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Helen A. Shafer
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I
MATHEMATICAL
RECREATIONS AND ESSAYS
^O-
. MACMILLAN A?fD CO.. Limited
XONDON • BOMBAY • CALCUTTA
MELBCURNE
THE 3rACMILLAN C03IPANY
NEW TOKK • BOSTON • CHICAGO
DALLAS • SAX FRANCISCO
THE MAC]\IILLAX CO. OF CANADA. Ltd.
TORONTO
MATHEMATICAL
RECKEATIONS AND ESSAYS
BY
W.^W. EOUSE BALL,
FELLOW OF TRINITY COLLEGE, CAMBRIDGE.
SIXTH EBITIOy.
MACMILLAN AND CO., LIMITED
ST MARTIN'S STREET, LONDON
1914
[All rights reserved]
Copyright
First Edition, February, 1892.
Second Edition, May, 1892.
Third Edition, 1896.
Fourth Edition, 1905.
Fifth Edition, 1911.
Sixth Edition, 1914.
1
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 additional matter has been
inserted since the work was first issued in 1892.
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
VI PREFACE
solutions are long or intricate I have been generally content
to indicate memoirs or books where 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 eleven chapters, mostly
dealing with Historical Questions. It is with some hesitation
that I have included in this part chapters 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 my readers.
I am indebted to my friend Mr G. N. Watson for his
kindness in reading the proof-sheets of this (the fifth) edition,
and for many helpful suggestions and comments.
w. w. rousp: ball.
Trinity College, Cambridge.
October, 1911.
NOTE TO THE SIXTH EDITION (1914).
In this edition, besides trivial corrections and changes, a
few additional recreations have be^n inserted, and Chapter XVIII
has been re-written.
Vil
TABLE OF CONTENTS.
PART I.
i¥latf)ematiral aaerreatioitsi*
Chapter I. Arithmetical Recreations.
Baehet. Ozanam. Montucla .....
Elementary Questions on Numbers ....
Determination of a number selected by someone
Prediction of the result of certain operations
Problems involving two numbers
Miscellaneous Problems ....
Problems with a Series of Numbered Things .
Medieval Problems
The Josephus Problem. Decimation
Addendum on Solutions
PAGE
2
3
4
7
10
12
14
18
23
27
Chapter II. Arithmetical Recreations (Continued).
Arithmetical Fallacies » ... 28
Paradoxical Problems 31
Permutation Problems 32
Bachet's Weights Problem 34
Problems in Higher Arithmetio 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
viii TABLE OF CONTENTS
Chapter III. Geometrical Recreations.
Geometrical Fallacies '^^
Paradoxical Problems • ... 52
Map-Colouring Theorem 54
Physical Coufiguration of a Country 59
Addendum on a Solution 61
Chapter IV. Geometrical Recreations [Continued).
Statical Games of Position 62
Three-in-a-row. Extension to p-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 Rings 80
Addendum on Solutions . « 81
Chapter V. Mechanical Recreations.
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 93
Boat Moved by a Rope inside the Boat 101
Hauksbee's Law 101
Cut on Tennis-Balls, Cricket-Balls, Golf-BalJs 103
Flight of Birds 106
Curiosa Physica .• 107
Chapter VI. Chess-Board Recreations.
Chess-Board Notation 109
Relative Value of the Pieces 110
The Eight Queens Problem 113
Maximum Pieces Problem 119
Minimum Pieces Problem . 119
Analogous Problems ... ..... 121
TABLE OF CONTENTS
IX
Re-Eutrant Paths on a Chess-Boaid
Knight's Re-Entrant Path
King's Re-Entrant Path .
Rook's Re-Entrant Path .
Bishop's Re-Entrant Path
Miscellaneous Problems
PAGE
122
122
133
133
134
134
Chapter VII. Magic Squares.
Notes on the History of Mi'.gic Squares .
Construction of Odd Magic Squares
Method of De la Loubere and Bachet
Method of De la Hire .
Construction of Even Magic Squares
First Method
Method of De la Hire and Labosne
Composite Magic Squares ....
Bordered Magic Squares ....
Magic Squares involving products .
Magic Stars, etc
Unsolved Problems .....
Hyper-Magic Squares
Pan-Diagonal Squares
Symmetrical Squares
Doubly-Magic and Trebly-Magic Squares
Magic Pencils
Magic Puzzles
Magic Card Square. Euler's Officers Problem
Reversible Square ....
Domino Squares ....
Coin Squares
Addendum on a Solution ....
138
139
140
142
144
145
149
152
152
154
154
155
156
156
162
163
163
166
166
167
168
169
169
Chapter VIII. Unicup.sal Problems.
Euler's Problem 170
Definitions 172
Euler's Results ... 172
Illustrations 176
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 OF CONTEI^TS
Chapter IX. Kirkman's School-Girls Problem.
PAGE
History of the Problem 193
Solutions by One- Step Cycles 19o
Examples when n=3, 9, 27, 33, 51, 57, 75, SI, 99 . . . 196
Solutions by Two-Step Cycles 199
Examples when w = 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, 93, 99 205
Solutions by the Focal MeiLod 209
Examples when w = 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
Walecki's Theorem 218
Problem of w^ Girls in n groups (Peirce) ...... 219
Examples when n= 2, 3, 4, 5, 7, 8 219
Example when n is prime 220
Kirkman's Problem in Quartets, &c. 221
A Bridge Problem. Arrangements in Pairs , 221
Sylvester's Corollary to Kirkman's Problem 222
Steiner's Combinatorische Aufgabe ..... .4 . 223
Chapter X. Miscellaneous Proble:.i3.
The Fifteen Puzzle
The Tower of Hanoi
Chinese Eings
Algebraic Solution ....
Solution in Binary Scale of Notation
Problems connected with a Pack of Cards
Shuffling a Pack ....
Arrangements by Rows and Columns
Bachet's Problem with Pairs of Cards
The Three Pile Problem .
Gergonne's Generalization
The Mouse TrajD. Treize .
224
228
229
230
232
234
235
237
238
240
241
245
TABLE OF CONTENTS
XI
PART II.
i^lferellaneoue? (Sefe^api^.
Chapter XL The Mathematical Tripos.
Medieval Course of Studies: Acts .
The Eenaissance at Cambridge
Eise of a Mathematical School
Subject-Matter of Acts at different periods
Degree Lists
Oral Examinations always possible .
Public Oral Examinations become customary, 1710-30
Additional work of Moderators. Stipends raised, 1731
Facilitates arrangement in order of merit
Scheme of Examination in 1750
Eight of M.A.s to take part in it
Scheme of Examination in 1763
>/Foundation of Smith's Prizes, 1768
Introduction of a Written Examination, circ. 1770
Description of the Examination in 177*2.
Scheme of Examination introduced in 1779
System of Brackets
Problem Paper in 1786 ....
Description of the Examination in 1791
The Poll Part of the Examination
A Pass Standard introduced .
Problem Papers from 1802 onwards
Description of the Examination in 1802.
Scheme of Reading in 1806
Changes introduced in 1808 .
Description of the Examination in 1816.
Introduction of Analytical Notation
Description of the Examination in 1820 and 1823
Alterations in Schemes of Study, 1824
Scheme of Examination in 1827
Scheme of Examination in 1833
All the papers marked .
Scheme of Examination in 1839
Scheme of Examination in 1848
Creation of a Board of Mathematical Studies
Scheme of Examiualion in 1873
PAGE
248
249
249
250
251
252
253
253
253
254
254
255
257
257
257
260
260
261
263
264
264
265
265
268
270
270
271
272
274
274
276
276
2T&
277
278
279
xu
TABLE OF CONTENTS
Scheme of Examination in 1882, 1883 . . . .
Fall in number of Students reading Matbematics
Scheme of Examination introduced in 1908 .
Origin of term Tripos .......
Tripos Verses
PAGE
280
281
281
281
282
Cha
les
Chapter XII. 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, Apollonius, and Diodes . 287
Solutions by Vieta, Descartes, Gregory of St Vincent, and Newton
The Trisection of an Angle
Ancient Solutions quoted by Pappus
Solutions by Pappus, Descartes, Newton, Clairaut, and
The Quadrature of the Circle .....
Incommensurability of tt
Definitions of tt
Origin of symbol tt
Methods of approximating to the numerical value of ir
Geometrical methods of Approximation .
Eesults of Egyptians, Babylonians, Jews
Eesults of Archimedes and other Greek Writers
Eesults of Eoman Surveyors and Gerbert
Eesults of Indian and Eastern Writers
Eesults of European Writers, 1200-1630
Theorems of Wallis and Brouncker
Analytical methods of Approximation. Gregory's series
Eesults of European Writers, 1699-1873
Geometrical Approximation
Approximations by the Theory of Probability
290
291
291
292
293
294
295
296
296
296
297
297
298
298
300
302
303
303
305
305
Chapter XIII. The Parallel Postulate
The Problem ....
The Earliest Proof. Thales .
Pascal's Proof ....
Pythagorean and Euclidean Proof
Features of the Problem .
Ptolemy's Proof of the Postulate
Proclus's Proof of the Postulate
Wallis's Proof of the Postulate
Bertrand's Proof of the Postulate
307
308
308
310
311
313
314
314
315
TABLE OF CONTENTS
Xlll
PAGE
Playfair's Proof of the Postulate 316
Playfair's Rotational 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 XIV. Insolubility of the Algebraic Quintic.
The Problem 327
Lagrange's Examination of Solutions of Algebraic Equations . . 327
Demonstration that the Quintic is Insoluble. Abel. Galois . . 329
Order of the Radicals in any Solution 330
Chapter XV. Mersenne's Numbers.
Mersenne's Enunciation of the Question
Results already known
Cases awaiting verification ....
Table of Results
History of Investigations
Methods used in attacking the Problem .
Lucas's Test by Residues
By trial of divisors of known forms
By indeterminate equations .
By properties of quadratic forms .
By the use of a Canon Arithmeticus
By properties of binary powers
By the use of the binarj' scale of notation
By the use of Fermat's Theorem .
Mechanical methods of Factorizing Numbers .
333
334
335
336
337
340
340
341
343
344
344
345
346
346
347
Chapter XVI. String Figures.
Two Types of String Figures or Cat's-Cradles
Terminology. Rivers and Haddon .
European Varieties of Cat's-Cradles
The Cradle ....
Snuffer-Trays ....
Cat's-Eye ....
Fish-in-a-Dish
Pound-of-Candles, Hammock, Lattice-Work, Other Forms
Sequence of Forms ...,,,.
348
349
350
351
352
352
353
354
366
XIV
TABLE OF CONTENTS
PAGE
Oceanic Varieties of Cat's-Cradles 357
Openings A and B. Movement T 357
A Door 358
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 . c 376
The Button-Hole Trick 377
The Loop Trick . . ....... 378
The Waistcoat Puzzle 378
Knots, Lashings , , 379
Chapter XVII. Astrology.
Astrology. An Ancient Art
Two Branches of Medieval Astrology: Natal and Horary
Rules for Casting and Eeading a Horoscope .
Houses and their Significations ....
Planets and their Significations ....
Zodiacal Signs and their Significations
Knowledge that rules were worthless ....
Notable Instances of Horoscopy
Lilly's prediction of the Great Fire and Plague .
Flamsteed's guess
Cardan's Horoscope of Edward VI . . .
380
381
381
381
384
386
388
389
390
390
391
TAIILE OF CONTENTS
XV
Chapter XVIII. Cryptographs and CrrrTEKS.
Authorities
Cryptographs. Definition. Illustration .
Ciphers. Definition. Illustration
Essential Features of Cryptographs and Ciphers
Cryptographs of Three Types ....
Transposition Type. The Eoute Method
Use of non-significant symbols. The Grille
Use of broken symbols. The Scytale .
Ciphers. Use of arbitrary symbols unnecessary
Ciphers of Four Types
Substitution Alphabets. Illustrations .
Conrad's Tables of Frequency
Ciphers of the Second Type. Illustrations
Shifting Alphabets. Illustrations .
Gronfeld's and the St Cyr Methods
The Playfair Cipher ....
Ciphers of the Fourth Type. Illustrations
Desiderata in a good Cipher or Cryptograph .
Cipher Machines
On the Solution of Cryptographs and Ciphers
Example
Historical Ciphers
Julius Caesar, Augustus, Bacon, Morse Cod
Charles I
Pepys
De Kohan, Marie Antoinette .
The Five Digit Code Dictionary .
PAGE
395
395
396
397
397
397
400
402
403
403
403
405
406
409
<109
411
412
413
414
414
415
418
418
419
, 420
421
421
Chapter XIX. Hyper Space.
Authorities ....
Two subjects of speculation
Space of Two Dimensions
Space of One Dimension .
Space of Four Dimensions
Existence in such a world
Arguments in favour of the existence of such a world
Use of, to explain physical phenomena
Non-Euclidean Geometries ......
Euclid's Axioms and Postulates. The Parallel Postulate
Hyperbolic and Elliptic Geometries of Two Dimensions
Non-Euclidean Geometries of Three or more Dimensions
Non-Archimcdian Geometries of Two Dimensions .
Non-Legendrian Geometry
Semi-Euclidean Geometry .....
Truth of Axioms
424
425
425
426
426
427
428
430
433
433
433
435
436
436
436
437
XVI
TABLE OF CONTENTS
Chapter XX. Time and its Measurement.
PAGE
Difficulty of the Subject 438
Simultaneity . , 438
Units for measuring Durations (days, weeks, months, years) . . 439
The Civil Calendar (Julian, Gregorian, Ac.) 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 XXI. Matter and Ether Theories.
Labile Ether
Subject of Chapter. Views formerly held about Matter
Hypothesis of Continuous Matter .
Atomic Theories
Popular Atomic Hypothesis .
Boscovich's Hypothesis .
Hypothesis of an Elastic Solid Ether
Dynamical Theories ....
The Vortex Eing Hypothesis
The Vortex Sponge Hypothesis
The Ether-Squirts Hypothesis
The Electron Hypothesis
Speculations due to investigations on Radio
Le Bon's Suggestion
Mendelejev's Hypothesis .
The Bubble Hypothesis.
Eequisites in a Theory ....
Conjectures as to the Cause of Gravity .
Conjectures to explain the finite number of specie
Size of the Molecules of Bodies
Mass of a Body affected by its Motion .
Theory of Relativity
Index .......
Notices or Books
Activity
s of Atoms
459
460
460
460
461
462
462
463
464
465
465
466
468
468
469
470
471
475
477
480
480
483
493
PART I.
i^latl^ematiral J^etrfatfonE!.
" Les hommes ne sont jamais jylus ingenieux que dans V invention
desjeux; V esprit s'y trouve d son aise Apres lesjeux qui dependent
uniquement des nomhres viennent les jeux ou entre la situation
Apres les jeux oil ne/Urent que le nomhre et la situation viendraient
les jeux ou entre le 7nouvement..,.Enjin il serait a souhaiter qu^on
eut un cours entier desjeux, traiies mathematiquement." (Leibnitz:
letter to De Montmort, July 29, 1715.)
B. R.
2
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
Prohlhnes plaisans et delectables, 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
Ozanam's Beci^eations 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
Biocjraphie Universelle allusion is made to subsequent editions
en. l] ARI'J HMETICAL 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 fur 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 conjuiing, I refrain from alluding to the
1 o
4 ARITHMETICAL RECREATIONS [CH. I
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 half of it.
(iii) Tell him to multiply the result of the second step by 3.
(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?i + 1, according as the result of step (i)
was even or odd.
The demonstration is obvious. Every even number is of
the form 2n, and the successive operations applied to this give
(i) 6n, which is even; (ii) J6w = 37i; (iii) SxSn = 9n; (iv)
^9n = n; (v) 2n. Every odd number is of the form 2?i + 1, and
the successive operations applied to this give (i) Qn + 3, which
is odd; (ii) i(6?i + 3 + 1) = 871 + 2 ; (iii) 3(8?i + 2) = 9/^ + 6;
(iv) ^ (9n + 6) = n+ a remainder ; (v) 2?i + 1, These results
lead to the rule given above.
Second Methodf. 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
* Baehet, Prohlejiies, Lyons, 1624, problem i, p. 53.
t A similar rule was given by Baehet, problem iv, p. 74.
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) bn; (ii) 5?i + 6 ; (iii) 20/1 + 24;
(iv) 20?i + 33, (v) 100;i + 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, 6. (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 acjhd is subtracted from this sum,
the remainder will be the number chosen originally.
For, if 71 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,
71 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 Methudf, 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 ^x + y, where a; :}> 9 and yi^^, and if r is the
* Bachet, problem v, p. 80.
t Educational Times, London, May 1, 180o, vol. xlviii, p. 234.
6 ARITHMETICAL RECREATIONS [CH. I
remainder when a — h-^o(c — d) is divided by 9, we have
0) = e, y = 9 —r.
The demonstration is not difficult. Suppose the selected num-
ber is 9^+1/. Step (i) gives 90a; + IO1/ + a. Let y -h a = Sn -\- b,
then the quotient obtained in step (ii) is 30a; + 3?/ + n. Step
(iii) gives .300a; + SOy + lOn + c. Let n + c = Sm + d, then the
quotient obtained in step (iv) is lOOx + lOy -{■ Sn -\- m, which I
will denote by Q. Now the third digit in Q must be x, because,
since y :^ S and a :|> 9, we have n :}> 5 ; and since n:^ 5 and c :^ 9,
we have w ::f> 4 ; therefore lOy + 8n + m r)^ 99. Hence the third
or hundreds digit in Q is x.
Again, from the relations y -\- a = Sn + b and n-\-c= 3m + d,
we have 9m — y = a — b + S(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 ob and y are both
known, and therefore the number selected, namely 9x-\-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 remairider 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 ^, 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 a'c'd' ... 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 h', and similarly find analogous numbers C\ D, Rules
for the calculation of A, B, G,... are given in the theory of
numbers, but in general, if the numbers a', h' , c\ ... are small,
the corresponding numbers A, J5, 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 + 56 + (7c + ..., and let M {x) stand for a
multiple of x. Now A = M{a')-\-l, therefore Aa = M {a') -\- a.
Hence, if the first term in N, that is Aa, is divided by a, the
remainder is a. Again, 5 is a multiple of ac'd' Therefore
Bh is exactly divisible by a Similarly Gc, Dd,... are each
exactly divisible by a\ Thus every term in N, 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-7i = M{a).
Similarly N~n = M(b'),
N-n = M{c'\
But a\ b\ c', ... are prime to one another.
.-. N-n = M(a'b'c\..) = M(p),
that is, N=M{jp) + 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,
y = 4, c'=5, p = 60, ^ = 40, 5 = 45, 0=36. If the number
chosen is less than 420, we may take a' = 3, 6' = 4, c' = b., d' = 1 ,
p = 420, il = 280, 5 = 105, (7=336, i) = 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, 6; (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 = \\, 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 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-\-^) 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 — g 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-^l).
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 A 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
* Bachet, 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.
(i) 237 100a + 106 + c
(ii) 732 100c + 106 + g
(iii) 495 lOO(a-c-l) + 90 + (10 + c-a)
(iv) _594 100(10 + c-a)+ 90 + (a-c-l)
(v) 1089 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 11^, 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 Reprints, 1890, vol. i.iii, p. 78.
10 ARITHMETICAL RECREATIONS [CH. T
The following analysis explains the rule, and shows that the
final result is independent of the sum written down initially.
£ s. d.
(i) a h 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 by ^. 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 byp. The
numbers, m = 7, n = 15, _p = 3, ^^ = 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. + 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 lO") + (0 X 10^) + (1 X 10) + 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, i.e. 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) ^a] (ii) 5a 4- 7; (iii) 10a + 14;
(iv) 10(1 + 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 Mathematicall 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
n. van Etten, published in 1G26.
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 b be
the tens and units digits of the birth-year. The successive opera-
tions give (i) 5a; (ii) 5a + 2; (iii) lOci + 4; (iv) 10a + 4 + 6;
(v) 120- (10a + 6), which is his age in 1916. The rule can be
easily adapted to give the age in any specified year.
Fourth Example^. Another 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 L' Illustration for July 13, 1895.
t L'lntermediaire des Matheinaticiens , 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 4- 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 it is known that
the sum of the digits of every multiple of nine is itself a
multiple of nine.
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, 5,
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, ... w, 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. I
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 w = 4,
I make jp = 88, (? = 264 ; if n = 5, _p = 231, ^ = 790 ; with the
four digits 0, 1, 2, 3, ^^ = 36, q — 40. The problem is not easy,
and these limits may be too low.
Four Fours Problem. Another traditional recreation is,
with the ordinary arithmetic and algebraic notation, to express
the consecutive numbers as far as possible in terms of four
" 4's." Everything turns on what we mean by ordinary notation.
I take it that this allows the use of the denary scale {ex. gr.
numbers like 44) and decimals ; the symbols for factorials and
square roots (repeated if desired a finite number of times) ; and
the symbols for addition, subtraction, multiplication, division,
and brackets; but I consider that indices (other than first
powers), not expressible by a " 4 " or " 4's," and roots (other
than square roots) are excluded ; and that though a number
like 2 can be expressed by one " 4," numbers like "2 and 22 are
inadmissible. On these assumptions we can express every
number up to and including 112. If we also allow the use of
subfactorials * we can thus express every number up to and
including 87 7 f. The similar problems of the expression by
four"9's" in ordinary notation up to 132, and by four"3's,"
with the use of subfactorials, up to 153, present no difficulty.
Problems with a series of things which are numbered.
Any collection of things which can be distinguished one from
the other — especially if numbered consecutively — afford easy
concrete illustrations of questions depending on these ele-
mentary properties 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. I present
them in these forms.
* Subf actorial n is written n| or n j and is equal to
n\ (l-l/l! + l/2!-l/3! + ...±l/n!).
+ For the method used to obtain these results and extensions of them, see
the Mathematical Gazette, May, 1912.
CH. l] AKITHMETKJAL 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, w, 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 + 1 ), &c.,
the (7i + 12)th 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, Viii, VII, 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 w^hich 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 (ti + 12 — m) is always
positive, since n is positive and m is not greater than 12, and
therefore if we make him pass over (?^+ 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 follow*ing is another well-kno^vn
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 from 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 ?ith 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; Ougbtied or Leake, Mathematkall Rccrea-
tiovs, London, 1653, 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 icth 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,
firom 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 (x-\- l)th, the next
as the (x + 2)th, and so on, in which case the card originally
chosen will be the (y + 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 a? is the number of pips on the bottom card of a pile,
* A particular case of this 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-^,) + (13-a'.3)+... + (13-^p) + r = 52,
.*. ISp — (x^-{-Xo-\- ... -\- Xp) + ?' = 52,
,\ {Ci-\-a;o+ ... +Xp = ISp — 52 + r
= 13(j^-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 + l)p + r — n. In an ecarte
pack 71= 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. Remove 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 ^; in the second line, the
10, 1, 2, ... 9, of suit B; 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
♦ Bachet, problem xix, 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 n. Then if the suit is A, 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?i + 6)th card ; and if the suit is D,
it will be the {4m + 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 eleaientary 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.
Fii'st 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 f. The next of these is a not uncommon
* Some similar problems were given by Bachet, Appendix, problem in,
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.
+ Bachet, 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's 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 + 1.
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 (saj-) 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 3, 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; ^ takes a 3, and scores 17 ; jB takes a 4, and scores
21 ; ^ 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, S; A, 1; B, Q; A, S; B, 4>; A, 2;
B,5;A,1;B,5;A,2;B,5;A,2;B,S, ^ 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 ; jB 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 RECREATIONS 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 ?i 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 8p be the sum of the coefficients of 2^ thus
determined. Thus Sp = dip + d^p + cl^p-^ ..., Then either Sq,
Sx,S^,... 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 Exajiiple. The following medieval problem is some-
what more elaborate. Suppose that three people, P, y, R,
* From a letter to me by Mr R. 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, Pj, Pg, ..., 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
Pji. 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 P^ had chosen the domino numbered h,
* Bachet, problem xxv, p. 187.
t Bulletin dcs Sciences Matliematiques, Paris, 1893, vol. xvii, pp. 105 — 107.
CII. l] ARITHMETICAL RECREATIONS 23
he would take n^k counters. The total number of counters
taken is '^n^k. Divide this by n, then the remainder will be
the number on the domino selected by Pq ; divide the quotient
by ??, 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 P^ ; 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 byP/^.
Thus in Bachet's problem with 3 people and 3 dominoes,
we should first give one counter to Q, and two counters to P,
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 P. 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 P.
Eojj^loration 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
^ a (1 + i + J + . . . + Ijn ) days.
TJie Josephiis 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 ea«ily tsolvcd ompiricaliy.
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 "f*. In this case we must arrange the men thus :
GGGGTTTTTGCTGGGTCTTGGTTTGTTGGT,
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 l,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 dny she said to him, " Dear Husband,
* De Bello Judaico, bk. in, chaps. 16 — 18.
t Bachet, problem xxiii, p, 174. The same problem had been previously
enunciated by Tavtaglia.
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 ^vife, 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-f-m)th place when
p-{-m is not greater than n+1, and the (p + m. — n — l)th.
place when p + m is greater than n + 1. Thus, provided there
are to be r men left, their original positions are each shifted
forwards along the circle ?n places for each addition of a single
man to the original group*.
Now suppose that with n men the last survivor (r = 1)
occupied originally the p\h place, and that with {n-\-x) men
the last survivor occupied the yih. 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 Scientljic Fa^en, Cambridge, vol. n. 1900, pp. 432—435.
2g ARITHMETICAL RECREATIONS [CH. I
For instance, take the Josephus V^o^^^.t ^ToZv^d
ThPn we know that the final survivor of 41 men o<=cupiea
Then we know ^^_^^ ^^^^ ^^^^^ ^^ T^^en
:7:7Z th t.^vor oL'upied originally the .th place.
ite:lr;onsider only the --J^ ^tl^I ^^^^^^^^^^^
, less than «' /^ ^^^X/^/f^tiJ^ X , ^sitive and
Now, we have to take a value ot x w ii ^ r
" = \^.;Sr;S 349956 52i934, or 787401; and the
69127 If 5536 233304^ 3 14 21 47, 158, 237, 533, 1199, 4046,
second place when »- J'f'^*' t'' ' nn65S. From these
cAflQ i^fi'i'5 46085, 103691, 1181 lO^.or ill iujt'- ...
lXhr.;pe-dapplicat.ns^^^
any intermediate values of « ^^^ P"— f,^ go^lth place ;
by the man last taken. JJ^^ ^'^fj" ^^^ ^ ^h 1000000 men,
with 100000 men, the 92620th place ana
.V, R'^TTQSth Dlace are those which would be seleccea oy
the 637798tn puce aie „„hiected to tnmation.
bimiiaiiy <* originally m the 26th
!='=,'^ « ?.t?° «? C« r- ,tt;'"^
CH. l]
ARITHMETICAL RECREATIONS
27
every Kh man is selected, all the Christians will be picked out
for punishment. The problem is to find a, 6, 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 A;th 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 h instead of h, or vice versa, and so consign all his
iriends 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 h. Obviously if c = 2, then for an arrangement like
T C G T Si solution is A = 4, A; = 3. If c = 3, then for an arrange-
ment like TO TOOT a solution is /t = 7, A; = 8. If c = 4, then
for an arrangement like T G T T G T G C a solution is h = 9,
A; = 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 50 + 49 + 1/2 + 38/76 = 100.
A solution of the nine digit problem is
1-234 + 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... 0... 0... 0...
First, make their contents 0... 8... 11... 5...
Second, make their contents ... 16... 8... 0... 0...
Third, make their contents ... 16 ... ... 8 ... ...
Fourth, make their contents ... 3 ... 13 ... 8 ... ...
Fifth, make their contents 3 ... 8 ... 8 ... 5 ...
Lastly, make their contents ... 8... 8... 8... 0...
Note. Pages 26 — 27. The simplpst solution of the five Christians and five
Turks problem isa=l, /i = ll, 6 = 9, A = 29.
28
CHAPTER 11.
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 o])inion, 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''- h\ .'. 6 (a - 6) = (a + b) (a - h).
.'. b = a-\-b. .-. b = 2b. .*. 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, 1839, vol. ii, p. 150 ; 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 x = to a; = l) of the integral to y (from
y = Otoy — l)oia, function <p should be equal to the integral to y (from y = to
y = l) of the integral to x (from x = to x = l) of (p, but if tp={x'^~ y^)l{x'^ + y")-,
this gives lir= - \ir.
CH. Il] ARITHMETICAL RECREATIONS 29
Second Fallacy. Another example, the idea of which is due
to John Bernoulli, may be stated as follows. Wc have (—1)'*= 1.
Take logarithms, .-. 2 log(- 1) = log 1 = 0. .*. log (- 1) = 0.
.-. -l=eo. .-. -1 = 1.
The same argument may be expressed thus. Let a; be a
quantity which satisfies the equation e*= — 1. Square both
sides,
• » 6/ • — ■ J. • • • ^*tf • — - yjm • • ^ — \Jm • . C/ — O •
Bute^=-1 and e'=i, .'. -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{l -\-oo) = x — ^x'^ + ^a:^—
If a;= 1, the resulting series is convergent; hence we have
log2 = l-i + i-i + i-i + |-i + i-....
.-. 21og2 = 2-l + f-i + |-J + |-i + f-....
Taking those terms together which have a common denominator,
we obtain
21og2 = H-i-J+i + f-i+4...
= i-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+i-i + i-i + ...
={a+j+i+...)+(4+i+K--)i-2(i+j+H--)
= 0.
Fifth Fallacy. We have
^ax \/b = \/ab.
Hence V- 1 x V- 1 = V(- 1) (- 1),
therefore, (V^)- = S^T, 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,
V« — y = i \/y — X (i),
where % 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 x — a and y = h,
,*. s/a — h^is/h — a (ii).
Next put x=^h and y = a,
.*. \/h — 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— 1 in both cases. Hence, from (ii) and (iii), we have
^/a-h^/b — a = ^^^/b — a^/a — b,
.•. 1 = i\
that is, 1 = — 1.
Seventh Fallacy. The following fallacy is due to D'Alem-
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
aib = c : d, and if in this proportion a>b, then c> d. Now if
we put a = d = l and b = c= — l 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, Marcli 1, 1894, vol. xlix, pp. 365—360, 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, .B 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 paj^ments 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 affected 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 l/?ith
of his money on an even chance (say tossing heads or tails
with a penny): he repeats this again and again, each time
betting 1/nth 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. 11
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!/, that is, 53644,737765,488792,839237,440000
* The Tribune, September 3. 1906.
t Mathematical Tripos, Cambridge, Part I, 1894.
CH. Il]
ARITHMETICAL RECREATIONS
33
possible diflPerent 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 5 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 pr/{ns + r).
The Knights of the Bound 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)(n — 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)(n — 2)/2 arrangements in which
no one has the same two neiglibours on any two occasions.
Solutions for various values of 7i 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.
1.2.3.4.5.6.7.8
; 1.2.5.6.8.7.4.3
; 1.2.7.8.4.3.5.6;
1.3.5.2.7.4.8.6
, 1.3.7.4.6.8.2.5
; 1.3.8.6.2.5.7.4;
1.4.2.6.3.8.5.7,
1.4.3.8.7.5.6.2,
, 1.4.5.7.6.2.3.8;
1.5.6.4.3.7.8.2
, 1.5.7.3.8.2.6.4^
1.5.8.2.4.6.3.7;
1.6.2.7.5.3.8.4
; 1.6.3.5.8.4.2.7
1.6.4.8.2.7.3.5;
1.7.4.2.5.8.6.3
; 1.7.6.3.2.4.5.8
, 1.7.8.5.6.3.4.2;
1.8.2.3.7.6.4.5
; 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. Bergbolt, May, 1906, see The Secretary
and The Queen, August, 1906. Mr Dudeney had given the problem for the case
when 71 = 6 in 1905, and informs me that the problem has been solved by
Mr B. D. Bewiey 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.
u.
34 ARITHMETICAL RECREATIONS [CH. II
The Manage Problem*. Another difficult permutation
problem, suggested by Lucas, is concerned with the number x
of possible arrangements 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,
and is far from easy. I content myself with noting the results
when n does not exceed 10. When n = 3, ^ = 1 ; when ?i = 4,
iz; = 2 ; when 9i = 5, ic = 13 ; when ?i = 6, a; = 80 ; when n = *7,
x = 5l9; when n = 8, a; = 4738; when w=9, 0^ = 43387; and
when n = 10, a;=439792.
Bachet's Weights Problem f. 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 Tartaglia|.
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 4- 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.
+ Bachet, Appendix, problem v, p. 215.
X Trattato de' numeri e misure, Venice, 1556, voL ii, bk. i, chap, xvi, art. 32.
CH. Il] ARITHMETICAL RECREATlONii 36
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-\-x- + ...-\-x^
into factors, each factor being of the form 1 + x^ -{- x^^ -{-... + x^"^;
the number of solutions depends on the composite character of
n -}- 1. The method for case (ii) consists in resolving the expres-
sion a:-" -f a;-"+^ + ...-|-^~^-f-l+a;-l-...-h a;"-^ + x'"- into factors,
each factor being of the form x'"^^ -+-... -I- x'"' -j-l -\-x^-\- ... + a;"""; ^
the number of solutions depends on the composite character of
2n -I- 1.
* See his article in the Quarterly Journal of Matheviatics, 16S6, vol. xxi,
pp. 367 — 373. An account of the method is given in Nature, Dec. 4, 1600,
vol. XLii, pp. 113—114.
3-2
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
a}~*° + x~^^ + , . . 4- 1 + . . . + '^"^ + ^^ First, there is the expres-
sion itself in which a = 1, m=40. Second, the expression is
equal to (1 — x^^)/a^^ (1 — x), which can be resolved into the
product of (1 —x^)la;{l — x) and (1 — a;^^)/a^^(l — 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, w = 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=l. 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 resulved 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 (6).
If we denote p weights each equal to w by w^, these eight
solutions are 1^°; 1, 3'^; l\ 9'; 1, 3, 9*; V\ 27; 1, S\ 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 Diophantus, 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 = :>2f 4^ 41-= 40•^ + 0^ 4l='= 28(r + lir>2,
and so on. Propositions such as the one just (jiiotcd 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 100895,598109
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 ?
Mersexne's Numhers* 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-Matliematica 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 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 fifteen values of p:
namely, 101, 103, 107, 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 rererences, see chapter x\ below.
38 ARITHMETICAL RECREATIONS [CH. II
make 2^ — 1 a composite number. The demonstration of the
prime character of 2^— 1 when jj = 127 has not been published :
and the verification in this case has not been corroborated by
independent work.
Mersenne's revsult 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 when ^ = 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 j9= 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'IntermSdiaire des Mathematiciens, Paris, 1S95, vol. n, 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] ARITHMETICAL RECREATIONS 39
Euler's BiQiTADRATE 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 = 859, 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 4?2 — 1 is the sum of a prime of
the fbrm 4n 4- 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 Arithmetic ae CoUectae, St Petersburg, 1849,
vol. I, pp. 473—476 ; vol. ii, pp. 450—456.
t Transactions of the Halle Academy {Naturforschung), vol. lxxii, Halle,
1897, pp. 5—214: see also L'IntermSdiaire des Mathematiciens, 1903, vol. x,
and 1904, vol. xi.
+ Nouveaiix Blemoires de VAcademie Royale des Sciences, Berlin, 1775, p. 356.
§ Letter of Oct. 18, 1G40, Opera^ Toulouse, 1079, p. 162 : or Brassinne's
Pricis, p. 143.
40 ARITHMETICAL 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 Euler* showed that if n= 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^'- + 1 must be primes of the form 64?i + 1.
During the last thirty years it has been shown f 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 full 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 Fermat 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 x, y, z can be found to satisfy
the equation x^ ^ y^ — z^\ if n is an integer greater than 2.
