THE UNIVERSITY OF CHICAGO
SCHOOL SCIENCE SERIES
NATURE-STUDY
Editor
ELLIOT ROWLAND DOWNING
OUR PHYSICAL WORLD
THE UNIVERSITY OF CHICAGO PRESS
THE BAKER & TAYLOR COMPANY
NEW YORK
THE CAMBRIDGE UNIVERSITY PRESS
LONDON
THE MARUZEN-KABUSHIKI-KAISHA
TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI
THE MISSION BOOK COMPANY
SHANGHAI
THE POND IN A CITY PARK WHERE CHILDREN RACE THEIR BOATS
OUR PHYSICAL
WORLD
A Source Book of Physical Nature- Study
By
ELLIOT ROWLAND DOWNING
The School of Education, University of Chicago
WITH A CHAPTER ON RADIO COMMUNICATION
BY FRED G. ANIBAL
Central High School, Kansas City, Mo.
Formerly Radio Officer, United States Air Service
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO ILLINOIS
COPYRIGHT 1924 BY
THE UNIVERSITY OF CHICAGO
All Rights Reserved
Published June 1924
Second Impression October 1925
Composed and Printed By
The University of Chicago pres»
Chicago, Illinois, U.S.A.
GENERAL PREFACE
Never before in this country has there been so insistent a de-
mand for a more thorough and more comprehensive system of
instruction in practical science. Forced by recent events to com-
pare our education with that of other nations, we have suddenly
become aware of our negligence in this matter. Industrial and
educational experts and commissions are united in demanding a
change.
While on the whole there has been a steady increase in the
amount of time given to science work in the secondary and ele-
mentary schools, the attention paid to it, especially in the ele-
mentary schools, has been somewhat spasmodic, and its admini-
stration has been more or less chaotic. This is not due to lack of
interest on the part of school officials but to their dissatisfaction
with the methods of instruction employed. There is no doubt that
superintendents would gladly introduce more science if they felt
sure that the educational results would be commensurate with the
time expended. This is indicated by a recent survey of about one
hundred and fifty cities in seven states of the Central West. The
survey shows that two-thirds of them have nature-study in the
elementary schools and that all are requiring some science for
graduation from the high school. The average high school is
offering three years of science. Since 1890 there has been a greater
increase in the percentage of students enrolled in science in the
high schools than in any other subject, and the present enrolment
in science is greater than in any other subject. Moreover, greater
attention is now being paid to the training of teachers in methods
of presentation of science.
The chief needs in science instruction today are a more effi-
cient organization of the course of study with a view to its sociali-
zation and practical application, and a clear-cut realization on the
vii
viii GENERAL PREFACE
part of the teacher of the aims, the principles of organization, and
the methods of instruction; it is to meet these needs that this
series is being issued. The books attempt to present such gen-
eralizations of science as the average pupil should carry away
from his school experience and to organize them for the prepara-
tion of the teacher and for presentation to the class. The volumes
are therefore of three kinds: (i) source books with accompanying
field and laboratory guides for the use of teachers and students in
normal schools and schools of education; (2) pupils' texts and
notebooks; and (3) books on the teaching of the various science
subjects. In the first the material is organized with special refer-
ence to the training of the teacher and the most effective methods
of presenting the subject to students. In the second the matter is
simplified, graded, and arranged in such a way that the books will
serve as guides in science work for the pupils themselves. More-
over, they will furnish texts for the grades and high school that
will simplify the teacher's task of presentation and will assure to
the pupil well-tried and well-organized experiences with natural
objects. This series of texts for elementary and secondary schools
will have dependent continuity and the subject matter will gradu-
ally increase in difficulty to accord with the increasing capacity of
the pupils. It will furnish a unified course in science. The third
type of book is for the teacher and deals with the history, aims^
principles of organization, and methods of instruction in the
several sciences.
AUTHOR'S PREFACE
Among animals, play often functions to prepare for adult
life. Wolf and dog puppies tussle in fun and so strengthen their
muscles and improve their strategy for the fights of maturity.
So the kitten plays with a stray spool or ball and goes through
all the antics she will use later in catching her prey. The play
activities of children are in many, instances imitative of adult
activities. Dolls are given as solicitous attention by the child
as is the baby of the household by the parents. The plan of the
play house built with blocks receives a deal of thought. The
play store must have its wares appropriately displayed; clerk
and purchaser must be properly decorous.
One need only go through the toy department of a city store
to see that toys have followed the trend of a scientific age and
are themselves replicas of adult appliances. There are construc-
tion sets, railroads and trains, telephones, radio sets, aeroplanes,
magic lanterns, chemical sets. It seems a great opportunity with
this interest in scientific toys to secure for the child through play a
variety of experiences that will give him some elementary appre-
ciation of those principles of science which are so important in
the social and industrial life of the adult.
It is the purpose of this book to organize the subject-matter
of elementary physical science or physical nature-study about
toys and familiar home appliances. It is hoped it may serve as
a guide in the workshop of the boy or girl who enjoys making
things, that it may help children understand how commonplace
appliances work and may aid parents and teachers in answering
the questions of inquisitive youngsters. It is a source book in
the sense that it brings together in one volume material else-
where scattered and difficult of access. This volume is supple-
mented by the practical constructions in the Field and Laboratory
ix
x A UTHOR'S PREFA CE
Guide in Physical Nature-Study already published. There are
introduced into this book some things more profound than most
grade children will undertake to understand. They are intended
to serve as a background for parent and teacher in order that
they may present the materials to the children in better perspec-
tive. They are suggested by the types of questions experience
has shown are most frequently asked by those preparing to teach
this subject-matter.
ELLIOT R. DOWNING
THE UNIVERSITY OF CHICAGO
THE SCHOOL OF EDUCATION
May i, 1924
TABLE OF CONTENTS
EAGE
LIST OF ILLUSTRATIONS xiii
CHAPTER
I. THE UNIVERSE IN WHICH WE LIVE i
II. THE EARTH'S ROCK FOUNDATIONS . . 43
III. THE CONQUEST OF THE AIR 77
IV. ALR AND WATER AS SERVANTS OF MAN 104
V. THE SLING, Bow, AND OTHER WEAPONS 130
VI. FIRE AND ITS USES 146
VII. THE NATURE OF MATTER 163
VIII. STEAM AND GASOLINE ENGINES 178
IX. DISCOVERIES IN MAGNETISM AND ELECTRICITY 199
X. ELECTRICAL INVENTIONS 211
XL RADIO COMMUNICATION 250
XII. DEVICES FOR SEEING BETTER, FARTHER, AND LONGER . . 281
XIII. CAMERAS AND PICTURE-MAKING 309
XIV. THE HOMEMADE ORCHESTRA 325
XV. SOME SIMPLE MACHINES 339
BOOK LIST 3Si
INDEX . 357
LIST OF ILLUSTRATIONS
CHAPTER I
PAGE
Children Sailing Their Boats on a City Park Pond, Frontispiece
FIGURE
1. The Corona of the Sun 5
2. Sun Spots 6
3. Diagram to Show Relative Sizes of the Planets .... 7
4. Diagram of the Earth in Its Orbit to Show Varying Lengths of
Day and Night and Change of Seasons 10
5. Diagram of Path of the Earth to Show Cause of the Tides . 14
6. Diagrams of the Earth's Equatorial Bulge and Its Action in
Causing the Precession of the Equinoxes 15
7. Ursa Major, the Big Bear 20
8. Bootes, the Hunter .......... 21
9. Cassiopeia 23
10. Diagram to Show the Method of Finding the Circumpolar Con-
stellations 24
11. Cepheus 25
12. Andromeda 26
13. Pegasus 27
14. Draco, the Dragon 28
15. Cygnus, the Swan . . 29
1 6. Auriga, the Charioteer 30
17. Taurus, the Bull 32
1 8. Orion and His Dogs, Canis Major and Canis Minor ... 33
19. Gemini, the Twins 34
20. Aries, the Ram 36
21. Diagram to Show the Method of Finding Some Zodiacal Con-
stellations 37
22. Leo, the Lion 37
23. Virgo, the Virgin . 38
24. A Group of Southern Constellations Named in Commemoration of
the Flood 41
CHAPTER II
25. Soil Underlain by Rock . . 46
26. Crystals 48
xiv LIST OF ILLUSTRATIONS
FIGURE PAGE
27. Feldspar, to Show Cleavage 49
28. A Zinc Mine 51
29. Basalt 63
30. Limestone, Showing Stratification 71
31. Fossils 72
32. Entrance to a Coal Mine 75
CHAPTER III
33. Diagram of the Decomposition of Forces 80
34. A Tetrahedral Kite in Flight 83
35. Besnier's Flight Apparatus . 86
36. De Bacqueville's Wings for Flight 86
37. Lillienthal's Glider 88
38. A Recent French Glider 89
39. Langley's Aeroplane 93
40. The Aeroplane Frame 98
41. Front View of Aeroplane Frame 98
42. Diagram of the End of the Block from Which Propeller Is Cut . 99
43. The Aeroplane Complete 101
44. Front View of Biplane Built by Seventh-Grade Pupils . . 102
CHAPTER IV
45. A Military Observation Balloon . 107
46. A Dirigible Balloon * . . no
47. Tin Can with Tubes in It to Show Water Pressure . . .113
48. A Coracle 121
49. An Old-fashioned Wind Mill 123
50. A Water-Power Plant 126
51. Diagram of a Lift Pump 129
CHAPTER V
52. The Crossbow 136
53. An Archer 136
54. The Catapult . 139
55. The Flintlock Musket . . . 141
56. An Old Cannon on Its Wooden Carriage 143
57. An Air Drill in a Quarry 144
CHAPTER VI
58. A Fire Drill 147
59. Diagram of a Fireplace . . 152
60. Diagram of a Hot-Water Heating System 1 54
61. A Fire 156
LIST OF ILLUSTRATIONS XV
FIGURE PAGE
62. A Weather Map of the United States 158
63. The Same for the Succeeding Day 159
64. A Blast Furnace 161
65. A Line of Old-fashioned Charcoal Kilns 161
CHAPTER VII
66. Diagram of a Helium Atom 166
67. (a) Diagram of a Sodium Atom . . . . . . . 169
(b) Diagram of a Fluorine Atom 169
68. An X-Ray Photograph . . 175
CHAPTER VIII
69. Diagram of Savery's Steam Pump 179
70. Diagram of Newcomen's Engine 180
71. Diagram of Watt's Engine 182
72. Diagram of a Modern Steam Engine 184
73. Diagram of the Governor of a Steam Engine .... 185
74. Harvesting and Binding Done by Hand 187
75. The Power Harvester and Binder 187
76. An Early Power Loom 188
77. The First Railroad Train in the United States . . . .190
78. Two Diagrams of a Gasoline Engine 193
79. Diagram of a Carburetor 195
80. Diagram of an Automobile Chassis and the Gear Shift . . 197
CHAPTER IX
81. Magnet Holding a String of Nails 201
82. Pattern of Iron Filings over a Magnet 202
83. Volta's Crown of Cups 206
84. A Simple Galvanoscope 208
CHAPTER X
85. Diagram of an Electric Telegraph 212
86. Telegraph Sending Key and Receiving Sounder . . . .213
87. Laying the Atlantic Cable (Copy of a Contemporary Print) . 214
88. Diagram of a Telephone Receiver 218
89. Diagram of a Microphone Transmitter 219
90. A Modern Telephone Exchange Switchboard . . . .220
91. Diagram of an Electric Bell 221
92. Diagram of Buzzer, Push Button, and Batteries Properly Con-
nected 222
93. Several Types of Batteries 224
xvi LIST OF ILLUSTRATIONS
FIGURE PAGE
94. Diagram of Batteries Connected "in Series" and "Parallel" and
of Water Tanks by Way of Analogy 226
95. Diagram of an Ammeter 227
96. Diagram of a Kilowatt-Hour Meter 229
97. Diagram of a Dry Battery 230
98. Diagram of a Storage Battery 232
99. Diagram of an Electric Motor Reduced to Simple Terms . . 233
100. Diagram of a Commercial Electric Motor 235
101. Diagram of a Toy Motor 237
102. The Electric Motor on a Sewing Machine 239
103. Diagram of a Vacuum Cleaner 239
104.' A Powerful Electric Engine 240
105. Diagram of a Simple Dynamo 242
106. A Dynamo with Cored Coils Set Like Cogs 243
107. A High-Power Transmission Line 244
108. Diagram of an Electric Light . 246
109. Diagram of Wiring for Electric House Lights .... 248
no. (a) An Electric Heater; (6) An Electric Percolator; (c) An
Electric Flatiron; (d) An Electric Toaster 249
CHAPTER XI
in. Diagram of the Wireless Telegraph Sending Outfit . . . 252
112. A Spark Gap 253
113. Diagram of a More Complex Sending Outfit 254
114. A Train of Damped Waves 256
115. Diagram of the Receiving Set 256
116. A Crystal Detector .257
117. The Radio Room, SS. "Leviathan" 258
1 1 8. Diagram of a Receiving Circuit 259
119. A Two-Slide Tuning Coil ... . 260
120. A More Elaborate Receiving Set 266
121. A Rotary Variable Condenser 267
122. Discontinuous and Continuous Waves 267
123. A Three-Electrode Vacuum Valve 268
124. The Use of the Vacuum Tube as a Detector 269
125. Power Tubes for Transmission. Radio tron Vacuum Tubes . 271
126. The Heterodyne. Use of the Vacuum Tube as a Generator . 272
127. Diagram of Voice Modulation of Continuous Waves . . . 275
128. The Radio Telephone Transmitter 276
129. The Operating Room of a Broadcasting Station .... 278
130. A Modern Receiving Set 279
LIST OF ILLUSTRATIONS xvii
FIGURE CHAPTER XII PAGE
131. Diagram of Varying Light Intensities 282
132. The Pinhole Camera 284
133. The Camera Obscura 285
134. Reflection in a Plane Mirror 287
135. Horizontal Section of Eyeball 288
136. Reflection from a Convex Mirror 290
137. Images Seen in Curved and Plane Mirrors . . . . . 290
138. Diagram of an Object Magnified by a Spherical Concave Mirror,
Object Being Inside of Focus 291
139. Figure of Coin in a Bowl of Water, to Show Refraction . . 293
140. Diagram of a Ray of Light Entering Glass 293
141. Diagram of Light Coming Out of Glass 294
142. The Action of the Burning Glass ........ 295
143. The Conjugate Foci of a Convex Lens 296
144. Lenses of Several Shapes 297
145. Diagram Showing How* a Magnifying Glass Magnifies . . 297
146. Diagram of a Compound Microscope 298
147. A Compound Microscope, Showing Parts ..... 300
148. Diagram Showing Operation of a Telescope 301
149. A Telescope and Its Mount 302
150. Diagram of the Stereopticon 303
151. Diagram of Refraction by a Prism . . . . . . . 304
152. The Correction of a Convex Lens by a Concave Lens . . . 305
153. Diagram of Wave Motion 305
154. Diagram of Marching Men 306
155. Formation of Rainbow 308
CHAPTER XIII
156. A Box Camera, the Brownie 309
157. A Plate Camera on Its Tripod 310
158. An Exposure Meter 312
159. Front of Camera Lens to Show Device for Setting the Time and
Diaphragm 314
1 60. Diagram of a Reflecting Camera 316
161. Some Darkroom Equipment 318
1620 and b. A Negative and a Print from the Same . . . ' . 321
163. Handling the Film ; 322
164. A Lantern Slide 323
CHAPTER XIV
165. Vibration of a Taut String 326
1 66. Sound Waves Radiating from a Bell 327
xviii LIST OF ILLUSTRATIONS
FIGURE PAGE
167. Strings Stretched across the Table 328
168. Cello and Violin 329
169. End of a Clarinet Showing Reed 330
170. Squawker Made of an Oat Straw 331
171. Fife Showing Changing Length of Air Column .... 332
172. String Vibrating as a Whole and in Halves 335
173. The Larynx . 336
174. Diagram of a Phonograph 336
175. Diagram to Locate a Gun by Sound 338
CHAPTER XV
176. A Pair of Scales 340
177. The Crowbar in Use 340
178. The Arm Showing Triceps Muscle . 342
179. Levers of Classes i, 2, and 3 . . . . . . . . 342
180. A Hammer as a Bent Lever . . 342
181. The Wheelbarrow as a Lever 343
182. Wheel and Axle Used in Steering a Boat 343
183. A Windlass 344
184. A Capstan 344
185. GearWheels . . . . . . . . . . .345
186. A Hand Derrick . 345
187. The Sprocket Wheel and Chain on a Bicycle .... 346
1 88. A Single-wheeled Pulley 347
189. Two Double Pulleys 347
190. Rolling a Barrel up an Inclined Plane . . . . . . 348
191. The Chisel as an Inclined Plane 349
192. A Screw Jack . . . . 349
193. Turning a Nut on a Bolt with a Wrench 350
194. A Planisphere. Part I facing 356
195. A Planisphere. Part II . . . . , . . . facing 357
CHAPTER I
THE UNIVERSE IN WHICH WE LIVE
Why does not someone teach me the constellations and make me at
home in the starry heavens, which are always overhead and which I don't
half know to this day. — CARLYLE.
Were you so fortunate as a child as to have some older
companion — father, mother, big brother, or teacher — who took
you out under the sparkling night sky and taught you to know
the conspicuous stars by name, pointed out some of the constella-
tions, and told you the marvelous myths connected with them
that have come down from the childhood of the race to delight
the modern child? Was the night a source of terror to you,
or was it a source of pleasure because the stars had come to seem
like old friends and you knew their names and some of the marvels
of their existence ? To how many a modern adult has the starry
sky come to be so commonplace that he is unaware of its
existence — perfectly oblivious to the glory of the heavens. If as
a child you had a speaking acquaintance with the stars, if you
knew them as distant suns, if you were made aware of their
immensity and the immeasurable distance of these familiar yet
usually unknown companions of the night, if you learned to
recognize the wandering planets, then the infinity of the universe,
the mystery, the awesomeness, made so deep an impression on
your childhood imagination that the nightly pageant can never
be commonplace. It seems as if some such impression should
be one of the inalienable heritages of childhood.
The sun, the moon, the stars, and the other heavenly bodies
have always been objects of great interest to man. Indeed, they
have been objects of mystery, of reverence, and of worship.
Primitive man recognized in the splendid sun the source of light,
of comfort, and of life. The stars were his guides by night, the
2 OUR PHYSICAL WORLD
moon, a welcome relief from the fearsome gloom. He was prone
to identify all these things as the dwelling-places or the very
incarnations of his gods, easily believing that they exerted a
potent influence for good or evil over his daily life. So astron-
omy, or its earlier prototype, astrology, is the oldest of the
natural sciences.
The early astrologers knew most of the planets, too, as bril-
liant bodies that are not fixed in their positions with relation to
each other as are the stars, but are constantly changing their
locations, apparently pursuing somewhat erratic courses among
the stars. Indeed, the Greek word from which the name planet
is derived means "a wanderer." The paths of these wandering
bodies the ancients knew with remarkable accuracy and they
even foretold their positions with certainty. The names of these
bodies still indicate their identification with the ancient gods.
These planets we now know revolve about the sun. The earth
is simply one of them. In order they are: Mercury, Venus, the
earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Mercury
is nearest to the sun, Neptune the farthest. Uranus and Neptune
were unknown to the ancients, for they are visible only with the
aid of the telescope. Uranus was discovered by accident by
Sir William Herschel in 1781, while he was making a systematic
survey of all the stars. The size, motion, and position of Nep-
tune were calculated before its discovery, for Uranus did not
move as it should, and astronomers felt certain it must be influ-
enced by some, as yet, undiscovered planet. Adams and
Leverrier, respectively an English and a French astronomer,
made the very difficult calculations to determine its position,
and Galle, a German astronomer, was the first to see it, Septem-
ber 23, 1846.
Between the orbits of Mars and Jupiter there are more than
500 small bodies, similar to planets except for their size. They
also revolve about the sun. These are known as planetoids.
The first of them was discovered the first day of the nineteenth
century by Piazzi at Palermo, Italy.
THE UNIVERSE IN WHICH WE LIVE 3
Naturally the most interesting, as it is the most conspicuous,
of all the heavenly bodies is the sun; it has been worshiped as a
diety by many primitive peoples. While astronomy has robbed
it of its mysticism, it has increased our wonder at the marvels it
displays. In the first place, it is tremendously large as compared
to our earth, having a diameter of 866,540 miles, about no times
that of the earth. More than 1,300,000 bodies the size of the
earth could be packed into the space occupied by the sun. It is
because of its enormous mass that the sun is the center of our
solar system, holding the planets in their orbits by its gravita-
tional pull.
The sun is the chief source of all of our energy — light, heat, and
chemical rays emanating from it. We all realize from experience
that the sun is the source of light and warmth. We know that its
chemical rays produce marvelous changes in the photographic
plate when a picture is taken. But few stop to think how very
dependent we are on the sun in all our daily activities. It is the
stored-up energy of the sun, caught and held by the plant, that
is released from the wood we burn to keep us warm. Coal isj
compressed vegetation, the imprisoned sunlight of ages long gone
by, so the heat that glows in our coal stove is really sunlight.
Plants cannot live without sunlight, for its energy is the source
of all their vital activities. It is this energy stored up in the
plant in the form of sugar, starch, and other plant products that
is released when we take these plant foods and burn them in
our bodies, so that really we live on condensed sunshine. Even
the meat we eat is that, too, for the source of animal energy is
that of the plant. The electric light which we turn on in our
homes is sunlight, for the electric current comes from a generator
run by steam that is made by heat which in turn comes from the
coal. Surely the sun is the immediate giver of all good gifts, and
it is quite comprehensible that the savage should see in this life-
giving orb the personification of divine power.
In spite of the fact that the sun does so much for the earth,
warming its surface, providing energy for all life's processes and
4 OUR PHYSICAL WORLD
for all industrial activities, still our little earth receives only a
minute fraction of the power the sun is continually giving off,
for the sun is radiating its energy, light, heat, and chemical
influence in all directions. The earth is only a tiny sphere some
8,000 miles in diameter, nearly 93,000,000 miles from the sun.
It, therefore, is hardly more than a speck compared with the
sphere 186,000,000 miles in diameter which the sun fills with its
energy. In fact, it is estimated that all the planets intercept
only about one hundred millionth part of the sun's flood of power
which is constantly pouring out into space.
Astronomers calculate that the sun gives off every hour as
much radiant energy as would be produced by the burning in that
time of a layer of hard coal 25 feet thick covering its entire surface.
This is equivalent to 140,000 horse-power for every square yard
of the sun's surface. If all the coal in Pennsylvania were mined
and then burned in one second it would not produce as much
energy as the earth receives from the sun in the same time.
Such figures are almost beyond comprehension. It is well-nigh
impossible to form any idea of the temperatures of the sun.
It is believed that the outer radiating portion registers about
10,000° F., while the temperatures of the inner portions probably
range above 50,000°.
But how can the sun remain so hot when it is spending its
energy at such a profligate rate? It seems probable that one
main source of its heat is the constant contraction that occurs
in it. We know that when a body takes up heat it expands. A
familiar example is the expansion of the mercury in the thermom-
eter bulb as it gets hotter, which causes the mercury to rise in
the tube. (See also experiment 94 in the Field and Laboratory
Guide in Physical Nature-Study.) The reverse is also true, that
when a body contracts it liberates heat. The sun is so very large
that it is estimated it need only contract 250 feet in diameter a
year to produce the energy it radiates into space. This is so
slight an amount as to be immeasurable from the earth, except
after the lapse of thousands of years. Quite probably, too, there
THE UNIVERSE IN WHICH WE LIVE 5
are sources of energy in the sun comparable to that of radium,
which we know can give off energy rays for a very long time
with scarcely an appreciable diminution of weight.
The outer layers of the sun, at least, are highly incandescent
gases filled with liquid particles. Possibly the central portions
are liquid. The intense heat makes volcanic activity and
storms exceedingly violent on the sun. Explosions carry flames
FIG. i. — The corona of the sun. Photographed at Matheson, Colorado,
June 8, 1918, by Edison Pettit, of Yerkes Observatory.
out from its surface to a height of 200,000 miles or so, with veloci-
ties as great as 600 miles per second. Indeed, some of the impal-
pable dust and gases seem to be forced up to very much greater
height, and appear as streamers running far out into space.
These are seen well at times of eclipse, when the intense glare
of the sun's surface is hidden behind the moon, and they form
what is known as the corona (Fig. i).
6 OUR PHYSICAL WORLD
Storms are perpetually raging and the furious movements of
the heated gases are seen even at our great distance. Sometimes
the down draft of the cooler outer portions appears to pour
through rifts in the gas mantle so swiftly as to cool off the fiery
interior a bit, and then the throat of the cyclonic movement
seems dark as seen against the brilliant deeper portions, and we
FIG. 2. — Sun spots. Photograph by Miss Mary Calvert, taken with the
12-inch telescope at Yerkes Observatory, August 7, 1917.
designate the object a sun spot (Fig. 2). Such spots are some-
times many thousands of miles in diameter, large enough to
be seen through a smoked glass by the naked eye. They are of
great interest since by watching them it was determined that the
sun rotates on its axis from west to east once in about twenty-five
days, a fact confirmed by other methods of observation. Their
THE UNIVERSE IN WHICH WE LIVE
Saturn
^
Jupiter
FIG. 3. — Diagram to show relative sizes of the planets. Drawn to scale
appearance seems to be coincident with electrical disturbances in
the earth's atmosphere that affect our weather.
Our earth is by no means the largest of the planets, in fact
it is a relatively small one. The equatorial diameters are given
in the following table :
Miles
Mercury 2,765
Venus 7 , 826
Earth 7,9*3
Mars 4,352
Miles
Jupiter 90, 190
Saturn 79 , 470
Uranus 34, 900
Neptune 32 , 900
These relative sizes are shown in the diagram by a series
of circles drawn to scale (Fig. 3.)
8 OUR PHYSICAL WORLD
Since Mercury and Venus are nearer the sun than is the earth,
their orbits are included within that of the earth, and they can
never appear on the opposite side of the earth from the sun, but
are always seen near the sun, either rising just ahead of it,
when they are called " morning stars," or setting shortly after it,
when they are known as " evening stars." Mercury is as bril-
liant as its namesake, the liquid metal familiar in the thermometer
bulb. Its orbit is so small that it is usually obscured by the sun's
intense light, since it can never get far from it. Venus, however,
with its larger orbit, may precede the sun or follow it at greater
distance, and therefore is not commonly obliterated by the glare
of the sun when it is a morning or evening star. Shining as it does
with a silvery sheen, it has ever been a noted object in the sky, and
may even be seen by day when one knows just where to look for it.
Mars glows with a ruddy light. Its blood-red appearance has
always associated it with war. Mars was the war-god. While
the surfaces of Mercury and Venus can never be studied with our
telescopes very satisfactorily because they are so near the sun,
Mars may be seen at times with a round disk, like a full moon;
and since it is our next-door neighbor, distant when nearest to us
only about half as far as the sun, its surface is plainly visible.
What appear to be polar snow caps may be seen, which increase
and decrease in size as the seasons change. Numerous straight
markings radiate from the Pole in various directions, often
intersecting. These have been thought by some astronomers to
indicate a complicated system of canals built by the inhabitants
of the planet to conduct water from the melting polar snows to
irrigate their lands, or possibly since they change color seasonally
they are lines of vegetation along such canals or along areas of
maximum rainfall. Since Mars is much smaller than the earth,
the force of gravity on its surface is only 38 per cent of that on
the earth, so that an object weighing 100 pounds here would
weigh only 38 pounds there. The inhabitants may grow, there-
fore, proportionately larger, and these giants might really dig
such great canals, since the material excavated would be so
THE UNIVERSE IN WHICH WE LIVE 9
much lighter there than here. But this is all mere speculation
even if it is fascinating. We know nothing about such inhab-
itants, or if there really be such.
We know even less about the four outer major planets than
the minor ones, our near neighbors. Jupiter is some 1,300
times as large as the earth, and is probably still in a partly gaseous
condition. Not long ago, astronomically speaking, it was glow-
ing with its own heat, but now has largely cooled. Saturn has
some unique rings about it, composed of myriads of tiny bodies
that whirl about the planet in parallel orbits.
All the planets revolve about the sun in orbits that are practi-
cally circles, that of Neptune being most nearly such. The orbits
are really ellipses, curves with one axis larger than the other.
Such curves may readily be drawn thus: Take a 1 6-inch length
of string and tie the ends together, making a loop. Stick two
pins through the paper into the drawing board, 5 inches apart,
and place the loop over the pins. Set the pencil point within
the loop and hold the loop out taut, so the string forms a triangle
with the pins a.t two corners, the pencil at the third. Now move
the pencil about, as if trying to draw a circle, when an ellipse will
be the result. The shape of the ellipse will vary as the distance
between the pins is altered, or as the length of the loop of string
is changed. The points occupied by the pins are known as the
foci, and in the solar system the sun occupies one focus of each
planetary orbit.
If the diameter of the orbit of Mercury be represented by
a line i inch long, then that of Venus would be approximately
1.9 inches; of the earth, 2.6 inches; of Mars, 3.9 inches; of Jupiter,
13.4 inches; of Saturn, 24.6 inches; of Uranus, 49.5 inches; and
of Neptune, 77.5 inches. In the case of the earth's orbit the differ-
ence between the long and short diameters is about 3,000,000
miles, not a great departure from a circle when it is remembered
the total diameter is sixty times this.
The plane passed through the orbit of the earth is known
as the plane of the ecliptic. If we should think, as did the
IO
OUR PHYSICAL WORLD
ancients, of the earth as floating, half -submerged, on a great
sea and as moving about the sun, also floating, half -submerged,
the surface of this sea would represent the plane of the ecliptic.
The planes of the orbits of the other planets are all nearly in the
plane of the ecliptic, that of Mercury being inclined to it at an
angle of 7°, the others at much smaller angles.
The axis of the earth, the imaginary line on which it seems to
rotate, so producing day and night (see diagram, Fig. 4), does
FIG. 4. — Diagram of the earth in four positions in its orbit about the sun.
a and c the summer and winter solstices; b and d, the equinoxes. The relative
sizes of the sun, the earth, and its orbit are necessarily incorrect.
not stand vertically to this plane of the ecliptic but is inclined to
it at an angle of 23^°. Note that the earth's axis is an imaginary
line. The North Pole is not a real pole sticking up out of the
earth. When Peary stood at the Pole there was nothing to
mark the spot. If he had stood there long enough he would
merely have turned about as one would if standing over the
pivot of a turntable.
It is evident that at position a in the diagram the days are
long and the nights short in latitude 40° in the Northern Hemi-
THE UNIVERSE IN WHICH WE LIVE n
sphere, since such a place is in the illuminated part of the earth
for a longer time than it is in the dark part. Moreover, the sun's
rays strike the earth more nearly vertically in this latitude than
they do in position c, and so they are more powerful. In position a
there is summer in the Northern Hemisphere, for the long days
give the sun time to impart more heat than is lost in the rela-
tively short nights, and the nearly vertical rays are very effective,
losing relatively little of their heat as they pass through the air.
In position c, however, the Northern Hemisphere is having
winter, for the days are short and the nights are long, while the
sun's rays strike the earth obliquely and so glance off readily;
they lose much of their heating effect also, since they must pass
through a long stretch of atmosphere that reflects much of their
heat.
In positions b and d the circle between the dark and the
illuminated sides passes through the North and South poles,
and so the days and nights are of equal length all over the earth.
These points of the earth's orbit are therefore known as the
equinoxes. Points a and c are called respectively the summer
and winter solstice, for the sun appears to cease its northward or
southward journeying and to stand still for a few days before it
begins to move back toward the celestial Equator.
Just as the planets travel in pathways about the sun, so there
are bodies that we call moons, that travel in orbits about the
planets. Our earth has one such, the queen of the night; Mer-
cury and Venus, so far as we know, have none; Mars has two
very small ones, probably not over 10 miles in diameter; Jupiter
has seven, four large ones and three small; Saturn has ten, one
of which is larger than the planet Mercury; Uranus has four and
Neptune, one.
Our moon has been from time immemorial a god or goddess
worshiped by primitive man. The Assyrians adored her as
Ashtaroth; the Egyptians, as Isis; the Greeks named the moon-
goddess Selene, or Phoebe and, later, Artemis, while the Romans
called her Diana or Luna. The ancient Aztecs adored her as
12 OUR PHYSICAL WORLD
Meztli, and regarded her as the wife of the sun-god. The son
of this pair was Inca, their national hero.
Our moon is not very far away as astronomical distances go,
only 238,840 miles. It has a djameter of 2,162 miles. The
moon has no atmosphere and apparently no moisture on its sur-
face. It is quite thoroughly cooled off, and the surface tempera-
tures there are probably 200° below zero, except as the sun's
rays heat it at noonday. Its contour is varied with great plains
that are quite smooth and seem dark, and with mountainous
areas whose numerous peaks reflect the light and so appear
bright, just as the numerous facets of salt or snow crystals reflect
the light and appear white. These patches of light and dark are
arranged so as to suggest the face of the "man in the moon" or
the "woman's face/' according to the way one looks at it.
These imaginary figures have given rise to many fables.
According to the Chinese legend, it is the man in the moon who
ties together with invisible yet unbreakable cords the young
man and maiden who are destined to marry each other. It has
been aptly suggested that he must be the man of the honeymoon.
The moon shines only by reflected light, the sunlight always
illuminating the half turned toward it. When the moon is on
the opposite side of the earth from the sun we see the illuminated
half, and the moon is full. The full moon, therefore, always
rises as the sun is setting. When the sun and moon are on the
same side of the earth and about in line with it, we do not see the
moon at all, for the side turned our way is the dark side.
Between these two positions we see first the new moon, just as a
narrow rim of light, then more and more of the illuminated por-
tion, as the moon proceeds to quarter and on to full. During
this time it is waxing more and more brilliant. Then gradually
it wanes, passing to third quarter and so on till the old moon
disappears.
The period of time occupied by these changes from new
moon to new moon is apparently the original month. The divi-
sion of this into four periods or weeks was likely facilitated by
THE UNIVERSE IN WHICH WE LIVE 13
the easy recognition of the new moon, the first quarter, the full
moon or second quarter, and the third quarter.
The ancients classed the sun and moon as planets, for they,
like the true planets which they knew, Mercury, Venus, Mars,
Jupiter, and Saturn, seemed to move about among the stars.
The names of the seven days of the week were given in honor of
these seven planets. Sunday, Monday, and Saturday are evi-
dently names from sun, moon, and Saturn. The French names
for the other days of the week show plainly their derivation from
the Greek or Latin gods. Mardi is Tuesday; merer edi, Wednes-
day; jeudi, a contraction of Joms dies, is Thursday; and vendredi
is Friday. Our English names have come to us through the
substitution of the corresponding Norse deities, Tyr's or Tiwes'
day, Woden's day, Thor's day, Freya's day. Thus we are
reminded daily of the old myths that were blended with the
early astronomical lore.
The moon exerts two very potent influences on the earth. It
is the chief cause both of the tides and of the precession of the
equinoxes. The sun is only a secondary cause, for, though it is
immeasurably larger, its greater distance makes it play the minor
role. We say that the moon revolves about the earth. As a
matter of fact, earth and moon revolve about a point that is
relatively near the earth's center. It is as if we should balance
on a point a rod with a large and very heavy ball at one end and a
small light one at the other (see Fig. 5, p. 14), then set it to whirling.
The small ball would move about the big one, but still the big
one would travel in a circular path about the balancing point.
So the earth constantly moves straight ahead and at the same
time toward the moon, making a nearly circular path around the
center of gravity of the pair. This path is not jerky as indicated
in the diagram, where first one movement is shown and then the
other, but quite smooth, since both movements occur simultane-
ously. Now, that part of the ocean near the moon moves toward
the moon most rapidly, the solid earth next most rapidly, and the
waters on the side opposite the moon least rapidly, since the pull
14 OUR PHYSICAL WORLD
of gravity varies inversely as the square of the distance. So there
is a heap of water in the ocean under the moon and one on the
opposite side of the earth also. When the earth revolves, the solid
land slips along through the water thus held by the moon, the
water level along the shore rises, and we say the tide is coming in.
As the shore passes out at the other side of the heap the tide falls;
so the tide rises and falls twice a day. When the sun assists the
moon, as it does when the moon is full or new, the tides are
highest. If sun and moon pull against each other, as they do
when the moon is at the quarter, the tides are slight. The
FIG. 5. — Diagram of a portion of the earth's path to show the cause of the tides
amount of the rise or fall of the tide is not great on the open
coasts, but when the tide runs up a narrowing bay the rise and
fall near the head of the bay may be 60 to 70 feet.
The friction of the land sliding along under these heaps of
water slowly retards the rotation of the earth on its axis and in
time must check it. It is supposed that the moon once possessed
oceans, and the tides occasioned by the earth's attraction caused
its rotation to slow down until now its period of rotation is the
same as its time of revolution about the earth, and therefore it
keeps the same face always toward us. Its waters have since
combined with its mineral materials to form the hydrated miner-
als (seep. 58).
THE UNIVERSE IN WHICH WE LIVE 15
The second effect the moon produces on the earth is the pre-
cession of the equinoxes. The sun again plays the minor role.
The earth is a sphere with an added bulge about the equatorial
regions (an oblate spheroid — see Fig. 6 A). The moon's orbit is
inclined to the plane of the ecliptic only 5°, so the sun and moon
are pulling on the earth practically in the same plane." Since
the earth's axis is inclined 23^°, this equatorial bulge is in large
FIG. 6. — 04) Diagram showing the earth's equatorial bulge and its action in
causing the precession of the equinoxes; (B) diagram showing the effect of the
moon's attraction on the motion of the earth.
measure above or below the plane of the ecliptic. Therefore,
the sun and moon tend to pull the bulge back into the plane;
that is, the pull of the moon (and sun) acting on the bulge along
the line a-b is resolved into two forces, one component acting
along b-c. Points along the Equator such as b (Fig. 6B) are
therefore under the stress of two forces, one be, this pull toward
the plane of the ecliptic, the other, the momentum of the earth's
rotation indicated by bd, the resultant being a motion along, say,
be; that is, a point on the Equator at every turn of the earth
16 OUR PHYSICAL WORLD
cuts the ecliptic a trifle before it would if this pull of the moon
were not acting on the bulge.
It is evident, then, that the equinoctial point is ever occurring
a trifle earlier than it would occur if it were not for this action of
moon and sun. This phenomenon is known as the precession
of the equinoxes. As a result, the North Pole of the earth's
axis does not point continually to the same spot in the celestial
sphere, but makes a rotation once in 25,868 years. As a matter
of fact, the motion is not as simple as described, for the moon, sun,
and earth are constantly changing their relative positions, so it
is quite irregular, though entirely predictable when the move-
ments and consequent relations of the three bodies are known.
It is a matter of relatively simple calculation to determine the
point in the sky to which the Pole pointed thousands of years
ago or will point in the future.
Some authorities claim that when the great pyramid at
Cheops, Egypt, was built it was so oriented that a narrow pas-
sageway over 300 feet long pointed to the star that was then the
polestar, alpha of the constellation Draco. This pyramid was
located quite exactly on 30° north latitude. Certain of its
dimensions apparently record the length of the year, the period
of the precession of the equinoxes, and other astronomical data,
so that it really is a record of quite wonderful astronomical
knowledge on the part of its builders.
As one looks up into the starry skies on any clear night, it
seems as if the stars were as numerous as the sand grains on
the seashore. Yet, as a matter of fact, there are only about
2,000 visible to the average eye at any one time. And even if
you should watch the heavens year in and year out from points
both north and south of the Equator, you would see only 4,000
to 6,000. These stars differ in brilliancy from the brightest
one, Sirius, down to those that are just visible to the naked eye.
They are consequently said to differ in magnitude, sixth-
magnitude stars being those that are only just visible, first-
magnitude stars those that are most brilliant. This latter group
THE UNIVERSE IN WHICH WE LIVE 17
has recently been subdivided into three magnitudes, as our
measures of brilliancy have become more exact, namely, stars
of — i magnitude, the most brilliant, those of magnitude o, and
those of magnitude i. A star of magnitude i is 2.5 times as
bright as one of magnitude 2, (2.5)2 or 6.25 times as bright as
one of magnitude 3, etc.
But the stars that are visible to the naked eye are but a
fraction of those that exist. The telescope shows thousands and
thousands that the eye cannot see. Indeed, every time a new
and more powerful telescope is made and pointed to the skies
it shows new stars beyond the range of the old, less powerful
telescopes; so that just how many stars there really are no one
knows. Some 200,000 have already been located and mapped,
and it is estimated that there are at least a half-billion of them
in our stellar system. The Milky Way, which seems like a band
of hazy light crossing the sky, is made of thousands of stars so
numerous and so distant that their radiance blends into a mist
of light. Then beyond the limits of our galaxy of stars, with its
half-billion or more, are possibly many other galaxies, so dis-
tant they seem like mere flecks of hazy light, even when seen in
powerful telescopes. How many such exist astronomers even
do not guess.
Many of these stars are almost inconceivably distant from
our earth. The nearest one, 61 in the constellation of Cygnus
(see p. 29), is so far away that, if we represent the distance from
the earth to the sun by i inch, the distance to this star would be
represented by a line yj miles long. Light traveling at the
enormous rate of 186,300 miles (seven and one-half times around
the earth) in one second, takes three and one-half years to reach
us from this star. Some of the stars are so far away that their
light only reaches the earth after traveling through space for
10,000 years, and that probably is not the limit.
Stars are really suns that in all probability, judging by our
sun, have planets revolving about them. Is it possible they too
are inhabited ? If so, by what sorts of beings ? And many of
1 8 OUR PHYSICAL WORLD
these distant suns we call stars are very much larger than
ours. Betelgeuse in the constellation Orion (p. 33) has been
recently measured and found to have a diameter 300 times that
of our sun, yet it is so far away it is not as brilliant as Sirius,
which, though only thirty times as large as our sun, is but eight
and one-half light-years away and outshines Betelgeuse.
There are only about twenty-five stars in the list of the old
first-magnitude stars, so it is not very difficult to learn to locate
and recognize these. They were all known to the ancients and
came down to us with ancient names.
Undoubtedly the stars served early man as a means of keep-
ing his directions when traveling by night, as they still similarly
serve us. The stars, too, were supposed to mark important
events. Thus, Sirius, the Dog Star, when it received its name, rose
just before the sun, at the time of the year that was intensely hot,
when dogs went mad, and so it appeared as a warning of the
approach of the season that must have had terrors for the early
hunter and shepherd peoples among whom dogs were likely as
abundant and as ill kept as they are today in the East.
Probably, too, important events in the history of the race
were connected with groups of stars, as well as with individual
stars, when such groups were particularly brilliant or in com-
manding positions at the time such events occurred, just as the
birth of Christ was connected with an unusual astronomical
phenomenon, the appearance of the "Star in the East." Many
of the star groups or constellations are still commemorative of
events that once had great historical significance, but the stories
have been so altered by constant repetition, as they have been
told and retold, that they come to us merely as legends, or myths.
Many of these legends have been transmitted from the earlier
primitive peoples through the fervid imaginations of the Greeks,
and so the heavens have come to be "a pictured scroll of Greek
mythology." One needs a large measure of this imaginative
power to see in the star -groups any likeness to the things the
ancients figured in their maps of the sky.
THE UNIVERSE IN WHICH WE LIVE 19
In latitudes such as those of mid-Illinois, Indiana, and Ohio,
or of Washington, B.C., or Denver, Colorado, all in the neigh-
borhood of 40° north latitude, the point directly overhead in
the celestial sphere, the zenith, is evidently 40° north of the
celestial Equator and 50° from the North (celestial) Pole. The
Pole is, in this latitude, about 40° above the horizon. There is,
therefore, a region of 40° around the Pole in which the stars never
set. There will be a broad band of sky running from 50° north
of the celestial Equator to 50° south that will be in part above the
horizon at any one time, and all of which may be seen by continu-
ous observation throughout the year or through any winter night.
It is evident that the constellations seen, say, at midnight
on December 20 are not the same as those visible at the same hour
on June 20, for at the first of these dates the dark or night side
of the earth (Fig. 4, position c) is turned toward one part of
the starry vault, while at the other date it is turned toward the
opposite portion. Since the earth rotates on its axis, a person
at latitude 40° north will see all the stars pass in view that are
located north of 50° south celestial latitude.
Probably the one constellation that everyone knows is the
"Big Dipper," seen in the latitude mentioned at any time of the
night, for it never sets but simply circles about the celestial Pole.
All the stars that make the Dipper are quite bright (see Fig. 7,
p. 20). The Dipper makes up part of the constellation known as
the Great Bear (Ursa Major). It is a curious fact that widely
separated ancient races like the Chaldeans (Abraham, it will be
remembered, came from Ur of the Chaldees) and the American
Indians called this star group by the same name, the Great Bear.
This is true of many constellations. They bear the same name
among Chaldeans, Chinese, Egyptians, Greeks, American Indians,
etc. It seems as if the name of many constellations must have
been given them before the races separated from that region that
was their common home.
The two stars forming the side of the Dipper away from the
handle are commonly called the pointers, for if the line drawn
20
OUR PHYSICAL WORLD
through them is extended toward the Pole it leads to Polaris,
now the polestar, situated not exactly at the North Pole but very
near it. This star is at the end of the handle of the Little Dipper,
which is made up of rather faint stars so that it is visible only on
very clear nights. The Little Dipper is included in the figure
of the Little Bear. The star in the middle of the handle of the
Big Dipper is an interesting double, both stars of which are
FIG. 7.— Ursa Major, the Big Bear
visible to the naked eye. They are named Mizar and Alcor,
the Horse and Rider.
If the curved line made by the stars of the handle of the Big
Dipper be extended for about the length of the Dipper, handle
and all, it leads to a star of the first magnitude, Arcturus, 70°
from the Pole, and this locates Bootes the Hunter, who is follow-
ing the Bears (Fig. 8). North of Arcturus and somewhat to the
east is a kite-shaped figure that is also included in Bootes.
THE UNIVERSE IN WHICH WE LIVE 21
The Greek myth of these constellations is as follows: Cal-
listo was so beautiful that she excited the jealousy of Juno, the
goddess, who changed her into a bear. While wandering in the
woods she met her son Arcos, who was about to strike her with
FIG. 8.— Bootes, the Hunter
V
his spear, when Jupiter in pity snatched both up to the sky, and
there they still are, the Big and Little Bears.
The Fox Indians believed the forest trees wandered about and
gossiped among themselves at night. Once a bear clumsily
bumped against the oak, king of trees, in his wanderings. The
22 OUR PHYSICAL WORLD
king, in anger, seized the bear by his short tail and so threw him
into the sky, stretching his tail in the process. Hence this bear
now has a long tail, an appendage quite foreign to his kind. In
the earlier star maps, the bear is figured without a tail, but in
later maps both Big and Little Bears possess tails.
Ursa Major is also figured, especially in England, as a wagon,
Charlemagne's cart or Charles's wain, and Bootes is then the
wagoner. Since the wagon turns about the polestar like the
hand of a clock on a great dial, its position was an index of time
to those familiar with it. So Shakespeare makes the Carrier say
in Act II, Scene i, of Henry IV: " Heigh-ho! An't be not four
by the day, I'll be hanged; Charles' wain is over the new chimney
and yet our horses not packed."
The constellation of the Little Bear was also known to the
ancients as "Transmountain," " beyond the mountain," for they
believed that the earth rested on the " mountain of the north" and
that beyond it the gods had their habitation. This idea is evi-
dent in such biblical passages as Isa. 14: 13 and Ps. 48:2. The
polestar in Transmountain is probably the most famous single
star in the sky. Shakespeare in Act III, Scene i, of Julius
Caesar makes Caesar say:
But I am constant as the northern star,
Of whose true-fixed and resting quality
There is no fellow in the firmament.
The skies are painted with unnumbered sparks;
They are all fire, and every one doth shine:
But there's but one in all doth hold his place.
On the opposite side of the Pole from the Big Dipper is a
group of fairly brilliant stars forming an open W or M that readily
serves to locate the constellation Cassiopeia (Fig. 9). Together
with one rather dim star the letter makes the figure of a chair and
is known as Cassiopeia's Chair, and on it the unfortunate queen
is seated in the ancient star charts. The other dramatis per-
THE UNIVERSE IN WHICH WE LIVE 23
sonae of this legend are also close about the Pole, and we may
use the stars of the Chair to find them.
FIG. 9. — Cassiopeia
Key to star magnitudes in this and succeeding figures, except Fig. n
A line drawn through alpha and beta Cassiopeia, which stars
form the ends of the legs of the Chair (see Fig. 10, p. 24) and
extended about once and a half the length of the whole Chair,
OUR PHYSICAL WORLD
leads to the star alpha of Cepheus, the King (Fig. n). Directly
in line from this toward Polaris is another star of the figure, so
that these stars may be used as pointers to the Pole quite as
well as the two of the Big Dipper. A third fairly bright star
of Cepheus may be located from the sketch.
FIG. 10. — The chief constellations about the Pole and the pointers to be used
in finding them.
Carry a line through the lower two stars of the Chair back, in
the opposite direction from Cepheus and about as far from Cas-
siopeia as Cepheus is, and there is seen a conspicuous star alpha
in Perseus, the hero of the tale. This same star is found by
passing a line through the one in the tip of the Dipper and the
THE UNIVERSE IN WHICH WE LIVE 25
basal star near the handle; alpha Perseus is about as far from
the Pole as is the latter star. Almost in line with alpha
Perseus but farther away from the Pole is another bright star
of Perseus, Algol.
Now alpha Cassiopeia, alpha Perseus, and a fairly bright
star of Andromeda make an equilateral triangle; so Andromeda
FIG. ii. — Cepheus
can be located. The other moderately bright stars of this
constellation are seen in Figure 12 (p. 26). If the line through the
pointers of the Dipper be carried beyond Polaris about twice as
far from the latter as are the pointers, it leads to a large figure of
four bright stars known as the square of Pegasus (Fig. 13,
p. 27). One of them is really in the figure of Andromeda.
The villain of the piece is the Dragon (Fig. 14, p. 28). Draw
a line from the basal star in the back of Cassiopeia's Chair through
26 OUR PHYSICAL WORLD
that polestar pointer in Cepheus nearest the Pole, and carry it
beyond Cepheus about as far as Cassiopeia is from Cepheus and
it reaches the head of the Dragon marked by four stars, the two
bright ones being the eyes. This same Dragon's Head is found
also by carrying a line through the two stars on the opposite
FIG. 12. — Andromeda
side of the bowl of the Big Dipper from the polar pointers.
The body of the Dragon is traced in a curving line of fainter
stars that lie between the Big and Little Bears. If the plani-
sphere (see Figs. 194 and 195 inserted at end of this volume) is
constructed and used, it will help locate these and the other
constellations.
THE UNIVERSE IN WHICH WE LIVE
27
Cepheus and Cassiopeia were the king and queen of Ethiopia.
The queen was very beautiful but also very vain — so much so
that she had the temerity to compare herself to the sea nymphs.
This so enraged them that they sent a monster of frightful mien
to ravage the coasts of the kingdom. The king and queen were
informed by the oracle that the only way to stop its awful visita-
tions was to chain their daughter Andromeda to the rocks and
FIG. 13. — Pegasus
allow the monster to have her; so Andromeda was prepared
for the sacrifice. Perseus was just then returning on his famous
charger Pegasus from his great adventure, during which he slew
the Gorgon and brought back its head. He saw the beautiful
Andromeda chained to the rock, slew the dragon, and so won her
for his bride.
If the same two stars in the back of Cassiopeia's Chair that
were used to point to Perseus be again used as pointers, but the
28 OUR PHYSICAL WORLD
line be extended through them in the opposite direction from
Perseus and about as far again from Cassiopeia as is Perseus, a
very bright star is encountered, Deneb, in the constellation of
Cygnus, the Swan (Fig 15). The chief stars of this constellation
are shown in the figure, and it is to be noted that some of them
are arranged in the form of a cross, so that the group is sometimes
known as the Northern Cross. Cygnus, or more correctly,
FIG. 14. — Draco, the Dragon
Cyncnus, the Swan, is the son of Mars and the most intimate
friend of Phaethon. Phaethon was the son of Phoebus, who
drove the chariot of the sun. He persuaded his father to let
him drive for one day. The steeds, feeling the strange driver,
ran away, bringing the sun so close to the earth as to scorch it.
Jove struck Phaethon with a thunderbolt, and he fell into the
river Eridanus (p. 40). Cygnus lingered at the spot, repeatedly
plunging beneath the flood to seek some relic of his lost com-
THE UNIVERSE IN WHICH WE LIVE 29
panion. The gods, angered, changed him to a swan that nightly
plunges into the sea.
The line drawn through the two stars of the bowl of the Dipper
opposite the polestar pointers not only reaches the head of the
Dragon, but if extended a bit farther reaches a first-magnitude
star, Vega, in the constellation of the Lyre. The polestar,
Deneb, and Vega mark the corners of a right triangle. The posi-
FIG. 15. — Cygnus, the Swan
tion of alpha in the constellation of the Dragon with reference
to this triangle of Polaris, Deneb, and Vega is shown in the dia-
gram. If now this line through alpha Draco and Polaris is
extended on the other side of the Pole about the same distance
from the Pole as is the Dragon's Head, another first-magnitude start,
is seen, Capella of the Charioteer (Fig. 16, p. 30). Auriga is the
Charioteer, who carried the goat, Capella, and the kids or Haedi
in his arms. It was the goat that suckled the infant Jupiter.
