also set so as to 
give the mean time at any other locality whose longitude relative 
to the place of observation is known. 

The sun-ring or ring-dial is another instrument for measuring 
solar time-f-. One of the simplest type is figured in the diagram 
below. The sun-ring consists of a thin brass band, about a 
quarter of an inch wide, bent into the shape of a circle, which 
slides between two fixed circular rims — the radii of the circles 
being about one inch. At one point of the band there is a 
hole; and when the ring is suspended from a fixed point 
attached to the rims so that it hangs in a vertical plane con- 
taining the sun, the light from the sun shines through this 
hole and makes a bright speck on the opposite inner or 
concave surface of the ring. On this surface the hours are 
marked, and, if the ring is properly adjusted, the spot of 
light will fall on the hour which indicates the solar time. The 

* An account of this sun-dial with a diagram was given in Knowledge, London, 
July 1, 1892, pp. 133, 134, 

t See Ozanam, 1803 edition, vol. m, p. 317 ; 1840 edition, p. 526. 



CH. XXl] 



TIME AND ITS MEASUREMENT 



453 



adjustment for the time of year is made as follows. The rims 
between which the band can slide are marked on their outer 
or convex side with the names of the months, and the band 
containing the hole must be moved between the rims until 
the hole is opposite to that month for which the ring is being 
used. 




For determining times near noon the instrument is reliable, 
but for other hours in the day it is accurate only if the time 
of year is properly chosen, usually near one of the equinoxes. 
This defect may be corrected by marking the hours on a 
curved brass band affixed to the concave surface of the rims. 
I possess two specimens of rings of this kind. These rings 
were distributed widely. Of my two specimens, one was bought 
in the Austrian Tyrol and the other in London. Astrolabes 
and sea-rings can be used as sun-rings. 

Clepsydras or water-clocks, and hour-glasses or sand-clocks, 
afford other means of measuring time. The time occupied by 
a given amount of some liquid or sand in running through 
a given orifice under the same conditions is always the same, 
and by noting the level of the liquid which has run through 
the orifice, or which remains to run through it, a measure of 
time can be obtained. 



454 TIME AND ITS MEASUREMENT [CH. XXI 

The burning of graduated candles gives another way of 
measuring time, and we have accounts of those used by Alfred 
the Great for the purpose. Incense sticks were used by the 
Chinese in a similar way. 

Modern clocks and watches* comprise a train of wheels 
turned by a weight, spring, or other motive power, and regu- 
lated by a pendulum, balance, fly-wheel, or other moving body 
whose motion is periodic and time of vibration constant. The 
direction of rotation of the hands of a clock was selected 
originally so as to make the hands move in the same direction 
as the shadow on a sun-dial whose face is horizontal — the dial 
being situated in our hemisphere. 

The invention of clocks with wheels is attributed by tradition 
to Pacificus of Verona, circ. 850, and also to Gerbert, who is said 
to have made one at Magdeburg in 996 : but there is reason to 
believe that these were sun-clocks. The earliest wheel-clock of 
which we have historical evidence was one sent by the Sultan 
of Egypt in 1232 to the Emperor Frederick II, though there 
seems to be no doubt that they had been made in Italy at least 
fifty years earlier. 

The oldest clock in England of which we know anything 
was one erected in 1288 in or near Westminster Hall out of 
a fine imposed on a corrupt Lord Chief Justice. The bells, and 
possibly the clock, were staked by Henry VIII on a throw of 
dice and lost, but the site was marked by a sun-dial, destroyed 
early in the nineteenth century, and bearing the inscription 
Discite justiciam moniti. In 1292 a clock was erected in Canter- 
bur}^ Cathedral at a cost of £30. One erected at Glastonbury 
Abbey in 1325 is at present in the Kensington Museum and is 
in excellent condition. Another made in 1326 for St Albans 
Abbey showed the astronomical phenomena, and seems to have 
been one of the earliest clocks that did so. One put up at 
Dover in 1348 is still in good working order. The clocks at 
Peterborough and Exeter were of about the same date, and 
portions of them remain in situ. Most of these early clocks 

* See Clock and Watch Making by Lord Grirathorpe, 7th edition, London, 

1883. 



CH. XXl] TIME AND ITS MEASUREMENT 455 

were regulated by horizontal balances: pendulums being then 
unknown. Of the elaborate clocks of a later date, that at 
Strasburg made by Dasypodius in 1571, and that at Lyons 
constructed by Lippeus in 1598, are especially famous: the 
former was restored in 1842, though in a manner which de- 
stroyed most of the ancient works. 

In 1370 Vick constructed a clock for Charles V with a 
weight as motive power and a vibrating escapement— a great 
improvement on the rough time-keepers of an earlier date. 

The earliest clock regulated by a pendulum seems to have 
been made in 1621 by a clockmaker named Harris, of Covent 
Garden, London, but the theory of such clocks is due to 
Huygens*. Galileo had discovered previously the isochronism 
of a pendulum, but did not apply it to the regulation of the 
motion of clocks. Hooke made such clocks, and possibly dis- 
covered independently this use of the pendulum : he invented 
or re-invented the anchor pallet. 

A watch may be defined as a clock which will go in any 
position. Watches, though of a somewhat clumsy design, were 
made at Nuremberg by P. Hele early in the sixteenth century 
— the motive power being a ribbon of steel, wound round a 
spindle, and connected at one end with a train of wheels which 
it turned as it unwound. Possibly a few similar timepieces 
had been made in the previous century; by the end of the 
sixteenth century they were not uncommon. At first they 
were usually made in the form of fanciful ornaments such as 
skulls, or as large pendants, but about 1620 the flattened oval 
form was introduced, rendering them more convenient to carry 
in a pocket or about the person. In the seventeenth century 
their construction was greatly improved, notably by the 
introduction of the spring balance by Huygens in 1674, and 
independently by Hooke in 1675— both mathematicians having 
discovered that small vibrations of a coiled spring, of which one 
end is fixed, are practically isochronous. The fusee had been 
used by R. Zech of Prague in 1525, but was re-invented by 
Hooke, 

• Eorologium Otcillatorium, Paris, 1673, 



466 TIME AND ITS MEASUREMENT [CH. XXI 

Clocks and watches are usually moved and regulated in the 
manner indicated above. Other motive powers and other 
devices for regulating the motion may be met with occasionally. 
Of these I may mention a clock in the form of a cylinder, usually 
attached to another weight as in Atwood's machine, which rolls 
doAvn an inclined plane so slowly that it takes twelve hours to 
roll down, and the highest point of the face always marks the 
proper hour*. 