* Commentarli Academiae Scientiarum Petropolitanae, St Petersburg, 1738,
vol. VT, p. 104 ; see also Novi Comm. Acad. Sci. Petrop., St Petersburg, 1764,
vol. IX, p. 101 : or Commentationes Arithmeticae Collectae, St Petersburg, 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. C. 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
Encyclopedic 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 Mather.iaticiens, Paris, 1908,
vol. XV, p. 2S4.
CH. Il] ARITHMETICAL RECREATIONS 41
This proposition has acquired extr.K^rdiriary 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
n = 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 J:. These
proofs depend upon showing that, if three integral values of
as, 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 ?i = 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 true when complex factors are
admitted. For instance 10 can be expressed as the product of
3+1 and 3 - i, or of 3 4- 2 and 3 - 1, or of 2, 2 + i, and 2 - 1.
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 subsequently 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 Diophanttts, note on p. 339; or Brassinne's Precis, p. 127.
+ Euler's Algebra (English trans. 1797), vol. n, chap, xv, p. 2-17.
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
establishing 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 Dirichletf gave one for n=14; and in 1840
Lame and Lebesgue J gave proofs for w = 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. Recently the method has
been developed by L. E. Dickson ||. His calculations show that
Fermat's result is true if n is less than 6857. 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 demon-
stration 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 Nombi'es, 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—215, 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 « is a prime. A prize * of 100;000 marks
has been offered for a general proof, to be given bet ore 2U07.
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 mngnitude, 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?'^ + 2 = 3/' : the solution
evidently being x = b, 2/ = 3- 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 \ 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 4n -f 1 is expressible as the sum of
two squares — may be established with comparative ease by
properties of such fractions.
♦ Ulntermidiaire des Mathematiciens, vol. xv, pp. 217—218, for references
and details.
t Fermat's Diophantus, bk. vi, prop. 19, p. 320; or Brassinne's Pricis,
p. 122.
I Fermat's Diophantus, bk. iv, prop. 31, p. 181 ; or Brassinne's Precis, p. 82.
•i^
CHAPTER TIL
GEOMETRICAL KECREATIONS.
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 rectanole,
equal to AB 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 KO 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 OAE 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. 20-4, 2Gu, where they appear in the
form in which I originally gave them.
46
GEOMETRICAL RECREATIONS
[CH. ill
00= OE. Similarly, since HO bisects GB and DA and is per-
pendicular to them, we have OD = OA. Also, by construction,
DC = AE. Therefore the three sides of the triangle ODC are
equal respectively to the three sides of the triangle OAE.
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, we
have the angle ODA equal to the angle OAD.
Hence the angle ADC (which is the difference of ODC and
ODA) is equal to the angle DAE (which is the difference of
OAE and OAD). But ADC 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 ABC 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 (7, and
cutting BG in D. From A draw AE perpendicular to BG.
The triangles ABG, ABD are equiangular; hence, by Euc.
VI. 19, /SABG : IS ABD = AG' : AD\
Also the triangles ABG, ABD are of equal altitude; hence, by
Euc. VI. 1,
l\ABG'. AABD = BG:BD,
.-. AG':AD'=BG : BD.
. AG^ _AD'
•'• BG ~ BD •
* See a note by M, Ooccoz in U Illustration, Paris, Jan. 12, 1895.
CH. Ill] GEOMETRICAL RECREATIONS 47
Hence, by Enc. ii. 13,
AB' + BG^ -2BG.be ^ AB'^ BD '-2BD . BE
BG ~ BD '
.\^+BG-2BE=^~ + BD- 2BE,
. ^^' BD-^^' EC
• ^^^ - B^ '^^ _ ^B^ - 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^ , , , ,D
Let ABG be a triangle. Complete the parallelogram of
which AB and BG are sides. Divide AB into n + 1 equal
parts, and through the points so determined draw n lines
parallel to BG. Similarly, divide BG into n + 1 equal parts,
and through the points so determined draw n lines parallel to
AB. The parallelogram ABGD 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 ^C 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 (r, J, i, ... will
approach indefinitely near the line AG, and ultimately will lie
on it; when this is the case the sum of AG and GH will be
equal to AH, and similarly for the other similar pairs of lines.
Thus, ultimately,
AB + BC^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 DO perpendicular to BG. Bisect the angle BAG by AO.
First. If DO and ^0 do not meet, then they are parallel.
Therefore ^10 is at ri-ht angles to BG. Therefore AB = AG.
Second. If DO and AO meet, let them meet in 0. Draw
OE perpendicular to AG. Draw OF
perpendicular to AB. Join OB, OG.
Let us begin by taking the case
where is inside the triangle, in
which case E falls on ^C and F on
BG.
The triangles AOF and AOE are
equal, since the side ^0 is common,
angle OAF= angle OAE, and angle OF A = angle OEA. Hence
AF=AE. Also, the triangles BOF axid GOE are equal. For
since OD bisects BG at right angles, we have OB = 00] also,
since the triangles AOF and AOE are equal, we have
0F= OE; lastly, the angles at F and E are right angles.
Therefoie, by Euc. I. 47 and I. 8, the triangles BOF and GOE
are equal. Hence FB = EG.
Therefore AF + FB ^ AE ■{■ EG, that is, AB = AG.
The same demonstration will cover the case where DO and
AO 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 AO meet outside the
triangle, and E and F fall on AG and AB produced. Draw
CH. Ill]
GEOMETRICAL RECREATIONS
49
OE perpenrlionlar to AC produced. Draw OF perpendicular
to ^^ 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 is, AB = A G.
Thus in all cases, w^hether 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 'ttJ^ is equal to ir/S. On the
hypothenuse, BO, of an isosceles right-angled triangle, DBG,
describe an equilateral triangle ABO, the vertex A being on
the same side of the base as D is. On QA take a point H so
that OH = GD. Bisect BD in K. Join HK and let it cut OB
(produced) in L. Join DL. Bisect DL at 31, 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 00, 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 OCD
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.
B. R.
4
50
GEOMETRICAL RECREATIONS
[CH. HI
OG common, hence (by Euc. I. 8) the angle OCD is equal to the
angle OGH. Hence the angle BGD 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 A BGD be a quadrilateral such that AB 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
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 = 00, 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
AOM, 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 directioa
* i^lathesis, October, 1893, series 2, vol. in, p. 224.
CH. Ill] GEOMETRICAL KECREATIONS 51
Therefore AD and DC, which are perpendicular to this direc-
tion, must be parallel. This result is not uuiversally 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 OM, 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'y 6" with Ox. Hence, by projection on Oy and on Ox,
we have
Z sin (9 = V sin 6' + I" sin 6\
Zcos(9 = rcos^'-}-rcos6'".
n sin 9' -I- sin 6"
\ tan^ =
• •
ncos6' + cos
/' >
where n^Vjl". This result is true whatever be the value of n.
But n may have any value {ex. gr. ?i=x, or n = 0), hence
tan 6 — tan 6' = tan 9"^ 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 ^irab. 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.
.'. ^7rah= |a X 26,
.-. 7r=8/3,
a result which obviously is untrue.
Ninth Fallacy. Ever-y ellipse is a circle. The focal distance
of a point on an ellipse is given (in the usual notation) in terms
* Tliis was communicated to me hy Mr 11, Chai ties.
1 o
52 GEOMETRICAL RECREATIONS [CH. Ill
of the abscissa by the formula r=a+ex. Hence dr/dx=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
{cf. Euc. L 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]
GEO:\IETUICAL RECREATIOMS
63
together form a figure containing 65 suoh 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
__J-
"7 ^
t-ZZ
===/
•^
■^
"^
^^
■*>,
\
■^^^
•^
^
^
B
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'\^ much greater than
its breadth. The diagrams show that the angle between the
two sides of this lozenge which meet at A is tan~^| — 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 5x13 — 8^=1.
Similar results can be obtained from the formulae
13x34-212 = 1, 34x89-552=1,...;
or from the formulae
5--3x8 = l, 132-8 X 21 = 1, 342-21 x55 = l,....
* I do not know who discovered this paradox. It \i given in various modem
books, but I cannot find an earlier reference to it than one in the Zeitachrijt
fUr Mathematik und Fhysik, Leipzig, 1868, vol. xiii, p. 102.
54 GEOMETRICAL RECREATIONS [CH. Ill
These numbers are obtained by finding convergents to the con-
tinued fraction
111
1+ 1 4.1 + 1 + —•
A similar paradox for a square of 17 cells, by which it was
shown that 289 was equal to 288, was alluded to by Ozanam*,
who gave also the diagram for dividing a rectangle of ll by 3
into two rectangles whose dimensions appear to be 5 by 4 and
7 by 2.
Turtons Seventy - Seven Puzzle. A far better dissection
puzzle was invented by Captain Turton. In this a piece of
cardboard, 11 inches by 7 inches, subdivided into 77 small
equal squares, each 1 inch by 1 inch, can be cut up and
re-arranged so as to give 78 such equal squares, each 1 inch
by 1 inch, of which 77 are arranged in a rectangle of the same
dimensions as the original rectangle from one side of which
projects a small additional square. The construction is in-
genious, but cannot be described without the use of a model.
The trick consists in utilizing the fact that cardboard has a
sensible thickness. Hence the edges of the cuts can be
bevelled, but in the model the bevelling is so slight as to be
imperceptible save on a very close scrutiny. The play thus
given in fitting the pieces together permits the apparent pro-
duction of an additional square.
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)
in such a way that no two contiguous districts shall he 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. Mobiusf in his
Lectures in 1840, but it was not until Francis Guthrie J com-
* Ozanam, 1803 edition, vol. i, p. 299.
t Leipzig Transactions {Math.-jyhys. Glasse), 1885, vol. xxxvir, pp. 1—6.
:J: See Proceedings 0/ the Royal Society of Edinburgh, July 19, 1880, vol. x,
p. 728.
CH. Ill]
GE(JMETRICAL REOIIEATIOXS
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 Cayley* recalled attention to it by stating that he could
not obtain any rigorous proof of it.
Probably the following argument, though not a formal
demonstration, will satisfy the reader that the result is true.
Let A, B, G be three contiguous districts, and let X be any
other district contiguous with all of them. Then X must
lie either wholly outside the external boundary of the area
ABC or wholly inside the internal 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 whicli shall
be contiguous with A,B,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 ^, By C are not con-
tiguous, each with the other, or if X is not contiguous with
A, B, and G, it is not necessary to colour them all differently,
and thus the most unfavourable case is that already treated.
* 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 difificulties ar« indicated.
56 GEOMETRICAL RECilEATlONS [CH. Ill
Moreover any 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 proposition 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 w^iere 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, Loudon, 1890, vol. xxiv, pp. 332 — 338; and 1897,
vol, XXXI, pp. 270—285.
t Proceedings of the Royal Society of Edinburgh, July 19, 1880, vol. x,
p. 729 ; Philosophical Magazine, January, 1884, series 5, vol. xvu, p. 41 ; and
Collected Scientific Papers, Cambridge, vol. ii, 1800, p. 93.
CH. Ill] GEOMETUICAL RECUEATIONS 57
meets any other line except at one of the vertices, \vhich 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
the lines is not regarded as such a vertex. Expressed in tech-
nical language the difficulty 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 2?ith order and each of the first degree, or into two graphs
of the 2nth. 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, VIntermedinire des MatMmaticiens, vol. v,
1898, pp. 225—227; and vol. vi, 1899, pp. 30—38. Also Acta Madiematica,
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 71 + 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 /3, 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 7 with the colour 0; the district adjoining J.
and divided from it by the line B with the colour I). Proceed
in this way so that a line /3 always separates the colours A
and B, or the colours G and D; a line 7 always separates
A and (7, or D and B; and a line S always separates A and D,
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 ^, 7, 8 as indicating certain
operations, then an operation like B may be represented as
equivalent to the effect of the two other operations y8 aad 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
>c
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 Maxwellf. 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. G. Dixon, Messenger of MatJiematics, Cambridtje, 1902-3, vol. xxxii,
pp. 81—83.
t Cayley on ' Contour and Slope Lines,' VhiloRophical Marjazine, London,
October, 1859, eeiies 4, vol. xvui, 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, p|). 421—127; 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 it, 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.
\i n-\-l 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
ni\i 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
passes.
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, p^ of double passes, and so on ; let / be the
whole number of forks, /i that of single forks, f of double
CH. Ill] GEOMETRICAL RECREATIONS (>1
forks, and so on ; let w be the number of watercourses, then
there will be also w watersheds. Hence, by the theureuis of
Cauchy and Euler,
A = 1 + j9j + 2j[;., + . . . ,
and w=^ (p, +/i) + 3 (p^ +/;) 4- ....
These results can be extended to the case of a multiply-
connected closed surface.
ADDENDUM.
N'ote. 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 OAlJC,
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.
K9
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 cliapter 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.
Three-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 n" 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 RECREATIONS 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. M(^st 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 — iy/8. 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
counters 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, Lc Jeiix dcs Anciens, second edition, Paris, 1873,
chop. XVIII.
t Educational Times Reprints, 18G8, vol. viii, p. lOO; Ihid. 188G, vol. xt.v,
pp. 127—128.
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 1 5 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 tesselatioD, 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/n.
Hence m (n — 2) w/n = 27r. Therefore (m - 2) (n - 2) = 4. Now
from the nature of the problems m is greater than 2, and so
is n. If m = 3, ?i = 6. If m > 3, then n<6, and since n>2,
CH. IV] GEOMETRICAL KECREATIONS 65
we have in this case only to consider the vahies n = 3, 7i=4,
and ?i = 5. If w = 3 we have m = 6. If n = 4 we have in = 4.
If n= 5, m is non-integral, and this is impossible. Thus the
only solutions are m — S and n = 6, m=4 and w = 4, ?/i=6
and n = 3 *.
If, however, we allow the use of unlike tiles (ex. gr. regular
hexagons, squares, and triangles), we can construct numerous
geometrical designs covering a plane area; though it is im-
possible to make such designs by the use of starred concave
polygons f.
The use of colours not only adds variety to the patterns, but
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 alter-
nately white and black, or sometimes white and red. 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
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
* 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 equaiions,
B. E. 6
66
GEOMETRICAL RECREATIONS
[CH. IV
each line has 10 white tiles and 6 black tiles, and in the re-
maining rows and columns each line has 6 white tiles and
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.
A
B
B
B
B
A
B
B
B
B
A
B
B
B
B
A
An Anallagmatic Isochromatic Pavement.
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 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
CH. IV]
GEOMETRICAL RECREATIONS
67
possible ' designs increases with startling rapidity*. I content
myself with giving two illustrations of designs of pavements
constructed with sixty-four such tiles, all exactly alike.
Examples of Tesselated Pavements.
If more than two colours are used, the problems become
increasingly difficult. As a simple instance of this class of
problems I may mention one, sent to me by a correspondent
who termed it Cross-Fours, wherein sixteen cardboard squares
or tiles are used, 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 : they can be
also arranged so that in each of these ten lines there shall be 6
colours and no more, or 5 colours and no more, or 4 colours and
no more. Puzzles of this kind are but little known ; they are
however not uninstructive.
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 diHPeient
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.
t By Major MacMahon ; an abstract of his paper, read before the London
Mathematicid Society on Feb. 9, 1893, was given in Naiwe, Feb. 23, 1893,
vol. xLvn, p. 406.
6-2
68 GEOMETRICAL RECREATIONS [OH. IV
thirty such cubes, no two of which are coloured alike. Take
anv 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, f. Let the cube K be put on a
table, and to fix our ideas suppose 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 (/; h\ 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 ; h^d, a, e). Similarly from the top south-west corner
of K we get the cubes (/; e ; 6, c, a, d) and (/; d\ b, c, e, a) :
and from the top south-east corner we get the cubes
(/; e; a, c, d, b) and (/; 6 ; 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 K,
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, NW, 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 corners of the top face; of course each placed
with the colour a at the top, while 7 and 1 have the colour 6 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, however, they permit of so
many movements of the pieces that any mathematical analysis
of them becomes too intricate to follow out completely. Games
in which the possible movements are very limited may be
susceptible of mathematical treatment. One or two of these are
given later : here I shall confine myself mainly to puzzles and
simple amusements.
Shunting Problems. 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, DBF, with two sidings, DBA
and FGA, 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 rnns up one siding, such as DBA, it must come back
the same way.
Initially a small blork of wood, P, coloured to represent
a wagon, is placed at 5 ; a similar block, Q, is placed at G \ and
a longer block of wood, R, 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 BGE 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=^BG== DE^l inch.
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 \>o F and in the
order 1, 2, 3, 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 ioop-
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 ^ to i^ 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 .s^oat, and a
basket of cal>bages ; 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
80 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 have 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 ?i — 1 people, but not m^re,
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 n=S. Let y denote the number of passages
from one bank to the other which will be necessary. Then it
has been shown that if 7i=3, 3/= 11; if w = 4, y = 9; and if
w > 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 if 71 = 2, then a; = 2, and y= 5. If n = S, then
a; = 2, and 3/ = 11. If n = 4, then a? =3, and y = 9. If n=o,
then a) = S, and 3/ = 11. And finally if n > 5, then ^ = 4, and
y = 2n—l.
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 Sn — S 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 fi-om 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 6n — 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 Moliammedan 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 high. 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 similar question propounded
at Cambridge in 1903, but I first saw it in print in the Daily Hail, 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; iiej^ce the distance is 40 feet.
w
«K
N.
Frohlems 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.
First 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 be3^ond 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 .*> on 8, then 1 on 6, and
lastly 9 on 4 ; or by putting 4 on 1, then 6 on 9, theu 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 ease the four couples finally formed are not equi-
distant. Here the transformation will be effected if we move
5 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 + 2n) counters, for if we
move 4 on 1 we have left on one side of this couple a row of
(8 + 2n — 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, oi a
somewhat similar kind, is of Japanese origin. Place four florins
(or white counters) and four halfpence (or black counteis)
alternately in a line in contact with one another. It is rec^uired
* Note by J. Fitzpatrick to a French translation of the third edition of tliis
work, Paris, 1898.
t Bibliotheca Mxthematica, 1896, series 3, vol. vi, p. 323; P. G. Tait, Philo-
Bophical Magazine, London, January, 1884, series 5, vol. xvii, p. 39; or Collected
Scientijic i'ajjers^ Cambridge, vol. ii, 1690, 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 . . . . xxabahabah.
After the first move . . baababaxxb.
After the second move . 6 a abxx a abb.
After the third move . . bxxbaaaabb.
After the fourth move . bbbb aaaax 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 uext 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, 4?7i + 2, 47n + 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 KECKEATlONS 77
Tait suggested a varir-.tion 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, ti + 1 moves are reijuired.
Problems on a Cliess-hoard 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 *' b " 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, thon
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 w^hite 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. Ill — 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
2w + l cells, and n black pawns at the other end, they can
be interchanged in n (m + 2) moves, by moving in succession
1 pawn, 2 pawns, 3 pawns, ... n — 1 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
b
b
b
b
b
b
b
b
b
b
b
b
b
a
a
a
a
a
a
a
a
a
a
b
b
b
b
b
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 " h " 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 is at some time or other vacant, and whenever that
* Lucas, vol. H, 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 2?i(n+l) white pawns and 2n{7i+l) black
pawns in a similar way on a square board of (2/1+1)^ cells,
we can transpose them in 2n (n + 1) (?z + 2) moves: of these
4?i(7i4-l) are simple and 2n^ (^+1) are leaps.
Third Problem with Fawns. The followinsf analoorous
problem is somewhat more complicated. On a square board
of 25 cells, place eight white pawns or counters on the cells
a
b
c
—
d
9
e
h
f
*
E
B
G
D
A
F
C
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 I have given a symmetrical solution in 48 moves, but
the following, due to Mr H. E. Dudeney, is efiected in 46 moves :
nhg*F/c*CBHh*GDFjehbacf'GABHEFjdg*Hhbc*CFj''GHh*
80 GEOMETRICAL RECKEATIONS [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 Vandermonde'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-hoard 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
difiiculty of mentally realizing the effect of geometrical altera-
tions in a figure unless they are of the simplest character.
Paradromic Rings. 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 Reiss, 'Beitrage zur Theorie des Solitdr-
Spiels,^ Crelle's Joiirnal, Berlin, 1858, vol. uv, pp. oil — 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 off-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
mir, 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 eJge. 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 A\j ^2) -^3? -^4> ^5, <^rid centre 0. The sides intersect in five
* Vorstadien zur Topologic, Die Studieii, Gottingen, 1647, part x.
B. R. 6
82 GEOMETRICAL RECREATIONS [CH. IV
points Biy ... B^. 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
AA'A!\ draw lines AB, AE 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 of AD, AE, AF, with the similar pencils of Unes drawn from A'y 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 Hnes .^= ±a, x= ±6, y= ±a, y= ±b, 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 ^, couples
Q to P, draws them both out to F, and then pushes them to E. (iii) P
is now uncoupled, R takes Q back to A, and leaves it there, (iv) R returns
to P, takes P back to (7, and leaves it there, (v) R running successively
through F, i>, B comes to -4, 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 ^nd. 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, C, 3,
7, e, 8, 5, 6, 7.
83
CHAPTER y.
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 meclianics,
and confine myself to questions which, like those treated in the
preceding chapters, are of an elementary character. The
results are well-known to mnthematicians.
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 iniiuence
one another.
6-2
84j 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.
Zeno's 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. The answer that the very idea of the motion
of the arrow implies the passage from where it is to where it is
not was rejected by Zeno, who seems to have thought that the
appearance of motion of a body was a phenomenon caused by
the successive appearances of the body at rest but in different
positions.
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
Zeno's Paradox on Time. The fallacy of Achilles and the
Tortoise is usually explained by saying that though the time
required to overtake the tortoise can be divided into an infinite
number of intervals, as stated in the argument, yet these
intervals get smaller and smaller in geometrical progression, and
the sum of them all is a finite time : after the lapse of that
time Achilles would be in front of the tortoise. Probably Zeno
would have replied that this explanation rests on the assump-
tion that space and time are infinitely divisible, an assumption
which he would not have admitted. He 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'b'c', a"h"c" are placed so that a, a\ a"
are vertically over one another, as also h, h\ h" and c, c', c".
Imagine the second line moved as a whole one span to the right
in the time r, 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 r (which by hypothesis is
indivisible) a' must have passed vertically over the space a"h"
and the space 6V. Hence the duration is divisible, contrary
to the hypothesis.
Tke 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 eventfuUy as it began, no part of his
biograpliy would have remained unwritten. For if he wrote the
events of ihe first day in the first year, he would write the
* B. A. W. Russell, Principles oj Mathematics, Cambridge, 1903, vol. i, p. 368.
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 1/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 ajn seconds, the next in ajn^ seconds, and
so on, and will finally reach the pole in
(a -f- ajn ■\- a/n^ -\- ajn^ -\- )
seconds, that is, in anl{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 A to B would
meet and pass on his way, assuming that the journey either
way takes 4J 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 iu 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 Philosophic de$
Mathematiques, Paris, 1903, 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 many of the tricks of acrobats,
CH. V] MECHANICAL RECREATIONS 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, intendins" 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 off 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 an}^
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 calculated the
mass of the earth and, assuming that a man could work inces-
santly at the rate of 116 foot-lbs. per second, which is a very
high estimate, he found that 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 would take about 74,000 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. n, p. 15 ; 1840 edition, p. 201.
CH. V] MECHANICAL RECREATIOiNS 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
three-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 G, 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 whicli one force
is the reaction of the table passing through C 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 drop^^ed 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
see boxes of tin soldiers mounted on lead hemispheres, and
when the lid of the box was taken off the whole reg-iment
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 fiuger. The centre of gravity is thus brought below
the point of support, and a small displacement given to the
■ Ex. gr. Oughtre>l, Muthematicall Rccrealions, p. 2-i ; Ozanam, 1803 eJilion,
vol. II, p. 14; 1810 edition, p. 2U0.
92 MECHANICAL RECREATIONS [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 man-f-. 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 fl -shaped
wire weighted at its ends. If the figure is placed on a narrow
sloping plauk or strip of wood passing between the legs of the
n, then owing to the -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
* Oughtred, p. oO; Ozanam, 1803 edition, vol. ii, p. 12; 1840 edition,
p. 199.
t La Nature, Paris, Much, 1891,
CH. V] MECHANICAL RECREATIONS 93
reeling motion. Shortly after the publication of the third
edition of this book an improved form of this toy, in the
shape of a walking elephant made in heavy metal, was issued
in England, and probably in that form it is now familiar to
all who are interested in noticing street toys.
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 egg 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 egg 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 egg cannot be made to
stand on its base.
Cones running up hill*. The experiment to make a double
cone run up hill depends on the same principle as the toys
above described ; namely, on the tendency of a body to take a
position so that its centre of gravity is as low as possible. In
this case it produces the optical effect of a body moving by
itself up a hill.
Usually 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.
Perpetual Motion, The idea of making a machine which
once set going would continue to go for ever by itself has been
* Ozunam, 18U3 edition, vol. ii, p. 49 ; 1840 editiou, p. 216.
94 MKCHANICAL RECREATIONS [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 suggested f,
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,
t Several of them have been described in H. Dirck's PerpeLaum Mobile,
London, 1861, 2ud edition, 1870.
CH. V]
MECHANICAL llECIlEATlO^^iS
95
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 throuo^h C 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, 1S03 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 satisfactoril}^
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 I'n? times,
while others which depend on area (such as the sustaining
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 : 843 {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 latio 64:49; thus
in some respects he would be less etficieut than before. Of
course the increased 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, Gesammelte Ahhandlunrjen, Leipzig, 1881, vol. i, p. 165.
B. R. 7
98 MECHANICAL RECREATIOXS [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 hloivs, 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 ; 1840 edition, pp. 540, 543.
CM. V]
MECHANICAL RECKEATiOiNb
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 throingh 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 accompanying figure, let BAR represent the keel
of a boat, B being the bow, and let SAL represent the sail.
Suppose that the wind is blov/ing in the direction WA with
a velocity u; and that this direction makes an angle 6 with
the keel, i.e. angle V, A R= 6. Suppose that the sail is set so
as to make an angle a with the keel, i.e. angle BAS = a, and
therefore angle WAL = 6 -\- 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 to WA
will be parallel to SL, if v sin a = w sin (^ + a) ; and in this case
the resultant pressure perpendicular to the sail vaoishes.
Thus, for steady motion we have v sin a = t( sin (0 + a).
Hence, whenever sin (^+ a) > sin a, we have v > u. Suppose,
7—2
100
MECHANICAL RECREATIONS
[CH. V
to take one instance, the sail to be tixed, that is, suppose a to
be a constant. Then ?; is a maximum if ^ + a = Jtt, that is, if
is equal to the complement of o.. In this case we have
i} — xi cosec a, and therefore v is greater than u. Hence, if the
wind makes the same angle a abaft the beam that the sail
makes with the keel, the velocity of the boat will be greater
than the velocity of the wind.
Next, suppose that the boat is running close to the wind,
so that the wind is before the beam (see figure below), then
in the same way as before we have v sin ol — u sin {6 + a),
or V sin o. — u sin 6, where <f> = angle ^VAS = 17— — a. Hence
v=u sin <jb cosec a.
Let w be the component velocity of the boat in the teeth
of the wind, that is, in the direction A W. Then we have
w = v cos BA W = V cos {a+ <l)) = u sin <^ cosec x cos (a + <^). If a
is constant, this is a maximum when (f> = lir — ^a; and, if (j>
has this value, then w = -^u {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 reg.'ittas, which affords a somewhat curious
illustration of certain mechanical principles. The only thing
supplied to the cre^v 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 illustrations of Hauksbee's Law.
Haiihshees 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 BesJint, Hydromechanics, Cambridge, 18G7, art. 119, where however
it is assumed that the pressure is proportional to the density. Hauksbee was
the earliest 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,
Camhridije PhilowpJdcal Traiiaactiona, 1830, vol. in, 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, is ^ = Tla"'^, 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 experimeQt is explicable by the same aigii-
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 other
causes will be also introduced which affect the phenomena : it
is however unnecessary for ray purpose to go into these details.
Cut on a Tennis-Bail. 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 remembered 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 ^ Die Abiveichung der Geschosse^ in the Ahliandlungen der
Alcademie der Wissenschajten, 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 Royal Society, Edinburgh,
vol. xxTii, 1893; or Collected Scientific Papers^ Cambridge, vol. u, 1900,
pp. 356—387, and references therein.
104 MECHANICAL RECREATIONS [CH. V
its centre of fio^ure, 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 FQ. 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 frictionless, 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 movincr 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.
Sp{7i 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 sViould 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-
currents. 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. Bi-yan in the Transactions of the British Association for 1896,
vol. Lxvi, pp. 726—723.
CH. V] MECHANICAL RECREATIONS 107
incessant occurrence of crusts of wind separated by lulls, and
Dr Br3'-an'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 lor ward
velocity.
Curios A Physica. When I was writing the first edition
of these Recreations, I put together a chapter, following this
one, on " Some Piiysical 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 surface, 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 : ??i + 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 t(jo 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 Transactions,
London, 1860, pp. 133 — 148) 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 mei-uoir 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 Prof. G. D. Liveing before the Cambridge Philosophical Society,
November 26, 1894.
10^
CHAPTER Yl,
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 I v.
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
85
84
15
25
35
45
55
65
75
14
24
34
44
54
64
74
13
23
33
43
53
63
73
83
32
12
22
32
42
52
62
72
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 of the mid-
points of the cells. I use this notation in the following pages.
A generalized board consists of n^ 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 n^ 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.
♦ Vlntermediaire 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-BOARD 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 rt? cells the chance is
2 (n - \)l{n' - 1), that is, 2/(n + 1).
A knight when placed on any of the 4 corner 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 (4x2-1-8x3-1- 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 8 (n — 2)/n'^ (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 -h 20 X 9 -i- 12 X ll-f 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 (2n — l)/3n (n + 1). When n is odd the analysis is longer,
owing 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)/3?i (w + 1).
112 CHESS-BOARD RECREATIONS [CH. VI
On the above assumptions the relative vahies 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 w^hen 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 (4 X 12 + 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 (/i + 1). For a knight all
checks are safe, and therefore the chance of a safe check is 1/12 ;
or on a. board of ?i^ 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)ISn^ (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 ri" cells, is 2 {n - 2) {5n - 3)/3?i2 {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
according 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 cells 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 -l){n- 2) (n^ + Sn- 2)ln^ (n^ - 1) and
(n - 1) (n - 2) (n' + 371^ - 20^1^ - 30n + lS2)/n' (n' - 1) (n^ - 2).
The Eight Queens Problem f. 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 71- cells are given by those terms, if any, of the determinant
Ch
0,
Ci
0.
tts
h
78
13.
a.
^4
7b
A
di
a.
^2n— 3
P2n— 2
^2n— I
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
* V Intcrmidiaire des Mathematiciens, Paris, 1897, vol. rv, p. 6, and 1901,
vol. VIII, p. 140.
t On the history of this problem see W. Ahrens, Mathematische Unter-
haliungen und Spiele, Leipzig, 1901, chap. ix.
X Grunert's Archiv der Mathematik und Physik, 1874, vol. LVi, pp. 281 — 202.
B. R.
8
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. VV.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 n + 1 queens on a board of {n-{-Vf cells could be deduced,
and that all the other solutions of w + 1 queens on a board of
{n + Xf 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
?i = 3. If n = 4 there are two terms in the determinant which
give solutions, namely, b^c^js/Ss and c^^jb^'y^. To find the solutions
when 71 = 5, Glaisher proceeded thus. In this case, Gllnther's
determinant is
di h Cs (h 65
13.2 as h C5 de
78 ^A «5 ^6 C7
^4 75 ^6 CI7 bs
^5 Se 77 A Ctg
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, h^c^y^^^a^ and CsfizhyzCto-
* Philosophical Magazine^ London, December, 1874, series 4, vol. xLvin,
pp. 457—487.
CH. Vl]
CHESS-BOARD KECEEATIUNS
115
The solutions which involve a^ e^ and 65 can be written down
by symmetry. The eight sohitions 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
h
Cz
d.
13.
«8
Ih
C5
da
7»
/54
(<5
h
C7
^.
75
/3.
Clr
hs
8e
77
13s
If, in this, we take the minor of h.2 and in it replace by zero
every term involving the letter b or the suffix 2 we shall get
all solutions involving 63. But in this case the minor at once
reduces to d^a^Bi^s- We thus get one solution, namely, bodss^ilSs-
The solutions which involve ,8.2, B^, 8^, ^s, ^s, dg, and d^ can be
obtained by symmetry. Of these eighfc 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
Cs
a.
h
C5
78
/3.
a.
be
C7
75
A
a,
77
If, in this, we take the minor of Cg, and in it replace by zero
every term involving the letter c or the suffix 3, we shall get
all the solutions which involve Cj. But in this case the minor
vanishes. Hence there is no solution involving Cs, and therefore
by symmetry no solutions which involve 73, 77, or C7. 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
tj^pe, no solutions of the third type, and no solutions of the
fourth type ; that is, forty solutions in all. If ?z = 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 difference. Thus on boards of 4^, 5^, 6^
7^ 8^ 92, 102 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. VI]
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 7th 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 difiiculty, and Mr Derrington has constructed the following
table of solutions :
2.4.1
2.4.1
3
3
for a board of 4? cells
2.4.6.1.3.5
2.4.6.1.8.5.7
2.4.6.8.3.1.7.5
2.4.1.7.9.6.3.5.8
2. 4. 6. 8. 10. 1.3. 5. 7. 9
2. 4. 6. 8. 10. 1.3. 5. 7. 9. 11
2. 4. 6. 8. 10. 12. 1.3. 5. 7. 9. 11
2 . 4 . 6 . 8 . 10 . 12 . 1 . 3 . 5 . 7 . 9 . 11 . 13
9 . 7 . 5 . 3. 1 . 13 . 11 . 6 . 4 . 2 . 14 . 12 . 10 . 8
15 . 9 . 7 . 5 . 3 . 1 . 13 . 11 . 6 . 4 . 2 . 14 . 12 . 10 . 8
2 . 4 . 6 . 8 . 10 . 12 . 14 . 16 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15
2 . 4 . 6 . 8 . 10 . 12 . 14 . 16 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17
2 . 4 . 6 . 8 . 10 . 12 . 14 . 16 . 18 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17
2 . 4 . 6 . 8 . 10 . 12 . 14 . 16 . 18 . 1 . 3 . 5 . 7 . 9 . 11 . 13 . 15 . 17 . 19
12 . 10 . 8 . 6 . 4 . 2 . 20 . 18 . 16 . 14 . 9 . 7 . 5 . 3 . 1 . 19 . 17 . 15 . 13
21 . 12 . 10 . 8 . 6 . 4 . 2 . 20 . 18 . 16 . 14 . 9 . 7 . 5 . 3 . 1 . 19 . 17 . 15
11
13.11
62
72
8-^
92
102
11"
12-
13-
142
152
1G2
172
182
192
20-
212
•t
i>
»»
>>
>i
>»
>»
»>
»>
>»
»>
>>
)»
»)
* 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 Mathematical Society, vol. xvii, 1898-9,
pp. 43—68.
CH. Vl] CHESS-BOARD RECREATIONS 119
and so on. The rule is obvious except when n is of the form
6m 4- 2 or Qin + 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. Dudcney has written on these problems m the WeeJdy 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 71 = 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 ?i = 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 the}?- are put on the cells 11, 35, and
53, giving 4 solutions in all. Four queens are required when
71 = 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=*I, 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 Mathematique au Jeu des
Echecs, St Petersburg, 1862, Appendix, p. 244 et seg.; see also L' Inter mediaire
des MatkcmaticienB, Paris, 1901, vol. viii, p. 88.
CH. Vl] CHESS-BOARD 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
5) 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 Ulntermidiaire des Mathe-
maticiens, Paris, 189G, vol. in, p. 58; 1897, vol. iv, pp. 15—17, 254; 1898, vol.
V, pp. 87, 230—231.