Having broken off one of his horns in play, Jupiter endowed it
30 OUR PHYSICAL WORLD
with the power of being filled with whatever its possessor might
desire, whence it was called the horn of plenty or cornucopia.
These kids were supposed to be a very unpropitious sign.
Tempt not the winds, forewarned of dangers nigh,
When the Kids glitter in the western sky.
— CALLIMACHUS, third century, B.C.
FIG. 16. — Auriga, the Charioteer
About 9:30 Christmas night or 7:30 a month later, the
Pleiades are on the meridian not far north of the zenith. This
group of stars is likely as well known as the Big Dipper. There
are six or seven stars visible to the naked eye, grouped somewhat
as a Little Dipper. Six stars are plainly visible, the seventh
only to very good eyes. The one dim star was long ago very
much brighter, so the cluster is also known as the " Seven
THE UNIVERSE IN WHICH WE LIVE 31
Sisters." The lost one, "Electra," is supposed to have run off
to the Great Bear and is now Alcor. Many more are visible
with a telescope and they are then seen to be enveloped in a great
nebula so they "glitter like a swarm of fireflies tangled in a silver
braid" (Tennyson). Onondaga Indians have a legend that the
Pleiades were a group of happy children skipping off into the sky
and having such a good time that they never came back. The
Greek legend makes them the daughters of Atlas, all very beauti-
ful. Jupiter assumed the disguise of a bull, Taurus, in order to
carry away Europa, whom he considered the most beautiful,
from her sisters when they were playing in the meadows.
Alcyone or Halcyone is the brightest star of the group. It
used to be thought that the kingfisher, Halcyone, nested about
the time this star culminated at the time of the winter solstice.
Ceyx, king of Thessaly and husband of Halycone, was drowned.
She, seeing his body floating, repeatedly rushed into the sea to
save him. Then the gods changed them both to halcyon birds, and
they go skimming across the waters and rushing into it always.
The Pleiades lie on the neck of Taurus, the Bull (Fig. 17, p. 32).
The head of the animal is indicated by a V-shaped figure to the
southeast of the Pleiades. One star of this group, Aldebaran,
is a first-magnitude star and is one eye of the Bull. The V-shaped
group forming the tips of horns, eyes, and the tip of the nose is
known as the Hyades. The Roman year began in March
when Taurus was just visible above the eastern horizon. Hence
Virgil's line: "The white bull opens with his golden horns the
year." Only the head and shoulders of the Bull are pictured as
visible in the old star maps, for his body is supposed to be sub-
merged in the sea, in which he is swimming to make his escape
with Europa after capturing her in the meadows near the shore.
A line run through the Pleiades and Aldebaran and still
farther to the south and east reaches three bright stars in line,
the belt of Orion, the Hunter. This constellation of Orion is the
most brilliant in the sky. To the south of the belt is a first-
magnitude star, Rigel, and to the north one that has a reddish
32 OUR PHYSICAL WORLD
cast, Betelgeuse. Two other stars about as bright as those of the
belt lie, one near Betelgeuse, and one near Rigel. There are also
several fainter stars in the figure, three of which in line near the
belt make a portion of the Hunter's dagger. The middle one
of this trio is imbedded in nebulous matter, the great nebula of
FIG. 17.— Taurus, the Bull
Orion. Orion, the Hunter, stands with club upraised about to
strike Taurus in an attempt to rescue Europa (Fig. 18).
Orion's father, according to another Greek legend, was an old
man and childless, a hunter by trade. One day three strangers
came to his hut, whom he entertained right royally. On leaving
they asked him what thing he most wanted. He replied "a
son." Jove, who was one of the strangers, granted his wish,
THE UNIVERSE IN WHICH WE LIVE
33
and when the boy was born he was named Orion. When grown
he became a mighty hunter, so tall he could wade the sea. He
found some beautiful girls playing ball one time, and ran after
FIG. 1 8. — Orion and his dogs, Canis Major and Canis Minor
them. The girls, exhausted, were changed to birds by the gods
and later into stars, the Pleiades. Later he met Diana, the
hunting goddess, and fell in love with her. Her brother Apollo,
fearing she would consent to marry him, seeing Orion approach
over the ocean, merely a black speck in the distance, challenged
34 OUR PHYSICAL WORLD
Diana to try her skill with her arrow and see if she could hit the
tiny thing. This she did, and when she found out that she had
killed Orion she was so much grieved that she gave him immortal-
ity among the stars and made him outshine all his rivals.
To the east of Orion (below in the winter evenings) is the
most brilliant star in the sky, Sirius, in the constellation of
FIG. 19. — Gemini, the Twins
the Great Dog, Canis Major. Sirius, Betelgeuse, and Procyon, a
first-magnitude star in the Lesser Dog, form an isosceles triangle,
Procyon forming the northern apex. The Great and Lesser
Dogs are following the Hunter, Orion.
Another larger triangle with Sirius and Aldebaran at the basal
corners has a bright star, Pollux, one of a pair, at its northern
THE UNIVERSE IN WHICH WE LIVE 35
apex. The other star is Castor, and the two mark the constella-
tion of Gemini, the Twins (Fig. 19). Another pair, similarly
spaced but farther south, are also in this constellation of Gemini.
Castor and Pollux were the sons of Leda, and Helen of Trojan
fame was their sister. "They accompanied the Argonautic
expedition, and, when on the return voyage the vessel was almost
overwhelmed in the storm, Orpheus with his lyre invoked Apollo,
who caused the two stars to appear on the heads of the twins and
so the tempest was allayed." So these stars became the protec-
tive portents of sea-going men as the gods Castor and Pollux were
the tutelary gods of sailors. Altars were erected to them in all
important seaports, and often a vessel carried as a figurehead on
her prow the symbol of Castor and Pollux, as did the ship in
which Paul sailed for Rome. St. Helen's fire or St. Elmo's, a
single flame on the mast head or spar, is an evil sign, but twin
flames are the sign of the presence of these gods and are propi-
tious. "By Gemini" was a favorite oath among seafaring folk,
and it still persists, modified to "by Jiminy."
Both Taurus and Gemini are among those constellations
known as the zodiacal constellations, which were exceedingly
important to the old astrologers in forecasting the future. The
zodiac is a girdle of constellations stretching around the celestial
sphere, among which the sun and the planets when seen from the
earth seem to wander. These constellations named in order in a
bit of doggerel are as follows:
The Ram, the Bull, the Heavenly Twins,
And next the Grab the Lion shines,
The Virgin and the Scales,
The Scorpion, Archer, and the Goat,
Water-Bearer and Fish with tails.
Draw a line from Aldebaran through the Pleiades, and
extend it beyond them half as far again as Aldebaran is dis-
tant from them, and it leads to two third-magnitude stars
near which, toward the Pole, are three third-magnitude stars
36 OUR PHYSICAL WORLD
forming a triangle (Triangulum) . The former two are in Aries,
a constellation in which alpha is a conspicuously bright star
(Fig. 20).
Cancer, the Crab, is made up of inconspicuous stars, but Leo,
the Lion, is easily recognized. When Castor and Pollux are on
the meridian about the middle of March, there is a sickle-shaped
group of stars to the east of them and about a third of the way
to the horizon that marks the Lion. Regulus, the brightest star
FIG. 20. — Aries, the Ram
of the constellation, is at the end of the handle of the sickle.
Castor, Sinus, and Regulus make a triangle-shaped figure (see
Fig. 21) that is the counterpart of the triangle formed by Castor,
Sirius, and Aldebaran. Regulus is also at one corner of a nearby
isosceles triangle with one of the pointers of the Dipper and
Denebola, another bright star of Leo, at the other corners. Leo
represents the Nemean lion, the fight with which formed the
first of the celestial labors of Hercules (Fig. 22).
Still later in the spring, about 8 : 30 P.M. in the last of April,
when the pointers of the Dipper are on the meridian, the next
THE UNIVERSE IN WHICH WE LIVE
37
FIG. 21. — Diagram to show the method of finding some zodiacal constellations
and their relation to Ursa Major. Face north and hold it above your head.
FIG. 22. — Leo, the Lion
38 OUR PHYSICAL WORLD
zodiacal constellation, the Virgin (Fig. 23), is readily located by
its first-magnitude star Spica. This star, Denebola, and Arcturus
form an equilateral triangle. Arcturus we have learned to find
by extending the handle of the Dipper.
FIG. 23. — Virgo, the Virgin
Libra, the Scales, is 'a group of low-magnitude stars that
originally formed a part of the Scorpion, the next constellation
in the zodiac that is marked by a first-magnitude star, Antares.
It is still low down in the southeast at 9:00 P.M. the last of May.
A line drawn through the pointer in the Dipper that is farthest
from the Pole and the star in the outer end of the handle leads
THE UNIVERSE IN WHICH WE LIVE 39
across the sky to Antares. It has a third-magnitude star close
on either side of it; a line drawn through these leads to two
second-magnitude stars, one on either side, that are about as
far from Antares as the length of the Dipper handle. These also
are in the Scorpion.
Vega in Lyra is about on the meridian at 10:00 P.M. the first
of August. A line drawn through the polestar and Vega and
continued nearly to the southern horizon leads to an irregular
group of third-magnitude stars, conspicuous only because there
are nine of them in a nearly horizontal group. These are in
Sagittarius, the Archer. The Goat and the Water-Bearer are
not marked by any conspicuous stars.
In the evenings of mid-September, when the square of
Pegasus is a conspicuous object in the sky, to the east of the
meridian there is an irregular V of dim stars on the side of
Andromeda away from the Pole. The point of the V is well
down, then, toward the horizon, the arms rising so as almost to
inclose one end of the square. This line is the constellation of
the Fishes, two of them caught on the ends of one line.
As suggested above, these zodiacal constellations were con-
sidered of great importance by the old astrologers. At the exact
time of a person's birth the heavens were divided by these sages
into twelve houses by great circles passing through the zenith
and nadir of the place of his birth. These houses beginning
in the east and passing around to the north, then west, were:
(i) life and health; (2) riches; (3) kindred; (4) inheritances;
(5) children; (6) sickness; (7) marriage; (8) death; (9) journeys;
(10) honor; (n) friends; (12) enemies. The position of the
planets and of the twelve signs of the zodiac in these houses
determined the forecast of the person's nativity. Certain
planets were fortunate, such as Jupiter, the sun, Venus, Mer-
cury. The others, Saturn, Mars, the moon, were unfortunate.
Thus Jupiter in the first house at one's birth meant long life and
excellent health. Each zodiacal sign was connected with certain
personal characteristics. Our language bears evidence still of
40 OUR PHYSICAL WORLD
the prevalence of such early notions. One is capricious if
Capricornus was in the ascendant house at birth, saturnian when
the baneful influence of this planet was potent at his birth.
The whole process of forecasting one's nativity or of predicting
what might be expected on a particular voyage or in special
circumstances was a complicated one; enough has been sug-
gested to give some simple notions of the basis on which the
astrologer proceeded. The books of the astrologers make strange
reading now.
There is one other procession of constellations that passes
in review in the southern skies that is exceedingly interesting,
because it seems to be a memorial of a great disaster, the flood,
the legend of which has come down to us in many literatures
also.
At 6: co P.M. in the middle of January there is a brilliant
first-magnitude star in the southwestern horizon, Formalhaut, in
the constellation of the Southern Fish. A line drawn down
through the two westernmost stars of the square of Pegasus leads
a little to the west of this star. West of this line and not quite
halfway from the square to Formalhaut is a line of three close-
set, third-magnitude stars that mark Aquarius, the Water-Bearer.
The ancient figures show Aquarius pouring a flood out into the
mouth of the great fish. In the same region are the Whale, the
Dolphin, and the other fish, already located. In the eastern sky
is to be seen the river Eridanus. It is an irregular line of stars
ending near Rigel and including all the plainly visible stars in
the southeastern sky at this time.
When Sirius is nearing the western horizon in the evenings of
spring, say at 8 : oo in the middle of April, there lies close to the
horizon, stretching from a point south of Cam's Major over past
the meridian, a string of dim stars with a few of the third magni-
tude that mark the figure of a ship or ark, if much imagination is
used, the constellation Argo Navis.
In the southeastern sky, symmetrically placed with respect
to the meridian with Sirius is a close crescentic group of five stars,
THE UNIVERSE IN WHICH WE LIVE
two quite bright, the constellation of the Raven, Corvus. Stream-
ing back from Corvus over toward Procyon is a line of dim stars
Acjuarlus,ihe \Vkterbearer
Argo Navalw, the Ship Corvus, -the Crow
Cetus,the Whale ~ Centaur
FIG. 24. — A group of southern constellations
42 OUR PHYSICAL WORLD
forming the monster, Hydra, on the back of which the Raven is
supposed to stand.
When Corvus is on the meridian about 9:00 P.M. on May 10
or thereabouts, the southeastern sky displays a cluster of five
third-magnitude stars and farther east two of second magnitude,
all in the constellation of the Centaur. When Arcturus is on the
meridian not far from the zenith at 9:00 P.M. in the middle of
June, the two bright stars in the Centaur (second magnitude)
are one on either side of the meridian close to the southern
horizon. This figure, designated the Centaur with his bow and
arrow in hand in the Greek star lore, was represented in the
earlier astrological charts as Noah placing a sacrifice on the
altar, while overhead stretched the bow of promise.
This whole series of constellations, the flood poured out by the
gigantic Water-Bearer, the numerous sea beasts, the mighty
river, the Ark, the Raven that was released to find dry land, and
finally the commanding figure of Noah, sacrificing on his altar
with the rainbow near at hand — all these emblazon on the
southern sky the interesting flood legend that runs continuously
during the evenings from January to midsummer.
CHAPTER II
THE EARTH'S ROCK FOUNDATIONS
Sermons in stones and good in everything. — SHAKESPEARE, As You
Like It.
Has it ever been your good fortune to be possessed with a
mania for collecting? It matters little what the material is,
whether butterflies, beetles, stamps, coins, shells, or minerals,
the young collector generates a degree of enthusiasm for his pet
hobby that stimulates endeavor, carries him through volumes of
learned scientific discussion, sends him to geographies, encyclo-
pedias, and histories for concentrated study that no school course
arouses, makes him a purposeful correspondent, and frequently
leads him to books on travel, or the biographies of great explorers
with an appreciative fellow-feeling that leaves an indelible
imprint on impressionable youth. The writer recalls to this day
the delights of a boyhood spent among the rocky hills of northern
Michigan. It is a mining region whence came great quantities of
the world's best iron and copper, with some silver, gold, and
other mineral products. Many a Saturday or holiday was
occupied in wandering with hammer and specimen bag over the
rock dumps at the mine, or in rambling over the hills in search
of new finds. And what thrills came when some new speci-
men was found to add to the cabinet! Of course there were
chums who were also enthusiasts. I recall how Charlie and
I had for months cast longing eyes on a "vug" or pocket
in a great quartz vein that went zigzagging down the face
of a rock wall in one of the open-pit mines. We knew that
such a place was likely to yield some fine quartz crystals. But
it was 50 feet down the precipitous side to the pocket, and
another sheer drop of as much more to the bottom. Finally, one
43
44 OUR PHYSICAL WORLD
Saturday we ventured. Twisted strands of clothesline, purloined
for the occasion from our respective back yards, were fastened
securely to the root of an old but sturdy stump above the vug,
and by its aid we scrambled down. The wall curved at the spot
we wanted to reach, however, and we were forced to swing in,
dangling on the rope, until our feet caught a rocky shelf just
below the pocket. Precariously perched there, we dug out
handfuls of soft, slimy hematite, bushels of it, it seems to me as
I recall it now, until at last we began to feel in the lumps the loose
crystals. These went into the collecting bag until it was well
loaded. When the vug was emptied of all its treasure, the bag
was lowered to the bottom of the pit. We slid and climbed down,
then made our way with our load up the steep surface of a pile
of waste rock with which the abandoned old mine was being
filled. I still have the most perfect of these clear crystals and
cannot bear to part with it for recollection's sake.
It is a splendid thing if some one of these enthusiasms of boy-
hood days carries over into adult life, an avocation to relieve the
strain of the serious life- vocation, if it does not lead, with added
years, to the vocation itself. A great lawyer of national reputa-
tion is known in scientific circles as an enthusiastic collector and
an authority on snails of the Middle West. A physician of my
acquaintance follows as his hobby, still, a boyhood delight in
collecting wasps, and' is a recognized specialist in this group.
A business man has camped during his vacations these many
years on the famous fossil-collecting grounds of many states,
adding each time to his splendid collection.
If the work in elementary science could impart to every boy
and girl a sufficient interest in minerals and rocks so that they
would, for a while at least, collect with enthusiasm, it would be
eminently worth while; for the pupils might get, then, some
glimpse of the wonderful history of our earth and the marvelous
processes that have been at work to make the rock-ribbed hills,
some appreciation of the striking series of episodes in the history
of even the commonest pebble.
THE EARTH'S ROCK FOUNDATIONS 45
Dig down into the soil anywhere and you finally come to the
solid rock on which the soil always rests (Fig. 25, p. 46). Some-
times the soil is only a thin cover for the rock that lies close to
the surface; again it may be a thick blanket. In the hill or
mountain regions the bare rock may cover miles of area with
scarcely a vestige of soil upon it. Bore down into the rock as
deeply as man has been able, or sink a mine shaft, and the going
is all the way through solid rock. True, our deepest mines and
borings explore only the outer part of the earth, penetrating but
a little over a mile. The J. H. Lake well at Fairmont, West
Virginia, is 7,579 feet deep. But they tell us that this outer por-
tion is all made of just such rock as we find somewhere at the
surface. Such rock may be a great mass of a single mineral or
it may be composed of grains or crystals of minerals all firmly
pressed together.
By a mineral we mean any inorganic substance composed
throughout of one definite or nearly definite chemical substance.
Limestone, the common bed rock of the Chicago area, is a rock,
and the mineral of which it is made is called calcite, which is
chemically a carbonate of lime. Sandstone, another widespread
rock, is made of grains of the mineral quartz, cemented together
with more or less lime. Granite, on the other hand, is made up of
bits of several minerals, quartz and feldspar certainly, and fre-
quently others as well, all making the solid rock.
Most minerals occur as solids, though a few are liquids in
a state of nature. Thus mercury may occur in drops, sulphur
in pools or even lakes of the molten mineral. If the term,
mineral, as defined is taken in its broadest sense it will include
certain gases like nitrogen, oxygen, and steam, but here it is
used with its more customary meaning.
A few minerals are chemical elements; that is, they cannot by
ordinary means be broken up into simpler things. Such, for
instance, are certain metals like iron, copper, gold, silver. These
occur in the rocks at times in grains or even in good-sized chunks
of pure or native metal. Masses of such native copper were highly
46
PHYSICAL WORLD
FIG. 25.— Soil underlain by rock
THE EARTH'S ROCK FOUNDATIONS 47
prized by the Indians and Eskimos, for from such they laboriously
cut off bits that could be hammered into arrow- and spearheads
or even shaped to make crude knives. The copper ore of the
famous Calumet and Hecla mine in upper Michigan is a rock
in which the native copper occurs in grains or threads. Gold
ordinarily occurs as grains, flakes, or strings in the quartz veins,
from which it is separated by crushing and washing.
Some of the non-metals occur similarly in the free state as
elements. Sulphur is a good example. In volcanic regions,
especially, great deposits of it are found. In some of the coastal
regions bordering the Gulf of Mexico it is very abundant as
grains or small masses in the deeper layers of the sand. Live
steam is forced down to melt it, and the melted sulphur comes to
the surface through pipes sunk for the purpose. Until these
deposits were discovered and a method of working them perfected,
the United States imported its sulphur largely from Italy and
Sicily. Now we export sulphur in quantity.
But for the most part the minerals are compounds; that is,
they consist of two or more elements united in a chemical com-
pound. Thus, while silicon is, next to oxygen, the most abundant
element in the earth's crust (making, it is estimated, one-fourth
of it), yet the element occurs nowhere in the earth free, but it is
united with other elements. Combined with oxygen it makes
silica or quartz, SiO2, one of the most widely distributed of
minerals. It is found as an element in many of the other com-
mon minerals which are complex silicates, as will be seen below.
Calcite, the mineral from which vast deposits of limestone and
marble are formed, is a compound of calcium, carbon, and oxygen,
CaC03.
While the mineralogist knows hundreds of minerals, most of
them are rare, and, fortunately for the beginning student, those
that occur as essential constituents of the common rocks are not
many, and they are, moreover, distinguished with comparative
ease. Before describing these it will be necessary to review some
of the important characters that serve as distinguishing features
48 OUR PHYSICAL WORLD
and to understand some descriptive terms. It would be well
for the reader- to obtain from some dealer in minerals and rocks
a collection of those described in the following pages in order
that he may have in hand a specimen to observe as he reads
the description.
In solid form, minerals may be crystalline or non-crystalline.
In the latter case they are described as amorphous, the terms
amorphous and non-crystalline being synonymous. The forms
of the crystals of any specific mineral are always constant. Thus
FIG. 26.— Crystals
quartz crystals are always six-sided prisms with a six-sided
pyramid on each end if the crystal is perfect. Hematite crystal-
lizes in cubes, pyrite in cubes, octohedra, or duodecahedra. The
very definite form of the crystals, if the mineral is crystalline,
is one means of distinguishing it (Fig. 26).
Many minerals break along definite planes so that the frag-
ments are bounded by smooth surfaces that meet always at the
same angle. This property is called cleavage. Thus galena
always cleaves into cubes, calcite and feldspar into rhombs,
though the angle between the faces of the rhombs are different
in the two cases. Mica cleaves into thin plates and asbestos
THE EARTH'S ROCK FOUNDATIONS
49
into needles or threads. Cleavage, then, is another physical
feature that aids in the determination of minerals (Fig. 27).
Certain minerals break in a characteristic way other than
along cleavage planes. The mineral is then said to possess a
peculiar fracture. Thus flint breaks with a conchoidal fracture,
the surface of the break being either concave or convex like a
clam shell.
FIG. 27. — Feldspar, to show cleavage
The fresh surface of many minerals so reflects the light as to
give it a peculiar luster. Thus quartz usually has a vitreous or
glassy luster, galena a metallic luster, selenite a pearly luster,
chalcedony a waxy luster.
Then many minerals when scratched or, better still, when
rubbed on a piece of unglazed white porcelain yield a streak that
is peculiar. In this manner hematite is distinguished from
limonite, which it often resembles, for the former yields a red
streak, the latter a yellowish-brown one.
50 OUR PHYSICAL WORLD
Finally, the hardness of the mineral is an important aid in
its determination. So important is this that a very definite scale
of hardness has been arranged, running from the very soft
minerals with a hardness of "one" to the diamond with a hard-
ness of "ten." This scale is as follows: talc with a hardness of i ;
gypsum, 2 ; calcite, 3 ; fluorite, 4 ; apatite, 5 ; orthoclase feldspar, 6 ;
quartz, 7; topaz, 8; corundum, 9; diamond, 10.
Minerals may be classed from the point of view of rock forma-
tion into essential and accessory. Quartz and orthoclase feldspar
are essential ingredients of granite. A rock would not be named
a granite unless composed largely of these two minerals. Other
minerals, such as mica, hornblende, etc., may be present in rela-
tively small quantity and the rock still be a granite. Such are
the accessory minerals. Essential minerals are those the pres-
ence of which determines the name of the rock. Accessory
minerals are those that may be present but need not be so neces-
sarily. The chief minerals that play essential roles are quartz,
calcite, the feldspars, mica, amphibole, pyroxene, dolomite,
serpentine, kaolin. These are not always essential; they may
at times be accessory. The accessory minerals are much more
numerous. Only a few of the more important can be mentioned,
such as magnetite, hematite, limonite, pyrite, chlorite, olivine.
Then there is a large group of minerals which are important
primarily as ores of the metals used so largely in industry.
Some of these, as already indicated, are accessory ingredients of
rocks. There are magnetite, hematite, limonite, oxides of iron;
pyrite, a sulphide of iron; siderite, a carbonate of iron; chal-
copyrite and bornite, copper iron sulphides; azurite and mala-
chite, copper carbonates; galena, lead sulphide; sphalerite or
"blackjack," a sulphide of zinc; cassiterite, an oxide of tin;
cinnabar, mercury sulphide; pyrolusite, an oxide of manganese
(Fig. 28).
Many other minerals are commercially valuable as sources of
chemicals needed in industry. Such are halite or rock salt;
borax, a borate of sodium; saltpeter, a nitrate of potash; soda
THE EARTH'S ROCK FOUNDATIONS 51
niter, a nitrate of soda; gypsum used as a fertilizer and in making
plaster of Paris; sulphur; and corundum, which is so hard it is
used in making grinding disks.
Mention should be made also of the very beautiful and rare
minerals that are used as gems. The diamond is crystallized
carbon. The sapphire and ruby are pellucid varieties of corun-
dum. Emerald and aquamarine are lustrous forms of beryl, a
silicate of berylum and aluminium. Topaz is a fluosilicate of
FIG. 28. — A zinc mine
aluminium. Garnet is also a silicate and the different varieties
vary in the metals present: lime, aluminium, iron, soda, chro-
mium, etc. Turquoise is a phosphate of aluminium.
Of all the minerals quartz is the most abundant in the rocks
at the earth's surface. Sand consists largely of grains of quartz
more or less rounded by water action. Sandstone, which is the
prevalent surface rock over wide areas and is extensively used
as a building stone, is simply sand cemented together to form
rock, and so is quartz in great measure. Quartzite is another
common rock made of quartz. It is really sandstone modified
52 OUR PHYSICAL WORLD
by heat and pressure so that the individual quartz grains are
fused together. Quartz veins occur in many rocks. When the
rock cracks under the terrific strains of crust movements, wide
fissures open that run for many miles in length and extend deep
into the earth. Such fissures are often later rilled with quartz
deposited from water. Such seams of quartz are known as
veins. Then again quartz is a very common constituent of
many rocks like granite, diorite, etc.
Pure quartz in the amorphous or uncrystallized state is a
milky-white rock that is so hard it cannot be scratched with a
knife blade. When you attempt to scratch it, the steel rubs off
on to the quartz, leaving a metallic streak. Quartz scratches
glass easily. Quartz breaks with a conchoidal fracture, and the
freshly broken surface has a glassy sheen, or, as the mineralogist
says, a vitreous luster. Quartz is so hard it is little subject to
the wear and tear of the elements. Heat and cold, rain and frost,
have little effect upon it, so that quartz veins usually stand out
of the rock in which they occur since the rock containing them is
likely to weather more readily than the quartz. Quartzite hills
are likely to be rugged for the same reason, the contours being
angular, the slopes precipitous.
Ultimately, of course, even resistent quartz is broken up under
the incessant attacks of the elements. It will crack as it is
alternately heated intensely by the mid-day sun and suddenly
cooled by the rain or the low temperature of night. Water
accumulating in the tiny cracks changes to ice in winter and in
changing expands, heaving the quartz apart and widening the
crevices. Thus even quartz breaks in time into angular frag-
ments. The pelting rain, acting through countless centuries,
will wear away the angular edges, rounding off the fragments.
The smaller pieces may be washed down the slopes into the
streams, rolled along by the spring freshets, and ground against
each other until they are worn down to rounded pebbles. In
time they may be carried to the lake or sea and further pulverized
by wave action until the quartz block is transformed into sand.
THE EARTH'S ROCK FOUNDATIONS 53
So, too, when such a rock as granite disintegrates under constant
weathering, the angular quartz grains wear down much less
readily than the other minerals. But still in time they are
rounded by water action and reduced to sand. Sand, the
grains of which are still angular and sharp edged, is called torpedo
sand.
Pure quartz, when crystallized, forms transparent crystals in
the form of six-sided prisms with a six-sided pyramid on each
end. Such crystals, because quartz is so nearly indestructible,
are much used for spectacle lenses and for lenses in optical instru-
ments such as microscopes. The crystals are very likely in
nature to form on a surface, the prisms standing up on end
capped with a pyramid at the free end but lacking the pyramid
at the base (Fig. 26, p. 48, right end).
While quartz does not dissolve readily in ordinary water, it
does dissolve with comparative ease in water that is charged with
carbon dioxide, especially if the water is hot and under pressure.
Now carbon dioxide results from the decomposition of organic
material. Soils usually contain a great deal of it, especially in
marshes and forests where much decaying plant and animal
material lies on or in the ground. Rain falling on the ground
percolates through it and absorbs much carbon dioxide as it goes.
If this water then finds its way down into the rock layers, running
through their cracks and crevices, and so sinks into the rock of
the earth's crust, it may become hot. As it heats it expands and
in the confined spaces may develop a high pressure. Then it
dissolves quartz readily. Later it may be forced to the surface
again, appearing as a hot spring. About the mouths of such hot
springs quartz is deposited abundantly, for as the water comes
to the surface it is free to expand, the pressure decreases, the
water cools and loses its carbon dioxide to the air, and so it can
no longer hold the quartz in solution.
Not infrequently such alterations in temperature, pressure,
and carbon dioxide content occur in part as the water flows into
a cavity in the rock, and then the cavity is lined with layer after
54 OUR PHYSICAL WORLD
layer of quartz, the innermost layer often being a layer of
upstanding crystals. Later on, the rock containing such a
hollow mass of quartz may disintegrate, freeing the quartz mass.
Such a rounded chunk with a hollow center lined with crystals is
known as a geode.
The layers of quartz deposited in a rock cavity or at the sur-
face about a hot spring may have a waxy luster. Such quartz
is known as chalcedony.
But the quartz in the process of solution in water and rede-
posit is very prone to become impregnated with impurities that
color it. So quartz either in the massive or crystalline condition
may assume almost any color. A very beautiful variety of mas-
sive quartz is tinged with pink and is known as rose quartz.
Quartz crystals may be tinged with purple and are then called
amethysts. They are so beautiful as to be in demand for gems.
So the crystals when tinged with yellow are mounted as topaz,
although they are false topaz, as the real gem is still harder and
more lustrous than quartz. Similarly, red quartz crystals
make false rubies: green, false sapphires. The crystals may be
hazy with dark coloring and are then known as smoky quartz.
The layers of quartz deposited in cavities or about the mouths
of hot springs may be colored with different tints as first one
impurity, then another, is predominant. If the layers are vary-
ing shades of red, onyx is produced. Rounded masses of quartz
deposited in varicolored layers in some small .cavity of the rock
and later set free by rock disintegration are known as agates.
The layers may be shades of red, varying degrees of dark colors,
blues, or yellows. Such an agate may look like an ordinary
rounded quartz pebble or bowlder when found, for the exterior
is rough and water-worn, but when broken open it displays the
concentric colored layers. When ground down and well polished
it is a thing of marvelous beauty. Some very exquisite vases and
bowls are made of agate, chalcedony, and onyx, and the latter is
used for table tops or even decorative pillars in the interiors of
costly buildings.
THE EARTH'S ROCK FOUNDATIONS 55
The most beautiful gem in the quartz group is opal. This is
a form of quartz found usually in volcanic rocks. It has a texture
that makes its luster exceptional, so that the stone gives off reflec-
tions of brilliant color that change according to the angle at which
it is viewed, now red, now green, blue, yellow. The most brilliant
opals are those that dart shades of red like flames, and such are
known as fire opals.
Next to quartz the commonest rock-forming mineral at the
earth's surface is calcite. This is a carbonate of calcium (CaC03) .
It crystallizes in a variety of forms of which the rhombohedron
is the most common. It then easily cleaves along the planes of
the crystal faces in three directions, so that the pieces are bounded
by plane faces like a cube, but unlike a cube the angles at which
the faces meet are not right angles but are about 78° and 102°.
The opposite faces are parallel to each other and alike, though
they are not squares, as in the cube, but quadrilaterals whose
sides meet at the same angles as the faces. These angles between
the faces are always the same in the fragments, so the fragments
are all rhombs.
Calcite may be quite transparent, when it is known as
Iceland spar because such beautiful specimens of the mineral are
to be found in that locality. This spar has a peculiar effect on
light that passes through it, so that when a piece of the spar is
placed on an object, such as a printed page, each letter appears
double. The spar is said to be doubly refractive.
Calcite, when pure, is transparent, translucent, or white, but
it may assume many different colors as it takes up various
impurities. It may be red or yellow from the presence of iron
oxide, or blue, green, and other tints from other substances. It
is a soft mineral with a hardness of 3, and so is easily scratched
with a knife. It decomposes readily in dilute acids, yielding an
abundance of carbon dioxide gas, so that when a drop of such
acid is placed on it, or a small fragment is put in acid, it effer-
vesces, the gas bubbles coming up through the acid as they do
in soda water. The softness, the rhombohedral cleavage, and
56 OUR PHYSICAL WORLD
this effervescence with dilute acids make it easy to determine
calcite. The only mineral with which it is likely to be confused
is dolomite, a carbonate of magnesium that is heavier, harder,
and effervesces in strong acids or in weak ones only when
powdered.
Calcite is very prevalent, forming great beds of rock. Lime-
stone, chalk, and marble are made of calcite. The calcite in
limestone is usually in grains, while in chalk it is still finer — a
dust. Marble is derived from limestone through alteration by
heat and pressure, and is crystalline ; the calcite in limestone and
chalk is non-crystalline.
Calcite is a representative of several minerals that are also
carbonates. The most important as a rock-forming mineral is
dolomite, a carbonate of magnesium. Marble which contains
much dolomite instead of calcite is known as dolomitic limestone.
There is one sulphate of calcium that is a frequent ingredient
of rocks and that forms extensive beds in certain localities. This
is gypsum. The very clear crystals of this mineral are known as
selenite, while the pure white amorphous form is called alabaster.
The term feldspar is used to designate a group of minerals
rather than one. They are of unlike chemical composition,
though closely similar. In this respect, therefore, the term
feldspar is not co-ordinate with quartz and calcite, for these
terms indicate single substances of a definite chemical composi-
tion. The feldspars are, however, very similar in appearance
and have like physical properties. They are all complex sili-
cates of certain basic elements, sodium, calcium, potassium,
and aluminium. Orthoclase is a silicate of potash and alumin-
ium (KAlSi3O8); albite, similarly, a silicate of sodium and
aluminium (NaAlSi3O8), while anorthite is a silicate of lime and
aluminium (CaAl2Si2Os).
These feldspars occur rarely in rocks as such, but freely as
mixtures, two of them being usually fused together. Orthoclase
and albite fuse in making a series of potassium-sodium-aluminium
silicates. If the orthoclase feldspar is largely dominant in the
THE EARTH'S ROCK FOUNDATIONS 57
mixture, as is usually the case, the fused product has the proper-
ties of this mineral and is still known as an orthoclase feldspar
or potash feldspar. The mixtures of anorthite and albite are
known as plagioclases or soda-lime feldspars. A distinctive name
has been given to that feldspar that is a product of the fusion of
anorthite and albite in about equal amounts. It is called
labradorite.
All the feldspars cleave readily in two directions, and the
cleavage faces are at right angles to each other (in the orthoclases)
or at slightly oblique angles in the plagioclases. The cleavage
faces of the plagioclases are striated with many fine parallel lines.
In directions other than along the cleavage planes feldspar breaks
with an uneven fracture. Even in small fragments found in such
rocks as fine-grained granite it is usually possible to see the
cleavage faces with the hand lens sufficiently distinctly to recog-
nize the mineral.
Pure feldspars are colorless, but they are seldom pure. Ortho-
clase is usually tinged with red, varying from pale pink to deep
brick red; the color seems due to the presence of fine particles
of iron oxide scattered throughout the mineral. Plagioclase is
commonly gray, while labradorite is likely to be dark, smoky
gray, or even black. The colors are not dependable as absolutely
reliable distinguishing features, however, since plagioclase is
sometimes red, and orthoclase may be gray or dark. The ready
cleavage in two directions at right angles or nearly right angles
to each other, the vitreous luster on fresh fractures in other planes,
and the hardness are the chief features to be relied upon in field
determination. The feldspars have a hardness of 6, scratching
glass, but being in turn scratched by quartz. The feldspars are
probably the most widely distributed of rock-forming minerals,
though not occurring in such large quantities as those previously
mentioned.
Chemically, the feldspars are representative of a large major-
ity of the minerals which, like them, are complex compounds of
various basic elements with some one of the series of silicic acids.
58 OUR PHYSICAL WORLD
All such are primary minerals; that is, they are formed directly
in the cooling of molten materials. Such primary minerals con-
tain no water. In addition there is a series of minerals, also
silicates, that contain water. The former are the anhydrous
minerals, the later the hydrous. These secondary minerals are
the result of alterations of the primary ones through the addi-
tion of water and other chemical changes. Among the important
primary anhydrous silicates are the pyroxenes, the amphiboles,
olivine. The hydrous silicates include mica, kaolin, the chlorites,
serpentine, talc.
The pyroxene group includes hypersthene, diopside, common
pyroxene, augite, and aegirite, all similar in physical properties
but differing in the relative amounts of magnesium, iron, calcium,
sodium, and aluminium that combine with the silicic acids to
form the mineral. Pyroxene and augite are the commonest
and may be taken as typical. Both consist of silicates of calcium,
magnesium and iron, the latter containing aluminium also. The
pyroxenes are dark green in color, the augite, black. They are
quite hard, 5-6. The fracture is uneven, but they cleave fairly
well in two planes that are so nearly at right angles to each other
that they appear such except on very careful measurements.
They crystallize usually in short, thick crystals that are eight-
sided prisms, the ends capped with four-sided pyramids, which,
however, are commonly very imperfect, frequently reduced to
two faces.
The amphiboles or hornblendes include also a series of min-
erals which are so much alike that for our purposes we may de-
scribe common hornblende as typical. If the beginner can dis-
tinguish it in the rocks it will be sufficient. Hornblende looks on
casual inspection much like pyroxene. It is green to black, has
a hardness of 5-6, and occurs in the same dark igneous or meta-
morphic rocks. However, it has a highly perfect cleavage in
two directions, the cleavage planes meeting at angles of 55°
or 125°; pyroxene, it will be recalled, cleaves at right angles and
not very perfectly. The crystals are long and slender, as a rule,
THE EARTH'S ROCK FOUNDATIONS 59
and are six-sided in cross-section, the faces meeting at angles
like those made by the cleavage faces. The luster on freshly
broken surfaces is bright and vitreous, while in pyroxene it is
commonly dull. Hornblende occurs sometimes in a finely
columnar or even fibrous form known as asbestos; then the
luster is silky.
Olivine is an olive-green to bottle-green mineral, harder than
pyroxene or hornblende (6.5-7). It is transparent to trans-
lucent. It cleaves only in one direction. It occurs in the igne-
ous rocks in grains, and might be mistaken for the preceding
minerals at first sight, but its greater hardness and cleavage in
only one direction will distinguish it.
The micas are readily distinguished because they cleave so
readily into very thin elastic plates. The commonest ones in
rocks are muscovite, a light-colored one, which is a hydrated
silicate of potassium and aluminium; and biotite, dark brown
to black, a hydrated silicate of iron, magnesium, and aluminium.
Kaolin, which is a very pure clay, results from the disin-
tegration of the feldspars or similar minerals in the presence of
water and carbon dioxide. It is a silicate of aluminium com-
bined with water (H4Al2Si2O9) . It usually occurs in great masses
or beds, is soft, white, and has a greasy feel when rubbed
between the fingers. It is readily tinged with impurities, becom-
ing yellow, brown, or gray. It also occurs in beds more or less
mixed with other substances — sand, mica, hematite, organic
matter, etc. — and so gives the ordinary clays. Such beds are
important in rock formation, for out of them have been made
some important sedimentary and metamorphic rocks.
Chlorite is another hydrous silicate resulting from the weather-
ing of the anhydrous sorts. In reality there are several chlorites,
but all are much alike and may be treated here under the one
heading. The color is green; the cleavage is much like that of
mica, but the flakes, while bending easily, are inelastic and remain
bent instead of springing back to their original form as do the
micas. Chlorite is so soft, too, that it is scratched by the finger
6o
OUR PHYSICAL WORLD
nail. Chlorite gives its green tinge to many igneous rocks known
commonly as green stones, and to some schists and slates.
Serpentine is usually massive, sometimes fibrous like asbestos
(chrysolite). It is green in color, occasionally so dark as to be
nearly black. It has a greasy feel, a waxy luster (pearly in the
fibrous sorts), and is quite soft (2.5-3). It is not only a common
accessory mineral in many igneous and metamorphic rocks but
also forms great bodies of rock itself.
Talc is readily recognized by its softness. It makes a light
streak even on cloth. It is usually white to green. It is some-
what laminated like mica, but the flakes are inelastic. It has
a distinctly greasy feel.
Such, then, are the common rock-forming minerals. The
student should be familiar with them before he goes on to a study
of the rocks. The following tabulation will serve to give the
characteristics in condensed form.
KEY TO COMMON ROCK-FORMING MINERALS
'Chalk, 0.5-2.5 White to gray, dull, crumbles in
fingers, no earthy odor when
breathed upon, effervesces with
acid.
So soft they can
be scratched with <
the finger nail
Chlorite, 1.5-4.0
Gypsum, 1.5-2.0
Kaolin, 0.5-2.5
Mica, 2.2-5.0
A green mineral of pearly to
vitreous luster with greasy feel-
ing. It usually occurs in grains
or scales in basic rocks.
Many colors, streak always
white. Massive (alabastine),
fibrous (satin spar), foliated (if
transparent called selenite).
Many colors, streak like color.
Feels greasy. Strong clay odor
when breathed on. Dull to
pearly luster; brittle.
Perfect cleavage; very thin
elastic scales can be obtained.
The black sort is biotite; the
colorless, gray, or pale green,
muscovite.
THE EARTH'S ROCK FOUNDATIONS
6l
KEY TO COMMON ROCK-FORMING MINERALS— Continued
Easily scratched
with a knife
Scratched with a
knife with dif-
ficulty
Scratched by
quartz but not
with a knife
Galena, 2.5
Serpentine, 2.5-4.0
Calcite, 3
Sphalerite, 3.5-4.0
(Zinc blende)
Chalcopyrite, 3.5-4.0
(Copper pyrite)
Dolomite, 3.5-4.0
(Pearl spar)
Mica, see above
Limonite, 5.0-5.5
Pyroxene or
Augite, 5-6
Amphibole or
Hornblendes, 5-6
^Hematite, 5.5-6.5
Lead gray, streak same. Metal-
lic luster. Very heavy; cleaves
in cubes.
Color, shades of green. Luster
greasy, waxy, or earthy. Feels
smooth or greasy. Compact and
amorphous, making a rock of
the same name.
Many colors, streak white to
gray. Always cleaves into
rhombs. Effervesces in dilute
acid.
Yellow, red, brown, black.
Luster resinous when yellow.
Perfect cleavage. Brittle.
Brass yellow, often tarnished,
then showing iridescence.
Streak green-black. Softer than
pyrite.
White, gray, green, black.
Streak white. Transparent to
translucent. Crystals curved
like saddles.
Dark brown, streak yellowish
brown. Of ten fibrous; if earthy,
color is yellow. In cubical
crystals.
Green to black. Fracture un-
even to conchoidal. Usually in
short, thick, eight-sided prisms.
Cleavage poor; faces meet at 90°.
Brown, green, or black, darker
than augite. Fracture as above.
Luster pearly on cleavage faces.
Crystals long, slender, six-sided,
faces finely cross-striate. Cleav-
age faces meet at 125°.
Cherry red to iron black; streak
red. Metallic luster, massive or
fibrous or scaly.
62
OUR PHYSICAL WORLD
KEY TO COMMON ROCK-FORMING MINERALS— Continued
Feldspar,* 6.0-6.5 Many colors, streak white.
Cleavage perfect, faces at nearly
right angles. Light colored,
orthoclase; darker, plagioclase.
Pyrite, 6.0-6.5 Brass yellow, tarnishes brown.
(Fool's gold) Streak greenish black. Metal-
lic luster. Crystals, cubes or
dodecahedra. Harder than
chalcopyrite.
'Olivine, 6.5-7.0 Green, streak white. Trans-
parent to translucent. Usually
occurs in rounded grains.
Quartz, 7 Color anything from black to
As hard as white. Luster vitreous or waxy
quartz in chalcedony. Fracture con-
choidal. Crystals six-sided
prisms ending in pyramids;
blue, amethyst, banded agate,
onyx, jasper. In massive
nodules occurs as flint.
* The term feldspar stands for a group of minerals. Orthoclase is a silicate of
aluminium and potassium — a "potash-feldspar." Its cleavage angle is a right
angle, or nearly so. It is usually light in color, white, gray, pink. It commonly
occurs in rocks in which quartz is present fairly abundantly and seldom associates
with the plagioclase group. This plagioclase group includes the soda-lime feld-
spars like oligoclase and labradorite. The plagioclases have an oblique cleavage
angle, and certain cleavage faces are marked with numerous fine parallel lines.
The plagioclases, especially the oligoclase and the labradorite, -are strongly basic,
seldom occur with quartz in any quantity, often are present with augite or horn-
blendes. They are usually dark colored, blues, grays, or dull reds.
Rocks are constantly forming nowadays. When from some
great volcano there is an outflow of lava, and this molten material
cools and solidifies, it forms rock (Fig. 29). Such rocks, formed
from the cooling of a molten mass, are known as igneous rocks.
The wear and tear of the waves, ocean currents, and other agents
of erosion disintegrate rocks, and the debris is carried out to sea.
Offshore this material is being deposited as great beds of sand
and mud. As this process goes on through countless years the
deposits thicken, and the lower strata, subject to the vast pressure
THE EARTH'S ROCK FOUNDATIONS 63
of the accumulating layers above and to the internal heat of the
earth, are transformed to rock. Just as man takes clay and by
pressure and heat transforms it into solid brick, so in nature the
loose sands and clays are by similar processes transformed to
rock. A bed of sand, for instance, will make sandstone. Such
rocks, the constituent materials of which are deposited by water
and solidified by heat and pressure, are known as sedimentary
rocks.
- FIG. 29. — Basalt
Such processes of rock formation and rock disintegration by
weathering and the re-formation from the debris have been going
on for a very long time on the earth. The very old rocks, how-
ever, are all igneous apparently. The earth was at one time
much hotter than now, volcanic activity was more intense, lava
outflows were very extensive, and the early crust was made of
the rocks obtained by cooling of this molten material. These
very old rocks have, in a large measure, been covered up by later
outflows of lava and by sedimentary deposits on top of them.
64 OUR PHYSICAL WORLD
Still there are regions in which the very old rocks are found at
the surface, later sedimentary rocks having been washed off
from them; or else they have been brought to the surface by
the folding and crumpling of the earth's crust. In mountain
regions where volcanic activity is present, igneous rocks are very
plentiful.
When a great mass of molten material like a great lava out-
flow cools, the surface layers cool first, naturally. These surface
layers are made up of the lighter materials which have come to
the top while the mass was still molten. Moreover, such molten
masses are full of gases that are escaping and bubbling up to the
surface. The rock, therefore, that first forms on the top of
such a lava mass is likely to be frothy, light in color, and light
in weight. Deeper down in the cooling mass the rock formed is
glassy in its texture. Still deeper down as cooling goes on much
more slowly, the ingredients of the molten mass crystallize as
they cool. When cooling goes on fairly rapidly, crystals that
form are very small; but as cooling goes on more and more
slowly the crystals tend to become larger and larger. It is very
evident that igneous rocks will vary in their structure according
to the rate at which the original molten mass cooled. We may
have rocks of spongy character, like pumice, glassy rocks such
as obsidian, or crystalline rocks, and these latter may be either
fine-grained like basalt or coarse-grained like gabbro. The
coarsely crystalline rocks of all groups are called plu tonic, for they
have been formed, as a rule, deep down in the throat of the
volcano. The finely crystalline, glassy, porous rocks are called
volcanic, for they have cooled upon the surface of the earth
as lava outflows.
The various minerals that enter into the composition of the
rocks do not crystallize out all at the same time. Some begin to
form as crystals when the molten mass is still quite hot. Others
wait until the material has cooled a great deal. The minerals
that contain large proportions of the heavier metallic elements,
such as iron and magnesium, crystallize early. Plagioclase
THE EARTH'S ROCK FOUNDATIONS 65
feldspars crystallize out before the orthoclase, and quartz seems
to be one of the last to crystallize. Not infrequently one finds
a rock composed of a finely crystalline ground-mass containing
large and distinct crystals of some constituent mineral. Such
rocks are designated porphyries. In the porous rocks the cavi-
ties formed by gas bubbles have in some cases later been filled
with some mineral deposited usually by water percolating through
the rock. Such rocks with more or less spherical masses of
mineral deposited in the cavities are known as amygdaloids.
Not only do the igneous rocks differ in texture but they differ
also in chemical composition according to the prevalence of
the various minerals. As noted already, most of the important
minerals entering into the formation of igneous rocks are silicates.
When metals combine with silica some of them take up large
quantities of silica, others relatively small quantities. This
depends upon the valence of the metal. Thus iron has a valence
of four, as does manganese; while sodium and potassium have a
valence of only one; calcium a valence of two. This means
that iron is capable of combining with four atoms of monovalent
substances, like hydrogen, say, while sodium can only combine
with one. When, therefore, such a metal is combining with silica
to form a silicate, the element with the greater valence will take
up much more of the silica. The silicates of such metals as
sodium and potassium, as we have seen in the orthoclase feld-
spars, are likely to be light in color and light in weight as com-
pared with the minerals that are silicates of the heavy metals like
iron and manganese, such as pyroxene and hornblende. The
rocks formed from the combination of such light-colored and
lightweight minerals are also prone to contain a great deal of
free silica in the form of quartz, whereas, for the reason just
given, the silica is not likely to be free in rocks made of the
darker and heavier metals.
On the basis of these two characters — the texture of the
mineral and the prevalence of certain constituent minerals — we
can classify the rocks. In the accompanying tabulation (p. 66) , the
66
OUR PHYSICAL WORLD
rocks are divided into certain groups according to the dominance
of certain minerals. In the granite group at the left, the domi-
nant minerals are quartz and orthoclase feldspar. As you read to
the right in this table through the succeeding groups, the quartz
becomes a less and less conspicuous element in the rocks. The
feldspars decrease in amount and those present are of the plagio-
TABLE OF IGNEOUS ROCKS
Granite-Rhyolite Group
Syenite-
Trachyte
Group
Diorite-Andesite
Group
Gabbro-Basalt
Group
Peridote
Group
Quartz and Orthoclase
Dominant
Drthoclase
Dominant:
Quartz
Absent or
Present in
Negligible
Quantity
Plagioclase and
Hornblende Dom-
inant: the Latter
Equaling or Ex-
ceeding the Feld-
spar in Amount
Feldspar (Lab-
radorite) and
Pyroxene Dom-
inant: the Latter
Equaling or Ex-
ceeding the Feld-
spar in Amount
Feldspar
Absent or
Nearly So.
Hornblende,
Pyroxene,
Olivine,
the Dominant
Minerals
Rhyolite pumice (porous)
Rhyolite obsidian (glassy)
Trachyte
(included in
the f elsites)
Andesite
(included in the
felsites)
Basalt tuff
Basalt breccia
Basalt
Dolerite
Granite (crystalline)
Other minerals may be
present but not dom-
inant giving biotite-
granite, hornblende-
granite, etc.
Syenite
Diorite
Diabase
(Olivine diabase,
olivine gabbro,
green stone)
Gabbro
Peridotite
Porphyritic granite
Diorite
porphyry
Diabase
porphyry
Pegmatite granite
clase varieties; finally the feldspars disappear entirely. While in
the granite group we may have hornblende or pyroxene present
in small quantities, in the right-hand groups these minerals come
to be the dominant ones.
Reading down in any one of the groups, the texture of the
rocks varies from a spongy texture, through a glassy texture to
the crystalline texture, and the latter is first fine, then coarse.
In some of the groups these spongy and glassy rocks are missing.
THE EARTH'S ROCK FOUNDATIONS 67
Thus in the granite group we have first pumice, then obsidian,
then granite, and the granites may vary from very fine-grained
to very coarse-grained granite, the latter being not infrequently
porphyritic.
Rhyolite pumice is a spongy glass. It is light in color, porous,
and, therefore, light in weight. It is found only in the regions
where volcanic action has occurred comparatively recently.
Rhyolite obsidian also occurs only in the regions of recent volcanic
activity. It is a glassy rock which breaks with a conchoidal
fracture. It varies greatly in color from a light to so dark a
tint that it is almost black.
The granites consist essentially of quartz and orthoclase
feldspar or at least of feldspars that have so large a mixture of
the orthoclase as to have its characters predominant. The
granites may be fine-grained or coarse-grained. If one constitu-
ent is very coarse-grained and the others more finely crystalline,
the granite is spoken of as a porphyritic granite. A number of
other minerals besides the two essential ones may be present;
mica, especially the biotite form, is very often present, hornblende
and pyroxene are frequent ingredients, but never dominant.
If these darker minerals are present in quantity, the granite, of
course, is very dark. Sometimes the quartz crystals are scattered
through the granite in rather regular lines and are frequently
twinned, making the rock appear like a slab of feldspar with
more or less regular lines of angular quartz figures giving the
appearance of Arabic writing; such granite is known as peg-
matite.
Granites are very widely distributed especially in the regions
where the older rocks are exposed; for a large proportion of these
older rocks are of granitic character. They are found con-
sequently as the core of mountain systems where the later sedi-
mentary rocks have been worn away from the crest of up-arched
strata. They occur abundantly in the Laurentian Highlands
of Canada, throughout northern Michigan, Wisconsin, Minne-
sota, along the Appalachian Mountains, running through New
68 OUR PHYSICAL WORLD
England, New York, the Virginias, and Carolinas, and down into
Georgia. They are similarly found in the Rocky Mountain
regions and in the Ozarks.