A water- clock made on a somewhat similar plan is described 
by Ozanamf as one of the sights of Paris at the beginning of 
the last century. It was formed of a hollow cylinder divided 
into various compartments each containing some mercury, so 
arranged that the cylinder descended with uniform velocity 
between two vertical pillars on which the hours were marked 
at equidistant intervals. 

Other ingenious ways of concealing the motive power have 
been described in the columns of La NatureX. Of such 
mysterious timepieces the following are not uncommon examples, 
and probably are known to most readers of this book. One 
kind of clock consists of a glass dial suspended by two thin 
wires; the hands however are of metal, and the works are 
concealed in them or in the pivot. Another kind is made of 
two sheets of glass in a frame containing a spring which gives 
to the hinder sheet a very slight oscillatory motion — imper- 
ceptible except on the closest scrutiny — and each oscillation 
moves the hands through the requisite angles. Some so-called 
perpetual motion timepieces were described above on page 96. 
Lastly, I have seen in France a clock the hands of which were 
concealed at the back of the dial, and carried small magnets; 
pieces of steel in the shape of insects were placed on 
the dial, and, following the magnets, served to indicate the 
time. 

The position of the sun relative to the points of the compass 

♦ Ozanam, 1803 edition, vol. ii, p. 39 ; 1840 edition, p. 212 ; or La Nature, 
Jan. 23, 1892, pp. 123, 124. 

t Ozanam, 1803 edition, vol. ii, p. 68 ; 1840 edition, p. 225. 
% See especially the volumes issued in 1874, 1877, and 1873, 



CH. XXl] TIME AND ITS MEASUREMENT 457 

determines the solar time. Conversely, if we take the time 
given by a watch as being the solar time — and it will differ 
from it by only a few minutes at the most — and we observe 
the position of the sun, we can find the points of the compass*. 
To do this it is sufficient to point the* hour-hand to the sun, 
and then the direction which bisects the angle between the 
hour and the figure xii will point due south. For instance, 
if it is four o'clock in the afternoon, it is sufficient to point 
the hour-hand (which is then at the figure iiii) to the sun, and 
the figure ii on the watch will indicate the direction of so'uth. 
Again, if it is eight o'clock in the morning, we must point the 
hour-hand (which is then at the figure viil) to the sun, and 
the figure x on the watch gives the south point of the 
compass. 

Between the hours of six in the morning and six in the 
evening the angle between the hour and xii which must be 
bisected is less than 180°, but at other times the angle to be 
bisected is greater than 180° ; or perhaps it is simpler to say 
that at other times the rule gives the north point and not the 
south point. 

The reason is as follows. At noon the sun is due south, 
and it makes one complete circuit round the points of the 
compass in 24 hours. The hour-hand of a watch also makes 
one complete circuit in 12 hours. Hence, if the watch is held 
in the plane of the ecliptic with its face upwards, and the 
figure XII on the dial is pointed to the south, both the hour- 
hand and the sun will be in that direction at noon. Both 
move round in the same direction, but the angular velocity 
of the hour-hand is twice as great as that of the sun. Hence 
the rule. The greatest error due to the neglect of the equation 
of time is less than 2°. Of course in practice most people, 
instead of holding the face of the watch in the ecliptic, would 
hold it horizontal, and in our latitude no serious error would be 
thus introduced. 

• The rule is given by W. H. Richards, Military Topography, London, 
1883, p. 31, though it is not stated quite correctly. I do not know who firct 
enunciated it. 



458 TIME AND ITS MEASUREMENT [ciI. XXI 

In the southern hemisphere where at noon the sun is due 
north the rule requires modification. In such places the hour- 
hand of a watch (held face upwards in the plane of the ecliptic) 
and the sun move in opposite directions. Hence, if the watch 
is held so that the figure xii points to the sun, then the direction 
which bisects the angle between the hour of the day and the 
figure XII will point due north. 



459 



CHAPTER XXIL 

MATTER AND ETHER THEORIES. 

Matter, like space and time, cannot be defined, but either 
the statement that matter is whatever occupies space or the 
statement that it is anything which can be seen, touched, or 
weighed, suggests its more important characteristics to anyone 
already familiar with it. 

The means of measuring matter and some of its properties 
are treated in most text-books on mechanics, and I do not 
propose to discuss them. I confine the chapter to an account 
of some of the hypotheses formerly held by physicists as to the 
ultimate constitution of matter, but I exclude metaphysical 
conjectures which, from their nature, are incapable of proof 
and are not subject to mathematical analysis. The question 
is intimately associated with the explanation of the phenomena 
of attraction, light, chemistry, electricity, and other branches of 
physics. 

I commence with a list of some of the more plausible of the 
hypotheses formerly proposed which accounted for the obvious 
properties of matter, and shall then discuss how far they explain 
or are consistent with other facts*. The interest of the list is 

* For the earlier investigations I have based my account mainly on Recent 
Advances in Physical Science, by P. G. Tait, Edinburgh, 1870 (chaps, xii, xiii); 
and on the article Atom by J. Clerk Maxwell in the Encyclopaedia Britannica or 
his Collected Works, vol. ii, pp. 445 — 484. For the more recent speculations 
see *J. J. Thomson, Electricity and Matter, Westminster, 1904 ; J. Larmor, 
Aether and Matter, Cambridge, 1900; and E. T. Whittaker, History of the 
Theories of Aether and Electricity, Dublin, 1910. 