122 CBESS-BOARD RECREATIONS [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.
KnigMs 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 Linde, Geschichte 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 dei Linceiy
Rome, 1872, vol. xxv, pp. 87 — 162 : also Applications de V Analyse Mathematique
au Jeu des Echecs, by C. F. de Jaenisch, 3 vols., St Petersburg, 1862-3; and
General Parmentier, Association Franraise pour I'avancement des ScienceSf 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
Montmort 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
34
49
22
11
36
39
24
"
21
10
35
50
23
12
37
40
48
33
62
57
38
25
2
13
'
20
51
54
63
60
41
26
I32
1
47
58
61
56
53r
14
3
|l9
8
55
52
59
64
27
42
46
31
6
17
44
29
4
15
7
18
45
30
5
16
A
28
30
21 6 15
28
19"
7
16
29 20
5
14
22
31 8
35
18
27
9
36
17
26
13
4
32
23
2
11
34
25
1
10
33
24
3
12
De Moivre^* Solution.
Euler^s Thirty-six 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 huve
previously suggested the problem. I do not know where they were first pub-
lished ; they were quoted by Ozanam and Moutucla, 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 ^vhich the circuit can be made
re-entrant.
The following example, mentioned by Legendre as one of
exceptional difficulty, illustrates the method. Suppose that
55
58
29
40
27
44
19
22
22
25
50
39
52
35
60
"I
60
39
56
43
30
21
26
46
27
40
23
36
49
58
53
34
57
54
59
28
41
18
23
20
24
21
26
51
38
61
56
59
38
61
42
31
8
25
46
17
41
28
37
48
3
54
33
62
53
32
37
a
47
16
9
24
20
47
42
13
32
63
4
55
50
3
52
33
36
7
12
15
29
16
19
46
43
2
7
10
1
34
5
48
b
14
c
10
18
45
14
31
12
9
64
5
4
49
2
35
6
11
d
13
15
30
17
44
1
6
11
8
Figure i. Figure ii.
Example of Euler^s 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, b, 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
' MSmoires de Berlin for 1759, Berlin, 1766, pp. 310 — 337 ; or Commentationes
Arithmeticae Collectae, St Petersburg, 1849, vol. i, pp. 337 — 355.
CH. Vl] CHESS-BOARD RECREATIONS 125
re-entrant. This is the case here ii 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 numl)ers 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, b, 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, b, 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
52
41
62
35
49
46
67
42
61
36
63
40
44
59
48
51
38
55
34
63
47
50
45
56
T
33
64
39
54
22
7
32
31
2
23
6
19
16
27
12
8
21
4
29
10
25
14
17
3
80
9
20
5
28
11
26
50
45
62
41
60
39
54
35
63
46
42
51
48
53
36
57
88
49
44
61
40
59
34
65
43
64
5
47
52
1
33
56
37
68
23
2
27
8
29
12
17
14
6
25
4
21
19
10
81
Is
3
22
7
28
9
30
13
Euler's Half-hoard Solution.
Eogefs 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] CHRSS-BOARD RECREATIONS 127
whose distances are respectively x and y from the opposite
sides a complementary cell. Thus the cells (a?, y) and (9 — x,
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 b}^ 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 fractions 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 fractions: 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'Hiistoire de VAcadeniie des Sciences for 1771, Paris, 1774, pp. 5GG— 574.
128 CHESS-BOARD 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 Roget, 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|. 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.
Wamsdorff 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 Probleme du Cavalier au Jeu des Echecs, Mannheim, 1778.
+ Studies of Chess, sixth edition, London, 1825.
X H. C. Wamsdorff, Des Rosselsprunges einfachste und allgemeinste Losung,
Schmalkalden, 1823 : see also Jaenisch, vol. ii, 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 re-
strictions : such as to require that the route shall be re-en-
trant, or more generally that it shall begin and terminate on
given cells.
The simplest solution with which I am acquainted — and
one which I believe is not generally known — is due to Rogetf.
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 (4ny cells: for
example, it could not be used on the board of the French jeu
des dames J 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 Z, 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 mav make the circuit re-entrant, and
* See ex. gr. T. Ciccolini's work Del Cavallo degli Scacchi, Paris, 1836.
t 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.
Some solutions, founded on Eoget's method, are given in the Leisure Hour^
Sept. 13, 1873, pp. 587—590 ; see also Ibid., Dec. 20, lt)73, pp. 813-815.
b. u. 9
130
CHESS-BOARD KECREATIONS
[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.
/
€
a
P
I
e
a
P
a
P
1
e
a
P
I
e
e
I
P
a
e
I
P
a
P
a
e
I
P
a
e
I
I
e
a
P
I
e
a
P
a
P
I
e
a
P
I
e
e
I
P
a
e
I
P
a
P
a
e
I
V
a
€
I
34
51
32
16
38
63
18
3
31
14
35
52
17
2
39
54
50
33
16
29
56
37
4
19
13
30
49
36
1
20
17
65
40
48
63
28
9
44
22
5
27
12
45
64
21
8
41
58
62
47
10
25
60
43
6
23
11
26
61
46
7
24
59
42
Roget'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, Y, in
the 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
p 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 I, 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.
* Comptes Rendiis, April, 1861 ; and Bulletin de la SorigtS Mathfmatique de
France, 1881, vol. ix, pp. 17 — 24.
t Bulletin de la Society Matlieinatique de France, 1880, vol. viii, pp. 82 —
102, 132—158.
9—2
132
CHESS-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 v^^hich 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 covering 32 cells, and together covering the board, are
remarkable as constituting a magic square*.
63 22
15
40
1
42
59
18
U
39
64
21
60
17
2
43
37
62
23
16
41
4
19
58
24
13
38
61
20
57
44
3
11
36
25
52
29
46
5
56
26
51
12
33
8
55
30
45
35
10
49
28
53
32
47
6
50
27
34
9
48
7
54
31
15
20
17
36
13
64
61
34
18
37
14
21
60
35
12
63
25
16
19
44
5
62
33
56
38
45
26
59
22
55
4
11
27
24
39
6
43
10
57
54
40
49
46
23
58
3
32
9
47
28
51
42
7
30
53
2
50
41
48
29
52
1
8
31
Jaenischh Solution.
Two Half Board Solutions.
It is as yet impossible to say how many solutions of the
problem exist. Legendref mentioned the question, but Minding J
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.
+ Theorie des Nomhres, Paris, 2nd edition, 1880, vol. ii, p. 165.
X Cambridge and Dublin Mathematical Journal^ 1852, vol. vii, pp. 147 — 156;
and Crelle's Journal, 1853, vol. xliv, pp. 73—82.
§ Jaenisch, vol. ii, p. 268.
II Bulletin de la Societe Mathematique 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
62
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
60
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
35
34
33
32
31
30
L!i
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.
Book'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 kiiig or a queen covering the board.
134 CHESS-BOARD RECREATIONS [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 chosen f; for instance,
starting from the cell 11, it may move successively to the cells
55, 82, 71, 17, 28, 46, 13, 31, 86, 68, 57, 48, 15, 51, 84, m, 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 66 in fifteen moves entering every cell once
* L'Intermediaire des Mathematiciens, Paris, 1901, vol. viii, pp. 153 — 154.
t H. E. Dudeney, The Tribune, Dec. 3, 1906.
CH. Vl] CHESS-BOARD RECREATIONS 135
and only once, and never cro.sying 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 33"95 inches.
Another familiar problem can be enunciated as follows.
Construct a rectangular board of vm cells by ruling in + 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
doTNTiwards or horizontally from left to right. The answer is
the number of permutations of m -I- n things, of which m are
alike of one kind and n are alike of another kind: this is equal to
{m-\-n)\ I 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 = n = 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— 1)! (?i — 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 |.
Guarinis Problem. One of the oldest European problems
connected with the chess-board is the following which was
• H. E. Dudeney, The Trihune, Oct. 3, 1906.
+ Ibid., Oct. 2, 1906.
+ See C. F. Sainte-Marie in L^Intermediaire de$ Mathematiciens, Paris,
vol. XI, March, 19U4, 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
D
b
C
A
d
B
c
two top corner 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 b and c, while the black shall occupy the cells
a and d.
The solution is tolerably obvious. First, move the pieces
from a to ^, from b to B, from c to C, and from d to D. Next,
move the pieces from A to d, from B to a, from C to 6, and
from D to c. The effect of these eight moves is the same as if
the original square had been rotated through one right angle.
Repeat the above process, that is, move the pieces successively
from a to ^, from b to B, from c to G, from d to D; from A
to d, from B to a, from G to 6, and from D to c. The required
result is then attained.
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.
» H. E. Dudeney, The Tribune, November 7, 1906.
137
CHAPTER VII.
MAGIC SQUARES.
<. A Magic 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 any row, column,
or diagonal is equal to \n {n^ ■\- \) \ 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 7t^ 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
5
10
11
8
9
6
7
12
.4
15
14
1
I ^
15
10
3
6
4
5
16
9
14
11
2
7
1
8
13
12
Figure L
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 loO 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 ampliiied 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. Giiuther's Gesehichte der mathematischen Wisseu.=^chaften, Leipzig, 1876,
chapter iv ; and W. Ahrens, Mathematische Unterhaltungen und Spiele, Leipzig,
1901, chapter sii. The references in the text are to Bachet, Problemes plaisans,
Lyons, 1624, problem xxi, p. 161 : Fr^nicle, Divers Ouvrages de Mathematique
par Messieurs de V Academic 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,
Traite des Quarres Sublimes, Brussels, 1704 : De la Hire, Mimoires de V Academic
des Sciences for 1705, Paris, 1706, part i, pp. 127—171 ; part ii, pp. 364—382 :
Sauveur, Construction des Quarr€s Magiques, Paris, 1710 : D'Ons-en-bray,
Memoires de VAcadimie des Sciences for 1750, Paris, 1754, pp. 241 — 271 : Des
Ournies, Mimoires de Mathematique et de Physique (French Academy), Paris,
1763, vol. IV, pp. 196 — 241 : Ozanam and Montucla, Recreations, part i,
chapter xii : Euler, Commentationes Arithmeticae Collectae, St Petersburg, 1849,
vol. II, pp. 503—602 : Violle, Traiti Complet des Carris Magiques, 3 vols, Paris,
1837-8.
140
MAGIC SQUARES
[CH. VII
♦ De la Louhhe*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 rov/ ; (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.
17^
•^
1
8
16
23
5'^
7
14
16
4
6
13
20
22
10
12
19
21
3-
11
18
25
2
9
15 + 2
20 + 4
+ 1
*
5 + 3
10 +6
20 + 3
+ 5
5f2
10 + 4
15 + 1
+ 4
5 + 1
10 + 3
15 + 5
20 + 2
5 + 5
10 + 2
15 + 4
20 + 1
0t3
10 fl
15 + 3
20 + 5
+ 2
5+4
Figure iii.
i
- V Vo 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 Loubere, Du Royaume de Siam (Eng. Trans.), London, 1693, vol. n,
pp. 227 — 247. De la Loubere was the envoy of Louis XiV to Siam in 1687-8,
and there learnt this method.
CH. VIl] 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, ..., 72; 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 ^ (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 — l). 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 inagic 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, \xiiether even or odd, other magic
squares of the same order can be formed by the mere inter-
14j2 magic squares [CH. VII
chano-e of the row and the column which intersect in a cell on
a diagonal with the row and the column which intersect m 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,
2/ = 0.
- De la Hires Method^. 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, ..., n, and
the other of multiples of the radix, namely, 0, /i, 2/i, . . . , (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 Meinoires de I'Academie des Sciences 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 m 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
1
5
2
2
3
4
1
5
5
2
3
4
1
1
5
2
3
4
4
\
5
2
3
15
20
5
10
20
5
10
15
20
5
10
15
5
10
15
20
10
15
20
5
18
4
21
10 "12]
2
23
9
11
20
25
7
13
19
1
6
14
15
17
3
24
IG
5
22
8
First Subsidiary Square.
Figure v.
Second Subsidiary Sqxiare. Resulting Magic Square.
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 oneL
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 corre6ponding
cells of these two squares, we shall obtain 25 numbers such
144 MAGIC SQUARES [CH. VII
that the sum of the numbers in every row, every column,
and each diagonal is equal to 15 + 50, 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 from 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 horizontal 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 skew interchange consists in the
interchange of the numbers in two skewly related cells. A
cross intercha nge consists in the change of the numbers in any
^eTland 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, ...,7i^in 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 fi:om 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 iV = ?i(?i^-|- l)/2; 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. xxm, pp. 65 — 69, and the Monist, Chicago,
1912, vol. XXII, 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. R. 10
146 MAGIC SQUARES [CH. VII
a^tli row fi'om the top is N — n^ (n — 1x 4- 1)/2. The sum of the
numbers in the complementary row, that is, the a;th row from
the bottom, is iV + 71^ (n — 2a; + 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 by 71 (?i — 2^7 + 1). Hence, if in these two rows we
make 7?/2 interchanges of the numbers which are situated in
vertically related cells, then we increase the sum of the numbers
in the icth row by ti x ?i {n — 2a) + l)/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 ?i/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 — 2y + 1)/2. The sum of the numbers originally in the
complementary column, that is, the yih column from the right-
hand side, is H + n(n— 2y + l)/2. Also the number originally
in any cell in the yth column was less than the number in the
cell horizontally related to it by 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 vertical 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, ..., ti^ 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 2m columns and 2m rows. In each of these columns
take m cells so arranged that there are also m cells in each row^j
and change the numbers in these 2??i^ cells and the 6m^ cells
associated with them by a cross interchange. The result is
equivalent to 27n interchanges in every row and in every
column, and therefore renders the square magic.
One way of selecting the 2m^ cells in the first quarter is to
divide the whole square into sixteen subsidiary squares each
a
b
b
a
b.
♦
a
b
//
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 b 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, constructed 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 2/?i
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 + 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 n/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 + 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 affecting 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. VIlJ MAGIC SQUARES 149
that, if w = 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 Hires 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, n, 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
* Tlie 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 Problemes, 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
6
2
4
3
5
1
6
5
3
4
2
1
1
5
3
4
.2
6
6
2
3
4
5
1
1
2
4
3
5
6
30
30
30
24
fi
24
24
C)
C
18
18
12
12
12
IB
12
12
18
18
18
12
C
24
G
6
24
24
30
30
30
1
35
34
3
32
6
30
8
28
27
11
7
24
23
16
IG
14
19
13
17
21
22
20
18
12
26
9
10
29
25
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 from 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.
J
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
y of the mnth order. For example, a square of 81 cells may be
V 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 Composite Magic Squares.
The second method, which was introduced by Frenicle, 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 — 2)^ numbers, the usual process
is to reserve for the 4(7i— 1) numbers in the border the first
2 (n - 1) natural numbers and the last 2 (n — 1) numbers. Now
the sum of the numbers in each line of a square of the order
CH. VIl]
BIAGIC SQUARES
153
(n - 2) is i (?? - 2) {(n - 2)»+ 1}, and the averasre is J{(n-2)Hll.
Similarly the average number in a square of the t?th 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 ^ (n^ + 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 n^-\-l—p
(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
19
20
23
8
18
IG
9
14
21
11
13
15
5
22
12
17
10
4
3
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 rules for
this purpose. It will illustrate sufficiently 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 each pair is 26, and obviously they must be placed
at opposite ends of any line.
154
MAGIC SQUARES
[CH. VII
I believe that with a little patience a magic square of any
order can be thus built up, and of course it will have the
property that, if each border is successively stripped off, the
square will still remain magic. This is the method of con-
struction commonly adopted by self-taught mathematicians,
some of whom seem to think that the empirical formation of
such squares is a valuable discovery.
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
Z + v-z
X
Y
V-v+z
Y-x + z
V
Z
X+x - z
X+x
Z-v-y
V-x + y
Y+v
32768
4
2
4096
16
512
1024
128
256
32
64
2048
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 %a is constant for every
line in the square, then TimP' 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. Lxxxm, Maj 26, 1910, pp. 368—369.
CH. Vllj
MAGIC SQUARES
155
intersecting sides of two equal concentric squares. It is required
to place the first 16 natural numbers on the corners 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 eycry
15
Magic Star.
Figure xv.
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 v/
can be presented in 7040 forms. De la Hire showed that,
* Communicated to me by Mr K. 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 hyper-magic.
Hypee-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
I 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, 5, 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 roAvs or
* Squares of this type were mentioned by De la Hire, Sauveur, and Euler.
Attention was again called to tbem 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 Mathe-
matic^, vol. xix, 1897, pp. 99—120.
CH. VIl]
MAGIC SQUARES
157
any two columns, and the two pieces be interchanged, the new
square so formed will be also pandiagoiially magic. Hence it is
obvious that by one vertical and one horizontal transj)osition
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 w, with the understanding that the unit-digits run from
1 to n, it is evident, as in the corresponding explanation of why
De la Loubere'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
13
21
9
17
5
19
2
15
23
6
25
8
16
4
12
1
14
22
10
18
5 + 2
15 + 5
0+3
10 \-\
20 + 4
10 + 3
20+1
5 + 4
15 + 2
+ 5
15 + 4
042
10 f 5
20 + 3
5 + 1
20 + 5
5 + 3
15+1
+ 4
10 + 2
+ 1
10 + 4
20 f 2
5 + 5
15 + 3
Figure xvi.
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 from cell to cell by steps, using the term step
(a, h) to denote going a cells to the right and 6 cells up. Thus
a step (a, h) will take us from any cell to the ath column to the
right of it, to the ^th row above it, to the (6 -I- 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 Loubere's rule, that the move-
ments along lines are taken cyclically ; thus a step n + a is
equivalent to a step a. Of course, also, if a means going a cells
158 MAGIC SQUARES [CH. 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 b cannot be zero, and that if a and b
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 (h, k) from the cell n to get to an
unoccupied cell in which we place the number w -h 1. Con-
tinuing the process with n — \ more steps (a, b) we get another
series of n numbers in various cells. If h and h 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
Qi, 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 oi n — \ uniform steps (a, b)
followed by one cross-step (A, k). The numbers inscribed after
the n cross-steps will be n + 1, 2w -♦- 1, 3n + l, ..., and these
will be themselves connected by uniform steps {u, v\ where
u = {n — \)a-\-h^k— a, mod. n, and v={n — \)b-{-k=k — b.
I proceed to investigate the conditions that a, 6, A, 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+ 1, ..., are con-
nected by steps (u, v), where « = h — a and v = k — b. Hence, if
these are to fit in their places, we must also have u and v
unequal, and u, v, n + 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, h) from any cell and q steps {u, v) from it,
w^here p and q are each less than n, should lead to the same cell.
Thus, to the modulus n, we cannot have pa = qit, = 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 + 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, h=0, k= — l, 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 6 -f 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 = — dq. 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
[CH. 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 — h) = pqb (h — a), that is, ^iq (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 ak — bh is prime to n shows that the cross-step (/i, 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 — — h,k = a. If n is prime our
conditions are then satisfied if h be any number jfrom 2 to
{n — 1)/2, if a be positive and less than b, and if a^ + 6^ be prime
to n. We can, if we prefer, take h = h, k = — a; but it is not
possible to take h = a and k = ^b, or h = — a and k = b, since
they make u=iO 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
18
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^ + b^ = 5. Hence the use of a knight's move
is not applicable when n is 5, or a multiple of 5.
CH. VII]
MAGIC SQUARES
101
The construction of singly-even pandiagonal sqnares (that is
those whose order is 4m + 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 4m. and as before let us
write the number in a cell in the scale 4m, that is, as 4mp + r
so that p and r are the radix and unit-digits, with the conven-
tion that r- cannot be zero. Place p„p„p„ ..„ p^^ ;„ ^rder
m the cells m the bottom row. Proceeding from p. by steps
(^m, 1) fill up 2m cells with it. And proceed similarly with
the other radix-digits. Next place r„ n, ..., r,„ in order in
the cells m 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 r„ r„ ..., r,„, the
the values 0, 1, .... 2m -1, 4m -1, .... 2m, the square will be
pand.agonally magic. I leave the demonstration to my readers
The resulting square in the case when m = 1, « = 4, and the p
and r subsidiary squares are shown below. This is the square
represented in figure ii on page 137.
3
2
l\
1
3
2
7j2
1
1
3
2
3
2
3
2
4
1
4
1
2
3
2
3
[T 4
1
4
12 + 3
8 + 2
+ 3
4 + 2
+ 4
4 + 1
12 + 4
8 + 1
12 + 2
— — —
8 + 3
+ 2
4+3
1 + 1
4 + 4
12 + 1 8 + 4|
Subsidiary p Square.
Figure xix.
Subsidiary r Sqtiare.
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
m the square delineated in figure ii on page 137 the numbers
1, 7, 10, 16 form a magic group whose sum is 34, connected
11
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 (eoo. gr. 22 and
28 in figure xviii) shall be constant and equal to n' + 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, 6 = 2, A = 2, A; = — 1, be
followed, this will lead to placing the number 1 in the (w+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^ -\- 1)/2, which I will denote by m, in the
middle cell, and work from it forwards with the numbers ni + l,
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 VI — 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
Bouhly-Magic Squares. In another species of hyper-magic
squares the problem is to construct a magic square of the «th
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
57
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, so 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 2/i + 2 lines, each containing
* See M. Coccoz in L' Illustration, May 29, 1897. The subject has been
studied by Messieurs G. Tarry, B. Portier, M. Coccoz and A. Rilly. More than
200 such squares have been given by the last-named in bis itude sur les
Triangles et les Carres Magiqu£s aux deux premiers degres, Troyes, 1901.
t See Magic Reciprocals by G. Frankenstein, Cincinnati, 1875.
11—2
164
MAGIC SQUARES
[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 2n + 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
02
Gl
GO
59
7
8
9
10
54
53
52
51
15
IG
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
5C
57
58
6
5
4
3
G3
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 ;S pencils, with vertices
A\ ... H'\ one pencil with its vertex at ; and one pencil with
its vertex on the axis of the last-named pencil.
The sum of the numbers in each of the 18 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. VIl]
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
Figure xxiv.
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 fi:om 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, h, c, d, e, f,
drawn fi:om 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 Eider^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, . . . , ti), 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 Comvientationes Arithmeticae, St Petersburg, 1849, vol. n, pp. 802—
361. See also a paper by G. Tarry in the Comptes rendus of the French Associ-
ation for the Advancement of Science, Paris, 1900, vol. ii, pp. 170—203 ; and
various notes in L' Inter mediaire des Mathematiciens, Paris, vol. m, 1896,
pp. 17, 90 ; vol. V, 1898, pp. 83, 176, 252 ; vol. vi, 1899, p. 251 ; vol. vn, 1900,
pp. 14, 311.
CH. VIl]
MAGIC SQUARES
1G7
For instance, if n = 3, with three red counters a^, a^, a^,
three white counters h^, b^, 63, and three black counters Ci, C2, Cg,
we can satisfy the conditions by arranging them as in figure xxv
below. If ?i = 4, then with counters a^, a^, a^, a^) ^1, ^2, ^3>
K; Ci, C3, C3, C4; di, di, di, di, we can arrange them as in
figure XX vi below. A solution when n ^ 5 is indicated in
figure xxvii.
->
a.
b.
C3
h
Ci
a._.
<-'.,
«3
^
«i
^a
C3
d.
C4
^3
'^2
b.
d,
Cl
^
63
f*4
<h
Ci
^h
f^-2
^3
<f.
^5
K
Cl
d.
<'a
"4
'-^
d.
^\
«,
^
d.
^*
«&
^
^■i
«3
^4
^6
Figure xxv. Figure xxvi. Figure xxvii.
The problem is soluble if w is odd ; it is insoluble if n is
of the form 2 (2m + 1). If solutions when n=a and when
n = b are known, a solution when n= ab 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
2Z
61
19
12
Z2
19
21
6Z
Meversible 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.
* Sec The Tribune, April 29, 1907.
168
MAGIC SQUARES
[CH. VII
Magic Domino Squares. 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: of these 56 blocks, eight are
• • •• •
• • • • •
• • • •• • jV»
• • •
• • • • •
•
•
•
• • •
• • •
• • •
• • •
•
•
•
•
•
•
•
•
•
• •
•
• •
• •
• •
• •
• •
• •
• •
•
• • •
• • •
•
•
e • •
• • •
•
•
•
•
• • •
• • •
•
•
• • •
• • •
•
9 «
e
• •
• •
•
• •
• •
• •
•
• •
•
e • •
• • •
•
•
•
•
Magic Domino Square.
Figure xxix.
blank, on each of eight of them is on.e 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 the
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 V Illustration, July 10, 1897.
CH. VIl]
MAGIC SQUARES
169
•
•
•
• « •
• 1 •
• • •
• • •
• •
• •
• •
•
•
•
•
•
• •
• •
• •
•
• •
• •
• • • •
• • • •
•
• • •
• • •
• • • • •
• • •
• • • • •
• • • • '"'."
• • •
• • • • •
■■■■^ —■■— I aHHiM mmmmt mmmima ^^imm
• • • • •
• • •
• • • . •
• • • • •
• • •
• • • • •
• • • •
• • • •
• • • • •
• • •
• • • • •
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 Cj
and a florin in the cell below it, namely, in 0.3, 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
figure xiv we must put a double-florin and a sixpence in cell Oj, 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 a2> a crown and a half-crown in
cell C2, a crown and a sixpence in cell 61.
See The Strand Magazine, Londou, December, 1896, pp. 720, 721.
170
CHAPTER VIII.
tJNICURSAL PROBLEMS.
I propose to consider in this chapter some problems which
arise out of the theory of unicursal curves. I shall commence
with Fillers Problem and Theorems, and shall apply ^he results
briefly to the theories of Mazes and Oeometrical 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 to take a walk in the town of
Konigsberg in such a way as to cross every bridge in it once
and only once.
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 problematis ad Geometriam situs pertinentis,' Gommentarii
Academiae Scientiarum Petropolitanae for 1736, St Petersburg, 1741, vol. viii,
pp. 128 — 140. This has been translated into French by M. Ch. Henry; see
Lucas, vol. I, part 2, pp. 21 — 33.
CH. VIIl]
UNICURSAL 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
Kdnigsberg problem this figure is of the shape indicated below,
the areas being represented by the points A^ B, G, D, and the
bridges being represented by the lines I, m, n, p, q, r, s.
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 from 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 Geometrical
Figures, Oxford, 1905.
y
CH. VIIl] UNICURSAL PROBLEMS 173
First, The number of odd nodes in a closed network is
even.
Suppose the number of branches to be b. Therefore the
number of hooks is 26. Let kn be the number of nodes of
the ?ith 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.
,*. 2k2 + •3^'3 + 4Ar4 + . . . + nkn + . .. = 26.
Hence Sk^ + bk^ + ... is even,
/. Arg + ^5 + ... is even.
Second. A figure which has no odd node can he described
unicursally 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 from 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 eveiy branch and come back to the initial point. But
174 UNICURSAL PROBLEMS [CH. VIII
if this random coupling produces anywhere an isolated loop, i,
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, (7, each
connected with both the others by two bridges. The most
unfavourable way of re-coupling the ends at A, B, C would be
to make ABA, AC A, and BOB separate loops. The loops
ABA and AC A are separate and touch at A; hence we should
re-couple the hooks at A so as to combine ABA and AC A into
A
one loop ABACA. Similarly, by re-arranging the couplings
of the four hooks at B, we can combine the loop BCB with
ABACA 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
iM-anch 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
he described unicursally hy a point which starts from one of the,
odd nodes and finishes at the other odd node.
CH. Vlll] 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 Z is a free end,
then we must travel from Z to some node, Y, 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
he 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 2n — 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
UMICURSAL PROBLEMS
[CH. 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 fi:om 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.
^4 ^^7
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
vyUta, the diphthong et 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 djdng 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] 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(w— 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 A,
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 — l) N'.
Similarly if the node A on the original figure is ot the order
2 {u + l)y and there are I independent closed loops which start
from and reiurn to -4, we shall have
N = 2hi (m -f- 1) (/z + 2) . . . (71 + ^ - 1 ) iV",
where N' is the number of routes by which the figure obtained
by suppressing these I loops can be described.
* See G. Tarry, Association J^'ranc^aise pour VAvancement de$ Sciences, 1886,
pp. 49 — 53.
B. R. 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 — 1 nodes. For suppose
that the node A is of the order 2n, Then the 2/1 paths which
meet at A may be coupled in n pairs in 1 . 3 . 5 ... (2?i— 1)
ways and each pair will constitute either a path through J.,
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 2/1 it is easily
seen that it can be described in 2 x {2n — 1)! ways.
We know that a figure with only two odd nodes, A and B,
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,
vol. u, pp. 253—263 ; see also Lucas, vol. iv, pp. 145—150.
CH. VIIl] UNICURSAL PROBLEMS 179
which are either blank or markod 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-li
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 bj
the rest of the dominoes must end on one side in a 5 and on the other in a 4.
12—2
180
UNICURSAL PROBLEMS
[CH. VIII
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 G,
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 6,
of describing figure C by c, and so on. The effect of suppressing
the node in the pentagon A is to give us three quadrilaterals,
5, C, B. And, in the above notation, we have a = h -\- 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 1 4, 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, d = 2e-^f. Similarly
b = 2e +/, and c = 2e +/. Hence a — b-^c-rd = Qe-\-Sf,
CH. VIIl] 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 y8 and
/9', and the two paths 4 3 by 7 and 7. Then we can couple /?
and 7, as also 13' and 7', or we can couple /S and y, as also /9'
and 7 : each of these couplings gives the figure 0. Or we can
couple /3 and 13', as also 7 and 7 : this gives the figure H. Thus
e=2g + h. Each of the figures G and H has only two nodes.
Hence by the formulae given above, we have ^ = 2.3.2=12,
and /i = 2.2.2 = 8. Therefore e = 2g+h = ^2. Next take
the figure F. This has a loop at 4. If we suppress this
Q^- ^0
Figure G. Figure H. Figure J.
loop we get the figure J, and /= 2j. But the figure /, 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 + Sf= 192 + 72 = 264. And therefore
iV=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
from 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 h= 129976320. Hence the number of possible
arrangements in line of the usual set of 28 dominoes, marked
up to double-six, is 28 . 3^ h, which is equal to 7959229931520.
The number of unicursal routes covering a polygon of nine
sides is n = 2l^ 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, Aniiali di Matematica, Milan, 1871, vol. v, pp. 63—120.
182 UNICURSAL PROBLEMS [CH. VIII
Mazes. Everyone 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 course 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. Tremaux 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 labyrinths seems
to have comprised any complicated building with numerous
* Lucas, vol. I, part iii, p. 47 et aecj.
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, c. 1, art. 37; Diodorus, bk. i,
cc. 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. 216 —
235, from which much of the historical information given above is derived.
CH. VIIl] UNICURSAL PROBLEMS 185
drawn in fisfiire 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 from 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, ajoparently 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
* Knotoledge, London, October, 1892.
t See ex. gr. Breton's Pompeia, p. 303.
f Ozanam, Graphia aureae urbis Romae, pp. 92, 178.
186
UNICURSAL PROBLEMS
[CH. VIII
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 jpater 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
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 UNICURSAL 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 ti^ee. 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 Bn the number of
trees with n free branches which are bifurcations at least,
then
{1 - x)-^ (1 - x^)-^^ (1 - a^)-^' = 1 + A,x + A^oc' + A,x^ + ..,,
(1 - x)-^ (l - x'')-^^- (1 - x')-^^ = l-\-x + 2B,x' + 2B,x' + ....
♦ Philosophical Magazine, March, 1857, series 4, vol. xra, pp. 172—176 ; or
Collected Works, Cambridge, 1890, vol. iii, no. 203, pp. 242—216 : see also the
paper on double partitions, Philosophical Magazine, November, 18G0, 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 in his report on trees to the British Association in
1875, Reports, pp. 257—305.
CH. VIIl]
UNICURSAL PROBLEMS
189
Using tliese formulae we can find successively the values of
Ai,A2, ..., and Bi, B^, .... The values of An when n = 2, 3, 4,
5, 6, 7, are 2, 4, 9, 20, 48, 115; and of B^ 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) Game 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 2Iagazine, January, 1884, series 5. vol. xvii, p. 42; also Lucas,
vol. II, part vii.
190 UNICURSAL PROBLEMS [CH. VIII
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 = rlry rl^r=lrl, lrH = r^, rl^r = l\
Also the operation Z^ or r^ brings us back to the initial point:
we may represent this by the equations
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 = ^ = ^2^3 = {rl^r) I' = {rl^Y = K (^^'^) ^1"
= {rH^rlY = jr^ (rl^r) IrlY = {r^Mrl]\
Therefore {r'l'(riyY=l (i),
and similarly {Z V (Zr^j^ = 1 (ii).
Hence on a dodecahedron either of the operations
rrrlllrlrlrrrlllrlrl ... (i),
II I r r r I r I r I I Irrrlrlr... (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 ^, but if we think of
ourselves as coming into F from E, the path FB would be
indicated by n The path from J5 to ^ is indicated by I,
and so on. Hence our first paths are indicated either by lllr
or hy rllr. 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 UTPONGDEJKLMQRSHGF,
and FBAVTSRKLMQPONGDEJHGF.
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 UNICURSAL PROBLEMS [CH. VITI
extended before it could be applied to these 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.
kirkman's school-girls problem.
The Fifteen School-Girls Problem — first enunciated b}'
T. P. Kirkman, and commonly known as Kirhnans Problem
— consists in arranging fifteen things in ditferent 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
row^s 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. R. 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
R. E,. Anstice who, in 1852 and 1853, described a method for
solving all cases of the form 12m + 3 when 6m +1 is prime.
He gave solutions for the cases when n=15, 27, 89. 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 12?7i + 8 and 24??i + 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 ?i = 15 and to all cases when n is
of the form 2""* — 1. It has been applied to find solutions
when 71 = 15 and ?i = 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 71= 27, S3, 51, 57, 69, 75, 87, 93, 111 ; and Eckenstein for
71= 27, 38, 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 195
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??! + 3, and seems to be
also applicable when n is of the form 24??i + 9. Somewhat
similar methods were used by Dudeney, save that he made
no use of geometrical constructions.
We have ?i = 2y -f 1 = 24??i + 3 or n= 2y -[-1 - 24m + 9.
We may denote one girl by k, and the others by the numbers
1, 2, 3, ... 2y. 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 2?/ — 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 2^—2 points.
These chords may be represented symbolically by [1], [2],
[3], ... [i/— 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 ii p -\- q is less
than y, we have p -\-q = r', and if ^ 4- ^^ is greater than y we
have p + q-\-r= 2y. If we like to use arcs larger than the
13—2
196 kirkman's school-girls problem [ch. IX
semi-circnmference we mav 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 jt?, g, 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 (3/ — l)/3 triangles with the corresponding diameter
will give an arrangement for one day. If the system be per-
muted cyclically 3/ — 1 times we get arrangements for the other
y — \ 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 (3/ — 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 y — \ 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, w = 3, m = 0, 3/ = 1. This involves no discussion,
the solution being (a. 6. c).
The next case is that of nine girls walking out for four days,
that is, ?i = 9, m = Oj 3/ = 4. The first triplet on the first day
is (1. h. 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, 3, and they satisfy the condition p + 5 = r. If a
I
CH. IX]
KIPKMANS SCHOOL-GTRLS 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, k, at the centre and the other 26
on the cu'cumference 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
trianoles may be of the lengths represented symbolically by
[1], [2], ... [12]. We have to group these lengths so that
p-{-q = rovp-\-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 liaving 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 a,rrangements for the re-
maining 12 days can be found immediately.
I proceed to give one solution of this type for every remaining case where n
is less than 100. From the result the triads or gi'oupings used can be obtained.
It is sufficient in each case to give an arrangement on the first day, since the
arrangements on the follov.iug 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 (fc. 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. 62. 3), (8. 10. 51), (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. ft. 41), the 13 triplets (3. 35. 42), (4. 10. 29), (5. 28. 56), (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 24??i + 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 (fc. 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. 63), (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 24m + 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, ueing 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 tlie arrangements
on the other days can be obtained by a two-step (but not by a one-step) cyclical
permutation.
It is uDnecessary 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.
Tiuo-Step Cycles. The method used by Legros was extended
by Eckenstein to cases where n is of the form 127?i-f 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(3/ — l)/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
cii'cumference 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 - !]• 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 sj^stem is rotated cyclically.