Because of their very wide distribution, the granites have
played an important part as the source of soils. The feldspar
which they contain weathers readily and, as a result of its weath-
ering, changes to kaolin, which, when permeated with such impuri-
ties as the oxides of iron, gives our common clays. The quartz
is, of course, more resistent to the weather but is sorted out by
the water, is more or less weatherworn, and is deposited as beds
of sand.
In the syenite-trachyte group only two rocks are given. The
syenite is the coarsely crystalline or plu tonic member; the
trachyte, the finely crystalline or volcanic member. The sye-
nites are not very common. They consist of orthoclase chiefly,
though other minerals, like mica, hornblende, pyroxene, may be
present. The quartz is either absent or present in such small
quantities as to be a negligible constituent. Trachyte is a very
fine-grained rock of similar constitution. It can usually be
recognized, in spite of the fact that the constituent minerals are
in such small particles that they are distinguished with difficulty,
by its light color and light weight.
The diorite-andesite group includes diorite, sufficiently
coarsely crystalline so that the constituent minerals may be dis-
tinguished, and andesite, very finely crystalline. In the diorite-
andesite group, the feldspar present is of the dark variety, plagio-
clase feldspar. Hornblende is also present and equals or exceeds in
its amount the feldspar. Quartz is also usually present, and there
may be other accessory minerals. It is evident from the composi-
tion that the diorites grade into the granites, on the one hand,
and it will be seen that they grade into the gabbros, on the other.
When the constituent minerals are present in very tiny grains
so that it is quite impossible to make out the individual com-
ponents, the rock is known as an andesite. When the constituent
minerals occur, any one of them in large crystals, while the rest
THE EARTH'S ROCK FOUNDATIONS 69
of them are relatively fine crystals, the rock is again known as a
porphyry; and in this case porphyritic diorite or diorite porphyry.
In the gabbro-basalt group, the rocks consist essentially of
pyroxene and feldspar, and the feldspar is usually of the dark
variety, labradorite being the commonest form, though we do
have gabbros in which orthoclase is abundant. The pyroxene in
these rocks equals or exceeds the amount of feldspar present.
These rocks are all dark in color, relatively heavy, and the amount
of quartz present is small. Very often there are accessory
minerals present such as mica in tiny flakes, particles of hematite
or magnetite, and often olivine in considerable quantity. In the
latter case the olivine gives the rock a distinctly greenish cast and
such rocks are commonly known as green stones. Gabbro is the
coarsely crystalline member of this group. Diabase is more
finely crystalline. If the crystals of which the rock is composed
are quite fine, the feldspar being recognizable but the accompany-
ing darker minerals in such fine particles that it is difficult to
distinguish them even with a lens, the rock is a dolerite.
The term, basalt, is used to include all of those dense, dark,
igneous rocks in which the constituent grains are so tiny as to be
unrecognizable. Sometimes one of the ingredient minerals will
be present in coarse crystals, when the rock is known as a basalt
porphyry. Basalt occurs in very large beds, covering immense
areas, especially in the regions occupied by the older rocks. As
the old lava cooled, giving rise to the basalt, often the mass so
contracted as to break into quite regular columns, and these
shattered into blocks by cross-fractures so that not uncommonly
basalt has a columnar structure. A similar phenomenon is
seen in beds of mud where the clay on drying cracks into polyg-
onal masses. In the latter case the phenomenon is due to loss
of water, whereas in the former it is due to the gradual contrac-
tion as the hot mass cools. Such columnar basaltic masses are
famous in the Giant's Causeway in Ireland, the Devil's Pile
Quarry in our western states (see also Fig. 29). Basaltic tuff is
very light, spongy rock, dark in color, and correspondingly
70 OUR PHYSICAL WORLD
heavier than rhyolite pumice. It was thrown out originally as
coarse ash from the throat of the volcano and later solidified.
If the ash were thrown out in coarse fragments and these
were later cemented together, the rock is known as a basaltic
breccia.
The trachytes, andesites, and basalts are so fine-grained
that it is difficult to distinguish them in the field, so for practical
purposes they are distinguished as felsites and basalts. If such
a fine-grained rock is very dark, grayish, greenish, purplish, or
black, the rock is called a basalt. If, however, the color is light,
medium gray, pink, red or even dark red, yellow, brown, or
light green, it is termed felsite.
Finally, the peridotites are very heavy rocks in which there
is very little or no feldspar, the dominant minerals being pyroxene
and hornblende together with considerable iron ore.
These igneous rocks would be largely wanting in the regions
covered by the sedimentary deposits, such, for instance, as the
states of the North and Central United States, were it not for
the fact that the great glacier which at one time covered this
region brought down with it great quantities of these rocks
imbedded in its mass or riding on its surface from the regions
occupied by the older rocks in Canada or the northern portions
of the states bordering the Great Lakes. When the glacier
finally melted and retreated, these rocks were deposited in the
soil as bowlders, so that the student even in regions where
the bed rock is sedimentary rock may find many samples of the
igneous rocks described above by collecting samples of these
bowlders.
The chief sedimentary rocks are limestone, sandstone, con-
glomerate, breccia, shale, slate. When shells of such animals as
clams, oysters, snails, are worn to sediment by wave action, or
when the hard parts of coral are similarly disintegrated and the
sediment deposited in the quieter depths of the sea, then later by
the pressure of overlying layers and the heat of the earth is
changed to rock, the result is limestone. One marvels that the
THE EARTH'S ROCK FOUNDATIONS
shells of animals or corals can exist in such quantity and be so
ground up as to form great beds of rock, yet the process can
readily be seen now going on in many localities, as along the coast
of Florida. The bed rock of that state is largely such limestone —
cochina limestone, of very recent formation — and the little clam,
the cochina, exists in countless
hordes in the ocean along its
shores. The area of the state
is constantly being thus ex-
tended. The soil of the states
of the Middle West, Ohio, In-
diana, Illinois, etc., lies in large
part on a limestone bed rock
deposited in the old seas that
once covered their present
sites. Such beds of limestone,
often hundreds of feet thick,
represent the accumulated re-
mains of untold numbers of
shells and countless genera-
tions of corals (Fig. 30). But
the time consumed in their
formation according to the
geologists mounts up into the
millions of years, which is
quite necessary for such a vast
procession of living things.
Limestone may be almost
as hard as feldspar or very
soft. It can always be scratched with a knife. It may be of
many colors though usually it is some shade of yellow or gray.
Since it is composed of calcium carbonate it effervesces with acid.
It often contains fossils, the remains of animals and plants that
were buried in the mud when the limestone was forming and were
altered with it to stone. Such fossils show with remarkable
FIG. 30. — Limestone, showing stratification
72 OUR PHYSICAL WORLD
fidelity all the details of structure. Sandstones and shales also
contain such fossils (Fig. 31).
FIG. 31. — A group of fossils: (a), (6), (d), (/), fern fronds; (c), an equisetum;
(e), an animal, a shrimp; (g), bark of Sigillaria.
THE EARTH'S ROCK FOUNDATIONS 73
Sandstone is composed of sand grains more or less thoroughly
cemented together, and may be quite hard or very soft and
friable. It often contains impurities, notably the oxides of
iron that impart various colors to it, chiefly yellow or red. It is
the result of the solidification of old beds of sand. Conglomerate
is merely a very coarse sandstone in which the component bits
are rounded, water- worn pebbles instead of sand grains. Breccia
is similar, but the bits of stone of which it is formed are still
angular.
Beds of clay when transformed to rock by pressure and heat
form shales. They are fine-grained rocks, usually soft and split
easily into layers. They give an earthy odor when breathed
upon. They also vary greatly in color, depending on the nature
of the contained impurities.
The soft coals are also sedimentary rocks. Along the margins
of the ancient seas there occurred sometimes extensive swamps,
especially at the mouths of rivers just as they exist today in
deltas. In these vegetation was very rank. As the trees, ferns,
rushes, and other forms matured and fell, they sank into the
shallow water which covered them and prevented their immediate
decay. Year after year, century after century, added to the
accumulation until the lower layers were compressed into peat.
So peat beds are forming nowadays in such locations. These
peat beds continued to form to great depths, the crust of the
earth sinking with the weight of the great accumulation. The
lower layers were still more powerfully compressed by the great
weight above them, and were heated from the earth's hot interior.
So lignite or brown coal was formed, and this in turn changed to
bituminous or soft coal as the gases and more volatile oils were
driven off. Often quantities of mud were brought down by the
rivers and deposited in such swamps. When compression
occurred these transformed to shales. Since the clay contained
much vegetable material, the shale formed from it is dark,
carbonaceous shale. It often is impregnated with the oils and
gases that distil off from the forming coal.
74 OUR PHYSICAL WORLD
Sedimentary rocks are all deposited in layers (see Fig. 30).
Throw a handful of sand into a tumbler of water and allow it to
settle thoroughly. There will then be layers of sand in the
bottom of the tumbler, the heavy coarse material having gone
down first, the lighter, finer material following. So the debris
resulting from the distintegration of shells and coral skeletons
or of the igneous rocks worn to bits by the forces of erosion as it
deposited in the quiet depths of the seas was sorted and laid down
in layers whose constituent particles were now coarse, now fine,
depending on the strength of the currents that brought them to
the place of deposit. Sometimes these layers are thin; so they
may readily be seen even in a hand specimen, again they are
thick and are only to be noted at the quarry or rock cut.
Now igneous and sedimentary rocks may be greatly altered
after their original formation by heat and pressure. When a
new lava stream forces its way up in the cracks of older rocks it
alters the rock with which it comes in contact. As old beds of rock
are heated and subjected to terrific strains and compression as
they are bent and upheaved when mountain chains are formed
they are much changed. This process is known as metamorphism
and the rocks so altered as metamorphic rocks. Thus limestone
changes to marble, sandstone to quartzite, shale to slate and
schist, bituminous coal to anthracite, while igneous rocks like
granite change to gneiss or schist. Gneiss contains the same
constituent minerals as the volcanic rock from which it is derived,
but the component grains are flattened and forced to lie with
their long axes in the same direction, thus giving to gneiss a
somewhat stratified appearance. Schists have the constituent
particles even more flattened, so they are scalelike. They are
often so soft they may be crumbled with the fingers. They are
named from the dominant mineral present, as chloritic schist,
micaceous schist. In slate the layers of the rock are easily
separable. Sometimes they are very thin, as in the familiar
school slates. Quartzite is very hard, like quartz. It also breaks
with a conchoidal fracture but shows the granular structure of
THE EARTH'S ROCK FOUNDATIONS 75
the sandstone, though the sand grains are indistinct through
partial fusion. Marble is crystalline, fairly hard though still
scratched with a knife, and effervesces with acid, though not as
violently as limestone unless the latter contains much silica, a
siliceous or cherty limestone.
Commercially the most valuable of all the sedimentary rocks
is the coal (Fig. 32). We are very fortunate in possessing such
vast quantities of it in this country. It is estimated that we
FIG. 32. — Entrance to a coal mine
have mined some 14,000,000,000 tons thus far in our history, and
that we still have left 17,000,000,000 tons of anthracite,
1,500,000,000,000 tons of bituminous, and 2,000,000,000,000 tons
of lignite, a coal of inferior quality but still usable. We are using
our coal at a much faster rate than ever before, for the industrial
demand for it is ever increasing. In 1921 we mined nearly two-
thirds of a billion tons. Just how long the available supply will
last it is very difficult to say or even to make an approximate
guess as there are so many factors involved. Some of the coal
76 OUR PHYSICAL WORLD
is so deep down or in such narrow seams it can scarcely be mined
with profit. Then other forms of energy production may take
the place of production by coal. We are already using water-
power very extensively.
Out of every 1,000 tons of coal, industry uses 350 tons; rail-
roads, 250 tons; domestic heating and cooking, 165 tons; coke,
130 tons; fuel at the mines, 35 tons; gas works, 10 tons; and we
export 60 tons. Our methods are still so wasteful that less than
half of the energy in the coal actually dug out of the earth gets
to the consumer in available form.
Oil and gas are also derived from the coal measures. Since
oil was struck in 1859 we have used 5,467,000,000 barrels, nearly
50 per cent of the estimated supply, and we are using it at the
rate of over 500,000,000 barrels annually, so that the available
supply of oil that can be pumped out of the earth in the United
States will necessarily soon be exhausted. Fortunately, there
are almost limitless supplies available in the oil shales from which
it can be distilled. This is a more costly process and oil prices
probably must rise, but still there is no danger of an oil famine
for generations. Indiana alone has oil shales estimated to yield
100,000,000,000 gallons.
We are burning about 800,000,000,000 cubic feet of natural
gas annually purposely, and there are many millions of cubic
feet going to waste as it escapes into the air or burns at wells
where it is not being utilized.
CHAPTER III
THE CONQUEST OF THE AIR
When I bestride him I soar, I am a hawk. — SHAKESPEARE
Primitive man was forced to find or produce food, to protect
himself from the inclemency of the weather and from his enemies,
and to transport himself and his belongings to new territory
when he had exhausted the resources of one spot. Production,
transportation, and self-defense are still problems of prime
importance in our modern life.
At first man found or produced what he needed by his own
unaided efforts. He made things by hand. His chief defense
was the strength of his bare arm or the speed of his legs. He
was his own beast of burden. In time he discovered how to
domesticate plants and animals, how to use tools and machines.
Then production, transportation, and defense were made rela-
tively easy. The history of man's progress along these lines, of
his inventions and their effect on social adjustment and organiza-
tion, is the most interesting and important phase of the history
of the race.
Much of the subject-matter to follow will deal with the
matters thus briefly outlined. The presentation will not follow
the logical order here suggested, however, but will begin with
such toys and appliances as usually enter into the pleasurable
experience of childhood, and proceed through the scientific
principles elucidated by them to an understanding of some of the
most valuable inventions man has made to aid him in the task
of making the forces of nature subservient to his needs.
No chapter in the history of man's subjection of Nature has
been more replete with thrilling incidents than that which deals
with the conquest of the air. Two major lines of endeavor have
77
78 OUR PHYSICAL WORLD
characterized the attempts to utilize the air as a means of further-
ing his purposes: (i) to harness the winds to provide power for
his machines; (2) to use the air as a medium of transportation.
Under the first heading may be mentioned windmills and sail
boats; under the second, kites, aeroplanes, and balloons.
Who the inventive genius was who first devised and flew a
kite we do not know. But probably it was some Chinaman,
for kites have been known in China and Malaysia for a very long
time, even before historic times; they are used there for decora-
tive effects at the numerous festivals. Not only the tailed
variety but also the tailless sorts are made, and these latter of
many curious designs — fish, birds, and geometrical figures of
pleasing shapes.
Kites have been largely playthings for the race until very
recent times, although occasionally some keen ancient mind
caught sight of their serious uses.
The first really serious use of kites that is historically authentic
occurred in 1749 when Dr. Alexander Wilson, an Englishman,
and Thomas Melville, an American, raised kites high up in the air
with thermometers attached to them to get the temperature of
the upper air. Since then kites have been used extensively for
carrying up thermometers, barometers, hygrometers, anemom-
eters, and other scientific instruments to get records of the
conditions up among the clouds. Such facts are of service in a
better understanding of the sudden changes and probable condi-
tions of the weather. Many of the United States Weather
Bureau stations are provided with kites which are regularly flown,
carrying up self-registering instruments to collect data. Kites
have been sent for such purposes so high that the recording
instruments showed a barometric pressure of only 4 inches and a
temperature of —87° C. Remembering that the air pressure at
sea-level is about 30 inches on an average, it is evident that the
kite had soared well above the great bulk of the air.
Kites have been used to take up cameras to get a bird's-eye
view of the underlying territory, to lift men as observers, and
THE CONQUEST OF THE AIR 79
to carry messages out of besieged cities, but these things are all
better done by small balloons, which will be considered later.
Lawrence Hargrave, of Sidney, New South Wales, invented
the box kite. This kite, it was discovered, has greater lifting
power than a kite that presents only one plane surface; besides
which it flies much more steadily, needing no tail. This dis-
covery was very suggestive to those inventors who were working
on the aeroplane at that time. In fact, the principles underlying
the flight of a kite are the ones that make possible the flight of
the aeroplane, and to understand why a kite flies is to understand
in large measure the principles upon which operate all those
machines of men that depend on currents of moving fluids such
as winds and streams of water for their motive or sustaining
power.
Every lad who has flown a kite knows it will fly well only in
a wind. By running swiftly while you hold one end of the long
string to the other end of which the kite is attached, you may
make the kite rise a bit on a still day, but it drops back to the
ground again the minute you stop running. While running you
pull the kite through the air, but when the wind is blowing the
air streams past the kite, sending it up. But just how does it
operate to accomplish this ?
When the wind is blowing, particles of the air in their forward
trike -against -the face of the kite. If the kite were
not held by the string, it would just blow along on the ground
in the direction in which the wind is blowing, like a loose sheet of
paper. But the string is so tied to the kite by means of the bridle
that the kite's surface stands inclined to the wind, and so the
moving air particles strike the kite at an angle, hitting a glancing
blow. When that happens the force with which the wind strikes
the kite is broken up into two components, one of which lifts the
kite up into the air.
A simple experiment may be readily performed to illustrate
the law of the composition and decomposition of forces. Set
three small nails into a large drawing-board or the floor at the
8o
OUR PHYSICAL WORLD
points of a triangle with sides at least 15 inches long (see Fig. 33).
Slip the ring of a spring balance on to each nail. Tie a string
to the hook of each spring balance, and then tie the other ends
of these strings together, making the tie so that each scale will
register some pull. It is evident that the amount registered on
any one scale is the resultant of the pulls of the other two.
The relation between these forces may be graphically cal-
culated as follows. Lay a good-sized sheet of paper on the
A
FIG. 33. — Diagram of the decomposition of forces
GD= i in. Scale A shows i Ib.
DE = 4 in. Scale C shows 4 Ib.
DF = 3! in. Scale B shows 3! Ib.
drawing-board underneath the three strings, its center about
under the knot. With a ruler draw lines immediately under and
parallel to the three strings, the three lines meeting immediately
under the central knot (D in the figure). Suppose the scale at
A measures i pound, the scale at C, 4 pounds. Lay off on line
DG i inch, on line DE, 4 inches. From point G, i inch from D,
draw a line parallel to DE, and from point E, 4 inches from D,
a line parallel to DG, thus making a parallelogram. Continue
the line BD, and it will make a diagonal of the parallelogram.
Its length in inches will be the pull on the scale at B in pounds.
THE CONQUEST OF THE AIR 81
Thus knowing the strength and direction of the pull of the two
combined forces, the resultant may be determined; or knowing
the resultant and the direction of the pull of the component forces,
the latter may be determined.
Forces acting along DE and DG in unison on point D in the
direction of the points C and A combine to produce the effect
of a force acting along line DF which is counteracted by the equili-
brant pull on the scale at B along line ED. As calculated above,
the magnitude of the operating forces and the resultant effect are
in proportion to the sides of the parallelogram and to the diagonal.
Conversely, suppose a particle of air is moving swiftly along
line ED from E toward Dy and at D it strikes the surface of the
kite KI. It hits the kite a glancing blow at D, and flies off along
DE. But the force of the blow at D is resolved into two factors,
one of which acting along DG lifts the kite. Successive air
particles as the wind blows hit repeated blows. The combined
effect sends the kite up. If the length DF represents the inten-
sity of the force striking Z), it will be resolved into two com-
ponents acting along DE and DG proportional to the lengths of
these lines, and the sum of the components will, of course, be
equal to DF. The wind is resolved into such factors only when
the string holds the kite against the wind.
The kite mounts in the air as the string is played out until the
down pull on the string and the weight of the kite equal the factor
DG. If the bridle is so adjusted that the kite lies at an acute
angle to the direction of the wind, the factor DG will be small,
the kite will fly almost directly overhead, and the pull on the
string will be slight. If, however, the bridle is adjusted so the
kite makes a relatively large angle to the wind, the factor GD
will be great. The kite will pull hard on the string and will
sail off to a distance, but will not mount very high. It is evident
that in a light breeze the bridle must be adjusted as in the second
case to get the kite to go up at all, for it will need much of the
force of the wind to raise the kite and its attached string. But
in a good stiff breeze the attachment indicated in the first case
82 OUR PHYSICAL WORLD
will be used to insure the kite carrying high into the air, nearly
straight overhead.
Directions for making and flying the various types of kites,
the ordinary tailed kite, the tailless bow, and box kites (Fig. 34),
are given in the Field and Laboratory Guide in Physical Nature-
Study. A method for making the odd bird kite will be given here.
Cut four very thin strips of bamboo or cedar 3 feet long.
Fasten two of these together to make a figure 8 with one loop
three or four times the size of the other. This may be done
by binding the overlapped ends and the intersection with stout
thread. Similarly, fasten together by their ends the other two
strips laid parallel and then spread their centers apart a foot.
Bind them, so spread, to the figure 8, fastening the mid-point of
one strip to the neck of the 8, the other one to the sides of the
large loop. The large loop of the 8 makes the frame for the body
of the bird, the small one for the head. The side extensions are
the wing frames. Cut two thin 1 5-inch strips of bamboo and
fasten one end of each to the sides of the larger loop of the 8,
halfway from the wing frame to the lower end. Cross them and
tie the crossing to the mid lower end of the loop so their free ends
spread fan-shaped beyond the 8 for the frame of the tail. Fasten
a taut string between their free ends.
Now lay the kite frame on a large sheet of tissue paper. Cut
from it a rectangular strip as long as the kite is wide from tip
to tip of the wing frames and 3 inches wider than these at their
widest point. Run paste all around the edges of this strip.
Place the kite frame on it and turn the edges of the paper over
the wing frames just far enough so the edge will stick to the
paper. This will allow the paper to bag more and more out to
the wing tips, where one edge of the paper will be stuck to the
opposite edge.
Cut the sheets to cover the head frame, and the body frame
2 or 3 inches larger all the way around than the frames, and stick
the edges over the frame as on the wings, so the paper on both
head and body will bag in, in the same direction as on the wings.
THE CONQUEST OF THE AIR 83
A triangular piece of paper is pasted flat on to the tail frame, its
edges overlapping the frame.
When the kite is dry, with small brush and ink paint eyes and
beak on the head, feet and legs on the body, and radiating lines
on the tail to suggest spread-out tail feathers.
To make the bridle to which the string is attached, tie three
1 8-inch lengths of string, each by one end, on to the frame, one
at the neck and one on each side where the wing frame crosses
the body frame. Then tie the other ends of these together,
making the strand to
the neck about 3 inches
shorter than the other
two, which are equal in
length. The string on
which the kite flies is
tied to the point where
these three are knotted
together.
The ordinary kite
must needs have a
tail. The wind is al-
ways fitful and gusty,
blowing with changing
velocities and con- / FIG. 34.— A tetrahedral kite
stantly shifting its direction as one flaw comes from one direc-
tion, another from a different point of the compass. The kite
in such a gusty wind is buffeted first to one side, then to the
other, and so tends to bob around. As the wind suddenly
increases in intensity, the kite rises quickly, pulls hard on the
string, and turns a somersault. The tail acts as a stabilizer, for
it makes an inert weight hanging below the kite which the kite
must carry along with it. A body at rest forcibly resists move-
ment as a force acts upon it to move it. It has inertia. So if the
body is in motion, because of inertia, it tends to remain in motion
in the same direction until some force acts upon it to deflect it
-84 OUR PHYSICAL WORLD
or to bring it to rest. The inertia of the tail tends to restrain
the kite and keep it from bobbing about erratically.
When a kite is built of several plane surfaces set at varying
angles to each other as in the box kite or the tetrahedral kite
(p. 83), or presents curved surfaces to the wind as in the bow kite
or bird kite, the gusts of wind strike these at such varying angles
that the kite is impelled in a dozen different directions simul-
taneously, with the result that these various impulses work
against each other, and so the kite remains quite steady in the
main air current. Such kites, therefore, fly well without a tail.
Kite-flying may be made to afford a great deal of amusement
and incidentally much experience with winds that gives the pupil
a real appreciation of their power and a first-hand acquaintance
with some of the problems involved in their utilization for man's
purposes. Indeed, it is a sport followed in many countries by
adults. It requires considerable skill to fly your kite higher
than any of your competitors. Tandem teams of box kites
will fly to great heights. Send up a box kite Jetting out 200 feet
of string, then fly another one on 50 feet of string and fasten the
free end of the string to the string of the first kite. Let out more
string and fasten on another.
You may slip colored paper windmills or disks of paper on to
the string of your kite, passing the latter through the hole at the
center of the disk or windmill and so let them go sailing or
twirling up to the kite. You may draw a face on your kite or the
head of an animal. A kite that is covered with black paper on
which are pasted tissue-paper circles for the whites of the eyes,
red tissue-paper nostrils and lips with white paper teeth is a
conspicuous and comical object in the air as it goes bobbing
about.
We boys used to fasten a sharp, narrow strip of tin on to the
kite string, then try to cut the other fellow's kite string, and so
see who could keep his kite up longest without accident. I
remember, too, our ambitious plan of building a great, big kite.
It was of the ordinary type but extraordinarily large — -15 feet
THE CONQUEST OF THE AIR 85
long and 10 wide. It was covered with heavy express paper.
Its tail was made of old rope, abandoned at the mine but still
strong enough for our purposes. We used small rope to fly it.
We found it necessary to rig a windlass on top of an old stump on
the hill behind our house to wind the big kite in. I shall never
forget the demonstration the monster gave of its lifting power as
it pulled me off my feet in one of our early attempts to fly it.
Civilized man has always been envious of the flight of birds.
It seemed strange that the lords of creation should be condemned
to progress by such a tiresomely slow method as walking, while
birds and even lowly insects mount the blue heavens on beating
wings and soar over the earth with speedy flight and wide vision.
So mythology has supplied its heroes with a winged horse, a
magic carpet, wings like those of Icarus, or some such contrivance
by which they, in story at least, might move swiftly from place
to place as do the birds.
It is related of Archytas, a Greek who was famous for his
knowledge of mathematics and mechanics, that he made a me-
chanical contrivance resembling a pigeon and that like a pigeon it
could fly. But this is very doubtful, and the tale probably
belongs with other Greek myths indicating desire rather than
achievement. Nevertheless, it does show that even these early
natural philosophers had it in mind to devise a flying machine.
One Simon, a magician in Rome in the days of Nero, so legend
says, actually went up in the air by means of some sort of a con-
trivance, just what we do not know. But he fell and was killed,
and the populace credited his performance to his alliance with
the devil.
It is apparently authentic history that a Benedictine monk
Elemus, at Malmesbury, England, in the eleventh century, built a
machine with wings and tried it from a tower. He glided for a
short distance, but, lacking the skill to balance his appliance,
fell with disastrous results.
A Scotchman, Albert Damien, undertook to fly in 1508 with
a pair of wings made to fit on to his arms and feathered with
86 OUR PHYSICAL WORLD
chicken feathers. He was so confident when his wings were in
the making that they would carry him readily that he proposed
to fly across the English Channel. When he tried them out,
however, jumping from an elevation for a preparatory flight, he
found flying no easy art and fell, breaking a leg and losing his
ambition. Besnier, in France, in the reign of Louis XIV devised
an apparatus of four folding planes that spread out on the down
FIG. 35. — Besnier's flight apparatus
stroke and closed on the upstroke (Fig. 35). They were carried
at the ends of light rods that balanced on the shoulders and were
worked by arms and legs. De Bacqueville, another Frenchman,
in 1744 tried to fly with four large planes, one attached to each
FIG. 36. — Marquis de Bacqueville's wings for flight
hand and each foot (Fig. 36). His idea was that one could
swim in air as in water, since both are fluids, provided hands and
feet could be sufficiently enlarged. He tried his scheme, jump-
ing from a balcony overlooking a river, but fell into a passing boat.
THE CONQUEST OF THE AIR 87
These are only a few of the more famous persons who all
down through the centuries have tried to fly by crude wings
operated by their own weak muscular energy. They were
doomed to failure, for it is estimated that a man can exert such
continuous muscular energy only to the extent of a third of a
horse-power (see p. 183) while it would take some two horse-
power to operate wings with sufficient power to lift him from the
ground.
There followed these first foolhardy attempts at flight, with-
out knowing anything of the principles underlying the process,
a period in which an attempt was made to get at the facts and
discover scientifically the principles. Sir George Cayley, an
English engineer and scientist, as a result of his study and experi-
ments, suggested the use of a steam engine to furnish motive
power for the flying machine and that the engine be made to
drive revolving propellers. He further advised that the wing
planes be curved from front to back instead of being flat, so as
to increase the lifting power. He predicted that a tail plane
would add materially to the stability of the machine. These
suggestions, published in Nicholson's Journal in 1809-10, were
not incorporated into an actual flying machine by Gayley. It
was not until Henson and Stringfellow, an Englishman and an
Australian respectively, built a model aeroplane in 1845 that
any of them took concrete form.
But while Cayley did not build a flying machine he did some-
thing that, at that stage of the development of air craft, was more
important. About 1797 he built a glider, as we should call the
appliance now, and experimented with it. It was really a large,
light plane like a kite but not kite-shaped. Cayley thought
that, if you can raise a kite, a small plane with its attached
string and tail, into the air, a big plane might raise itself and a
man if he would run into the wind with it, holding it tipped up
slightly at the front so the wind could get under it and exert
its power. Cayley 's glider actually worked and lifted him from
the ground, carrying him some distance.
88
OUR PHYSICAL WORLD
The glider has played an important part in the development
of the aeroplane, for it was quite necessary that some skill should
be achieved in balancing the glider before it was possible to fly in
an aeroplane. Without such skill an aeroplane might be made to
rise, but it would dash itself and its occupant to almost certain
destruction. The Lillienthal brothers in Germany, Santos-
Dumont in France, Chanute at Chicago, and the Wright brothers
at Dayton, Ohio, became quite expert in balancing themselves
on their gliders, and succeeded in making fairly long flights.
Lillienthal, by taking advantage of ascending air currents,
occasionally rose above the elevation from which he started
FIG. 37. — Lillienthal's glider
(Fig. 37). Santos-Dumont had his glider towed by a boat after
the manner of a boy running with a kite. Otto Lillienthal met
his death when he tried to fly in a glider to which an engine had
been added. Santos-Dumont and the Wright brothers were
more fortunate, although they were not the first to go up in an
aeroplane carrying a man, as will be related below.
Not only did the glider help in the development of the aero-
plane, but the experience gained in flying aeroplanes has in turn
developed skill in balancing and in making adjustments to the
air currents that have enabled men to make sustained flights in
gliders. H. P. Henzen, a student at the Hanover Technical
School, flew for three hours and a few minutes in an engineless
THE CONQUEST OF THE AIR
89
glider, taking advantage of the air currents to keep him in the
air. This was in the summer of 1922. The accompanying pic-
ture shows one of the French contestants at the gliding contest
at Clermont-Ferrand. He was in the air two minutes, thirty-one
seconds, in this particular flight (Fig. 38).
FIG. 38. — A French glider in flight over the field at Clermont-Ferrand.
Courtesy of the New York Times.
Three methods of getting a heavier-than-air machine to
rise and move through the air have been devised. Naturally,
the first method was by means of beating wings like those of a
bird. A second was by the use of a plane like that of a kite,
which, instead of passively flying on the wind, should be driven
90 OUR PHYSICAL WORLD
by a propeller through the air, cutting it so as to force itself up
as well as forward. Third, just as a propeller drives a boat
through the water, so it was thought might a propeller blade,
rapidly turning, screw itself and the attached machine up into
the air; then possibly a second propeller could drive the machine
in the desired direction.
Now Cayley not only tried the glider successfully, but he was
apparently the first to make a helicopter, as this last-named
device is called. Truly it was only a toy affair, but it contained
the germ of an idea from which much is yet anticipated. The
directions for making a simple flier on the principle of the heli-
copter are given in the Field and Laboratory Guide in Physical
Nature-Study, page 3 1 . Helicopters have recently been built and
flown with success, carrying both pilot and passenger. They
have this advantage over the aeroplane, that they can rise
straight up and do not need a large field from which to start or
on which to land.
M. A. Penaud, a Frenchman, in 1865 built a toy on the
principle of a flying bird, and it worked, the first successful
machine of its type. Later (in 1874) he built another model,
a miniature aeroplane, the motive power of which was twisted
strands of rubber. This worked even better than his orthopter.
Herbert Wenham, an Englishman, coined the word " aero-
plane" (1868) and applied it to the glider. He had the idea also
that such a glider could be forced to rise and carry a man if an
engine could be mounted on it. He was the first to suggest
that two planes mounted one above the other in an aeroplane
would have greater lifting power than the single plane.
The first aeroplane actually to carry an engine and fly was
a model built by Stringfellow, the Australian. He was a skilful
mechanic, and his engines were exquisitely built. He and Henson
had worked together to plan such a flying machine, but it was
Stringfellow who actually completed the work. It was in 1845
that he finished his 8^-pound model, the engine and boiler making
up 5 pounds of this weight. This model was a monoplane, and
THE CONQUEST OF THE AIR 91
it really flew. Later he built several other models; one, a tri-
plane, was exhibited in London in 1868.
About 1 88 1 Horatio Phillips built the first full-sized aeroplane.
He believed that many narrow planes would give greater lifting
power than one broad one, so he rigged fifty planes 22 feet long
and only i| inches wide on a frame so they looked like a large
Venetian blind. Each plane was curved from front to back as
Cayley had suggested, though the hump of the curve was not at
its center, but near the front edge, an improvement that Phillips
devised. This machine of Phillips was mounted on wheels that
ran on a track, and it was held down so it could not fly off and
wreck itself. It registered a lift of 72 pounds besides its own
weight.
Sir Henry Maxim, of Maxim gun fame, was the next to build
an airship. It was a big biplane, 105 feet from tip to tip of its
wing planes. Its four engines each developed 180 horse-power.
They ran two wooden propellers, canvas-covered, that were
1 8 feet long. This machine had a small horizontal plane in
front that could be tilted up and so start the aeroplane on its
rise from the ground. It also had a vertical tail plane that was
movable and could serve as a rudder. Both these additions of
Maxim's were valuable contributions to the structure of the
aeroplane that have been retained, more or less modified, in later
types. This machine of Maxim's ran on a track also and was
held down by guide rails. It developed so much lifting power,
however, that it broke away, raised itself from the ground,
toppled over, and was wrecked.
Clement Ader, a Frenchman, was working on the aeroplane
about this time. He built several machines with batlike wings,
the cloth cover stretched on bamboo and hollow wood-spar
frames. His propellers were four-bladed ones. His machine
ran on wheels on the ground and was free to rise. He called the
machine an avion. In 1890 it actually rose into the air, covering
about 50 yards. It was wrecked when it landed. The French
government gave him a generous grant to continue his experiments
92 OUR PHYSICAL WORLD
and in 1897 an improved avion rose and skimmed over the
earth for 300 yards, the first successful ascent of a heavier-than-
air machine with a man on board. One of Ader's machines
is still exhibited at the Institute of Arts and Sciences in Paris.
This ascent could hardly be called a successful flight, for the
aviator was at the mercy of his machine rather than having it
under control. It was badly damaged when it descended. It
remained for those men who had acquired skill in balancing the
gliders to make the first real flight. Before describing their
experiences, however, mention should be made of the work of an
American inventor, S. P. Langley, then secretary of the Smith-
sonian Institution at Washington.
He began experimenting in 1887 with the avowed purpose of
producing an aeroplane. He made many models, powered with
rubber bands, that flew successfully. He was so encouraged
that he made some larger models that were driven by steam, and
these flew also ; one especially made flights of nearly a mile. The
United States government then put funds at his disposal to build
a large machine. This was provided with a gasoline engine and
tried in 1903. It was the first aeroplane to carry a gasoline
engine — a distinct advance in the power plant of the aeroplane.
The machine carried a weight equal to that of a man as pilot.
It was launched from the deck of a houseboat on the Potomac
River, but the tip of one wing caught on a wire stay on the boat,
and the aeroplane toppled over into the water as it rose from the
deck of the boat. Langley 's funds were now exhausted, so the
machine was housed as a curiosity in the Museum of the Smith-
sonian Institution. It is interesting to note, however, that this
machine was taken out and flown by Orville Wright in 1914
(Fig. 39). This, however, was after the inventor's death.
Meanwhile Orville and Wilbur Wright, of Dayton, Ohio, had
been learning to use the Chanute biplane glider and had altered
and improved it. The fixed tail of the Chanute glider was
replaced by a plane that was movable so it would steer the biplane
up or down. The wings were also capable of movement so that
THE CONQUEST OF THE AIR 93
they could be warped a little, throwing the front edges up or
down as necessity required. In all experiments with the glider
the chief difficulty encountered was found to be the balancing
of the plane. The sea of air in which the pilot launches his
glider is not a calm sea but is full of waves and cross-currents.
Every large obstruction like a hill on the surface of the earth
throws the wind up into a billow. The wind does not blow
steadily but in gusts and flaws that come first from one point
of the compass and then from another. Lillienthal had
FIG. 39. — Langley's aeroplane
endeavored to balance his machine by movements of his body.
He supported himself in his machine by resting on the frame
with the supports under his arms, leaving his body from the
shoulders down free to swing in any direction. If a gust of wind
tended to lift one wing of his machine he threw his body over
toward that wing, so shifting his weight as to bring the wing back
again into its horizontal position. In such a position his body
offered large surface to the wind that tended to retard the flying
of the machine. The Wright brothers were accustomed to lie
on the lower plane, thus reducing the air resistance of their
bodies, and near at hand were the levers that controlled the
94 OUR PHYSICAL WORLD
warping movements of the plane. When a gust of wind tended
to throw up one wing, the front edge of that wing was turned
down while the front edge of the opposite wing was turned up.
As the wings cut the air in this new position, the machine regained
its balance.
Anyone who has undertaken to ride a bicycle will appreciate
in some measure the difficulties encountered in learning to balance
the aeroplane. In balancing the bicycle one has only to avoid
falls to right or left. If you tend to tip over to the right you
turn the front wheel to the right and so bring the line of support
of the two wheels underneath your center of gravity. In the
aeroplane, however, you are not riding on the solid ground but
in unstable air. You are likely to be buffeted by the winds that
blow up and down as well as by cross-currents that come from
right or left. The Wrights, however, became very skilful in
flying their gliders, and then they attached a gasoline engine
to drive such a glider. This engine operated two propellers
by means of chains. They launched their machine from an
inclined rail, a rather bungling contrivance for getting under way ;
but in December, 1903, they made their first successful flight.
This was made in an out-of-the-way place in South Carolina.
After demonstrating to their own satisfaction that they could
really fly, the machine was packed away while they were getting
patents on their various devices.
French inventors were also busy in building and perfecting
aeroplanes. Santos-Dumont, after becoming somewhat skilful
with the glider, undertook flights with an aeroplane which was
constructed for him by the Voisin brothers. This was in 1906.
In November of that year he made a flight of some 230 yards.
He did not follow up his success, however, but abandoned this
machine and undertook the construction of an aeroplane which
should rise from the water. In 1907 another Frenchman who
later became famous in air work, Henri Farman, began practice
with a Voisin machine and before the end of the year made a
flight of nearly half a mile. Another French aviator who was
THE CONQUEST OF THE AIR 95
to become famous, Louis Bleriot, was practicing with a machine
of his own construction. All these French machines were pro-
vided with light wire wheels by means of which they could run
along the ground until sufficient speed was attained to carry
them into the air. In 1908 Wilbur Wright took his machine to
France to demonstrate its abilities, and astonished the French
aeronauts by his easy control of the machine. He was able to
climb rapidly, turn with precision, and dive easily. More than
that, he remained in the air for a much longer time than the
French pilots had been able to, flying steadily for more than two
hours and a half.
The year 1909 was notable in aeroplane achievements, for
there occurred near Rheims the first International Meet. Several
new types of aeroplanes made their appearance, and there were
a number of exceedingly interesting contests. Hubert Latham,
an Englishman, won the prize for the greatest height achieved—
500 feet. He drove an Antoinette biplane. Farnam won the
endurance test, remaining in the air for three hours and four
minutes. He was flying a machine of his own design, a biplane
with a new type of engine, the Gnome. In this engine the
cylinder's revolved, thus cooling themselves as they whirled
rapidly through the air. One of the great difficulties which the
air man had encountered up to this time was the overheating
of his engine. Glenn Curtis, an American, won the speed con-
test with a biplane of his own construction, achieving 47 miles
an hour.
When one realizes that the first successful flight in an aero-
plane was made in 1903, and when comparison is made of the
achievements of this first International Meet and present-day
accomplishment, the remarkable celerity with which the aero-
plane has been developed is truly wonderful. J. A. MacReady,
an American army officer, in 1921 attained a height of 40,800
feet. At such a height the air is so rare and the temperature so
low that the aeroplane and aviator must both be equipped with
special devices. A condenser is added to the engine equipment
96 OUR PHYSICAL WORLD
so as to deliver air to the engine cylinders at normal sea-
level pressure. The aviator wears electrically heated clothing
and a mask which is connected with an oxygen tank so he may
be supplied with the necessary oxygen for respiration. In the
flight of Mr. MacReady, when in 1920 he attained a height of
36,020 feet, the valves of his oxygen apparatus failed to work
properly as they rose into the very thin air. The aviator lost
consciousness; the machine, out of control, fell, but, luckily,
MacReady regained sufficient consciousness at a height of
some 2,000 feet to get control of his machine and make a
landing.
In the Pulitzer trophy race at St. Louis, Missouri, in 1923 an
American, Lieutenant A. J. Williams, won, driving a blue Curtiss
navy plane over the 12 5-mile course at a speed of 243.67 miles
an hour. On a short, straight course a speed of nearly 400
miles an hour has been achieved. Already the Atlantic has
been crossed in a single flight. Furthermore, the aeroplane has
been developed to the point where it is commercially valuable.
Regular mail routes are now established both in Europe and in
this country. New York mail is carried to San Francisco, and
the Pacific Coast mail back to New York. New York, Cleve-
land, Chicago, Minneapolis, St. Louis, Omaha, San Francisco,
Portland, are all connected now by the regular air-mail routes.
Regular passenger service is established. The time from Paris
to London is two hours. The passenger rides in a coach that is
quite as stable, comfortable, and luxurious as a Pullman car.
A simple but very effective type of aeroplane is made as
follows: Cut a f-inch square strip of white pine 22 inches long
(or use a piece of bamboo f inch wide). This strip should be
straight-grained and free from knots, for it serves as the back-
bone of the machine and must bear the strain of the twisted
rubber bands that serve to run the propeller.
Cut a strip of tin 4§ inches long and f inch wide. Bend it
2 inches from one end into a sharp V. Holding it with the long
arm to the left, bend this long arm i inch from its end so that the
THE CONQUEST OF THE AIR 97
bent portion turns to the right and lies at right angles to the rest
of this side of the V. Bend the other arm of the V in the same
direction i inch from its end so that the bent portion is parallel
to that of the first arm of the V. These two parallel parts
should now be bound tightly with coarse linen thread to the end
of the backbone, their long axes coincident with its long axis.
This end is the front end of the machine. Near the tip of this
V and in its midline punch a hole through both sides so that a
stiff wire axle that bears the propeller may run through the holes
parallel to the long axis of the backbone.
For the skids, cut two thin strips of bamboo f inch wide and
6 inches long, and one 4! inches long. Bind these together with
the linen thread in the form of a triangle, letting their ends over-
lap J inch. Bind this to the backbone i inch back of the tin
propeller bearing, the juncture of the two long sides above the
backbone and on the opposite side from the point of the tin strip.
Let the plane of the triangle be at right angles to the backbone.
Cut two more such thin strips 5 inches long and bind one end of
one to the midpoint of one of the long sides of the triangle, the
other end to the backbone about z\ inches back of the point to
which the apex of the triangle is affixed. The other strip will be
bound to brace the other side of the triangle in a similar way.
Cut two more thin strips 5 inches long. Set one on each side
of the backbone i inch from its rear end at right angles to the
backbone and perpendicular to the base of the forward triangle.
Bind them on tightly at their midpoints. Fasten a brace of
bamboo from the upper end of this pair of strips to the backbone
about 3 inches in front of the point where the pair of 5-inch strips
is bound to it.
Cut three strips of bamboo 3 inches long and so thin that each
can be bent into a U over the end of the finger without breaking.
Bind one of these by its ends to the lower end of each side of the
bamboo triangle and one to the lower end of this last support
near the rear, the plane of each U parallel to the longitudinal
axis of the backbone. These three loops form skids on which
98
OUR PHYSICAL WORLD
the aeroplane stands, and they slip along the floor or sidewalk
as the machine takes flight (Figs. 40 and 41).
FIG. 40. — The aeroplane frame
FIG. 41. — Front view of aeroplane frame
THE CONQUEST OF THE AIR 99
Shape a 9-inch propeller out of the tin of a coffee can
similar to the one cut for the flier (p. 90). If the longitudinal
axis of the propeller is made, to coincide with the length of the
can, the curve of the tin will give about the right curve to the
propeller after it is bent according to the instructions. Or a
propeller may be fashioned out of white pine, white wood, or
cedar that is straight-grained and free from knots. Cut the
block of f-inch stuff 9 inches long and 2 inches wide. Bore a
hole at the middle of one broad face just large enough to take the
stiff wire that must be used as the axle for the propeller. Draw
a square i inch on each side, its center coincident with the hole,
its sides parallel to the sides and ends of the block. Draw lines
from its corners to points on the adjacent sides 2 inches from each
corner of the block. Cut
away the sides of the block
along these lines. Mark the
ends of the block according
to the diagram (Fig. 42),
and saw away the wood from
both sides of the diagonal FIG. 42. — Diagram of the end of the
Strip down to the central block from which a propeller is cut.
square. By sandpaper held over the thumb to give a curved
surface or with bits of broken glass having rounding edges work
away the wood of the blades to make them thin and curved
according to the heavy line of the diagram. The blades may
be shaped so that their outer ends are rounded similar to these of
the flier. Cut away the corners of the central block so that it
joins the blades in flowing surfaces.
Pass one end of a 6-inch length of stiff wire through the hole
in the center of the propeller so that it protrudes £ inch. Bend
this protruding end down to the wood center and tack it securely.
If the tin propeller is to be used, stick the wire through one hole
i J inches and b'end it so that the end can be thrust back through
the other hole and twisted on the long wire so as to hold the
propeller securely. A short block of wood set on the back of the
ioo OUR PHYSICAL WORLD
propeller between the holes and included in the loop of wire will
help to hold the propeller solidly.
Put a flat, good-sized bead on the free end of the wire, then
pass the end through the holes in the tin propeller bearing and
make a triangular loop on the wire just back of the bearing to
take the strands of rubber that make the motor. The bead
used helps to reduce friction. Make another small triangle of
wire and bend the free ends so that they can be bound securely
to the front of the rear skid strut about i inch from the backbone.
Pass the long strand of rubber that can be bought for this pur-
pose through this rear wire loop, then through the one on the
rear end of the propeller shaft, and so back and forth until about
ten strands are laid on. Tie the ends of the rubber together to
complete the last strand.
To make the planes, cut two thin bamboo strips | inch wide
and 22 inches long and two 5 inches long, and bind their crossed
ends together so as to make a rectangular parallelogram of the
strips that will serve as the frame for the forward plane. In the
same way make the rear frame for the plane 10 by 4^ inches.
Cover the frames with strong but light paper, folding the paper
over the edge of the frame i inch and gluing it down. Fasten
the forward plane horizontally to the backbone, its long axis
at right angles to the latter, its front edge just back of the struts
that support the forward skids. Tack it lightly in place with
thread. The rear plane is fastened similarly with its hind edge
just in front of the brace that supports the rear strut. When the
planes are in place balance the machine on the forefinger placed
under the backbone near its center. If the planes do not lie
horizontally but tend to dip to one side or the other, their posi-
tion may need to be changed slightly. When they do balance
well, fasten them securely in place, daubing the bindings with
glue so that they will not slip. Guy threads may then be
run from the outer tips of the planes to the adjacent struts to
make them sufficiently rigid to stand the strains of flight
(Fig. 43).
THE CONQUEST OF THE AIR 101
Observe which way the propeller, which is at the front of the
machine, should turn in order to carry the machine in the air,
then turn it about 150 times in the opposite direction. Head
the aeroplane into the wind, set it down on a smooth surface,
like a cement sidewalk, release the propeller and the machine
should rise and fly. If at first it is not successful try shifting
the planes slightly forward or back or changing their inclination.
Possibly you can reduce the weight of the machine. It is
imperative to keep in mind while building the aeroplane that it
FIG. 43. — The aeroplane complete
must be exceedingly light in order to fly and that the parts must
not be made any heavier than is absolutely necessary.
A still larger aeroplane with two propellers is made by making
a triangular frame of f-inch square strips 42 inches long with a
io^-inch strip of the same stuff for the base of the triangle. The
apex of the triangle is in this case to be the front end of the plane
and is provided with a pair of hooks to take the rubber bands,
one set of which runs along under each long side of the propeller
bearing at its hind end. The forward plane is small, about
12 by 4 inches, and is fastened in the plane of the triangle about
6 inches back of its tip, its longitudinal axis perpendicular to the
altitude of the triangle. The rear plane is 36 by 5 inches and
102 OUR PHYSICAL WORLD
attaches in a similar position 6 inches from the hind end of the tri-
angular frame. This plane takes two g-inch propellers. If the pull
of the tightly twisted rubber bands tends to bend the long sides
of the triangle, run fine wires one from the rear of each side to
the apex of the triangle over a 2-inch upright of light stuff set
on the middle of each side and bound in place. Skids may be
provided as in the other plane, but they are not as necessary, for
this plane is started off from the hands, each hand holding one
propeller and letting go as the plane is launched by a shove out
FIG. 44. — Front view of a biplane built by seventh-grade pupils
from the shoulders as the person launching it stands upright.
(See also Fig. 44.)
A very simple aeroplane propelled from a sling shot instead
of by a propeller is made thus: Split a J-inch square wood strip,
10 inches long, at one end. Insert a light card if by 3 inches
so that the ends of the card stick out equally on either side of
the stick and its rear edge is if inches from the end of the stick.
Bind it in place. Tack another card on the stick, the same size as
this, its surface at right angles to the first, its rear edge at the
end of the stick, its ends projecting equally from the sides of the
THE CONQUEST OF THE AIR 103
stick. Parallel to this card, at the other end of the stick, fasten
one 8 by 1 1 inches, its middle on the stick. Notch the stick
under this near the end. Bend a piece of telephone wire in the
form of a Y. Tie one end of a rubber band to the tip of each
arm of the Y. Tie one end of a 6-inch string to the free end of
one band, the other end to the other band. Hold the base of the
Y in the left hand. Hold the aeroplane by the end near the small
cards, between thumb and finger of the right hand, the string
of the sling in the notch near the front end. Pull it back,
stretching the rubbers, and release it for its flight.
CHAPTER IV
AIR AND WATER AS SERVANTS OF MAN
He that will use all winds must shift his sail. — FLETCHER
While the aeroplane has recently come into prominence as a
means of aerial transportation, it was for a long time eclipsed by
the balloon. The first balloon of which we have any record was
manufactured by two Frenchmen, brothers, Jacques and Joseph
Montgolfier. These men observed that clouds floated in the
air, that the smoke from a fire which appeared very cloudlike
rose into the air (see Fig. 61, p. 156), and they conceived the idea
that if one could inclose a cloud or a cloudlike smoke in a thin bag,
it might carry the bag up also. Their father was a paper manu-
facturer and so they could secure some large paper bags. They
tried the experiment of inflating these with the smoke over a fire
and found that they would rise. They then had a large bag
made, some 30 feet in diameter, and proposed to make a public
demonstration of their balloon. This occurred June 5, 1783, at
Annonay, France. People came for miles around to this little
village to see the spectacle, not knowing exactly what it was
they were to see. The huge paper bag, reinforced with cotton
fabric, was held by ropes over a smoldering fire of chopped straw.
Gradually it was inflated, and when finally the restraining ropes
were cast off, it sailed up into the air amid the cheers of the wildly
enthusiastic crowd. The balloon rose rapidly until it was esti-
mated to be a mile high; then, as the hot air in it cooled, it sank
back to earth, having been up about ten minutes.
The fame of this marvelous event quickly spread through
France. The king was desirous of having a demonstration, so that
on September 19, of the same year at Versailles, the Montgolfier
brothers sent up another balloon. This was still larger than the
preceding one and oval in outline, the mouth of the balloon
104
AIR AND WATER AS SERVANTS OF MAN 105
being at the narrow end. A basket was attached to this balloon
for the purpose of carrying passengers. No one, however, was
bold enough to undertake the ascent, for it was not known at
that time whether the upper air was fit to breathe, or in fact if
there was any air up as high as the balloon might go. So the first
three passengers to make a balloon ascent were a sheep, a rooster,
and a duck. The ascent was eminently successful, the balloon
sailed off some distance into the country, and came down in the
field of a peasant. The peasant was thoroughly frightened by
this visitation out of the skies, but the animal passengers were
found to be none the worse for their experience.
On October 15, 1783, the first balloon ascension was made
with a human being as passenger. The daring man to undertake
this was a Frenchman, Pilatre de Rozier. In this first ascent
it was deemed advisable to have the balloon attached to the
ground by ropes so that it might not sail too high. De Rozier
went up 100 feet, remaining in the air for some twenty-five
minutes, and when he came down was enthusiastic over the
delightful sensations of the ascent and the unobstructed view
of the surrounding country that he obtained. In November of
the same year this same man, together with the Marquis
d'Arlandes, made the first free balloon flight. It was then
looked upon as a foolhardy attempt. Their friends bade them
goodbye as if they were going to certain death. The balloon used
was again a hot-air balloon, and they ascended to a height of
about 500 feet, remaining in the air about five minutes.