460 MATCER AND ETHER THEORIES [CH. XXTI 

largely historical, for within the last few years new views as to 
the constitution of matter have been propounded, which cannot 
be discussed satisfactorily in a book like this. 

I. Hypothesis of Continuous Matter. It may be 
supposed that matter is homogeneous and continuous, in which 
case there is no limit to the infinite divisibility of bodies. This 
view was held by Descartes*. 

This conjecture is consistent with the facts deducible by 
untrained observation, but there are many other phenomena for 
which it does not account ; moreover there seems to be no way 
of reconciling such a structure of matter either with the facts 
of chemical changes or with the results of spectrum analysis. 
At any rate the theory must be regarded as extremely im- 
probable. 

II. Atomic Theories. If matter is not continuous we 
must suppose that every body is composed of aggi-egates of 
molecules. If so, it seems probable that each such molecule is 
built up by the association of two or more atoms, that the 
number of kinds of atoms is finite, and that the atoms of any 
particular kind are alike. As to the nature of the atoms the 
following hypotheses have been made. 

(i) Popular Atomic Hypothesis. The popular view is that 
every atom of any particular kind is a minute indivisible article 
possessing definite qualities, everlasting in its form and properties, 
and infinitely hard. 

This statement is plausible, but the difficulties to which it 
leads appear to be insuperable. In fact we have reason to think 
that the atoms which form a molecule are composite systems in 
incessant vibration at a rate characteristic of the molecule, and 
it is most probable that they are elastic. 

Newton seems to have hazarded a conjecture of this kind 
when he suggested f that the difficulties, connected with the 
fact that the velocity of sound was one-ninth greater than that 
required by theory, might be overcome if the particles of air 

* Descartes, Prlncipia, vol. ii, pp. 18, 23. 
t Newton, Pnncipia, bk. ii, prop. 50. 



CH. XXIl] WAITER AND ETHEI^ THEORIES 461 

were little rigid spheres whose distance from one another under 
normal conditions was nine times the diameter of any one of 
them. This was ingenious, but obviously the view is untenable, 
because, if such a structure of air existed, the air could not be 
compressed beyond a certain limit, namely, about l/1021st part 
of its original volume, which has been often exceeded. The 
true explanation of the difficulty noticed by Newton was given 
by Laplace. 

(ii) BoscovicKs Hypothesis. In 1759 Boscovich suggested* 
that the facts might be explained by supposing that an atom 
was an infinitely small indivisible mass which was a centre of 
force — the law of force being attractive for sensible distances, 
alternately attractive and repulsive for minute distances, and 
repulsive for infinitely small distances. In this theory all action 
between bodies is action at a distance. 

He explained the apparent extension of bodies by saying 
that two parts are consecutive (or similarly that two bodies are 
in contact) when the nearest pair of atoms in them are so close 
to one another that the repulsion at any point between them 
is sufficiently great to prevent any other atom coming between 
them. It is essential to the theory that the atom shall have 
a mass but shall not have dimensions. 

This hypothesis is not inconsistent with any known facts, 
but it has been described, perhaps not unjustly, as a mere 
mathematical fiction, and certainly it is opposed to the apparent 
indications of our senses. At any rate it is artificial, though 
it may be a prejudice to regard that as an argument against 
its adoption. To some extent this view was accepted by 
Faraday. 

Sir William Thomson, afterwards Lord Kelvin, showed f that, 
if we assume the existence of gravitation, then each of the 
above hypotheses will account for cohesion. 

* Philosophiae Naturalis Theoria Redacta ad Unicam Legem Ftnum, Vienna, 
1759. 

t Proceedings of the Boyal Society of Edinburgh, April 21, 1862, vol. iv, 
pp. 604—606. 



462 MAITER AND ETHER THEOKIES [CH. XXII 

(iii) Hypothesis of an Elastic Solid Ether. Some physicists 
tried to explain the known phenomena by properties of the 
medium through which our impressions are derived. By postu- 
lating that all space is filled with a medium possessed of many 
of the characteristics of an elastic solid, it was shown by Fresnel, 
Green, Cauchy, Neumann, MacCullagh, and others that a large 
number of the properties of light and electricity may be 
explained. In spite of the difficulties to which this hypothesis 
necessarily leads, and of its inherent improbability, it has been 
discussed by Stokes, Lam^, Boussinesq, Sarrau, Lorentz, Lord 
Rayleigh, and Kirchhoff. 

This hypothesis was modified and rendered somewhat more 
plausible by von Helmholtz, Lommel, Ketteler*, and Voigt, 
who based their researches on the assumption of a mutual 
reaction between the ether and the material molecules located 
in it : on this view the problems connected with refraction and 
dispersion have been simplified. Finally, Sir William Thomson 
in his Baltimore Lectures, 1885, suggested a mechanical ana- 
logue to represent the relations between matter and this ether, 
by which a possible constitution of the ether can be realized. 
He also suggested later a form of labile ether, from whose 
properties most of the more familiar physical phenomena can 
be deduced, provided the arrangement can be considered stable ; 
a labile ether is an elastic solid, and its properties in two 
dimensions may be compared with those of a soap-bubble film, 
in three dimensions. 

It is, however, difficult to criticize any of these hypotheses 
as a theory of the constitution of matter until the arrangement 
of the atoms or their nature is more definitely expressed. 

III. Dynamical Theories. In more recent years the 
suggestion was made that the so-called atoms may be forms of 
motion (ex. gr. permanent eddies) in one elementary material 
known as the ether ; on this view all the atoms are constituted 
of the same matter, but the physical conditions are different for 
the different kinds of atoms. It has been said that there is an 

* Theoretische Oj)tik, Braunschweig, 1885. 