Of course a vertex cannot be at the point 1 or y, 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 h
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 wsed 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. TX] KIRKMAN'S SCHOOL-GIRLS PROBLEM 201
a time, we get a solution of the problem for the seven cla3^s
required. Each of these solutions b^ 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, 6], [2, 3, 5], [3, 4, 7], [1, 5, 6] ; or [2, 4, 6], [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 i*n 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, G, 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 last 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). Tlie 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-stop cyclical jsolution.
202 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. 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 (fc. 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 (fc. 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 (&. 1. 32),
(57. 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 (fc. 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. 50), (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 (ft. 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. 87. 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. 56), (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 aj, 6i, a^, h.^, 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-gtrls problem 203
all cases except those where n is of the form 24m + 21. These
I have failed to bring undt^r 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 hand, 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 cti, 6i, Ci, a^, h., Ca, ..., and make a cyclical
permutation of the suffixes one step at a time), we cannot
get arrangements for more than 7i/3 days. Hence there will
remain (n — l)/2 — ??/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 n/S 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 18??i-}-3, or 18m 4-9, or
18m + 15. Most of the difficulties in using it arit^e in the case
when n is of the form 18m -f- 15.
204 KIRKMAN'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, ... n, or letters a,, 6i, Ci,
o^2> &2> C.2, — 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/o 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 ?i/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 njZ 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], ... [(?i — l)/2], whose values denote the lengths of
the sides p, q, r of the possible triangles, also, we have either
p + q — r OY p + q + 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-f-l, and once from a number of the form Sm + 2.
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 n, every triangle, one of whose
CH. iX] KIRKMA.N*S SCHOOL-GIRLS PROBLEM 205
sides 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 occur together only once, each diiference when
added to a number must start each time from a different
number. I will not go into further details as to how these
trianoles 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
p -^q z=,r OT p + q + 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 a^b^a^, hiC^h^, CiOfaCg. From this, by simple cyclical per-
mutations of the suffixes, we get arrangements for the second
and third days. Lastly, the triplet UibiCi gives, by cyclical
permutation of the suffixes, the arrangement for the fourth day,
namely, (ii&iCi, a^b^c^, a^biCz.
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 ma}^
appear necessitates that six of the triads shall involve a 3 or
a 6. Hence only one triad will be available for the second part
pf 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 6m -l- 1 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), (15. 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), (21. 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. 50). 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 girls, 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. 53. 84). From each of the 15 triplets
(1. 14. 42), (2. 15. 43), (3. 16. 41), (1. 33. 44), (2. 34. 45), (3. 35. 46), (1. IT. 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 aj, a.j, 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. cl). 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. c-i) 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 10th day
from the triplet (cl. rt2. 64). Thus with seven suffixes we keep 7 for each symbol
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-srep can also be obtained for
numbers of the form 18»i-f3; 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 18j7H-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. 612. 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), («8.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 the first day consists of the 7 triplets
(al. alO. «9), (a5. a8. a3), (a4. al9. alo), (a7. al4. 621), (a20. 611. cl2),
(al6. 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. &11. al2),
(cl6. 613, 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. al5. a28),
(all. a22. 633), (a32. 630. c23), (rt31. 627. cl3), (a29. 621. c26), (a25. 69. cl9),
(al7. 618. c5), and the 22 analogous triplets. 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 which, 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 18??i-i-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 18ni-|-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 Qm + 5 days are
obtained by three-step cyclical permutations. In the second part, we obtain the
first triplet on each of the remaining Sm -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. 15. 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. ix] kirkman's school-girls problem 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. 20), (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 is (23. 46. 69), (31. 34. 43),
(11. 8. 68), (54. 57. 66), (58. 52. 28), (35. 29. 5), (45. 51. 6), (16. 1. 49),
(62. 47. 26), (39. 24. 3), (4. 55. 42), (50. 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. 18), (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. 56. 81), (55. 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. S), (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. 16. 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 24?m + Sp, where p = 6g + 3. It is due
to Eckeustein. Here it is convenient to use a geometrical
repre.sentation by denoting 24;/i + 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 Sq + 1 days ; here the foci and the
numbered points are treated separately, the former being
a R. 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 •}-2q + l, 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^/ + 1 = c, where x = or a) = l, and a the highest multiple
of 6 included in x + y, then sokitions of not less than y-hl—a
cases can be deduced. Thus from a 27 circle arrangement
where c = 9, y = 2, x = 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, 2/ = 3, ^ = 0, a = 0, we can deduce
four solutions, namely when n = 81, 93, 105, 117.
I have no space to describe the method fully, but I will give
solutions for two cases, namely for 33 girls (?i = 33, m = 1, p = 3)
where there are 3 foci, and for 51 girls (?i = 51, m = 1, p = 9)
where there are 9 foci.
For 33 girls we have 3 foci which we may denote by a, b, 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), (b. 20. 25),
(c. 15. 30), (1. 2. 14), (16. 17. 29), (4, 28. 2G), (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. 6. 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
d, 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. .SI), (e. 19. 34), (/. 40. 13), (g. 39. 16), {k 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. 33), (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), {h. 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 n 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 (ri — 1)/2 is prime, of the form 6u + l,
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 + l, and 3/ 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 cc. The
residues to the modulus y of the 6w successive powers of oc form
a series of numbers, el, e2, e3, . . . , comprising all the integers from
1 to 6w, 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, 3/= 13, u=2, 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, b2, ... 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 ' b' and the remaining six ' h '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 Sa 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, cZ2, c?3, ... such
that any Su consecutive terms comprise all the integers from
1 to Su. The first half of this series may be divided into three
equal divisions thus: (1) dly d^, dl, ... ; (2) d2, do, d8, ...;
(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 wdth 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
8. 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 sufBxes of the '6's.
We have now to use six of these 'b'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),
(ao. al. 612), (alO. al. 611), (62. 66. 6 5), (6 3. 69. 61),
{k al3. 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's on the 13 circle. This gives the following solution,
in which the first six triangles are isosceles: (a2. aS. h5),
(a4. aS. 610), (a6. all. 62), (al2. a9. 64), (a5. a7. 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
2h—l 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 IS — d in a 26 circle, and every residue e in a 13 circle
will correspond to e or 1'3 + e in a 26 circle. Further, a
triplet in the 26 circle must either have tliree 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 24m + 3.
216
KIRKMAN S SCHOOL-GIRLS PROBLEM
[CH. IX
The process by pairing when 3/ is a prime of the form 6u-\-l
is extremely rapid. For instance, in the case of 15 girls we
have n = 15, 3/ = 7, w = 1, a?= 5. The order of the residues is
5, 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, a 4, 62), (a 6. a 2. 61),
(a3. al. 64), (63. 65. 66), (k. al. 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), (a6. a2. 64), (a3. al. 62), (63. 65. 66), {k. al. hi).
Again, take the case of 39 girls. Here we have n = 39,
2/= 19, i^=3, ^=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 sufiixes 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. 16
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. al5. 612), (aS. a1. hll), (a2. al6. 69), {aQ, alO. 68),
(al8. all. 65), (al4. all. 66), (a4. al3. 618), (ttl2. al. 616),
(63. 610. 61), (62. 613. 67), (614. 615. 611), (A;. al9. 619).
In the case of 39 girls we may also extend the method
used above by which 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. 30); (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 affected, 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 ha«
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.
Walecki's Theorem. It should be noticed that Walecki —
quoted by Lucas — has shown that if a solution for the case of
n girls walking out in triplets for (n — l)/2 days is known, then
a solution for Sn girls walking out for (3n — l)/2 days can be
deduced.
For if an arrangement of the n girls Oi, ag, ..., a^ for
(n— l)/2 days is known ; and also one of the n girls h^, h^, ...,&»;
and also one of the n girls Ci, Ca, ..., Cn\ then obviously an
arrangement of these 3n girls for (n — l)/2 days can be written
down at once. A set of n triplets for another day will be given
by ami>m+kCm+2k, where m is put equal to 1, 2, ..., n successivel3\
Here k may have any of the n values, 0, 1, 2, ..., (n — 1) ; but,
wherever a suffix is greater than n, it is to be divided by n and
only the remainder retained. Hence altogether we have an
arrangement for (n — l)/2 + n days, i.e. for (Sn — l)/2 days. We
might also form the known order of arrangements for (n — 1)/2
days for the three sets of girls, and then arrange the girls in
triplets thus: (ci. a^. hn), (02- cl^. &n-iX ••• ('^n- ^7f &i)« from this,
arrangements for n days can be obtained by one-step cyclical
permutations of the a and h suffixes, keeping the c suffixes
unchanged. Hence altogether we get, as before, arrangements
for (ri — l)/2 + n days, i.e. for (8n — l)/2 days.
I have given solutions for all values of n less than 100.
Other solutions for some of these values could also be found by
Walecki's Theorem. By that theorem we can also write down
CH. ix] kirkman's school-girls problem 219
solutions for 3'"/i girls where n has any of the values given
above.
Extension to n^ Girls. Peirce suggested the corresponding
problem of arranging n^ girls in n groups, each group containing
n girls, on w + 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 on the circumference.
When ?i = 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 solu-
tion for the other day is obtained b}- a one-step cyclical
permutation.
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 same triangle rotated through 180°,
thus occupying the position (5. 6. 3). 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, (k. 5. 10. \o);
a quadrilateral (1. 2. 4. 8) ; the same quadrilateral rotated
through 120° and 240°, thus occupying the positions (6. 7. 9. 13)
and (11. 12. 14. 3). 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, (/j. 6. 12. 18. 24);
a pentagon (2. 3. 5. 13. 22) ; the same pentagon rotated through
90°, 180°, 270°, thus occupying the positions (8. 9. 11. 19. 4),
(14. 15. 17. 1. 10), (20. 21. 23. 7. 16). 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 — (j has baffled all attempts to
find a solution.
220
KIRKMAN S SCHOOL-GIRLS PROBLEM
[CH. IX
When 72 = 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 the same group rotated through
60°, 120°, 180°, 240°, 300°. 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 Eck en stein.
If the first group is {k. 9. 18. 27. 36. 45. 54. 63), the second
groupiseither(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
heing obtained by successive addition of 9 to the number in the
second group.
When n is composite no general method of attacking the
problem has been discovered, though solutions for various par-
kl, k2, kn.
al, a2, an.
61, 62, bn.
kl, al, 62, c3,
k2, a2, 64, c6,
kS, a3, 66, c9,
kn, an, bn, en,
Arrangement on First Day
Arrangement on Second Day
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
kl, k2, kd, fe4, ko, fc6, kl.
al, a2, a3, a4, a5, a6, al.
61, 62, 63, 64, 65, 66, 67.
cl, c2, cB, c4, c5, c6, c7.
dl, d2, d3, d4, do, dQ, dl.
el, e2, eS, e4, e5, e6, el.
fh /2, /3, /4, /5, /6, fl.
Arrangement on the First Day
I
kl, al, 62, c3, d4, eo, /6.
k2, a2, 64, c6, dl, e3, /5.
^•3, aS, 66, c2, do, el, /4.
fe4, a4, 61, c5, d2, e6, /3.
fc5, a5, 63, cl, d6, e4, /2.
k6, a6, 65, c4, d3, e2, fl.
kl, al, hi, cl, dl, el, fl.
Arrangement on the Second Day
arrangement on the first day. Then on the second day we may
take as an arrangement that shown in the right half of the
CH. ix] kirkman's school-girls problem 221
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 A; being
unaltered. An example will make this clearer. For instance,
when n = 7 the rule gives the arrangements for the first and
second days shown in the second table.
Kirkmans Problem in Quartets. The problem of arranging
4??i girls, where m is of the form 3n + 1, in quartets to walk out
for (4/Ai,— 1)/3 days, so that no girl will walk with any of her
school-fellows in any quartet more than once has been attacked.
Methods snuilar 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 plav toofether 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. To obtain cyclic solutions we may
proceed thus. Denote the members by a point k at the centre
of a circle and by 4?7i — 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 arrangement, (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. Solutions have been
published for several values of m. In the following examples
for m = 2, 3, 4, 10, I give an arrangement of the card tables
for the first rubber : the arrangements for the subsequent
rubbers being thence obtained by one-step cyclical permutations
of the numbers. If 7?i=2, such an initial arrangement is {k. 1
against 5. 6) and (2. 4 against 3. 7). If m = 3, one such initial
222 kirkman's school-girls problem [ch. ix
arrangement is (k. 1 against 5. 6), (2. 11 against 3. 9) and (4. 8
against 7. 10). If m = 4, such an inibial arrangement is {k. 1
against 6. 11), (2. 3 against 5. 9), (4. 12 against 13. 15), and
(7. 10 against 8. 14). One example of a higher number will
suffice; if m = 10 such an initial arrangement is (k. 1 against
14. 27), (2. 9 against 26. 6), (3. 17 against 12. 11), (5. 33 against
23. 21), (15. 39 against 19. 22), (16. 25 against 24. 30), (18, 36
against 34. 7), (28. 32 against 35. 13), (29. 37 against 4. 38) and
(31. 8 against 20. 10). In some cases there are also solutions
not reducible to single cycles.
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 co girls walking in rows of a abreast can be
arranged so that every possible combination of b 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 cc 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 oo/a. I confine myself
to these cases. 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 combination of h girls may
walk together once, and only once.
An example where the solution is obvious is if a; = 2n,
a— 2, 6 = 2, in which case 3/ = ?i(27i — 1), z=2n — \. If
we take the case a; =15, a = 3, 6=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 ^=7. More generally,
if a?= 5 X 3'^ a = 3, 6= 2, we find y = ^{x-\)ftx, z=-{x-\)l2.
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 x=9,
(X=3, b = o, then y = 84, ^ = 28; and if x=lo, a = 3, 6 = 3,
2/ =455, ^ = 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 x — 2^\ a = 4, 6=3, but his
analysis is open to objection. In 1867 S. Bills showed that
if 57 = 15, a = 3, 6 = 2, then y = 35: and the method used
will give the value of y, if a;=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 Problem. 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
Steiners Combinatorische Aufgabe. 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 ?i 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 tetrad ? And generally when an arrangement in ^^-ads
has been made, is it possible to arrange the things in sets of
k -{- 1-ads so that no /i-ad, where ^ :|> A:, is obtained in a
A; + l-ad, and so that every A;-let tliat is not or does not
contain an /i-ad is contained in one and only one k-v 1-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 Avith another, and I jot them down almost 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
* Thdi'e are two articles on the subject in the American Journal of
Matheviatics, 1879, vol. ii, by Professors Woolsey Johnson and Storey ; but
the whole theory is deducible immediately from the proposition I give in
the tesct.
CH. X]
MISCELLANEOUS PROBLEMS
cell or small square empty; the puzzle is to move them so
that finally they occupy the position shown in the first of the
annexed figures.
D Top of Box C
n 4 1 ta
Bottom of Box
B
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 from that in the first diagram by six
interchanges ; namely, by interchanging the counters 1 and 2,
15
B. R.
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;
11, 8, 1;
13, 2, 3, 4, 12, 7, 6, 10, 14, 15, 9; 5.
2, 3, 4, 12, 7, 6, 10, 14, 15, 9, 13; 5.
Thus, iLin 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
CH. X] MISCELLANEOUS PROBLEMS
227
to make AB 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 arrang:ement 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 m/i - 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) from the peg on which they initially rested to one of the
other pegs.
The method of efiecting 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
n — l discs from B to 0, which will necessitate x transfers.
Hence, if it requires oc transfers of simple discs to move a tower
of n — 1 discs, then it will require 2^7+1 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 RiNGSf. 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.
+ It waa described by Cardan in 1550 in his De Subtilitate, bk. xv,
paragraph 2, ed. Sponius, vol. iii, p. 587 ; by Wallis in his Algebra, Latin edition,
1693, Opera, vol. ii, chap, cxi, pp. 472—478 ; and allusion is made to it also
in Ozanam's Recreations, 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.
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**+^ — 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
free 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+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 (?7i+ 2)th ring is on and the first m 4- 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 — \ 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 — \ 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
{in-\- l)th ring off, which requires x—\ steps: these movements
require [2 (a; — 1) + 1} steps.
Therefore, if when the first in rings are off it requires x
steps to take off the {in + 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 (^- 1) + 1},
that is, is 4a7.
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 ;
^ 4,2
• • >> »» «-' )5 )> ^ 3> ii
In this way we see that the number of steps required to take
off the first 2n-\-\ rings is 1 + 4 + 4- -f ... + 4", which is equal
to J- ( 22'*+=^-!).
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 ;
• • « » >» " » )J ^ ^ -X 5, f,
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 | (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
2^^ and 2''''-'- - 1.
I give the above analysis because it is the direct solution
of a problem attacked unsuccessfully by Cardan in 1550 and by
WalKs 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.
* Theorie 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 off 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
m 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.
O OOP
o o o
o o o o o
OOP
o o o o
^ Q
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* + 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?i 4- 1 rings requires
(1 + 2- + . . . + 2-'') steps, that is, i (22^+2 _ j) g^eps ; and to put
on a set of 2n rings requires (2 + 2^ + . . . + 2^"~^) steps, that is,
J (2""+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.
Initia
. position
After
1st step
>i
2nd „
»
3rd „
ji
4th „
?>
5th „
j>
6th „
jj
7th „
jj
8th „
>j
9th ,,
jy
10th „
o
o
o
o
o
P
o
o
o
1
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
o
L2L
o
o
o
o
o
r^
o
o
o
o
o
o
o
o
10101
10110 1
lOlllJ
11000
11001 1
iioio)
11011
11100
11101
11110
11111.
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 hy Rows and Columns, the
Determination of a Pair out of ^n{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 theo:^ 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 Qp + 4 cards, the (2jo 4- 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 10/) + 2 cards is shuffled in this way, the
{2p + l)th and the (6^ + 2)th cards will interchange places. For
instance, if an 6carte pack of 32 cards is shuffled as described
above, the 7 th 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 o^oth place to the x-fii place, where
a^i = -^ (2p -\- Xq-\-V) if Xq is odd, and Xi = \ (2p — Xo + 2) if Xo 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
Xoth. place to the a^^th place, where
2»"+ia;;^ = (4j9 + l)(2"^-^±2"^-2± ... ±2 ± 1) ± 2^o + 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 ?i, 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 des
Sciences, Paris, 1773, pp. 390 — 412, Among those who have studied the subject
afresh I may in particular mention V. Bouniakowski, Bulletin physico-viatMma-
tique de St Petersbourg, 1857, vol. xv, pp. 202 — 205, summarised iu the Nouvelles
annales de mathematiques, 1858, pp. G6 — 67 ; T. de St Laurent, Memoires
de V Academic de Gard, 1865 ; L. Tanner, Educational Times Re})rints, 1880,
vol. xxxiii, pp. 73 — 75; M. J. Bourget, Lioxiville's Journal, 1882, pp. 413 — 434;
and H. F. Baker, Transactions of the British Association for 1910, pp. 520 — 528.
236 MISCELLANEOUS PROBLEMS [CH. X
the number of shuffles required for a pack of 2p cards, it is
sufficient to put oom = ^o ^'Hd find the smallest value of m which
satisfies the resulting equation for all values of ccq from 1 to 2p.
It follows that, if m is the least number which makes 4"* — 1
divisible by 4p+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 2^ + 1 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 fall pack of 52 cards, twelve shuffles are
sufficient; with a pack of 13 cards ten shuffles are sufflcient ;
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 after
the 1st, 2nd, ..., pth. shuffles by the cards Ai, Ao, ..., Ap re-
spectively, and that initially the position is occupied by the
card J-o- Suppose further that after the ^th shuffle Aq returns
to its initial position, therefore Aq = Ap. Then at the second
shuffling -ia succeeds ^1 by the same law by which Ai succeeded
-4o at the first; hence it follows that previous to the second
shuffling A2 must have been in the place occupied by Ai pre-
vious to the first. Thus the cards which after the successive
shuffles take the place initially occupied by A^ are A^, A3, ...,
Ap, All that is, after the pth shuffle Ai has returned to the
place initially occupied by it: and so for all the other cards
xi^} ■"■%} •••, -0._p.-i.
Hence the cards At,, 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, r^ ... such
cycles, whose sum is n ; then the L. c. m. of p, q, r, ... is the
• Educational Times Reprints^ London, 1865, vol. 11, 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 180180.
Arrangements by Rows and Columns. A not uncommon
trick, which rests on a species of shuffling, depends on the
obvious fiict 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
1
5
9
13
2
6
10
14
3
7
11
15
4
8
12
16
1
2
3
4
5
6
8
9
10
11
12
13
14
15
16
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
4?i(7i + l) GIVEN PAIRS*. Another common trick 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
1
4
■
2
9
3
10
5
11
7
1
13
6
8
12
14
15
18
16
19
17
20
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 239
The first 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 mih, 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 ^th and suppose q>p.
Then that one of the required cards which is in the qth line
will be the (q — ^)th card which is below the first of the clue
squares in the pth. line. The other of the required cards is in
the pth line and is the {q —p)th 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 ti + 1 cards ; for there are
^n(n + l) 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 Matas dedit nomen Cocis. 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, iy 0, c, d, m, n, s, or t respectively. Now the sentence Matas
dedit nomen Cocis 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 xvni, p. 143, but his analysis
of it is insufficient.
CH. X] MISCELLANEOUS PROBLEMS 24»1
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 br
the middle one in the pack, that is, if 27 cards are used it will
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 7?i"* 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 kth. Then when the cards are collected
after the mth deal the selected card will be nth from the top
where
if m is even, n = Jcni^-^ — pi^-^ + ... ^-hni —.a + l,
if m is odd, n = hn^"'~'^ — jm''^~' + ... —hm + a.
For example, if a pack of 256 cards (i.e. w = 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
• Gergonne's Annates de Mathimatiques, Nismes, 1813-4, voL I7, 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 hth, after the third cth, and after the fourth
dtli, the card will be the (64>d — 16c + 4ib — 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 + 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 + 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 fi:'om (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 hth : then (i) at the top of the
pack are 9(6—1) cards; (ii) next are 9 — Sa 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 S — a cards will be taken from
(ii), the next card will be one of the three cards in (iii), and the
remaining S — Sh + 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 ti = 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 ?ith. In this case we
have to find values of a, 6, c which will satisfy the equation
n = 9c — 36 + ct, where a, 6, c can have only the values 1, 2,
or 3.
Hence, if we divide w 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 n/3 : then Sp = 9c — 36, therefore ^ = 3c — 6. Hence 6 is
16—2
24<4« MISCELLANEOUS PROBLEMS [CII. 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=fl? + l, 6 = 3 — 3/, 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 ^z -\- Zy -\- x — n — 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 m"* 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, . . . , /j. 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 PKOBLEMS 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 ecarte 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.
Cayley* 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. Steenf 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 — 5] ; the
number of arrangements in Avhich 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
PAKT 11.
* 1^0 man of science should think it a waste of time to learn
something of the history of his own subject ; nor is the investigation
of laborious methods now fallen into disuse, or of errors once commonly
accepted, the least valuable of mental disciplines."
" The most wo7'thless book of a bygone day is a record worthy of
preservation. Like a telescopic star, its obscurity m,ay render it
unavailable for most purposes ; but it serves, in hands which know
how to use it, to determine the 2^^<^c^s of more important bodies."
(De Morgan.)
24>S
CHAPTER XL
THE MATHEMATICAL TRIPOS.
The Mathematical Tripos has played so prominent a part
in the history of education at Cambridge and of mathematics
in England, that a sketch of its development* may be inter-
esting to general readers.
So far as mathematics is concerned the history of the
University before Newton may be summed up very briefly.
The University was founded towards the end of the twelfth
century. Throughout the middle ages the studies were or-
ganized on lines similar to those at Paris and Oxford. To
qualify for a degree it was necessary to perform various
exercises, and especially to keep a number of acts or to oppose
acts kept by other students. An act consisted in effect of a
debate in Latin, thrown, at any rate in later times, into
syllogistic form. It was commenced by one student, the
respondent, stating some proposition, often propounded in the
form of a thesis, which was attacked by one or more oppone^its,
the discussion being controlled by a graduate. The teaching
was largely in the hands of young graduates — every master of
arts being compelled to reside and teach for at least one year —
* The following pages are mostly summarized from my Histoi-y of the Study
of Mathematics at Cambridge, Cambridge, 1889. The subject is also treated in
Whewell's Liberal Education, Cambridge, three parts, 1845, 1850, 1853;
Wordsworth's Scholae Academicae, Cambridge, 1877 ; my own Origin and
History of the Mathematical Tripos, Cambridge, 1880 ; and Dr Glaisher's
Presidential Address to the London Mathematical Society, Transactions, vol.
xvm, 1836, pp. 4—38.
CH. Xl] THE MATHEMATICAL Till PCS 249
though no doubt Colleges and private hostels supplemented
this instruction in the case of their own students.
The Reformation in England was mainly the work of
Cambridge divines, and in the University the Renaissance was
warmly welcomed. In spite of the disorder and confusion of
the Tudor period, new studies and a system of professional
instruction were introduced. Probably the science (as distinct
from the art) of mathematics, save so far as involved in the
quadrivium, was still an exotic study, but it was not wholly
neglected. Tonstall, subsequently the most eminent English
arithmetician of his time, migrated, perhaps about 1495, from
Balliol College, Oxford, to King's Hall, Cambridge, and in
1530 the University appointed a mathematical lecturer in the
person of Paynell of Pembroke Hall. Most of the subsequent
English mathematicians of the Tudor period seem to have been
educated at Cambridge ; of these I may mention Record, who
migrated, probably about 1535, from Oxford, Dee, Digges,
Blundeville, Buckley, Billingsley, Hill, Bedwell, Hood, Richard
and John Harvey, Edward Wright, Briggs, and Oughtred.
The Elizabethan statutes restricted liberty of thought and
action in many ways, but, in spite of the civil and religious
disturbances of the early half of the 17th century, the mathe-
matical school continued to grow. Horrox, Seth Ward, Foster,
Rooke, Gilbert Clerke, Pell, Wallis, Barrow, Dacres, and Morland
may be cited as prominent Cambridge mathematicians of the
time.
Newton's mathematical career dates from 1665 ; his repu-
tation, abilities, and influence attracted general attention to
the subject. He created a school of mathematics and mathe-
matical physics, among the earliest members of which I note
the names of Laughton, Samuel Clarke, Craig, Flamsteed,
Whiston, Saunderson, Jurin, Taylor, Cotes, and Robert Smith.
Since then Cambridge has been regarded as, in a special sense,
the home of English mathematicians, and from 1706 onwards
we have fairly complete accounts of the course of reading and
work of mathematical students there.
Until less than a century ago the form of the method of
250 THE MATHEMATICAL TRIPOS [CH. XI
qualifying for a degree remained substantially unaltered, but
the subject-matter of the discussions varied from time to time
with the prevalent studies of the place.
After the Renaissance some of the statutable exercises were
" huddled," that is, were reduced to a mere form. To huddle
an act, the proctor generally asked some question such as Quid
est noinen? to which the answer usually expected was Nescio.
In these exercises considerable license was allowed, particularly
if there were any play on the words involved. For example,
J. Brasse, of Trinity, was accosted with the question. Quid est
aes ? to which he answered, Nescio nisi finis examinationis. It
should be added that retorts such as these were only allowed
in the pretence exercises, and a candidate who in the actual
examination was asked to give a definition of happiness and
replied an exemption from Payne — that being the name of the
moderator then presiding — was plucked for want of discrimina-
tion in time and place. In earlier years even the farce of
huddling seems to have been unnecessary, for it was said in
1675 that it was not uncommon for the proctors to take
" cautions for the performance of the statutable exercises, and
accept the forfeit of the money so deposited in lieu of their
performance.**
In medieval times acts had been usually kept on some
scholastic question or on a proposition taken from the Sentences.
About the end of the fifteenth century religious questions, such
as the interpretation of Biblical texts, began to be introduced.
Some fifty or sixty years later the favourite subjects were
drawn either from dogmatic theology or from philosophy.
In the seventeenth century the questions were usually philo-
sophical, but in the eighteenth century, under the influence of
the Newtonian school, a large proportion of them were
mathematical.
Further details about these exercises and specimens of acts
kept in the eighteenth century are given in my History of
Mathematics at Cambridge. Here I will only say that they
provided an admirable training in the art of presenting an
argument, and in dialectical skill in attack and defence. The
CH. Xl] THE MATHEMATICAL TRIPOS 251
mental strain in a contested act was severe. De Morgan,
describing his act kept in 1826, wrote*:
I was badgered for two hours with arguments given and answered in Latin
— or what we call Latin — against Newton's first section, Lagrange's derived
functions, and Locke on innate principles. And though I took off everything,
and was pronounced by the moderator to have disputed magno honore, I never
had such a strain of thought in my life. For the inferior opponents were made
as sharp as their betters by their tutors, who kept lists of queer objections drawn
from all quarters.
Had the language of the discussions been changed to English,
as was repeatedly urged from 1774 onwards, these exercises
might have been retained with advantage, but the barbarous
Latin and the syllogistic form in which they were carried on
prejudiced their retention.
About 1830 a custom grew up for the respondent and
opponents to meet previously and arrange their arguments
together. The discussions then became an elaborate farce,
and were a mere public performance of what had been already
rehearsed. Accordingly the moderators of 1839 took the re-
sponsibility of abandoning them. This action was singularly
high-handed, since a report of May 30, 1838, had recommended
that they should be continued, and there was no reason why
they should not have been reformed and retained as a useful
feature in the scheme of study.
On the result of the acts a list of those qualified to receive
degrees was drawn up. This list was not arranged strictly in
order of merit, because the proctors could insert names any-
where in it, but by the beginning of the eighteenth century
this power had become restricted to the right reserved to the
vice-chancellor, the senior regent, and each proctor to place
in the list one candidate anywhere he liked — a right which
cjntinued to exist till 1828, though it was not exercised after
1792. Subject to the granting of these honorary degrees, this
final list was arranged in order ot merit into wranglers and
senior optimes, junior optimes, and poll-men. The bachelors
on receiving their degrees took seniority according to their
* Biuhjct of Paradoxes, by A. De Morgan, London, 1872, p. 305.
252 THE MATHEMATICAL TRIPOS [CU. XI
order on this list. The title wrangler is derived from these
contentious discussions; the title optime from the customary-
compliment given by the moderator to a successful disputant,
Domine..., optime disputasti, or even optime quidem disputasti,
and the title of poll-man from the description of this class as ol
The final exercises for the B.A. degree were never huddled,
and until 1839 were carried out strictly. University officials
were responsible for approving the subject-matter of these acts.
Stupid men offered some irrefutable truism, but the ambitious
student courted reputation by affirming some paradox. Prob-
ably all honour men kept acts, but poll-men were deemed to
comply with the regulations by keeping opponencies. The
proctors were responsible for presiding at these acts, or seeing
that competent graduates did so. In and after 1649 two
examiners were specially appointed for this purpose. In 1680*
these examiners were appointed by the Senate with the title of
moderator, and with the joint stipend of four shillings for
everyone graduating as B.A. during their year of office. In
1688 the joint stipend of the moderators was fixed at £40
a year. The moderators, like the proctors, were nominated by
the Colleges in rotation.
From the earliest times the proctors had the power of
questioning a candidate at the end of a disputation, and
probably all candidates for a degree attended the public schools
on certain days to give an opportunity to the proctors, or any
master that liked, to examine themf, though the opportunity
was not always used. Different candidates attended on different
days. Probably such examinations were conducted in Latin.
But soon after 1710 1 the moderators or proctors began the
custom of summoning on one day in January all candidates
whom they proposed to question. The examination was held
* See Grace of October 25, 1680.
t Ex. gr. see De la Pryme's account of his graduation in 1694, Surtees
Society, vol. liv, 1870, p. 32.
X W. Eeneu, in bis letters of 1708-1710 describing the course for the B.A.
degree, makes no mention of the Senate-House Examination, and I think it is
a reasonable inference that it had not then been established.
CH. XlJ THE MATHEMATICAL TRIPOS 253
in public, and from it the Senate-House Examination arose.
The examination at this time did not last more than one day,
and was, there can be no doubt, partly on philosophy and partly
on mathematics. It is believed that it was always conducted
in English, and it is likely that its rapid development was
largely due to this.
This introduction of a regular oral examination seems to
have been largely due to the fact that when, in 1710, George I
gave the Ely library to the University, it was decided to assign
for its reception the old Senate-House — now the Catalogue
Room in the Library — and to build a new room for the
meetings of the Senate. Pending the building of the new
Senate-House the books were stored in the Schools. As the
Schools were thus rendered unavailable for keeping acts,
considerable difficulty was found in arranging for all the candi-
dates to keep the full number of statutable exercises, and thus
obtaining opportunities to compare them one with another:
hence the introduction of a supplementary oral examination.
The advantages of this examination as providing a ready
means of testing the knowledge and abilities of the candidates
were so patent that it was retained when the necessity for
some system of the kind had passed away, and finally it became
systematized into an organized test to which all questionists
were subjected.
In 1731 the University raised the joint stipend of the
moderators to £60 " in consideration of their additional trouble
in the Lent Term." This would seem to indicate that the
Senate-House Examination had then taken formal shape, and
perhaps that a definite scheme for its conduct had become
customary.
As long as the order of the list of those approved for
degrees was settled on the result of impressions derived from
acts kept by the different candidates at different times and on
different subjects, it was impossible to arrange the men in
strict order of merit, nor was much importance attached to the
order. But, with the introduction of an examination of all the
candidates on one day, much closer attention was paid to
254 THE MATHEMATICAL TRIPOS [CH. XI
securing a strict order of merit, and more confidence was felt in
the published order. It seems to have been consequent on
this that in and after 1748 (N.S.) the final lists were circulated,
and it was further arranged that the names of the honorary
optimes should be indicated. The lists from this time appear
in the University calendars : the earlier lists from 1499, edited
with biographical notes by C. M. Neale, were published in
1909.
Of the detailed history of the examination until the middle
of the eighteenth century we know nothing. From 1750 on-
wards, however, we have more definite accounts of it. At this
time, it would seem that all the men from each College were
taken together as a class, and questions passed down by the
proctors or moderators till they were answered : but the ex-
amination remained entirely oral, and technically was regarded
as subsidiary to the discussions which had been previously held
in the schools. As each class contained men of very different
abilities a custom grew up by which every candidate was liable
to be taken aside to be questioned by any M.A. who wished
to do so, and this was regarded as an important part of the
examination. The subjects were mathematics and philosophy.
The examination now continued for two days and a half. At
the conclusion of the second day the moderators received the
reports of those masters of arts who had voluntarily taken part
in the examination, and provisionally settled the final list;
while the last half-day was used in revising and re-arranging
the order of merit.
Richard Cumberland has left an account of the tests to
which he was subjected when he took his B.A. degree in 1751.
Clearly the disputations still played an important part, and it is
difficult to say what weight was attached to the subsequent
Senate-House Examination ; his reference to it is only of a
general character. After saying that he kept two acts and two
opponencies he continues*:
The last time I was called upon to keep an act in the schools I sent in three
questions to the Moderator, which he withstood as being all mathematical, and
* Memoirs of Eichard Cumberland, London, 1806, pp. 78 — 79.
CH. Xl] THE MATHEMATICAL TIUPOS 255
required me to conform to the usage of proposing one metaphysical question in
the place of that, which I should think fit to withdraw. This was ground I
never liked to take, and I appealed against his requisition: the act was
accordingly put by till the matter of right should be ascertained by the statutes
of the university, and in the result of that enquiry it was given for me, and my
question stood I yielded now to advice, and paid attention to my health, till
we were cited to the senate house to be examined for our Bachelor's degree. It
was hardly ever my lot during that examination to enjoy any respite. I seemed
an object singled out as every man's mark, and was kept perpetually at the
table under the process of question and answer.
It was found possible by means of the new examination to
differentiate the better men more accurately than before ; and
accordingly, in 1753, the first class was subdivided into two,
called respectively wranglers and senior optimes, a division which
is still maintained.