The difficulty with the hot-air balloon was that the air inside
the bag cooled off rapidly. This of course could be overcome by
carrying a basket below the balloon in which a fire could be built,
and De Rozier, accompanied by the Marquis, made several ascents
in a balloon of this type. The balloonists stood on the platform
below the balloon and fed fuel into the fire which kept the air
hot. They realized that this was risky since the balloon was
constructed of paper covered with cloth and varnished to prevent
the escape of hot air. They accordingly carried a bucket of
io6 OUR PHYSICAL WORLD
water and a sponge. On one of their voyages when they sailed
across the city of Paris, the balloon repeatedly took fire, but they
were fortunate in dashing the wet sponge on to the burning spots
before the flames had done much damage. In this particular
ascent they were in the air twenty-five minutes.
About this time (1784) Cavallo, in England, discovered what
we now know as hydrogen gas, then called inflammable air.
This gas is very much lighter than air, and Cavallo at once saw
that it would be a good substance with which to fill a balloon.
He tried to do this, but he could not get a bag that was suffi-
ciently impervious to prevent the escape of the hydrogen. He
did, however, blow soap bubbles with this gas, and they arose with
celerity. Two French brothers by the name of Roberts and
another Frenchman by the name of Charles did succeed that same
year in building a balloon and inflating it with hydrogen gas.
With such a balloon it was much easier to make prolonged ascents.
In 1794 Monsieur Blanchard, accompanied by a Benedictine
monk, made an ascent at Paris with a hydrogen balloon, reaching
a height of 9,600 feet. In January the next year Blanchard and
an American physician by the name of Jeffries undertook to
cross the English Channel. They started from Dover and were
slowly carried by the wind toward the French coast. It was
only after they had thrown out all their ballast and much of their
clothing in order to lighten the load that they reached shore
safely not far from Calais.
The Frenchman, De Rozier, determined not to be outdone
by any newcomer in the field of aeronautics, also undertook to
make the trip across the Channel. To prevent his balloon set-
tling into the sea as Blanchard's had so nearly done, he fastened
below his hydrogen balloon a hot-air balloon with a fire-basket
underneath it to keep the air hot. When out in mid-channel, at
a height of 3,000 feet, his balloon was seen to burst into flames,
an explosion followed, and De Rozier fell to his death.
The French military authorities were prompt to see the possi-
bilities of the balloon in war time. In 1794 they used a captive
AIR AND WATER AS SERVANTS OF MAN
107
balloon as a means of observing the movements of the enemy,
the Austrians. In this year Captain Coutelle, at the Battle of
Mayence, went up 1,000 feet in a captive balloon which at that
time was beyond the range of the Austrian guns. Here he sat
and dropped written messages giving the French information
regarding the position and movements of the Austrian troops.
The Austrians protested this unfair method of waging war but
the protests were in vain for the balloon came into universal
use in military work. It was used frequently by the northern
armies during the
American Civil War,
again at the Siege of
Paris, 1871, and the
British made use of it
during the Boer War.
During the Great War
with Germany and
her allies, the balloon
was in constant use as
an observation sta-
tion. The old spheri-
cal balloon was early
abandoned during
this war for it was
too unstable, bobbing around with every shift of wind. The
Germans were the first to use the kite balloon, a long, sausage-
shaped affair with a bag at the tail end. The mouth of this bag
faced the wind so that it was blown full of air and served to steady
the balloon as a tail steadies a kite (Fig. 45). The French kite
balloon was an improvement on this, having three of these
balloonettes at the hind end, one on each side and one below.
Now practically all the armies of the world are supplied with
observation balloons and the means of transporting them quickly
by means of automobile and inflating them on the field wherever
they are needed. The observer is in touch with headquarters
FIG. 45. — A military observation balloon
io8 OUR PHYSICAL WORLD
below by telephone, the wire of which runs down the cable by
means of which the balloon is held. This cable winds on to a
drum that is revolved by a small engine so the balloon may be
brought down quickly if desired.
In all of the early balloon ascensions the balloonist was at the
mercy of the winds, but early in the history of the balloon
attempts were made to propel it. It was early suggested that
a balloon might be equipped with sails and a rudder, as is a ship,
but, of course, it was found that such a balloon, because it offered
so large a surface to the wind, still drifted before the wind and
could not get headway enough to steer. A French general by
the name of Meusnier had a balloon made equipped with large
cloth-covered oars and a rudder. The oars were torn away on
the experimental flight by the winds, and the experiment was a
failure as far as controlling the balloon was concerned.
Meusnier's balloon, however, was an improvement in one
respect — it was a long, cigar-shaped affair so built as to offer less
resistance to the air. He suggested another improvement. One
of the difficulties in the early balloons was that the gas would
escape and the balloon would become shrunken and out of shape.
He proposed putting a bag into the balloon which might be
pumped full of air as the gas escaped and so maintain the shape.
Another Frenchman by the name of Giffard was the first man to
attempt to drive a dirigible by means of an engine. Giffard's
engine, however, was not sufficiently powerful as it developed
only three horse-power. His balloon was cigar-shaped, 144 feet
long, 40 feet in diameter at its thickest point. When no wind
was blowing he could drive the balloon at the rate of about
4 miles an hour. Experiments, however, continued to improve
the balloon and its engine. Electric motors driven by storage
batteries were substituted for the steam engine. It was not,
however, until the gasoline engine was introduced as the motive
power that real success came to the dirigible. The two men who
are conspicuously connected with the success of the modern
balloon are Santos-Dumont, a young Brazilian who was work-
AIR AND WATER AS SERVANTS OF MAN 109
ing in France, and Count von Zeppelin in Germany. Santos-
Dumont made the flight from outside of Paris over the city,
going around the Eiffel Tower. Zeppelin's dirigible was built
on a somewhat different plan from that of the Frenchman's.
In the German dirigible a rigid framework of light metal con-
struction contained the numerous gas bags. The car and the
motors were attached to this rigid framework. In the French
balloons the gas bags were held in a net to which, below the
balloon, there was attached the car for the aeronaut and engine.
Later a long metal beam that hung below the gas bags held the
car, thus making a semi-rigid balloon. The shape of the balloon
was maintained by keeping the gas bags well inflated. These two
types of dirigibles, rigid and non-rigid, are still maintained and,
as is well known now, the dirigible was greatly developed during
the war, although it did not accomplish what was anticipated,
especially by Zeppelin, it would do in offensive warfare. How-
ever, the dirigible is now driven by sufficiently powerful engines
to maintain headway even against a stiff wind. Such dirigibles
have made long flights. In 1920, a British dirigible with both
British and American men on board crossed the Atlantic from
Ireland to America and returned. In 1923 the Zeppelin L-72,
rechristened the "Dixmunde" by the French, its new owners, made
a flight of 4,500 miles in a non-stop flight of 118 hours. There
is keen competition between the dirigible and the aeroplane to
see which wfll be more serviceable in the transportation of goods
and passengers, with every prospect that both will serve, each
in its particular field, in solving some of the difficult problems of
transportation (Fig. 46).
It is an easy matter for the child to repeat some of the experi-
ments that mark the discovery of the principles underlying the
operation of the balloon. He may readily make the hot-air
balloon, and directions for this are given in the Field and Labora-
tory Guide in Physical Nature-Study, page 49. He may make
hydrogen gas and inflate soap bubbles as directed in the same
book, page 54, and it is well for the child to go through such
no OUR PHYSICAL WORLD
experiments as a foundation for the comprehension of the science
that is involved. Sooner or later he is bound to know why the
balloon rises. The little child may be satisfied temporarily by
some analogy to experiments with which he is more or less famil-
iar. Thus you may tell him that just as a cork rises and floats
on the surface of the water so the balloon tends to rise to the
upper levels of the air. He may have had the actual sensation
of being lifted off his feet when in swimming, and if he has learned
FIG. 46. — A dirigible balloon, the "Shenandoah," over New York Harbor
to float his experiences may lead him to some appreciation of
the way in which a balloon rises, but in time he will persist in
knowing more in detail the forces that are operative.
In order to understand why the balloon goes up, the child
must have, as a rule, a number of new experiences that will
clarify and render exact his hazy conceptions. The balloon
rises because of the pressure of the air, and the child is neither
familiar with gases nor with the law of pressure. When informed
that the air is a gas he gains little notion of the characteristics of
AIR AND WATER AS SERVANTS OF MAN in
gas, because the air and illuminating gas, which the term gas
usually suggests, are both invisible and not readily handled in a
way that leaves much impression on the mind of the child. Some
experiments with such a visible gas as chlorine, for instance, is
therefore worth while to render his conceptions more definite and
exact. Directions for making chlorine gas are given in the Field
and Laboratory Guide in Physical Nature-Study, page 54. It may
be readily seen; it is heavier than air, therefore it may be poured
from one bottle to another as water might be poured. Iodine
gas may also be readily formed by heating crystals of iodine.
This also is a colored gas and heavier than air. If the child can
see some experiments of this sort he readily gains the notion that
gases are somewhat similar in their properties and behavior to
water, and he will more readily believe that the laws of fluids
apply both to liquids and gases.
Some experiments can be readily performed to demonstrate
air pressure. One of the classic experiments, historically, was
an experiment performed at Magdeburg. Two large metal
hemispheres were placed together so as to form a sphere, their
edges being ground smooth so as to fit together quite perfectly.
The air was then pumped out from the sphere, and when two
horses pulling in opposite directions, one on each hemisphere,
were unable to separate them, it was a striking demonstration of
the pressure of the air on the outside of the two hemispheres.
This apparatus is still known as the Magdeburg sphere and prob-
ably may be borrowed from the physics department together
with an air pump to make the experiment for the children of the
grades.
Light tin cans can now be obtained with a tin cover that
presses into the opening of the can. Such cans are used in home-
canning processes. They are also commonly used as containers
of paint and molasses. Barely cover the bottom of such a can
with water and then set the can on a stove or over a Bunsen
burner and bring the water to a boil. The cover of the can may
be laid on the opening but not forced on tightly. The can now
112 OUR PHYSICAL WORLD
fills with steam driving out the air. When this happens, remove
the can from the stove or flame and force the cover in tightly.
As the can cools the water which was in the form of steam con-
denses and becomes water again. There was, of course, very-
little water in the can to start with, so that as condensation occurs
the water occupies only a very small part of the space. Since the
air was driven out by the steam there is little or no air in the can.
The air pressure on the outside will crumple in the can.
Take a glass tube some 3 feet long and close one end of it
by heating it in the flame. Directions for handling the glass
tube in the flame are given in the Field and Laboratory Guide in
Physical Nature-Study. When the tube has cooled, fill it with
water, put the finger over the end and set the tube held in a
vertical position, mouth down, in a bowl of water. When the
mouth of the tube is under water remove the finger. Since
there is no air in the tube above the water to exert its pressure on
the column of water, the water in the tube is held up by the
pressure of the air on the surface of the water in the bowl. If
possible, repeat this experiment, using mercury in place of water.
Mercury is much heavier than water so the air pressure will only
support a column of mercury in the neighborhood of 30 inches
high. The column of water supported in a similar way by air
pressure is 33 feet high, for mercury is nearly fourteen times as
heavy. This apparatus is the barometer and as the height of the
mercury varies it shows the variations in the pressure of the air.
At sea-level normal pressure is about 30 inches; but this may vary
considerably, dropping as the pressure decreases or rising as the
pressure increases. A barometer is a very useful instrument to
indicate sudden changes of atmospheric pressure that herald the
approach of storms.
Such experiments will help the child to appreciate the fact
that air has pressure. When we say that air has pressure we
simply mean that air has weight. We may demonstrate the
fact that air has weight in another way. On a pair of scales lay
a football that is distended but not blown entirely full and bal-
AIR AND WATER AS SERVANTS OF MAN 113
ance it exactly by weights in the other pan. Now blow up the
football, forcing just as much air into it as you can. Put it on
the scale pan again and you will notice that it is slightly heavier,
due, of course, to the added air that has been forced into it.
Next we need to demonstrate that the pressure of a fluid
is exerted equally in all directions. A tin can with glass tubes
set into its top, sides, and bottom and filled with water will show
that the water stands at the same level in all the tubes (Fig. 47).
See the Field and Laboratory Guide in Physical Nature-Study,
page 51. Pressure, therefore, of the water must be exerted in
all directions in order to maintain the columns of water in all
of these tubes. The pressure in the tube let
into the bottom of the can must be downward
at the end of the tube that projects into the
can. It must be likewise sideways at the
ends of the tubes inserted into the sides of
the can. Another simple device for showing
that the pressure in the water is equal in all
directions is made as follows: Tie a piece | M\
of sheet rubber tightly over the mouth of
a thistle tube. Cut off the stem of the FlG- 47.-Diagnun
,,. ,, , , f ,. ,. , ... , of can with tubes in it
thistle tube (see directions for cutting glass to show water pressure,
in the Field and Laboratory Guide, p. 50)
about i inch from the bulb of the tube. Slip a 3 -foot piece
of rubber tubing on to the short stem of the thistle tube. Draw
out the cut-off stem of the thistle tube at about its middle
point so as to make a fine glass tube. Break this at the
middle point and put one of the pieces into the other end of the
rubber tubing, the fine end out. Press lightly on the stretched
rubber over the mouth of the thistle tube, insert the fine end of
the glass tube into red ink, and slowly release the pressure. The
red ink is drawn up now into the fine end of the tube. By rubber
bands fasten this glass tube on to a ruler or meter stick. Now
stick the thistle tube down into a larger beaker of water. As the
thistle tube goes down notice that the red ink moves in the tube.
H4 OUR PHYSICAL WORLD
The increasing pressure of the water drives in the rubber dia-
phragm, exerts pressure on the air in the apparatus which forces
the red ink to move. Note the depth to which the thistle tube
has been sunk and also the position of the red ink against the
scale. * Now place the diaphragm of the thistle tube at the same
depth in the water but turned up instead of down and note the
pressure. Let it be turned sideways at the same depth. In all
these positions the drop of red ink in the small tube will register
the same on the scale, showing that the pressure of the liquid is
the same in all directions.
Having given the child now some notion of the nature of
gas and an appreciation of fluid pressure and the fact that the
pressure is exerted equally in all directions, we must next give
some conception of what happens when a body is immersed in a
fluid. Cut from a block of plasticine a piece i centimeter wide,
i centimeter thick, and 5 centimeters long. Cut this as accu-
rately as possible. Fasten a piece of thread to this so that it
may be lowered into a glass graduate of 100 cubic centimeters
capacity. Fill the graduate up to the 50 cubic-centimeter
mark with water and then lower into this the piece of plasticine.
The water in the graduate will now rise to the 55 cubic-
centimeter point, and, since the block of plasticine contained
5 cubic centimeters, it is evident that a body immersed in water
displaces its own volume of water. Withdraw the block of
plasticine and press it out of its regular shape between the fingers,
then lower it again into the water. We still, of course, have
5 cubic centimeters of plasticine in the block, and it will still dis-
place the same volume of water, but now the child knows that
this law holds true even with irregular objects.
The history of the discovery of this law is interesting. A
certain king of Greece had given to his artificers of metal a lump
of gold which was to be made into a crown. The king suspected
that his workmen had abstracted some of the gold and that the
crown was made in part of silver — a much less valuable metal—-
which had been substituted for the gold by his craftsmen. He
AIR AND WATER AS SERVANTS OF MAN 115
called in the Greek scientist, Archimedes, and assigned him the
task of determining whether the crown was pure gold. The king
required that he solve the problem within a specified time or
lose his life. Archimedes, therefore, went to the task with much
energy. He knew, of course, that gold is much heavier than
silver, and if he could but know the volume of the crown, knowing
the weight of gold, he could tell how much it should weigh. His
chief problem, therefore, was to find the volume of the crown.
He could not, of course, pound it into a lump that could be
measured, and so he pondered intently on the task of measuring
the volume of the crown. The story relates that he went to take
his bath and rather absent-mindedly filled the tub too full, and
when he got into the tub the water overflowed. Archimedes
saw at once that a body that is immersed in water displaces
its own volume and herein was the means of determining the
volume of the crown. He was so excited that he ran home from
the bath, crying, "Eureka! Eureka! I have found it!" much to
the astonishment of the citizens, for he had not waited to put
on his clothes.
Now cut out another block of plasticine the same size as the
one used above, and weigh it carefully. Again fasten it to the
thread and fasten the thread to the pan of the scales or the hook
of a spring scale. Immerse the plasticine in water as before and
note what it weighs as it hangs in the water. Remove the
plasticine and exactly balance a glass graduate on the scales,
then add 5 cubic centimeters of water. It will be found that
the water weighs 5 grams and also that the difference in the
weight of the plasticine in air and in water is 5 grams. In
other words, the plasticine immersed in water loses as much
weight as the weight of the water which it displaces.
The reason for this is perfectly evident when we consider the
block of plasticine immersed vertically in the water. The
pressure on opposite sides of the block will evidently be identical
since the opposite sides are exactly of the same area and are at
the same depth in the water. The downward pressure on the
n6 OUR PHYSICAL WORLD
top of the block as far as the water is concerned is evidently
equal to the weight of a column of water i centimeter square and
as high as the distance from the top of the block to the surface
of the water. The upward pressure of the water on the underside
of the block is equal to the weight of a column of water i centi-
meter square and as tall as the distance from the underside of
the block to the surface of the water. The upward pressure on
the underside of the block, therefore, exceeds the downward
pressure on the top of the block by the weight of 5 cubic centi-
meters of water, which, as we have seen, is 5 grams.
If now the immersed object weighed less than 5 grams, the
pressure on the underside of the block would evidently force it up
to the surface, and it would rise out of the water until the portion
of it in the water displaced a volume of water equal in weight
to the weight of the object. If you float a cube of cork on water,
and mark the line at which the surface of the water stands on
the cork, then cut the cork in two along this line, you will find
that the weight of the cork is the same as the weight of a volume of
water equal to the part of the cork that was below the water-level.
It is now easy to see why a soap bubble filled with hydrogen
gas, which is lighter than air, rises, but one more thing must be
explained before the rise of the hot-air balloon is clear, that is,
that heat expands things. Unscrew a nut from a bolt and heat
the screw end of the bolt until it is real hot. Now try to put
the nut on. It is evident that the end of the bolt has enlarged,
expanding in the process of heating. Fill a small flask one-third
full of water to which a little red ink has been added to color it.
Bore a hole in a cork that will fit the flask and insert in this hole
tightly a small glass tube, so that when the cork is in the flask
the end of the tube will dip into the water. Insert the cork in
the flask tightly. The colored fluid will rise part way up the
tube. Now hold the flask in your warm hand and watch the
level of the water in the tube. It is very evident from this
experiment that the air in the flask as it warms expands and
forces the water higher up the tube.
AIR AND WATER AS SERVANTS OF MAN 117
When the hot-air balloon is held over a fire the heat
expands the air within the balloon and some of this air must
therefore escape out of the bottom of the balloon. Since there
is less air now in the balloon than there was to start with, the
volume of air in the balloon weighs less than the same volume
of air outside of the balloon, and therefore the balloon will rise if
this difference is greater than the weight of the balloon.
The principles which have just been explained and illustrated
are the ones on which depend the floating of a boat. It seems
strange at first thought that a boat may be constructed entirely
out of iron and steel, substances which will themselves sink
promptly in water, and yet the boat built of them will not only
float but will carry a great load of freight. The explanation is,
of course, perfectly simple. The boat is not solid steel but is a
hollow affair. When it is put into the water it settles down until
the weight of the water which it displaces is equal to its own
weight. As you load the boat it settles deeper and deeper,
displacing an amount of water equal in weight to the weight of
the load added. If you continue to load it, it settles until finally
the edge of the boat is flush with the water. Then, added load
will sink it. All of this may be experimentally verified with a
thin glass vessel or a tin pan floated on water and loaded with
weights. You may mark the level of the water on the vessel
and get the weight of the water it would contain up to that
mark. There will be a slight discrepancy between the weight
of the contained water and the weight of the vessel and its load,
for the contained water measures the volume of the inside of the
vessel while the water is displaced by the outside of it. If the
glass vessel is very thin this discrepancy will be very slight.
When the wind hits the sail of a vessel the force with which
it strikes is resolved into two factors and one of these serves to
drive the boat forward. In a similar way it will be recalled
(p. 80) that the force of the wind is broken into two factors as it
strikes the kite and one element lifts the kite into the air. If
the boat is running before the wind then its sails are set at right
u8 OUR PHYSICAL WORLD
angles to the axis of the boat so as to catch the full force. Still
it can go no faster than the wind is blowing for the sails would
then act as drags and hold it back. But in a good breeze a boat
with its sail set at an angle to the wind (frontispiece) may go
faster than the wind is blowing for the factor that shoves the boat
ahead may be much greater than the resistance the water and
air offer to the hull and superstructure of the boat.
This art of sailing a boat with the sails set so the wind strikes
them at an angle is a fairly recent innovation. In old times the
sailboat simply ran before the wind. It was not until 1537
that Fletcher, an Englishman of Rye, discovered it was possible
by proper adjustment of sails and rudder to sail a boat into the
wind — a discovery of great importance commercially, for ships
now sail to their destination even with a head wind. The dis-
covery was of great importance historically, too. When the
great Spanish " Armada" set sail to conquer England the ships
were of the old type — high out of the water to catch all pos-
sible wind. They were able only to run before the wind in the
storm that struck them. The English boats were low-lying
vessels that could sail into the wind and could easily gain posi-
tions to rake the Spaniards with their broadside of cannon fire
and get away before the Spanish gunners could return it. Be-
tween the storm and the new type of sailing vessel that had
come out of Fletcher's discovery, with the new skill in handling
such craft, the course of events in history was turned quite
unexpectedly.
In the twenty-five years or so prior to our Civil War no sailing
craft in the world were as famous for speed as our American Clip-
pers. The American merchantmen were then the world's greatest
carriers, and our foreign carrying trade was exceeded by no other
nation. The " Flying Cloud" in a trip from New York to San
Francisco ran 1,256 miles in four days. The "Sovereign of the
Seas" in one day's run sailed 411 miles while the " Lightning,"
record-maker, sailed 436 miles in one day. These are good records
even for steamers.
AIR AND WATER AS SERVANTS OF MAN 1 19
When the boat is driven by a propeller the force with which
the propeller blades strike the water is decomposed, one element
serving to drive the boat ahead just as the propeller of the aero-
plane carries it through the air (p. 99). At present the record
for speed boats is held by " Miss America II." Her official record
is 80.56 miles per hour, made in 1921. Nothing like this speed
is maintained in commercial craft. Still the best of the trans-
atlantic liners now make 23 to 25 knots per hour, and the latest
battleships make 35, while destroyers run at still higher speeds.
We do not know at all who first devised the boat, and we
can only guess the steps by which its discovery progressed. Still
the very primitive types of boats yet in use help us to formulate
guesses that are probably nearly correct. As far back as history
goes sailboats were used and these, quite pretentious ones. On
an old vase, now in the British Museum, which was found in an
Egyptian tomb is the relief of a sailboat. This boat was also
manned by many oarsmen, for on the Nile wind is not always a
dependable motor power. This vase is one of the oldest relics
of ancient Egyptian civilization that has come to light, probably
3,000 years old or more. In the oldest code of laws yet dis-
covered, laws written on the clay tablets of the ancient peoples in
the Tigris and Euphrates valleys, there were strict regulations
in regard to the course of vessels and their movements when
passing each other or in coming to port. Marble models of
boats from this same time, probably votive offerings, show their
general shape and structure, the holes for the masts and the
rigging being visible still, though masts and shrouds are gone to
dust.
Probably boats have been devised and used independently
by various peoples in different parts of the world, and we shall
never know exactly by whom the various types of primitive craft
have been invented. One can readily imagine, however, how
the savage, desirous of crossing a stream, straddled a floating
log and trusted to the current or the wind to land him on the other
shore. He must soon have found that his arrival at his destina-
120 OUR PHYSICAL WORLD
tion was rendered more certain by poling his way across. Boats,
or rather rafts, consisting of a few light logs or poles fastened
together with thongs or ropes of grass, are still to be found in
China, Japan, and other countries where the light bamboo makes
ideal material for such craft. One wonders how many centuries
it was before the primitive boatmen learned to use the pole as
a paddle when they worked out into deep water. Then some
inventive genius fashioned a paddle more skilfully, widening its
blade, and so contributed to the advance of mankind. Then it
is to be presumed some tribes living along shore, instead of
trusting to luck to find a suitable log when needed, pulled up the
logs, once used, on shore to be used repeatedly. One can
readily conceive how some fellow, brighter than the rest, chipped
off with his stone hatchet a place to sit, so as to make his log
more secure and more comfortable. In time the log was all
flattened, for standing on a slippery rounded log is precarious
business. Finally the log was dug out so as to hold the fish, the
products of the chase, or the boatman's belongings when he
went on long expeditions.
Such dugouts are still widely used and are no mean boats.
A huge log is shaped by the patient labor of many workers toiling
with crude tools. It may be chipped out or hollowed by fire.
Such a craft may hold thirty or forty warriors. These war
canoes are used by the people of Africa, South America, Asia,
and by the South Sea Islanders. The latter tribes have increased
the stability of the canoe by fastening long, light logs out at each
side by means of poles. These outriggers prevent the canoe from
capsizing and make it quite seaworthy.
Possibly the next step in advance was taken when it occurred
to some early man that he might save labor by fashioning a frame-
work of light sticks and covering it with skin or bark, thus
avoiding the task of cutting out the hard heartwood of the log.
Perhaps such canoes were made first in a region where timber
was scarce. At any rate, boats of this type are still familiar, such
as the birch-bark canoe of the American Indian, the skin-covered
AIR AND WATER AS SERVANTS OF MAN
121
kyaks of the Eskimos, and the curious basket-like coracles of
the Welsh (Fig. 48).
Probably very early in his primitive life man discovered the
value and use of some of the simpler machines such as the lever
and the wedge. Much later, he devised the more complex con-
trivances to aid him in his tasks. The windmill and the water
wheel are among the earliest of them to appear.
The windmill is a rimless wheel the spokes of which are flat
or slightly curved blades set at an acute angle to the plane of the
wheel. The principle of operation is simple. When particles
of air moving along the surface of the earth as a wind strike
these blades, the mill
headed into the
wind, the force of
the blows is resolved
into two compo-
nents, just as in the
case of the kite
(p. 80), and one
of these component
forces turns the mill
around. A crank
arm attached to the FlG- 48'~A coracle
axle of the wheel which turns with the wheel transmits the power
to the pump or other machine to be operated by the mill.
To make the paper windmill, take a 6-inch square of paper,
preferably colored paper. If the paper is not already cut in such
form, proceed as follows to cut a 6-inch square out of any rec-
tangular sheet of larger size. From any corner of the sheet
measure 6 inches along each adjacent side, and mark the points.
Fold the corner over and crease the paper along the line connect-
ing the marked points. With the scissors, cut the paper close
to the folded-over edges.
Draw lines on the 6-inch square, running from each of two
adjacent corners to the diagonally opposite corners. Cut in
122 OUR PHYSICAL WORLD
from the corners along these lines to within a half-inch of the
intersecting lines. Lay the left hand, back down, on the paper,
the fingers about at the center. With the right hand fold in any
one corner and hold it with thumb and finger of the left hand.
In the same way fold in every alternate corner around the square,
and when all are in hand run a pin through the four infolded
corners and also through the center of the square. Thrust this
pin into a wood handle and the windmill is complete.
An eight-point windmill may be made in place of the four-
point, as follows: It makes the mill more attractive if paper of
two colors is used. Cut a 6-inch square of paper of each color,
and cut in from the corners as before. On one paper make a
half-inch cut at the inner end of each diagonal cut on the left-
hand blades, making it at right angles to the edge. Lay this
square upon the table, the second square upon it so that the
centers coincide and so that the corners of the upper sheet are
midway between the corners of the lower sheet. Then insert
each alternate edge of the upper blades into the cuts on the lower
blades. Then fold over all the inner points as before and run
the pin through them and through the centers of the two sheets.
Stick the pin into a handle.
To make the wooden windmill, cut two 8-inch lengths of wood
| inch square. Find the middle of each piece and mark a cross-
line at this point. Draw two lines parallel to this, one at each side
of it, TV inch distant from it. Saw into the strip on each of these
two lines, cutting halfway through the strip. Cut out the central
block. The two strips may now be put together at right angles
to each other, the space formed by cutting out the block fitting
over the remaining section of the other stick. See that they fit well.
With a knife shave off the opposite angles of one arm until a
thin blade of wood is left. The central region is not cut away,
but bevels on the thin blade. Cut each of the other arms in the
same way, so that the blades are inclined in the same direction.
Fasten the mill thus formed securely to a cylindrical stick some-
what larger than a pencil.
AIR AND WATER AS SERVANTS OF MAN
123
The base of the windmill is built thus: Cut a 3-inch length
of ^-inch stuff that is i inch wide. At each end with small
brads fasten on a 2 -inch length of the same material at right
angles to the 3 -inch strip, the two shorter strips parallel to each
other and on the same side of the 3-inch strip. Bore a hole near
the top of each 2 -inch piece, the holes in line so that the cylin-
drical piece fastened to the windmill may be run through them.
Bore a hole in the middle of the 3-inch
piece. This is fastened to the upright
piece, which should be f inch square
and 8 inches long. Cut a thin piece of
wood out of a cigar box or simi-
lar material to form the vane of
the mill. Let this be 6 inches
long and 4 inches wide, with a
projecting piece sticking out
from the 4-inch side, the projec-
tion to be i inch long and J inch
wide. Tack this projection to
the 3 -inch strip that makes the
base of the structure that carries
the mill so that the vane pro-
jects from the base in a vertical
plane parallel to the cylindrical
strip that serves as the axle for
the mill.
When this vane is on the basal strip, fasten the base to the
upright support by running a flat-headed wire nail through the
hole bored in the basal piece; drive it in through the center of
the end of the supporting upright. Put the axle of the mill
through the holes bored in the supports and drive a couple of
small brads through the axle, one on either side of one of the
supports, so that the mill will be held in place.
The blades of the old-type windmills were wooden frames
covered with cloth and were often spoken of as the sails (Fig. 49) .
FIG. 49. — An old-fashioned windmill
124 OUR PHYSICAL WORLD
The mill usually bore four sails. In the modern mill the blades
are smaller, more numerous, and made of wood or steel.
In many parts of the Old World as well as in America the
country landscape is dotted with such mills raised into the breeze
on towers. They furnish the farmer with power for pumping
water, for running his electric plant, his churn, and many other
small farm machines. They have been used, too, for power to
grind his grain. When not in use the mill is turned with the
edge of the wheel into the wind instead of its face. This is
easily accomplished with the modern, small, light mill but it
was not so easy a task with the old mill with its great expanse of
sails. Sometimes the sails were furled as on a ship. Again a great
slanting beam was attached at one end to the axle of the mill
while the other end rested on the ground or was attached to a
wheel on the ground. Then horses or oxen could be attached to
this end so the mill could be turned on a pivot into the desired
direction. Another scheme was to have the tower or perhaps
merely its top rotate on its axis and turn by means of a rack and
pinion that could be operated by a great hand crank.
The early water wheel was a paddle wheel. The blades were
wide, usually four in number, and radiated from the hub with
their faces set at right angles to the plane of the wheel. Such a
mill wheel might be set so its blades dipped one after another
into a stream of water that ran under it, the undershot wheel, or
the water coming from some source above the wheel was led by
a trough or flume so it fell on the tip of the blade, first one,
then another, as the wheel was made to revolve by the falling
water, the overshot wheel.
Now, however, it is much more customary to set a wheel like
a windmill at the bottom of a vertical pipe through which water
is flowing from some height when the wheel is turned by the
passing water just as the windmill is turned by the passing air.
Such a wheel is known as a turbine. We have seen (p. 112) that
water in a vessel exerts a pressure of about 15 pounds per square
inch for every 33 feet of height of the water. So that if the
AIR AND WATER AS SERVANTS OF MAN 125
column of water in the pipe is several times 33 feet the force
exerted on the blades of the turbine is as many times 1 5 pounds
to the square inch. This force is resolved into two factors, one
of which pushes the wheel around. Such a turbine wheel set
at the bottom of a waterfall with water filling the pipe as it
flows into its upper end at the top of the falls may develop a
tremendous horse-power. So they are using a part of the water
at Niagara Falls to develop power for manufacturing plants.
Every stream with a rapid current may be dammed, and the fall-
ing water be used in a similar way (Fig. 50, p. 126). In Switzer-
land the railroads are to be run entirely by electricity developed by
power plants that are to be operated by such means. The work
of installing the necessary turbines and power stations is proceed-
ing rapidly, and some sections of the lines are now operated by
electric locomotives. The Lake Ritom power-house receives the
water from a source far above the station. It is led in through
cement conduits and has a head of 2,580 feet, so giving a pressure
at the turbines of 1,150 pounds per square inch. Six turbines
are installed that yield 70,000 horse-power.
Some of our own transcontinental lines are using electric
locomotives in the mountain sections, the power being furnished
by hydroelectric plants, and are finding that they can haul the
trains more rapidly and more economically. About 20 per cent
of the freight-hauling capacity of our railroads is now used in
distributing the fuel needed to supply their engines, while another
10 per cent is used in hauling the coal in the engine tenders.
Electrification would save this wastage.
The utilization of our water power — white coal, it has been
aptly termed — will relieve greatly the demand made now on our
fuel supply. It is estimated by the United States Department
of the Interior that we have in this country an available water
supply of 60,000,000 horse-power, and that of this we are now
using some 10,000,000 horse-power, thus saving annually about
33,000,000 tons of coal. Some of the states are very fortunate
in possessing many streams with precipitous descents — notably
126
OUR PHYSICAL WORLD
AIR AND WATER AS SERVANTS OF MAN 127
the mountain states — from which they may develop immense
power for factory purposes. Vermont is already using its water
power nearly to the limit; Illinois is using about 50 per cent of
that which is available; Washington, about 5 per cent.
One of our great national problems is the careful develop-
ment of this power so that the rights to its use may not fall into
the hands of private interests without ample compensation to
the people of the states and nation for its use. Timber lands,
coal lands, and mineral lands belonging to the people as a whole
have been sold to private concerns and to individuals for a mere
pittance, and these lands have yielded millions of dollars to such
private interests with no return to state or nation other
than the meager purchase price. Thus the iron and copper
lands of northern Michigan were sold in many cases for
$1.25 an acre. One mine, the Calumet and Hecla, yielded
over $13,000,000 worth of copper to enrich its owners. Thou-
sands of acres of government land on which stand the great
western forests, the finest in the world, have similarly been sold
when the lumber from a single tree will pay the purchase price
many times over. One of the great redwoods yields enough
lumber to build several bungalows. It remains to be seen
whether we as a people will part with our water power in the
same careless manner.
Another device that has been of inestimable value to man is
the pump. It, too, depends on these principles of fluid pressure,
although it was in use long before the principle of its operation
was understood. Both lift pump and force pump may be readily
constructed and the method of operation will be better under-
stood after they have been made and operated.
The lift pump is made readily as follows: Take a length of
good-sized glass tubing 12 inches long, a paraffined mailing-
tube, or a piece of bamboo. Cut a piece of wood 15 inches long
and about as large around as a lead pencil, for the plunger
handle. At one end of this fit a slice of cork for a plunger and
fasten it securely. The cork should fit the tube snugly. Punch
128 OUR PHYSICAL WORLD
a hole through the cork and then with a small tack fasten a flap
of leather so that it will cover the hole on the handle side, the
tack being placed at one side of the hole. The cork should be
free to slip up and down rather tightly in the tube when worked
by the lift handle. Put a cork in the lower end of the tube,
having first made a hole in it, and cover the hole with a leather
flap held by a tack, the flap being on the inner face of the cork.
Put this corked end of the tube in the water and work the plunger
back and forth. If properly constructed, the water rises in the
tube and is pumped out at the top. A tube made of rolled paper
may be set with glue in the mailing tube or bamboo, to serve as a
spout.
To make a squirt gun fit a cork into one end of a good-
sized glass tube or length of bamboo, but before inserting it
file or cut a groove on one side. Make a plunger, as was done
for the pump, except that there will be no valve in this. Put
the head of the plunger into the free end of the tube or length of
bamboo, drive it down nearly to the cork, put the corked end
under water, draw the plunger back slowly, lift the corked end
above the water, and drive the plunger rapidly down. This
squirt gun illustrates the principle of the force pump.
As the stream of water comes from the force pump of the
waterworks into the faucets in the house, or from the hose nozzle
connected to the fire engine, the stream is a steady stream and
not a succession of spurts. This change is brought about by the
addition of an air chamber, which has an inlet and an outlet.
The water coming in, in a succession of spurts, crowds up against
the cushion of elastic air, the pressure of which sends the water
out in a steady stream. Replace the cork in the squirt gun with
one having two holes, one for intake, one for outlet. Put short
lengths of glass tubing in each so that the ends are flush with the
small end of the cork. Attach a leather valve over the intake
tube so that it will let water in but not out. Attach a rubber
tube to the intake pipe and let its free end set in a glass of water.
Fit a cork with two holes intp a 4-ounce wide-mouthed bottle.
AIR AND WATER AS SERVANTS OF MAN
129
Put lengths of glass tubing into the cork, the end of one flush
with the inner end of the cork, the end of the other reaching
nearly the bottom of the bottle. Put a valve over the end that is
flush with the cork so that it will let water in. Connect this one
by a short length of rubber tubing to the outlet of the squirt
gun, now to be used as a force pump. Connect a short rubber
tube to the outlet of the small bottle and put a pipette glass into
the other end of this rubber tube. Then
operate the pump and a steady stream
will issue from the pipette " nozzle."
When the handle of the ordinary
pump is brought up (see Fig. 51) the
plunger is forced down in the cylinder,
the air escaping through the valve in it.
When the handle is forced down the
plunger rises, the valve closes at once,
and so a vacuum tends to form under
the plunger. The pressure of the air
on the surface of the water forces the
water part way up the pipe. The valve
in the bottom of the pipe lets the
water in but prevents its escape as the
plunger descends again. This process
is repeated until the water rises to the
plunger, when it flows through the
valve opening as the plunger is forced down. The water is then
raised above the plunger until it flows out of the spout. If the
plunger and valve get dry so they leak air, they must be made
air-tight by "priming" the pump, pouring water into the pump
from above. Such a pump cannot work when the distance from
the plunger to the surface of the water in the well is over 33 feet.
FIG. 51. — Diagram of a lift
pump.
CHAPTER V
THE SLING, BOW, AND OTHER WEAPONS
Fight, gentlemen of England! fight, bold yeomen!
Draw, archers, draw your arrows to the head!
— SHAKESPEARE, Richard III.
We think of this modern age as the age of great inventions,
and justly so, for more inventions of major importance to civiliza-
tion have been made in the last hundred years than in any like
period. The aeroplane, automobile, gas engine, telephone,
telegraph, locomotive, steamboat, harvester, spinning jenny, and
many others occur to one on a moment's reflection, all belonging
to the years since 1 800. And yet we must not forget that our very
early forebears also made great discoveries and that we are
indebted to them for many of the most important inventions
that are fundamental to our activities. They discovered how
to produce and use tools, weapons, language, fire, how to plant
and cultivate crops, how to domesticate animals, how to cook
food, build houses, make clothing. Should not that savage who
first conceived and put into practice the idea of planting seeds
where he wanted them to grow instead of searching for his grains
and fruits where they had planted themselves, or that one who
first cultivated his garden patch with a sharp stick, be accorded
quite as great glory as he who perfected the harvester? I
wonder what savage first used a sharp-edged flake of flint to cut
the meat from the dead beast instead of tearing it off with fingers
or teeth, who first used a stone as a hammer, or first found he
could hurl a stone and kill his quarry. Such primitive tools and
weapons are a far cry from our modern machine tools and engines
of destruction, yet they were prime discoveries, and since their
invention we have merely improved them.
130
THE SLING, BOW, AND OTHER WEAPONS 131
I suppose the first weapon was a club that the savage dis-
covered increased the reach of his arm and force of his blow, or
possibly it was a stone held in his hand to add to the power of his
punch. Then he learned to throw the stone or hurl his club as
a crude spear. Finally, he discovered how to shape his spear
to make it more effective, how to make devices that would hurl
the stone or spear farther than he could unaided, and so came
the sling, bow, blowgun, and other similar appliances.
Such progress as is here briefly sketched in few words took
long ages to accomplish. Man has come up very slowly from a
savagery that was next door to animal existence. For tens of
thousands of years his language was made up of grunts and
gestures. He built no shelter, made no clothes, had no tools, no
weapons, ate raw foods, since he had not learned the use of fire,
and trusted largely to chance for them, eating only as luck
gave him a meal. In fact, his existence was a bestial one with
merely a shade of advantage over his animal competitors because
of his increased cunning. This is not merely guesswork, for we
have discovered the skeletons of these early men in caves where
their bones, together with the bones of some of the animals that
lived there, have been covered up and preserved- by deposits
of lime or accumulated clay. There are no vestiges of tools,
weapons, utensils, no evidence of fire or clothing, as are found in
similar situations among the remains of the men of later ages.
One of the very early weapons of mankind was the sling.
Every child is familiar with the story of David and Goliath and
will recall that it was with the sling that David killed Goliath.
This sling is made out of a piece of leather large enough to hold
the stone that is to be thrown. A leather thong or string some
30 inches in length is tied on each side of the leather; the free
ends of the strings are held in the hand, one firmly, the other so
it can be readily released. The sling with the contained stone
is then swung round the head and, when the stone is swinging
with great rapidity, the thong is released and the stone flies out
of the sling. The boy who undertakes to use this sling for the
132 OUR PHYSICAL WORLD
first time should go well away from buildings and companions,
for at first the stone is likely to be thrown in a direction quite
different from that intended, and it requires much practice to
become skilful in hitting a mark.
This simple weapon is illustrative of several important scien-
tific principles. Primitive man, of course, did not comprehend
these. In fact, we usually acquire control over the forces of
nature by a trial-and-error method. We learn first how to do
things and later inquire why things behave as they do. It is
always interesting, however, when we can understand the reason
why. When any object is at rest it requires the application of
force to move it from this position of rest, and when a thing is in
motion it tends to continue that motion in a straight line unless
something acts upon it to stop it or start it moving along another
line. This is called the law of inertia. When the stone is
swinging rapidly around in the sling and one thong is released
the stone moves in a straight line in the direction that it was going
at the moment of release, and it keeps on going until it is stopped
by striking some object. If no other object is struck it is, of
course, striking particles of air all the time and gradually these
check its movement and it drops to earth pulled down by the
earth's attraction, what we call the force of gravity. The stone
is held in the sling because every moment it tends to fly off in a
straight line, and so presses against the leather which restrains
it. Probably most children have amused themselves by taking
a small pail partly full of water and holding the handle of the
pail in the hand have swung this around in a vertical circle. Of
course, when the pail is directly overhead with its mouth down,
the water would spill out of the pail if the pail were not being
swung rapidly. B ecause of the inertia the water tends to fly away
from the center of the circle in which the pail is being swung and
therefore presses against the bottom and sides of the pail, so remain-
ing in the pail, a demonstration of the so-called centrifugal force.
This experiment will help one understand why the stone stays in
the sling when it is merely laid in the leather and not fastened to it.
TEE SLING, BOW, AND OTHER WEAPONS 133
It is this centrifugal force that causes a flywheel or rapidly re-
volving grindstone to break and fly in pieces, sometimes doing
much damage. This same force is used in the cream separator and
the centrifugal laundry wringer. In the latter the clothes are
put in a rotating drum with perforated sides, out of which the
water is thrown as the drum whirls. In the cream separator,
water, casein, and other heavy parts of the milk are thrown out
from the rapidly rotating bowl while the light cream remains at
the center.
The common top is an admirable illustration of this same law
of inertia. When the top is set spinning each particle of it
travels in its own path and resists any force that acts to move it
out of that path, so that while it would not for a moment stand
straight up on its peg if it were not spinning but would promptly
topple over, when it is set going it resists the pull of gravity and
stands erect as it spins. When the top is spinning on your hand
you may incline your hand but the top remains upright. The
skilful lad even lets the top spin down a string stretched from
hand to hand, one end lower than the other, and the top main-
tains a fixed inclination as it slides along instead of falling off, for
its inertia resists the pull of the earth.
A passenger boat is just being put on the route from New
York to England that has in its hold a great metal disk weighing
100 tons that is set on an axle in a frame so it may be rotated
with great speed. It is expected that this great rotating disk
will resist the force of the waves that makes a ship roll and keep
it steady, a gyroscope stabilizer. If the device is as successful
as its designer expects, many passengers will be delighted to
have the good ship spin its top all the way over.
The bow and arrow are very old weapons. Crude, chipped-
stone arrowheads are found very deep in piled-up strata of soil,
clays, sand, and gravel that must have taken many thousands of
years to accumulate. In the same beds in which the arrowheads
are found, there have been discovered in Europe parts of the
skeletons of very primitive men and of ancient animals that man
134 OUR PHYSICAL WORLD
then hunted but which are no longer living in Europe, such as
the straight-tusk elephant, the mammoth, the hippopotamus,
the giant beaver, bison, and the lion. The wounds made by the
hunters' stone- tipped arrows are still discernible in some of the
bones of the well-preserved animal skeletons. Just how old
these early arrowheads are, there is difference of opinion, but
probably they were made by primitive man well over 100,000
years ago. There are still savage tribes who hunt with the bow
and arrow, so that it is a weapon that has been used by man
these many hundreds of centuries.
The bow and arrow are largely confined to those savage
peoples inhabiting regions where some very elastic wood grows.
It is essentially an arm of the natives of North America and
Asia. In the latter territory the bamboo is used chiefly in its
construction; in North America, however, a great variety of
woods enter into its construction. The Indians of California
used the desert juniper; the plains Indians, the osage orange,
called by the French Bois d'Arc, or bow wood. In many cases
the bow was backed with deer sinew glued on and strengthened
by encircling bands of sinew along the bow. Among the
Eskimos the sinew furnishes the elasticity entirely, the wood
being applied in small bits for the sake of rigidity, for it is scarce.
The bow has played no mean part in the history of the civilized
world. Apart from its service in obtaining food and clothing for
man by bringing down the quarry for the huntsman, it has been
the deciding factor in many a hard-fought battle. The armies
of the ancient peoples, the Babylonians, Egyptians, Greeks, and
Romans, all have had bodies of trained archers. The Hebrews
found some of their foes so well trained in the use of the bow that
they were compelled to adopt it, also, and train their archers.
It was not until the long bow was perfected by the Scotch and
English that the bowmen came to be really formidable. This
long bow was 6 feet or more in length, was made of stout yew
or lance wood, and drove a feathered arrow 30 inches in length
with such tremendous force that it would go entirely through
THE SLING, BOW, AND OTHER WEAPONS 135
a deer at 300 yards. The Indian buffalo hunter often drove his
arrow through the huge beast, firing from horseback as he rode
beside the herd. In such famous battles as Crecy and Agincourt,
the lance, sword, and bow were the weapons in use, but the last
was the most important. So thickly did the arrows fly that
armored knights were in a perfect storm of them, and woe betide
the warrior whose armor offered the slightest opening for the
expert bowman.
The manufacture of the bow and arrow was a craft by itself.
The weapons needed to be made with as much nicety and as
much care in the selection of the material as the modern firearm.
The bow was usually made of several strips of wood glued together
and not infrequently was made in parts, a central portion and
end pieces. Sometimes several different kinds of wood entered
into the composition of the bow, but the best of the English
weapons were made of yew, carefully selected, thoroughly sea-
soned, and free from all blemish. The bow, when strung, was
curved, the string standing about 6 inches from the middle of
the bow. The arrow was also made with great care and precision.
The best of them were perfectly straight, uniform in diameter
throughout, tipped with a metal point, and feathered at the
opposite end so as to make them fly true. Peacock feathers were
generally used for this part of the arrow as the web of the feather
is tough and retains its shape well.
The Indian arrow-maker was an exceedingly skilful crafts-
man. It was a hard day's work to make one arrow. The stems
of the reed, Phragmites vulgaris, straight willow wands, the
so-called white cedar or arbor vitae, the red cedar, striped maple,
and many other woods were used. The material was carefully
selected, seasoned with care, scraped down to uniform size,
straightened by laying on a hot, grooved stone and bending to
take out slight irregularities. The tip of chipped stone or of
fire-hardened wood was fastened in with sinew cord and glue
and the feathers were applied to the base. Three half-feathers
were bound on equidistant from each other by sinew cord or
136
OUR PHYSICAL WORLD
FIG. 52. — The crossbow
vegetable fiber. The shaft of the feather lay parallel to the long
axis of the arrow or perhaps slightly inclined— the latter to make the
arrow rotate as it flew.
Among some savage
tribes the arrowheads
are barbed with
thorns, fish spines, or
porcupine quills to in-
flict as bad a wound
as possible and to
make them difficult to
withdraw.
The crossbow was
used in European
armies as an improve-
ment on the bow. A very strong bow
was set at the end of a grooved stick.
A small windlass at the other end drew
back the string which could be released
by a trigger. The arrow or bolt lay in
the groove and was driven at the foe
or game by the bowstring. The cross-
bowmen made a formidable part of
the army (Fig. 52).
The bow and crossbow as weapons
in war and in the chase were replaced
by the gun, when powder was intro-
duced into Europe. Archery still
exists, however, as a national sport
among many peoples. The Royal
Scottish Archers, the Woodsmen of
Arden, and similar organizations still
keep alive the use of the bow and arrow in England, and there are
several archery associations in this country. It is no mean art to
acquire — this handling of such a powerful bow (Fig. 53). The
FIG. 53. — An archer in correct
position.
THE SLING, BOW, AND OTHER WEAPONS 137
bow is held about its midpoint in the left hand, the arm fully
extended. The arrow is laid upon the first finger of the hand
that grasps the bow, the notch of the arrow is placed upon the
bowstring at its midpoint. Three fingers of the right hand are
laid upon the string, one above the base of the arrow, two
below. Just the tips of the fingers are on the string. The string
is then pulled back, the base of the thumb going back against
the cheek. The bowman then quickly sights along the arrow
and releases the arrow by a movement of the wrist, turning the
hand slightly to the right.
It requires a strong arm to pull back one of these bows until
the head of the arrow is drawn back to the bow, and when
released the arrow flies with great speed. The archer ordinarily
wears leather caps for the fingers of the right hand that hold the
bowstring and wears on his left arm a leather protector so that
the bowstring when released will not injure the arm. The archer
must of course allow for the direction and force of the wind and
for the drop of the arrow in response to the pull of gravity when he
is shooting at long range. The sport is a very attractive one
and may be begun in a very simple way. Directions for making
the beginner's bow and arrow, the crossbow, and the target are
given in the Field and Laboratory Guide in Physical Nature-Study.
When a bow is bent, then springs back to its original shape
when released, the wood is manifesting what is called elasticity. It
is a familiar property of many substances. It is the elasticity
of the steel in the watch spring that keeps the watch running,
the elasticity of a rubber ball that makes it bounce, the elasticity
of the air in the automobile tire that makes the machine so
springy, the elasticity of the wood that makes the springboard
toss one into the air. The molecules of solid substances are
definitely arranged and spaced in relation to each other, so that
the solid in many cases seems to resist any distortion of this
arrangement. This does not mean that the molecules are fixed,
for they are moving with tremendous rapidity in a tangle of
interweaving pathways, yet on the whole the general pattern
138 OUR PHYSICAL WORLD
of their arrangement remains constant. When this arrangement
is disturbed the elastic body tends to resume its normal condition
the moment the strain is removed and rebounds with as great a
force as was applied to produce the distortion. Similarly, gases are
made up of molecules much more widely spaced than those of solids
or liquids, and these molecules are moving in relatively wide
pathways with still greater speeds than those of solids. When the
gas is compressed or crowded into smaller space, moving mole-
cules repel each other more forcefully and hit the sides of the
container much more frequently, because there are more of them
moving in a given space, and so they exert upon the walls of the
container an ever increasing pressure. Gases exhibit elasticity
to perfection.
Various engines of war from the days of the primitive bow-
men to the present have largely depended upon this property of
elasticity for their efficiency. A device in use by ancient people
was the catapult (Fig. 54). It consisted of a heavy, inclined
plank with one end fixed firmly in a framework, the other free
to move, and levers and pulleys so mounted that the free end
of the plank could be pulled back until it was bent like one end
of a huge bow. A great rock was then placed on this end,
which was suddenly released, throwing the missile at the enemy.
The huge plank was bent back by the labor of many men
working with levers or windlass for considerable time, and this
energy stored in the bent plank was suddenly released to act
upon the rock.
These ancient engines of war were replaced by the gun when
powder was introduced into Europe from China. Just when it
was discovered there is not known, but old pictures of naval
engagements show the vessels obscured in clouds of smoke,
presumably made by the firing of guns many centuries prior to
the introduction of powder into Europe. This event occurred
in the fourteenth century. The early gun was a metal tube on
the end of a straight stick. The powder was touched off through
a small hole in the base of the tube by means of a lighted stick.
THE SLING, BOW, AND OTHER WEAPONS 139
This gun could not be aimed with accuracy, but was merely
pointed in the general direction of the enemy. From this primitive
arm to the modern high-powered rifle or the great coast-defense
guns is a far cry, and yet the steps have been merely improvements
on the primitive weapons, not the application of new principles.
The force exerted on the bullet in the gun is that of the elas-
ticity of the gases formed when the powder is burned. Gun-
powder consists of a mixture of several solids; charcoal, sulphur,
and saltpeter have been the ones most commonly used. When
charcoal burns it unites with the oxygen gas in the air and forms
the gas known as carbon dioxide. Saltpeter is a substance
FIG. 54. — The catapult
containing a large amount of oxygen which it readily gives up.