CH. XXI l] MA'ITER AND ETHER THEORIES 463 

initial difficulty in any such hypothesis, since the all-pervading 
elementary fluid must -possess inertia, so that to explain matter 
we assume the existence of a fluid possessing one of the chief 
characteristics of matter. This is true as far as it goes, but it 
is not more unreasonable than to attribute all the fundamental 
properties of matter to the atoms themselves, as is done by 
many writers. The next paragiaph contains a statement of 
one of the earliest attempts to formulate a dynamical atomic 
hypothesis. 

(i) The Vortex Ring Hypothesis. This hypothesis assumes 
that each atom is a vortex ring in an incompressible frictionless 
homogeneous fluid. 

Vortex rings — though, since friction is brought into play, 
of an imperfect character — can be produced in air by many 
smokers. Better specimens can be formed by taking a card- 
board box* in one side of which a circular hole is cut, filling it 
with smoke, and hitting the opposite side sharply. The 
tendency of the particles forming a ring to maintain their 
annular connection may be illustrated by placing such a box on 
one side of a room in a direct line with the flame of a lighted 
candle on the other side. If properly aimed, the ring will 
travel across the room and put out the flame. If the box is 
filled only with air, so that the ring is not visible, the experiment 
is more effective. 

In 1858 von Helmholtz* showed that a closed vortex 
filament in a perfect fluid is indestructible and retains certain 
characteristics always unaltered. In 1867 Sir William Thomson 
propounded f the idea that matter consists of vortex rings in a 
fluid which fills space. If the fluid is perfect we could neither 
create new vortex rings nor destroy those already created, and 
thus the permanence of the atoms is explained. Moreover the 
atoms would be flexible, compressible, and in incessant vibration 

* CrelU's Journal, 1868, vol. lv, pp. 25 — 65; translated by Tait iu the 
Philosophical Magazine, June, 1867, supplement, series 4, vol. xxxiii, pp. 485 — 
512. 

t Proceedings of the Royal Society of Edinburgh, Feb. 18, 1867, vol. vi, 
pp. 94—105. 



464 MA.T1'ER AND ETUliR THEORIES [CH. XXII 

at a definite fundamental rate. This rate is very rapid, and 
Sir William Thomson gave the number of vibrations per second 
of a sodium ring as probably being greater than 10". 

By a development of this hypothesis Sir J. J. Thomson* 
showed, some years ago, that chemical combination may be 
explained. He supposed that a molecule of a compound is 
formed by the linking together of vortex filaments representing 
atoms of different elements : this arrangement may be compared 
with that of helices on an anchor ring. For stability not more 
than six filaments may be combined together, and their 
strengths must be equal. Another way of explaining chemical 
combination on the vortex atom hypothesis has been suggested 
by W. M. Hicks. It is known f that a spherical mass of fluid, 
whose interior possesses vortex motion, can move through liquid 
like a rigid sphere, and he has shown that one of these 
spherical vortices can swallow up another, thus forming a 
compound element. 

(ii) The Vorteos Sponge Hypothesis. Any vortex atom 
hj^pothesis labours under the difficulty of requiring that the 
density of the fluid ether shall be comparable with that of 
ordinary matter. In order to obviate this and at the same time to 
enable it to transmit transversal radiations Sir William Thomson 
suggested what has been termed, not perhaps very happily, the 
vortex sponge hypothesisj: this rests on the assumption that 
laminar motion can be propagated through a turbulently 
moving inviscid liquid. The mathematical difficulties con- 
nected with such motion have prevented an adequate dis- 
cussion of this hypothesis, and I confine myself to merely 
mentionino: it. 

These hypotheses, of vortex motion in a fluid, account for 
the indestructibility of matter and for many of its properties. 
But in order to explain statical electrical attraction it would 

* A Treatise on the Motion of Vortex Rings, Cambridge, 1883. 

t See a memoir by M. J. M. Hill in the Philosophical Transactions of the 
Eoyal Society, London, 1894, part i, pp. 213—246. 

t Philosophical Magazine, London, October, 1887, series 5, vol. xxiv, pp. 342 
—353. 



CH. XXIl] MATTER AND ETHER THEORIES 466 

seem necessary to suppose that the ether is elastic; in other 
words, that an electric field must be a field of strain. If so, 
complete fluidity in the ether would be impossible, and hence 
the above theories are now regarded as untenable. 

(iii) The Ether-Squirts Hypothesis. Karl Pearson* sug- 
gested another dynamical theory in which an atom is conceived 
as a point at which ether is pouring into our space firom space 
of four dimensions. 

If an observer lived in two-dimensional space filled with 
ether and confined by two parallel and adjacent surfaces, and 
if through a hole in one of these surfaces fresh ether were 
squirted into this space, the variations of pressure thereby 
produced might give the impression of a hard impenetrable 
body. Similarly an ether-squirt from space of four dimensions 
into our space might give us the impression of matter. 

It seems necessary on this hypothesis to suppose that there 
are also ether-sinks, or atoms of negative mass ; but as ether- 
squirts would repel ether-sinks we may suppose that the latter 
have moved out of the universe known to our senses. 

By defining the mass of an atom as the mean rate at which 
ether is squirting into our space at that point, we can deduce 
the Newtonian law of gravitation, and by assuming certain 
periodic variations in the rate of squirting we can deduce some 
of the phenomena of cohesion, of chemical action, and of electro- 
magnetism and light. But of course the hypothesis rests on 
the assumption of the existence of a world beyond our senses. 

(iv) The Electron Hypothesis. MacCullagh, in 1837 and 
1889, proposed to account for optical phenomena on the as- 
sumption of an elastic ether possessing elasticity of the type 
required to enable it to resist rotation. This suggestion has 
been recently modified and extended by Sir Joseph Larmorf, 
and, as ilow enunciated, it accounts for many of the electrical 
and magnetic (as well as the optical) properties of matter. 

* American Journal of Mathematics, 1891, vol. xin, pp. 309 — 362. 

t Philosophical Transactions of the Royal Society ^ London, 1894, pp. 719 

822 ; 1895, pp. 695—743. 