The semi-official examination by M.A.s was regarded as the
more important part of the test, and the most eminent residents
in the University took part in it. Thus John Fenn, of Caius,
5th Avrangier in 1761, writes* :
On the following Monday, Tuesday, and Wednesday, we sat in the Senate-
house for public exainination ; during this time I was officially examined by
the Proctors and Moderators, and had the honor of being taken out for
examination by Mr Abbot, the celebrated mathematical tutor of St John's
College, by the eminent professor of mathematics Mr Waving, of Magdalene,
and by Mr Jebb of Peterhouse, a man thoroughly versed in the academical
studies.
This irregular examination by any master who chose to take
part in it constantly gave rise to accusations of partiality.
In 1763 the traditional rules for the conduct of the examin-
ation took more definite shape. Henceforth the examiners used
the disputations only as a means of classifying the men roughly.
On the result of their " acts," and probably partly also of their
general reputation, the candidates were divided into eight
classes, each arranged in alphabetical order. The subsequent
position of the men in the class was determined solely by the
Senate- House Examination. The first two classes comprised all
who were expected to be wranglers, the next four classes included
* Quoted by C. Wordsworth, Scholae Academicae, Cambridge, 1877,
pp. 30—31.
256 THE MATHEMATICAL TRIPOS [CH. XI
the other candidates for honours, and the last two classes con-
sisted of poll-men only. Practically anyone placed m either
of the first two classes was allowed, if he wished, to take an
aegrotat senior optime, and thus escape all further examination:
this was called gulphing it. All the men from one College were
no longer taken together, but each class was examined separately
and vivd voce ; and hence, since all the students comprised in
each class were of about equal attainments, it was possible
to make the examination more effective. Richard \Vatson, of
Trinity, claimed that this change was made by him when
acting as moderator in 1763. He says*:
There was more room for partiality... then [i.e. in 1759] than there is now;
and I attribute the change, in a great degree, to an alteration which I introduced
the first year I was moderator [i.e. in 1763], and which has been persevered in
ever since. At the time of taking their Bachelor of Arts' degree, the young
men are examined in classes, and the classes are now formed according to the
abilities shown by individuals in the schools. By this arrangement, persons of
nearly equal merits are examined in the presence of each other, and flagrant
acts of partiality cannot take place. Before I made this alteration, they were
examined in classes, but the classes consisted of members of the same College,
and the best and worst were often examined together.
It is probable that before the examination in the Senate-House
began a candidate, if manifestly placed in too low a class, was
allowed the privilege of challenging the class to which he was
assigned. Perhaps this began as a matter of favour, and was
only granted in exceptional cases, but a few years later it
became a right which every candidate could exercise ; and I
think that it is partly to its development that the ultimate
predominance of the tripos over the other exercises for the
degree is due.
In the same year, 1763, it was decided that the relative
position of the senior and second wranglers, namely, Paley, of
Christ's, and Frere, of Caius, was to be decided by the Senate-
House Examination and not by the disputations. Henceforward
distinction in the Senate-House Examination was regarded as
the most important honour open to undergraduates.
* Anecdotes of the Life of Richard Watson by Ilimselff London, 1817,
pp. 18—19.
CH. Xl] THE MATHEMATICAL TRIPOS 257
In 1768 Dr Smith, of Trinity College, founded prizes for
mathematics and natural philosophy open to two commencing
bachelors. The examination followed immediately after the
Senate-House Examination, and the distinction, being much
coveted, tended to emphasize the mathematical side of the
normal University education of the best men. Since 1883 the
prizes have been awarded on the result of dissertations*.
Additional prizes, awarded at the same time, and associated
with the name of Lord Rayleighf, were founded in 1909.
Until now the Senate- House Examination had been oral,
but about this time, circ. 1770, it began to be the custom to
dictate some or all of the questions and to require answers to
be written. Only one question was dictated at a time, and a
fresh one was not given out until some student had solved that
previously read : a custom which by causing perpetual inter-
ruptions to take down new questions must have proved very
harassing. We are perhaps apt to think that an examination
conducted by written papers is so natural that the custom is of
long continuance, but I know no record of any in Europe earlier
than the eighteenth century. Until 1830 the questions for the
Smith's prizes were dictated.
The following description of the Senate-House Examination
as it existed in 1772 is given by JebbJ.
The moderators, some days before the arrival of the time prescribed by the
vice-chancellor, meet for the purpose of forming the students into divisions of
six, eight, or ten, according to their performance in the schools, with a view
to the ensuing examination.
Upon the first of the appointed days, at eight o'clock in the morning, the
students enter the senate-house, preceded by a master of arts from each college,
who... is called the "father" of the college, ...
After the proctors have called over the names, each of the moderators sends
for a division of the students: they sit with him round a table, with pens, ink,
and paper, before them : he enters upon his task of examination, and does not
dismiss the set till the hour is expired. This examination has now for some
years been held in the english language.
* See Grace of October 25, 1883 ; and the Cambridge University Beporter^
October 23, 1883.
t See Grace of February 11, 1909, and the Cambridge University Reporter,
December 8, 1903.
J The Works of J. Jcbb, London, 1787, vol. ii, pp. 290—297,
B. R. 17
258 THE MATHEMATICAL TRIPOS [CH. XI
The examination is varied according to the abilities of the students. The
moderator generally begins with proposing some questions from the six books
of Euclid, plain {sic) trigonometry, and the first rules of algebra. If any person
fails in an answer, the question goes to the next. From the elements of
mathematics, a transition is made to the four branches of philosophy, viz.
mechanics, hydrostatics, apparent astronomy, and optics, as explained in the
works of Maclaurin, Cotes, Helsham, Hamilton, Eutherforth, Keill, Long,
Ferguson, and Smith. If the moderator finds the set of questionists, under
examination, capable of answering him, he proceeds to the eleventh and
twelfth books of Euclid, conic sections, spherical trigonometry, the higher parts
of algebra, and sir Isaac Newton's Principia ; more particularly those sections,
which treat of the motion of bodies in eccentric and revolving orbits; the
mutual action of spheres, composed of particles attracting each other according
to various laws ; the theory of pulses, propagated through elastic mediums ;
and the stupendous fabric of the world. Having closed the philosophical
examination, he sometimes asks a few questions in Locke's Essay on the
human understanding, Butler's Analogy, or Clarke's Attributes. But as the
highest academical distinctions are invariably given to the best proficients in
mathematics and natural philosophy, a very superficial knowledge in morality
and metaphysics will suffice.
When the division under examination is one of the highest classes, problems
are also proposed, with which the student retires to a distant part of the
senate-house, and returns, with his solution upon paper, to the moderator, who,
at his leisure, compares it with the solutions of other students, to whom the
same problems have been proposed.
The extraction of roots, the arithmetic of surds, the invention of divisers,
the resolution of quadratic, cubic, and biquadratic equations ; together with
the doctrine of fluxions, and its application to the solution of questions " de
maximis et minimis," to the finding of areas, to the rectification of curves, the
investigation of the centers of gravity and oscillation, and to the circumstances
of bodies, agitated, according to various laws, by centripetal forces, as unfolded, and
exemplified, in the fiuxional treatises of Lyons, Saunderson, Simpson, Emerson,
Maclaurin, and Newton, generally form the subject matter of these problems.
When the clock strikes nine, the questionists are dismissed to breakfast : they
return at half-past nine, and stay till eleven ; they go in again at half-past one,
and stay tiU three ; and, lastly, they return at half -past three, and stay till five.
The hours of attendance are the same upon the subsequent day.
On the third day they are finally dismissed at eleven.
During the hours of attendance, every division is twice examined in form,
once by each of the moderators, who are engaged for the whole time in this
employment.
As the questionists are examined in divisions of only six or eight at a time,
but a small portion of the whole number is engaged, at any particular hour,
with the moderators ; and, therefore, if there were no further examination,
much time would remain unemployed.
But the moderator's inquiry into the merits of the candidates forms the
least material part of the examination.
CH. Xl] THE MATHEMATICAL TRIPOS 259
The "fathers" of the respective colleges, zealous for the credit of the
societies, of which they are the guardians, are incessantly employed in ex-
amining those students, who appear most likely to contest the palm of glory
with their sons.
This part of the process is as follows :
The father of a college takes a student of a different college aside, and,
sometimes for an hour and an half together, strictly examines him in every part
of mathematics and philosophy, which he professes to have read.
After he hath, from this examination, formed an accurate idea of the student's
abilities and acquired knowledge, he makes a report of his absolute or comparative
merit to the moderators, and to every other father who shall ask him the question.
Besides the fathers, all masters of arts, and doctors, of whatever faculty
they be, have the liberty of examining whom they please ; and they also report
the event of each trial, to every person who shall make the inquiry.
The moderators and fathers meet at breakfast, and at dinner. From the
variety of reports, taken in connection with their own examination, the former are
enabled, about the close of the second day, so far to settle the comparative merits
of the candidates, as to agree upon the names of four-and-twenty, who to them
appear most deserving of being distinguished by marks of academical approbation.
These four-and-twenty [wranglers and senior optimes] are recommended to
the proctors for their private examination ; and, if approved by them, and no
reason appears against such placing of them from any subsequent inquiry,
their names are set down in two divisions, according to that order, in which
they deserve to stand ; are afterwards printed ; and read over upon a solemn
day, in the pres'^uce of the vice-chancellor, and of the assembled university.
The names of the twelve [junior optimes], who, in the course of the examina-
tion, appear next in desert, are also printed, and are read over, in the presence
of the vice-chancellor, and of the assembled university, upon a day subsequent
to the former. . . .
The students, who appear to have merited neither praise nor censure, [the
poll-men], pass unnoticed : while those, who have taken no pains to prepare
themselves for the examination, and have appeared with discredit in the schools,
are distinguished by particular tokens of disgrace.
Jebb's statement about the number of wranglers and senior
optimes is only approximate.
It may be added that it was now frankly recognized that
the examination was competitive*. Also that though it was
open to any member of the Senate to take part in it, yet the
determination of the relative merit of the students was entirely
in the hands of the moderators f. Although the examination
did not occupy more than three days it must have been a severe
* "Emulation, which is the principle upon which the plan is constructed."
The Works of J. Jebb, London, 1787, vol. in, p. 261.
t Tlie Works of J. Jebb, London, 1787, vol. in, p. 272.
17—2
260 THE MATHEMATICAL TRIPOS [CH. XI
physical trial to anyone who was delicate. It was held in winter
and in the Senate-House. That building was then noted for
its draughts, and was not warmed in any way : and, according
to tradition, on one occasion the candidates on entering in the
morning found the ink in the pots on their desks frozen.
The University was not altogether satisfied* with the
scheme in force, and in 1779 f the scheme of examination was
amended in various respects. In particular the examination
was extended to four days, a third day being given up entirely
to natural religion, moral philosophy, and Locke. It was
further announced J that a candidate would not receive credit
for advanced subjects unless he had satisfied the examiners in
Euclid and elementary Natural Philosophy.
A system of brackets or " classes quam minimae " was now
introduced. Under this system the examiners issued on the
morning of the fourth day a provisional list of men who had
obtained honours, with the names of those of about equal merit
bracketed, and that day was devoted to arranging the names in
each bracket in order of merit: the examiners being given
explicit authority to invite the assistance of others in this work.
Whether at this time a candidate could request to be re-ex-
amined with the view of being moved from one bracket to
another is uncertain, but later this also was allowed.
Under the scheme of 1779 also the number of examiners
was increased to four, the moderators of one year becoming, as
a matter of course, the examiners of the next. Thus of the four
examiners in each year, two had taken part in the examination
of the previous year, and the continuity of the system of ex-
amination was maintained. The names of the moderators
appear on the tripos lists, but the names of the examiners were
not printed on the lists till some years later.
The right of any M.A. to take part in the examination was
not affected, though henceforth it was exercised more sparingly,
and I believe was not insisted on after 1785. But it became
* See Graces of July 5, 1773, and of February 17, 1774.
+ See Graces of March 19, 20, 1779.
t Notice issued by the Vice-Chancellor, dated May 19, 1779.
CH. Xl] THE MATHEMATICAL TRIPOS 261
a regular custom for the moderators to invite particular M.A.s
to examine and compare specified candidates. Milner, of
Queens', was constantly asked to assist in this way.
It was not long before it became an established custom that
a candidate, who was dissatisfied with the class in which he had
been placed as the result of his disputations, might challenge it
before the examination began. This power seems to have been
used but rarely ; it was, however, a recognition of the fact that
a place in the tripos list was to be determined by the Senate-
House Examination alone, and the examiners soon acquired the
habit of settling the preliminary classes without exclusive
reference to the previous disputations.
The earliest extant paper actually set in the Senate-House,
to which we can with certainty refer, is a problem paper set in
1785 or 1786 by W. Hodson, of Trinity, then a proctor. The
autograph copy from which he gave out the questions was
luckily preserved, and is in the library* of Trinity College.
It must be almost the last problem paper which was dictated,
instead of being printed and given as a whole to the candi-
dates. The paper is as follows :
1. To determine the velocity with which a Body must be thrown, in a
direction parallel to the Horizon, so as to become a secondary planet to the
Earth ; as also to describe a parabola, and never return.
2. To demonstrate, supposing the force to vary as — , how far a body must
fall both within and without the Circle to acquire the Velocity with which a
body revolves in a Circle.
3. Suppose a body to be turned (sic) upwards with the Velocity with which
it revolves in an Ellipse, how high will it ascend ? The same is asked supposing
it to move in a parabola.
4. Suppose a force varying first as ^-^ , secondly in a greater ratio than .=-^
but less than y-g , and thirdly in a less ratio than ■=-, , in each of these Cases to
determine whether at all, and where the body parting from the higher Apsid
will come to the lower.
5. To determine in what situation of the moon's Apsid they go most
forwards, and in what situation of her Nodes the Nodes go most backwards,
and why ?
• The Challis Manuscripts, iii, 61. There are two copies almost identical,
one dated 1785, the other 1786. Probably the paper printed in the text was set
in 178d.
262 THE MATHEMATICAL TRIPOS [CH. XI
6. In the cubic equation x^ + qx + r = which wants the second term ; sup-
posing x = a + b and Sab= - q^ to determine the value of x. {sic.)
7. To find the fluxion of o:^ x (y** + 2"^)a .
•
8. To find the fluent of -^ .
a+x
9. To find the fluxion of the vi^^ power of the Logarithm of x,
10. Of right-angled Triangles containing a given Area to find that whereof
the sum of the two legs AB + BC shall be the least possible. [This and the two
following questions are illustrated by diagrams. The angle at B is the right
angle.]
11. To find the Surface of the Cone ABC. [The cone is a right one on
a circular base.]
12. To rectify the arc DB of the semicircle DBV.
In cases of equality in the Senate-House Examination the
acts were still taken into account in settling the tripos order :
and in 1786 when the second, third, and fourth wranglers came
out equal in the examination a memorandum was published
that the second place was given to that candidate who dialectis
magis est versatvs, and the third place to that one who in scholis
sophistarum melius disputavit.
There seem to have been considerable intervals in the
examination by the moderators, and the examinations by the
extraneous examiners took place in these intervals. Those
candidates who at any time were not being examined occupied
themselves with amusements, provided they were not too
boisterous and obvious: probably dice and cards played a
large part in them. Gunning in an amusing account of his
examination in 1788 talks of games with a teetotum* in which
he took part on the Wednesday (when Locke and Paley formed
the subjects of examination), but " which was carried on with
great spirit... by considerable numbers during the whole of the
examination."
About this time, 1790, the custom of printing the problem
papers was introduced, but until 1828 the other papers con-
tinued to be dictated. Since then all the papers have been
printed.
* H. Giinning, Reminiscences, second edition, London, 1855, vol. r, p. 82.
CH. Xl] THE MATHEMATICAL TRIPOS 26
•">
I insert here the following letter* from William Gooch, of
Caius, in which he describes his examination in the Senate-
House in 1791. It must be remembered that it is the letter of
an undergraduate addressed to his father and mother, and was
not intended either for preservation or publication: a fact which
certainly does not detract from its value.
Monday | aft. 12.
We have been examin'd this Morning in pure Mathematics & I've hitherto
kept just about even with Peacock which is much more than I expected. We
are going at 1 o'clock to be examin'd till 3 in Philosophy.
From 1 till 7 I did more than Peacock ; But who did most at Moderator's
Kooms this Evening from 7 till 9, 1 don't know yet ;— but I did above three times
as much as the Sen' Wrangler last year, yet I'm afraid not so much as Peacock.
Between One & three o'Clock I wrote up 9 sheets of Scribbling Paper so you
may suppose I was pretty fully employ'd.
Tuesday Night.
I've been shamefully us'd by Lax to-day ; — Tho' his anxiety for Peacock
mu?t (of course) be very great, I never suspected that his Partially {sic) w"^ get
the better of his Justice. I had entertain'd too high an opinion of him to
suppose it. — he gave Peacock a long private Examination & then came to me
{I hop'd) on the same subject, but 'twas only to Bully me as much as he could,
— whatever I said (tho' right) he tried to convert into Nonsense by seeming to
misunderstand me. However I don't entirely dispair of being first, tho' you see
Lax seems determin'd that I shall not. — I had no Idea (before I went into the
Senate-House) of being able to contend at all with Peacock.
Wednesday evening.
Peacock & I are still in perfect Equilibrio & the Examiners themselves can
give no guess yet who is likely to be first ; — a New Examiner (Wood of St. John's,
who is reckon'd the first Mathematician in the University, for Waring doesn't
reside) was call'd solely to examine Peacock & me only. — but by this new Plan
nothing is yet determin'd. — So Wood is to examine us again to-morrow morning.
Thursday evening.
Peacock is declar'd first & I second, — Smith of this Coll. is either 8'^ or 9'^
& Lucas is either 10'^ or ll*'^. — Poor Quiz Carver is one of the ol ttoWoI; — I'm
perfectly satisfied that the Senior Wranglership is Peacock's due, but certainly
not 80 very indisputably as Lax pleases to represent it — I understand that he
asserts 'twas 5 to 4 in Peacock's favor. Now Peacock & I have explain'd to
each other how we went on, & can prove indisputably that it wasn't 20 to 19 in
his favor ; — I cannot therefore be displeas'd for being plac'd second, tho' I'm
provov'd {sic) with Lax for his false report (so much beneath the Character of
a Gentleman.) —
N.B. it is my very particular Request that you dont mention Lax's be-
haviour to me to any one.
* C. Wordsworth, Scholae Academicae, Cambridge, 1877, pp. 322—323.
i>64 THE MATHEMATICAL TRIPOS [CH. XI
Such was the form ultimately taken by the Senate-House
Examination, a form which it substantially retained without
alteration for nearly half-a-century. It soon became the sole
test by which candidates were judged. The University was
not obliged to grant a degree to anyone who performed the
statutable exercises, and it was open to the University to refuse
to pass a supplicat for the B.A. degree unless the candidate had
presented himself for the Senate-House Examination. In 1790
James Blackburn, of Trinity, a questionist of exceptional abilities,
was informed that in spite of his good disputations he would
not be allowed a degree unless he also satisfied the examiners
in the tripos. He accordingly solved one " very hard problem,"
though in consequence of a dispute with the authorities he
refused to attempt any more*.
It will be recollected that the examination was now com-
pulsoiy on all candidates pursuing the normal course for the
B.A. degree. In 1791 the University laid down rules f for its
conduct, so ffir as it concerned poll-men, decreeing that those
who passed were to be classified in four divisions or classes, the
names in each class to be arranged alphabetically, but not to be
printed on the official tripos lists. The classes in the final lists
must be distinguished from the eight preliminary classes issued
before the commencement of the examination. The men in the
first six preliminary classes were expected to take honours ;
those in the seventh and eighth preliminary classes were primd
facie poll-men.
In 1799 the moderators announced J that for the future they
would require every candidate to show a competent knowledge
of the first book of Euclid, arithmetic, vuglar and decimal
fi-actions, simple and quadratic equations, and Locke and Paley.
Paley's works seem to be held in esteem by modern divines,
and his Evidences, though not his Philosophy, still remains
(1905) one of the subjects of the Previous Examination, but his
contemporaries thought less highly of his writings, or at any
* H. Gunning, Beminiscences, second edition, London, 1855, vol. i, p. 182.
t See Grace of April 8, 1791.
X Communicated by the moderators to fathers of Colleges on January 18,
1799, and agreed to by the latter.
CH. Xl] THE MATHEMATICAL TRIPOS 265
rate of his Philosophy. Thus Best is quoted by Wordsworth*
as saying of Paley's Philosophy, " The tutors of Cambridge no
doubt neutralize by their judicious remarks, when they read it
to their pupils, all that is pernicious in its principles": so also
Richard Watson, Bishop of Llandaff, in his anecdotal autobio-
graphy -|-, says, in describing the Senate-House Examination in
which Paley was senior wrangler, that Paley was afterwards
known to the world by many excellent productions, " though
there are some principles in his philosophy which I by
no means approve."
In 1800 the moderators extended to all men in the first four
preliminary classes the privilege of being allowed to attempt
the problem papers : hitherto this privilege had been confined
to candidates placed in the first two classes. Until 1828 the
problem papers were set in the evenings, and in the rooms of
the moderator.
The University Calendars date from 1796, and from 1802 to
1882 inclusive contain the printed tripos papers of the previous
January. The papers from 1801 to 1820 and from 1838 to 1849
inclusive were also published in separate volumes, which are to
be found in most public libraries. No problems were ever set
to the men in the seventh and eighth preliminary classes, which
contained the poll-men. None of the bookwork papers of this
time are now extant, but it is believed that they contained but
few riders. Many of the so-called problems were really pieces
of bookwork or easy riders: it must however be remembered that
the text-books then in circulation were inferior and incomplete
as compared with modern ones.
The Calendar of 1802 contains a diffuse account of the ex-
amination. It commences as follows :
On the Monday morning, a little before eight o'clock, the students, generally
about a hundred, enter the Senate-House, preceded by a master of arts, who on
this occasion is styled the father of the College to which he belongs. On two
pillars at the entrance of the Senate-House are hung the classes and a paper
denoting the hours of examination of those who are thought most competent to
contend for honours. Immediately after the University clock has struck eight,
* C. Wordsworth, Scholae Academicae, Cambridge, 1877, p. 123.
+ Anecdotes of the Life of Richard IVutson by Himself, London, 1817, p. 19.
266 THE MATHEMATICAL TRIPOS [CH. XI
the names are called over, and the absentees, being marked, are subject to
certain fines. The classes to be examined are called out, and proceed to their
appointed tables, where they find pens, ink, and paper provided in great
abundance. In this manner, with the utmost order and regularity, two-thirds
of the young men are set to work within less than five minutes after the clock
has struck eight. There are three chief tables, at which six examiners preside.
At the first, the senior moderator of the present year and the junior moderator
of the preceding year. At the second, the junior moderator of the present, and
the senior moderator of the preceding year. At the third, two moderators of
the year previous to the two last, or two examiners appointed by the Senate.
The two first tables are chiefly allotted to the six first classes ; the third, or
largest, to the ol iroWoi.
The young men hear the propositions or questions delivered by the ex-
aminers ; they instantly apply themselves ; demonstrate, prove, work out and
write down, fairly and legibly (otherwise their labour is of little avail) the
answers required. All is silence ; nothing heard save the voice of the examiners ;
or the gentle request of some one, who may wish a repetition of the enunciation.
It requires every person to use the utmost dispatch ; for as soon as ever the
examiners perceive anyone to have finished his paper and subscribed his name
to it another question is immediately given
The examiners are not seated, but keep moving round the tables, both to
judge how matters proceed and to deliver their questions at proper intervals.
The examination, which embraces arithmetic, algebra, fluxions, the doctrine of
infinitesimals and increments, geometry, trigonometry, mechanics, hydrostatics,
optics, and astronomy, in all their various gradations, is varied according to
circumstances : no one can anticipate a question, for in the course of five
minutes he may be dragged from Euclid to Newton, from the humble arithmetic
of Bonnycastle to the abstruse analytics of Waring. While this examination is
proceeding at the three tables between the hours of eight and nine, printed
problems are delivered to each person of the first and second classes ; these he
takes with him to any window he pleases, where there are pens, ink, and paper
prepared for his operations.
The examination began at eight. At nine o'clock the papers
had to be given up, and half-an-hour was allowed for breakfast.
At half-past nine the candidates came back, and were examined
in the way described above till eleven, when the Senate-House
was again cleared. An interval of two hours then took place.
At one o'clock all returned to be again examined. At three the
Senate-House was cleared for half-an-hour, and, on the return
of the candidates, the examination was continued till five. At
seven in the evening the first four classes went to the senior
moderator's rooms to solve problems. They were finally dis-
missed for the day at nine, after eight hours of examination.
CH. Xl] THE MATHEMATICAL TRIPOS 267
The work of Tuesday was similar to that of Monday: Wednesday
was partly devoted to logic and moral philosophy. At eight
o'clock on Thursday morning a first list was published with all
candidates of about equal merits bracketed. Until nine o'clock
a candidate had the right to challenge anyone above him to an
examination to see which w^as the better. At nine a second
list came out, and a candidate's right of challenge was then
confined to the bracket immediately above his own. If he
proved himself the equal of the man so challenged his name
was transferred to the upper bracket. To challenge and then
to fail to substantiate the claim to removal to a higher bracket
w^as considered rather ridiculous. Revised lists were published
at 11 a.m., 3 p.m., and 5 p.m., according to the results of the
examination during that day. At five the whole examination
ended. The proctors, moderators, and examiners then retired
to a room under the Public Library to prepare the list of
honours, which was sometimes settled without much difficulty
in a few^ hours, but sometimes not before 2 a.m. or 3 a.m. the
next morning. The name of the senior wrangler was generally
announced at midnight, and the rest of the list the next
morning. In 1802 there were eighty-six candidates for honours,
and they were divided into fifteen brackets, the first and second
brackets containing each one name only, and the third bracket
four names.
It is clear from the above account that the competition
fostered by the examination had developed so much as to
threaten to impair its usefulness as guiding the studies of the
men. On the other hand, there can be no doubt that the
carefully devised arrangements for obtaining an accurate order
of merit stimulated the best men to throw all their energies
into the work for the examination. It is easy to point out
the double-edged result of a strict order of merit. The
problem before the University was to retain its advantages
while checking any abuses to which it might lead.
It was the privilege of the moderators to entertain the
proctors and some of the leading resident mathematicians the
night before the issue of the final list, and to communicate that
268 THE MATHEMATICAL TRIPOS [CH. XI
list in confidence to their guests. This pleasant custom survived
till 1884. I revived the practice in 1890 when acting as senior
moderator, but it seems to have now ceased.
In 1806 Sir Frederick Pollock was senior wrangler, and in
1869 in answer to an appeal from De Morgan for an account of
the mathematical study of men at the beginning of the century-
he wrote a letter* which is sufficiently interesting to bear
reproduction :
I shcall write in answer to your inquiry, all about my books, my study, and
my degree, and leave you to settle all about the proprieties which my letter
may give rise to, as to egotism, modesty, &c. The only books I read the first
year were Wood's Algebra (as far as quadratic equations), Bonnycastle's ditto,
and Euclid (Simpson's). In the second year I read Wood (beyond quadratic
equations), and Wood and Vince, for what they called the branches. In the
third'year I read the Jesidfs Newton and Vince's Fluxions ; these were all the
books, but there were certain mss. floating about which I copied — which be-
longed to Dealtry, second wrangler in Kempthorne's year. I have no doubt
that I had read less and seen fewer books than any senior wrangler of about
my time, or any period since ; but what I knew I knew thoroughly, and it was
completely at my fingers' ends, I consider that I was the last geometrical and
fiuxional senior wrangler ; I was not up to the differential calculus, and never
acquired it. I went up to college with a knowledge of Euclid and algebra to
quadratic equations, nothing more ; and I never read any second year's lore
during my first year, nor any third year's lore during my second ; ray forte was,
that what I did know I could produce at any moment with perfect accuracy. I
could repeat the first book of Euclid word by word and letter by letter. During
my first year I was not a ' reading ' man (so called) ; I had no expectation of
honours or a fellowship, and I attended all the lectures on all subjects —
Harwood's anatomical, Woollaston's chemical, and Parish's mechanical lectures
— but the examination at the end of the first year revealed to me my powers.
I was not only in the first class, but it was generally understood I was first in
the first class ; neither I nor any one for me expected I should get in at all.
Now, as I had taken no pains to prepare (taking, however, marvellous pains
while the examination was going on), I knew better than any one else the value
of my examination qualities (great rapidity and perfect accuracy) ; and I said to
myself, 'If you're not an ass, you'll be senior wrangler' ; and I took to 'reading'
accordingly. A curious circumstance occurred when the Brackets came out in
the Senate-house declaring the result of the examination : I saw at the top the
name of Walter bracketed alone (as he was) ; in the bracket below were Fiott,
Hustler, Jephson. I looked down and could not find my own name till I got to
Bolland, when my pride took fire, and I said, ' I must have beaten that man, so
I will look up again ' ; and on looking up carefully I found the nail had been
passed through my name, and I was at the top bracketed alone, even above
* Memoir of A. de Morgan, London, 1882, pp. 387—392.
CH. Xl] THE MATHEMATICAL TRIPOS 269
Walter. You may judge what mj feelings were at this discovery; it is the only
instance of two such brackets, and it made my fortune — that is, made me
independent, and gave me an immense coDege reputation. It was said I was
more than half of the examination before any one else. The two moderators
were Hornbuclde, of St John's, and Brown (Saint Brown), of Trinity. The
Johnian congratulated me. I said perhaps I might be challenged; he said,
•Well, if you are you're quite safe — you may sit down and do nothing, and no
one would get up to you in a whole day.'
Latterly the Cambridge examinations seem to turn upon very different
matters from what prevailed in my time. I think a Cambridge education has
for its object to make good members of society — not to extend science and make
profound mathematicians. The tripos questions in the Senate-house ought not
to go beyond certain limits, and geometry ought to be cultivated and encouraged
much more than it is.
To this De Morgan replied :
Your letter suggests much, because it gives possibility of answer. The
branches of algebra of course mainly refer to the second part of Wood, now
called the theory of equations. Waring was his guide. Turner — whom you
must remember as head of Pembroke, senior wrangler of 1767 — told a young
man in the hearing of my informant to be sure and attend to quadratic equations.
• It was a quadratic,' said he, ' made me senior wrangler.' It seems to me that
the Cambridge revivers were [Woodhouse,] Waring, Paley, Vince, Milner.
You had Dealtry's mss. He afterwards published a very good book on
fluxions. He merged his mathematical fame in that of a Claphamite Christian.
It is something to know that the tutor's ms. was in vogue in 1800-1806.
Now — how did you get your conic sections? How much of Newton did you
read? From Newton direct, or from tutor's manuscript?
Surely Fiott was our old friend Dr Lee. I missed being a pupil of Hustler
by a few weeks. He retired just before I went up in February 1823. The echo
of Hornbuckle's answer to you about the challenge has lighted on Whewell,
who, it is said, wanted to challenge Jacob, and was answered that he could not
beat [him] if he were to write the whole day and the other wrote nothing. I do
not believe that Whewell would have listened to any such dissuasion.
I doubt your being the last fluxional senior wrangler. So far as I know,
Gipps, Langdale, Alderson, Dicey, Neale, may contest this point with you.
The answer of Sir Frederick Pollock to these questions is
dated August 7, 1869, and is as follows :
You have put together as revivers five very different men. Woodhonse was
better than Waring, who could not prove Wilson's (Judge of C. P.) guess about
the property of prime numbers; but Woodhouse (I think) did prove it, and
a beautiful proof it is. Vince was a bungler, and I think utterly insensible of
mathematical beauty.
Now for your questions. I did not get my conic sections from Vince. I
copied a ms. of Dealtry. I fell in love with the cone and its sections, and
270 THE MATHEMATICAL TRIPOS [CH. XI
everything about it. I have never forsal<en my favourite pursuit ; I delighted
in such problems as two spheres touching each other and also the inside of
a hollow cone, &c. As to Newton, I read a good deal (men now read nothing),
but I read much of the notes. I detected a blunder which nobody seemed to be
aware of. Tavel, tutor of Trinity, was not ; and he argued very favourably of
me in consequence. The application of the Principia I got from mss. The
blunder was this: in calculating the resistance of a globe at the end of a
cylinder oscillating in a resisting medium they had forgotten to notice that
there is a difference between the resistance to a globe and a circle of the same
diameter.
The story of Whewell and Jacob cannot be true. Whewell was a very, very
considerable man, I think not a great man. I have no doubt Jacob beat him
in accui-acy, but the supposed answer cannot be true; it is a mere echo of what
actually passed between me and Hornbuckle on the day the Tripos came out —
for the truth of which I vouch. I think the examiners are taking too practical
a turn ; it is a waste of time to calculate actually a longitude by the help of
logarithmic tables and lunar observations. It would be a fault not to know
how, but a greater to be handy at it.
A few minor changes in the Senate-House Examination
were made in 1808*. A fifth day was added to the exami-
nation. Of the five days thus given up to it three were devoted
to mathematics, one to logic, philosophy, and religion, and one
to the arrangement of the brackets. Apart from the evening
paper the examination on each of the first three days lasted six
hours. Of these eighteen hours, eleven were assigned to book-
work and seven to problems. The problem papers were set
from 6 to 10 in the evening.
A letter fi'om Whewell dated January 19, 1816, describes
his examination in the Senate-House f.
Jacob. Whewell. Such is the order in which we are fixed after a week's
examination... I had before been given to understand that a great deal depended
upon being able to write the greatest possible quantity in the smallest time,
but of the rapidity which was actually necessary I had formed the most distant
idea. I am upon no occasion a quick writer, and upon subjects where I could
not go on without sometimes thinking a little I soon found myself considerably
behind. I was therefore surprised, and even astonished, to find myself
bracketed off, as it is called, in the second place ; that is, on the day when a
new division of the classes is made for the purpose of having a closer examina-
tion of the respective merits of men who come pretty near to each other, I was
not classed with anybody, but placed alone in the second bracket. The man
* See Graces, December 15, 1808.
t S. Douglas, Life of W. Whewell, London, 1881, p. 20.
CH. Xr] THE MATHEMATICAL TRIPOS 27 1
who is at the head of the list is of Cains College, and was always expected to be
very high, though I do not know that anybody expected to see him so decidedly
superior as to be bracketed off by himself.
The tendency to cultivate mechanical rapidity was a grave evil,
and lasted long after Whewell's time. According to rumour the
highest honours in 1845 were obtained by assiduous practice in
writing*.
The devotion of the Cambridge school to geometrical and
fluxional methods had led to its isolation from contemporary
continental mathematicians. Early in the nineteenth century
the evil consequence of this began to be recognized ; and it
was felt to be little less than a scandal that the researches
of Lagrange, Laplace, and Legendre were unknown to many
Cambridge mathematicians save by repute. An attempt to
explain the notation and methods of the calculus as used on
the Continent was made by R. Woodhouse, who stands out as
the apostle of the new movement. It is doubtful if he could
have brought analytical methods into vogue by himself; but his
views were enthusiastically adopted by three students, Peacock,
Babbage, and Herschel, who succeeded in carrying out the re-
forms he had suggested. They created an Analytical Society
which Babbage explained Avas formed to advocate "the principles
of pure d-ism as opposed to the dot-Sige of the University." The
character of the instruction in mathematics at the University
has at all times largely depended on the text-books then in use,
and the importance of good books of this class was emphasized
by a traditional rule that questions should not be set on a new
subject in the tripos unless it had been discussed in some treatise
suitable and available for Cambridge students *!■. Hence the
importance attached to the publication of the work on analytical
trigonometry by Woodhouse in 1809, and of the works on the
differential calculus issued by members of the Analytical Society
in 1816 and 1820.
In 1817 Peacock, who was moderator, introduced the symbols
for differentiation into the papers set in the Senate- House
* For a contemporary account of this see C. A. Bristed, Five Years in an
English Univenity, New York, 1852, pp. 233—239.
t See ex, gr. the Grace of November 14, 1827, referred to below.