When the gunpowder is exploded the supply of oxygen, to combine
with the carbon, is thus obtained, not from the air, but from
the solid saltpeter. Sulphur and oxygen also readily unite to
form a gas, and they unite at a considerably lower temperature
than do carbon and oxygen; the sulphur, therefore, is put into
the gunpowder so that it may be readily touched off. When heat
is applied to the gunpowder the oxygen of the saltpeter combines
with the sulphur and the carbon to form gases that occupy a
large amount of space. The gases formed occupy at atmos-
pheric pressures from 300 to 500 times the space occupied by
the solid substances of the gunpowder. A small quantity of
140 OUR PHYSICAL WORLD
gunpowder set off in the open does not explode but merely burns
rapidly. If, however, this same gunpowder is put in a confined
space as it is when rammed down in the gun barrel and then
touched off, the gases formed need to occupy so much more space
than the solids that the elastic force exerted is very great. The
bullet, therefore, is hurled out of the gun barrel with great speed.
If the powder is confined in a hole bored in rock and then
touched off the expansive force of the gases is so great it bursts
the rock.
The first crude gun was rapidly improved. The metal tube
or barrel was fitted to a stock that was shaped so as to rest
against the shoulder, enabling one to aim the piece and lessening
also the effect of the recoil. In some of these early guns a small,
toothed, steel wheel, bearing upon a piece of flint or pyrite, was
rotated rapidly by a little crank, so furnishing the spark that
set off the powder in the pan. Later a hammer carrying a piece
of flint struck a piece of steel so producing the spark (Fig. 55).
The flintlock musket was the arm of the British army until
1844. Most of these old guns were loaded from the muzzle.
The charge of powder, as also the ball, was held in place by
wadding and was rammed down with the ramrod.
The next improvement of prime importance was the substitu-
tion of a percussion cap for the flint and steel. The hammer
struck the cap which was set on a hollow post over the powder
charge. The cap, a small, cup-shaped metal affair, contained a
substance that when struck with the hammer exploded and drove
a flame down to ignite the powder. This improvement was used
on sporting guns for some time before it was used on the guns
furnished the armies, for it was an expensive proposition to
change the type of gun for an entire army.
Next came the breech-loading gun. The breechloader was
devised long before it came into general use, in fact there were
breech-loading guns made back in the fifteenth century, but it
was always a difficult matter to make the breech so tight that
the explosion would not blow it out. When this was finally
THE SLING, BOW, AND OTHER WEAPONS 141
accomplished, the breechloader came rapidly into use, for it could
be loaded so much more rapidly than the old muzzle-loading gun.
A shell containing powder charge and bullet and with a percussion
cap fixed in one end was introduced into the stock end of the
barrel. The hammer struck a movable pin that rested against
the percussion cap in the shell. The Prussian army was fur-
nished such breech-loading guns at the time of the war with
Austria, 1866. The war was of very short duration, for the new
type of arm was so much more efficient than the old muzzle-
loading gun that the Austrians were repeatedly routed with
exceedingly heavy losses. This war was such a conclusive
FIG. 55. — The flintlock of an old musket
demonstration of the value of the breechloader that all the
European nations proceeded to furnish these guns to their
armies.
In the old guns the bullet was a leaden ball, slightly smaller
than the bore of the barrel. It could not be fired at long range
with any great accuracy for it struck first one side of the
bore, then the other, as it was shot out and never went very
straight. It was so large in cross-section it offered great resist-
ance in its passage through the air, and its speed was rapidly
checked. When attempts were made to use long, slender,
sharp-pointed bullets to overcome this difficulty, they would go
tumbling end over end through the air in irregular courses.
142 OUR PHYSICAL WORLD
Finally, however, a device was found to overcome this. The
gun barrel was grooved with spiral grooves. The base of the
bullet was hollowed out and a sharp-pointed metal or wooden
ping was set in the hollow so that the explosion of the powder
drove in this peg, expanding the base of the bullet so the lead
was forced into the grooves, thus giving the bullet a twisting
motion about its long axis as it sped away from the gun, and it
would keep going straight. Moreover, this device made the
bullet fit the barrel tightly so no gases escaped around it as had
happened when the loosely fitting ball had been used. Now
there are added to the gun a magazine to hold a number of shells,
a shell ejector, and accurately gauged sights.
The old muzzle-loading guns could not be fired very rapidly
for the loading process was slow and one must stand up to accom-
plish it. Loading at the breech was quicker, and it could be
done lying flat on the ground, so offering little target for an enemy
to shoot at. Added speed in firing was possible with the inven-
tion of the magazine gun. Several cartridges are carried in a
chamber in the stock. By the movement of a lever the empty
shell is ejected and a loaded one is brought up from the magazine
and slid into position ready to fire. In some rapid-fire machine
guns the force of the recoil is made to eject the old shell and bring
the next one into position. The shells are introduced in a long
belt, and the gun keeps up a continuous fusillade of shots, a
steady roar of discharge.
When the bullet leaves the gun, gravity at once begins to pull
it down to earth at a rate of 16 feet the first second. The bullet
fired from the modern high-powered rifle has a velocity when it
leaves the muzzle of a half-mile a second. If the gun is aimed
at an object only 100 yards distant, the bullet is pulled down by
gravity only a few inches before it reaches its mark. But if the
object is a half-mile away, then the bullet must be fired several
feet above the object in order to hit it. The sights in the modern
gun can be set for various distances, and the muzzle is elevated
more and more for increasing distances. In recent tests of
THE SLING, BOW, AND OTHER WEAPONS
143
machine-made ammunition manufactured in an American arsenal
the 3-foot bull's-eye was hit 176 times in succession at a range of
800 yards, and 41 times in succession at 1,200 yards, over two-
thirds of a mile.
The early cannon were made of wood, later of brass, were
muzzle-loaders, touched off with a blazing torch, and the shot were
at first stones, then solid iron balls. They were small, primitive
affairs, inaccurate in their fire, and were first used in Europe
with no expectation of killing people but merely to scare the
horses on which the armored knights were riding. In the fa-
mous "Constitution" that won renown in our War of 1812, the
guns were about as long as a
man, mounted on crude, wooden-
wheeled carriages, and the muzzle
was lowered by driving a wedge-
shaped, wooden block in under
the butt of the gun (see Fig. 56).
The improvements in the
cannon followed along the same
lines as the changes in the small
arm. Breech-loading took the
place of muzzle-loading. The
round shot was changed to a
pointed cylinder which was given a rotary motion by spiral grooves
in the barrel. Now the great guns are sometimes 75 feet long, and
throw a shell that weighs more than a ton 20 miles. The bore of
such a gun is 12, 14, or 1 6 inches. Naturally, such a gun could
only be manufactured when machinery had been devised for
handling it, forging it, boring it accurately. Ways have to be
devised also for strengthening it, for the pressures of elastic
gases formed by firing the charges of powder — hundreds of
pounds — are terrific. The gun must resist a bursting pressure of
50 tons or more per square inch.
The same elasticity of gases that is used to work such awful
devastation in war is also immensely serviceable to man in peace.
FIG. 56. — An old cannon on its
wooden carriage.
144
OUR PHYSICAL WORLD
Powder and other more powerful explosives are essential in
blasting out coal, the ores of the metals in the mine, the stone
in the quarry. Air compressed by powerful engines is sent
down by iron pipes and hose into mines and quarries to furnish
the power for drills that make the holes in which the charge of
FIG. 57. — A drill operated by compressed air in a quarry
explosive is placed to smash the rock into pieces that can be
handled (Fig. 57). The hand-power air pump is familiar to
most boys and girls who have ridden a bicycle and to the auto-
mobile driver, for by its aid the tires are inflated. The principle
of operation is very simple. A piston-head fits a metal cylinder
tightly. In this head is a valve that lets air into the cylinder on
THE SLING, BOW, AND OTHER WEAPONS 145
the upstroke, but closes when the down stroke begins. A second
valve lets the air out of the cylinder on the down stroke into the
tire, As more and more air is pumped into the tire the elasticity
of the air increases and so the pressure against the inner wall of
the tire is greater and greater. When the valve is open between
the cylinder of the pump and the tire, the air in the pump is
exerting the same pressure on the plunger and walls of the cylinder
as is exerted on the walls of the tire. If the plunger were larger
the upward pressure upon it would be difficult to overcome, and
it would take more power than the average person has to force
the plunger down. Sometimes the automobile pump is made
of two cylinders with a plunger in each. The upstroke of the
plunger in the large cylinder drives the air into the small cylinder,
and the down stroke drives the somewhat compressed air into
the tire. The cross-section of the plunger in the small cylinder
may have an area of, say, only one-quarter square inch. It
would have to be pushed down, therefore, only with a force
slightly exceeding 15 pounds to overcome a pressure in the tire
of 60 pounds per square inch. Yet the large cylinder has capa-
city enough so that the tire can be pumped up quite rapidly.
CHAPTER VI
FIRE AND ITS USES
Fire is a good servant and a bad master. — OLD DANISH PROVERB
It is difficult to say which of the discoveries primitive man
made in his gradual conquest of nature was most important, yet
certainly the ability to make and utilize fire was one of the most
important, possibly the most important. It added very greatly
to his creature comforts, and opened up the way to a multitude
of added discoveries in the arts and industries.
Undoubtedly he came to use fire before he knew how to make
it. Possibly he took it from some red-hot lava stream, some
flaming vent of natural gas that flowed in his neighborhood, from
a forest fire started by lightning. Once he knew its value, he
guarded the glowing embers with jealous care. It seems to have
been one of the functions of the early priestly caste to keep the
fire blazing, and that blaze may well have been regarded as
sacred, so important was its continuance in the life of the com-
munity. Possibly the savage went to the sacred places to renew
his own home fire. In the pioneer days in our own country it
was no uncommon thing to go miles to the nearest neighbor to
borrow fire to start the blaze on the hearth when it accidentally
went out. Even in historic times savage peoples have been found
who did not know the use of fire. Magellan in his exploring
trips found such on islands of the Pacific.
In these modern days when we start a fire so easily with a
match it is difficult to realize that the match is a recent invention,
and that for many centuries flint, steel, and tinder box were used
to start a fire, or possibly the fire stick, the fire drill, or some such
cumbersome device. There are still primitive peoples that use
the fire stick and fire drills. The former is a sharp-pointed stick
FIRE AND ITS USES
147
of hard wood that is held in the hand and plowed back and forth
in a groove in a block of soft wood. A fine wood dust is thus
made in the groove which is ignited as the friction of the two
pieces of wood develops heat. The glowing spark is nursed
with shreds of dry bark or punk, blown into flame, and so the
fire is started.
The fire drill (Fig. 58) works in much the same way except
that the stick of hard wood is given a rapid rotary motion while
its point is pressed
down into a shallow
hole in the softer wood.
Among some tribes
this rotary motion is
imparted to it while it
is held between the
palms of the hands.
In other cases the
thong of a bow is
wrapped about the
drill a time or two,
and as the bow is
drawn back and forth
the drill is turned
rapidly. The upper
end of the drill rests against a leather pad or wood block placed
against the chest. The operator of the drill kneels, bends
over the drill, and so has both hands free to operate the bow
or thong.
The first match was devised by Chancel. It consisted of a
bit of wood tipped with a gum containing chlorate of potash
and sugar. This was dipped into strong sulphuric acid to ignite
it. Most persons preferred to carry flint and steel and tinder
box rather than sulphuric acid, for the latter burns badly and
makes holes in clothing wherever it touches. So this type of
match was never widely used. It was not until 1835 that the
FIG. 58. — Parts of a fire drill and its use
148 OUR PHYSICAL WORLD
friction match was invented. It rapidly replaced the flint and
steel. The bit of dry wood is tipped with a paste containing
some substance that ignites at low temperatures, such as phos-
phorus, one that burns readily like sulphur, and a substance that
parts with its oxygen easily as do potassium chlorate or man-
ganese dioxide. The friction of striking the match generates
heat enough to ignite the phosphorus, which lights the sulphur,
which makes heat enough to start the wood burning. The white
phosphorus used in these early matches was poisonous, and
sometimes a child was killed by eating the heads of matches
carelessly left where it could get them ; and the workers who made
matches were affected by a very painful disease, a result of inhal-
ing the fumes. The sulphur used produced choking fumes when
the match burned. So the phosphorus is now replaced by sub-
stances like antimony sulphide or phosphorus sulphide, which
also ignite at a low temperature but are safe; and paraffin is
used in place of sulphur. In the safety match the potassium
chlorate and antimony sulphide or similar substance is used in
the head, and the red phosphorus is present in small quantity
in the prepared surface on which the match must be scratched
to light it readily.
Break a lump of sugar into smaller lumps and these into
still smaller bits. You might think you could keep on doing
this indefinitely if eyes were sharp enough to see the finer particles
and fingers were skilful enough to use fine-pointed instruments
to do such a delicate job. But the chemist and physicist tell us
that this is wrong and that sugar (and, in fact, every substance)
is made up of very minute particles called molecules that cannot
be broken up without destroying the sugar as such. True, the
molecule is made up of still smaller particles, the atoms, but when
the sugar molecule is broken up into its atoms we have carbon,
hydrogen, and oxygen, simple substances having properties quite
unlike sugar.
Now atoms of substances like carbon, hydrogen, and oxygen
have a very strong attraction for one another and tend to rush
FIRE AND ITS USES 149
together in intimate associations or molecules like sugar. Oxygen
atoms have a strong attraction for carbon atoms, uniting vigor-
ously to form carbon dioxide. Similarly, oxygen and sulphur
unite to form sulphur dioxide, and phosphorus and oxygen unite.
These atoms rush together with such energy that the molecules
are set into rapid vibration. So heat is generated together with
light, and we say the substance burns. A burning substance as
usually understood is one whose atoms are uniting with oxygen
so rapidly as to produce heat and light. Oxidation, the union of
a substance with oxygen, may go on slowly and no heat or light
be noticeable. When iron rusts, it is uniting with oxygen, but
slowly. Other substances may unite chemically so rapidly as to
produce heat and light. Thus, if powdered antimony is sprinkled
into chlorine gas there is so rapid a union of chlorine and antimony
to produce antimony chloride that heat and light are produced.
We might say the antimony burns in an atmosphere of chlorine.
It is a simple matter to generate oxygen and to collect it in
quantity (see Field and Laboratory Guide in Physical Nature-
Study^ p. 60). When a splinter of wood is lighted and allowed
to burn a moment, then the flame is blown out, leaving a glowing
ember, and this is stuck into a jar of oxygen, the splinter bursts
into flame again. A bit of sulphur when lighted burns sluggishly
in the air, which is only about one-fifth oxygen, but introduced
into a jar of oxygen it burns freely with a bright light. Iron
picture wire, which does not burn at all in air, burns vigorously
in oxygen, throwing off showers of sparks.
The explanation of the process of burning is now so simple
that the child may get a reasonably clear notion of it. Yet it
quite mystified our great-great-grandparents. In their day the
four elementary things were earth, air, water, and fire. Every-
thing was made of these mixed in varying combinations and
proportions. True, the notion of atoms had occurred to the old
Greek philosophers, but it had been a shrewd guess rather than
a scientific theory based on anything like adequate evidence.
Even this was lost sight of during those dark ages that followed
150 OUR PHYSICAL WORLD
the submergence of the old civilization by the hordes of
barbarians. Joseph Priestly, an English clergyman, who also
delighted to make chemical experiments, discovered oxygen
(1774), and described its properties with considerable accuracy.
He and the chemists of his time were beginning to realize that
these four so-called elements were not elements at all. Priestly
showed that one of the substances in the air was oxygen, or as he
called it "dephlogisticated air." He used the term air as we use
the term gas. Thus hydrogen he also knew as ( 'inflammable air/7
and carbon dioxide as "fixed air," because it was fixed or united
with other substances in limestone. He believed as did most of
the chemists of his day that fire was due to the escape of
an "inflammable principle" called phlogiston from substances
when they burned. Since oxygen would not burn as inflammable
air did, he thought the phlogiston had been in some way taken
out of it, so it was "dephlogisticated."
Lavoisier, a French contemporary of Priestly, proved that
when a substance burned it gained weight rather than lost it,
and so must take up something instead of giving it off. He was
convinced that burning was the union of oxygen with the burning
substance. Priestly had visited Lavoisier and talked this matter
over with him and yet the old notion of phlogiston was so fixed
in his mind he could not see the truth.
We really owe a very great deal to the scientists who devote
themselves to discovering truth for its own sake. It is only when
we understand the nature of things and of the forces that operate
about us that we can make rapid progress in the invention of
those devices that make life easier and more agreeable.
When the nature of fire was understood there soon came
discoveries of new applications of it to the arts and industries
and improvements in the old ways of doing things. Primitive
man warmed himself beside a fire built in the open, and cooked
his food over it too. When he built a fire in his cave or shelter
the smoke made its escape as best it could. Cottager and
nobleman alike among our Anglo-Saxon forebears must choose
FIRE AND ITS USES 151
between cold dwellings and suffocating smoke. The chimney
did not appear in England until the thirteenth century and then
it was merely a hole in the wall over the fireplace. The built-
up chimney with extension above the roof is a modern conven-
ience. The fireplace was the best means of heating the house
and of cooking until 100 years ago, when the iron stove came
into general use. Brick or tile stoves were used back in the
Middle Ages — really a fireplace set out in the room. Cardinal
Polignac, of France, invented an iron stove in 1709, but it was
Benjamin Franklin who devised improvements that made it
really practicable (1745).
We have seen in an earlier chapter why the hot-air balloon
rises. The heat expands the air in it so that some of it must
flow out. The balloon, therefore, contains less weight of air
than a corresponding volume of surrounding air. Since the
upward pressure on the underside of the balloon is greater than
the downward pressure on its upper surface by an amount equal
to the weight of the air the balloon displaces, the balloon rises,
provided this difference in weights is greater than the weight of
the balloon and its trappings.
In a similar way the column of air in the chimney is heated
by the fire in the stove or the fireplace, and, expanding, it over-
flows. The column of air in the chimney, therefore, weighs less
than a corresponding column outside because there is less of it.
The air in the chimney is forced up and out of its top as the cool
and heavier air rushes in at the bottom. This air is in turn
heated and so the draft up the chimney is continuous.
An efficient fireplace is built with slanting sides and rear wall
so as to reflect the heat out into the room, with a large smoke
outlet whose cross-section area is not less than one-eighth that
of the fireplace opening, and with the front edge of the latter
opening considerably below the smoke outlet so smoke will not
get out into the room. Its depth should be about the same
as the length of the rear wall and the height of the front open-
ing not over three-fifths of its length (see diagram, Fig. 59),
152
OUR PHYSICAL WORLD
FlG> 59 —A fireplace: (a) face; (b) vertical section showing plan; (c) floor plan
FIRE AND ITS USES 153
The stove has numerous advantages. As its radiating sur-
face is relatively large, much more of the heat from the fire is
radiated into the room, and much less goes up the chimney. By
dampers set in the stovepipe and drafts below the fire box that
may be opened and closed, the flow of air through the fire can
be controlled and so the rate at which the fire burns. On the
cook stove the utensils may be heated by contact with the hot
surface, and not get covered with soot as they are in cooking over
an open fire. Certain metals are very good conductors of heat,
such as aluminium and copper, while others conduct it poorly.
It is an advantage to have the heat conducted rapidly to the
thing in the kettle or pan that is to be heated. So the teakettle
often has a copper bottom, and cooking utensils made of alumin-
ium are often used.
You may readily demonstrate that there is a difference in
the heat conductivity of various substances. Take a piece of
No. 1 8 copper wire 8 inches long and one of iron wire of the same
size and length. Twist them together at one end so as to form
a V. Fix a little ball of paraffin or beeswax as big as a pea on
each wire halfway from the point of the V to the end. Holding
the V by its ends stick the point of the V in a flame, the arms
horizontal. Continue holding it thus until both wax balls fall off.
You will find that the one on the copper wire melts enough to
fall long before the one on the iron wire, for copper is a better
heat conductor than iron. We shall find that it is also a much
better conductor of electricity.
We put coverings of poor conductors like asbestos felt on
steam pipes and furnace pipes to prevent loss of heat. We
build the fireless cooker (see Field and Laboratory Guide in
Physical Nature-Study, p. 58) by inclosing the pail in which the
cooking is to be done in a box packed with some non-conductor
like chopped straw or else surround it with several air spaces
separated by sheets of asbestos. Dry air is itself a very poor
conductor. So the thermos bottle is merely a bottle surrounded
by several air spaces, or, better still, spaces in which there is no
154
OUR PHYSICAL WORLD
air. These, of course, must be air-tight. Any hot substance
in the bottle cannot lose heat to the surrounding air, while if
the bottle contains a cold substance the heat of the surrounding
air cannot get to it to warm it. We put a storm sash on our
windows to inclose a layer of air between the window and the
storm sash. These numerous
substances in the path of the
radiating heat tend to reflect it
and prevent its escape, for heat
is reflected just as is light. Put
a thermometer bulb at the point
at which the light from a lamp
is brought to a focus by a con-
cave mirror and the mercury
rises rapidly.
Now, too, to avoid dust and
dirt in our homes the heating
plant for the house is often put
in the basement. Hot-air pipes
from the furnace conduct the
heated air to the rooms above
on the same principle that the
chimney carries the hot air up.
In the same way the hot- water
pipes conduct the hot water up
to the radiators and as it cools
off in them, it flows back to the
heater, so forcing up the hot
water. In the hot-water sys-
FIG. 60. — Diagram of a hot-water plant
tern an expansion tank must be used, because when the cold
water is heated it expands and unless there were a chance for an
overflow it would burst the pipes and radiators (see diagram,
Fig. 60).
In the steam-heating plant, the water is boiled in the base-
ment, the steam goes up through pipes to the radiators to warm
FIRE AND ITS USES 155
the house, there condenses to water again as it cools, which flows
back to the heater through return pipes.
Since dry air is a poor conductor of heat it is important to
keep the air in the house moist; otherwise the heat from the
radiator does not readily pass to your body. It is quite as
important to have a hygrometer in the living-room to see that
the air is moist as it is to have a thermometer to see that the
temperature is correct. One feels comfortable when in fairly
moist air at 68° F., whereas in dry air the temperature may have
to be 75° to give the same feeling of comfort. Evidently it is
good economy to keep the air moist. This may be accomplished
by a water pan kept well filled in the hot-air furnace or by pans
of water hung on the radiators in hot-water or steam-heating
systems.
Just as the fire in the fireplace or stove causes the heated air
to rise in the chimney because the heavier cool air forces it up,
causing a draft, so any mass of heated air surrounded by cooler
air rises as the cooler air pushes it up and comes in with a rush
as it takes its place. A great fire in the open heats the air
above it, and the surrounding cool air blows in as the hot air
rises, causing local winds (Fig. 61). When the air becomes
heated over any area on the earth as over a desert, it rises and the
cool air around it blows in. The equatorial regions of the earth
are hot, and the air over them rises. We do not notice rising
or falling air as a wind — only air that is moving horizontally
along the surface of the earth. So in the equatorial regions there
is a belt of calms. The cooler air flowing in from north and
south along the surface of the earth on the edges of this belt of
calms makes winds. These do not blow straight from the
north or south, for the air is coming from regions where it is
rotating with the earth at less speed than that of the equatorial
region. Because of inertia the inflowing air tends to keep its
slower rate of rotation, and the more rapidly moving equatorial
region slips along under it from west to east, so that the winds
s_eem to come from the northeast north of the equatorial belt of
156
OUR PHYSICAL WORLD
calms, from the southeast south of it. These constant winds are
known as the trade winds.
The temperature of the air is not the only factor that deter-
mines its weight or pressure. If it is carrying a great deal of
moisture, it is lighter than when it is dry, because the water
vapor displaces air and the latter is heavier than the former.
FIG. 61. — A fire. Note the piece of roofing carried up by the hot air
The combination of these factors with others produces belts of
high pressure from which the trade winds move toward the
Equator, and other less steady winds known as "the westerlies"
which move toward the poles. The air does not move due north
and south toward the poles, but for the reason already indicated
these constant winds blow from the southwest in the Northern
Hemisphere, and from the northwest or nearly west in the
FIRE AND ITS USES 157
Southern. The course of both trades and westerlies is further
made irregular by the irregularity of the distribution of land
and water. Still they are sufficiently regular to be of much
importance in commerce, and were much more so in the days of
sailing vessels.
In addition to these general air movements from temperate
regions toward the Equator and poles along the earth's surface
and in the reverse direction high up in the air, there are local
winds produced as variations in heat and moisture develop local
areas of high and low pressure. The winds blow along the earth's
surface from the high-pressure areas to the low-pressure. Daily
reports of atmospheric pressure are sent from many stations all
over the country to the Weather Bureau at Washington so that
with a knowledge of the location of high- and low-pressure areas,
the country over, the probable direction of the wind at any
locality can be predicted. If the difference in pressure between
adjacent high and low areas is very great, the winds will be
strong; severe blows can be foretold in time to warn vessels
and persons interested in such forecasts.
When moist air is rising into the upper atmosphere which is
cool, the moisture will be condensed to form clouds, and if the
rising air is very moist, the condensation produces rain. The
air coming into a low-pressure area from the south is usually
warm and moist; therefore clouds and showers may be expected
on the south side of a low-pressure area. On the other hand, the
air coming in from the north is cool and dry, and since it grows
warm as it moves southward it can take up additional moisture.
On the north side of a low-pressure area fair weather may be
expected. Having reports from many stations on the humidity
of the air as well as on temperature and pressure, the Weather
Bureau embodies these in the daily weather map on the basis of
which the predictions are made (Figs. 62 and 63, pp. 158, 159).
Improved industrial processes offer very many illustrations
of the way in which our knowledge of fire and its methods of
control have contributed to the advance of civilization and the
158
OUR PHYSICAL WORLD
multiplication of creature comforts; one must suffice here.
Primitive man used chipped-stone implements because he did
not know how to obtain anything better. Our American Indians
used copper to some extent. They found bits of float copper
brought by the glaciers from the great deposits in northern
Michigan or in similar locations, and fashioned an occasional spear
head or knife from it, but the Indian was still largely in the
stone-implement stage when Columbus came to this shore.
There came a time when early man learned how to extract the
FIG. 62. — A weather map of the United States
metals from their ores. That was so very long ago we do not
know what his methods were. But following the man of the
chipped-stone age and of the polished-stone age, there came
peoples who made bronze utensils, and that time is known as the
Bronze Age. Bronze is made by melting together tin and copper.
So those people must have know how to extract tin from its ores.
We know the tin mines of Cornwall, England, were worked during
Roman times and probably very much earlier.
Then came the age of iron implements. Some savage tribes
have today very crude processes for extracting iron from its ore.
FIRE AND ITS USES
159
Possibly the process was discovered when some savage used an
easily reduced ore of iron like siderite (see p. 50) to build a fire-
place, and found after many fires a bit of iron in it that could be
hammered out into serviceable shape. At any rate, the iron
forge among some African and Asiatic tribes is today simply a
hole dug in a high clay bank to serve as a fireplace in which a
charcoal fire is built and bits of iron ore and limestone are added,
then more charcoal, limestone, and iron ore, layer after layer.
The wind may furnish the draft, or simple bellows made of the
FIG. 63. — The weather map one day later than Figure 62
skin of an animal may be used. After the fire has been kept
going for many hours it is allowed to go out, and at the bottom
of the hole there is dug out of the ash and debris a bit of iron.
Such a process of reduction is exceedingly slow. The quantity
of iron produced is small and it, therefore, is very costly. Iron
was, among early peoples, often used as money.
The modern furnace does not differ in principle of operation
from such a primitive affair as that described. The ores of iron
most commonly used are oxides of iron, chemical unions of iron
and oxygen. They melt at high temperatures at which the
l6o OUR PHYSICAL WORLD
oxygen of the ore unites with the carbon present as charcoal or
coke and forms gaseous oxides of carbon. Some impurities in
the ore such as phosphorus and sulphur also unite with the
oxygen to form their oxides, also gases, while others like silicon
unite with the limestone or "flux" and form a glassy slag.
It was found as the furnace stack was made larger that the
melted iron because of its weight sank to the bottom of the stack
while the melted slag, being lighter, floated on top of it. The slag
could be drawn off, and then the iron through a lower hole, and so
the furnace could be run continuously instead of letting the fires go
out to get the iron. The stack came to be larger and larger,
was built of brick and lined with firebrick, and the bellows was
operated by power. Still later a rotary fan was used to blow
the draft into a furnace. The ores, flux, and charcoal were taken
to the top of the stack by elevator, piled on a movable lid on
the top of the stack, and fed into the stack when this lid was
opened by machine power. When the iron was drawn off it
was run into a trough in the molding-sand floor adjacent to the
stack, and from this was led into small tributary troughs where it
hardened into "pigs," so called because they lay side by side
like a row of nursing pigs. Iron thus produced was called pig
iron (Fig. 64).
There was a vent from such a stack carrying off into the air
the inflammable gases. Now these are brought by great pipes
down under the boilers to make steam for power to handle the
ore, flux, and charcoal or coke, and under great steel stoves that
heat the air to be driven into the furnace so the fires in the stack
may not be cooled by the entrance of cold air. The pipes that
carry this hot-air blast into the furnace have their points cooled
by a jacket of constantly changing water so that they will not melt
in the intense heat. The ore, flux, and fuel are handled by
machinery, so that human labor is reduced to a muiimum.
All the products which distil off as the wood isheated in the char-
coal kilns (Fig. 65) or the coal is made into coke in the ovens, and
which at one time were turned into the air as wastes, are now
FIRE AND ITS USES
iCJi
FIG. 64. — A blast furnace. Courtesy of the Pioneer Furnace Co., Marquette,
Michigan.
FIG. 65— A lii
162 OUR PHYSICAL WORLD
caught, and by proper treatment are turned into valuable com-
mercial products. Thus, wood alcohol, acetic acid, creosote,
tar, heavy oils, dyes, and many other valuable by-products are
saved. Indeed, it is said that the by-products are now so valu-
able that they pay the expense of operation, and the iron itself
is sold at a clear profit. In many furnaces the iron is no longer
run into "pigs" but is received as it runs from the stack in
caldrons on cars that take it to the puddling furnace or Bessemer
converters, where it is made at once into steel.
The improvements in the process make it possible now to
produce more iron and steel in a single year than existed in the
whole world when Columbus discovered America. Then all the
iron existing would have made a pile 8X6 feet and less than a
mile long; now, a year's output is a pile of like size that would
reach from New York City beyond the Mississippi River ! Conse-
quently we use it lavishly, and its relative cheapness makes pos-
sible the immense quantity of labor-saving machinery now in use
in factories, on farms, and in homes. It has made possible our
great system of transportation, our railroads, locomotives, steel
freight cars, great steamships, automobiles, and trucks. This
is the age of steel.
CHAPTER VII
THE NATURE OF MATTER
In Nature's infinite book of secrecy
A little I can read. — SHAKESPEARE, Antony and Cleopatra.
As we noted in a previous chapter, the physicist believes that
every substance is made up of very tiny particles called molecules,
and that if these are broken up into their component atoms the
nature of the substance is completely changed except in the case
of elements. Thus a drop of water might be divided into smaller
drops and these into still tinier droplets. But such subdivision
cannot go on indefinitely. Ultimately a division would give mole-
cules of water. If these were again split, the product would no
longer be water but hydrogen and oxygen, the two elements that
make up water whose properties are entirely unlike those of
water. The physicist believes in molecules although he has
never seen them, because this molecular theory enables him to
explain and predict the many physical phenomena. Even ele-
ments exist in molecular form and while, when the molecules of
an element are split into the atoms, we have no new substance,
yet the properties of an element in its atomic state are usually
quite different from its properties in the molecular state.
In spite of the fact that the molecule is so small it has never
been seen, yet its size has been calculated from experimental
data, with reasonable accuracy. A hydrogen molecule has a
diameter of about one eleven-billionth of an inch and weighs
about one ten-sextillionth of an ounce, figures that are meaning-
less, because they are so far removed from experience. It is
difficult to put them in terms that are comprehensible. A
bubble of hydrogen gas under ordinary conditions with a diam-
eter as great as that of the cross-section of a pin would contain
163
1 64 OUR PHYSICAL WORLD
fifteen quintillion molecules. There would be some three million
of them just along the line of its diameter. If such a bubble
were magnified to the size of the earth the molecules would be
somewhat over an inch in diameter (i.i). This is a magnifica-
tion of about twelve and a half billion diameters. The most
powerful microscopes now at our disposal magnify about ten
thousand diameters.
These molecules are not standing still but, due to the radiant
energy imparted to them in the form of heat, they move in
straight lines at the rate, in the case of hydrogen, of a mile a
second, or in our magnified bubble at a rate over 12,000,000,000
miles a second. Oxygen gas with a molecule whose mass is
sixteen times as great travels only a quarter as fast. Such
molecules are, therefore, constantly bumping into each other and
against the sides of the container, and so must constantly be
shifting the direction of their movement. Hydrogen molecules
at ordinary conditions of temperature and pressure average
about 10,000,000,000 collisions every second. It is the constant
impact of the molecules of a gas against the walls of the contain-
ing vessel that makes the gas exert its pressure.
The velocity of molecular movement increases with an
increase in temperature and diminishes with its decrease. It is
calculated that all molecular movement would cease at what is
called absolute zero, 271.3° C. below the freezing-point of water,
a temperature which has recently been nearly achieved in the
laboratory. The molecules move less rapidly and are closer
together in liquids than in gases and are still more closely spaced
and move still less freely in solids. When great quantities of
heat are absorbed without a rise in temperature, as occurs when
a solid is changed to a liquid, as in the melting of ice, or when a
liquid is changed to a gas, as in the change of water to steam,
the absorbed heat is used to impart the more vigorous motion to
the molecules, which necessitates their wider spacing and the
consequent increase in volume of the substance changed. When
the reverse process goes on, the latent heat again becomes sensible.
THE NATURE OF MATTER 165
So it is quite commonly observed that a thunder shower does not
cool the air but makes the heat more oppressive, and that severe
winter temperatures are moderated by a snow storm.
If the tiny bubble of hydrogen were to be magnified as indi-
cated above, you would not see the molecules as solid objects
like golf balls, for each molecule is made up of relatively small
particles traveling in orbits or possibly oscillating in pathways.
Just as we say that the solar system, the central sun and the
bodies revolving about it, has a diameter of nearly 560,000,000
miles, though only a minute portion of this space is actually
occupied by the sun, planets, and moons, so the bodies that com-
pose a molecule really occupy but a small part of the space
assigned to it. The molecule consists of atoms, two in the case
of elements (or rarely one atom), moving in pathways about
some center. In complex compounds a molecule may consist of
hundreds of atoms. Each atom of hydrogen consists of a cen-
tral mass carrying an excess of one charge of positive electricity
and revolving about it one charge of negative electricity — a bit
of disembodied force. The latter is known as the electron, the
body that carries the positive charge, the proton. It is the path-
ways of these that occupy the space assigned to the atom, the
diameter of which, in the case of hydrogen, is about half that of
the molecule.
Protons and electrons are so small that they would still be
invisible if the tiny bubble of hydrogen gas were only magnified
to be as large as the earth. Suppose it were enlarged to a sphere
with a diameter that of the orbit of the earth. Then the mole-
cules would be some two-fifths of a mile in diameter, the electron
about one-fourth inch in diameter while the proton would be one
eighteen-hundredth of that. In spite of this disparity in size
the mass of the proton is about 1,800 times that of the electron.
Chemists used to believe that there are eighty or more ele-
ments such as copper, iron, oxygen, which enter into various com-
binations forming compounds. Common salt, for instance, is a
combination of the elements sodium and chlorine. And this
1 66 OUR PHYSICAL WORLD
distinction between elements and compounds is still maintained.
But now it appears that elements in turn are made of protons
and electrons, and the difference in their properties is due merely
to the difference in the number and arrangement of these com-
ponent units in their atoms. The nucleus of every atom is
made of one or more protons, each holding at some distance an
electron that moves about the nucleus. Possibly the nucleus
is made of both protons and electrons, but if so the protons are
in excess, and it is the excess protons that hold the electrons that
move about the nucleus. It will simplify matters here to con-
sider only the excess protons and their attendant circling electrons.
The atom of hydrogen, as we have seen,
consists of one excess proton and one elec-
tron. The helium atom has two protons in
its nucleus and two electrons that lie on
opposite sides of this nucleus. These two
electrons have pathways which are included
in a sphere that is relatively distant from
FIG. 66.-The helium ^ central nucleus just as was the case
atom. In this and the .
succeeding diagrams no m tne hydrogen atom. The electrons are
attempt is made to repre- symmetrically arranged with reference to
sent relative sizes and ^ nucleuS (Fig. 66). In all such Cases
distances accurately. A, , , , ,. , .
the element seems to be relatively inactive
chemically, and helium is a very inert gas. It is a very light gas,
not as light as hydrogen, but it is used in place of the latter in
filling balloons, for it is safer. It is obtained from natural gas.
Hydrogen is very active chemically, and forms with oxygen an
explosive mixture.
The lithium atom, the next in the series, has three protons
in its nucleus and three electrons about the nucleus, two in a
sphere similar to that of helium, the third in a sphere
twice as far from the nucleus as the first sphere. Its mole-
cule is, therefore, larger than that of helium. Then come
beryllium with four electrons, boron with five, carbon with six,
nitrogen with seven, oxygen with eight, fluorine with nine, and
o
THE NATURE OF MATTER 167
neon with ten. In each of these, two of the electrons are in the
inner sphere, the remainder in the outer. Now neon has eight
in the outer sphere, which seems to be its capacity, and these
eight are apparently symmetrically arranged. Neon like helium
is very inactive and ends the second series. In the first place we
have helium with two electrons in a sphere about the nucleus, then
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
2+1 2 + 2 2+3 2+4 2 + 5 2+6 2 + 7 2 + 8
These form what may be termed the second series, the electrons
arranged in a second sphere. Again we have a series of eight
elements, the third series, each one with one additional electron
in its atom, and these seem to be in a third sphere nearly
coincident with the preceding one, as follows:
Sodium
2+8+1
Magnesium
2+8+2
Aluminium
2+8+3
Silicon
2+8+4
Phosphorus
2+8+5
Sulphur
2+8+6
Chlorine
2+8+7
Argon
2+8+8
In argon the third sphere is full, 2+8+8. The fourth series is a
double series, the electrons being in a sphere with a radius three
times that of the helium electrons, therefore capable of holding
more electrons. The fourth series ends with krypton. This has
two electrons in the first sphere, eight in the second, eight in the
third which is nearly coincident with the second, eighteen in
the fourth, thirty-six altogether. The fifth series is also a double
series, the electrons being in a sphere that is nearly coincident
with the fourth, and also has room for eighteen electrons. It
ends with xenon, which has fifty-four electrons. The sixth series
is a triple series. The electrons are in a sphere with a diameter
four times that of the first sphere, which sphere therefore has
capacity for 42X2 electrons or 32. Niton, with eighty-six
electrons, ends the sixth series. The added electrons of the
seventh series are in a sphere coincident with the sixth and
therefore with a capacity of thirty-two. However, the later
1 68 OUR PHYSICAL WORLD
elements in this series are unknown, uranium with ninety-two
electrons in its atom being the heaviest known substance.
Chemists believe that the elements differ in the construction
of their atoms, as indicated above, for several reasons, the chief
of which is that when the elements are arranged in such a scheme
they are in the order of their increasing atomic weights, and their
properties are a function of their position in the scheme.
The first clear apprehension that the elements are so related,
that they form several series in which correspondingly placed
members in these series exhibit similar properties, was due to
Mendeleeff. The law has come to be known as the periodic
law or, since any element has properties closely approximating
the eighth one before or after it, if the elements are arranged
on the basis of the atomic weights, it is also known as the law
of octaves. The arrangement of the elements in the periodic
scheme is shown in the table on pages 1 70 and 171. The explana-
tion of their atomic structure in terms of protons and electrons
is very recent, and is a tentative theory that may have to be
much modified.
When elements unite to form a chemical compound, a positive
element usually unites with a negative one. Thus positive
sodium unites with negative chlorine to make common salt or
sodium chloride. Positive elements do not unite with positive or
negative with negative. Moreover, elements always unite in
definite proportions by weight. That is one reason the atomic
theory was adopted. If one atom of sodium always unites with
one of chlorine to form a molecule of sodium chloride, then evi-
dently they must unite in amounts proportional to the relative
weights of the atoms. Sometimes, however, one atom of one
element unites with two of another. Thus Mg unites with Cl
to form MgCl2, which means that one atom of magnesium has
united with two atoms of chlorine to form one molecule of
magnesium chloride. The number of bonds an atom of one
element has, by which it attaches itself to the atom of another
element, is designated the valence of the element. It will be
THE NATURE OF MATTER
169
noted in the groups of elements under the periodic law that the
elements in the first group, after the inert substances of group o,
like neon, argon, have a valence of one, those of the second two,
the third three, and these are all positive. Substances in the
fourth group may behave either as positives or negatives, and
their valence is four. The fifth, sixth, and seventh groups have
decreasing negative valences, three, two, one respectively, or
they may rarely behave as positives, with valence of five, six,
,o
o
o
o
o
o
o
o
c
o
o
o
6
c
o
o'
o
o
o
FIG. 67. — (a) Diagram of the sodium atom, with a group of protons at the
center, two electrons indicated by dotted lines in the first sphere, eight in the next
lying at the corners of a cube (suggested by lines) in the second sphere, and one
electron of the next sphere. (6) Diagram of the fluorine atom.
seven respectively. Now this is all easily explicable on the
basis of the structure of their atoms. Thus sodium has eleven
protons and electrons arranged as shown in Figure 67^, while
fluorine has nine arranged as in Figure 676. Sodium has only
one lonesome electron in its outer sphere. It needs seven more
to fill up this sphere to satisfaction. Fluorine has its outer
sphere full except for one electron. Now if fluorine takes this
lonely electron in the outer sphere of sodium into its outer
sphere to make up the eight, then this electron will jointly be a
member of the sodium atom and of the fluorine atom. The two
atoms are tied together and are united to form sodium fluoride.
170
OUR PHYSICAL WORLD
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THE NATURE OF MATTER
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172 OUR PHYSICAL WORLD
In a similar way magnesium has twelve electrons, two in the
inner sphere, eight in the second sphere, and two in the third.
Six more would be needed to supply this outer sphere, and it is
difficult to get them. But oxygen has eight electrons, two in
its inner sphere, six in its second sphere, and needs two more to
satisfy this sphere. If an atom of magnesium and one of oxygen
unite by using the two electrons in the third sphere of the mag-
nesium atom to fill up the second sphere of the oxygen atom,
we will have the substance known as the oxide of magnesium.
Evidently it would take two atoms of fluorine to unite with one
of magnesium to make magnesium fluoride whose formula is
written, then, as MgFl2.
But valences are not the only properties of the elements that
seem to be sequentially arranged on the basis of this periodic
law. The elements in any one column are very similar to each
other in their physical properties and chemical behavior. Thus
all the elements in the zero group or column are very inactive
chemically. They may be regarded as having no tendency to
combine with other substances — they have a valence of zero.
The metals are more vigorously metallic in their characters as
you go down the columns, and the non-metals are less vigorous
in their non-metallic characters. Thus in column VII, fluorine
is the most vigorous non-metal known, chlorine slightly less so,
etc. Fluorine, chlorine, bromine, and iodine are so much alike
they have been grouped together as the "halogens" for a long
time. Color, density, and solubility of similar salts increase down
each column. Thus fluorine is pale yellow, chlorine greenish yel-
low, bromine red, iodine purplish black. The melting-point of
the elements decreases as you go down each column while the
boiling-point increases.
The elements in the right column (VIII) do not fit well into this
scheme, and chemists suspect that this periodic law is but a par-
tial expression of the truth. In time we shall discover a better
statement of it which will take in these apparent exceptions. It
is, however, a working hypothesis and helps one to recall atomic
THE NA TURE OF MA TTER 1 73
weight, valence, and other physical and chemical properties of the
elements. It has been, too, a valuable aid in the discovery of
new elements. For instance, when Mendeleeff first stated it the
element scandium was unknown, as indeed were several others
now known. He was able to predict the discovery of this ele-
ment and to give its probable atomic weight, valence, and many
of its physical and chemical properties. Chemists were therefore
on the lookout for it, and it was only a few years after the predic-
tion of its discovery before this was accomplished. The properties
of the new element agreed remarkably well with the predictions.
One of the most startling discoveries of modern chemistry is
that the elements which the old chemists thought were the sim-
plest forms of matter and could not be resolved into still simpler
things are capable of such resolution. The more complex ones
like uranium and radium are giving off emanations by which
they change to other so-called elements. Three things seem to
be emitted from such decomposing substances: (i) what are
known as alpha rays which seem to be streams of helium mole-
cules, moving at about 18,000 miles per second; (2) beta rays or
streams of electrons, moving with a very high velocity, about
that of light, 186,000,000 miles per second; and (3) gamma
rays or X-rays, a form of vibratory impulse. Bacquerel first
discovered radioactive substances when he found that uranium
would make a shadow picture on a photographic plate even
through a protecting layer of black paper, and this in a perfectly
dark place. This was in 1896. Professor and Madame Curie
discovered polonium and radium, much more active substances,
two years later. Now we know the uranium decomposes in
time to form radium, which passes through several stages and
gives rise to niton and this to polonium, which in turn by loss
of these emanations becomes lead. The time consumed in these
transitional changes varies greatly with the different substances.
Thus, it takes some 5,000,000 years for half of a given mass of
uranium to change to radium, but only about 136 days for po-
lonium to change similarly to lead. These changes are as yet
174 OUR PHYSICAL WORLD
beyond the control of man. They persist in going on under any
and all conditions. He cannot stop them or start them.
Here is a possible source of energy that may some day be
under man's control. If we could start an element to giving off
this energy of decomposition and check it at will, it might put
at our disposal the greatest source of energy available. We do
use the emanations of radium now. When the alpha rays strike
certain chemicals like the sulphate of zinc they make a visible
splash of light. So we coat the hands of a watch with a paint
in which there is such a chemical and a very tiny amount of
a salt of radium, and the hands are then visible in the dark.
Radium salts are used in the treatment of cancer and other
pathological conditions. But they must be handled with extreme
care for the radiant energy shot off causes the death and rapid
decomposition of living tissue, making bad " burns, " and they
go through most any substance, penetrating the armor plate of
a battleship as if there were nothing in their way. Lead seems
to be relatively impervious to them.
The chief source of these radioactive substances is a mineral
called carnotite. It is found in this country abundantly in
Colorado and in less quantity elsewhere. Radium forms a very
small part of it, so that it takes a trainload of the ore to make a
thimbleful of the radium salt. Yet the energy given off by this
amount is very great. It would make enough luminous paint
to cover the state of Illinois.
These radioactive substances are not the only sources of
streams of electrons and of X-rays. These were produced by elec-
trical discharges through tubes from which the air or other gases
had been largely exhausted (vacuum tubes) for some time before
radioactive substances were discovered. The streams of elec-
trons were known as cathode rays. The X-rays have been used
in medical diagnosis for many years now. They penetrate flesh
but are stopped in part by bone, metal, and other foreign sub-
stances so that it is possible to get pictures of broken or deformed
bones, foreign substances such as bullets or pins that have lodged
THE NATURE OF MATTER
175
in the tissue, and help the surgeon in determining the proper
treatment (Fig. 68).
m m
• •-.-=
I
FIG. 68.— An X-ray photograph of a child's wrist
176 OUR PHYSICAL WORLD
We have already been using in this and preceding chapters
some chemical terms, and shall need to use others in later chap-
ters. It is a very simple matter, however, to get in mind such
elementary chemical concepts as are needed to understand the
simple chemical processes treated in this book.
The difference between a physical change and a chemical
change must be apparent from the discussion of burning in the
preceding chapter. Heat a substance like solid ice and it
changes to a liquid, and this in turn to steam, a gas. These,
however, are merely three different physical states of water.
So solid sulphur may be changed to liquid and solid iron to molten
iron by heat. Heat sulphur still more in the air until it reaches
its ignition point and it burns or unites with oxygen and forms a
new substance, oxide of sulphur. So when iron burns in oxygen
or rusts slowly in the moist air, a new substance is formed, an
oxide of iron, with properties quite unlike iron.
Chemists have devised a sort of shorthand for writing out
these reactions, and indicate the elements by the initial letter
of their English or sometimes their Latin names. In case two
or more elements begin with the same letter, it is necessary to
use in such cases two letters from the name; thus C is carbon;
Cl, chlorine; N, nitrogen; Na, sodium (Latin, natrium}. Thus
when sulphur burns the reaction is written:
S+O2=SO2.
This means that one atom of sulphur unites with two of oxygen
to form one molecule of sulphur dioxide. Such a statement to be
an equation must, of course, have equal numbers of atoms of
each substance on opposite sides of it.
Most chemical substances are classed as bases, acids, or
salts. For our purpose we may define these simply. A base is
a positive substance, like a metal, combined with OH, and is
named a hydroxide. Thus KOH, Ca(OH)2, are potassium
hydroxide and calcium hydroxide respectively. The valence of
the OH radical is one, of potassium one, but of calcium, two.
THE NATURE OF MATTER 177
An acid is a negative or non-metallic substance combined with
hydrogen ; thus HC1 is hydrochloric acid. When a base and acid
are brought together, the positive component of the base usually
combines with the negative element or radical of the acid to form
a salt. The positive component thus takes the place of the
hydrogen of the acid.
NaOH+HCl=NaCl+H2O.
The NaCl is a salt and in this particular case it is the salt we
call table salt. •
The hydro- acids, like hydrochloric or chlorhydric, have no
oxygen. So HBr is hydrobromic acid. Knowing the -ic acid,
like HC103, chloric acid, you can always give the formulas of
others of the same series, for the per ic acid, like HC1O4,
perchloric acid, has one more atom of O than the -ic acid; the
-ous acid, like HC102, or chlorous acid, has one less atom of O,
and the hypo ous acid, like HC1O, or hypochlorous acid,
has two less than the -ic acid.
The salts formed from the acids are readily named :
Hydr- acids give -ide salts. NaCl is sodium chloride.
-ous acids give -ite salts. NaC102 is sodium chlorite.
-ic acids give -ate salts. NaC103 is sodium chlorate.
per ic acids give per ate salts. NaClO4 is
sodium per chlorate.
hypo ous acids give hypo ite salts. NaCIO is
sodium hypochlorite.
CHAPTER VIII
STEAM AND GASOLINE ENGINES
Soon shall thy arm, unconquered steam, afar
Drag the slow barge or drive the rapid car;
Or on wide, waving wings expanded, bear
The flying chariot through the fields of air.
— ERASMUS DARWIN (1731-1802).
No application of fire since man's early discovery of, the
methods to produce it at will has been more revolutionary in its
effects on society than its application to the production and use
of steam in the steam engine. Like so many other great inven-
tions the steam engine is a cumulative product. Hero of Alex-
ander one or two centuries before Christ devised a metal sphere
with radiating elbow-shaped pipes about its equator which, when
water was boiled in it, would revolve on its axis, propelled by
the jets of steam that came out of the pipes which all opened
in the plane of its equator and on the same side of their re-
spective radii. But this was a curiosity and served no practical
end. Branca, an Italian, early in the seventeenth century made
a wheel rotate by jets of steam that struck paddles or blades
along its circumference much as a water wheel is made to revolve
by the water striking its paddles. He connected this wheel to
a contrivance that he used for pulverizing drugs, so his steam
engine was actually harnessed to do work. A Frenchman, Denis
Papin (1647-1712), devised the piston and cylinder to operate
by steam in 1690. Though born at Blois, he lived in London
much of his life. He fitted a disk with an attached rod to a
cylinder, closed at one end, the rod protruding at the open end.
Steam was let into the closed end of the cylinder, and the disk was
shoved along toward the open end. He suggested that by
spraying water on to the closed end of the cylinder the steam
178
STEAM AND GASOLINE ENGINES
179
within would be condensed to water and a vacuum would tend
to form. The pressure of the atmosphere on the disk would
then drive it back toward the closed end. But it remained
for an Englishman, Newcomen, to devise (1705) a means of
making practical application of this idea.
Before this was
accomplished, how-
ever, Thomas Savery
devised a scheme for
pumping water by
the use of steam
(Fig. 69). A pipe
some 30 feet long
dipped into the water
at its lower end. At
its upper end was a
chamber that could
be cut off from the
pipe by a stopcock
and that also had a
vent pipe and a steam
pipe both capable
of being closed by
stopcocks. Steam
was let into the
chamber, and the air
FIG. 69. — Diagram of Savery's improved steam
pumping engine. Steam generated in a flows into b
and fills it, after which the valve is closed and cold
water from pipe d pours over the outside of b. Thus
the steam condenses and water comes up through pipe
e, which extends down into well or mine and fills b.
Vessel c has been so filled, and now steam is entering
it, forcing the water up pipe/ toward the surface.
let out while the cock
to the water pipe was closed. When the chamber was full of
steam, vent pipes and steam pipes were closed by the cocks.
Then cold water was sprayed on the outside of the chamber until
the steam inside condensed making a vacuum. The cock in the
water pipe was then opened and the air pressure drove the water
up the pipe into the chamber, when the water-pipe cock was
closed and the vent pipe opened so the water could run out as
steam was let in. So the process started all over again. This
i8o
OUR PHYSICAL WORLD
device worked slowly, for the chamber had to be heated by the
flow of steam for some time, else the steam would condense
as rapidly as it entered. The cocks were operated by hand by
an attendant. Savery later improved this by adding a second
chamber in order that while the steam was flowing into one it
could be condensing in the other.