B. E. SO 



466 MATTER AND ETHER THEORIES [CH. XXII 

The hypothesis is however very artificial. The assumed 
ether is a rotationally elastic incompressible fluid. In this fluid 
Larmor introduces monad electric elements or electrons, which 
are nuclei of radial rotational strain. He supposes that these 
electrons constitute the basis of matter. He further supposes 
that an electrical current consists of a procession of these 
electrons, and that a magnetic particle is one in which these 
entities are revolving in minute orbits. Dynamical considera- 
tions applied to such a system lead to an explanation of nearly 
all the more obvious phenomena. By further postulating that 
the orbital motion of electrons in the atom constitute it a fluid 
vortex it is possible to apply the hydrodynamical pulsatory theory 
of Bjerknes or Hicks and obtain an explanation of gravitation. 

Thus on this view mass is explained as an electrical mani- 
festation. Electricity in its turn is explained by the existence 
of electrons, that is, of nuclei of strain in the ether, which are 
supposed to be in incessant and rapid motion. Whilst, to 
render this possible, properties are attributed to the ether which 
are apparently inconsistent with our experience of the space it 
fills. Put thus, the hypothesis seems very artificial. Perhaps 
the utmost we can say for it is that, from some points of 
view, it may, so far as analysis goes, be an approximation 
to the true theory; in any case much work will have to be 
done before it can be considered established even as a working 
hypothesis. 

(v) Recent Developments. Most of the above was written in 
1891. Since then investigations on radio-activity have opened up 
new avenues of conjecture which tend to strengthen the electron 
theory as a working h3rpothesis. More than thirty years ago 
Clerk Maxwell had shown that light and electricity were closely 
connected phenomena. It was then believed that both were 
due to waves in the hypothetical ether, but it was supposed 
that the phenomena of matter on the one side and of light 
and electricity on the other were sharply distinguished one 
from the other. The difierences, however, between matter and 
light tend to disappear as investigations proceed. In 1895 



CH. XXIl] MATTER AND ETHER THEORIES 467 

Rontgen established the existence of rays which could produce 
light, which had the same velocity as light, which were not 
affected by a magnet, and which could traverse wood and certain 
other opaque substances like glass. A year later Becquerel 
showed that uranium was constantly emitting rays which, 
though not affecting the eye as light, were capable of producing 
an image on a photographic plate. Like Rontgen rays they 
can go through thin sheets of metal ; like heat rays they burn 
the skin; like electricity they generate ozone from oxygen. 
Passed into the air they enable it to conduct the electric 
current. Their speed has been measured and found to be 
rather more than half that of light and electricity. It was soon 
found that thorium possessed a similar property, but in 1903 
Curie showed that radium possessed radio-activity to an extent 
previously unsuspected in any body, and in fact the rays were 
so powerful as to make the substance directly visible. Further 
experiments showed that numerous bodies are radio-active, but 
the effects are so much more marked in radium that it is 
convenient to use that substance for most experimental pur- 
poses. 

Radium gives off no less than three kinds of rays besides 
a radio-active emanation. In these discharges there appears to 
be a gradual change from what had been supposed to be an 
elementary form of matter to another. This leads to the belief 
that of the known forms of matter some, perhaps even all, are 
not absolutely stable. On the other hand, it may be that only 
radio-active bodies are unstable, and that in their disintegration 
we are watching the final stage in the evolution of stable and 
constant forms of matter. It may, however, in any case turn 
out that some, or perhaps all, of the so-called elements may be 
capable of resolution into different combinations of electrons or 
electricity. 

At an earlier date J. J. Thomson had concluded that the 
glow, seen when an electric current passes through a high 
vacuum tube, is due to a rush of minute particles across the 
tube. He calculated their weight, their velocity, and the charge 
of electricity transported by or represented by them, and found 

30—2 



468 MATTER AND ETHER THEORIES [CH. XXII 

these to be constant. They were deflected like Becquerel rays. 
All space seems to contain them, and electricity, if not identical 
with them, is at least carried by them. This suggested that 
these minute particles might be electrons. If so, they might 
thus give the ultimate explanation of electricity as well as 
matter, and the atom of the chemist would be not an irreducible 
unit of matter, but a system comprising numerous such minute 
particles. These conclusions are consistent with those subse- 
quently deduced from experiments with radium. In 1904 
the hypothesis was carried one stage further. In that year 
J. J. Thomson investigated the conditions of stability of certain 
systems of revolving particles; and on the hypothesis that an 
atom of matter consists of a number of particles carrying 
negative charges of electricity revolving in orbits within a 
sphere of positive electrification he deduced many of the pro- 
perties of the different chemical atoms corresponding to different 
possible stable systems of this kind. His scheme led to results 
agreeing closely with the results of Mendelejev's periodic hypo- 
thesis according to which some or all of the properties of an 
element are a periodic function of its atomic weight. An 
interesting consequence of this view is that Franklin's description 
of electricity as subtle particles pervading all bodies may turn 
out to be substantially correct. It is also remarkable that 
corpuscles somewhat analogous to those whose existence was 
suggested in Newton's corpuscular theory of light should be 
now supposed to exist in cathode and Becquerel rays. 

(vi) These facts have been utilized by G. Le Bon who 
suggested that electricity and matter may be regarded as 
intermediate stages in the flux of the ether, the former being 
one stage in the incessant dissociation of matter which arises 
from ether and ultimately is resolved again into ether. The 
theory is ingenious but appears untenable. 

(vii) Ether as matter. Recently another hypothesis as to 
the nature of the ether was put forward by D. I. Mendelejev, the 
distinguished chemist. In the grouping of the elements ac- 
cording to his periodic law, there were originally twelve series 



CH. XXll] MATTER AiND ETHKU THEORIES 469 

and eight groups. All the elements since discovered have 
fallen into place in the sequence, and possess properties in 
general accordance with his grouping, but helium, argon, and 
other similar inactive elements, constitute a ninth or zero 
group. 