272 THE MATHEMATICAL TRIPOS [CH. XI
Examination. But his colleague continued to use the fluxional
notation. Peacock himself wrote on March 17 of 1817 {i.e.
shortly after the examination) on the subject as follows*:
I assure you... that I shall never cease to exert myself to the utmost in the
cause of reform, and that I will never decline any office which may increase my
power to effect it. I am nearly certain of being nominated to the office of
Moderator in the year 1818-19, and as I am an examiner in virtue of my office,
for the next year I shall pursue a course even more decided than hitherto, since
I shall feel that men have been prepared for the change, and will then be
enabled to have acquired a better system by the publication of improved
elementary books. I have considerable influence as a lecturer, and I will not
neglect it. It is by silent perseverance only that we can hope to reduce the
many-headed monster of prejudice, and make the University answer her char-
acter as the loving mother of good learning and science.
In 1818 all candidates for honours, that is, all men in the
first six preliminary classes, were allowed to attempt the
problems : this change was made by the moderators.
In 1819 Peacock, who was again moderator, induced his
colleague to adopt the new notation. It was employed in the
next year by Whewell, and in the following year by Peacock
again. Henceforth the calculus in its modern language and
analytical methods were freely used, new subjects were in-
troduced, and for many years the examination provided a
mathematical training fairly abreast of the times.
By this time the disputations had ceased to have any im-
mediate effect on a man's place in the tripos. Thus Whewell *[*?
writing about his duties as moderator in 1820, said :
You would get very exaggerated ideas of the importance attached to it [an
Act] if you were to trust Cumberland ; I believe it was formerly more thought
of than it is now. It does not, at least immediately, produce any effect on a
man's place in the tripos, and is therefore considerably less attended to than
used to be the case, and in most years is not very interesting after the five or
six best men : so that I look for a considerable exercise of, or rather demand
for, patience on my part. The other part of my duty in the Senate House
consists in manufacturing wranglers, senior optimes, etc. and is, while it lasts,
very laborious.
Of the examination itself in this year he wrote as follows J :
* Proceedings of the Royal Society, London, 1859, vol. ix, pp. 538—539.
t WhewelVs Writings and Correspondence, ed. Todhunter, London, 1876,
vol. II, p. 36.
J S. Douglas, Life of Whewell, London, 1881, p. 56.
CH. Xl] THE MATHEMATICAL TRIPOS 27S
The examination in the Senate House begins to-morrow, and is rather close
work while it lasts. We are employed from seven in the morning till five in
the evening in giving out questions and receiving written answers to them;
and when that is over, we have to read over all the papers which we have
received in the course of the day, to determine who have done best, which is
a business that in numerous years has often kept the examiners up the half of
3very night ; but this year is not particularly numerous. In addition to all
this, the examination is conducted in a building which happens to be a very
beautiful one, with a marble floor and a highly ornamented ceiling; and as it
is on the model of a Grecian temple, and as temples had no chimneys, and as
a stove or a fire of any kind might disfigure the building, we are obliged to take
the weather as it happens to be, and when it is cold we have the full benefit of
it — which is likely to be the case this year. However, it is only a few days,
and we have done with it.
A sketch of the examination in the previous year from the
point of view of an examinee was given by J. M. F. Wright*,
but there is nothing of special interest in it.
Sir George B. Airyf gave the following sketch of his recol-
lections of the reading and studies of undergj-aduates of his time
and of the tripos of 1823, in which he had been senior wrangler :
At length arrived the Monday morning on which the examination for the
B.A. degree was to begin.... "We were all marched in a body to the Senate-House
and placed in the hands of the Moderators. How the "candidates for honours"
were separated from the ol iroWoi I do not know, I presume that the Acts and
the Opponencies had something to do with it. The honour candidates were
divided into six groups: and of these Nos. 1 and 2 (united), Nos. 3 and 4
(united), and Nos. 5 and 6 (united), received the questions of one Moderator.
No. 1, Nos. 2 and 3 (united), Nos. 4 and 5 (united), and No. 6, received those of
the other Moderator. The Moderators were reversed on alternate days. There
were no printed question-papers : each examiner had his bound manuscript of
questions, and he read out his first question ; each of the examinees who
thought himself able proceeded to write out his answer, and then orally called
out "Done." The Moderator, as soon as he thought proper, proceeded with
another question. I think there was only one course of questions on each day
(terminating before 3 o'clock, for the Hall dinner). The examination continued
to Friday mid-day. On Saturday morning, about 8 o'clock, the list of honours
(manuscript) was nailed on the door of the Senate-House.
It must be remembered that for students pursuing the
normal course the Senate-House Examination still provided
the only avenue to a degree. That examination involved a
* Alma Mater, London, 1827, vol. ii, pp. 58—98.
t See Nature, vol. xxxv, Feb. 24, 1887, pp. 397—399. See also his Auto-
biography, Cambridge, 1896, chapter ii.
B. R. IS
274 THE MATHEMATICAL TRIPOS [CH. XI
loiowledge of the elements of moral philosophy and theology,
an acquaintance with the rules of formal logic, and the power
of reading and writing scholastic Latin, but mathematics was
the predominant subject, and this led to a certain one-sidedness
in education. The evil of this was generally recognized, and
in 1822 various reforms were introduced in the University
curriculum ; in particular the Previous Examination was estab-
lished for students in their second year, the subjects being
prescribed Greek and Latin works, a Gospel, and Paley's
Evidences. Set classical books were introduced in the final
examination of poll-men ; and another honour or tripos examin-
ation was established for classical students. These alterations
came into effect in 1824; and henceforth the Senate-House
Examination, so far as it related to mathematical students, was
known as the Mathematical Tripos.
In 1827 the scheme of examination in the Mathematical
Tripos was revised. By regulations* which came into opera-
tion in January, 1828, another day was added, so that the
examination extended over four days, exclusive of the day of
arranging the brackets ; the number of hours of examination
was twenty-three, of which seven were assigned to problems.
On the first two days all the candidates had the same questions
proposed to them, inclusive of the evening problems, and the
examination on those days excluded the higher and more
difficult parts of mathematics, in order, in the words of the
report, " that the candidates for honours may not be induced to
pursue the more abstruse and profound mathematics, to the
neglect of more elementary knowledge." Accordingly, only
such questions as could be solved without the aid of the
differential calculus were set on the first day, and those set on
the second day involved only its elementary applications. The
classes were reduced to four, determined as before by the
exercises in the schools. The regulations of 1827 definitely
prescribed that all the papers should be printed. They are
also noticeable as being the last which gave the examiners
power to ask viva voce questions, though such questions were
* See Grace, November 14, 1827.
CH. Xl] THE MATHEMATICAL TRIPOS '^ 275
restricted to " propositions contained in the mathematical works
commonly in use at the University, or examples and explanations
of such propositions." It was further recommended that no
paper should contain more questions than well-prepared students
could be expected to answer within the time allowed for it, but
that if any candidate, before the end of the time, had answered
all the questions in the paper, the examiners might propose
additional questions viva voce. The power of granting honorary
optime degrees now ceased ; it had already fallen into abeyance.
Henceforth the examination was conducted under definite
rules, and I no longer concern myself with the traditions of the
examination.
In the same year as these changes became effective the
examination for the poll degree was separated from the tripos
with different sets of papers and a different schedule of subjects*.
It was, however, still nominally considered as forming part of
the Senate-House Examination, and until 1858 those who
obtained a poll degree were arranged in four classes, described
as fourth, fifth, sixth, and seventh, as if in continuation of the
junior op times or third class of the tripos. The year 1828
therefore shows us the Senate-House Examination dividing into
two distinct parts ; one known as the Mathematical Tripos, the
other as the poll examination. Historically, the examination
known as "the General" represents the old Senate-House
Examination for the poll-men, but gradually it has been moved
to an earlier period in the normal course taken by the men.
In 1851 admission to the Classical Tripos f was allowed to others
than those who passed the Mathematical Tripos, and thus
provided a separate avenue to a degree. In 1852 another set
of examinations, at first called " the Professor's Examinations,"
and now somewhat modified and known as " the Specials," was
instituted for all poll-men to take before they could qualify for
a degree. In 1858 the fiction that the poll examinations were
part of the Senate- House Examination was abandoned, and
subsequently they have been treated as providing an independent
• See Grace, May 21, 1828, confirming a Eeport of Maixb 27, 1828.
t See Grace of October 31, 1849.
18—2
276 THE MATHEMATICAL TRIPOS [CH. XI
method of obtaining the degree : thus now the Mathematical
Tripos is the sole representative of the old Senate-House
Examination. Since 1858 numerous other ways of obtaining
the degree have been established, and it is now possible to get
it by showing proficiency in very special, or even technical
subjects.
Further changes in the Mathematical Tripos were introduced
in 1833*. The duration of the examination, before the issue
of the brackets, was extended to five days, and the number of
hours of examination on each day was fixed at five and a half.
Seven and a half hours were assigned to problems. The ex-
amination on the first day was confined to subjects that did
not require the differential calculus, and only the simplest
applications of the calculus were permitted on the second and
third days. During the first four days of the examination the
same papers were set to all the candidates alike, but on the
fifth day the examination was conducted according to classes.
No reference was made to viva voce questions, and the preliminary
classification of the brackets only survived in a permission to
re-examine candidates if it were found necessary. This per-
missive rule remained in force till 1848, but I believe that in
fact it was never used. In December, 1834, a few unimportant
details were amended.
Mr Earnshaw, the senior moderator in 1836, informed me
that he believed that the tripos of that year was the earliest
one in which all the papers were marked, and that in previous
years the examiners had partly relied on their impression of
the answers given.
New regulations came into force f in 1839. The examina-
tion now lasted for six days, and continued as before for five
hours and a half each day. Eight and a half hours were assigned
to problems. Throughout the whole examination the same
papers were set to all candidates, and no reference was made to
any preliminary classes. It was no doubt in accordance with
the spirit of these changes that the acts in the schools should
* See Grace of April 6, 1832.
+ See Grace of May 30, 1838.
CH. Xl] THE MATHEMATICAL TRIPOS 277
be abolished, but they were discontinned by the moderators of
1839 without the authority of the Senate. The examination
was for the future confined* to mathematics.
In the same year in which the new scheme came into force
a proposal to again reopen the subject was rejected (March 6).
The difficulty of bringing professorial lectures into relation
with the needs of students has more than once been before the
University. The desirability of it was emphasized by a Syndicate
in February, 1843, which recommended conferences at stated
intervals between the mathematical professors and examiners.
This report foreshadowed the creation of a Mathematical Board,
but it was rejected by the Senate on March 31.
A few years later the scheme of the examination was again
reconstructed by regulations f which came into effect in 1848.
The duration of the examination was extended to eight days.
The examination lasted in all forty-four and a half hours,
twelve of which were devoted to problems. The first three
days were assigned to specified elementary subjects; in the
papers set on these days riders were to be set as well as book-
work, but the methods of analytical geometry and the calculus
were excluded. After the first three days there was a short
interval, at the end of which the examiners issued a list of those
who had so acquitted themselves as to deserve mathematical
honours. Only those whose names were contained in this list
were admitted to the last five days of the examination, which
was devoted to the higher parts of mathematics. After the
conclusion of the examination the examiners, taking into account
the whole eight days, brought out the list arranged in order of
merit. No provision was made for any rearrangement of this
list corresponding to the examination of the brackets. The
arrangements of 1848 remained in torce till 1873.
* Under a badly-worded Grace passed on May 11, 1842, on the recommencla-
tion of a syndicate on theological studies, candidates for mathematical honours
were, after 1840, required to attend the poll examination on Paley's Moral
Philosophy, the New Testament and Ecclesiastical History. This had not been
the intention of the Senate, and on March 14, 1855, a Grace was passed making
this clear.
t See Grace of May 13, 1846, confirming a report of March 23, 1846.
278 THE MATHEMATICAL TRIPOS [CH. XI
In the same year as these regulations came into force, a
Board of Mathematical Studies (consisting of the mathematical
professors, and the moderators and examiners for the current
year and the two preceding years) was constituted* by the
Senate. From that time forward their minutes supply a per-
manent record of the changes gradually introduced into the
tripos. I do not allude to subsequent changes which only
concern unimportant details of the examination.
In May, 1849, the Board issued a report in which, after
giving a review of the past and existing state of the mathe-
matical studies in the University, they recommended that the
mathematical theories of electricity, magnetism, and heat should
not be admitted as subjects of examination. In the following
year they issued a second report, in which they recommended
the omission of elliptical integrals, Laplace's coefficients, capillary
attraction, and the figure of the earth considered as hetero-
geneous, as well as a definite limitation of the questions in lunar
and planetary theory. In making these recommendations the
Board were only giving expression to what had become the
practice in the examination.
I may, in passing, mention a curious attempt which w^as
made in 1853 and 1854 to assist candidates in judging of the
relative difficulty of the questions asked. This was effected by
giving to the candidates, at the same time as the examination
paper, a slip of paper on which the marks assigned for the
book-work and rider for each question were printed. I mention
the fact merely because these things are rapidly forgotten and
not because it is of any intrinsic value. I possess a complete
set of slips which came to me fi'om Dr Todhunter.
In 1856 there was an amusing difference of opinion between
the Vice-Chancellor and the moderators. The Vice-Chancellor
issued a notice to say that for the convenience of the University
he had directed the tripos lists to be published at 8.0 a.m. as
well as at 9.0 a.m., but when the University arrived at 8.0 the
moderators said that they should not read the list until 9.0.
Considerable changes in the scheme of examination were
* See Grace of October 31, 1818,
CH. Xl] THE MATHExMATICAL TKIPOS 279
introduced in 1873. On December 5, 1865, the Board had
recommended the addition of Laplace's coefficients and the
figure of the earth considered as heterogeneous as subjects of
the examination; the report does not seem to have been
brought before the Senate, but attention was called to the fact
that certain departments of mathematics and mathematical
physics found no place in the tripos schedules, and were
neglected by most students. Accordingly, a syndicate was
appointed on June 6, 1867, to consider the matter, and a scheme
drawn up by them was approved in 1868* and came into effect
in 1873. The new scheme of examination was framed on the
same lines as that of 1848. The subjects in the first three days
were left unchanged, but an extra day was added, devoted to
the elements of mathematical physics. The essence of the
modification was the greatly extended range of subjects intro-
duced into the schedule of subjects for the last five days, and
their arrangement in divisions, the marks awarded to the five
divisions being approximately those awarded to the three days
in proportion to 2, 1, 1, 1, 2/3 to 1 respectively. Under the
new regulations the number of examiners was increased from
four to five.
The assignment of marks to groups of subjects was made
under the impression that the best candidates would concentrate
their abilities on a selection of subjects from the various divi-
sions. But it was found that, unless the questions were made
extremely difficult, more marks could be obtained by reading
superficially all the subjects in the five divisions than by
attaining real proficiency in a few of the higher ones : while
the wide range of subjects rendered it practically impossible to
thoroughly cover all the ground in the time allowed. The
failure was so pronounced that in 1877 another syndicate was
appointed to consider the mathematical studies and examina-
tions of the University. They presented an elaborate scheme,
but on May 13, 1878, some of the most important parts of it
wtre rejected and their subsequent proposals, accepted on
* See Grace of June 2, 1868. It was carried by a majority of only five in a
house of 75.
280 THE MATHEMATICAL TRIPOS [CH. XI
November 21, 1878 (by 62 to 49), represented a compromise
which pleased few members of the Senate*.
Under the new scheme which came into force in 1882 the
tripos was divided into two portions : the first portion wns taken
at the end of the third year of residence, the range of subjects
being practically the same as in the regulations of 1848, and the
result brought out in the customary order of merit. The second
portion was held in the following January, and was open only to
those who had been wranglers in the preceding June. This
portion was confined to higher mathematics and appealed chiefly
to specialists. The result was brought out in three classes, each
arranged in alphabetical order. The moderators and examiners
conducted the whole examination without any extraneous aid.
In the next year or two further amendments were madef,
moving the second part to the June of the fourth year, throwing
it open to all men who had graduated in the tripos of the
previous June, and transferring the conduct of the examination
in Part II to four examiners nominated by the Board : this put
it largely under the control of the professors. The range of
subjects of Part II was also greatly extended, and candidates
were encouraged to select only a few of them. It was further
arranged that Part I might be taken at the end of a man's
second year of residence, though in that case it would not
qualify for a degree. A student who availed himself of this
leave could take Part II at the end either of his third or of his
fourth year as he pleased.
The general effect of these changes was to destroy the
homogeneity of the tripos. Objections to the new scheme
were soon raised. Especially, it was said — whether rightly
or wrongly — that Part I contained too many technical subjects
to serve as a general educational training for any save mathe-
maticians ; that the distinction of a high place in the historic
list produced on its results tended to prevent the best men
* See Graces of May 17, 1877; May 29, 1878; and November 21, 1878: and
the Camhridge University Reporter, April 2, May 14, June 4, October 29,
November 12, and November 26, 1878.
t See Graces of December 13, 188o; June 12, 1884; February 10, 1885;
October 29, 1885 ; and June 1, 1886,
CH. Xl] THE MATHEMATICAL TRIPOS 281
taking it in their second year, though by this time they had
read sufficiently to be able to do so ; and that Part II was so
constructed as to appeal only to professional mathematicians,
and that thus the higher branches of mathematics were neg-
lected by all save a few specialists.
Whatever value be attached to these opinions, the number
of students studying mathematics fell rapidly under the scheme
of 1886. In 1899 the Board proposed* further changes. These
seemed to some members of the Senate to be likely to still further
decrease the number of men who took up the subject as one of
general education. At any rate the two main proposals were re-
jected, February 15, 1900, by votes of 151 to 130 and 161 to 129.
A few years later f the Board brought forv\^ard another
scheme, proposing changes so sweeping as almost to destroy
the identity of the Tripos. Under this the examination in
Part II was abolished — a change on which all parties were
agreed. There was introduced an examination, called Part I,
confined to elementary mathematics, which could be taken as
early as the second term of residence, and for which in certain
cases of failure a student could present himself again, but this,
although an examination for honours, did not qualify for a
degree. In the new Part II, taken normally at the end of the
third year of residence and qualifying for a degree, candidates
were given some option in the subjects of their examination,
and order of merit was abolished. The first examination under
this scheme was held in 1908.
The curious origin of the term tripos has been repeatedly
told, and an account of it may fitly close this chapter. Formerly
there were three principal occasions on which questionists were
admitted to the title or degree of bachelor. The first of these
was the comitia priora, held on Ash-Wednesday, for the best
men in the year. The next was the comitia posteriora, which
was held a few weeks later, and at which any student who had
distinguished himself in the quadragesimal exercises subsequent
* See Reports dated November 7, 1S09, and January 20, 1900.
t See the Reports of the Special Board, Cavibridye UniverRity Reporter,
May 29 and November 20, 1906, and the Graces of Feb. 2, 1907. The voting
on the first Grace was 770 Placet and 641 Non-Placet.
282 THE MATHEMATICAL TlllPOS [CH. XI
to Ash-Wednesday had his seniority reserved to him. Lastly,
there was the comitia minora, for students who had in no special
way distinguished themselves. In the fifteenth century an im-
portant part in the ceremony on each of these occasions was
taken by a certain "ould bachilour," who sat upon a three-legged
stool or tripos before the proctors and tested the abilities of the
would-be graduates by arguing some question with the " eldest
son," who was selected from them as their representative.
To assist the latter in what was often an unequal contest his
" father," that is, the officer of his college who was to present
him for his degree, was allowed to come to his assistance.
Originally the ceremony was a serious one, and had a certain
religious character. It took place in Great St Mary's Church,
and marked the admission of the student to a position with new
responsibilities, while the season of Lent was chosen with a
view to bring this into prominence. The Puritan party objected
to the observance of such ecclesiastical ceremonies, and in the
course of the sixteenth century they introduced much license
and buffoonery into the proceedings. The part played by the
questionist became purely formal. A serious debate still some-
times took place between the father of the senior questionist
and a regent master who represented the University ; but the
discussion was prefaced by a speech by the bachelor, who came
to be called Mr Tripos just as we speak of a judge as the bench,
or of a rower as an oar. Ultimately public opinion permitted
Mr Tripos to say pretty much whafc he pleased, so long as it was
not dull and was scandalous. The speeches he delivered or the
verses he recited were generally preserved by the Registrary,
and were known as the tripos verses: originally they referred to
the subjects of the disputations then propounded. The earliest
copies now extant are those for 1575.
The University officials, to v/hom the personal criticisms in
which the tripos indulged were by no means pleasing, repeatedly
exhorted him to remember " while exercising his privilege of
humour, to be modest withal." In 1740, says Mr Mullinger*,
* J. B. Mullinger, The University of Cambridge^ Cambridge, vol. i, 1873,
pp. 175—170.
CH. Xl] THE MATHEMATICAL TRIPOS 283
"the authorities after condemning the excessive license of the
tripos announced that the comitia at Lent would in future be
conducted in the Senate-House ; and all members of the Uni-
versity, of whatever order or degree, were forbidden to assail or
mock the disputants with scurrilous jokes or unseemly witticisms.
About the year 1747-8, the moderators initiated the practice
of printing the honour lists on the back of the sheets containing
the tripos verses, and after the year 1755 this became the
invariable practice. By virtue of this purely arbitrary con-
nection these lists themselves became known as the tripos; and
eventually the examination itself, of which they represented the
results, also became known by the same designation."
The tripos ceased to deliver his speech about 1750, but the
issue of tripos verses continued for nearly 150 years longer.
During the latter part of this time they consisted of four sets
of verses, usually in Latin, but occasionally in Greek, in which
current topics in the University were treated lightly or seriously
as the writer thought fit. They were written for the proctors
and moderators by undergraduates or commencing bachelors,
w^ho were supposed each to receive a pair of white kid gloves
in recognition of their labours. Thus gradually the word tripos
changed its meaning " from a thing of w^ood to a man, from a
man to a speech, from a speech to sets of verses, from verses to
a sheet of coarse foolscap paper, from a paper to a list of names,
and from a list of names to a system of examination *."
In 1895 the proctors and moderators, without consulting
the Senate, sent in no verses, and thus, in spite of widespread
regret, an interesting custom of many centuries standing was
destroyed. No doubt it may be argued that the custom had
never been embodied in statute or ordinance, and thus w^as
not obligatory. Also it may be said that its continuance was
not of material benefit to anybody. I do not think that such
arguments are conclusive, and personally I regret the dis-
appearance of historic ties unless it can be shown that they
cause inconvenience, which of course in this case could not be
asserted.
* "Wordsworth, Scholae Academicae, Cambridge, 1877, p. 21.
284
CHAPTER XII.
THREE CLASSICAL GEOMETBICAL 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 triscction 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 graduated, 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, Vortrage iiher ausgeiocihlte Fragen der Elementargeometrie,
Leipzig, 1895. It is said that the earliest rigorous proof that the problems ■
were insoluble by Euclidean geometry was given by P. L. Wantzell in 1837. J
CH. XIl] THREE GEOMETRICAL PROBLEMS 285
we have to find a line of length x, 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 4^^ =Hx — 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 a^ -h y"^ -\- ax -\- hi/ + c = 0)
and straight lines (whose equations are of the form or^+^y -f 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.
TJie 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 sufferinsf from
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 Hiatorla Prohlematis cle Cubi BupUcatione by N. T. Reimer, Gottingen,
1798; and Historia Prohlematis Cubi DupUcandi 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 Philoponus ad Aristotelis Analytica Posteriora, bk. i, chap. vii.
286 THREE GEOMETRICAL PROBLEMS [CH. XII
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 Jater.
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, a; : 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 \/2 : 1, but it did
not therefore follow that they could not find the ratio by
* Archimedis Oj^era cum Eutocii Commentariis, ed. Torelli, Oxford, 17S2,
p. 144; ed. Heiberg, Leipzig, 1880-1, vol. iii, pp. 104 — 107.
CH. Xli] THREE GEOMETRICAL PROBLEMS 287
geometry: in fiict, 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 a?
and y, we have a : x — x : y = y : 2a, fi:om which it follows that
cc^ = 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 a 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 OP 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, ill, 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 Nicomedcs (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, 453.
t Proclus, ed. Friedlein, pp. 212—213.
X Archimcdis Opera, ed. Torelli, p. 14S ; ed. Heiberg, vol. iii, pp. 93 — 103.
288 THREE GEOMETRICAL PROBLEMS [CH. XII
show analytically that the construction is correct, take OA as
the axis of x, 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 cf) ; and for that of the cone
sin 6 cos (t>=2' These three surfaces cut in a point such that
sin^ = ^, and therefore (r sin Oy = 2a^. Hence the volume of
the cube whose side is r sin 6 is twice that of the cube w^hose
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 BC, parallel, and their hypothenuses, AC and
BD, at right angles, then if these hypothenuses cut in P, we
have PC :PB = FB : 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 y"^ — 2ax and x^ = ay, they
intersect in a point whose abscissa is given hj c(^ = 2aK It is
probable that this method was suggested by the form in which
Hippocrates had cast the problem : namely, to find x and y so
that a : X = X : y = y : 2a, whence we have x^ = 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
* ArcJdmedis Opera, ed. Torelli, p. 135 ; ed. Heiberg, vol. iii, pp. 66 — 71.
t IMd., ed. Torelli, pp. 141—143 ; ed. Heiberg, vol. iii, pp. 92—99.
CH. Xll] 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 xy — 2l\ These cut in
a point determined by x^ = 21^ and y^ = 4^^ Hence
I : X = X : y = y : 21.
The solution of Apollonius*, 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 ^jB in G. With G as centre describe
a circle cutting OA produced in a and cutting OB produced in
h, so that aDh shall be a straight line. If this circle can be so
described, it will follow that OA : Bb = Bb : 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 Apollonius 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 G and let A produced
cut it in D. Find a point E on BG so that if BE 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
E, 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. iii, 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—84,
90-93.
X Opera Varia, Leyden, 1724, pp. 393—396.
B. R, 19
290 THREE GEOMETRICAL PROBLEMS [CH. XII
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 AB to E so that BE = AB.
Through A draw a line AF parallel to OE. Through draw a
line DOCi^G, cutting the circumference in D and G, cutting AF
in F, and cutting BA produced in G, so that GF= GA. If this
line can be drawn then AB : GG =GG : GA = GA : GB.
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 |. 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 = ab and x^ + y^ = ay + bx cut in a point
(x, y) such that a : x = x : y=y : b.
One of the constructions proposed by Newton is as folio ws§.
Let GA be the greater of two given lines. Bisect GA in B.
With centre and radius GB describe a circle. Take a point
C on the circumference so that BG is equal to the other of the
two given lines. From draw GDE cutting A G produced in
D, and BG produced in E, so that the intercept DE = GB. Then
♦ Opera Mathematica, ed. Schooten, Leyden, 1646, prop, v, pp. 242—243.
t Geometria, bk. iii, ed. Schooten, Amsterdam, 1659, p. 91.
X Gregory of St Vincent, Opus Geometricum Quadraturae Circuli, Antwerp,
1647, bk. VI, prop. 138, p. 602.
§ Arithmetica Universalis, Ralphson's (second) edition, 1728, p. 242; see
also pp. 2i3, 245.
CH. Xll] THREE GEOMETRICAL PROBLEMS 291
BC : OD = OD : CE = GE : OA. Hence OD and CE are two
mean proportionals between any two lines BC 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 FM perpendicular to
OA. Through P draw Pi2 parallel to 0^. On il/P 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 = ^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~^ (b/a). Construct the hyper-
bola xy — ab, and the circle (x — of + {y — hf = ^ (a^ -I- 6^). Of
the points where they cut, let x be the abscissa which is greatest,
then PR = x— a, and tan~^ (^/^) = i tan~^ (^/<^)-
The second construction is as follows. Let AOB be the
given angle. Take OB = OA, and with centre and radius OA
describe a circle. Produce AO indefinitely and take a point G
on it external to the circle so that if GB cuts the circumference
in D then GD 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 VIntermidiaire
des Mathematiciens, Paris, May and June, 1904.
t Pappus, Mathematicae CoUectiones, bk. iv, props. 32, '6'6 (cd. Commandino,
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. XII
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 CA'
to >S^ so that A'S=GA\ On AS describe a segment of a
circle to contain the given angle. Let the orthogonal bisector
oi 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 modern 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 f is equivalent to finding the points of intersection,
other than the origin, of the parabola y^ = ios and the circle
rjQ^^y'i — 13.x + ^ay = 0. The ordinates of these points are given
by the equation 4y = 3y — 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 follows |. 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 AS 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
PAS 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 00
cutting AB in L. Then AH — 2.HL. With focus A, vertex
if, and directrix OC, describe a hyperbola. Let the branch of
* Pappus, Mathematicae Collectiones^ bk. iv, prop. 34, pp. 99 — 104.
+ Geometria, bk. in, 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 GoiiicSf
by G. Taylor, Cambridge, 1881, No. 308, p. 126.
CH. XIl] THREE GEOMETR[CAL 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 nght-angled
* Traite des sections coniques, Paris, 1865, art. 37, p. 36.
+ See Montucla's Histoire des Recherches sur la Quadrature du Cercle,
edited by P. L. Lacroix, Paris, 1831 ; 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
Ajchimedes, Huygens, Lambert, and Legendre on the subject, with an intro-
duction on the history of the problem, Leipzig, 1892.
X Archimedit Opera, KvkKov fiirpricis, prop, i, ed. Torelli, pp. 203 — 205 ; ed.
Heiberg, vol. i, pp. 258—201, vol. iii, pp. 2G9— 277.
294 THEEE GEOMETRICAL PROBLEMS [CH. XII
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 Lindemann| 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
* Memoires de VAcademie de Berlin for 1761, Berlin, 17G8, pp. 265—
322.
t Legendre's Geometry, Brewster's translation, Edinburgh, 1824, pp. 239—
245.
X Ueber die Zahl tt, Mathematische Annalen, Leipzig, 1882, vol. xx, pp. 213 —
225. The proof leads to the conclusion' that, if re 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 iri 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— 4G2.
II Principia, bk. i, section vi, lemma xxviii.
CH. XIl] THREE GEOMETRICAL PROBLKMS 295
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 different
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 tt should not exist.
I have quoted elsewhere an anecdote, w^hich 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 ir does not afford the best analytical defini-
tion of TT, and is only one of its properties.
296 THREE GEOMETRICAL PROBLEMS [cH. Xll
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 perimeters^. 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 ir 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 Mathematica, Stockholm,
1889, vol. m, p. 28; Ibid., 1890, vol. iv, p. 22.
:;: The history of this method has been written by K. E. I. Selander, Uistorik
ajver Ludolphska Talet, Upsala, 1868.
§ For the methods used in classical times and the results obtained, see the
notices of their authors in M. Canter'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, 1858, 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. ii, pp. 119—128; and Ihid., 1874, vol. in,
pp. 27—46.
CH. XIl] THREE GEOMETRICAL PROBLEMS 297
The ancient Egyptians* took 256/81 as the value of tt, this
is equal to 3'1605... ; 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, Ilippias,
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 3f and greater
than 3ff, that is, it lies between 3-1428... and 3*1408... . He
established II 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
d — 'TT/QQ: the values of sin^ and tan ^ were deduced by
Archimedes fi'om those of sin ^tt and tan ^tt by repeated
bisections of the angle. With a polygon of n sides this
* Ein mathematisches Handbiich der alten Aegypter {i.e. the Ebiiid papyrus),
by A. Eisenlohr, Leipzig, 1877, arts. 100—109, 117, 124.
t Oppert, Journal Asiatique, August, 1872, and October, 1874.
X 1 Kings, ch. 7, ver. 23 ; 2 Chronicles, ch. 4, vcr. 2.
§ These results can be deduced from Euc. iv, 15, and iv, 8: see also book xii,
prop. 16.
II Archimedis Opera, KiJkXou iJ.hpr)(xis, prop, iii, ed. Torelli, Oxford, 1792,
pp. 205 — 216; ed. Heiberg, Leipzig, 18S0, vol. i, pp. 263 — 271.
298 THREE GEOMETRICAL PROBLEMS [CH. XII
process gives a value of tt correct to at least the integral part
of (2 logri — 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.
Apollonius 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 + ^ + -g^gg = Sj^^ = 3*1416.
The Roman surveyors seem to have used 3, or sometimes 4,
for rough calculations. For closer approximations they often
employed 3^ 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-BhataH, circ. 530, gave 62832/20000, which is equal
to 31416. 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 2n sides, then
6^ = J — J (1 — a'^y\ From the side of an inscribed hexagon, he
found successively the sides of polygons of 12, 24, 48, 96, 192,
* Blensurae, ed. Hultscli, Berlin, 1864, p. 188.
t 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, arta.
26—28.
IF Legons de calcul d'Aryabhata, by L. Rodet in the Journal Asiatique, 1879,
series 7, vol. xiii, pp. 10, 21.
CH. XIl] THREE GEOMRTRICAL PROBLEMS 299
and 884 sides. The perimeter of the last is given as equal
to V9*8694, from which his result was obtained by approxi-
mation..
Brahmagupta*, circ. 650, gave VlO, which is equal to
81622.... 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^, Vim, V9^, V9^ respectively; and to
have assumed that 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 in-
certain wliether this was not given only as an approximate
value.
Among the Arabs the values 22/7, VIO, and 62832/20000
were given by Alkarismi§, circ. 830; and no doubt were derived
from Indian sources. He described the first as an approximate
value, the second as used by geometricians, and th3 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 Japanesejj 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.
X Ibid., p. 95.
§ The Algebra of Mohammed ben Musa, ed. by F. Eosen, 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. XII
Leonardo of Pisa*, in the thirteenth century, gave for tt the
value 1440/458^, which is equal to 3'1418.... In the fifteenth
century, Purbachf gave or quoted the value 62832/20000,
which is equal to 3'1416 ; Cusa believed that the accurate
value was } (V3 + \/6), which is equal to 31423...; and, in
1464, RegiomontanusJ: is said to have given a value equal to
314243.
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^ 1^ = 1 — cos ^. He also gave|| a result equivalent to the
formula
2 _ V2 V(2 + V2) V{2 + V(2 + V2)}
7r~ 2 2 2
The father of Adrian MetiusH, 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 tt 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 fche
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), Eome,
1862, p. 90.
t Appendix to the De Triangulis of Regiomontanus, Basle, 1541, p. 131.
J In his correspondence with Cardinal Nicholas de Cusa, De Quadratura
Girculi, 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, 1579, pp. 56, 66 : probably
this work was printed for private circulation only, it is very rare.
II Vietae Opera, ed. Schooten, Leyden, 16i6, p. 400.
^ 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. XIl] THREE GEOMETRICAL PROBLEMS 301
L. van Cenlen devoted no inconsidorable 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^ Jil. 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 i* in St Peter's Church, Le3^den. His posthumous
arithmetic J: 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 from 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 places 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 the circle
was intermediate between them; whereas Snell constructed
• Vanden Circkel, Delf, 1596, fol. 14, p. 1 ; or De Circulo, Leyden, 1619, p. 3.
t The inscription is quoted by Prof, de Haan in the Messenger of Mathematics,
1874, vol. Ill, p. 25.
X De Arithmetische en Geometrische Foudavienten, Leyden, 1615, p. 163; or
p. 144 of the Latin translation by W. Snell, published at Leyden in 1615 under
the title Fundamenta Arithmetica et Gcometrica. This was reissued, togetlier
with a Latin translation of the Vanden Circkel, in 1619, under the title De
Circulo; in which see pp. 3, 29 — 32, 92.
§ Cyclometricus, Leaden, 1621, p. 55.
302 THREE GEOMETRICAL PROBLEMS [CH. XII
from the sides of these polj^gons 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 J B), 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 ii 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 Grienberger J, 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 ...
Ji X.O.O.O.0.4.1 ...
and quoted a proposition given a few years earlier by Viscount
Brouncker to the effect that
7r_ P 32 5^
4~ ^2+ 2 + 2 + ...,
* It is quoted by Montucla, ed. 1831, p. 70.
t Vera Circuli et Hyperbolae Quadratura, prop. 29, quoted by Huj'gens,
Opera Varia, Leyden, 1724, p. 447.