Newcomen built a vertical cylinder closed at its lower end
and connected at the same end with a steam pipe from the
water -fcuik
cylinder
valves
— outlet pipe
steam boiler
FIG. 70. — Diagram of Newcomen's engine
boiler (see Fig. 70). In this pipe there was a valve. Three other
pipes also connected with this cylinder, each having a valve.
One of these connected with a water tank so cold water could be
sprayed into the cylinder, another was an outlet pipe for water,
and the third an outlet pipe for air. The disk was connected
by a chain to one end of a lever to the other end of which beyond
the fulcrum there was attached another chain that fastened to a
weight and to a pump. The attendant would open the air vent
STEAM AND GASOLINE ENGINES 181
and the steam inlet. The steam pressure used was slight, and it
did not push the disk up. This was raised by the weight on the
end of the lever, which weight also pushed the pump rod down.
When the air was all expelled from the cylinder, and it was full of
steam, the valve on the air vent was closed as also was the one
on the steam inlet. Then the valve on the water pipe was opened
and cold water let into the cylinder. This condensed the steam
to water, which occupied only one two-thousandth of the space of
the steam. Then air pressure forced the disk down, which brought
down the arm of the lever to which it was attached and raised
the other end with the attached weight and pump rod. The
valves in the air pipe and in the water vent were now opened, the
water let out of the cylinder, and the process was started over
again. In spite of the fact that this engine was very crude and
that the valves were operated by hand it was used to pump
water out of the British mines, for it was an improvement on
hand- or horse-power.
It remained for a resourceful Scotch lad, Humphrey Potter,
who tended the valves on such a pumping engine at a mine, to
rig ropes from -the valve handles to moving parts of the engine
so that they were opened and closed at the proper times. The
engine thus became automatic. This arrangement was called a
"scroggin" — a Scotch word, meaning "lazy."
A model of Newcomen's engine in the museum of the Uni-
versity of Glasgow was turned over for repair in the year 1763
to James Watt, an instrument maker connected with the univer-
sity. This led him to think of various means of improving this
crude device and to the invention of a real steam engine, one in
which steam alone furnished the propulsive power. Watt called
his engine a "fire engine " (Fig. 71, p. 182). He saw that the
expansive power of the steam itself could be used to force the
piston head first one way and then the other in the cylinder.
He built the cylinder of his engine closed at both ends with the
piston rod coming out at one end through a steam-tight packing
of greased tow. He arranged the valves in a way to let steam in
182
OUR PHYSICAL WORLD
at one end of the cylinder while a valve at the other end was
open to let out the exhaust steam. Then these valves closed and
others opened to reverse the process. A second very important
improvement he thought out was the addition of condensing
beam
FIG. — 71. Diagram of Watt's steam engine
chambers. Instead of condensing the steam in the cylinder itself
the exhaust steam went to a separate chamber where it was
cooled by water. Since this chamber was a partial vacuum, the
exhaust steam rushed out of the cylinder into it the moment the
valve was opened so the pressure in one end of the cylinder was
very slight while that at the other was high because live steam
STEAM AND GASOLINE ENGINES 183
was entering it. This made the thrust of the piston very power-
ful. He also incased the cylinder in a larger one in order to keep
steam in the space between them. This kept the inner cylinder
hot so the steam entering it would not condense in part and thus
lose its power. In the fourth place he attached the free end of the
piston rod to a heavy flywheel in order to make it whirl round.
The stroke of the piston is a back-and-forth stroke, and at each
end of the stroke there is a moment when it stands still and is
exerting no pressure to make the machine go. The momentum
of the revolving flywheel carries the piston past this dead point
and makes the engine run smoothly rather than jerkily. The
governor was the fifth major improvement that Watt devised.
When an engine is working, the load on it is necessarily a variable
one. Thus it is more work to lift the water in a mine pump than
it is merely to drop the pump plunger for the next stroke. The
engine thus tends to slow down when hard work is being done and
to race when the load is lessened. Watt's governor automatically
partially closed the valve on the steam inlet pipe when the engine
speeded up and opened it wider when it slowed down. The
method of operation will be described below. It is evident from
what has been said here that Watt was the real inventor of the
steam engine. He did so much more than his predecessors
toward making it a practical machine that he deserves the
lion's share of the credit.
He not only largely created the steam engine, but he devised
the measure which we still use to express its work capacity.
Since the "fire engine" was taking the place of the horse as a
means of doing work, it was natural that its ability to work should
be expressed in horse-power. Watt concluded that a good horse
could draw 1,000 pounds up a hill 33 feet high in one minute and
so he adopted this as the unit of measure to indicate the power
of an engine. He rather overestimated the power of a horse,
but we use his horse-power today to measure the work capacity
of an engine. A fifty-horse-power engine is one that could raise
50,000 pounds 33 feet in one minute.
1 84
OUR PHYSICAL WORLD
The general method of operation of the modern steam engine
is very much the same as that of Watt's fire engine, though very
many improvements in details have been made in it. The
boiler is commonly what is known as the tubular type in which
the draft carries the heat from the fire box up between numer-
ous pipes or tubes containing the water that is to be turned to
steam. These tubes present a much larger heating surface than
the old type of kettle-like boiler, and steam can be made much
more rapidly. The modern boiler is so well made that it stands
high pressures, and the steam is sent to the cylinders with a
pressure of several hundred pounds to the square inch.
eccerrfHc
jimp
cranh
crowhcad
connt-
rcd
FIG. 72. — Diagram of a modem steam engine
When the engineer opens the throttle of the engine it lets
steam from the boiler into the steam chest that lies next to the
cylinder. Sliding valves between steam chest and cylinder let
steam first into one end and then into the other, at the same
time others open to let out the exhaust steam. These valves
are operated by a rod attached to the eccentric or similar device.
The method of operation of this portion of the engine should be
plain from the study of the accompanying diagram (Fig. 72).
The exhaust steam from the high-pressure cylinder may be dis-
charged directly into the air through the smokestack or it may
go to a condensing chamber in the so-called condensing engine
or it may enter another steam chest and cylinder that works at
less pressure before going to the condenser, for the work power
of the steam in high-pressure engines is not taken out of it entirely
STEAM AND GASOLINE ENGINES
185
in the first cylinder. These latter engines are called double-
expansion or, if three cylinders receive the steam one after
another, triple-expansion engines.
The free end of the piston rod is attached by a movable
joint to the crank shaft — a shaft with a right-angled bend to it
like the crank for an automobile or that on a grindstone or coffee
mill — so that the back-and-forth motion of the piston rod is trans-
formed to a rotary motion of the shaft and its attached flywheel.
The governor on many
engines now is very like
the one devised by Watt.
A solid vertical rod has
firmly fixed near its lower
end a wheel which by
teeth or belt is geared to
a rotating shaft of the
engine and the rod thus
rotates rapidly about
its longitudinal axis
(Fig. 73) . Two arms are
jointed by one end to
opposite sides of the
upper end of this rod.
Near the lower free end
of each arm there is fixed a heavy metal ball. A rod is attached near
the end of each free arm and runs thence to a collar that encircles
the rod several inches below the level of the balls. The rods
attach to this collar by a movable joint. This collar fits into
another one just below it so that the lower one must move up and
down with it but need not revolve with it. As the vertical rod
rotates, the balls attached to the arms whirl about and stand
away from the rod on account of centrifugal force. The faster the
rotation, the farther away they move. As they move out the
rods attached just above them pull the collar up on the vertical
rod. To the second collar a rod attaches that runs to the valve
FIG. 73. — Diagram of the governor of a steam
engine.
i86 OUR PHYSICAL WORLD
in the steam intake, which is thereby closed as the collar rises:
When the engine slows down, the balls move in closer to the
vertical rod, the collar is pushed down, and the valve is opened.
In this way the engine is made to run at a nearly uniform speed.
The exhaust steam is made to heat the water before it is sent
into the boiler until it is almost ready to boil. Since the pressure
in the boiler is great the water has to be driven in by force. An
injector is generally used for this purpose.
The stationary engine came rapidly into use late in the eight-
eenth and early in the nineteenth century, for running machines
that were being invented to aid man in his labors. Up to this
time manufacture had been largely a household process. The
shoemaker made the shoes at home and his wife and children
all helped. Wool was combed, corded, spun into thread, dyed,
and woven into cloth, all in the home. The spinning-wheel
and hand-power loom were part of the necessary equipment in
the home of the weaver and everybody worked, including father.
On the farm everything was done by hand (Figs. 74, 75). In
town and country it took the combined labor of all the family to
pay for the necessary food, clothing, and shelter. Even the little
children found some tasks. But the steam engine and power
machinery began to shift manufacture from the home to the
factory. Workmen saw machines doing the work of 100 hand
operatives (Fig. 76, p. 188). They were afraid the factories were
going to deprive them of the chance to work, for children and
women could tend machines. Mobs tried to burn the mills and
destroy the machines and in many cases they succeeded. But
what appeared temporarily as a menace to labor proved a great
blessing, for steam power and machinery increased production.
A single steam engine can do the work of 10,000 men, and do it
ceaselessly and tirelessly.
The more expeditiously man can obtain raw materials, like
iron, coal, wood, grain, and manufacture them into the things
he needs, the more rapidly he accumulates wealth. William E.
Gladstone once estimated that the wealth of the world increased
STEAM AND GASOLINE ENGINES
FIG. 74.— Harvesting grain by hand
FIG. 75. — Reaping and binding grain by machine power
i88
OUR PHYSICAL WORLD
as much in the first fifty years of the nineteenth century, due
largely to the use of steam, as it had in the preceding fifty cen-
turies. It doubled again in the next twenty-five years, and was
doubling even more rapidly before we learned to spend with such
prodigality in the Great War.
Because of this great increase of wealth children, at least in
their early years, were released from the slavery of production,
and were free to go to school. The laborer could begin to have
FIG. 76. — An early power loom
some leisure. The working day was cut to twelve, then ten,
then eight hours. Women were freed to devote themselves to
home duties rather than labor in field or factory. Public schools
began to serve the children of the common people about the time
this industrial and social revolution was coming on, due to power
production. They appeared somewhat earlier in this country
of ours whose virgin resources made the production of wealth
relatively easy from the first. Still in 1800 the average child
in this country was getting only eighty-two days' schooling,
while in 1900 this had increased to 1,040 days. The age of com-
STEAM AND GASOLINE ENGINES 189
pulsory school attendance has constantly advanced until it
stands at seventeen years in some states, sixteen in not a few,
and fourteen pretty generally. The first part of the nineteenth
century saw the public graded schools gradually fill up so that
since 1870 there has been no increase of the percentage of the
population that is attending them. But there has been a marked
increase in the attendance in the public high schools. Since
1900 high-school attendance has increased seven fold, college and
university attendance twelve fold, while the increase in the general
population has not even doubled. It might be a fit tribute if
the school children of the world should erect monuments to
Papin, Newcomen, and Watt, inventors of the steam engine that
has made possible their commercial freedom, their public schools,
and yet perhaps the boys and girls themselves, happy in their
increased opportunities, are their best imperishable monuments.
While the stationary engine was rapidly increasing produc-
tion, attempts were being made to use steam power for distribu-
tion also. The first practical steamboat, also commercially
successful, was built by Symington and put to service on the
Forth and Clyde Canal in the year 1802. Fulton's famous
steamer, the "Clermont," laboriously made its way up the
Hudson River first in 1807, and plied regularly after that between
New York and Albany. The "Clermont" was not Fulton's first
steamboat, for while in France in 1803 he had built and operated
a small one on the river Seine.
The locomotive appeared in 1804 but it was a very primitive
affair. It ran on a road of flat iron plates with the outer edges
turned up so the engine would not run off. The toothed drive
wheels played into toothed strips on the roadbed. It was used
for hauling cars of coal at the mines. The rolled malleable iron
rail with the flange on the wheels of engine and car came into
use first about 1820. It was considerably later, however, before
smooth rails and smooth-faced wheels were used or even tried,
for it was so perfectly evident that the smooth wheel would not
grip a smooth rail enough to give traction that no one ever
i go
OUR PHYSICAL WORLD
thought of trying them. The carriages on many early railroads
were pulled by horses, and they were merely stage coaches fitted
for riding the rails. When in 1828 the Liverpool and Manchester
Railway was under construction, there was prolonged discussion
among its directors as to whether horses or engines should be
used to draw the carriages. It was the influence of Mr. George
Stephenson that finally decided the matter in favor of steam
power. His engine, the "Rocket," took the prize offered by
the directors. It weighed 4^ tons, and drew a train of coaches
weighing nearly 13 tons at an average speed of 14 miles an hour
FIG. 77. — The first railroad train in the United States
and a maximum of 29. A serious article in that most serious
English periodical, the Quarterly Review, for March, 1825,
expresses the hope "that Parliament will in all railways it may
sanction limit the speed to 8 or 9 miles per hour which is as great
as can be ventured on with safety." Smile's Life of George
Stephenson is well worth reading to obtain some notion of the
difficulties and opposition the early railroads encountered and
overcame. The first railroad train in the United States made
its maiden trip in 1831 (Fig. 77).
As early as 1770 a Frenchman, Cugnot by name, built and
operated a small wagon with three wheels that was propelled
by a steam engine mounted on it. This, I believe, was the first
STEAM AND GASOLINE ENGINES 191
motor car. Constant improvements were made in such steam-
motor cars and their engines and by the middle of the nineteenth
century steam-motor busses were in use to some extent, and the
steam-motor car while still a novelty gave promise of general use.
Such promise would undoubtedly have been realized had not the
gasoline engine been rapidly developed. In 1900 there were
about 700 automobiles in the United States, all of which were
steam cars except a few imported ones. In 1910, 400,000 cars
were in use here and very few were steam-driven — nearly all
makers having adopted the gasoline engine.
The gasoline engine has many advantages over the steam
engine, especially where a portable power plant is required. It
develops a greater horse-power in proportion to its weight than
does the steam engine. It wastes less of the power that is
developed than does the steam engine. In the latter there is a
great loss of energy through radiation of heat, by friction, and in
other ways, so that only from 6 to 1 2 per cent of the energy gen-
erated by burning the coal is actually delivered as mechanical
energy to do the work required. A good gasoline engine delivers
from 20 to 40 per cent of the energy of the gasoline.
Gasoline is a highly volatile liquid composed largely of
carbon and hydrogen. When it burns or unites chemically with
oxygen it gives rise to carbon dioxide (or carbon monoxide, a
very poisonous gas, if the oxygen supply is limited) and water
vapor or steam. These gases are produced in large volume
from a very small amount of gasoline so that, if the latter
is mixed well with air so it will burn quickly and thoroughly
and the mixture is fired in a confined space, an explosion occurs
just as happens when gunpowder is set off in a small space.
It is the elasticity of these confined gases that exerts the pressure
on the piston head in the cylinders. In general, the plan of
operation of the gas engine is similar to that of the steam engine;
the piston, however, is driven only in one direction by the force of
the explosion. It is forced back again by the action of other cyl-
inders that fire later and are coupled up with the same crank shaft.
192 OUR PHYSICAL WORLD
The gasoline engine consists essentially of at least two
cylinders in which the gas explosions occur alternately, the
pistons which connect by their rods with the crank shaft that
bears the flywheel, the spark plugs, one in the end of each cylinder
where occurs the electric spark that fires the gas, the carburetor
in which the gasoline vapor is mixed with air before it is drawn
into the cylinders, and a storage battery, or else a magneto, which
supplies the electric current to the spark plugs. There are
many accessory parts (Fig. 78).
The gasoline engine is usually at least a two-cylinder engine,
the cylinders firing alternately, and in most automobile engines
the cylinders are still more numerous, four, six, or twelve.
Then they work in groups, the explosion and out stroke (or
power stroke) occurring in part of them, while in others the piston
head is moving in to compress the gases (compression stroke), in
still others to drive out the gases after burning (exhaust stroke).
The crank shaft to which one end of each piston rod attaches by
a movable joint is a forged and accurately turned steel shaft
with as many right-angled bends in it, like squares with one side
open, as there are piston rods. Each piston rod fastens loosely
to one bend, and helps to rotate the crank shaft as hand and arm
rotate the crank on a coffee mill. In a two-cylinder engine the
two bends are in the same plane but face in opposite directions.
In a four-cylinder engine the pairs of bends are similarly placed,
one pair facing one way, the other in the opposite direction. In
the six-cylinder engine there are three pairs of bends that lie
in three planes that are 120° apart. By such an arrangement the
crank shaft is rotated by a succession of thrusts of the piston rods
rather than having them all push at once, and so the engine runs
smoothly.
There are really four phases to a complete cycle in any
cylinder. Beginning with the explosion: (i) the piston head
(and rod) moves out, then (2) it moves in to expel the gases
formed by the explosion, (3) it moves out to draw in the new
charge of gasoline vapor mixed with air, and finally (4) it
STEAM AND GASOLINE ENGINES
193
1 94 OUR PHYSICAL WORLD
moves in to compress the mixture after which the explosion
occurs and the cycle begins over again.
Evidently there must be valves arranged so as to open and
let out the burned gas, others to let in the fresh mixture of air
and gasoline, and these must open and close at just the right
times. These valves are usually opened by rods that are raised
and lowered by eccentrically placed disks called cams revolving
on a cam shaft (see Fig. 78$). The valves are closed by springs.
In some engines the valves operate by means of a rotating sleeve
that fits inside the cylinder with holes in the sleeve and in the
cylinders that coincide when gases are to enter or leave, but are
closed at other times.
The continued burning of gasoline in the cylinders would
naturally keep them very hot. They are cooled either by a draft
of air or more often by a jacket of water that is forced to circulate
in the spaces about them. This water is kept cool by circulating
also in the radiator, a honeycomb metal device with water in the
hollow comb and air drawn through its holes by a fan operated
by a belt or chain drive to the crank shaft.
The carburetor is very variable in different makes (one is
diagrammed here, Fig. 79), but its purpose is the same in
all, namely, to saturate partially the air with gasoline vapor
before it is drawn into the cylinders. Gasoline is either
carried to the carburetor by gravity from the gasoline tank or
pumped up to it. Usually there is a " choke" attached to the
carburetor, a sort of damper which regulates the air intake.
When it is wide open, the air goes in rapidly, and is not as com-
pletely filled with gasoline vapor as it is when it is closed so the
air enters slowly. In the former case the mixture is said to be
lean, in the latter rich. In starting the engine a rich mixture
is used. After it has been running a short time and the cylinders
get heated, the mixture becomes hot also, and will fire even if it is
lean. When one "steps on the gas," a throttle in the pipe
between the cylinder and the carburetor is opened, thus allowing
more of the mixture to flow in and make the explosions more
STEAM AND GASOLINE ENGINES
195
forceful, as the car speeds up. This same valve may be operated
by a lever on the steering wheel.
The mixture of gas and air in the cylinder is ignited by an
electric spark. A spark plug is set into the end of the cylinder
or just at one side of the end. This bears two metallic points at
its inner end between which an electric spark passes when the
mixture is properly compressed, and since the mixture is all
around the spark the latter ignites it. The electricity is fur-
nished either by the storage battery or by a magneto, an electric
Mixiure to
Cylinder
* Auxiliary
Air Inlet
Adi'ustir
Zen*
FIG. 79. — Diagram of a carburetor
generator, that is run by a belt or cogwheels attaching to the
crank shaft or other moving part. It takes a high-voltage
alternating current to send this spark across the gap, a much
higher voltage than the battery furnishes, so the current is sent
through an induction coil to change the low- voltage direct current
of the battery to a high- voltage alternating current. This will be
better understood after reading the chapters on electricity (p. 254).
The ammeter on the instrument board shows the strength of the
current that is being furnished by the battery. A dynamo,
power to run which comes from an axle or from the crank shaft,
ig6 OUR PHYSICAL WORLD
sends a current to the battery to replace the electricity used
constantly at the spark plugs and in the lights.
The current from the induction coil to the spark plugs must
also pass through the distributor and the timer. The distributor
sends the current first to one cylinder then to another and so on,
so they will be fired in the proper order. This is usually not the
order in which the cylinders stand in their row. The firing order
in a four-cylinder engine may be cylinder one, then three,
then four, and, finally, two, rather than one, two, three, four,
for the vibration of the engine is usually less when the order is
not in the regular succession. The timer determines the exact
moment at which the spark fires the mixture with reference to
the position of the piston. When the engine is running slowly,
firing can come at the moment of greatest compression as the
piston head has reached the top of its stroke and is just about to
begin the descent. But when the engine is running rapidly, the
spark must come slightly sooner else the piston head will be well
on its down stroke before the gases will develop their maximum
pressure. The timer in most machines is now automatic in this
adjustment, but a lever is put on the steering wheel to advance
or retard the spark when speeds are very extreme.
When the valves are opened to let the gases out of the
cylinders after the gasoline is burned, they are still under high
pressure, and if discharged directly into the air they would come
out with a noise like that of a pistol shot. They are therefore
discharged through a muffler, a long tube of increasing diameter
with numerous incomplete cross-partitions. The gases go into a
succession of constantly enlarging chambers, and thus expand
gradually instead of suddenly. When the muffler is not in use
the "cut out" is said to be "open," and the exhaust is noisy.
As explained, the piston rods are so attached to the crank
shaft as to make it turn around. On the rear end of this crank
shaft is a heavy flywheel which helps to keep the engine running
smoothly and which also serves to transmit the engine's power to
the rear wheels of the car. Through a device known as a clutch
STEAM AND GASOLINE ENGINES
197
it transmits its rotation
to a secondary shaft on
which are cogwheels of
various sizes (Fig. 80).
The operation of the
clutch may be illustrated
thus: Set the eraser of
your pencil down on a
card or sheet of paper
on a smooth table, then
give the pencil a rotary
motion between your
fingers. If the rubber is
at the same time pressed
on to the card, the latter
will also turn around.
One face of the solid fly-
wheel has pressed against
it a disk on the end of
the secondary shaft, and
so this shaft turns with
the wheel. The pressure
is maintained by a spring
except when the clutch
pedal is in. In most
machines now, the clutch
is of a multiple-disk
variety in which several
disks on the secondary
shaft engage correspond-
ing projecting plates on
the flywheel.
By means of the
gear-shift lever, cog-
wheels of several sizes
198 OUR PHYSICAL WORLD
on the transmission shaft may one at a time be brought into such
position that their teeth interlock with the teeth on the cogwheels
on the secondary shaft, and the transmission shaft is set rotating.
Through a flexible joint it conveys the rotary- motion to the
rear wheels. If a large cogwheel on the transmission shaft is
geared into a small one on the secondary shaft, it will take several
turns of the latter to turn the former once and the transmission
shaft and the rear wheels will turn slowly. If, on the contrary,
a small wheel on the transmission shaft is geared into a large
one on the secondary shaft, then the rear wheels will turn rapidly
and the car will run fast. When the gear shaft is set " at neutral, "
no wheel on the transmission shaft is playing into the wheel on
the secondary shaft.
There are many bearings in an automobile engine that need
constant lubrication. Thus the crank shaft may make 1,000
or more revolutions a minute when the machine is running
rapidly. This would create much friction unless the bearings
were well oiled. The oiling is partly accomplished by having
below the engine a pan of oil, which splashes up and keeps the
moving parts lubricated. In addition an oil pump forces oil along
small tubes bored in the center of the shafting and out of tiny
holes in the bearings. Frequently an oil gauge is put on the
instrument board connected with this system to show that the
oil is moving properly.
An electric motor is connected with the storage battery
so that when a current is sent into it, it turns a cogwheel that
plays into cogs on the circumference of the flywheel and the
engine is "turned over" to start it. You step on the starter or
press a button to accomplish this. Just as soon as the cylinders
have drawn gas and air into themselves and the mixture is set
of! by the sparking of the plugs, the engine begins to run of itself,
the starter is disconnected, and the electric motor stopped.
Sometimes the engine is turned over by hand by means of a
crank temporarily fitted on to the forward end of the crank shaft.
But the self-starter is in quite general use.
CHAPTER IX
DISCOVERIES IN MAGNETISM AND ELECTRICITY
He snatched the lightning from the heaven and scepters from tyrants. —
Inscription on Franklin's Bust.
In these days when streets and houses are lighted by electric
lamps, when the telephone is a necessity and the telegraph a
commonplace, when the electric motor furnishes power, not only
for the shop, but for the washing machine and sewing machine
in the home, when old and young alike are amusing themselves
with radio concerts and lectures, it is hard to realize that all
these electrical contrivances are recent inventions which people
not yet old saw introduced. To most of us they are still mysteri-
ous. What child has not wondered how they make the electric
current that produces the light as he presses the button, or how the
telephone can reproduce so clearly the voice of his chum, or how
that very modern marvel, the radio, can send messages without
even the semblance of connecting wires? What boy has not
stood lost in wonder at the window of the telegraph office and
watched with fascination the messages sent and received, or
envied the electrician at the power-house who seemed to know
all about the great dynamo whose smooth, whirring speed sends
out the current ? Even our playthings now are electrical, and it
is not difficult for the child to repeat experiments that once were
great discoveries, and gain from them in his play a knowledge of
the principles that underlie these magnetic and electrical appli-
ances that have so largely helped to revolutionize the modern
commercial world.
Very ancient peoples knew there was a kind of a stone to be
found that attracts bits of iron. It was called the lodestone, or
magnet, because it was found quite commonly near Magnesia,
199
200 OUR PHYSICAL WORLD
a city in Ionia, a province of Greece. This lodestone is one of
the ores of iron, an oxide of iron, known as magnetite. They knew
also that a piece of iron rubbed on such a stone became a magnet.
We know now other and better ways of making a magnet, as
will appear below.
In the city of Naples, Italy, is a monument to Flavio Gioja,
a man who lived in the city of Amain, and the legend on the
monument ascribes to him the discovery of the compass in the
year 1302. This is undoubtedly an error, for Peter de Maricourt,
a Frenchman, also known as Peregrinus, had devised a compass
with pivoted needle and graduated scale as early as 1269, and
mention is made of it in cruder form nearly a hundred years
earlier. This primitive compass consisted of a magnetized
needle that floated on a cork in a basin of water. Gioja did
make improvements in the compass. At that time there was no
Italy. Amain, once an independent republic, then belonged to
the kingdom of Naples, whose ruler was of the royal family of
France. So Gioja marked the north-pointing end of his compass
needle with the fleur-de-lis, symbol of the iris, the flower of
France that appears on her coat-of-arms. It still is usually so
marked.
Amain was once a great center of commerce whose ships ruled
the Mediterranean and brought her great wealth. Now the
stone wharves where her ships unloaded are lying below the
sea, due to a submergence of that portion of the coast. Her
prestige is gone. Still she will long be remembered, for the
compass which came from her in its improved form was a boon
to commerce. By it a vessel finds its way from port to port
even when clouds obscure the stars and the mariner has no guide
but the little steadfast needle.
The end of the magnet that points north when the magnet
is freely suspended is called the north pole and the other end the
south pole. When two such magnets are brought together end
to end, they repel each other if the poles are alike, but attract if
they are unlike. This fact may readily be discovered by any
MAGNETISM AND ELECTRICITY 201
child who has a pair of magnets to play with, and to make a
discovery like this for one's self is really thrilling.
If a nail or other bit of iron is brought near the end of a bar
magnet, it leaps toward it and is held firmly by it (Fig. Si).
When a second nail touches the end of the first, it is held to the
first, for the nail in contact with the magnet has also become a
magnet. So quite a chain of nails may be held by the bar
magnet, and a great cluster of tacks or iron filings will cling to
it and to each other. If you make a little paper or wooden
boat and put a nail in it, the magnet will draw it about when it
floats in a basin of water
even when the magnet is
quite a distance away,
for this magnetic force
works through paper,
wood, glass, or other
substances.
If you lay a bar mag-
net down on a table with
a sheet of cardboard or
stiff paper over it, then
sprinkle iron filings on
the paper and gently tap
the latter, the filings arrange themselves in a strange pattern
(Fig. 82). They seem to lie along lines of force that radiate
from one pole and turn around to converge at the other. If
a sheet of blue-print paper is used in place of ordinary paper
and the experiment is set in bright sunshine, when the filings
have arranged themselves, the peculiar design will leave its
shadow on the paper permanently. After the paper has stood
until it begins to assume a bronzed tint, take it out of the sun,
shake off the iron filings, and wash it in water thoroughly; then
pin it up to dry. The design will appear white on a blue ground.
If a compass is set on the sheet of cardboard in the foregoing
experiment, its needle will assume a position parallel with the
202
OUR PHYSICAL WORLD
line of force that runs through it. This and other bits of evidence
make scientists think that the earth is a great magnet with such
lines of force running from pole to pole, so making the compass
needle point northward. The magnetic poles, however, do not
quite coincide with the geographic poles, so the compass needle
does not point exactly to the north in most places. This devia-
tion must be taken into consideration in setting a ship's course.
FIG. 82. — Pattern of iron filings on a sheet of paper over a magnet
Possibly it is currents of electricity that course around the earth
that make of the earth a magnet, just as we shall see it is possible
to make a bar of iron into a magnet by sending an electric current
through a wire coiled about it. But we must know something
of electricity to appreciate this.
The ancients knew a little about electricity as well as about
magnetism. They knew that if one rubs a piece of resinous
substance, like amber, on cloth it will then attract light substances
like bits of straw or dry pith. Gilbert, an English physician,
MAGNETISM AND ELECTRICITY 203
discovered that sulphur, sealing wax, alum, and many other
substances behave in the same way when rubbed on cloth,
and he published the first book about electric phenomena in
1600 A.D., though he called such phenomena magnetic not electric.
There were thus many centuries during which nothing had been
added to the simple knowledge of the ancients in regard to
electricity. Then Otto Guericke, the man who made the first
air pump and who tried the famous experiment with the hemi-
spheres at Magdeburg (p. in) to show how great is air pressure,
discovered that electrified bodies may repel each other as well
as attract. You can easily repeat his experiment. Hang up a
pith ball or even a small round wad of tissue paper by a fine silk
thread. The results will be more emphatic if the ball is covered
with lightweight tin foil. Rub a glass rod or tube or a stick of
sealing wax with a piece of silk or wool cloth and bring the rod
near the ball. The ball promptly flies to the rod, adheres to it a
few moments until its surface also is charged with electricity-
like that on the rod, and then it flies away from the rod. Rub
again and present the rod to the ball, and now the ball is strongly
repelled, for both are charged with the same kind of electricity.
So Guericke said that a body charged with electricity draws to
itself one that is not charged but repels it the moment it is also
charged.
It was not until 1762 that DuFay discovered that there were
apparently two kinds of electricity. When the ball is charged
with electricity from the glass rod rubbed with silk, it is repelled
by the glass rod, but if there be then presented to it a stick of
sealing wax rubbed with silk it is strongly attracted. Furthermore,
while it is repelled by the glass it is attracted to the surface of the
silk that has been used to rub the glass. So DuFay said there
are two sorts of electricity. Unlike kinds of electricity attract
each other but like sorts repel. An amusing method of demon-
strating the attraction and repulsion is as follows. Cut some
tiny dolls or figures of animals a half-inch high out of tissue
paper. Scatter these on a table so they will lie under a good-
204 OUR PHYSICAL WORLD
sized piece of window glass supported between the leaves of
two books so it is a little over a half-inch above the table. Rub
the upper surface of the glass briskly with a piece of silk or wool
cloth. Shortly the figures will dance as they fly up to the glass
on which the electricity is developed, become charged with it, so
fly away again to the table to which the charge is discharged,
when the process is repeated.
This electricity that is developed by friction is known as
frictional electricity. You have probably heard it crackle while
combing your hair when it is dry and cool, or have felt and
seen the spark fly when, after shuffling across the rug, you have
presented your finger to some metal object like the radiator or
water pipe. The two kinds that are developed, one on glass
when it is rubbed with silk, the other on amber or sealing wax
when it is so rubbed, were at first called vitreous (glassy) elec-
tricity and resinous electricity. But later they were designated
positive and negative respectively, for when they come together
they neutralize each other and no charge is apparent. They
appear to be present in equal quantities in such substances and are
merely separated by rubbing.
In 1749 Benjamin Franklin performed his famous kite experi-
ment. By this time men knew how to make quite powerful
frictional electric machines, so he knew from his work with these
that the electric spark has a zigzag course, crackles as it appears,
may set things on fire, can even kill small animals, and dis-
charges most readily from pointed conductors. He knew that
in many respects lightning behaved similarly, and so he surmised
that lightning was electricity discharging from cloud to cloud
or from a cloud to the earth, and that buildings might be pro-
tected from lightning stroke by setting in the ground near them
tall, pointed, metal rods in order that the electric discharge
would pass through them instead of through the buildings. This
seemed very absurd even to the scientists of his day, and his
suggestion was received only with amusement. But Franklin
was not to be easily discouraged. He decided to try an experi-
MAGNETISM AND ELECTRICITY 205
ment to test his theory. He told no one about it but his son,
who was to be a witness. In a thunder shower he sent up a silk-
covered kite. At first nothing happened, but as soon as kite
and string were sufficiently wet to serve as good conductors, the
current came down the string and jumped, in a succession of
sparks, from a key that Franklin had tied to it, to any good con-
ductor presented to it. Franklin was holding the kite string
with a piece of dry silk which is not a good conductor so that
the current would not pass into his body, for that might have
been dangerous.
When the tiny particles of water are carried up, as warm air
rises from the earth, they rub against the surrounding air, and so
by friction generate electricity. Such electricity is carried on
the surface of the object that is charged with it. These tiny
electrified particles merge to form larger and larger drops that
make a visible group of them, which we call a cloud. Finally,
they may become so large and heavy that they can no longer
float in the air and they fall as rain. As two of these particles
fuse, the surface of the resultant droplet is not as great as their
combined surfaces, for surfaces increase only as the square of
the radius, while volumes increase as its cube. Surface does
not increase as rapidly, therefore, as volume. So the electricity
on the drops, growing constantly larger, becomes crowded. The
cloud becomes overcharged and finally much of its electricity
leaps toward another part of the cloud, to another cloud that
happens to have a charge of the opposite kind, or toward some
portion of the earth so charged. This discharge heats the air
and the dust particles through which it passes, the latter to
brilliant incandescence as the electric current heats the filament
in an incandescent light, so we see the flash of lightning.
Furthermore, the great heat expands the air suddenly, and the
thunderclap is produced just as a gun makes a loud noise when
it goes off because the confined gases suddenly expand.
In 1789 what was supposed to be another sort of electricity
was discovered. Galvani, an Italian, found quite by accident
206 OUR PHYSICAL WORLD
that the muscles of the leg of a dead frog will twitch if the nerve
in them is excited by frictional electricity. Having prepared
several frogs' legs for further experiments, he hung each by a
copper wire to an iron railing of the balcony outside his window.
As they blew about in the wind, he noted with surprise that when-
ever one of the legs was thrown against the iron it was convulsed
with a contraction. He thought the electricity that caused
this must be generated by the animal and resided in its tissues.
When Alessandro Volta, a professor of natural philosophy
at the University of Pavia in Italy, heard of this he repeated
the experiment, but suspected that the electricity was coming
from the copper and iron, bathed with moisture from the
tissues. So he placed several cups in a small circle on the
table, and filled them partly
full of water. In each cup he
stood a strip of zinc and, op-
posite it, one of copper so that
the upstanding end of one
copper strip leaned against the
zinc strip in the next cup. The
FIG. 83.-Volta's crown of cups cup
not touch each other (Fig. 83) . If, now, one copper strip was sep-
arated, by ever so little, from the zinc strip against which it leaned,
a tiny spark appeared at the gap, showing that a current of electri-
city was being developed. He tried adding various substances
to the water in the cups to see if the strength of the current
might be increased. He found that any acid would do this
very efficiently. Then he improved his apparatus. He piled
up alternate plates of zinc and copper, separating them by
flannel pads wet with acid, but connecting each plate with those
on each side of it by short wires. One end of a wire, the
other end of which was attached to the lowest zinc plate, was
brought close to the free end of another similarly attached to the
top copper plate. A much brighter spark showed a more power-
ful current. This device is still known as a voltaic pile, and we
MAGNETISM AND ELECTRICITY 207
should now call Volta's "crown of cups" a group of batteries
connected in series, as will be explained later. The electricity
generated by such means came to be known as galvanic electricity
from its discoverer, Galvani, who, however, misunderstood its
source. Galvanic and frictional electricity are identical.
One day in 1819 when Hans Oersted, professor of physics in
the University of Copenhagen, Denmark, was working with
electric currents he noticed that a compass needle that happened
to be standing on the table moved every time an electric current
was sent through a wire near it. He began to investigate, and
found (1820) that, when a wire is held over or under the
magnetic needle and parallel to it, and an electric current is sent
through the wire, the needle turns and tends to stand at right
angles to the wire. If the current is strong it will assume such
a position ancl keep it while the current is maintained.
Andre Ampere, who was a professor at the Polytechnic School
in Paris, heard of this law that Oersted had discovered. He
repeated the experiments, verified Oersted's results, but found
out something more. He noticed that when the wire was held
over the needle the north pole was deflected in one direction, but
when held under the needle it turned in the opposite way.
Furthermore, if the current in the wire held over the needle
was going in one direction, the north pole of the needle was
deflected one way, but if the current was reversed the north pole
swung in the opposite direction. This law may now be stated
thus: If you imagine yourself swimming, breast toward the
needle, along the wire in the direction the current is going, the
north pole of the needle will swing to your left. You may readily
try this experiment for yourself with a compass and a copper wire,
the ends of which connect with the binding-posts of an ordinary
dry battery. The current is said to flow through the wire from
the carbon at the center of the dry cell to the zinc at its edge.
Ampere applied this knowledge he had discovered to the con-
struction of an instrument for detecting electric currents. A
compass was wound with many turns of insulated copper wire
208
OUR PHYSICAL WORLD
running parallel to the needle. The wire passed over the needle
in one direction, under it in the opposite direction. When even
a weak current is sent through the wire the needle is deflected.
By noting in which direction it swings one can tell the direction
of the current in the wire, and the amount of the swing tells
something of the strength of the current. This instrument is
called a galvanoscope (Fig. 84).
Ampere made another important discovery, namely, that, if
currents of electricity are sent in the same direction, through two
wires, set side by side and free to
move, the wires repel each other and
move apart. If the current is sent
in opposite directions the wires at-
tract each other and move together.
You can verify this for yourself in
this way. Fill a dish partly full of
dilute sulphuric acid made by pour-
ing the acid into the water. (The
acid is likely to spatter if you pour
water into it, and it burns badly.)
Fasten with tacks a strip of zinc on
one side of each of two good-sized
corks so that it sticks below the cork
an inch or two, and on the opposite
side of each cork tack a similar strip
FIG. 84.-A simple galvanoscope ofcOpper_ Wind good-sized insulated
copper wire (No. 16) about a pencil to make a right-handed coil as
long as the diameter of the cork. Lay one of these coils on the
top of each cork and fasten the ends of the wire, one to the tack
that holds the zinc, the other to the tack that holds the copper.
Now float the corks on the sulphuric acid in the dish. A current
flows through each coil, for we have made a battery. The cur-
rent flows in the wire from the copper to the zinc. Bring the
corks close together with zinc strip facing zinc strip, and the
corks come together, for the currents flow in the adjacent coils in
MAGNETISM AND ELECTRICITY 209
opposite directions. But let copper strip face zinc strip and the
corks tend to float apart.
Ampere perceived from these experiments that there must
be some intimate relation between magnetism and electricity,
and he wondered if it might not be possible to make a bar of
steel into a magnet by using electric currents. He tried various
ways of doing this and finally hit upon this plan. He wound
about a steel bar many turns of copper wire, covered with silk
so that the electricity would not escape into the iron, and let a
current of electricity run through the wire for some time. When
he . removed the windings from the steel bar, he found it was a
magnet. This experiment is worth repeating. Wind a fairly
coarse insulated copper wire about a bolt or nail, making many
turns, and connect the ends of the wire with the binding-posts of a
dry battery. You will find now without removing it from the wind-
ings that it is a magnet — an electromagnet, since it is made by elec-
tricity. Such a soft iron core does not remain a magnet when the
current is turned off; it is a temporary magnet. It was later
discovered, as we have shown above, that a coil of wire behaves
as a magnet when a current is running through it. Its magnetic
property is strengthened if the coil is wound about a core of soft
iron.
An explanation somewhat as follows will serve to give a mental
picture of what probably goes on in the iron bar when it is changed
to a magnet. Conceive that the molecules of the iron are each
a tiny magnet. They do not lie, in the unmagnetized bar, with
their like poles pointing in the same direction but rather in any
and all directions. They do not pull together, therefore, but at
cross-purposes and so neutralize each other. When, however, the
electric current flows in the wire wo;und about the iron bar, it
causes the molecules to assume a position in which like poles all
point toward the same end of the bar, when it becomes a magnet.
In a soft iron bar the molecules resume their varying positions
when the current ceases; but in a steel bar, which has greater
rigidity since the molecules do not move readily, they remain
210 OUR PHYSICAL WORLD
pointing in one direction after the current ceases and the bar is
therefore a more or less permanent magnet.
Michael Faraday, of the Royal Institute in London, heard of
Ampere's work, and thought that if a magnet can be made by
passing a current of electricity through wire wound around an
iron bar, the reverse of this also might be true, namely, that if
a magnet were put into a coil of wire it would make an electric
current flow in the wire. So he made a hollow coil of many
turns of insulated wire, and connected the ends of the wire with a
galvanoscope. Then he introduced one end of a strong bar
magnet into the center of the coil, and saw that the magnetic
needle did actually show a current. He found, however, that
when the magnet was lying quietly in the coil no current was
produced. It was only when the magnet was moving into or
out of the coil that the current was manifest, and it flowed in one
direction when the magnet was moving into the coil and in the
opposite direction when it was moving out.
CHAPTER X
ELECTRICAL INVENTIONS
Invention breeds invention. — EMERSON
Now all these discoveries, besides being interesting in them-
selves, led to a number of practical inventions of great impor-
tance. It has repeatedly been true that men have sought out
nature's secrets to satisfy their curiosity without any thought
of their immediate use, only to find in. later years that the facts
discovered were of immense value to man in increasing his
happiness and well-being. So we support scientific investiga-
tions of all sorts in the belief that the facts discovered will some
day be of use, even if at the present they cannot be turned to
commercial account. They satisfy our longing to understand
the universe about us, and this mental satisfaction is really quite
as important as physical comfort and luxury.
The first of these great practical inventions in electricity was
the telegraph. Two types of telegraph instruments were invented
and put into general use. Wheatstone and Cook of England
in 1837 patented an instrument that depended on the facts that
a magnetic needle is deflected when an electric current is sent
through a wire that passes over and under it, and that the direc-
tion of the deflection depends upon the direction of the current.
The sending instrument consisted of a device for making and
breaking the current and for reversing its direction at will. The
receiving instrument was simply a magnetic needle mounted in
a coil in such a way as to be free to swing in a plane at right
angles to the plane of the coil. Then, by previously agreeing
upon a set of signals to indicate the letters of the alphabet, it
was perfectly possible to send a message. Thus, one swing of
the north end of the needle to the left meant Cj one to the left
212
OUR PHYSICAL WORLD
followed by one to the right meant a, one left and two right
meant wt etc.
Morse, in the United States, devised an instrument depending
on the fact that, when an electric current is sent through a coil
wound about a core of soft iron, the latter becomes a magnet
but ceases to be one the minute the current stops. A sending in-
strument makes and breaks an electric current at the will of the
operator (see diagram, Fig. 85). This instrument consists of a
FIG. 85. — Diagram of an electric telegraph
metal bar hinged at one end to a post, and at the other end held
directly over a second post by a spring. One wire from a
battery attaches to this second post; the other battery wire
runs to the ground. A wire attached to the bar runs through the
receiving instrument of the second station. When the operator
depresses the bar the. circuit is made ; when the pressure ceases,
the bar springs back and the circuit is broken. The receiving
instrument has a small iron bar held by a straight spring close to
one end of the soft iron core within the coil (Fig. 86). When the
current is made by the sending instrument, it passes to the coil
of the receiving instrument and magnetizes the core. The bar is
then forcibly drawn to the core, which it strikes hard enough to
ELECTRICAL INVENTIONS
213
produce a click. When the current is broken, the bar springs back
and strikes a post with a click. If the current is made and
broken immediately, the two clicks sound almost as one and
represent a dot; if the current is allowed to run for a moment, the
two clicks are distinctly separate and the signal stands for a
dash. By various combinations of dots and dashes the letters
of the alphabet are indicated. The Morse Code is given (p. 214)
and the Continental
Code is shown in paren-
thesis where it differs
from the Morse Code.
There is a sending
and a receiving instru-
ment at each station.
When one is receiving
a message, he closes a
switch in his sending
instrument, so the cur-
rent can pass through it
to battery, ground, and
back to the Sending FIG. 86. — Telegraph instruments, (a) sending
station(Figs.85and86). key; ^ Diving sounder.
When the early telegraph instruments were installed, two
wires were run from station to station connecting the instruments.
Later it was discovered that only one wire was necessary, for
the earth would serve to complete the circuit. Now one wire is
run from each instrument to a metal plate buried in moist earth;
this is called the ground wire. At first, too, it was difficult to
send messages a very long way, for it took a very powerful cur-
rent to overcome the resistance in a long wire. Now, relay
batteries that add to the strength of the passing current are
introduced along the way. This, of course, is impossible in
the long cables that carry the current under the sea from con-
tinent to continent, and in these a strong current must be used.
In 1857 a wire was laid on the bottom of the sea between Dover,
214 OUR PHYSICAL WORLD
England, and Cape Grisnez, France, and telegraphic communica-
tion was established for a few days until wave action broke the
connection. The next year communication was re-established
THE MORSE1 TELEGRAPHIC CODE
4* * ~~~
B — • • -
C • • •
D — • •
E •
F
G
H • • • --
I • •
J •-•
K
L
M
N — •
P . . . .
Q
R • • •
S • • •
T — Attention
U • • — Separation
V • • • — End of message
W
X
Y • • • •
Z • • • •
through a well-protected cable. It was in 1858 also that the
first attempt was made to lay a transatlantic cable, but com-
munication was maintained for only a few hours. The first
1 The International Code when different is given in parentheses.
ELECTRIC A L INVENTIONS 2 1 5
successful transatlantic cable was laid in 1860 by the famous
old steamer, the " Great Eastern" (Fig 8 7). Since then many
other cables have been laid across the Atlantic and across the
Pacific.
FIG. 87. — Laying the Atlantic cable, splicing the ends in mid-ocean. (Copied
from the Scientific American, February 14, 1857.)
The following quotation from a contemporary account of the
laying of the first Atlantic cable taken from the Chicago Daily
Press and Tribune of Friday morning, August 6, 1858, shows the
spirit of daring achievement that flavored these early attempts :
THE GREAT WORK OF THE AGE COMPLETED
DISPATCH FROM CYRUS W. FIELD
QUEEN VICTORIA TO SEND THE FIRST MESSAGE
TRINITY BAY, N.F., AUG. STH
To THE ASSOCIATED PRESS:
The Atlantic Telegraph Fleet sailed from Queenstown on Saturday,
July 17. Arrived at mid-ocean on Wednesday, the 28th; made the splice
at i P.M. on Thursday, the 29th, then separated, the Agamemnon and
Valorous bound to Valentia, Ireland, and the Niagara and Georgian fof
216 OUR PHYSICAL WORLD
this place, where they arrived yesterday, and this morning the end of cable
will be landed.
It is 1,698 nautical or 1,950 statute miles from the telegraph house, at
the head of Valentia Harbor, to the telegraph house at Bay of Bulls, Trinity
Bay, and for more than two- thirds of this distance the water is more than
two miles in depth.
The cable has been laid out from the Agamemnon at about the same
speed as from the Niagara. The electrical signals sent and received through
the whole cable are perfect. The machinery for paying out the cable worked
in the most satisfactory manner, and was not stopped a single moment from
the time the splice was made until we arrived.
Captain Hudson, Messrs. Everett and Woodhouse, the engineers Und
electricians, and officers of the ships, and in fact every man on board the
Telegraph Fleet, have exerted themselves to the utmost to make the expedi-
tion successful and by the Divine Providence it has succeeded. After the
end of the cable is landed and connected with the land line of the telegraph,
and the Niagara has discharged some cargo belonging to the Telegraph
Company, she will go to St. Johns for coal, and proceed at once to New
York.
CYRUS W. FIELD
LETTER FROM MR. FIELD TO THE PRESIDENT,
PHILADELPHIA, August 5th. — The President, who is at Bedford, received
the first intimation of the successful laying of the Atlantic Cable through
the Associated Press. The following is a copy of Mr. Field's message
to the President of the United States, at Washington:
DEAR SIR: The Atlantic Telegraph cable on board the U.S. steam frigate
Niagara and her British Majesty's Agamemnon was joined hi mid-ocean,
July 29th, and has been successfully laid; and as soon as the two ends are
connected with the land lines, Queen Victoria will send a message to you,
and the cable will be kept free until your reply has been transmitted.
With great respect,
I remain
Your obd't serv't,
CYRUS W. FIELD
Not only is it now possible to send messages by telegraph,
which are then printed at the receiving station by the electric
receiving apparatus, but signatures and photographs can also
be faithfully transmitted. The principle of the transmission of a
photograph is perfectly simple even if it is marvelously ingenious.
ELECTRICAL INVENTIONS 217
The photographic print is moved back and forth between two
terminal points of an electric circuit, one touching the upper
surface of the picture, the other the under surface. These points
move along a series of parallel lines, from one end of the print
to the other. The current that flows in the circuit varies accord-
ing to the amount of silver deposit at every point of the print.
Where the silver deposit is heavy so that the print is dark, the
metal acts as a good conductor and the current flows readily, but
where the print is light, the flow of the current is weak. At the
receiving station a piece of sensitized paper is made to move
mechanically in correspondence with the movement of the print.
A beam of light strikes at a point on this paper, and as the paper
moves this point of light runs over its surface in parallel lines
corresponding to those over which the terminal points are moving
upon the print. This beam of light is focused on the paper
through a piece of selenium, through which also flows the current
coming from the transmitting station. Selenium has this peculiar
property, that the stronger the electric current flowing in
it, the more readily it permits light to pass through it. When,
therefore, the terminal points are traveling over a dark part of
the print, the current transmitted is strong, the selenium permits
much light to pass through it, the sensitized paper is strongly
acted upon, and prints dark. Thus the sensitized paper repro-
duces point by point the dark and light areas of the original print.
When the telegraph was invented, it seemed wonderful enough
that men could send intelligible messages over a wire for hundreds
of miles, but it seemed past belief when it was announced that
one could talk into a small instrument and be heard distinctly
miles away by another person who held to his ear a receiver
connected only by a wire with the sending instrument.
As in the case of most inventions the possibility of the tele-
phone occurred to several persons, and rude attempts were made
to produce it years before the practical instrument was devised.
Credit is due to Page, an American (1837), to Froment (1850),
to Bour-Seul (1854), and to Philippe Reiss, a science teacher in a
218
OUR PHYSICAL WORLD
little German town, who in 1860 applied the name " telephone"
to his invention. But Alexander Graham Bell is looked upon as
the real inventor of the telephone, although a few hours after
he had filed his papers at the Patent Office in Washington (1876),
Elisha Gray, of Chicago, filed his papers covering the invention
of an instrument for a similar purpose. Bell's was the more
practical as well as the prior invention, and the present instru-
ment is still called the Bell telephone, although his original
device has been greatly modified.
Bell, the son of an Edinburgh clergyman, received a literary
education. As a young man he emigrated to the United States
and became instructor in an institution for deaf-mutes in Boston.
This experience centered
his attention on sound and
hearing. He realized that,
in hearing, the ear drum is
made to vibrate by waves
of sound, and that, in
speaking, such waves are
caused by the vibrations of
the vocal chords. He con-
ceived the idea that such
FIG. 88. — Diagram of a telephone receiver
sound waves might be produced by a vibrating membrane
operated by an electric current in harmony with another mem-
brane at some distance, whose vibrations were produced by the
voice of a person speaking against it. He used to remark that
it was well he had received a literary rather than a scientific
education, for if he had known anything about electricity he
would never have had the audacity to think such a thing possible.
He was, however, encouraged by Joseph Henry, of Philadelphia,
then the American master of electrical science.
In the early instruments the transmitter and the receiver
were much alike. Each consisted of a thin steel diaphragm
mounted near one end of a soft iron core, wound with insulated
wire (Fig. 88). One of the two wires of the operating battery
ELECTRICAL INVENTIONS 219
ran to the ground, the other was joined to the wire wound about
the transmitter core. From this coil the current ran through a
wire connecting with the other station, where, after passing
through the coil about the receiver core, it was carried to the
ground which served to complete the circuit. When one spoke
into the transmitter, his voice caused a vibration of the dia-
phragm. As the diaphragm bent toward or away from the soft
core, magnetized by the flowing current, it caused a fluctuation in
the intensity of the current, because it was itself an induced
magnet moving in relation to a wire coil. These changes in the
intensity of the current
carried from the trans-
mitter caused the core of
the receiver to vary the
magnetic pull on its dia-
phragm in accordance with
the vibrations of the dia-
phragm of the transmitter,
and the receiving dia-
phragm vibrated so as to re-
produce the speaking voice.