In this scheme hydrogen, with an atomic weight 1*008, is in 
the first series and first group. It has the lowest atomic weight 
of any element yet known, but there may be lighter elements, 
ex. gr. one in the first series and zero group. Mendelejev has 
however suggested that just as a zero group has been now 
discovered so there may be a zero series ; and that the element 
(if there is one) in the zero group and zero series might be 
expected to possess properties closely resembling those of the 
hypothetical ether. It would be the lightest and simplest form 
of matter, of great elasticity, and with an atomic weight of 
perhaps about I/IO*' as compared with hydrogen. The velocity 
of its atoms would be so great as to make it all pervading, 
and it would appear to be capable of doing all that is required 
from the mysterious ether. The hypothesis is attractive and 
intelligible. 

(viii) The Bubble Hypothesis*. The difficulty of conceiving 
the motion of matter through a solid elastic medium has been 
met in another way, namely, by suggesting that what we call 
matter is a deficiency of the ether, and that this region of 
deficiency can move through the ether in a manner somewhat 
analogous to that in which a bubble can move ir^ a liquid. To 
express this in technical language we may suppose the ether to 
consist of an arrangement of minute uniform spherical grains 
piled together so closely that they cannot change their neigh- 
bours, although they can move relatively one to another. 
Places where the number of grains is less or greater than the 
number necessary to render the piling normal, move through 
the medium, as a wave moves through water, though the grains 
do not move with them. Places where the ether is in excess of 
the normal amount would repel one another and move away 

* 0. Reynolds, Submechanics of the Universef Cambridge, 1903. 



470 MATTER AND ETHER THEORIES [CH. XXII 

out of our ken, but places where it is below the normal amount 
would attract each other according to the law of gravity, and 
constitute particles of matter which would be indestructible. 
It is alleged that the theory accounts for the known phenomena 
of gravity, electricity, and light, provided the size of its grains 
is properly chosen. Re3molds has calculated that for this 
purpose their diameter should be rather more than 5 x 10~^^ 
centimetres, and that the pressure in the medium would be 
about 10* tons per square centimetre. This theory is in itself 
more plausible than the electron hypothesis, but its consequences 
have not yet been fully worked out. 

Returning from these novel hypotheses to the classical 
theories of matter, we may now proceed a step further. Before 
a hypothesis on the structure of matter can be ranked as a 
scientific theory we may reasonably expect it to afford some 
explanation of three facts. These are (a) the Newtonian law of 
attraction ; (b) the fact that there are only a finite number of 
ultimate kinds of matter — such as oxygen, iron, etc. — which can 
be arranged in a series such that the properties of the successive 
members are connected by a regular law; and (c) the main 
results of spectrum analysis. 

In regard to the first point (a), we can say only that none 
of the above theories are inconsistent with the known laws of 
attraction ; and as far as the ether-squirts, the electron, and the 
babble hypotheses are concerned, they have been elaborated into 
a form from ^hich the gravitational law of attraction can be 
deduced. But we may still say that as to the cause of gravity 
— or indeed of force — we know nothing. 

Newton, in his correspondence with Bentley, while declaring 
his ignorance of the cause of gravity, refused to admit the possi- 
bility of force acting at a finite distance through a vacuum. " You 
sometimes speak of gravity," said he*, "as essential and inherent 
to matter : pray do not ascribe that notion to me, for the cause 
of gravity is what I do not pretend to know." And in another 

* Letter dated Jan. 17, 1693. 1 quote from the original, which is in the 
Library of Trinity College, Cambridge j it is printed in the Letters to Bentley, 
London, 1756, p. 20. 



CH. XXIl] MATTER AND ETHER THEORIES 471 

place he wrote*, "'Tis inconceivable, that inanimate brute 
matter should (without the mediation of something else which 
is not material) operate upon and affect other matter without 
mutual contact; as it must if gravitation in the sense of 
Epicurus, be essential and inherent in it... That gravity should 
be innate, inherent, and essential to matter, so that one body 
may act upon another at a distance thro' a vacuum, without the 
mediation of anything else, by and through which their action 
and force may be conveyed from one to another, is to me so 
great an absurdity, that I believe no man who has in philo- 
sophical matters a competent faculty of thinking can ever fall 
into it. Gravity must be caused by an agent acting constantly 
according to certain laws, but whether this agent be material or 
immaterial, I have left to the consideration of my readers." 

I have already alluded to conjectural explanations of gravity 
dependent on the ether-squirts, the electron, and the bubble 
h3rpotheses. Of other conjectures as to the cause of gravity, 
three, which do not involve the idea of force acting at a distance, 
may be here mentioned : 

(1) The first of these conjectures was propounded by 
Newton in the Queries at the end of his Opticks, where he 
suggested as a possible explanation the existence of a stress 
in the ether surrounding a particle of matter f. 

This was elaborated on a statical basis by Clerk Maxwell, 
who showed J that the stress would have to be at least 3000 
times as great as that which the strongest steel would support. 
Sir William Thomson suggested § a d3m.amical way of producing 
the stress by supposing that space is filled with an incom- 
pressible fluid, constantly being annihilated by each atom of 
matter at a rate proportional to its mass, a constant supply 

♦ Letter dated Feb. 25, 1693 ; Letters to Bentley, London, 1756, pp. 25, 26. 

+ Quoted by S. P. Eigaud in his Essay on the Principia, Oxford, 1838, 
appendix, pp. 68—70. On other guesses by Newton see Rigaud, text, pp. 61 — 
62, and references there given. 

X Article Attraction^ in Encyclopaedia Britannica, or Collected Works, vol. ii, 
p. 489. 