X Elementa Trigonometrica, Home, 1630, end of preface.
§ De Gircula Magnitudine Inventa, 1654 ; Opera Varia, pp. 851 — 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 Infiiiitorum, Oxford, 1656, prop. 191. An analysis of the
investigation by Wallis was given by Cayley, Quarterly Journal of Mathematics^
1839, vol. xxiii, pp. 165—169.
CH. XIl] 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 ^ - 1 tan^ e-\-^ tan' (9 - ...,
the result being true only if 6 lies between — {tt and J tt.
The first mathematician to make use of Gregory's series
for obtaining an approximation to the value of tt 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 0=^7r in Gregory's series.
Machin§, earlier than 1706, gave the result to 100 places
(all correct). He calculated it by the formula
J TT = 4 tan~^ ^ — tan~^ ■^.
De Lagnyll, in 1719, gave the result to 127 places of
decimals (112 correct), calculating it by putting 6 = ^ir in
Gregor3^"'s series.
HuttonlF, in 1776, and Euler**, in 1779, suggested the use of
• See Euler's paper in the Novi Commentarii Academiae Scientiarum,
St Petersburg, 1763, vol. viii, pp. 157—168.
t See the letter to Collins, dated Feb. 15, 1671, printed in the Coinmercium
EpistoUcum, London, 1712, p. 25, and in the Macclesfield Collection, Corre-
fpondence of Scientific Men of the Seventeenth Century^ Oxford, 1841, vol. ii.
p. 216.
X 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 Synopsis Pahnarionim, London, 1706, p. 243; and Maseres,
Scriptores Logarithmici, London, 1796, vol. iii, pp. vii— ix, 155 — 164.
il Histoire de VAcad€mie for 1719, Paris, 1721, p. 144.
H Philosophical Transactions ^ 1776, vol. lxvi, pp. 476 — 492.
•* Nova Acta Academiae Scientiarum Petropolitanae for 1793, St Petersburg,
1798, vol. XI, pp. 133 — 149: the memoir was read in 1779.
304 THREE GEOMETRICAL PROBLEMS [CH. XII
the formula Jtt = tan~^ J + tan~^ i or Jtt = 5 tan"^ f + 2 tan~^ ^,
but neither carried the approximation as far as had been done
previously.
Vega, in 1789*, gave the value of tt to 143 places of
decimals (126 correct); and, in l794f, to 140 places (136
correct).
Towards the end of the eighteenth century Baron 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 coiTect).
In 1837, the result of a calculation of vr to 154 places of
decimals (152 correct) was published^.
In 1841 Rutherford§ calculated it to 208 places of decimals
(152 correct), using the formula J7r=4 tan~^ -J— tan~^ Y^^j^+tan"^ ■^.
In 1844 Dase || calculated it to 205 places of decimals (200
correct), using the formula ^ir = tan~^ J + tan~^ -J + tan~^ J.
In 1847 Clausen IF carried the approximation to 250 places
of decimals (248 correct), calculating it independently by the
formulae ^tt = 2 tan~^ J + tan'^ f and ^tt = 4 tan"^ i - tan"^ gfg-
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 decimals JJ.
These were calculated from Machin's formula.
In 1853 Richter, presumably in ignorance of what had been
* Nova Acta Academiae Scientiarum Petropolitanae for 1790, St Petersburg,
1795, vol. IX, p. 41.
t Thesaurus Logarithmorum {logarithmisch-trigonometrischer Tafeln), Jjei^^zig,
1794, p. 633.
X J. F. Callet's Tables, etc., Precis Elenieataire, Paris, tirage, 1837. I have
not verified this reference.
§ Philosophical Transactions, 1841, p. 283.
II Crelle's Journal, 1844, vol. xxvii, p. 198.
H 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. XIl] THREE GEOMETRICAL PROBLEMS 805
done in England, found the value of tt to 333 places* of
decimals (330 correct) ; in 1854 he carried the approximation
to 400 places -f ; and in 1855 carried it to 500 placesj.
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:
TT^l 1 J_ K3 1
6~2"'"2"3.2=='^2.4'5.2^
an(
i = 22 tan- ^ + 2 tan- ^^ - 5 tan- j^^ - 10 tan- ^-j^ ;
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 ic.
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 it 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 I, 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
* GriinerVs Archiv, vol. xxi, p. 119.
t Ibid., vol. xxm, p. 476: the approximation given in vol. xxii, p. 473, is
correct only to 330 places.
X Ibid., vol. XXV, p. 472; and Elbinger Anzeigen, No. 85.
§ VIntermediaire des Mathtmaticiens, Paris, Dec. 1896, vol. in, p. 276.
B. R. 20
306 THREE GEOMETRICAL PROBLEMS [CH. XII
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 tt can be found. In 1855
Mr A. Smith* of Aberdeen made 3204 trials, and deduced
7r = 3*1553. 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 TT = 3-1419.
Other similar methods of approximating to the value of tt
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:^
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 QJir^ = 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 w by R. Chartres, Philosophical Magazine, London, series 6,
vol. XXXIX, March, 1904, p. 315.
307
CHAPTER XIII.
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 aftbrd 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, Ist 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, Flanim^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, Veher
die Entwicklang 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 the more plausible
eli'orts.
20—2
308 THE PARALLEL POSTULATE [CH. XIII
Earliest Proof. Thales. 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 ABB 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 C
and CAD is equal to one right angle. Hence the sum of the
angles B, G, BAD, and CAD, that is, the sum of the angles of
the original triangle ABC, 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 Jrom Thales to Euclid, Dublin, 1889,
chap. i.
CH. XIIl] THE PARALLEL POSTULATE 809
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 C, we get B = FDB, A=FDE, and C = EDC\ hence
A + B^-C = EDF -h FDB + EDG = it. But we can only prove
these relations on the assumption that when the paper is folded
over BF and ^F will lie along DF, and thus that BF= FA = FB,
and similarly that CE = 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. XIII
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 t, 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 Pe3Tard, 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
modern 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. XIIl] 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 BC. 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 ACB to the angle KAC. Hence the sum of the three
angles of the triangle ABC must be equal to the sum of the
angles HAK, KAC, and CAB, 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 Philosophie des Mathema-
tiques, Paris, 1903, pp. 81—84.
312 THE PARALLEL POSTULATE [CH. XIII
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. XIll] THE PARALLEL POSTULATE 313
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 Deinonstrations 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 GD cannot meet when produced, that is they are
parallel.
E
Conversely, if AB and CD be parallel, then ^^and CG are
not less parallel than FB and GD ; and therefore whatever be
the sum of the angles AFG and FGC, such also must be the
* Procius, ed. G. Friedleiu, Leipzig, 1873, pp. C62— 368.
314! THE PARALLEL POSTULATE [CH. XIII
sum of the angles FGD and BFG. But the sum of the four
angles is equal to four right angles, and therefore the sum of
the angles BFG and FGD must be equal to two right angles.
This proof is not valid. Apart fix)m 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 thouo:h the sum of the ansfles 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 catohes 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 demonstra-
tion oftered hy J. Wallis, Sa\*ilian Professor of Geometry, in a
lecture deiiveicd at Oxlord on July 11, 1663. The substance of
CH. XIIl] THE PARALLEL POSTULATE 315
his argijment 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 BAC, ACD is less than two right angles, then AB
and CD must (if produced) meet. One of the angles BAC, ACD
must be acute; suppose it is BAC. He first showed that, in
this case, fi*om any point B in AB vi^e can draw a line BE
which will cut J.C in ^, so that the sum of the angles BEC,
ECD is equal to two right angles; hence the angle BE A is
equal to the angle DC A. Then if we take the triangle BAE
(drawn on J.^" as base and \v4th B as vertex) and construct
a similar triangle on J. C as base, he proved that its vertex
must be at a finite distance from 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 pc'stulate if it be thought desirable.
Bertrand's Proof of the Postulate. The following is another
interesting demonstration. It was originally given by Bertrand
HA B K
of Geneva "*•. Supp'>5e AX and BY are two lines which meet
a third line HABK so that XAB + YBA < tt. It is required
to show that AX and BY must cut. For simplicity, I draw
the figure so that XAB ^ir '2 and therefore YBA <'ir.2,
* J. Wallis, Opera, Oxford, 1693, vol. n, pp. 674—678.
t I do not know where ox when it was firs: published. It was given by
9. F. Lacroix in his ElemenU de Gionntrie, Paris, 1802, p. 23.
r)
16 THE PARALLEL POSTULATE [CH. XIII
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 w/tt of the
space round B above UK, that is, oc/tt of the area of the
plane above HK. Also the area between AX and BZ is the
fraction ABjHK of the plane above HK. Now, however small
a may be, w/tt is a definite 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 5 F 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/airs 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 Ptolem3^'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. XIIl] THE PARALLEL POSTULATE 317
Personally I agree with those who consider Playfliir'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.
Play fair s 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
H
of a vector. Let ABC be any triangle. Suppose we have
a rod A L placed along AB with one end dX A. If we rotate
it clockwise round ^ as a pivot through the angle BAO it
will move from AL to AK. It will make no difference if we
now slide the rod along AK ^o that the end moves from A
* See A. Cayley, British Association Report, London, 1883, p. 9.
t 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 sum of the interior angles, but the method is the same as that
given above.
318 THE PARALLEL POSTULATE [CH. XIII
to G. If we now turn the rod, in the same direction as before,
round (7 as a pivot through the angle ACB it will move from
CK 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 ^ 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.
Legendres 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.
Legendres 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. XIIl]
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 (7,
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 EBG 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 ajR,
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 inJ&nite ; if
so, any term in the expression for A which involved ajR
320 THE PARALLEL POSTULATE [CH. XIII
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,
* Memoires de Vlnstitut 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. XIll] THE PARALLEL 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 bv 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
jB in AB s. line BY be 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
(Durer, 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. Dodgsou, Curiosa Mathematica, London, 1890, p. 35.
B. R. 21
322 THE PARALLEL POSTULATE [CH. XIII
in a circle is greater 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 Clavius, and to give a definition which involves
a disputed assumption is worse than fi^ankly 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.
f W. Killing, Grundlagen der Geome'rie^ Paderborn, 1898.
CH. XlTl] 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 pei'pendicular 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 smiilar 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 from his correspondence in 1829 and 1831,
but even before then he had shown 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 affect the treat-
ment of the problem by other writers.
* See R. Bonola, La Geometria Non-Eiiclidea, Bologna, 190G ; and D. M. Y.
Somerville, Bibliography of Non-Euclidean Geometry, St Andrews, 1911.
21—2
324 THE PARALLEL POSTULATE [CH. XIll
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 from 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
CH. XIIl] THE PAKALLEL POSTULATE 825
resulting geometry may 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
9-s 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.g. the distances of
the fixed stars) are immensely smaller than any unit natural
326 THE PARALLEL POSTULATE [CH. XIII
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 xix, some other problems
connected with different kinds of space.
azi
CHAPTER XIV.
INSOLUBILITY OF THE ALGEBRAIC QUINTIC.
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 quartic 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 effected. 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^ = \ and oc^—l 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
■^o equations of an order higher than four. Lagrange was the
earliest writer 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.
328 INSOLUBILITY OF THE ALGEBRAIC QUINTIG [CH. XIV
For instance, take the quadratic equation x"^ -{■ acc-\-h = 0.
It has two roots Xi and x.2. We can reduce the solution to
that of a simple equation because we can form a function of
^1 and X2 which has only one value, and this value can be deter-
mined. Such a value is y = (x^ - x^y, since (xi — x.^'^ = (x^ — Xif.
Moreover we have y = (xi — x^Y = (x^ + os^Y — 4}X^X2 — a^— 46.
Thus y is known. Hence the roots of the quadratic are given by
Xl "T X^ ^ Q/y
Xi X2 ^^ V2/*
Similarly, in the cubic equation x^ + ax^ ^hx + c==0, if
y = (^1 + ft>^2 + <w^^3)^ where co is one of the complex cube
roots of unity, then y is a function of the roots which has
only two values, namely (^1 + (0X2 + co^x^y, which may be also
written in the form (X2 + cox^ + (o'^x^y or (x^ + cox^ + co^x^y, and
(x2 + coxi + (o^Xsf, which may be also written in the form
(^3 + (0X2 + oy^x^y or (xi + (oXs + (o^Xzy. Let y and z denote these
two values, that is, y = (xi + (0X2 + (o^XsY, and z = (x2 •{• (oxi + co'^x.^y.
Then y-^z=-2ci'-^9ab-27c=A, say, and y2={a^-Sby=B, say.
Thus y and z are the roots of t^ — At + B — 0, an equation which
can be solved. And the roots of the cubic are determined by
X^ "T" X2 ~T~ X^ -— ~" Ctf
Xl + (0X2 + w^a^s = \/y,
WXi +X2+ CO'X^ = i^/z.
Again, in the quartic equation x* + ax^ + bx'^-\-cx-\-d~0, if
y = (xi — X2 + X3 — x^y, then 3/ is a function of the roots which has
only three values, namely, (xi— X2+ x^ — x^y or (a?2 — ^1+^4 — ^3?,
(xi — X3 + x^ — X2y or {xs — Xx-\- X2 — ^4)^ and {x^ — x^-\r X2 — x.^^
or (^4 — Xi-\- Xs — x^f. If these values be denoted by y, z, and w,
we have y-^z-\-\i — Sa^ — 86 = ^, say, yz-\- zu-{-uy= 3a* - 16a-6
+ 166" + 166c - 64(^ = -B, say, and yzu = (a^ - 4a6 + 86)- = G, say.
Hence y, ^r, u are the roots of the equation P — At^ ■\- Bt -^ C = 0.
And the roots of the quartic are determined by
Xy-^- X2-\- x^-\- x^ = — a,
a^i - ^2 + ^3 - ^4 = "^y*
Xi-X2 — x-i-\-x^ = V^,
Xi-]-X2 — X-^ — Xi= VUc
CH. XIV] INSOLUBILITY OF THE ALGEBRAIC QUINTIG 829
Lagrange showed that for an equation of the ??th 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 foiled
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 rci, Xr,, x^, ... Xn be the roots of the equation
f{x) = x^-\-ax"-^+...+k = 0.
Any one of these roots, ajj, is a function (which will generally
involve radicals) of the coefficients a, b, .... These coefficients
are symmetrical functions of the roots, namely, a = Xx^,
h = 'ZxiX2, .... If these values of a, h, ... 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.2 are the roots of the
quadratic equation a^-{-ax-{-b = 0, we have
o^i=h[-a + V{a2 - 46}]
= i [(x^ + x^) + V{(^i + ^2)' - ^J^i^^a}]
= i [(^1 + ^2) + (^1 - ^2)].
* See Liouville's Journal, Paris, 1846, vol. xi, pp. 417—433.
330 INSOLUBILITY OF THE ALGEBRAIC QUINTIC [CH. XIV
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 oc^ in terms 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 2?th
root of H, where we may without loss of generality take p as
prime. Of course H is rational: it involves a, 6, c, ..., and
therefore is a symmetrical function of x^, X2, .... Thepth root
of H also will be rational, but it will not be symmetrical : let us
denote it by (f){xi, x^, ... Xn). For instance, in the quadratic
equation H=a' — 4:h = {xi + x.,)"^ — 4^1^72, which is a rational and
symmetrical function of x^ and x^. But i\/H = <j) = Xi — x.2, which
is not symmetrical, though it is rational.
In the general case (j> is rational but not S}Tnmetrical. It
involves the two roots, Xi and X2, and therefore it must change
in value if they are interchanged. Further, since the values
of (j) are determined by ^^ = H, and H does not vary when the
roots are interchanged, one of the values of (f> must be deducible
from the other by multiplying it by co, where w is a pih root
(other than unity) of unity, that is.
For instance, in the quadratic equation x- + ax-hh = 0, we
observe that, if in </)(^i, Xo), that is, in iCi — a-o, we interchange
a?i and a^g, we necessarily get ^{x.2, x^ — — <\i{x^, x^ since the
two values of <^ are roots of (j>^ = H, where H is invariable.
Hence if one is + »JH, the other must be + ^/H
The relation thus reached,
(p \X2, Xif X2, . ..^ = 0)Cp \Xif X2, X^y . . .Jf
is an identity; hence if we interchange Xi and Xr,, we have
(j>(Xi, X2, Xs, ...) = 0X^(572, a^i, Xs, ...).
Hence &)*=!. And as co^l, we have a) = — 1. Since &)- = !,
we have p = 2: this shows that in the expression for a root of
CH. XIV] 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 concludes 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^, 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 x^ 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 K, 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 a^i,
at a radical, say the 5th root of /i, 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 9th root of -ST as a rational function of
the roots "^{xi.x^, ... a?„), and, from the nature of the case, "^
takes different values when three roots are permuted cyclically.
The values of y^ are roots of -v/r? = K. Accordingly, following
the same argument as that given above, we have
Y \X2, X^y Xij ... Xji) = CO 'Y' {^Xif X^f X-^, ... Xn)y
where o) is a 5th root (other than unity) of unity. If we
permute x^, x.2, x^ cyclically, we have
T \'^3 J '^1 > ^) ^ii . . . ) = O) Y' yXo , X^, Xiy X^, • '•)}
and -^ (xi, x^, x^, Xt, ...) = a)-v/r(a'3, x^, x^, x^, ...).
Hence co^ = 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 [CH. XIV
our analysis. The solution of the cubic a^ -\-bx + c = is
generally written as
x={- c/2 + sju]^ + {- c/2 - y/u\^,
where u = cV^ 4- h^/27. 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 (/> and similar
square roots, we arrive at a radical ^^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 co^ =1, where o) is a root
(other than unity) of w^ = 1. Hence q = S. Also if we permute
five roots cyclically, we obtain by a similar argument o)" = 1.
Thus q = 5. These equations for co 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 Xi 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 coefficients.
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.
333
CHAPTER XV.
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 Mersennes 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 xxviii, libri de Numeris, non esse omnes Perfectos,
quippe 20 sunt imperfect!, adeovt [adeunt?] solos octo perfectos habeat qui
sunt 6 regione tabulae Bungi, 1, 2, 3, 4, 8, 10, 12, et 29 : quique soli perfecti
sunt, vt qui Buugum habuerint, errori medicinam faciant.
Porr6 numeri perfecti adeo rari sunt, vt vndecim dumtaxat potueriut hac-
tenus 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. Vndecimua denique,
potestas 25^, minus 1, hoc est potestas 257, vnitate decurtata, multiplicata per
potestatem 256.
• Cogitata Physico-MatJiemaiica, Paris, 1644, Praefatio Generalis, article 19.
334 mersenne's numbers [ch. xv
Qui vndecim alios repererit, nouerit se analysim omnem, quae fuerit hacte-
nus, superasse: memineritque interea nullum esse perfectum a 17000 potestate
ad 32000; & nullum potestatum interuallum tantum assignari posse, quin detur
illud absque perfectis. Verbi gratia, si fuerit exponens 1050000, nullus erit
Humerus progressionis duplae vsque ad 2000000, qui perfectis numeris seruiat,
hoc est qui minor vnitate, primus existat.
Vnde clarum est quam rari sint perfecti numeri, <fc quam merito viris per-
fectis comparentur ; esseque vnam ex maximis totius Matheseos difficultatibus,
praescriptam numerorum perfectorum multitudiuum exhibere ; quemadmodum
<fc agnoscere num dati numeri 15, aut 20 caracteribus constantes, sint primi
necne, cum nequidem saeculum integrum huic examini, quocumque modo
hactenus cognito, sufi&ciat.
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 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 cor-
rections we have no reason to doubt the truth of the statement,
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 N 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 jo=23 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 N were
given by Euler. He also proved that N was prime when
p = 31. Reuschle gave the factors of N 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 p = 61,
Cunningham gave the factors when p = 71, 163, 173, and 197,
Cole the factors when p = 67, Woodall the factors when p = 181,
and Powers proved that iV was prime when p = 89. It has
been asserted that the prime character of N when p = 127 has
been established, but the proof has not been published or verified.
CH. xv] mersenne's numbers 335
Thus there are 16 values of p for which Mersenne's state-
ment still awaits verification. These are 101, 103, 107, 109,
127, 137, 139, 149, 157, 167, 193, 199, 227, 229, 241, 257.
For these values N is (according to Mersenne) prime when
j[) = 127 and 257, and is composite for the other values. If we
admit that the character of N is known when p=12l, the
number of cases yet to be verified is reduced to 15.
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 to his list) 43 of the 56 primes less than 258 make
N composite and the remaining 13 primes make N prime. In 29
out of the 43 cases in which N is said to be composite we know
its factors, and in 14 cases the statement is still unverified. In
11 out of the 13 cases in wdiich it is said that N is prime the
statement has been verified, and in 2 cases it is still unverified.
From the w^ording 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. Pie 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, wdth whom Mersenne was in constant corresjoondence,
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 N as are less than
a million can be verified easily: they have been known for
a lonoj time, and I need not allude to them in detail.
336
MERSENNES NUMBERS
[CH. XV
TABLE OF MERSENNE'S NUMBERS.
1
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47
53
59
61
87
71
73
79
83
89
97
101
103
107
109
113
127
131
137
139
149
151
157
163
167
173
179
181
191
193
197
199
211
223
227
229
233
239
241
251
357
Value of N=2r'-1
Character
Discoverer
1
prime
3
prime
7
prime
31
prime
127
prime
2047 = 23x89
composite
8191
prime
131071
prime
524287
prime
8388607 = 47x178481
composite
Fermat
536870911 = 233x1103x2089
composite
Euler
2147483647
prjiue
Euler
137438953471 = 223 x 616318177
composite
Fermat
2199023255551 = 13367 x 164511353
composite
Plana
8796093022207 = 431 x 9719 x 2099863
composite
Euler
2351 X 4513 X 13264529
composite
Eeuschle
6361 X 69431 x 20394401
composite
Landry
179951 X 3203431780337
composite
Landry
2305843009213693951
prime
Seelhoff
= 0(193707721)
composite
Cole
= (228479)
composite
Cunningham
= 0(439)
composite
Euler
= (2687)
composite
Le Lasseur
= 0(167)
composite
Euler
618970019G42690137449562111
prime
Powers
= (11447)
composite
Le Lasseur
2535301200456458802993406410751
*
«
10141204801825835211973625G43007
»
»
162259276829213363391578010288127
»
•
(549037107316853453.366312041152511
*
»
= (3391)
composite
Le Lasseur
170141183460469231731087303715884105727
♦
*
= (2G3)
composite
*
»
Euler
•
=o*('i8i2ij
♦
composite
*
composite
*
composite
»
Le Lasseur
*
=o*(iV6287)
Cunningham
*
s0(73d753)
Cunningham
= (359)
composite
Euler
= 0(43441)
composite
\Voodall
= 0(383)
composite
*
composite
*
composite
Euler
*
=0 (7487)
Cunningham
*
=0(15193)
Le Lasseur
= 0(18287)
composite
*
*
composite
Le Lasseur
»
= 0(1399)
Le Lasseur
= (479)
composite
■ti-
composite
Euler
«
= (503)
Euler
♦
»
CH. xv] meksenne's numbers 337
The factors of N when p = ll, 23, and 37 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
6tez 2 ou 8 ou 32 Ac, les nombres premiers moindres de I'uuite qu'un multiple
du quaternaire, qui mesureront le reste, feront I'effet requis. Comme de 25,
qui est un quarr6, otez 2; le reste 23 mesurera la 11* puissance ~1; otez 2 de
49, le reste 47 mesurera la 23« puissance - 1. Otez 2 de 225, le reste 223 mesu-
rera la 37° puissance - 1, &c.
Factors of N when p = 29, 43, and 73 were given by Eulerf
in 1732 or 1733. The fact that N is 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 8n + 7 are primes, then 2'^''+^ -1 = (mod. Sn + 7).
This was proved by Lagrange | in his classical memoir of 1775.
The proposition also covers the cases of ^ = 11 and p = 23.
This is the only general theorem on the subject which has been
yet established.
The fact that iV is prime when ^ = 31 w^as 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 7ip + 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 if ^ is a given number we can define n
much more closely, and Euler showed that, if p = 31, the prime
factors (if any) of N w^ere necessarily primes of the form 248?i + 1
or 248n + 63 ; also they must be less than ^JSf, that is, than
• Oeuvres de Fermat, Paris, vol. ii, 1894, p. 210; or Opera Blathematica,
Toulouse, 1679, p. 164; or Brassiune's Precis, Paris, 1853, p. 144.
f Commentarii Academiae Scientiarum Petropolitanae, 1738, vol. vi, p. 105;
or Commentationes Arithmeticae Collectae, vol. i, p. 2.
X Nouveaux Memoires de VAcademie des Sciences de Berlin, 1775, pp. 323 —
35G.
§ Histoire de VAcademie des Sciences for 1772, Berlin, 1774, p. 36. See also
Legendre, Thiorie des Nombres, third edition, Paris, 1830, vol. i, pp. 222 — 229.
II Novi Commentarii Academiae Scientiarum Petropolltanae, vol. i, p. 20; or
Commentationes Arithmeticae Collectae, St Petersburg, 1849, vol. i, pp. 55, 56.
B. s. 22
338 mersenne's numbers [ch. xv
46339. 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 iV when ^ = 41 were given by Plana f in 1859.
He showed that the prime factors (if any) are primes of the
form 328n + l or 328w + 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 w^as 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 iV, if
p is not greater than 101.
That N is prime when ^5 = 127 seems to have been verified
by Lucas § in 1876 and 1877. The demonstration has not been
published.
The discovery of factors of N for the values ^ = 53 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 N when p = 79 and 113 were given first by
Le Lasseur, and were quoted by Lucas in the same
memoir||.
A factor of N when p — 233 was discovered later by Le
Lasseur, and was quoted by Lucas in 1882 IF.
* C. G. Eeuschle, Neue Zahlentheoretische Tdbellen, Stuttgart, 1856, pp. 21,
22, 42—53.
+ G. A. A. Plana, Memorie della Eeale Accademia delle Scienze di Torino,
Series 2, vol. xx, 1863, p. 130.
X Ibid., p. 137.
§ Su/' la Theorie des Nomhres Premiers, Turin, 1876, p. 11; and Becherches
sur les Ouvrages de Leonard de Pise, Rome, 1877, p. 26, quoted by A. J. C.
Cunningham, Proceedings of the London mathematical Society, Nov. 14, 1895,
vol. XXVII, p. 54.
II American Journal of Mathematics, 1878, vol. i, pp. 234 — 238.
H Recreations, 1882-8, vol. i, p. 241.
CH. xv] mersenne's numbers 339
Factors of N when p = 97, 151, 211, and 223 were deter-
mined subsequently by Le Lasseur, and were quoted by Lucas*
in 1883.
That N is prime when ^ = G1 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 J, 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 fector of N
when p = 163; in 190911 that 228479 is a factor of N when
_p=7l; and in 1912** that 730753 is a factor of i\^ when J3=173.
The factors of iV when p=:7l were discussed independently by
Ramesam in 1912 ff.
That iV'is not prime when ^=67 seems to have been verified
by Lucas II in 1876 and 1877. The composite nature of N,
when p = 67, was confirmed by E. Fauquembergue§§, and was
also implied by Lucas in 1891. The fiactors were given by
Cole|l|linl903.
A factor of N when 2^ = 181 was discovered by Woodall in
1911M.
♦ Recreations, 1882-3, vol. n, p. 230.
t P. H. H. Seelhoff, ZeiUcUrift fur Mathematik und Physik, 1886, vol. xxxi,
p. 178.
X See Weber-Wellstein, Encyclopaedic der Elementar -Mathematik, p. 48;
and F. N. Cole, Bulletin of the American Mathematical Society, December,
1903, p. 136.
§ A. J. C, CunningTiam, Proceedings of the London Mathematical Society^
March 14, 1895, vol. xxvi, p. 261.
II Ibid., April 30, 1908, vol. vi (2ad series), p. xxii.
% L'Intennediaire des Matheniaticiens, Paris, 1909, vol. xvi, p. 252.
** Proceedings of the London Mathematical Society, April 11, 1912, vol. xi,
p. xxiv.
tt Nature, London, March 28, 1912, vol. lxxxix, p. 87.
XX Sur la Theorie des Nombres Premiers, Turin, 1876, p. 11, quoted bj A. J. C.
Cunningham, Proceedings of the London Mathematical Society, Nov. 14, 1895,
vol. xxvii, p. 54, and Recherches sur les Ouvrages de Leonard de Pise, Rome,
1877, p. 26.
§§ L' Intermediaire des Matheniaticiens, Paris, Sept. 1894, vol. i, p. 148.
nil F. N. Cole, "On the Factoring of Large Numbers," Bulletin of the
American Mathematical Society, December, 1903, pp. 134 — 137.
nil H. J. Woodall, Nature, London, July 20, 1911, vol. lxxxvii, p. 73.
22—2
340 meksenne's numbers [ch. xv
That N is prime when ^ = 89 was proved by Powers in
1911*
Bickmore in the memoir i* cited below showed that 5737
is another factor of iV if p = 239. Cunningham has also shown
that 55871 is another factor of iV if ^ = 151, and that 54217 is
another factor of iV^ if ^ = 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 iV 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, ... -m^, constructed
according to the law Un+i = tin — 2, then N is prime if the first
residue which is zero lies between the (p— l)/2th and the ^th
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^ — h\ that is, of
the form 8i ± 1, and 2 must be a quadratic residue of q.
The theory of residues is, however, of but little use in finding
* K. E. Powers, American Mathematical Monthly, November, 1911, vol. xvni,
pp. 195—197.
t C. E. Bickmore, Messenger of Mathematics, Cambridge, 1895, vol. xxv, p. 19.
:J: American Journal of Mathematics, 1878, vol. i, p. 316.
§ Legendre, Theorie des Nomhres, third edition, Paris, 1830, vol. i, § 143.
In the case of Mersenne's numbers, B = h=l.
CH. XV] 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 iV has been summed up
in the statement f that a prime factor of the form 2pt-^l 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 2ptt'-\-l
can be found indirectly by a method due to Bickmore when
^ = 1, 3, 4, 8, or 12, and t' 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-{-l 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 t is odd,
will divide N when ^=1, if p=ll, 23, 83, 131, 179, 191, 239^
or 251 ; when t = 3, if ^ = 37, 73, or 233 ; when t = 5,if p = 43 [
when t = 15, if ;? = 113; when ^ = 17, if p = 79; when ^=19'
ifp = 29, or 197; when t=25, if p = 47; when ^ = 41, if
p^22S; when t = 59,ifp = 97; when ^ = 163, if jt^ = 41 ; when
t = 461, if p = 163 ; when t = 1525, if p = 59 ; when t = 1609, if
i? = 71. Similarly for even values of t, a prime of this form will
divide N when « = 4, if ^ = 11, 29, 179, or 239; when i = 8,
if ^ = 11; when ^=12, if ;) = 239; when ^ = 36, if j9 = 29,'
or 211; when ^ = 60, if p = 53, or 151; when t=120, if
^ = 181; when j5 = 2112, if |)=173; and when i= 1445580 if
jo = 67.
Of the 29 cases in which we know that the statement of
the composite character of i\r 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-ic residue. Proceedings of the London Mathematical Society
1895-6, vol. xxvn, pp. 85-122; and on Haupt-exponents of 2, Quarterly Journal
0/ Pure and Applied Mathematics, Cambridge, 1906, vol. xxxvii, pp. 122—145.
t Transactions of the British Association for the Advaticemcnt of Science
(Ipswich Meeting), 1895, p. 614.
342
mersenne's numbers [ch. XV
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 m
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 (^5 = 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 {p = 71, 163, and 173) are due to Cunningham
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 2^^ + 1.
Of the 11 cases in which we know that the statement ol the
prime character of N is correct all save two (namely, when ^5=61
and p = 89) 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 m
the cases of 2^ = 41 due to Plana, of ^9 = 59 due to Landry, of
p = 61 due to Seelhoff, of p=67 due to Cole, of ;) = 71, 163,
and 173 due to Cunningham, of p = 181 due to Woodall, and of
;> = 89 due to Powers. In fact, save for these nine 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
still 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. XV] MERSENNES NUMBERS 343
diminish the number of possible factors to be tried, but it will
be observed that the values of iV 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 v/hen p = 41, we proceed thus. Take the power
of 2 nearest to q or to its square-root. We have, to the
modulus g,
2'^ =16384 = 3017 = 7x431,
,-. 2^^ =49 (-1377) = -638,
.-. 2=' =-319,
.-. 2"+^ = (3017) (-319) = !,
.-. 2" =1.
Second. We can proceed by reducing the problem to the
sdution of an indeterminate equation.
It is clear that we can obtain a factor of N if we can express
it is the difference of the squares, or more generally of the Tith
povers, of two integers ii and v. Further, if we can express
a nultiple of N, say mN, in this form, we can find a factor of
mJS and (with certain obvious limitations as to the value of ?/i)
ihit 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 a3ertain form, a certain continuant* defined by the form of
the continued fraction is a factor of N.
Snce N can always be expressed as the difference of two
squajes, this method seems a natural one by which to attack
the poblem. If we put
iV = n^ + a = (71 + by - (6^ + 2hn - a),
we cai make use of the known forms of u and v, and thus
* Set J. G. Birch in the Messenger of Mathematics ^ Cambridge, 1902, vol.
XXII, pp.52 — 55.
344 mersenne's numbers [ch. xv
obtain an indeterminate equation between two variables x and
y of the form
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 8j9s + 1, hence
= 2P-1
= 2(2P-i-l) + l,
.'. As^-t + Spst = {2P-^-l)lp
= (say) a + 8/;/3.
Hence 45 + i = a4-8p^, and st-^ — x,
where x'if- ^ and t is odd. These results again lead to an
indeterminate equation.
But, in both cases, unless p is small, the resulting equatiois
are intractable.
Third. A not uncommon method of attacking problens
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 tiat
it would have been employed by a mathematician of the
seventeenth century I here content myself with alludng
to it.
Fourth, There is yet another way in which the prollem
might be attacked. The problem will be solved if we can find
an odd prime q so that to it as modulus ^P^y = Zy and ^ = z,
Avhere y and z may have any values we like to choost. If
such values of q, y, and z can be found, we have 2^ (2^ — T) = 0.
Therefore 2^ = 1, that is, 5' is a divisor of N, \
* For a sketch of this see G. B. Mathev/s, Theory of Numbers, part '., Cam-
bridge, 1891, pp. 261—271. See also F. N. Cole's paper, " On the Fact»ring of
Large Numbers," Bulletin of the American Mathematical Society, Decembr, 1903,
pp. 134 — 137; and Quadratic Partitions by A. J. C. Cunningham, Londai, 1904.
CH. xv] mersenne's numbers 345
For example, to the modulus 23, we have
2« =3,
Also 2' =32.
Therefore 2^*^ - 2^ = 0,
.-. 21^-1 =0.
Without going further into the matter we may say that the
d priori 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 q, 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 2pt + 1, and
if the remainders were the same when x = u and (v = v, then to
the modulus q we should have 2^ = 2", and therefore 2"~" = 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 the problem by this
method. Cunningham's Canon gives the solution of the con-
gruence 2^ = R 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 i? or ^ 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
if p = 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 Fermat* had paid
attention to numbers of this form.
* For instance, see above, pp. 39, 40.
346 mersenne's numbers [ch. xv
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 ^p or may-be
8^ — 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 x-\-\ is a prime
then 2^ — 1 is divisible by ^ + 1. Hence if 2pt + 1 is a prime
we have, to the modulus 2pt -I- 1,
^~" 2*^« -1=0,
/. (2P - 1) (1 + 2^ + 2^^^ + . . . + 2(2«-i)p) = 0.
Hence, a divisor of 2^ — 1 will be kno^wn, if we can find a
value of t such that ^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 2^ + 1 is a prime, we have,
to the modulus 2p + 1,
(22>-l)(2^-|-l) = 0.