In 1856 Du Moncel
discovered that, when a
rod Of carbon forms part FIG. 89.- Diagram of a microphone transmitter
of an electric circuit, compression of the carbon facilitates
the flow of the circuit. In 1877 Edison applied this principle
for making an improved telephone transmitter (Fig. 89). The
diaphragm of this transmitter rests lightly against carbon gran-
ules held in a shallow cup of hard rubber. As the current
introduced through metal strips flows through these carbon
particles, its intensity is increased or decreased according to the
pressure of the diaphragm. This makes a much more sensi-
tive transmitter than the earlier type.
In its early history, when two people wished to talk to each
other over the telephone, their two instruments were connected
220
OUR PHYSICAL WORLD
directly by a wire. As telephones multiplied, it was evidently
impossible to have wires running from each instrument to every
other with which the owner of one might wish to communicate.
A central station was therefore established to which the wires of
all instruments were run and where they might then be connected
as desired. Each wire running from a subscriber's telephone to
FIG. 90. — A modern telephone exchange switchboard. (Courtesy of the
Illinois Bell Telephone Co.)
" central" is bifurcated, one branch ending in a plug socket on a
switchboard, the other in the plug. Directly over the socket,
and wired to it, is a tiny electric lamp, which lights when the
subscriber rings up "central," and remains lighted until she con-
nects her receiver with this socket and learns what subscriber
is wanted. She then disconnects her receiver and connects the
plug of the desired subscriber's wire with the socket of the calling
subscriber.
ELECTRICAL INVENTIONS
221
In a great city, where there are hundreds of thousands of sub-
scribers (there are over 6,000,000 in Chicago), there must be a
number of centrals, for since most calls are between neighbors a
nearby exchange can care for these without the expenditure
necessary to carry all the wires to a single office. When you
call a person in a distant part of the city, the local central con-
nects your wire with that of the
distant exchange, and the operator
there plugs the wire from your local
central into that of the subscriber
with whom you wish to talk. There-
fore, it is necessary when calling to
give not only the desired number but
also the name of its local exchange
(Fig. 90).
Recently the automatic switch-
board is being introduced to replace
the operators at central, for an electric
device is cheaper and more depend-
able than a person, and the task of an
operator is very fatiguing. These new
automatic centrals will free human
beings for more worth-while tasks.
It seems very remarkable that a
mechanical contrivance can so effi-
ciently replace the intelligent action of the central operator.
The electric bell has a hammer attached to an iron bar so
mounted that it will be forcibly drawn to the magnetized soft
iron core of a coil when an electric current is sent through the
latter. As the bar moves, the hammer strikes the bell. It will
be seen that the principle of operation is very much like that of
the sounder or receiver of the Morse telegraph (Fig. 91). In the
bell, however, an ingenious device causes the hammer to strike
the bell repeatedly. The current goes to the coil through two
points which are in contact when the hammer is at rest, but
FIG. 91. — Diagram of an elec-
tric bell.
222
OUR PHYSICAL WORLD
which are separated when the hammer is pulled over so as to
strike the bell. When the current ceases to flow in the coil, its
core ceases to be a magnet, and the iron bar with its attached
hammer is drawn back by a spring to its initial position. Thus
contact between the points is again established, the core of the
coil again becomes a magnet, and the hammer again strikes the
bell. This process is repeated much more rapidly than it can be
described. The bell therefore rings with a rolling note like that
of a drum. The electric buzzer is similarly constructed and
operated, but since it has neither bell nor hammer, only a rat-
tling noise is produced as the iron bar strikes first the core of the
coil and then the post that bears the contact point (Fig. 92).
utn
FIG. 92. — Diagram of a buzzer, push button, and batteries connected up
properly/
The rapid increase in the use of telegraphic communication
created a great demand for more efficient types of batteries.
Since Volta first discovered how to make a battery to produce
the so-called galvanic electricity, very many types of batteries
have been produced, though the principle of operation is much
the same in all. The succession of events that produces the elec-
tric current may be described for one or two types of cells.
When a strip of copper and a piece of carbon, such as an old
electric light carbon, are partially immersed in dilute acid at the
opposite sides of a tumbler, and their free ends are connected
by a wire, a simple battery is made, and an electric current flows
through the wire. The copper replaces the hydrogen of the acid,
forming copper chloride, CuCl2. This in part ionizes, separating
ELECTRICAL INVENTIONS 223
into copper and chlorine ions. The copper, which a moment
before was in a neutral molecular state, now in its ionic condition,
bears an excess of two positive charges on each ion. To accom-
plish this change two electrons or negative charges have been left
on the copper plate. Since countless numbers of copper mole-
cules are rapidly making this change, the copper plate is nega-
tively charged. At the opposite side of the battery, metallic
copper is depositing on the carbon. The positively charged copper
ions change to a neutral molecular state by drawing negative
electrons from the carbon, so that the latter is left with a positive
charge. There is thus produced a difference in electric pressure,
and a current flows in the wire in consequence. In all the litera-
ture of batteries it has been the custom to speak of the current
as flowing in the wire from the positively charged carbon to the
negatively charged copper. Now physicists believe that it is the
movement of the electrons from the copper plate to the carbon that
makes the current in the wire. In spite of this, however, we follow
the old custom and speak of the current as flowing from the posi-
tive to the negative pole. Before the free ends of the copper and
the carbon are connected in such a simple battery, it will be
noticed that chemical action is going on rapidly at the copper
strip, while little or no action occurs at the carbon. Hydrogen
bubbles are rapidly evolving as the copper takes the place of the
hydrogen in the acid. When, however, the elements of the
battery are connected by the wire, the hydrogen nearly ceases
to appear at the copper pole, but accumulates rapidly on the
carbon. The copper of the plate drives off the hydrogen in the
molecules of acid next to it. This nascent hydrogen is very
active, and replaces the hydrogen of the next adjacent molecules.
So the hydrogen is passed from molecule to molecule across the
battery somewhat as a football might be passed down a line of
players. Thus it arrives at the carbon pole without being
visible in transit.
Such a battery will not operate very long, however, for the
bubbles of gas accumulate on the positive plate, and prevent the
224
OUR PHYSICAL WORLD
passage of the current. This difficulty is overcome in several
ways: First by using chemicals which will not liberate hydrogen
as in the gravity battery described below, or secondly by the use
of some chemical which unites with the hydrogen. Thus in the
chromate battery a solution of potassium bichromate, K2Cr207,
is used. This readily gives up a part of its oxygen, and the
oxygen and hydrogen unite to form water.
In general, when two substances like plates of two metals
are partially immersed in a chemical and chemical action occurs,
FIG. 93. — Several types of batteries: (a) gravity battery; (b) bichromate
battery (La Clanche); (c) Bunsen battery; (d) Daniell battery.
the electric current passes in a wire connecting the plates from
the one where chemical action is less rapid to the one where it is
more rapid. Thus if a zinc and a copper strip were used in the
simple battery described above, the copper would be the positive
plate and the zinc the negative.
The development of the electric current is explained in the
gravity battery somewhat as follows (Fig. 93). This battery
consists of a jar with a copper plate at its bottom and a zinc plate
near its top. A solution of common salt is used to fill the jar,
into which some copper sulphate crystals are thrown. Some of
ELECTRICAL INVENTIONS 225
this copper sulphate goes into solution, but since its specific
gravity is high the solution remains at the bottom of the jar,
the salt solution above it. The zinc replaces the copper in the
copper sulphate solution, and the zinc sulphate ionizes. The
zinc thus changes from the neutral molecular condition to the
ionic condition with an excess of two positive charges to each
ion, by discharging two electrons from each atom on to the zinc
plate, which as this process continues becomes negatively charged.
The copper moves to the copper plate, and is deposited as metallic
copper. As it makes this change from the ionic to the molecular
state, it must take on electrons, drawing them from the zinc
plate through the connecting wire. Since the copper plate is
constantly giving up electrons, it has an excess of positive charges
and is positive. The flow of electrons is, therefore, from the
zinc to the copper plate. In this battery the zinc gradually
disappears, the copper sulphate is also used up, and crystals of
zinc sulphate are deposited.
There is an instructive analogy between the flow of water
through pipes connected with a reservoir and the flow of electri-
city through the wires connected with a battery. In the former
the amount of water discharged depends, first, upon the head
of water in the tank or upon the pressure at the opening (the
greater the pressure, the more rapid the flow), and, second, upon
the character of the pipe; a long pipe diminishes the flow by the
friction of the water on its sides more than does a short pipe
of the same diameter; a pipe with rough interior reduces the
flow more than one with a smooth lining, and a small pipe carries
less water than a large one of the same material. (See p. 113.)
Similarly, the flow of electricity from a battery depends, first, on
the electric pressure developed by the battery (the greater the
pressure, the greater the flow) and, second, upon certain proper-
ties of the wire; a long wire offers more resistance than a short
one of the same substance and diameter; a fine wire offers more
resistance than a coarse one ; copper, which is a good conductor,
offers less resistance than iron, and both are better conductors
226
OUR PHYSICAL WORLD
than glass, which scarcely permits any electricity to flow through
it, and so is called a non-conductor. Recall the heat conductivity
of these substances, page 153. Furthermore, if a pipe carrying
water branches, the flow in each branch will be in proportion to
its capacity; if one branch has a cross-sectional area twice that
of the other, it will carry twice as much water. Similarly, if a
wire carrying a current branches, the flow of current in each
branch will be proportional to its capacity; if the circuit supplied
by one branch offers much resistance, while the other offers little,
the latter will carry the major part of the current.
FIG. 94. — Diagrams of batteries connected (a) in series and (b) parallel, and
of water tanks to correspond.
Batteries are said to be connected in series when the positive
plate of one is connected by a wire with the negative plate of the
next. One of the terminal wires of the series will come from a
positive plate, the other from a negative. Batteries are said to
be connected parallel when the positive plates of all are connected
by one wire, and the negative plates of all are connected by
another wire. When two or more batteries are connected in
series, the effect is similar to that of mounting one water-tight
reservoir above another and connecting them by pipes. The
pressure of water in the upper tank is added to that of the lower,
ELECTRICAL INVENTIONS
227
and the water outflows from the latter with a force equal to the
sum of the pressures. If batteries are connected parallel, the
effect is similar to connecting a small pipe running from each of
several water tanks standing at the same level with one large
pipe (Fig. 94). The combined outflow is greater, but the pres-
sure in the large pipe is no greater than it is in a small pipe run-
ning from one tank. Stated in electrical terms, we say that when
batteries are connected in series the current has a voltage equal
to the combined voltages of the several batteries, but the amper-
age is no greater than that of one of the batteries. When con-
nected in parallel, the amperage is increased while the voltage
remains the same.
Resistance is measured in ohms. The ohm is about the
resistance offered by 9.3 feet of No. 30, American gauge copper
wire. To overcome high re-
sistance, high electric pressure
must be used. Electric pres-
sure is expressed in terms of
volts. Just as with liquids,
so with electric currents, the
greater the pressure, the
greater the flow, other things
being equal. The unit that
is used in measuring the rate
of flow of electricity is the
ampere. It is denned as that amount of current which, while flow-
ing through a standard solution of silver nitrate, such as is used in
silver plating, will deposit a specified amount of silver (0.001118
grams) per second. The electric pressure that will force a cur-
rent of one ampere through a resistance of one ohm is desig-
nated the volt.
The instrument used for measuring the amount of current
flowing in a wire at any minute is called the ammeter (Fig. 95).
A soft iron core wound with insulated wire is pivoted at its mid-
point, and so mounted between the ends of a permanent magnet
FIG. 95. — Diagram of an ammeter
228 OUR PHYSICAL WORLD
that, when the current to be measured is sent through the wire,
the core turns on its pivot, repelled by the magnetic poles. When
the current ceases to flow, the iron core is returned to its original
position by the action of a spring. A hand like that of a watch
is attached to the core over the pivot, so that its free end moves
over a graduated scale on the face of the ammeter. The greater
the amperage, the greater the deflection of this hand.
The voltage of the current is measured by a similar instru-
ment, the voltmeter. In this meter, a part of the main current
is shunted off through a fine wire wound about the core. The
greater the voltage, the greater the current that flows in this
wire and the more the needle is deflected.
Both instruments may be combined in one, the voltammeter.
In this instrument the needle is deflected in one direction for
measuring the amperage and in the opposite direction for measur-
ing the voltage.
It will be seen later that electrical energy may easily be
transformed into mechanical energy by means of the motor, and
that mechanical energy may be transformed into electrical
energy by means of the dynamo. Electrical energy is turned
into heat by such devices as the electric flatiron, the hot-point
heater, etc. It is convenient, therefore, to have exact equivalents
of electrical energy in terms of mechanical energy and of heat.
It is found that a current of one ampere working under
pressure of one volt will do work equivalent to 1/746 of one horse-
power. This unit is known as the watt. It is evident then that
volts multiplied by amperes divided by 746 equals horse-power.
Electric current is usually sold at so much per kilowatt-hour, the
unit being a thousand watts of electrical energy furnished every
hour. The instrument for measuring this consists of a small
motor that runs on the current and turns cogs that operate the
hands on the dials by which the meter is read (Fig. 96).
The kilowatt-hour equals 3,600,000 joules of heat energy.
Or one may express the heat equivalent of electrical energy in
calories by stating that the number of small calories equals .24
ELECTRICAL INVENTIONS
229
of the resistance expressed in ohms, multiplied by the square of
the current intensity expressed in amperes, multiplied by the
time expressed in seconds.
Each of the many kinds of available batteries possesses
certain advantages but also certain disadvantages. There is,
FIG. 96. — Diagram of a kilowatt-hour meter
therefore, no single battery which is superior for general use.
A battery must be selected for the particular work it is intended
to accomplish. The several kinds of batteries differ from each
other chiefly in their length of life and in the voltage of the
current produced, its electromotive force, its constancy, its
230
OUR PHYSICAL WORLD
cost of production. Furthermore, some batteries discharge
undesirable fumes.
The gravity battery already described is long-lived, and gives
a very steady current. It is much used for telegraph and tele-
phone lines, though in large plants dynamos are now replacing
batteries.
In a common style of bichromate battery, a zinc rod is
immersed in dilute sulphuric acid held in a tall, porous earthen-
ware cup at the center of the battery jar. Outside this cup is
the solution of potassium bichromate and at
the periphery of the jar is a cylindrical sheet
of copper. The porous cup prevents the
mingling of the bichromate solution with the
acid, but permits the passage of the current
and of the hydrogen, which, when oxidized
by the bichromate, forms water. This battery
gives a current of considerable voltage (2 volts) ,
and permits intermittent use without much
deterioration. It is serviceable for running
electric lights that are only occasionally used
(Fig. 93, p. 224).
The Bunsen cell has a carbon rod im-
mersed in nitric acid in a porous cup at the
center of a battery jar, while a zinc plate is
immersed in sulphuric acid outside the porous cup. This battery
is inexpensive to run, and gives a current of good voltage and
great constancy, but unfortunately gives off disagreeable fumes
of nitrous oxide. Still, it is a serviceable battery for furnishing
the current for electrical experimentation.
Since it is not always convenient to use a battery containing
liquid, the so-called dry battery has been devised (Fig. 97).
This is familiar in the pocket flash light and in the bicycle head-
light, and is now used for ringing door bells and for similar
domestic purposes. Such a battery is really not a perfectly
dry cell, but the moisture which is essential to any battery is
FIG. 97. — Diagram
of a dry battery.
ELECTRICAL INVENTIONS 231
held in an absorbent substance as water is held by a sponge.
A cylindrical copper or sheet zinc cup is nearly filled with a
moist mixture of ammonium chloride, manganese dioxide, and
charcoal, each powdered. A rod of carbon is placed in the center
of the cup, whose open end is then sealed with asphalt or a
similar substance, through which the rod protrudes. One of the
binding-posts foi the wires is attached to this projecting rod,
the other to the copper or sheet zinc cup. The ammonium
chloride reacts with the copper giving ammonia and copper
chloride, which later ionizes. The ammonia is oxidized by the
manganese dioxide which becomes the simple oxide. The charcoal
serves to hold the moisture and to absorb excess of gases formed.
The so-called storage battery commonly used in automobiles
to furnish current for the starter and for the spark plugs is not
a battery in the same sense as those described. It does not
produce electric energy, but merely stores it.
Early in the nineteenth century it was accidentally discovered
that when a current of electricity is sent from a strong battery
into a weak one, the latter becomes charged and will, when used,
give off a relatively strong current. It was not, however, until
in 1859, when Plante discovered the peculiar adaptability of
lead for use in the storage battery or accumulator, that such
batteries became really serviceable. The principle of operation
is simple. When a current is sent into a storage battery, its
energy is there used to accomplish certain chemical changes.
Then, when this charged battery is used, these chemical changes
reverse, and the battery gives off the electric current that was
used in their production. This current comes off in a reverse
direction from that of the charging current.
The most commonly used storage battery (Fig. 98) consists
of two sets of lead plates, those of one set closely alternating with
those of the other, and all immersed in dilute sulphuric acid
(15-30 per cent in distilled water). When the battery is being
charged, one set of plates is connected with the positive pole
(anode), and the other with the negative pole (cathode), of a
232
OUR PHYSICAL WORLD
battery or other source of electricity. As a result of the charge,
the plates connected with the cathode are coated with lead per-
oxide (Pb20s). Now, when the battery is being used, the lead
plates that were connected with the anode become the cathode,
and those that were connected to the cathode become the anode.
The lead peroxide (Pb2Os) readily breaks down, yielding oxygen
which unites with the hydrogen of the sulphuric acid, thus
liberating S04, which goes to the anode and unites with the lead,
forming lead sulphate, PbS04. As
the lead peroxide, Pb20s, changes to
the oxide, PbO2 (Pb2Os = 2 Pb02+0),
the lead is becoming less positive by
taking on electrons drawn from the
anode plate, thus leaving it positive.
At the other plate where the lead
sulphate is forming and ionizing,
neutral molecular lead is changing to
positive lead ions by giving up elec-
trons to the cathode, which is there-
fore negative. When the battery is
charged, the reverse process takes
place. The electricity flowing into the
battery decomposes the water, the
hydrogen going to one pole, the oxygen to the other. The oxygen
now changes the PbO2 to Pb20s, while the hydrogen displaces the
lead in the lead sulphate, thus forming sulphuric acid. The
lead so displaced deposits on the plate. During the discharge
of the current, the movements of the electrons are just the reverse
of those described above.
WHEN CHARGING
Anode or PbSo4 H2O Cathode
positive decomposes decomposes or
plate <- Pb So4 <- Ha O -> unites with PbO2 negative
unite to to form Pb2Os -> plate
form
FIG. 98. — Diagram of a stor-
age battery.
ELECTRICAL INVENTIONS
233
WHEN DISCHARGING
PbA
decomposes
yielding
H2So4 Pb20s <- Anode
decomposes
Cathode unites with \|/ ^
->Pb <- SO4 H2
forming PbSo4 unite to form
H2O
The lead plates in stationary storage batteries are usually
honeycombed in order that they may carry a larger amount of
peroxide and may also
present a greater surface
for chemical action. But
in storage batteries that
are subject to constant
jar, like those of automo-
biles, plain plates must be
used since the jarring
detaches flakes of the
peroxide which are likely
to lodge between the
anode and cathode and so
short-circuit the battery.
Batteries in automobiles
must, therefore, be fre-
quently recharged, while
stationary batteries can
. , , .-, .n FIG. go. — Diagram of an electric motor
take a charge that will reduced ^ simple terms,
last a long time.
When Ampere discovered that a current flowing in a wire
makes the wire behave as a magnet, the foundation was laid for
the electric motor. An electric motor reduced to very simple
terms may be thus made (Fig. 99). Lay two bar magnets on
the edge of a table with the north pole of one and the south pole
of the other projecting over the edge an inch or so, and the two
poles about 2 inches apart. Take a piece of i6-gauge insulated
234 OUR PHYSICAL WORLD
copper wire some 20 inches long and bend it so as to make a
rectangle 2 inches long and i^ inches wide of several parallel
turns of the wire. Bend out one end of the wire at the middle
of one end of the rectangle so that it extends out about ^ inch in
the plane of the rectangle and at right angles to its end. Simi-
larly bend out about i| inches of wire at the other end of the
rectangle and strip off its insulation. We will call this last end
of the rectangle its top. Now bend this wire at the top of
the rectangle so as to make in it a square open on one side, the
plane of the square to lie at right angles to the plane of the
rectangle. File off the ends of the wire until they are smooth
and rounded. Cut a small piece of sheet copper about i X^ inch.
At one end of this make a dent, and near the other end punch
a hole with a nail point. Fasten one end of a 2 -foot length of
copper wire to this copper strip through the hole and the other
end to a free pole of three dry batteries connected in series.
Connect one end of another 2-foot length of wire to the other
free pole of the batteries and bend the other end, which has been
bared, to make a small semi-circle. Lay the bit of copper sheet
on the end of the middle finger of your left hand, set the tip of
the wire that projects from the bottom of the rectangle in the
dent in this, and hold the other end of the wire of the rectangle
against the ball of your thumb. Hold the rectangle thus, verti-
cally, between the ends of the bar magnets. Now hold the second
wire in your right hand and bring its free curved end into contact
with the wire of the open square. The wire rectangle should
now rotate rapidly on its axis. You may have to start the rota-
tion with a light push of the ringer. Suppose the bit of sheet
copper is connected with the carbon (positive) pole of the battery.
The current enters the wire through it, and in our diagram passes
up the wire at the right side of the rectangle. If the adjacent pole
of the magnet is the north pole, the wires tend to attract it, or
since the wires are free to move and the heavy magnet does
not move readily, the wire will be attracted by the magnet and
so turns to the right. (A more exact explanation is given later,
ELECTRICAL INVENTIONS 235
see p. 238.) On the other side of the rectangle the current is
moving down instead of up, and consequently will be attracted
by the south pole. The curved wire loses contact with the wire
of the open square as the rectangle turns, but the momentum
of the rectangle carries it on around until contact is again made
and the process just described is repeated.
FIG. ioo. — Diagram of a commercial electric motor, skeleton view. (After
Trevert.)
The commercial motor (Fig. ioo) is made up of a number of
such simple rectangular units, each consisting of many turns of
wire instead of a few. .These units may be so mounted that they
have a common axis, which is also the axis of an iron cylinder.
The tops and the bottoms of the rectangles are, therefore, diam-
eters of the ends of the cylinder. The sides of the rectangles
lie in the surface of the cylinder, separated from each other by a
few degrees of space. This structure is mounted on an axle
coincident with the axis of the cylinder. The ends of the wire
of each unit are both brought to the same end of the rectangle
236 OUR PHYSICAL WORLD
and attached to narrow copper strips that lie on opposite sides
of the axle in the plane of the unit. This pair of copper strips is
completely insulated from the next adjacent pair. This series
of metal plates borne on the axle is known as the commutator.
The current enters and leaves the units by two copper strips
that are applied on opposite sides of the commutator.
Instead of using permanent magnets the commercial motor
uses electromagnets. A part of the current entering the motor
is shunted off through a branch wire, which is wound about a
horseshoe-shaped core, transforming it into a magnet. The
cylinder of rectangular units, known as the armature, revolves
on its axis between the poles of this magnet. The explanation of
the rotation is the same as that given for the simple unit; but no
sooner has one unit revolved so that its strips on the commutator
have lost contact with the strips that supply the current than
another unit receives the current. This unit is forced to rotate
in the same direction, and so the armature continues to revolve.
There are many types of commercial motors, which differ
from each other principally either in the method of winding the
wire on the armature, in the arrangement of the coils, or in the
arrangements of the magnets. Each of these various types
possesses certain advantages, and each is adapted to a particular
sort of work. Some of them will run only on an alternating
current, others on a direct current. This point will be better
understood after the discussion of magnetos and dynamos.
One other simple type of motor may be described that is often
used in schools as a demonstration motor, and that is sold in
shops as a toy. Imagine a rimless wheel, with three equidistant
iron spokes, to be so mounted that it will whirl between the ends
of a horseshoe-shaped electromagnet (Fig. 101). The current
flowing into the motor is divided. Part of it goes through a
wire that is wound first about one arm of the horseshoe-shaped
iron in an anticlockwise direction, and then in a reverse manner
on the other arm of the iron. Thus the current makes one pole
the north pole and the other the south pole of the electromagnet.
ELECTRICAL INVENTIONS
237
The remainder of the current goes to a binding-post at d, thence
through metal strips that are held in contact with the commutator
by their springiness, and out at e. The commutator here con-
sists of three pairs of copper strips each insulated from its neigh-
bors. The members of each pair are fastened to the opposite
sides of the axle of the armature. (See II and III of Fig. 101).
The members of each pair are also attached to the ends of a
wire wound about one of the radial iron spokes or cores. Thus
/ and g are attached to the ends of a wire that is wound in a
FIG. 101. — Diagram of a toy motor. I, sectional view of the motor; II, the
armature enlarged; III, diagrammatic side view of commutator.
clockwise direction about the upper one of these radial soft-iron
cores. When the current is flowing through this wire, the free
end a of the core is a south pole. It is, therefore, attracted to
the nearby north pole of the electromagnet and so causes the
armature to whirl in an anticlockwise direction. In a sixth of a
revolution of the armature, h and i are in contact with the metal
strips bringing in the current. They connect with the wire that
is similarly wound about the core whose free end is at b. But
the current goes through this wire in a reverse direction from
that which it had in the first coil; the free end b is, therefore,
a north pole, and is repelled by the adjacent north pole of the
238 OUR PHYSICAL WORLD
magnet. This turns the armature so that the free end c comes
into the position first occupied by a; it is made a south pole, and
thus the whole process is repeated.
How the wires on the horseshoe magnet and the radiating
cores must be wound to produce the results described will be
clear if a simple law already learned is recalled (p. 207). If
an electric current is sent through a circular loop of wire placed
about a magnetic needle or small bar magnet that is free to swing
on its mid-point in a horizontal plane at right angles to that of
the loop, the needle or magnet is deflected. If a strong current
is used or if many turns of wire in a flat coil be used in place of
the single loop, the deflection is very marked, and the needle will
assume a position such that its long axis is perpendicular to the
plane of the coil. Furthermore, the direction of deflection will
be constant. If you imagine yourself swimming along the wire
in the direction in which the current is flowing, your chest toward
the needle, the north pole of the needle is always turned to the
left. Evidently the loop of wire is the equator of a magnetic
field whose south pole attracts the north pole of the needle
and coincides with it when the needle attains its maximum
deflection.
If a current is sent through a spirally coiled wire whose turns
run in the same direction as the hands of a clock, or as the turns
of the thread on a right-handed screw, the spiral coil behaves as a
magnet, and magnetizes a soft-iron core placed within it. Apply-
ing the same swimming figure for determining its polarity, evi-
dently the end of the coil toward which the current is moving is
the south pole, while, if the turns of the wire are left-handed or
anticlockwise, the pole is a north pole.
The electric motor is an exceedingly convenient device for
the application of power. It may be mounted directly on the
shaft that is to be turned, instead of being connected with it by
a crank shaft or by belting and pulleys, as is usually necessary
when a steam engine is used as the source of power. As it
occupies little space in proportion to the power developed, it
ELECTRICAL INVENTIONS
239
can be mounted on the tool itself as on the drill, the planer, the
sewing-machine (Fig. 102), the wringer, the vacuum cleaner,
FIG. 102. — The motor on a sewing machine. The light machine stands on
any table. The wires to the motor come from an ordinary electric-light socket,
running through a control switch operated by the foot. This switch is here shown
beside the machine.
FIG. 103. — A vacuum cleaner
etc. (Fig. 103). The power can be carried by flexible wires al-
most anywhere and to great distances from the central generating
240
OUR PHYSICAL WORLD
station. All sizes of motors can be built, from those of a frac-
tion of one horse-power to those of thousands of horse-power.
They can, therefore, be used for the most delicate operations,
such as running the dentist's drill, as well as for tasks requir-
ing tremendous power, such as the operation of electric trains
(Fig. 104). The power is generated so as to produce continuous
rotation rather than a back-and-forth motion which, as in the
piston rod of an engine, must be transformed into rotation at a
considerable loss of energy. Therefore, very high rotary speeds
FIG. 104. — Electric locomotive, weight 265 tons, used on the Chicago, Mil-
waukee, and St. Paul Railway over the Cascade Mountains. (Courtesy of the
Chicago, Milwaukee & St. Paul Railway.)
may be achieved. The motor of a rapidly running automobile
makes a thousand or more revolutions per minute, while motors
built especially for speed, such as those used on drills, centrifugal
pumps, etc., may run at rates ten, twenty, or more times as
great.
Faraday, it will be recalled, reasoned that if a current moving
in a wire will cause motion in a nearby magnetic needle, then a
moving magnet should produce a current in a nearby wire. He
verified this hypothesis by introducing a bar magnet into a coil
ELECTRICAL INVENTIONS 241
of wire. It will be remembered that it was only when the magnet
was moving that a current was produced; only, in other words,
when the wire was cutting through the lines of force of the
magnetic field. The current moved in the wire in one direction
when the magnet was being introduced into the coil, and in a
reverse direction when the magnet was being withdrawn from
the coil. This principle is the basis of the dynamo.
If the armature of a motor is forced to revolve by sending a
current through its wires, it might be expected that a current
would be generated in the wires by the rapid mechanical revolu-
tion of the armature. This is true. If the magnetic field in
which the armature revolves is produced by permanent magnets,
the machine is called a magneto; if the magnetic field is produced
by electric magnets, the machine is called a dynamo.
Let us follow in detail the events that would happen if the
armature in a motor were revolved mechanically (Fig. 101).
Such revolutions may be accomplished by a steam engine, a water
wheel, a windmill, or by hand. As the upper of the three coils
turns so as to approach the north pole of the magnet, it cuts
through the lines of force of the magnetic field, and a current is
produced in its wire. Since the current flowing through the
wire in a clockwise direction produced a motion of this coil
toward the north pole of the electromagnet when the machine
was operating as a motor, therefore motion of the coil toward
this north pole will now produce a current flowing in the wire in
a clockwise direction. Or to state the matter in a slightly differ-
ent way: The free end of the core a tends to become a south pole
of an induced magnet as it approaches the north pole of the
electromagnet, and the current in the wire must similarly tend
.to make this end of the coil a south magnetic pole. The current
so induced now flows through the coil to the armature strips
and thence through the spring clips and binding-posts, coming
from the machine at e. But now b is moving away from the
north pole, is becoming less strongly a south pole, and so the cur-
rent in the coil about the core whose free end is b is flowing in an
242
OUR PHYSICAL WORLD
anticlockwise direction. But since the strips on the armature
are in the reverse position from those of the coil about the core
whose free end is a, the current will flow from the machine in
the same direction as in the first case. The coil about the core
whose free end is c is now turning into the position occupied at
first by the coil about the core whose free end is a, and so the
process continues. Such a dynamo, therefore, gives rise to a
direct current. The core of the electromagnet remains mag-
netized sufficiently while the dynamo is idle to start the pro-
cess described when it is
again used, and then its
power is increased by
the current generated.
If a current is mov-
ing along a circular loop
of wire in the direction
indicated by the arrow
(Fig. 105), the south
pole of its magnetic field
is at the left of the loop,
the north pole at the
right. Recall the figure of the swimmer. If now the loop with-
out a current in it were rotated between the poles of a magnet
in a clockwise direction, the position previously occupied by
the south pole of the loop is approaching the north pole of the
magnet. The current generated in the wire will flow in such a
direction as would produce a south pole in this position if a
current were flowing: in other words, so as to develop a pole
opposite in character to the one which the wire is approaching.
Or you may use the "rule of thumb" to determine the direction
of the flow of the induced current. Hold the thumb and the
extended index and middle fingers of the left hand at right angles
to each other. Point the index finger in the direction of the lines
of magnetic force, the middle finger in the direction of the move-
FIG. 105. — Diagram of a simple dynamo
ELECTRICAL INVENTIONS
243
ment of the wire through the magnetic field. Then the thumb
held parallel to this wire will point in the direction of the flow
of the induced current. If, however, the same wire is considered
in the position indicated by the dotted line, the position of the
south pole is moving away from the north pole of the magnet,
and so the current in the loop is reversed. Rapidly rotating
such a loop would therefore produce an alternating current in
the wires connecting with its ends.
Or suppose the armature of a dynamo is a wheel having a
number of cored coils set like cogs on its rim (Fig. 106). The
wire of the coils is continuous and wound in each coil in the same
direction. The two ends of the
wire each run to a circular metal-
lic band fixed to the axle of the
armature. The current gener-
ated is taken from these bands by
spring clips that are in contact
with them. If such a wheel re-
volves with its rim inside of a
circle of north magnetic poles,
every time a coil approaches a
pole it produces a current in
one direction, and as it leaves the pole it produces a current
in the opposite direction. This type of dynamo therefore gen-
erates on alternating current.
When electric power is sent a long distance over wires from
a central generating plant to neighboring cities for running their
lights or factories, it is sent at high pressure. A long wire offers
much resistance, and it is found that less power is lost in leakage
to the air and to objects on the way when the current sent is of
high voltage. We have become familiar with these high-power
lines, as the distribution of electrical power has become common
(Fig. 107). The wires are usually supported on steel towers,
and huge porcelain insulators are used instead of the small glass
ones familiar on telegraph and telephone lines, which carry cur-
FIG. 106. — A dynamo with cored
coils set like cogs.
244
OUR PHYSICAL WORLD
rents of low voltage. Commonly, too, when the lines cross
highways, the towers bear signs to the effect that the wires are
dangerous. A shock produced by contact with a low-voltage
current usually causes surprise and more or less discomfort, but
FIG. 107.— A high-power transmission line
death is often the result of shock from a high-voltage current.
In case of severe shock the means of resuscitation are the same
as in the case of drowning, namely, the maintenance of artificial
respiration and of the body temperature.
It is usually unsafe as well as undesirable from a mechanical
point of view to use a high-voltage current for ordinary purposes
ELECTRICAL INVENTIONS 245
until it is " stepped down" to a lower voltage by a transformer.
One type of transformer easily comprehended may be briefly
described. A fine insulated wire is wound many times around a
small cylinder of soft iron outside of which is a larger cylindrical
frame wrapped with a few turns of coarse insulated wire. When
an alternating current of high voltage is sent through the inner
coil, it induces a low-voltage current of greater amperage in the
coarse wire. Thus a current of 5,000 volts and i ampere might
be stepped down to one of 100 volts and 50 amperes. Conversely,
if an alternating current of low voltage is sent through an inner
coil of coarse wire it will induce a high-voltage current of pro-
portionately less amperage in the fine wire of an outer coil. In
the latter case the transformer is built to step up the current.
When the transformer is used on a continuous current it is
provided with an interrupter, one type of which is similar to the
device used to rapidly make and break the current of an electric
bell. For it is only when the moving lines of magnetic force pro-
duced by one coil cut the wires of the other coil that a current is
produced in this second coil. This occurs incessantly with an alter-
nating current since the direction of the flow is constantly chan-
ging; it is assured in a constant current by the use of the interrupter.
It was not until the invention of the dynamo made it possible
to produce electricity cheaply and abundantly that motors,
electric lights, electric heaters, and similar contrivances became
commonplace.
When an electric current is forced under high voltage through
a fine wire, the electric energy is partly transformed into heat
energy. If you send a current from a dry battery through a
fine copper wire, you will feel the wire become hot, or if you wrap
the wire several times about the bulb of a thermometer, it will
very soon register a rise in temperature. The incandescent
electric light is made by sending a current through such a fine
resistant wire that a strong glow of light is the result. In the
earlier types of electric lights a fine filament of carbon was used
in place of a wire. Since carbon heated to the glowing point in
246
OUR PHYSICAL WORLD
the air would promptly combine with oxygen or burn, it was
necessary to exhaust the air from the electric-light bulb or to
fill the bulb with some gas like nitrogen or argon that does
not unite with carbon.
It was an American, W. Starr, who in 1844 invented the first
incandescent lamp. A thin strip of carbon in a glass capsule
from which the air had been exhausted
produced a light as the carbon glowed
with the current sent through it. The
next year a Frenchman, De Changy,
used lamps with filaments of platinum
for lighting the workings in coal mines.
Progress was gradually made, but the
incandescent lamp remained a crude
affair until Edison worked on it in 1878,
when he made so many improvements
that he is looked upon as its real in-
ventor. He used a carbonized fiber of
bamboo which was attached to plati-
num wires so fused into the glass of
the bulb that the latter could be made
sufficiently air-tight to hold the vacuum
for a long time. Recently, in place of
the carbon, filaments of metals like
tantalium or tungsten are used. Rare
ores are made to yield these metals, and
the invention of methods for making
them malleable and ductile was very
difficult. This task was necessary,
however, as naturally these metals are very brittle and fragile.
But now a single pound of tungsten makes 30 miles of fila-
ment that stands a temperature of 5,000° F. A carbon
filament could not stand even half the temperature neces-
sary to produce the more intense incandescence of the wire
(Fig. 108).
FIG. 108. — Diagram of an
electric light.
ELECTRICAL INVENTIONS 247
In wiring a house for electric lighting, it is customary to
connect the lamps in parallel rather than in series as the latter
method would offer more resistance, since all the current must
then go through each fine filament (Fig. 109). When the lights
are connected in parallel, the current flows through all the fila-
ments simultaneously, which is equivalent to being carried in
one wire as many times as large as one filament as there are
lamps.
The current goes to the several lamps through wires that
connect with a fuse in a fuse box. The fuse is merely a strip
of some easily melted alloy inclosed in a tube or porcelain box.
If through the accidental crossing of wires an unduly strong
current should be sent into the light circuit, this strip or fuse
would become hot, melt, and so sever the connections before
enough heat could be generated in the wiring of the house to
start fires in the woodwork along which the wires might be laid.
In the arc light the current is made to flow through two carbon
pencils whose tips are opposed at a slight distance from each
other. As the current jumps this space it carries with it nu-
merous highly incandescent particles from the positive to the
negative carbon and so produces the arc. The tip of the positive
carbon is, therefore, always hollowed while the negative is
always pointed.
The temperature of the glowing tip of the positive carbon is
about 6,000° F. Such temperature makes the electric furnace
possible. A crucible of heat-resistant substance is fitted about
the ends of a pair of large carbons adjusted like those of the arc
light. A heavy current sent through the carbons melts exceed-
ingly refractory substances placed in the crucible. Carbon so
melted under high pressure forms artificial diamonds.
In electric heat devices of various sorts, e.g., the heater,
the toaster, the percolator, the curling-iron heater, the bed
pad, the flatiron, etc. (Fig. no), the current is sent through
coils of wire or metallic plates that become more or less heated
according to their resistance and the strength of the current.
248
OUR PHYSICAL WORLD
ELECTRICAL INVENTIONS
249
As far as the principle of operation is concerned, such devices
can be readily understood from the diagrams of Figure no.
FIG. no. — (a) An electric heater; (6) an electric percolator sectioned to show
inside; (c) an electric flatiron, showing diagram of inside; (d) an electric toaster.
In the toaster, for instance, the current going through the
wires on the frame causes them to become red hot. The slice
of bread on the rack is exposed to their heat and so is toasted.
CHAPTER XI
RADIO COMMUNICATION1
There's music in the air.
— G. F. ROOT.
All of the inventions of electrical appliances described above
that have succeeded one another with such rapidity have been
marvelous, but no other one has so taken hold of the popular
interest as has wireless or radio. It has seemed incredible and
little short of the supernatural, yet it is quite simple and easily
comprehensible as science now explains it.
Transmission of telegraphic and telephonic messages by radio
is accomplished by setting up in the ether an electrical wave
motion, which, when intercepted by a suitable receiving appara-
tus, will in turn set this receiving apparatus into vibration similar
to the electrical vibration of the transmitting station. Thus
the original dots and dashes or the speech or musical sounds
originating at the sending station may be reproduced at the
receiving station, sometimes many thousands of miles distant.
The ether is a highly elastic medium that is supposed to fill space.
It must be understood that the actual vibrations in the ether
of the space separating the stations are inaudible, and produce
sound only after they have set the apparatus of the receiving
station into vibration and these electrical vibrations have been
converted into less rapid vibrations that produce sound or leave
a permanent record, as in the case of automatic recorders.
Since the whole system of radio transmission depends on
wave motion in an elastic medium, it can be compared with other
wave motions which are more familiar. Recall how a stone
thrown into a quiet pond starts a series of waves that in ever
1 This chapter has been prepared by Fred G. Anibal, formerly radio officer,
U.S. Air Service.
250
RADIO COMMUNICATION 251
widening circles run to the shore of the pond, and there set to
rocking the weeds or grasses that are growing along shore. A
bell when set in vibration will cause the surrounding air to be
set in motion, and this wave motion when it strikes the ear will
set up there a similar vibration which is transformed to nervous
impulse and transmitted to the brain, so we hear the sound
(Fig. 166, p. 327).
Sometimes it has been noticed that certain notes struck on a
piano will cause objects in a room to vibrate; other notes will
seem to have no effect on these same objects. Thus, if a violin
string be tuned so it gives off the C note if bowed and this note be
struck on the piano, the violin in the same room will be found to
also sound this note faintly. The violin string is set in motion be-
cause the sound waves regularly striking it have the same period
of vibration as is now natural to it, and so gradually produce in it
the same rate of vibration as the vibrating wire originally struck
in the piano. The violin is said to be "in tune" with the note,
and so will respond to notes of this rate of vibration. Similarly,
the radio receiving apparatus must be adjusted so as to be "in
tune" with the sending station. This adjustment may be
changed so that, although many stations may be sending out
vibrations at the same time, only the one with which the receiving
apparatus is "in tune" will produce noticeable effects. Sending
stations are also capable of adjustment so that at different times
they may send out vibrations of different rates.
Radio apparatus then consists of two types of appliances:
those that create the waves, the large transmitting and broad-
casting stations, and the appliances which receive the waves,
or the many thousands of small receiving sets distributed over
the country.
The sending station consists of apparatus which will produce
electrical vibrations of such high frequency that they will set
the ether into vibration, and thus radiate through space in every
direction from a point. Hence the expression "radio broad-
casting." A system of control must also be included so that the
252
OUR FHYSICAL WORLD
series of vibrations may either be broken into long and short
groups, as with the wireless telegraph when transmitting dashes
and dots, or modifications made in the nature of the wave so that
sounds of various pitch may be transmitted as in the case of
the radio telephone.
A very simple amateur wireless telegraph sending outfit
may consist of a source of electrical power, such as a battery,
*
k
FIG. in. — A simple wireless sending outfit
a key for controlling the power, an induction coil and spark gap
by means of which the battery current is transformed into high-
frequency electrical current, and an antenna or electrical con-
ductor extending some distance above the earth, so that the
electric waves may readily radiate into the ether with little
interference. Such an arrangement is shown by diagram in
Figure in. The source of electrical power is shown at (&), and
consists of a battery of several cells. The key for interrupting
RADIO COMMUNICATION 253
the primary circuit is shown at (&). An induction coil and spark
gap for transforming the low-voltage direct current into a high-
voltage, high-frequency oscillating current are shown at (i) and
(sg). The antenna or aerial conductor is shown at (a) and the
other side of the spark gap is grounded at (g).
When the primary circuit is closed, sparks will jump across
the gap (Fig. 112), and since these are in reality electrical dis-
charges of very high frequency they will set up in the antenna
and ground circuit a very high frequency electrical current.
This current will set the ether surrounding the antenna into vibra-
tion, and thus will radiate into space long and short series of vibra-
tions corresponding to the dots and dashes of the telegraph code.
It is in the circuit
consisting of antenna, i
spark gap, and ground ( uHO
connections that the
radio vibrations origi- «
nate. A condenser f
which will withstand
high potential electrical FlG" II2-~A spark gap
charges of several thousand volts may be connected across the
spark gap, and then a coil of heavy wire with adjustable con-
nectors may be included in the antenna circuit. With these
additions we have a typical radio-frequency oscillating circuit
as is shown in the second diagram (Fig. 113, p. 254).
The condenser consists of two sets of sheets of tinfoil or other
good conductor, the sheets of one set alternating with those of
the other, and each sheet is carefully insulated from its adjacent
fellows. The ends of the fine wire on the transformer each attach
to one of these two sets. One set also fastens to a wire that runs
to the aerial, and that also branches to connect with one of the
metallic knobs of the spark gap; the other set fastens to a wire
that runs to the ground, and that branches to the other knob of
the spark gap. This spark gap is made of two adjustable metal-
lic rods, mounted close together in the same straight line. Each
254
OUR PHYSICAL WORLD
rod bears at the end opposite its fellow a metallic knob; these
knobs, by the adjustment of the rods, may be spaced as desired.
As a current flows in the coarse wire of the induction coil, it
induces a high-tension current in the fine wire coil. Electrons
then discharge on to one set of sheets of foil in the condenser,
say the set connected with the aerial. These repel similar charges
FIG. 113. — Diagram of a more complex sending outfit
on the other set of sheets, and drive them off at the same time
they draw up positive charges from the earth on to them. Such
positive charges help hold more electrons on the first set which
draw more positive charges to the second set. This " condensa-
tion" continues until a strong charge of high- voltage electricity
accumulates, when finally there is a discharge back and forth
across the gap, and simultaneously the current surges up into
the antenna and sets going radio waves in the ether.
RADIO COMMUNICATION 255
The rate of frequency of the vibrations set up in this cir-
cuit depends essentially on two factors, capacity and inductance.
The condenser furnishes the capacity, and the number of turns
used in the helix (coil) of the antenna circuit determines the
inductance. As we increase the number of turns of the helix
included in the antenna circuit, the greater inductance makes
the condenser accumulate a heavier charge before the discharge
occurs, so the intervals between discharges are longer and the
waves created are therefore longer. This arrangement makes it
possible to tune the sending station within limits depending upon
the size of the induction coil, condenser, length and height of
antenna, etc. Usually small amateur stations, are tuned so as to
have the maximum output of energy from the antenna within
government frequency regulations for amateur stations.
The electrical vibrations which actually occur in the oscillat-
ing circuit and which are radiated into the ether from the antenna
are really a series of wave- trains. The more numerous the waves
are in each wave- train, the shorter each wave is; or the greater
the frequency, the shorter the wave-length. This frequency is
very high in radio waves, ranging from ten thousand up to several
million per second, and is known as radio frequency. Since the
waves travel in ether with the speed of light, or 300,000,000 meters
per second, we can easily determine the wave-length of a sending
station if we compute the frequency from the capacitance and
inductance values. Thus a frequency of 730 kilo-cycles (730,000)
would have a wave-length of about 411 meters.
The rate at which the wave-trains succeed each other is
much lower than radio-frequency rates, and is within the range
of audio frequencies or the rate of vibration of sound waves,
usually around 500 to 1,000 cycles per second. The pitch or
note of the incoming wave from a damped wave-sending station
depends on the frequency of the wave-trains. Damped waves
are those that gradually die out like the waves of a wave-train
(Fig. 114). For comparison a standard A tuning fork vibrates
435 times* per second.
256
OUR PHYSICAL WORLD
The simple wireless receiving equipment consists of appliances
for intercepting these trains of high-frequency ether waves and con-
verting them into electrical vibrations which can bemade to produce
mechanical vibrations of audible frequency in a telephone receiver.
f
wave-
o
wave-
FIG. 114. — A train of damped waves
A simple wireless receiving outfit may consist
of (Fig. 115), (a) the antenna for receiving the
ether waves, (d) a detector for converting these
waves into electrical impulses of audio frequency,
(r) a telephone receiver for converting the elec-
trical impulses into mechanical vibrations, and
(g) a ground connection.
The receiving antenna is not necessarily so
large as the sending antenna, and may consist
of a single wire suspended between high points
above surrounding buildings or trees and about
1,000 feet in length. Much simpler antennas
have been found to be very successful. Wires
suspended in an attic are sometimes employed,
and even small loops of wire within a room are
very efficient with sensitive receiving equipment.
Even bed springs and fire escapes give fair results
when not many miles from the sending station.
The detector is the distinctive part of the
radio-receiving circuit. There are a great num-
ber of types of detectors. They all consist of
an arrangement whereby the electrical oscillations are rectified
or made to flow principally in one direction with the result
that a pulsating current of audio frequency flows through
<\
)r
C/
FIG. 115.—
Diagram of a
simple receiving
set.
RADIO COMMUNICATION
257
the telephone receiver. The commonest type of detector in
use is the crystal detector. This piece of apparatus consists
of a piece of mineral, usually galena or iron pyrites imbedded in
a fusible alloy and so mounted that a fine wire may be adjusted
to touch the surface at one point. Since some points on the
mineral are more sensitive than others, the wire is made adjust-
able so that a sensitive point may be easily found while trying
to pick up signals caught by the antenna (Fig. 116).
FIG. 1 1 6.— The crystal detector. (Photo by the Radio Corporation of America . )
The action of this crystal type of detector as a rectifier is
much the same as that of a check valve in a pump. When the
oscillating current from the antenna, which is a back-and-forth
surge, attempts to pass through the crystal from the wire point,
the back-surge may be stopped so that current in one direction
only will pass through to the telephone receiver, and so on to the
ground, completing the circuit. The effect is that of a pulsating
direct current of audio frequency which will produce one click
in the telephone receiver for each wave-train. Since a dot in
the telegraph code is a short series of wave-trains, it will be
reproduced in the telephone receiver by a short succession of
258 OUR PHYSICAL WORLD
clicks at audio frequency, producing a short buzz. A dash will
be a long buzz.
The telephone receiver usually employed consists of two
watch-case receivers mounted on a head band in such a manner
that one receiver will be pressed on each ear. Such a piece of
equipment is called the head set (Fig. 117). The ordinary tele-
FIG. 117.— Radio room of the SS. "Leviathan." (Courtesy of the Radio
Corporation of America.)
phone receiver is not sensitive enough for the faint radio signals,
and, therefore, much more sensitive receivers with very thin
diaphragms and a resistance of around 1,500 ohms are employed.
In order that signals of different frequencies may be picked
up, the receiving equipment must include apparatus for varying
the inductance and capacity of the circuit. By variation, the
receiving circuit may be tuned to respond to the vibrations of
the sending station. It will be recalled that the violin in the
RADIO COMMUNICATION
259
room with the piano will vibrate only when a certain note is
sounded. The receiving circuit can be adjusted by changing
the values of capacity and inductance, so that it will respond
to any frequency or wave-length desired. The reception will
rcTX
FIG. 1 1 8. — A simple receiving circuit
not be interfered with by waves sent from other stations operating
unless the wave from such stations is at the same frequency as
the wave sought to be intercepted. A very simple receiving
circuit that may be tuned by varying inductance only while the
capacity is fixed is shown by the diagram in Figure 118. This
260
OUR PHYSICAL WORLD
circuit shows the type commonly employed on simple receiving
circuits, and is in reality two circuits. This method of connection
does not introduce the resistance of the head set into the antenna
circuit, and permits the vibrations from the antenna to flow more
freely.
The tuning coil (/) consists of one layer of insulated wire (about
No. 20) on a cardboard tube about 5 or 6 inches long (Fig. 119).
The insulation is removed in two strips on opposite sides of the
coil to permit connection by a slider which touches one turn of
wire at a time. In this manner the number of turns of wire
between the antenna and the ground can be varied by moving
FIG. 119. — A two-slide tuning coil
the slider. The turns of wire themselves constitute the con-
denser in this case so that the capacity is also varied when the
slider is moved. This circuit from the antenna (a) (Fig. 118)
through the turns of the tuning coil (/) to the ground (g) consti-
tutes the primary circuit.
The secondary circuit uses the same coil but a different portion
of it, part of which may overlap the primary inductance, as
shown in the diagram. This closed circuit is from the second
slider on the tuning coil (/) through the detector (d), through
the phones (r), and back to the other end of the tuning coil.
By moving these sliders we can change the inductance in
both the primary and secondary circuits, and thus place the
receiving outfit in tune or in electrical resonance with the send-
RADIO COMMUNICATION 261
ing station from which the signals are desired. When one tuning
coil is used with two sliders in this manner, and so becomes a
part of two circuits, it is known as an auto-transformer. In tun-
ing such an outfit the primary circuit or open oscillating circuit
must be tuned to respond to the frequency of the sending sta-
tion, and then the secondary or closed oscillating circuit must be
tuned to the primary circuit. Sometimes a small fixed condenser
is shunted across the phones, permitting the vibrations to flow
more easily. This is known as the phone condenser, and has a
capacity usually of about .001 microfarads. This condenser
(re) is sometimes left out of the circuit, and in such case the
phone cords themselves act as a condenser.
A simple receiving set such as the one described, but equipped
with a simple tuning coil, may easily be made and assembled
as follows. For the tuning coil procure a cardboard tube about
five or six inches long and three and a half to four inches in
diameter. Round cardboard oatmeal boxes serve this purpose
very well. This tube is to be wound with insulated wire and
mounted horizontally on a board, which may serve also as the
base for the detector and the terminals for the phone connection.
For the base secure a piece of material, wood or fiber, about
two inches wider than the diameter of the tube and four or five
inches longer than the tube. The tube is provided with end
pieces of the same material as the base. Tliese end blocks are
to be cut one inch less in width than the base board, of a height
equal to the width of the base board. In each end piece is now
cut a shallow groove to receive the ends of the cardboard tube.
This groove should be the same distance from the top of the end
piece as from the sides. The guides for the sliders and the sliders
themselves for the tuning coil may be secured cheaply at any
radio supply shop or ten-cent store. The guide rods should be of
square metal material of f - to J-inch stuff. They must be as long
as the length of the tube and end pieces when the tube is fitted
into the grooves in the end pieces. The sliders are small blocks
of wood with a square notch cut on one side to fit snugly over the
262 OUR PHYSICAL WORLD
slider guide. A piece of metal, brass or copper, is tacked or
screwed on the under side of the block to hold it in place on the
guide rod. This piece of metal should be cut with a narrow
strip which may be bent down and then back under the slider
so that it makes spring contact with the turns of wire on the
cardboard tube. In order that contact may be made successively
with each turn of wire on the tube, the insulation must be scraped
off in a narrow strip extending the full length of the tube, directly
under each slider rod. The pressure of this sliding contact on
the wire must be strong enough to insure positive connection
between slider and each separate turn, but not so strong as to
wear the wire rapidly or to require much force to move it along
the slider. Contact with the slider rod is made by the metal
covering over the groove on the slider. The end of this metal
piece may be cut slightly so that it can be pressed tightly against
the guide rod.