§ Proceedings of the Royal Society of Edinburgh, Feb. 7, 1870, vol. vii, 
pp. 60—63, 



472 MATTER AND ETHER THEORIES [CH. XXII 

being kept up at an infinite distance. It is true that this 
avoids Clerk Maxwell's difficulty, but we have no right to 
introduce such sinks and sources of fluid unless we have other 
grounds for believing in their existence. The conclusion is 
that Newton's conjecture is very improbable unless we adopt 
the ether-squirts theory: on that hypothesis it is a plausible 
explanation. 

I should add that Maclaurin implies* that though the above 
explanation was Newton's early opinion, yet his final view was 
that he could not devise any tenable hypothesis about the cause 
of gravitation. 

(2) In 1782 Le Sage of Geneva suggestedf that gravity 
was caused by the bombardment of streams of ultramundane 
corpuscles. These corpuscles are supposed to come in all 
directions from space and to be so small that inter-collisions 
are rare. 

A body by itself in space would receive on an average as 
many blows on one side as on another, and therefore would 
have no tendency to move. But, if there are two bodies, each 
will screen the other from some of the bombarding corpuscles. 
Thus each body will receive more blows on the side remote from 
the other body than on the side turned towards it. Hence the 
two bodies will be impelled each towards the other. 

In order to make this force between two particles vary 
directly as the product of their masses and inversely as the 
square of the distance between them, Le Sage showed that 
it was sufficient to suppose that the mass of a body was pro- 
portional to the area of a section at right angles to the direction 
in which it was attracted. This requires that the constitution 
of a body shall be molecular, and that the distances between 
consecutive molecules shall be very large compared with the 
sizes of the molecules. On the vortex hypothesis we may 
suppose that the ultramundane corpuscles are vortex rings. 

* An Account of Sir Isaac Newton's Philosophical Discoveries, London, 1748, 
p. 111. 

t Memoires de VAcademie des Sciences for 1782, Berlin, 1784, pp. 404 — 432 : 
see also the first two books of his Traits de Fhysiqtic, Geneva, 1818. 



CH. XXIl] MATTER AND ETHER THEORIES 473 

This is ingenious, and it is possible that if the corpuscles 
were perfectly elastic the theory might be tenable*. But the 
results of Clerk Maxwell's numerical calculation show, first, that 
the particles must be imperfectly elastic; second, that merely 
to produce the effect of the attraction of the earth on a mass 
of one pound would require that Le Sage's corpuscles should 
expend energy at the rate of at least billions f of foot-pounds 
per second; and third, that it is probable that the effect of 
such a bombardment would be to raise the temperature of all 
bodies beyond a point consistent with our experience. Finally, 
it seems probable that the distance between consecutive mole- 
cules would have to be considerably greater than is compatible 
with the results given below. 

Tait summed up the objections to these two hypotheses 
by sayingj, "One common defect of these attempts is... that 
they all demand some prime mover, working beyond the 
limits of the visible universe or inside each atom: creating 
or annihilating matter, giving additional speed to spent cor- 
puscles, or in some other way supplying the exhaustion suffered 
in the production of gravitation. Another defect is that they 
all make gravitation a mere difference-effect, as it were ; thereby 
implying the presence of stores of energy absolutely gigantic in 
comparison with anything hitherto observed, or even suspected to 
exist, in the universe ; and therefore demanding the most delicate 
adjustments, not merely to maintain the conservation of energy 
which we observe, but to prevent the whole solar and stellar sys- 
tems from being instantaneously scattered in fragments through 
space. In fact, the cause of gravitation remains undiscovered." 

(3) There is another conjecture on the cause of gravity 
which I may mention §. It is possible that the attraction of 
one particle on another might be explained if both of them 

* See a paper by Sir William Thomson in the Proceedings of the Royal 
Society of Edinburgh, Dec. 18, 1871, vol. vii, pp. 577—589. 

t I use billion with the English (and not the French) meaning, that is, a 
biUionrrlO^a. 

X Properties of Matter, London, 1885, art. 164. 

§ See an article by myself in the Messenger of MatheniaHct, Cambridge, 1891, 
vol. XXI, pp. 20—24, 



474 MATTER AND ETHEK THEORIES [CH. XXII 

rested on a homogeneous elastic body capable of transmitting 
energy. This is the case if our three-dimensional universe rests 
in the direction of a fourth dimension on a four-dimensional 
homogeneous elastic body (which we may call the ether) whose 
thickness in the fourth dimension is small and constant. 

The results of spectrum analysis lead' us to suppose that 
every molecule of matter in our universe is in constant vibra- 
tion. On the above hypothesis these vibrations would cause a 
disturbance in the supporting space, {.e, in the ether. This 
disturbance would spread out uniformly in all directions; the 
intensity diminishing as the square of the distance from the 
centre of vibration, but the rate of vibration remaining un- 
altered. The transmission of light and radiant heat may be 
explained by such vibrations transversal to the direction of 
propagation. It is possible that gravity may be caused by 
vibrations in the supporting space which are wholly longitudinal 
or are compounded of vibrations which are partly longitudinal 
and partly transversal in any of the three directions at right 
angles to the direction of propagation. If we define the mass 
of a molecule as proportional to the intensity of these vibrations 
caused by it, then at any other point in space the intensity of 
the vibration there would vary as the mass of the molecule 
and inversely as the square of the distance from the molecule ; 
hence, if we may assume that such vibrations of the medium 
spreading out from any centre would draw to that centre a 
particle of unit mass at any other point with a force proportional 
to the intensity of the vibration there, then the Newtonian law 
of attraction would follow. This conjecture is consistent either 
with Boscovich's hypothesis or with the vortex theory. It would 
be iuteresting if the results of a branch of pure mathematics so 
abstract as the theory of hyper-space should be found to be 
closely connected with one of the most fundamental problems of 
material science. 

I should sum up the effect of this discussion on gravity on 
the relative probabilities of the hypotheses as to the constitution 
of matter enumerated above, by sayiug that it does not enable 
us to discriminate between them. 