Hence 2^ = 1 if 2^ + 1 is prime to 2p + 1. This is the case
if ^ = 4w + 3. Hence 2p + 1 is a factor of iV if ^ = 11, 23,
83, 131, 179, 191, 239, and 251, for in these cases 2p-\-l
is prime.
The problem of Mersenne's Numbers is a particular case
of the determination of the factors of a^—1. This has been
* Comptcs Rendns de VAcademie des Scierices, Paris, 1877, vol. lxxxv,
pp. 950—952.
CH. xv] mersenne's 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 machines t 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. XXXIII, p. 49.
+ F. W. Lawrence, Messenger of Mathematics , Cambridge, 1894-5, vol. xxiv,
pp. 100 — 109 ; Quarterly Journal of Mathematics , Cambridge, 1896, vol. xxviii,
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 Rendus, Association
Franqais pour I' Avanccmerit des Sciences, 1891 (Marseilles), vol. xx, pp. 165 — 168.
348
CHAPTER XVI.
STRING FIGURES.
An amusement of considerable antiquity consists in tlie
production of figures, known as Cat'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. C.
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. XVl] STilING FIGUKES 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
with, 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 idnar; 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 \V. H. E. Rivers and A. C. Haddon, Man,
London, 1902, pp. 147, 143.
350 STRING FIGURES [CH. XVI
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 diagrams 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,
F 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
forminof 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. XVIJ
STKINQ FIGURES
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 Snuffer-Trays. From Snuffer- Trays we can obtain
forms known as a Pound- of- Candles, Cat's-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, pahn towards
pahn, 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. XVI
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 — is 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 beinor
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-Pond.
Figure ii. Sauffer -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.
Gafs-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.
STRING FIGUKES
353
Figure iii. Cafs-Eye.
Cafs-Eye can be got from Scuffer-Trays thus. I suppose tbat 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 Snuffer-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'a 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-Bish.
B. B.
23
354 STRING FIGURES [CH. XVI
Fish-in-a-Dish is produced from Cat's-Eye thus. I suppose that Cat's-Eye
is held by P in the norraal 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 Pound-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, Hfts 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-Cot 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. XVl] 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.
Lattice-Work can be produced from the Hammock (assumed to be held
^y Q) ^7 the same process as that described above for the production of
Snuffer-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 Snuffer-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, Double-Crowns,
Suspension-Bridge, Tridents, 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
booking 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 httle-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 throagh two right angles. Suspension-Bridge can be also
obtained from Double-Crowns.
Tridents is produced from Suspension-Briuge by the second player releasing
the left-hand thumb and index-tiuger, and then drawing his hands ai art.
23—2
356
STRING FIGURES
[CH. XVI
See-Saw is an arrangement of the string which each pla5'er 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 Saio 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 romid 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
SnuSer-Trays
Cat's-Cradle
1
See-Saw
Cat's-Eye
Fish-in-a-
Dish
(Numerous
Forms)
1
Donble-
Crowns
Pound-of-
Candles
Manger
Cat's-Eye
1
Trellis-Bridge
of them. The movements and forms described above will how-
ever illustrate the process sufiSciently.
CH. XVl] 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 the
right hand. The hands are now drawn apart. This is called the " first position."
358
STRING FIGURES
[CH. XVI
Next with the back of tho index of the right hand take up from the proximal
side {i.e. from below) the left palmar string, 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 puUed 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. XVl]
STflING FIGURES
359
Figure v. A Door.
The Door is produced 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 dorsal 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. Eub 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. XVl
the straight lines representing the tree will be upright, and
this is the way in which the figure is normally 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 Climbing a Tree.
The Boy Climbing a Tree may be produced thus. First. Take up tihe string
in the form of Opening A. Second. Pass the little-fingers over four strings
(namely, all the strings except the near or radial thumh 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.
Throioing 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. XVl]
STKING 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 thus. 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.
I describe a few of these.
Triple Diamonds. This is an interesting figure — see
fig^ure 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 FIGURES
[CH. XVI
Next, separate the ri.elit thumb and little-finger, take the loop off 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. Release 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. Right hand out. Right little-finger and
thumb through index loop. Right index takes up palmar string. Each thumb
over one and takes up one. Right index picks up near right index string and
left index picks up far left thumb string. T to thumb loops. Release 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. Release 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 litfcle-finger string, and return. Make a
similar movement with the other hand. Fourth. Pass each thumb away from
CH. XVl] STRING FIGURES 863
you over the near index string, and take up from below with the back of the
thumb the 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 thiimb 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 snia,ll
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 Diarnonds 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 Httle-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. Mxtltiple Diamovda.
Multiple Diamonds may be formed thus (Jayne, pp. 1.50—156). First. Take
up the string in the form of Opening A. Srcond. With the teeth draw the far
little-finger string towards you over all the strings. Then, bending the left
864
STRING FIGURES
[CH. XVI
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, and return. Next betid
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. Eighth. 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-finger 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 Stars. 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 Stars.
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. XVl]
STRING FIGURES
*S(io
Many Stars. The formation of this figure has been described by Haildon
{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 Openiiig 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. Ttiird. 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 FIGURES
[CH. XVI
ma}^ be talsen 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
np 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 example 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.
CH. XVl] STIUNG FlGUliES 307
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 Hadlon, 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-fiuger 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. TJdrd. 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 eacli 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-fingor loops to make thum
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 sun.
368 STRING FIGURES [CH. XVI
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 oft 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 victorious
over the right loop, and vice versa.
The Parrot Gage. 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. XVl] STRING FIQUKES 3G9
through the middle-finger loop, and then put the index loop on the thumb. Pull
all the strinps taut. Kepeat 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
wliich 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. Eepeat 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-Saiv. 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 stniight little-finger string,
and pulls it towards P. Finally. F releases all but the index loops. The
sawing movement can then be made.
Ligldning. 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. Zig-zag Lightning.
Lightning is produced thus (Jayne, pp. 216—219). Fir»t. 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. R. 24
370 STRING FIGURES [CH. XVI
tlie 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 from 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. XVl]
STRING FIGURES
371
the index, eephrate the thumb from the index. Make movement T on the thumb
loops. Repeat tlie 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 y. With the right thumb and index grasp the loops a and
/S 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 /3,
and the left thumb into 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 strmgs
in the figure, whereas it is left free if other strings are pulled.
2i— 2
372 STRING FIGURES [CH. XVI
The other comprises tricks where the string is released from or
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 through
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. XVl] 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 Jayne 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 Javne'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
BC, beginning with the B end, round the left thumb, above the thuml) when
moving towards the body and under the thumb when moving away from the
body. Leave about 6 inches at the end C unwound. Third. Out of this
6 inches, make a small loop by carrying the end of C 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 whicli 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 iiud 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 ^^lm, London,
October, 1902, pp. 141, 153. All, except the Hand-Cnfif, are described in
C. F. Jayne's Strinp Figures, New York, 1906, chapter viii. See, also,
W. I. Pocock, Folk- Lore, London, 1906, pp. 349—373.
374 STRING FIGURES [CH. XVI
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 oft 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. XVl] 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 rest of the loop hang down
in front of you. Second. Pass the right string round the neck 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 Xose. 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
80 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. Finalbj. 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-Cuff 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. XVI
arms. The ends of this piece are then tied one on Q's 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 himself 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, pa-.^ses it, from the elbow
to the finger side, between Q's wrist and the loop on that wrist. Next, P draws
this loop L sufficiently 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. Release 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
imder two. Little fingers, pick up near string. Thumbs, pick up near string.
Release loop from left index.
CH. XVl] STRING FIGURES 377
There also are some string tricks which must be fjimiliar
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. First. Pass a loop of string
through a button-hole or key-ring. Second. Hold the thumbs npripht, 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 Pocoek's paper in Folk-
Lore, 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 ou each side of the x*ing. 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, hojks the same string at some point between // and P'a
right thumb and draws it towards himself. Thus there is now one loop on
the back of Q's left index-finger and another on 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 pusbes
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 the radial string.
Sixth. Q transfers this index loop to P's right thumb, by touching the tips ns
378 STRING FIGURES [CH. XVI
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,
P 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. XVl] STKING FIGURES 379
armhole. The effect of this is to leave the left arm hole of the waistcoat at the
back of the neck. TJiird. 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, Erweiteruvg dcr Gauss'schen Theorie der Venchlinqungen,
Stuttgart, 1876 ; and P. G. Tait, Collected Scientific Papers, Cambridge, vol. i,
1898, pp. 273 — 347. The more common forms of knots and lashings and their
special uses are described in most, naval, engineering, and scouting manuals.
380
CHAPTER XVII.
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 chapterf , 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 Eaphael — whose real
name was R. C. Smith — London, 1828, to which the references to Eaph6,el
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 Astronomie und Astro-
logie, Leipzig, 1857.
t On the influences attributed to the planets, see The Dialogue of Bardesan
on Fate, translated by W. Cureton in the Spicilegium Syriacum, London, 1855.
CH. XVIl] ASTROLOGY 381
All the rules of the medieval astrology — to which I confine
myself — are based on the Ptolemaic astronomy, and originate
in the Tetr-abiblos* which is said, it may be falsely, 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,
so 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 cast 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— 1U9.
382
ASTROLOGY
[CH. XVII
were described as the first house, the second house, and so on.
The dividing lines were termed cusps: 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. XVIl] ASTROLOGY 383
SO 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
influence 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 facsimile 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 alLquestions 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. — "
• Eaphiiel, pp. 118—131.
384 ASTROLOGY [CH. XVII
I proceef\ 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 of the Sun indicates
respectability and moderate success: that of Venus 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 Eapliael, pp. 129—131.
CH. XVll] 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 born under Mercury, that is, ^\^th
Mercury in the first house, would be of a mercurial nature,
while anyone born 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.
♦ Eaphael, pp. 132—170.
B. R. 25
386 ASTBOLOGY [CH. XVII
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: ex. gr. Aries was
associated especially with events in England, France, Syria,
Verona, Naples, &c.
CH. XVIl] ASTKOLOGY 3§V
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 (7/^) 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 {in) 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 effect 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. 01 — 09.
25—2
386
j68 astrology [CH. XVII
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 w^as 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 from
* Cicero, De Divinatione, ii, 42.
f Pliny, Historia NaturaliSy vii, 49 ; xxix, 1.
CH. XVIl] ASTROLOGY 389
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 sorry 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;]: of Louis XI of France.
♦ St Augustine, De Civitate Dei, bk. v, chap, iii; Opera Omnia, ed. Migne,
vol. vii, p. 143.
t Annales, vi, 22 : quoted by Whewell, History of the Inductive Sciences,
vol. I, p. yi3.
+ Personal Characteristics from French History, by Baron F. RotLscliilJ,
390 ASTROLOGY [CH. XV 11
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 w^as 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 Scott in Quentin
Durward, chap. xv.
* Manual of Astrology, p. 37.
t The story, though in a slightly different setting, is given in The London
Chrovicle, Dec. 3, 1771, and it is there stated that Flamsteed attributed the
result to the direct action of the devil.
CH. XVIl] ASTROLOGY ,Sbx>
her property for her. At hist, 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 w4fe 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
39^
>2 ASTROLOGY [CH. XVII
ph}-icians had disagreed*. On his return through London,
Carcxn stopped with Sir John Cheke, the Professor of Greek at
CamHdge, who was tutor to the young king. Six months
previously, Edward had been attacked by measles and small-pox
whch 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 Geniturarum
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 J: 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 ; Gonsilia 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 se^'
X De Reruvi Varietate, p. 285.
CH. XVIl]
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
woi-k, 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 J 57 nicimdecimoSlcb.nor,
lymmUS'd VieridielondinJ,
R*^ Hv^tU Uiodrdi SexH,
./ 9j -V -iv "^
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 i^ost consideravi de eodemf.
In effect his explanation is that a weak nativity can never be
* Geniturarum Exempla, p. 19.
t Ibid., p. 23.
394 ASTROLOGY [Cli. XVII
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 frequentl}^ 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
beinof 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.
X See ex. gr. the life of Cellini, chap, siii, Eoscoe's translation, pp. 144 —
146. See also Sir David Brewster's Letters on Natural Magic,
395
CHAPTER XVIII.
CRYPTOGRAPHS 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
modern 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, 1909. 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 cryptograph 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
396 CRYPTOGRAPHS AND CIPHERS [CH. XVIII
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 deveileb 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 6, b by c,
and so on, and finally 2 by a. In this cipher the above message
would read:
fofnz cfm^fwfe 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
ZTifof efiufjmfc pufc hojsjufs uspqfs opjujtptq op ttpsd ebps.
If the message were written in a foreign language it would
further diminish the chance of it being read by a stranger
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. XVIIl] CRYi'TOGRAPHS 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 fixct 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 messasfe is con-
cealed or conveyed and have nothing to do with cryptographs
or ciphers. IMany 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.
Cryptographs. 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. XVIII
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 smret 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
written thus:
eio tnlit
sntioeth
acsncnse
ereuecep.
The 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 ?i -I- 1 letters in each
of the first r lines, and n letters in each of the last c — r lines,
CH. XVIIl]
CRYPTOGRAPHS 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
eseiua eqtonu sfdth enzce sjteo ttqiz yczht zjioa rhqet tjrfe sftnz
mroom ohyea rziaq neorn hreot lennk aerwi zesju asjod eziojz
zszjb rritt jnfjl weuzr oqyfo htqay eizsl eopji dihal oalhp epkrh
eanaz srvli irnosi adygt pekij scerq vvjqj qctjqn yjint kaehs hhsnh
goaot qefqe miesa yquni tpebq stzam ztqrj, and the clue number
were 17 we should put 16 letters in each of the first 15 lines
and 15 letters in each of the last ^ lines. The communication
could then be discovered by reading the columns downwards :
the letters J, q and z marking the ends of w^ords.
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 aivotn 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
8
16
5
pre-determined manner. Thus, to take a very simple example,
the words might be written in tabular form in the order shown
^QQ CEYPTOGEAPHS AND CIPHEES [CH. XVIU
in the diagram, certain spaces being filled with dummy words
xvz,..., and the message being sent in the order U, 8 13 2,
15 4 w X 1 .. This method was used successfully by the
Fe'de^als iA the American Civil War, 1861-1865, 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 woxdd 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
tvne is one in which the message is expressed m ordinary
writing, but in it are introduced a number of dummies or non
significant letters or digits thus concealmg 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 perniits
only such letters as are on certain portions of it to be v.sibla
Such a card is known as a grille. An example of a grille with
four openings is figured below. A communication made in this
\
way may be easily concealed from anyone who does not possess
a card of the same pattern. If the recipient possesses such
CH. XVIIl]
CKYl'TOGRAPHS AND CIPHERS
401
a card he has 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 i^ifles se. On applying the grille with the
H
K
981
NTT
261
070
523
479
100
OKI
SON
SON
AHY
DTG
BFS
PUM
OLT
KFE
LJO
EGX
AEU
QJT
EGO
FLE
HVE
WLA
FML
AES
REM
REM
ODA
TIM
^SE
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
26
B. R.
402
CRYPTOGRAPHS AND CIPHERS
[CH. XVIII
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 comj)osed 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
^■^^
4;
^m^^^ wm ^ -\^^^-h' \^m:^ ^ ^
B
B
c^^SH
is said to have been given to the Young Pretender during his
wanderings after Culloden. If it be creased along the lines
BB and (70 (CO being along the second line of the second score),
and then folded over, with B inside, so that the crease lies
CH. XVlIl] 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 of con-
cealing himself must have been so patent that it was hardly
necessary to communicate it by a cryptograph.
Cipliers. 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 h, 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. XVIII
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
ahcdefghijhlmnopqrstuviuxyz
olkmazsqxeufy rthcwbvnidgjp
In such a scheme, we must in our communication replace a
by 0, h by I, 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 wTite 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 lines thus :
pretoiahc dfg h
z y X lu V u s qn 111 I 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. XVIIl] 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; ^, 7'4; n, 7-3; s, 6-8; r, 59; h, 57; d, 3-9; l, 3-6; u, 3*0
c, 2-8; m, 2-7; / 2-5; p, 1-9; g, 1-8; y, 1-8; 6, 1-7; w, VI] v, 11
k, 0*6; j, 0'3; q, 03; x, 0*3; z, 0"2. 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, ISf^— 186, 241—242,
473—475. See also the (Jolleclcd Works oj E. A. Foe in 4 volumes, vol. i, p. 30
et seq^.
406 CRYPTOGRAPHS AND CIPHERS [CH. XVIII
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 bnjt.
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. XYIIl]
CRYPTOGRAPHS AND CIPHERS
407
numbers in an order like this, which suggests their meaning,
but it will serve 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 smoking-cap we shall have 100
S
31
K
I
N
—
P
z
J
t
d
n
X
G
c
A
i
8
m
p
•
t
d
n
b
r
s
a
b
1
V
c
m
w
8
q
a
d
n
h
r
b
1
V
e
y
i
s
c
m
g
q
a
k
e
o
y
h
r
b
1
M
~K
k
u
e
f
P
z
I
o
V
f
P
z
w
g
q
N
G
y
i
j
t
k
u
C
A
s
P
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 loug as when it is in clear.
Like the similar cipher with numbers this can be improved
408 CRYPTOGRAPHS AND CIPHERS [CH. XVllI
by introducing after every 7?2th 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 ksTcam sssgn nncae kknoo mklisc
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 ^th 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. XVIIl] 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) q, or that the
nmnbcr 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 : — Cisvg vuniya
vijnp vgzsi yhpjp tuoiy. 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 CIPHERS [CH. XVI II
key number. The difficulty of discovering the key number is
increased if after every (say) 77ith 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: atpzn hvaocu xhiep xafwg
hzniy prpsi khdkz yygkq prgez uytlk ohldi fehzm xlpog quyit
cmgxk ckuex vsqka ziagg sigay tnvvs styvu aslyw gjuzm csfct
qhpwj vaepf xhibw pxiul txlav vtqzo xivkvt uvvfh cqhxn 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 Hiut fhziedvhi
bujjy pxwmqmhg erh er^vmrq max zirteig station or as Hyy
svvkunthm lehx uuhzgmiq tvd gvcciq mqe frcoanr atpkcrr.
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. XVIIl]
CRYPTOGRAPHS AND CIPHERS
411
enciphered and decoded by non-experts: the St Cyr code fulfils
this requirement.
a
h
c
d
f
g
h
i
k
1
m
n
T
q
3
s
t
u
V
w
X
y
z
P
a
r
s
t
u
V
w
X
y
z
a
b
c
d
e
f
h
i
k
1
m
n
r
s
t
u
V
w
X
y
z
a
b
c
d
e
f
g
h
i
k
1
m
n
o
P
q
u
V
w
X
y
z
a
b
c
d
e
f
g
h
1
k
1
m
n
P
q
r
•a
s
t
d
e
f
g
h
i
k
1
m
n
P
q
r
s-
t
u
V
w
X
y
z
b
c
d~
e
f
g'
h
1
k
1
m
n
P
q
r
s
t
u
V
w
X
y
z
a
b
c
n
t
i
P
q
r
s
t
u
V
TV
z
y
z
a
b
c
d
e
f
g
h
i
k
1
m
s
u
V
w
X
y
z
a
b
c
d
e
f
g
h
i
k
i
m
n
P
q
r
k
1
m
u
P
q
r
s
t
u
V
w
X
y
z
a
b
c
d
e
f
g
a
b
c
d
e
f
g
h
i
1
k
1
m
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 gihe 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 v>^ords is equivalent to using 26 digits instead of
10 : thus the key word hings 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
7?ith 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
CllYPTOGRAPHS AND CIPHERS
[CH. XVIU
letter which occurs more than once) and the remaining letters
of the alphabet, thus : —
m
a
n
c
h
e
s
t
r
b
d
f
9
i
u
k
I
P
V
10
X
y
z
Only 25 cells are available, so k has been used for both h and g.
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 tn oz on ; then be put in cipher as yf iiiu ha hv sr xp Ih gt ux
xt] and finally be sent as yfuwk ahvsr xplhg tuxxt. It is curious
that this cipher is not used more extensively, for the discovery of
the key is a most difficult problem, 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. XVIIl] 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:
T h e f X j um 2^ e d over the garden gate,
ah c d ef g h i j k I mnop qrs t u vw x y z ah c.
Then we write the message replacing a hy t or a, b hy h
or t, c hy e, d by/, and so on. Here is such a message.
Mfoemho nea ge eoo jmdJiohg avfteg ev ume afrmeo. But it will
be observed that in the cipher a may represent a or u, d may
represent I or w, e may represent c or k or o or 5 or x, g may
represent t or z, h may represent b 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 afforded 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 r n e me n t provisoire.
ah cdefghijklmn o p qr st uv xy.
Hence in putting her message into cipher she replaced a by l^
h hy 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. XVIII
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 cryptograph ically, 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 frequency 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 Eendus, Association Frangais four I'Avancement des
Sciences, vol. xx (Marseilles), 1891, p. ICO et tteq.
CH. XVIll] CUYPTOGKArHS AND CIPHERS 415
though we must allow for the possibility that dummy letters,
like^', 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 hajopens 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 key 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 CIPHERS [cH. XVIII
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 :
Q 9 y 3 9 y v y
i V a n z h w
s u V p s p
V m i 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 ^, 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,
i, a, ?', a.
CH. XVIIl] CRYPTOGRAPHS AND CIPHERS 417
In the second line no letter occurs more than once, so we
get no chie 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, i, n, s, r, h. These give respectively 6= 11, 22, 15, 1,
7, 2, 23, 24, 8, and ^ = 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 z= e, s = h, OT 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 successivel}^ 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, f, 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 ^=1, and the slip derived from ^ = 4 in the
fourth line give w . rra . tis . ued . or a . res . ofj . 7ies, 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
D. K. 27
418 CRYPTOGRAPHS AND CIPHERS [CH. XVIII
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 ( ), b ( • • ), c ( — ), d ( ), ^ (•)>
/( \9{ Xh(-'''\i{''),j( ),k( ),
l( ), ^( X^( )>o( \p( \q( ),
^( )> s('--),t{—), u{ ), v{ ), w ( ),x{ ),
* Of some of Caesar's correspondence, Suetonius says (cap. 56) si quis
investigare et persequi velit, quartam elementorwn literani, id est, d pro a, et
perinde reliquas commutet. And of Augustus he says (cap. 88) quoties autemper
notas scribit, b pro a, c pro b, ac deinceps eadem ratione, sequentes literas ponit;
pro X autem duplex a.
CH. XVIIl] CRYPTOGRAPHS 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, c^ by 5, g 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, 5 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 France, 189 for the King,
260 for the Queen, 354 foi: Prince Rupert, and so on. Further,
there were a few special symbols, thus ^1 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 Report of the Royal Commisiion on Historical Mayiimcrijits, 1870,
pp. 2, 4.
27—2
420 CR^PTOGRAPJdS AND CIPHERS [CH. XVllI
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
difiScult 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, 6 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 known by his
subsequent title of Marquis of Worcester, in 1645; each of these
* The document, its translation, and the key used, are given iu 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 similar system by W. Cartwright was issued by J. Eich
under the title Semographie, London, 1644.
CH. XVIIl] CRYPTOGRAPHS AND CIPHERS 421
was a cipher of the first type arirl iiad 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 yontj,
Im ct idgc 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 pvisonnier, 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 t, 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, p, z, a, h, I, m\ the next,
0, p, q, m, n; the next /j, 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 Inventions , London,
1683, sections 3, 4, 5.
+ Various systems, includinej those used in classical and medieval times, are
described in the History of Shortiiand by T. Anderson, London, 1862.
422 CRYPTOGRAPHS AND CIPHERS [CH. XVIII
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. XVIIl] 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 w^e 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, 4G656 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.
^24
CHAPTER XIX.
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 phj^sical
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. Kodwell, Nature, May 1, 1873, vol. viii, pp. 8, 9 ;
and E. A. Abbott, Flatland, London, 1884.
On Non-Euclidean geometry, see chapter xiii above. The theory is due
primarily to N. I. Lobatschewsky, Geomctrische Untersuchtingen zur Theorie der
Parallellinien, Berhn, 1840 (originally given in a lecture in 1826) ; to C. 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 Tcntamen, 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, Ueher die Hypothesen welche der Geo-
metric zu Grunde liegen, written in 1854, Gottinger Abhandlungen, 1866-7, vol.
xin, pp. 131 — 152 (translated in Nature, May 1 and 8, 1873, vol. viii, 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 Geometria non-Euclidean
Naples, 1868, and the Annali di Matematica, series 2, vol. ii, 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. ii,
1900; and D. M. Y. Somerville, Bibliography of Non-Euclidean Geometry y
St Andrews, 1911.
CH. XIX] 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 tw^o
dimensions, other than those which are treated in the usual
text-books : some aspects of the latter question have been
already considered in chapter xiil. 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 surflice 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. XIX
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 would 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 Mothematische 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. Somerville, Messenger of Mathe-
matics, N.S., vol. XXXVI, 1907, pp. 139—144.
CH. XIX] 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 Hatland 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 dim.ension, 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. XIX
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 to 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. XIX] 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 C — 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 we may enquire whether the hypothesis of our exist-
ence in a space of four dimensions affords an explanation of
430 HYPER-SPACE [CH. XIX
any difficulties or apparent inconsistencies in our physical
science*. The current conception of the luminiferous ether,
the explanation of gi'avity, 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.
CH. XIX] 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
as were in their plane, 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. XIX
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 ; whereas 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 (p {x, y, z) — 0, we can obtain
some idea of its form by taking a series of plane sections by
planes parallel to 2: = 0, and mentally superposing them. In four
dimensions the boundary of a body would be <^ (a?, 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 t(; = 0. 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. in, pp. 1 — 14.
CH. XIX] HYPEK-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. XIX
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 w^as moved away from some point of the plane — as it
might do by a fall of temperature. Then a distance, which we
CH. XIX] HYPER-SPACE 435
should describe as finite, micrht when measured by this rule
appear to be infinite, since repeated applications of the ever-
shortening 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 geometiy*.
Thus the conception of hyperbolic, 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.
Kiemann 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 Mailieinatical Fa^era, Cambridge, 1396, vol. xi,
p. -iiio et seq.
28—2
436 HYPER-SPACE [CH. XIX
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 gTeater 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 X 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, Matheinatischen Annalen, Leipzig, 1900, vol. Lvn, pp. 404—439.
CH. XIX] 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.
438
CHAPTER XX.
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. XX] TIME AND ITS MEASUREMENT 439
mainly with questions concerninc^ 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 ai'titicial, and apparently is derived £iom the
Babyionians,
440 TIME AND ITS MEASUREMENT [CH. XX
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 jfind 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 oJi.
CH. XX] TIME AND ITS MEASUREMENT 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
ii) 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 life.
44)2 TIME AND ITS MEASUREMENT [CH. XX
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 was 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 Poman emperors, and, except
during the French Pevolution, 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 tlie earth,
CH. XX] 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, Mars'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 gradually 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 Egj^ptians 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. XX
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 j^ear 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
* Herscbel, Astronomy, London, lltli ed. 1871, arts. 916 — 919.
CH. XX] 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 eveiy 134 years, being rejected by Lilius and Clavius for
reasons which are not known.
Clavius believed the year to contain 365*2425432 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
di\dne 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 eifected 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 Galendarii a Greg. XIII Eestituti Explication Kome, 1603.
t Chronologia ReJ'ormata, Bonn, 1669, vol. n, p. 206.
446 TIME AND ITS MEASUREMENT [CH. XX
The rale 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
14J 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, Com;panion to the Almanac, London, 1815, pp. 1 — 36 ; ibid.,
1846, pp. 1—10.
CH. XX] 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, 6, c respectively, (ii) Divide 19c + m by 30, and let d be the
remainder, (iii) Divide 2ci + 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 + c? + e)th of March
or the (cZ + e — 9)th day of April, except that if the calculation
gives (i = 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-
day 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, n = 6.
In the Gregorian calendar we have, from 1582 to 1699 in-
clusive, m = 22, 71 = 2; from 1700 to 1799, m=23, n = 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 Zacb's Monatliche Coirespondeiiz,
August, 1800, vol. u, pp. 221—230.
448 TIME AND ITS MEASUREMENT [CH. XX
w = 5; from 2100 to 2199, m = 24, n = 6; from 2200 to 2299,
m = 25, n = 0; from 2300 to 2399, m=26, 7^ = 1; and from
2400 to 2499, m=25, n=l. 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
h 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/oo} denotes the integral part of the quotient when
N is divided by x, then m is the remainder when 15 + ^ is
divided by 30, and n is the remainder when 6 + 77 is divided
by 7 : where, in the Julian calendar, f = 0, and 77 = ; and, in
the Gregorian calendar, ^= {i\^/100} - {i\^/400} - {i\^/300}, and
97={iV/100}-{iV/400}-2.
If we use these values of m and n, and if we put for
a, b, c their values, namely, a = iV^-4 (i\^/4}, h = N-'7 {N/7},
c = iV^— 19 {i\^/19}, the rule given on the last page takes the
following form. Divide 19iY- {N/19} + 15 + f by 30, and let
the remainder be d. Next divide 6(N+d + l)- {N/4i} +7)
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 d = 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 A^= 1920, we divide
19 (1920) - 101 -1- 15 -t- (19 - 4 - 6) by 30,
CH. XX] TIME AND ITS MEASUREMENT 449
which gives d=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 Zcller, for determining the
day of the week corresponding to any given date. Suppose that
the pih day of the ^th month of the year N anno domini is the
7'th day of the week, reckoned from the preceding Saturday.
Then r is the remainder when
;? + 2^+ {3 (5 + \)ib]-\-N[Nm-'n
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, 17 = 0. If
we divide 12 + 20 + 6 + 1492 + 373 by 7 we get r = 6 ; hence it
was on a Friday. Again, Charles I was executed on January 30,
1G49 N.S. Here ^ = 30, g=13, xV=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,
q = Q, N = 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 Astrononiie, Paris, 1814, vol. iii, chap, xxxviii ; and his Histoire de
Vustronornie modcrnc, Paris, 1821, vol. i, chap, i: see also A. De Moi^'an, The
Book of Almanacs, London, 1851 ; S. Butcher, The Ecclesiastical Calendar,
Dublin, 1877; and C. Zeller, Acta Mathematica, Stockholm, 1887, vol. ix,
jip. 131 — 136: on the chronological details see J. L. Ideler, Lehrbuch der
Chronologie, Berlin, 1831.
B. R. 29
450 TIME AND ITS MEASUREMENT [CH. XX
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 Rome 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 iheShipman'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. ni, p. 321 ; 1840 edition, p. 529.
Cfl. XX] 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', 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 QFF as its extremity moves
from Q to P, it will advance through the angle FFF' as its
extremity moves from P to P\ and it mil regrede through the
angle P'FQ' 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 F is within the space bounded by this
branch of the hyperbola and its as3niiptotes. 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 i\',
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 Z, let this great circle cut the celestial meridian in A
* 2 Kii)gs, chap, xx, vv. 9—11.
29-2
452 TIME AND ITS MEASUREMENT [CH. XX
and A\ and of the arcs AL, A'L suppose that AL i's> the less
and therefore is less than a quadrant. If the style is pointed to
Ay 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 (o sec \(o) — cot~^ {sin © cos (Z — Jo) (cos^ I — sin^ to) ~ ^),
where I is the latitude of the place and w 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 ov ring-dial is another instrument for measuring
solar time-|*. 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. iii, p. 317 ; 1810 edition, p. 526.
CH. XX]
TIME AND ITS MEASUREMENT
4.33
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. XX
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-
bury 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 Maldng by Lord Griinthorpe, 7th edition, London,
1883.
CH. XX] TIME AND ITS MEASUKEMENT 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 Lippcus 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 Co vent
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. Hole early in the sixteenth century
— the motive power being a ribbon of steel, wound round a
spindle, and comiected 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.
* Jlorologiuiu Oscillatoiium, Paris, 1G73.
456 TIME AND ITS MEASUREMENT [CH. XX
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
down 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 KatureX- 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. n, 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.
X See especially the volumes issued in 1874, 1877, and 1878.
CH. XX] 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 south.
Again, if it is eight o'clock in the morning, we must point the
hour-hand (which is then at the figure viii) 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 xil 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 first
enunciated it.
458 TIME AND ITS MEASUREMENT [CH. XX
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.
4,5y
CHAPTER XXI.
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 Itecent
Advances in Physical Science, by P. G. Tait, Edinburgh, 1876 (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, IlistM'y of the
'Theories of Aether and Electricity, Dublin, 1910.
460 MATTER AND ETHER THEORIES [CH. XXI j
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 aggregates 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, Principia, vol. ii, pp. 18, 23.
t Newton, Principia, bk. n, prop. 50.
CH. XXl] MATTER AND ETHER 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 1/1 021st part
of its original volume, which has been often exceeded. The
true explanation of the difficulty noticed by Newton was given
by Laplace.
(ii) Boscovich's 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 sapng
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 NaUtralis Theoria Redacta ad Unicam Legem Firnim, Vienna,
1759.
t Proceedings of the Eoyal Society of Edinburgh, April 21, 1SG2, vol. iv,
pp. 60i— 606.
462 MATTER AND ETHER THEORIES [CH. XXI
(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 Ojptik, Braunschweig, 1885.
CH. XXl] MATTER 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 paragi-aph 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
* Crelle\^ Journal, 1858, vol. lv, pp. 25 — 55 ; translated by Tait in 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.TTER AND ETHIOR THEORIES [CH. XXI
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 Vortex 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 hypothesis |: 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
mentioning 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.
+ See a memoir by M. J. M. Hill in the Pliilosophical Transactions of the
Royal Society, London, 1894, part i, pp. 213—246.
J Philosophical Magazine, London, October, 1887, series 5, vol. xxiv, pp. 342
—353.
CH. XXl] MATTER AND ETHER THEORIES 4G5
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 from 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) Tlie Electron Hypothesis. MacCullagh, in 1837 and
1839, 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 now enunciated, it accounts for many of the electrical
and magnetic (as well as the optical) properties of matter.
* American Journal of Mathematics, 1891, vol. xiii, pp. 309 — 362.
t Philosophical Transactions of the Royal Society, Loudon, 1894, pp. 719
822 ; 1895, pp. 695—743.
B. li. 30
466 MATTER AND ETHER THEORIES [CH. XXI
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. Djmamical 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 hypothesis. 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 difterences, however, between matter and
light tend to disappear as investigations proceed. In 1895
CH. XXl] MATTER AND ETHER THEORIES 4G7
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 show^ed 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
SO— 2
468 MATTER AND ETHER THEORIES [CH. XXI
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 h3^pothesis 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 J). I. Mendelejev, the
distinguished chemist. In the grouping of the elements ac-
cording to his periodic law, there were originally twelve series
CM. XXl] MATTER AND ETHER 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. Mendelejcv 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 1/10^ 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
fi:om 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 in a liquid. To
express this in technical language we may suppose the ether to
consist of an arrangement of minute uniform spherical gi-ains
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 Universe, Cambridge, 1903.
470 MATTER AND ETHER THEORIES [CH. XXI
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. Reynolds 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 3^et 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
bubble h5rpotheses are concerned, they have been elaborated into
a form from which 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; it is printed in the Letters to Bentley,
London, 1756, p. 20.
CH. XXl] MATTER AND ETHER THEORIES 471
place he v/rote*, "'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
hypotheses. 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 matterf.
This was elaborated on a statical basis by Clerk Maxwell,
who showedj 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 dynamical way of producing
the stress by supposing that space is filled with an incom-
pressible fluid, constantly being amiihilated 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. Rigaud 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.
Ij: 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. XXI
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 Neiotonh 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 Traite de Physique, Geneva, 1818.
CH. XXl] 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 gome 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
billion = 10^2.
:J: Properties of Matter, London, 1885, art. 164.
§ See an article by myself in the Messenger of Mathematics, Cambridge, 1891,
vol. XXI, pp. 20 — 24.
474 MATTER AND ETHER THEORIES [CH. XXI
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, i.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 h3rpothesis or with the vortex theory. It would
be interesting 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