CH. XXU] MATTER AND ETHER THEORIES 475 

The fact that the number of kinds of matter (chemical 
elements) is finite and the consequences of spectrum analysis 
are closely related. The results of spectrum analysis show that 
every molecule of any species of matter, such as hydrogen, 
vibrates with (so far as we can tell) exactly equal sets of 
periods of vibration. Thi^ then is one of the characteristics 
of the particular kind of matter, and it is probable that any 
explanation of why the molecules of each kind have a definite 
set of periods of vibration will account also for the fact that 
the number of kinds of matter is finite. 

Various attempts to explain why the molecules of matter 
are capable only of certain definite periods of vibration have 
been made, and it may be interesting if I give them briefly. 

(1) To begin with, I may note the conjecture that it 
depends on properties of time. This, however, is impossible, 
for the continuity of certain spectra proves that in these cases 
there is nothing which prevents the period of vibration from 
taking any one of millions of different values : thus no explana- 
tion dependent on the nature of time is permissible. 

(2) It has been suggested that there may have been a 
sorting agency, and only selected specimens of the infinite 
number of species formed originally have got into our universe. 
The objection to this is that no explanation is offered as to what 
has become of the excluded molecules. 

(3) The finite number of species might be explained by 
supposing a physical connection to exist between all the mole- 
cules in the universe, just as two clocks whose rates are nearly 
the same tend to go at the same rate if their cases are connected. 

Clerk Maxwells objection to this is that we have no other 
reason for supposing that such a connection exists, but if we 
are living in a space of four dimensions as suggested above in 
chapter xix, this connection does exist, for all the molecules 
rest on one and the same body. This body is capable of trans- 
mitting vibrations, hence, no matter how the molecules were 
set vibrating originally, they would fall into certain groups, 
and all the members of each group would vibrate at the same 
rate. It was the possibility of obtaining thus a physical 



476 MAITER AND ETHER THEORIES [CH. XXII 

connection between the various particles in our universe that 
first suggested to me the idea of a supporting medium in a 
fourth dimension. 

(4) If we accept Boscovich's hypothesis or that of an 
elastic solid ether, and if we may lay it down as axiomatic 
that the mass of every sub-atom is the same, we may conceive 
that the number of ways of combining the sub-atoms into a 
permanent system is limited, and that the period of vibration 
depends on the form in which the sub-atoms are combined 
into an atom. This view is not inconsistent with any known 
facts. I may add that it is probable that the chemical atom is 
the essential vibrating system, for the sodium spectrum, to take 
one instance, is the same as that of all its compounds. 

(5) In the same way we may suppose that the vortex rings 
are formed so that they can have only a definite number of 
stable forms produced by interlinking or kinking. 

(6) Similarly we may modify the popular hypothesis by 
treating the atoms as indivisible aggregates of sub-atoms which 
are in all respects equal and similar, and can be combined in 
only a limited number of forms which are permanent. But 
most of the old difficulties connected with the atoms arise again 
in connection with the sub-atoms. 

(7) I am not aware that Clerk Maxwell discussed any other 
hypotheses in connection with this point, but it has been sug- 
gested recently that, if the various forms of matter were evolved 
originally out of some one primitive material, then there may 
have been periodic disturbances in this matter when the atoms 
were being formed, such that they were produced only at some 
definite phase in the period*. 

Thus, if the disturbance is represented by the swinging of 
a pendulum in a resisting medium, it might be supposed that 
the atoms were formed at the points of maximum amplitude, 
and we should expect that the atoms successively thrown off 
would form a series having the properties of its successive 
members connected by a regular periodic law. This conjecture, 

* See Nature, Sept. 2, 1886, vol. xxxiv, pp. 423—432, 



CH. XXIl] MATTER AND ETHER THEORIES 477 

when worked out in some detail, led to the conclusion that some 
elements which ought to have appeared in the series were 
missing, but it was possible to predict their properties and to 
suggest the substances with which they were most likely to be 
found in combination. Guided by these theoretical conclusions 
a careful chemical analysis revealed the fact that such elements 
did exist. 

That this hypothesis has led to new discoveries is some- 
thing in its favour, but I do not wish to be understood to say 
that it is a theory which leads to results that have been verified 
subsequently. I should say rather that we have obtained an 
analogy which is sufficiently like the truth to suggest new 
discoveries. Such analogies are often the precursors of laws, 
so that it is not unreasonable to hope that ere long our 
knowledge of this border-land of chemistry and phj^sics may be 
more definite, and thus that molecular physics may be brought 
within the domain of mathematics. It is however very re- 
markable that J. J. Thomson's conclusions on the stability of 
the orbital systems he devised should agree so closely with 
Mendelejev's periodic law. 

On the whole Clerk Maxwell thought that the phenomena 
point to a common origin of all molecules of the same kind, that 
this was an event not belonging to that order of nature under 
which we live, but must have originated when or before the 
existing order was established, and that so long £is the present 
order exists it is immutable. 

This is equivalent to saying that we have arrived at a point 
beyond which our limited experience does not enable us to carry 
the explanation. 

That we should be able to form an approximate idea of 
the size of the molecules of matter is a testimony to the 
extraordinary development of mathematical physics in the 
coui'se of the nineteenth century. 

Sir William Thomson suggested* four distinct methods of 

• See Nature, U&TQh 31, 1870, vol. i, pp. 551—553 ; and Tait's Recent Advances, 
pp. 303 — 318. The fourth method had been proposed by Loschmidt in 1863. 



478 MATTER AND ETHER THEORIES [CH. XX IT 

attacking the problem. They lead to results which are not 
very different. 

The first of these rests on an assertion of Cauchy that the 
phenomena of prismatic colours show that the distance between 
consecutive molecules of matter is comparable with the wave- 
lengths of light. Taking the most unfavourable case this 
would seem to indicate that in a transparent homogeneous 
solid or ^liquid medium there are not more than 64x10^ 
molecules in a cubic inch, that is, that the distance between 
consecutive molecules is greater than 1/(4 x 10^)th of an 
inch. 

The second method is founded on the amount of work 
required to draw out a film of liqu