Two of these sliders with guide rods are to be provided. One
is to be mounted directly over the tube with the ends of the rod
secured to the end pieces. Square notches may be cut in the end
pieces and the rod fitted snugly into these and secured by a screw
at one end through a hole in the rod, and by a binding-post at the
other to which the connection may be made. The slider rod is to
be mounted at the side of the tube and directly over the center.
The winding on the tube is to be of No. 22 insulated wire.
Enameled wire may be used. First shellac the tube. Punch
two holes in the tube about a quarter of an inch apart and one
half-inch from one end. Pass about ten inches of wire through
one hole from the outside and then secure it by bringing it up
through the other hole and then again through the first hole and
back out through the second hole. Now wind the wire closely
and smoothly over the tube to within about half an inch of the
other end. Secure the wire on the same side of the tube and in
the same way as before, allowing about ten inches for connection.
To hold the wire in place, a second coat of shellac may now be
applied.
RADIO COMMUNICATION 263
Before the tube is mounted on the base, the detector should
be procured or made, and provision made for the phone terminals
at one end of the base, and for the binding-post for the ground
connection on the other end of base (Fig. 119).
It is suggested that a crystal detector be purchased. This
detector may be of the type that can be adjusted or one that is
always in adjustment. If it is desired that the detector be
made, it would be well to investigate various devices on the
market and duplicate one of the numerous types. Essentially
the detector merely consists of a small crystal of selected galena
or iron pyrites, which is touched with light pressure by a small
spring wire known as a "cat whisker." One connection is made
to the whisker and the other to the crystal. The crystal may be
imbedded in fusible alloy, and secured to the base board with
screws. A binding-post may be set in the base close to the crystal,
and the whisker secured to the binding-post in such a manner
that it loops over with its point resting lightly on the surface of
the crystal.
A small radio-phone condenser should be purchased and
mounted at one end of the base between two binding-posts
provided with holes to take the terminals of the cord leading to
the phones. The spacing of these binding-posts is determined
by the phone condenser, which is a small strip with holes in each
end for the binding-posts.
The set is now ready to be mounted on the base board and
connected up. As small holes are to be drilled in the base board
for the connecting wires, it is advisable to assemble the set first
in order to determine the position of these holes before securing
the parts permanently.
The tuning coil is mounted at one end of the base with the end
piece one inch from the end of the base and one-half inch from
each edge of the base. Wire finishing nails may be used to secure
the end pieces of the coil to the base board. The tube with its
layer of wire is glued into the grooves with the ends of the winding
down. These ends are threaded through holes previously drilled
264 OUR PHYSICAL WORLD
through the base board. The wire nearest the end of the base
is fitted into a groove made on the under side of the base and
securely connected to the binding-post set in the center of this
end of the base. This binding-post receives the ground con-
nection.
The other end of the winding is also led through a hole in the
base and along a groove to the under side of the binding-post
which carries the cat whisker of the detector. Another style
of connection, which may work better in some localities, is
that of leaving this end of the coil unconnected, and con-
necting this side of the detector directly to the ground binding-
post.
The detector is mounted midway between the end of the coil
and the binding-posts for the phones. These phone connections
are placed close to the end of the base and midway between the
sides so that the strip condenser will be parallel to the end of the
base. One of these phone terminals is connected to the crystal
of the detector and the other to the slider mounted on the side of
the coil. The connection wire is to be led through a groove on
the under side of the base to a hole directly under the binding-
post on the slider rod. The slider rod on the slide is then secured
to the ends of the tuning coil on its front side by means of the
binding-post in such manner that this binding-post will be on the
end nearest the detector. The other slider rod is mounted on
top of the coil with its binding-post on the other coil end. This
slider is connected to the aerial wire or antenna.
The phones must be purchased. They may be either single
or double. A double head set of 2,000 ohms resistance is recom-
mended. When the phone cords are connected to the binding-
posts provided, the set is ready for operation as soon as the
antenna and ground are connected.
The ground connection is made with a bare copper wire (about
No. 10 or No. 12) to a water-pipe or to a metal plate buried
about three or four feet in the earth. If the earth is very dry
this plate may have to be buried deeper.
RADIO COMMUNICATION 265
The antenna is made and installed as follows. First decide
upon its location. The wire should be suspended between
two high points so that it does not come in contact with any-
thing between these points. The wire should be stranded if
possible and as nearly 150 feet long as the location will permit.
Attach to the end of this stranded wire, near that insulator which
is closest the set, an insulated wire which is led through a tube
insulator into the room where the set is located. Around the
groove in the knob insulator, or to the other end of the strain
insulator, attach wires or ropes and secure these to the high
points selected. For these points poles may be erected on the
roof of a building, or trees may be used. The antenna need be
only high enough to clear immediately surrounding obstacles.
To adjust the set for receiving, fit the receivers to the ears
and adjust the whisker on the detector so that it just touches the
crystal lightly. Now move the sliders back and forth one at a
time until locations are found at which the signals are heard.
After a little practice the proper positions of the sliders will be
more readily located and it will be possible to adjust the detector
with greater nicety.
A more elaborate and yet very simple receiving outfit is
shown in Figure 120 (p. 266). The difference between this outfit
and the one in Figure 118 is found in the substitution of a receiv-
ing transformer in place of the tuning coil and the addition of a
variable condenser. The receiving transformer consists of two
cardboard tubes each wound with a single layer of wire and
adjusted so that one will slide within the other. The wire on
both tubes is the same size. When a current flows in the circuit
from antenna to ground going through the outer coil, it induces a
current in the other coil whose strength depends on how far the
second coil is shoved into the first. The variable condenser
(Fig. 121) consists of two sets of metal plates, those of one set
alternating with and parallel to those of the other, to which they
lie very close without being in contact. One set, the rotor, is
so mounted that its plates may be moved so as to lie wholly or
266
OUR PHYSICAL WORLD
only partly between those of the other set. By adjusting the
plates, the capacity can be varied and the natural vibration
frequency of the primary circuit can be changed. This type of
FIG. 120. — Diagram of a more elaborate receiving set
outfit permits much closer tuning than the outfit shown in Figure
119, and since the coupling is adjustable, more interference can
be cut out.
Many large commercial wireless telegraph stations and ship
stations still employ the same method of transmission of signals
RADIO COMMUNICATION
267
as the simple wireless amateur station. Such sending stations
are known as discontinuous wave stations because they radiate
into the ether these series of wave-trains. In such large stations
the source of power is usually an alternating current dynamo, and
a high-frequency transformer is used in
place of the induction coil. The helix may
consist of many turns of heavy copper wire
or rod, and the condenser usually is made
up of many rows of large Leyden jars in
parallel. (See Field and Laboratory Guide
in Physical Nature-Study, p. 69.)
The more modern method of radio trans-
mission employs what is known as the con-
tinuous wave. As the name indicates, the
wave motion which is radiated from the
antenna is not broken into a series of wave-trains each of which
dies out before the next begins. The continuous wave is one
long series of waves of radio frequency, which are sustained, and
have the same strength as long as the circuit at the sending
station is closed. A simple diagram to illustrate this continuous
wave in comparison with a discontinuous wave is shown in
Figure 122.
FIG. 121. — A rotary
variable condenser.
FIG. 122. — Discontinuous and continuous waves
The methods employed to produce these continuous waves are
of various sorts. Sometimes an arc between a carbon and a
copper electrode is used. The arc is placed in a circuit with induc-
tance and capacity, and when properly balanced such a circuit will
268
OUR PHYSICAL WORLD
oscillate at radio frequency and send out on the antenna a con-
tinuous wave. Sometimes the dynamo itself is a radio-frequency
alternator generating a current of such a large number of alter-
nations or cycles per second that when connected in a circuit with
suitable capacity and inductance it can be employed to produce
directly oscillations of radio frequency.
Perhaps the most popular method of producing continuous
waves for radio transmission is by means of the three-electrode
vacuum valve. Since this piece of apparatus is also very gener-
ally used as a detector for radio reception, a very
brief treatment of its construction and mode of
operation will be given.
The vacuum tube or three-electrode vacuum
valve (Fig. 123) depends upon the emission of a
stream of electrons or particles of negative
• i electricity from a hot wire or filament. In con-
Ik $&i struction it is similar to an incandescent electric-
light bulb. A wire filament is inclosed in a glass
globe from which the air has been exhausted.
S* B In addition to the filament, which is counted as
l^^T one of the three electrodes, there is also placed
within the tube a metal plate. Between the
plate and the filament is supported a grid or rack
with many strands of wire stretched across much
This plate and grid constitute the other two
electrodes. Both ends of the filament, the plate, and the grid
lead to terminals outside the tube so that there are four connec-
tions to the three-electrode vacuum valve.
When used as a simple detector of damped waves, the three-
electrode vacuum valve is connected into the receiving circuit
as shown in the wiring diagram of Figure 124. It will be noted
that this receiving circuit is practically the same as for the
crystal detector circuit, and likewise consists of primary and
secondary circuits. The additional feature is that the oscillat-
ing circuit is connected on one side of the variable condenser
FIG. 1 23. —Three-
electrode vacuum
valve.
like a fence.
RADIO COMMUNICATION
269
to the grid of the vacuum valve and on the other side to the fila-
ment. The plate is then connected in a third circuit through
a high-potential battery of about 40 volts, through the telephone
receivers, and back to the filament.
The action is about as follows. When the filament is lighted
by the current from the battery at a, which is controlled through
|b battery
FIG. 124. — Diagram showing the use of the vacuum tube as a detector:
(a) antenna; (0 receiving inductance; (g) ground; (r) head set; (p) plate in tube;
(g) grid in tube; (/) filament in tube; (vc) variable condenser.
a rheostat, a stream of negative particles of electricity or electrons
passes from it between the wires of a grid and strikes the plate,
which is positively charged by the high-voltage battery. This
stream of electrons constantly striking the plate will cause a
current of electricity to flow through the plate circuit and through
the telephone receivers. Any variation in this current, then,
will produce an effect in the telephone receivers.
270 OUR PHYSICAL WORLD
As long as the grid is neutral, the plate current is steady and
direct. When the incoming signals set the receiving circuit into
electrical vibration, the potential of the grid will change from
positive to negative very rapidly as each wave-train passes.
When the grid is negative it will repel the negative particles of
electricity and so stop the flow of the plate current. The effect
will be a pulsating current of audio frequency through the tele-
phone receivers each time a wave- train affects the grid. Since
slight changes in potential of the grid produce large changes in
the current through the plate circuit, the vacuum tube is said
to act as an electrical valve, allowing current to flow through the
plate circuit in one direction only.
As was stated above, the vacuum tube is also used to pro-
duce continuous waves. Larger power tubes, of course, are used
in the large continuous-wave transmitting stations. The tubes
for this use bear the names of pliotrons, oscillions, or other
names derived from characteristic features in their construction
(Fig. 125). It has been shown that slight variations in the grid
circuit of a tube produce large variations in the plate current.
This action is made use of by causing the plate current to flow
through an inductance placed close to a similar inductance in the
grid circuit. When oscillations are started in the grid circuit
they produce oscillations in the plate circuit which are "fed back "
into the grid circuit through this inductive coupling of the grid
and plate circuits. These inductances can be so adjusted that
the oscillations will be sustained, and a continuous wave will be
produced in the antenna circuit.
The reception of continuous wave signals cannot be accom-
plished with the ordinary rectifying detector. Although the
incoming wave may be rectified and caused to pass through the
telephone receivers, its frequency is so great that the diaphragm
of the telephone receiver will not respond to it, and so some
means must be introduced to produce a frequency of audible
range in the telephone receiver. This production of audio-
frequency vibrations in the telephone receiver is accomplished
RADIO COMMUNICATION
271
by introducing into the receiving circuit a vacuum valve to act
as a generator of continuous waves. When two tuning forks of
slightly different pitch are sounded near together, a pulsating
sound is heard. This is due to the sound waves reinforcing each
other and interfering with each other at regular intervals. The
number of pulsations per second will be equal to the difference
FIG. 125. — Power tubes for transmission. (Photo by Radio Corporation of
America.)
in the rates of vibration of the two notes. Identically the same
principle is used in the reception of continuous-wave telegraph
signals. The local oscillating tube generating the continuous
wave in the receiving circuit may be part of a separate circuit as
in the case of heterodyne reception. Or the detector tube may
be used for generating continuous waves as well as for acting as a
detector, and then we have autodyne reception. The rate of
oscillation of the receiving circuit may be varied, and the differ-
272
OUR PHYSICAL WORLD
ence in rates of vibration between the incoming wave and the
locally generated wave thus adjusted to any audible frequency so
that the signal may be easily heard in the telephone receiver.
The result produced is a succession of clear, whistling notes of
long and short duration, corresponding to dots and dashes.
FIG. 126. — The heterodyne. Diagram showing the use of the vacuum tube
as a generator of continuous waves: (p) plate in tube; (g) grid in tube; (/) fila-
ment in tube; (cf) the fixed condenser; (la) grid inductance; (/3) plate inductance;
(vc) variable condenser.
The method employed to cause the vacuum valve to act as
a generator of continuous waves may be understood by refer-
ence to Figure 126. The inductive coupling between the plate
circuit and the grid circuit is shown at m. The inductive coils,
between which this coupling is made, are shown at 12 and /3,
and are commonly known as the grid inductance and the plate
inductance respectively.
RADIO COMMUNICATION 273
Small models of such a continuous wave generator are used
as the source of the local continuous wave employed in connec-
tion with the receiving circuit to produce the "beat" effect
required in receiving continuous wave signals. When so used
this circuit is known as the heterodyne.
Practically the same system as shown in Figure 126, and
explained above, may be used for producing the continuous wave
sent out by transmitting stations. The vacuum tubes used in
such stations are necessarily much larger than the small tube.
Since, in transmitting, considerable energy must be supplied
to the antenna circuit, it is necessary to withstand heavy voltage
on the plate. The vacuum in the power tube must be extremely
high; otherwise the effect of this high plate potential will be to
produce a blue glow in the tube and impair its action. Com-
paratively large plate currents, due to this high plate potential,
cause the transmitting tubes to become very hot. To prevent
this excessive heating, the power tube is supplied with cooling
devices such as heat radiating fins on the plate connection outside
the tube. Devices are also now being employed which make
use of circulating systems of water to carry away the excess heat.
In order to supply the high-voltage plate current, direct-
current dynamos are installed as part of the transmitting equip-
ment. Such a dynamo usually has two commutators so that the
current for lighting the filaments of the power tube may be taken
from the same dynamo that supplies the plate with the high-
voltage current.
Because of the high vacuum required and the necessity for
getting rid of the heat, the size of these tubes is limited. For
large-power output several tubes are connected in parallel, so
that it is possible to radiate considerable energy from the antenna
of the continuous- wave transmitting station.
Since it would be quite impractical to break the dynamo cur-
rent supplying the tubes, in order that dots and dashes could be
sent from the antenna, some other means must be employed for
modifying this antenna current to produce the desired signals.
274 OUR PHYSICAL WORLD
Several turns of the antenna inductance are shorted by large
relays. These relays are actuated by a current which can be
controlled by the telegraph key or by some mechanical sending
device. The effect of shorting a portion of the antenna induc-
tance is to change the frequency of the transmission wave at
intervals, corresponding to dots and dashes. The result at the
receiving station will be a succession of notes at two different
pitches which can readily be interpreted by the receiving opera-
tor into the dots and dashes of the telegraph code. If the tuning
of the receiving station is sufficiently accurate, the only note
heard will be the one caused by the frequency produced when the
key at the sending station is closed. The wave which is sent out
by the transmitting station when the key is not depressed is
called the compensating wave. Very accurate tuning at the
receiving end is necessary to tune out this wave. Later practice
has been to ground this compensating wave through the water-
cooling system of the tube so that it does not cause confusion at
the receiving station.
It remains now to explain how speech and music may be
sent out by radio. The principle of the radio telephone trans-
mission is fundamentally the same as the principle of continuous-
wave transmission, with the addition of some means of impressing
on the continuous wave the sound or audio-frequency modula-
tion. This modification is made, not in the frequency of the
transmitting wave, but in its current strength or amplitude.
This impressing of the speech wave upon the continuous wave
is known as voice modulation, and is shown in the diagram of
Figure 127. The continuous wave in this case is called the carrier
wave. Its frequency is very high, between five hundred thousand
and one million double vibrations per second. This high fre-
quency is necessary in order that the voice tones, with their
varying frequencies of around five hundred to one thousand
double vibrations per second, may be faithfully reproduced.
Thus each wave of the sound will be outlined by the increasing
and decreasing amplitudes of about one thousand radio waves.
RADIO COMMUNICATION
275
Not this many radio vibrations are shown in Figure 127, but a
sufficient number are indicated to show how the change of
amplitude will impress on the high-frequency carrier wave the
lower-frequency sound vibrations. A crude analogy may help
to make this plain. If one drops a stone into a pond whose
surface is covered with little wind-made waves, the wave emanat-
ing from the point of the splash will be a resultant jointly of the
wind and the falling stone. The shore grasses, when the waves
1 Pi p
— soum
fv ^
^s ^-i rl
d wave 1
ft
J LJU IJ^
FIG. 127. — Diagram of voice modulation of a continuous wave
reach them, will not sway regularly as when only the wind
waves hit them, but irregularly, moved by the waves that also
bear the impress of the stone's disturbance. So the vibrations
of the human voice are carried along with the high-frequency
waves of the wireless telephone sender and register on the receiv-
ing apparatus.
This change in current strength of the carrier wave, without
changing its frequency, may be accomplished by inserting a
microphone in the antenna circuit of the transmitting station.
This microphone is a telephone transmitter adapted for heavier
276
OUR PHYSICAL WORLD
currents than the ordinary telephone transmitter. While this
method of using the microphone in the antenna circuit is possible
within very narrow limits of current strength, it is not practical.
The reason for this impracticability is that large current strength
in the antenna circuit is necessary for long-distance transmitting
and broadcasting of lectures and musical programs.
-plate voltaic > <— -filament voltage-
01£
FIG. 128. — The radio telephone transmitter: (/) telephone transmitter;
(tr) telephone transformer; (m) modulation tube; (0} oscillating tube; (/x, 12, /a)
inductances; (c8) fixed condenser; (a) antenna; (g) ground.
In order to produce this sound modulation in large radio
telephone transmitting stations, recourse is again had to the
vacuum tube. When used for this purpose, it is called a modu-
lator. The connections for this use of the vacuum tube as a
modulator in a radio telephone transmitting circuit are shown
in Figure 128.
By a study of this diagram (Fig. 128) it will be noted that one
tube (o) is connected into the circuit as a generator of continuous
waves. The telephone transmitter or microphone (/) is con-
RADIO COMMUNICATION 277
nected through a small transformer (tr) into the grid circuit of
the modulator tube (m). The double commutator dynamo is
shown at (d). This dynamo supplies both the filament current
and the plate potential to both tubes. Inductances (/2) and
(73) are placed in the oscillating circuits. In actual operation
several modulator tubes are connected in parallel to increase the
strength of the speech-input current. There are also several
generator tubes connected in parallel in order to increase the
strength of the outgoing or carrier wave.
Very briefly the action may be explained as follows. When
words are spoken into the transmitter, or microphone, a speech
wave of audio frequency is impressed on the grid circuit of the
modulator tube. This change in potential of the grid will pro-
duce corresponding changes in the plate current of the modulator.
This oscillation of the plate current of the modulator causes this
tube to build up or absorb energy from the antenna. This build-
ing up and reducing process corresponds to the vibrations of the
sound taken in by the microphone. The carrier wave, then, is
oscillating at regular radio frequency during the whole time the
station is sending. At the same time the current strength of the
antenna circuit, or the amplitude of the carrier wave, is vibrating
at audio frequency. This audio-frequency vibration reproduces
exactly all the sounds that strike the diaphragm of the microphone.
It will be recalled that a receiving circuit employing a simple
crystal detector is used to pick up signals from discontinuous- wave
sending stations. This result was explained as possible because
the wave-trains were at audio frequency. Now, when such a
receiving circuit is tuned to the frequency of the carrier wave
from a radio telephone transmitting station, the frequency of the
carrier wave is too fast to actuate the diaphragm of the telephone
receiver. The result will be that no sound is produced by the
carrier wave itself. The current intensity of the carrier wave is
vibrating at audio frequency, corresponding to the sounds strik-
ing the diaphragm of the microphone at the sending station.
This fluctuation in current strength will cause the diaphragm of
278
OUR PHYSICAL WORLD
the telephone receiver to vibrate in exactly the same manner
as the diaphragm of the microphone at the sending station. Thus
the same receiving set used for receiving the dots and dashes from
a discontinuous wave station is used for receiving the programs
from the radio telephone broadcasting station (Fig. 129).
FIG. 129. — The operating room of a broadcasting station. (Photo by Sweeny
Automotive and Electrical School, Kansas City, Mo.)
Much more elaborate systems of receiving equipment are
commonly used for receiving educational lectures and musical
programs from large broadcasting stations. The principle of
their operation is identically the same as that of the simpler
receiving sets previously described. In addition to the simple
receiving circuit of these elaborate assemblies of equipment,
there is usually an arrangement of vacuum-valve circuits whereby
the incoming signal is very much amplified (Fig. 130). Loud-
RADIO COMMUNICATION 279
speakers with megaphone horns are also employed so that a
group may enjoy a musical program without each person being
required to listen to the music from a small telephone receiver.
Government regulations require radio telephone broadcast-
ing stations to employ wave-lengths or frequencies which are
assigned to them on such a schedule that no large stations near
each other will be sending on the same wave-length. Thus
when two broadcasting stations operate in the same city, one
FIG. 130. — A modern receiving set. The tubes (amplifying and detector),
condenser, coupler-coil, and tuner are shown mounted behind the panel.
station might have a wave-length of 411 meters while the other
might be operating on a wave-length of 260 meters. This
difference in wave-length, or frequency, enables the person receiv-
ing to choose one or the other, so that the musical programs or
signals from one station will not be confused with those from the
other station.
Development in radio transmission and receiving has been
so rapid in the few years succeeding the war that any predic-
tion as to its future use may easily be exaggerated. It seems
quite within reason, however, to expect the radio methods of
280 OUR PHYSICAL WORLD
communication to take over a very large part of the work now
being handled by the commercial wire telephone and telegraph
systems. Especially will this superseding of the wire systems
by the radio systems occur where long-distance transmission is
concerned. Radio communication is not subject to the serious
limitations in expense of right-of-ways for pole lines and cables,
the cost of maintaining large central stations, and the inter-
ference of communication because of the effects of such devastat-
ing elements as storms, floods, and fire.
CHAPTER XII
DEVICES FOR SEEING BETTER, FARTHER, AND LONGER.
Eyes are bold as lions, roving, running, leaping, here and there,
far and near. — EMERSON.
When men observe a sequence of events in nature that is
constant, the statement of such a constant sequence is called
a law of nature. While we realize in general that nature con-
forms to law, yet we daily see repeated many phenomena or fre-
quently make use of commonplace appliances without any
appreciation of the laws that underlie their operation 'or even
without a realization that there are laws governing such operation.
One sees it grow light long before the sun is visible, and the
strange fact does not challenge attention; or one plays a flute,
turns on the electric lights, or uses the telephone, and yet is
not even curious in regard to the laws that make such acts
possible.
But the appreciation of some laws is so vital to our existence
that they force themselves on our attention. We know them
in practice, at least, even if we do not formulate them in words.
Such is the law that light travels in straight lines. Very familiar
experiences need only be recalled to make one realize the truth
of this statement. When you see an object you want you reach
straight for it, and you do not expect to see around corners
unless a mirror is employed. The hunter sights along the
straight arrow or gun barrel, and lets fly his missile at the animal
he desires to kill. Nearly everyone has observed the straight
beam of light revealed by the dust particles in its course in a
partly darkened room. If you look at some object like a candle
flame through holes punched in each of two cards held a foot
apart, the flame and the holes must be in the same straight
281
282 OUR PHYSICAL WORLD
line if the former is to be seen. Light then travels out from its
source in all directions in straight lines.
It follows from this law that the intensity of illumination
varies inversely as the square of the distance of the illuminated
object from the source of light. Cut a piece of card i inch
square and hold it 6 inches from a candle flame or small flash
light in a dark room. Its shadow on a large white card or
screen held at 12 inches from the light will be a square 2 inches
on each side, or 4 square inches in area. The light, therefore,
that covers i square inch at 6 inches from the source would
cover 4 square inches at twice this distance. If the screen
FIG. 131. — Diagram showing varying light intensities
be held 18 inches away, the shadow will be 3 inches on each
side, or will cover 9 square inches. From the diagram (Fig. 131)
it is evident that this law follows mathematically from the
proposition that the area of the bases of similar pyramids vary
as the squares of their altitudes, which is easily demonstrated
by one familiar with geometry.
Practical application of this law is commonly made in
measuring the relative intensity of illumination from different
sources of light. This is usually expressed in terms of candle
power. Thus we say that an electric light is a fifty-candle-
power light. The standard is a carefully made candle of pure
sperm, £ inch in diameter, that burns 120 grams an hour with
DEVICES FOR SEEING 283
a flame of uniform intensity. The intensity of the light from
an ordinary candle is quite variable.
Suppose we wish to measure the candle power of an electric
light of unknown power. We may stand a nail or similar object
upright on the table so its shadow will fall on a white paper or
a ground-glass screen. Then place a lighted standard candle
on the table so it will throw a shadow beside that made by the
electric light. Move the candle nearer to or farther from the
nail until the two shadows are equally dark. The comparison
is easily made when the shadows are side by side on the paper
or screen. Suppose the candle is then i foot from the nail and
the electric light is 10 feet away. The relative intensity of the
two lights is as the square of these distances. The electric
light is, therefore, one of 100 candle power. (An ordinary
candle may be used to show the principle of the experiment,
but the result will not be exact.)
Another interesting application of this principle that light
travels in straight lines is seen in the pinhole camera. This
may be made as follows. Secure a small light-tight wooden
or pasteboard box — a starch box or chalk box. In the center
of one end bore a tiny hole, like a pinhole. Cut out the other
end of the box, and over the opening fasten a piece of white
tissue paper or, better still, tracing paper or tracing cloth. Set
this box on the sill of an open window, pinhole out. Throw
a dark cloth or your coat over your head and also over the end
of the box covered with the tracing paper. Hold the cloth or
coat tightly around the box so that no light gets to your eyes.
Look, now, on the tracing paper and you will see an inverted
image of the landscape in front of the camera. Every point in
that landscape is sending a tiny beam of light in a straight line
through the pinhole to the paper to make a part of the image (Fig.
132). If a second hole were punched near the first, another image
would be formed that would overlap and blur the first. Then if
the hole made in the end of the box is large instead of small like
a pin prick the overlapping images are all indistinct, and the
284
OUR PHYSICAL WORLD
tracing paper is illuminated but shows no distinct picture of
objects.
If, in place of the tracing paper, a photographic plate is
set so as to cover the opening opposite the pinhole with its
sensitive or film side which is the dull side toward the hole, you
can take a picture with this camera. You must take the plate out
of the box or package in which you buy it, in a room that is
entirely dark except for the photographer's lamp used to give you
light (see " darkroom " below), and fasten it in place. Cover that
end of the box and the plate with the dark cloth and keep your
finger over the pinhole until the camera is in position on the
FIG. 132. — The pinhole camera
window sill. Then uncover the hole for three or four minutes
if the sun is shining and it is the middle of the day, much longer
if the day is cloudy. The plate must then be developed to
bring out the picture (see below).
A modification of the pinhole camera is used in sketching
objects or in mapping landscapes. The device is known as a
camera obscura. Take a good-sized wooden box that is light-
tight and large enough to receive your head and shoulders.
Remove the top of it. Paint or stain the inside dull black. In
the middle of one side, 6 inches from one end, bore a small
hole with a drill. At the middle of the end adjacent to the
hole set a 6-inch post at right angles to the end. Mount on this
a plane mirror facing the drill hole and inclined 45° to the post
DEVICES FOR SEEING
285
so that the light entering the hole will be reflected by the mirror
down on to the other end which is to be the base of the instrument.
Tack an ample, dark curtain on to the open top of the box,
fastening it at the end near the drillhole and to the adjacent
sides so that when head and shoulders are introduced into the
box it will cover them and exclude the light. Set the instrument
base down on a table out of doors or on legs fastened to the base.
Lay a piece of white paper on the base inside the box. Light
now coming through the drillhole is reflected by the mirror
on to the paper, and forms there an image of the object to be
sketched or of the land-
scape to be mapped.
With pencil in hand and
your head and shoulders
under the curtain you
can trace the outline of
the picture desired. The
image will be much
brighter if a long-focus
camera lens is used in
place of the drillhole be-
cause it will admit much
more light (Fig. 133).
The ray of light will
be bent out of the
straight course in which
it usually travels (i) when it strikes a reflecting surface like
that of a mirror; (2) when it enters or leaves a substance more
or less optically dense than the one in which it is traveling,
as when it enters the water from the air or passes through a
glass lens. We must undertake to comprehend some simple
laws of reflection and refraction in order to understand such
instruments as the magnifying glass, telescope, camera, and
other contrivances that man has invented in order to see better,
and farther, and longer.
FIG. 133. — The camera obscura
286 OUR PHYSICAL WORLD
Some of the principles that underlie reflection are matters
of familiar experience. You know that when one looks at
himself in a mirror his right hand seems to be on the left side
of his image. If his hair is parted on the left, the image wears
its parted on the right. If he winks his right eye, the image
winks its left. A person and his mirror image face each other
in the same relative position as two persons facing each other.
A movement of the right hand toward the right appears in the
image as a movement of its left hand toward the left. We have
grown so accustomed to performing certain actions before the
mirror, such as combing the hair or tying a tie, that we are not
confused by the reversal. But undertake some unusual task,
looking at your action in the mirror, and it is difficult. Thus, as
you sit at the table, stand a book on edge on the table in front of
you. Behind it on the table lay a piece of writing paper. Stand
a mirror on the table beyond the paper. Now place your hand
on the paper ready to write and adjust the mirror so you can
see your hand and what you write, in the mirror, but cannot see
them by direct vision because the book is in the way. Then
write your name so you can read it in the mirror.
It is a more or less familiar fact that the image as seen in
a plane mirror seems as far back of the mirror as the object
is in front of it. We all know, too, how curved mirrors distort
images. As a child you probably amused yourself by looking
at your face in the back of a shiny spoon and then in its bowl,
seeing your distorted image upright at first and then upside
down. All these phenomena pertaining to mirrors are easily
understood when one fixes in mind a very simple law, namely,
that the ray of light which strikes a reflecting surface is sent
off from it at the same angle at which it strikes, or, in other
words, the angle of reflection equals the angle of incidence.
This will be appreciated by a simple experiment. Stand a
mirror on a table so that the surface of the mirror is at right
angles to the surface of the table. On the table in front of the
mirror lay a sheet of paper, one edge against the edge of the
DEVICES FOR SEEING
287
mirror. Set a pin in the paper some distance in front of the
mirror and considerably to one side of its center. With the
eye at the level of the table and near the opposite edge of the
paper from the pin, lay a ruler upon the paper, its edge in line
with the eye and the image of
the pin seen in the mirror.
Extend this line to the mirror.
From the point where it meets
the mirror draw a line to the
pin. The angles these two
lines make with the edge of
the paper that coincides with
the face of the mirror will be
equal, and may be roughly
proved so by cutting one out
and laying it on the other.
A similar law is practically
familiar to every child who
throws a ball against a wall or
the sidewalk and catches it as
it rebounds. It is still more
evident if one person throws
the ball against wall or ground
and another person, at some
distance, tries to strike it, as in
handball or tennis. The angle
at which the ball hits the wall
or ground is the same as the
angle at which it rebounds,
due allowance being made for inequalities in the surface and
the twisting motion of the ball. The billiard player depends con-
stantly on this principle as the balls rebound from the cushions
on the edge of the table. Suppose ab (Fig. 134) represents
the surface of a mirror, c and d the eyes of a person looking in
the mirror, and e the tip of his left ear. Beams of light from
FIG. 134. — Reflection in a plane mirror.
The image seems as far behind the mirror
as the object is in front of it.
288
OUR PHYSICAL WORLD
e strike the mirror at / and g, and are reflected into the eyes
of the observer. He sees the image of e at e'. Similarly, he
sees c and d at c' and dr respectively. But these imaged eyes
appear to face him from back of the mirror. The ear er of the
image is at the right of its eyes, while the ear of the observer e
is at the left of his eyes. The eye df is the right eye of the image,
while the corresponding eye, d, of .the observer is his left eye.
Note that e' appears as far to the rear of the mirror as e is
in front of it, because we judge the distance of an object by the
angle between the rays of light entering the two eyes from it.
This angle is evidently the
same after reflection from
the mirror as when the rays
start from e. The eyeballs
are turned in their sockets
by delicate muscles that are
richly supplied with sensi-
tive nerves (Fig. 135). So
we are able to sense just
how much the axes of the
two eyeballs converge
when we fix our eyes on
an object. The axes evi-
dently converge strongly
when a very near object
is examined, e.g., the tip of one's own nose, less strongly as the
object is more and more distant. That the two eyes are used
in such estimation of the distance of an object is made apparent
by a simple experiment. Tie a finger ring to one end of a
piece of fine wire or thread. Fasten the other end of the wire
to some object, such as an electrolier or a door frame, so the ring
hangs freely about breast high. Step away from the ring 2 or
3 yards and face its edge. Take a pencil in your hand, close
one eye, then walk up to the ring and pass the pencil through
it from right to left, with the eye still closed.
FIG. 135. — Section of the eyeball
DEVICES FOR SEEING 289
The knack of judging distances is one that we acquire very
early as we correlate repeatedly the play of these muscles that
move the eyeball with our experience in reaching for objects or
in walking to them. Other factors enter into our judgment of
distance, such as the operation of the muscles that control the
focus of the lens of the eye, the haziness of the image when
objects are very distant; but they may be neglected in this
discussion of the apparent position of the mirror image.
You may have been amused and possibly confused by going
into a mirror maze — a room whose walls are set with mirrors
projecting at various angles. You see yourself in many places
simultaneously, and when you try to find the door to go out it is
difficult to tell which of the many doors you see is the real one.
The production of such multiple images may be illustrated with
a simple experiment. Stand two long mirrors on edge, one end
of each near the margin of a table, so that they are parallel
and face each other a foot or so apart. Between their ends
that are distant from the edge of the table, set some object,
say a spool. With your eye between the other ends of the
mirrors see how many images of the spool you see. Change
the position of the mirrors so they stand at an angle to each
other instead of lying parallel. How does this affect the number
of images visible? One of the most fascinating illustrations
of multiple images is found in the child's toy — the kaleidoscope.
Directions for making this are found on page 83 of the Field and
Laboratory Guide in Physical Nature-Study.
Suppose ab (Fig. 136) represents the surface of a cylindrical
mirror whose center of curvature is shown at c. The eye of
an observer is shown at/. The points d and e are the tips of an
arrow, the image of which is seen in the mirror by the observer.
If ab were a plane mirror, the image would appear as large
as the object and would be seen as far behind the mirror as the
arrow is in front of it. But since the light is now reflected
from a convex surface, the rays from d to the eye will be rendered
more divergent than they would be if reflected from a plane
2 go
OUR PHYSICAL WORLD
surface. When, therefore, they are produced back of the mirror
to meet at the point d', they meet nearer the mirror than is d.
d'
FIG. 136. — Reflection from a convex mirror. The image of the large arrow
at the right is seen by the eye at the left and is relatively small.
Similarly, e' is nearer the mirror than e, and df and e' are closer
together. The image of the arrow is, therefore, smaller than the
arrow itself. An observer, seeing himself in such a cylindrical
B
FIG. 137.— Images of a man: A, as seen in a convex cylindrical mirror; B, as
seen in a plane mirror; C, as seen in a concave cylindrical mirror. D, Diagram
showing why the concave mirror broadens the face.
mirror when its long axis is parallel to his height, will see himself
narrowed from side to side while his vertical size will be
unchanged.
If now one looks at himself in the concave surface of a
cylindrical mirror when its long axis is parallel to his height,
DEVICES FOR SEEING 291
evidently just the reverse will be true, and his image will appear
broader than he is (Fig. 137).
Consider next the case of a concave mirror whose surface
is the segment of a sphere. If one looks for the image of a
candle flame in such a mirror, there are three possible positions
which the candle flame may occupy: it may be (i) at the focus
of the mirror, (2) outside the focus, (3) within the focus. If
such a source of light should be at the center of curvature of
the mirror, all the rays will be reflected back to the same point,
since they move out along the
radii of the curved surface,
which radii are perpendicular
to that surface. If parallel rays
of light strike such a mirror,
they will all meet after reflec-
tion in a point known as the
focus, and this point must be
halfway between the mirror and
,. T . T , FIG. 138. — An object and its image
its center of curvature. Light formed by a concave spherical mirror>
emanating from the focal point
will be reflected evidently as parallel rays, while rays emanating
from a source nearer the mirror than the focus will be reflected
as divergent rays.
Rays coming from a source farther from the mirror surface
than the focus will meet at a point. These two points, the one
from which the rays emanate, the other the one to which they
converge, are known as conjugate foci.
If an object like a candle flame is at ab (Fig. 138), the mirror
will form an inverted image of it at a'b', which image may readily
be seen on the screen at this position. If, however, the candle
flame were at a'bf (turn the figure upside down), the image
would evidently be at ab.
If the object is nearer the mirror than is the focus, no actual
image will be formed; but if the eye catches reflections in the
mirror from such an object, the object will appear magnified.
292 OUR PHYSICAL WORLD
Suppose, for instance, points a and b (Fig. 13 yZ)) represent the
opposite ends of an arrow seen reflected in a concave mirror,
these points being slightly nearer the mirror than its focus.
Follow two rays of light, the outer or the marginal rays of a
pencil of light, from point a to the mirror. When these are
reflected into the pupil they are less divergent than when they
left a. They will seem, therefore, to come from a point back of
the mirror and farther from the mirror than a is in front of it.
Similarly, b will appear at b', and the arrow will seem larger than
it is. So a dentist uses a small concave spherical mirror to see
his work on a tooth, and thereby magnifies the cavity he is
cleaning and filling.
An image is formed by a lens because the light entering and
leaving it is bent from its straight course or is refracted. Such
refraction always occurs when rays of light go into or out of
an optically more or less dense medium than the one in which
they were traveling, and the refraction occurs at the line of
demarcation of the two media. Thus light entering water from
air is refracted as it enters the water. Optical density and
ordinary physical density must not be confused. Thus carbon
disulphide is a liquid and not physically as dense as glass; yet
optically it is more dense than most glass, that is, it bends the
ray of light entering it more than does glass, or to put it in another
way, it has a higher refractive index than glass.
One may perform a simple experiment that will help clarify
this notion of refraction. Put a penny in a bowl that sits on
the table. Stand where you can just see the penny over the
edge of the bowl, and then step back until you just cannot see
it. Have some other person pour water into the bowl carefully
so as not to move the penny. The far side of the penny begins
to appear, and as the level of the water rises you see more and
more of it until it is all in sight. Evidently the rays of light,
coming from the penny over the edge of the bowl, go above your
eyes before the water is added, and after that are bent down
so that they enter your eyes. (See Fig. 139.) If one draws a
DEVICES FOR SEEING
293
line perpendicular to the surface of the water at the point at
which the ray of light leaves the water and enters the air, point b,
Figure 140, one may state the direction of the refraction as
away from the perpendicular when the
ray passes from an optically dense
medium to a less dense one (water to
air), and toward the perpendicular
when the light moves in the opposite
direction, as would be the case if eye
and coin interchanged positions in this
experiment. In spearing a fish, from
behind it, one must aim the spear at
its tail in order to hit its body, or
FIG. 139. — Diagram show-
ing refraction of light from an
object in water.
if it is lying in deep water the spear must be thrust at a point
back of the fish in order
to hit it at all.
The amount of the
refraction depends on
b the relative density of
the two media. Air is
taken as the standard,
and when we say that
a given sort of glass
has an optical density
of 1.5, we mean that it
is half again as dense
as air, or that light
travels through it only
two- thirds as rapidly as
through air. Practi-
cally we apply this in
tracing the course of the ray as follows : Suppose ab (Fig. 140) is
a ray of light which at b enters the plane surface of a piece of
glass with a refractive index of 1.5. With point b as a center
and a radius of i (in the diagram the radius is i inch), strike
FIG. 140. — Diagram showing method of finding
the path of a ray of light entering glass.
2Q4
OUR PHYSICAL WORLD
off the arc de. With b as a center and a radius of 1.5, strike off
the arc fg. Continue the line ab toward c, and from the point
h where this line intersects the arc de erect a perpendicular to
the .surface of the glass and extend it until it intersects the
arc fg at i. Through b and i draw a line, and this will be the
course of the ray after refraction. It is evident that the ray ab
is refracted at b toward the perpendicular bj erected at b.
Now suppose that the ray of light is coming out of a block
of glass with refractive index of 1.5 (Fig. 141). The ray ab
strikes the surface of
the glass at b and
enters the air. If it
were not refracted, it
would continue toward
c. This ray, on enter-
ing the air from the
glass, will be refracted
away from the per-
pendicular. (Recall
the experiment with
the penny and bowl.)
To determine its
course, proceed thus :
With b as a center and
a radius of i, strike off
an arc de, and similarly a second arc/g, with radius of 1.5.
From h, the point where the extended ray intersects the arc fg,
drop a perpendicular to the extended face of the glass. This
cuts the arc de at i. Draw the line bi, and this will be the
course of the ray. One can readily judge whether the perpen-
dicular is to be dropped from the intersection of the extended
ray with the arc whose radius is i, or the arc whose radius is
1.5, by thinking whether the refraction is to be toward or away
from the perpendicular; and the experiment with bowl and
penny will recall this. If the refractive index of the glass were
FIG. 141. — Diagram to show method of finding
the path of a ray of light leaving glass.
DEVICES FOR SEEING
295
1.25 instead of 1.5, then the
radius of the second arc would
be taken as 1.25 inches.
If the ray of light were to
strike the glass surface at a
small angle, as the ray kb, it
would be refracted back into
the glass if it could get out.
At such an angle it is, there-
fore, totally reflected at b to I.
Some simple experiments
with any convex lens like a
magnifying glass or a reading
glass will help make clear some
things that it is necessary to
understand in order to com-
prehend the working of cam-
eras, microscopes, telescopes,
or other instruments using
lenses. If you hold such a
lens so that the rays of sun-
light will strike it squarely,
the light is brought to a single
point on any surface such as a
sheet of paper held at the
proper distance from the lens
(Fig. 142). The sun is so far
away that the rays of light
entering the lens are practi-
cally parallel. The point at
which these rays meet is
known as the focus of the lens,
and the distance from that
point to the lens is the focal
length of the lens. To be
FIG.
a focus
glass.
142. — The beam of light brought to
by a plano-convex lens, or burning
296
OUR PHYSICAL WORLD
exact, the measurement should be made from the focus to the
optical center of the lens, but the rough measurement to the
face of the lens is adequate for our purpose.
Set a lighted candle on the table. Hold the lens in your
left hand a foot from the candle flame. Hold a sheet of paper
in your right hand on the opposite side of the lens from the
candle flame, and move this sheet closer to, or farther from,
the lens until a clear image of the candle flame is seen on the paper.
Note the size of the image. Move the lens to about 6 inches from
the flame. Note now that the image is no longer distinct. To
obtain a distinct image the screen must be moved farther away
FIG. 143. — Candle and screen are at the conjugate foci of the lens. Two pencils
of light are shown, focusing to form two points of the image; similar pencils ema-
nate from other points of the candle, and are brought to a focus to form correspond-
ing points of the image.
from the lens, and the image will be much larger than before.
On the other hand, if the lens is moved so that it is 2 feet
from the candle flame, the screen must be brought nearer the
lens, and the image will be smaller than in either previous position
of the lens. It is evident that the light emanating from the
candle flame is brought to a focus at the place where the image is
formed. If the candle and the screen were interchanged in
position, there would still be a sharp image of the flame upon
the screen. These two points are known as conjugate foci, and
the nearer one of these is to the lens, the farther away the other
must be (Fig. 143).
Lenses may be either convex or concave, the former bringing
parallel rays of light to a focus, the latter making such rays
DEVICES FOR SEEING
297
diverge. The convex lens may have both faces convex, one
plane and one convex, or one less concave than the other is
convex. Similarly, concave lenses
may be double concave, plano-
concave, and the concave meniscus
(see Fig. 144).
If the principles of operation
of a convex lens, given above, FlG- ^.-Lenses of severalshapes
have been grasped, it will be easy to understand the operation
of many optical instruments. Let us see why it is that a magni-
fying glass magnifies. The object to be examined must be
FIG. 145. — Diagram showing how a magnifying glass magnifies
placed nearer to the lens than is its focal point. Rays emanating
from a point in such an object, as from a in the diagram (Fig. 145),
will be less divergent after passing through the lens than they
were on entering the lens. When such rays enter the eyes
they will be referred back to a point at their intersection, and
this point a' is much farther from the lens than is the point from
which they really came. Similarly, point b of the little arrow
will be referred back to b', and intermediate points of the objects
to a position between a' and b'. One therefore sees the object
enlarged. Under these conditions no actual image is formed,
but the image seen is spoken of as a virtual image.
FIG. 146. — Diagram of a com-
pound microscope. An object
represented by the small arrow
at the bottom of the figure is so
placed that rays of light leaving
it are brought to a focus at ab
after passing through the object
lens or objective, there forming
an inverted image. The rays
pass on through the eye lens or
eyepiece, diverging less as they
pass, and the eye seems to see the
magnified virtual image at a'b'.
OUR PHYSICAL WORLD
You may make lenses for yourself
in either one of two ways that will
serve for the time being. First, good-
sized lenses may be made from two
watch crystals of the same size. Smear
their edges with vaseline. Immerse
them in water, and bring them edge
to edge so that the space between
them is filled with water. Be careful
not to include air bubbles. Hold the
two firmly together between the thumb
and fingers of the left hand, lift them
out of the water with their contained
water, wipe the edges dry and bind
them together with a strip of surgeon's
adhesive tape as you would passe-par-
tout a picture. The tape may be pur-
chased at any drugstore, and the i-inch
width is best. If the water runs out
from between the watch crystals and
air leaks in during this process, try it
again. It will do no harm if a small
bubble of air gets in, but it should
not occupy more than one-fifth or one-
sixth of the interior. Such a lens will
work well as a magnifying glass.
A second method of making a small
lens is as follows : Take a circular cover
glass such as is used in the prepara-
tion of microscopic mounts. Hold it
in a pair of spring forceps such as
the bacteriologist uses, and drop on
to it some liquid glass or thick Can-
ada balsam. Heap up as much as
it will hold without running off, then
DEVICES FOR SEEING 299
turn the cover glass over so the liquid glass or balsam will hang
from the under side, the lower surface of it in the shape of the
segment of a sphere. Allow this to stand until it hardens. A
lens of this sort may be used in making a microscope or for the
eyepiece of a telescope. The large-sized lenses made from watch
crystals are serviceable also as objectives for telescopes or magic
lanterns or as condensers for magic lanterns. Directions for
making a microscope, telescope, and magic lantern are given
in the Field and Laboratory Guide in Physical Nature-Study. The
principle of operation may be explained here.
The microscope consists of two lenses mounted at the opposite
ends of a tube which is about i inch in diameter and several
inches long. One of these lenses, the one through which you
look, is the eyepiece; the other, which is brought close to the
object to be examined, is the objective. The object to be
examined is brought near enough to the front of the objective so
that an image is formed up in the tube of the instrument just below
the eyepiece. This image is then examined by the eyepiece, which
serves as a magnifying glass (Fig. 146) . Recalling our experiment
with the convex lens and the candle flame, it will be remembered
that when the flame was near the lens the image was relatively far
from the lens and larger than the object. The image formed below
the eyepiece is therefore enlarged, and when the eyepiece magnifies
it still more, one sees the object hundreds or even thousands
of times larger than it really is. There are some accessory
parts to the microscope (Fig. 147), which make it more con-
venient, but the lenses held by the tube are the essential things.
There is usually a heavy base on which the instrument stands,
and a pillar that carries the tube on a movable arm. This pillar
also bears the stage on which the object to be examined is placed,
and a mirror to throw light on the object to be examined. In
addition there is a coarse adjustment that moves the tube
rapidly up and down by means of rack and pinion, and a
fine adjustment that moves it very delicately. The objec-
tives, especially of a good microscope, are built of several lens
300
OUR PHYSICAL WORLD
elements so as to free the image from distortion and fringes of
color.
The telescope is very much like the microscope, except that
the object to be examined is a long way off, but the objective
--"RACK a PINICN
COARSE ADJUSTMENT.
•^^
GRADUATED SHORT SLIDE — .
HEVOLV
STAGE
ADJUSTABLE
SPRING FINGER
CONDENSER; MOUNTING ON,.
DROP SWNG ARM'—"""
LOWER IRIS DIAPHRAGM-'
FOR OeUftUE Ll6HT.
STACE CENTERING
MIRROR
MIRROR FORK
MIRROR BAR
"RflCK & "PiNION
BUTTON.
FIG. 147. — A compound microscope. (Courtesy of the Spencer Lens Co.)
still forms an image of the object which is examined by the eye-
piece that magnifies it (Fig. 148). So that this image may be
as large as possible, the tube of the telescope is often very long
(see Fig. 149). In both telescope and microscope the tube is
not essential, but it is convenient to shut out the light from
DEVICES FOR SEEING
301
surrounding objects so that the image is seen
on a dark background. If you will take two
convex lenses, one in each hand, and hold one
at arm's length as an objective, the other
near your eye as an eyepiece, and hold them
both in line with some distant object, you
can, by varying the distance between them,
get the effect of the telescope without a tube.
In the magic lantern or stereopticon, the
light from some source of illumination, as an
electric lamp, is made to converge by convex
lenses on to the transparent glass slide that
bears the picture to be shown. The picture
is printed on the gelatine film on the slide
and must, of course, be transparent. The
light from the condenser goes through the slide
to the objective. The slide is at one of the
conjugate foci of this convex lens which we
call the objective, whose other focus is at the
screen. Since the slide is near the objective,
the screen will be far away and the image
formed will be much larger than the picture
on the slide. (See Fig. 150.)
In the more expensive types of lenses in
the camera, microscope, and telescope, the
lens is made of several elements or separate
lenses that are mounted together to make the
so-called lens. This is necessary because of
two defects in any single lens: (i) spherical
aberration, (2) chromatic aberration.
If you will hold in your hand any large
convex lens like a large reading-glass and look
through it toward the window, then move it
nearer to or farther from your eye until you see
the image of the window, you will note that the
302
OUR PHYSICAL WORLD
t
f
I
DEVICES FOR SEEING 303
vertical lines of the window frame that bound the panes of glass
appear not as perfectly straight lines but as more or less curved
lines. This is due to the fact that the rays passing through the
margin of such a lens and those passing through its center do not
come to a focus at exactly the same spot. If you will cut a small
circular opening one-half inch in diameter in a piece of cardboard
or thick paper and lay it on the lens so that all the lens is covered
except its central area and try the foregoing experiment again,
you will find that the image which you see is largely freed from
this spherical aberration. So you will find a diaphragm inserted
in the lens of many optical instruments to accomplish this
correction. The iris of the eye is in part for this purpose. When
FIG. 150. — Diagram of a stereopticon
one is out at night, the pupil is very large to admit as much
light as possible, as you will readily see if you look at your eye
in a mirror immediately on coming in from the dark. Because
the pupil is so large, the image is not very distinct, and we often
mistake commonplace objects for terrifying things.
The curved surfaces of a convex lens are segments of spheres.
If the surfaces could be paraboloid surfaces instead of spherical,
this defect would not occur. But it is very difficult to grind
lenses with paraboloid surfaces and very easy to grind them
with spherical surfaces. A piece of glass to be made into the
form of a lens is cemented to the end of a stiff rod; the other
end of the rod is pivoted at a point above a horizontal rotary
grindstone so that the glass presses on the surface of the grind-
stone. It is evident that the rod is the radius of a sphere, and
3°4
OUR PHYSICAL WORLD
that, as the glass is ground down, the surface formed will be
a spherical surface. The amount of curvature of the surface
will depend upon the length of the rod used.
If you look through a glass prism at some object such as the
window sill, you will demonstrate first that the prism must be
so placed as to allow the ray of light coming from the window
sill to enter your eye after its refraction. If you will think how the
ray of light is refracted (see Fig. 151) on entering and leaving
an optically denser medium
than the air, you will have
no difficulty in placing it in
approximately the correct
position at your first trial.
You will note, secondly, that
the window sill seems sur-
rounded with a halo of
color. A convex lens may
be thought of as a series of
prisms, and you will observe
as you look through your
large convex lens that the
image of an object seen
does have a fringe of color about it. This defect of the lens
is known as chromatic aberration.
This defect is remedied in large measure by making the
lens of several elements. This power of glass, or similar refractive
media, to spread the component color rays of white light so that
they form a color band as in the rainbow is known as its dispersive
power. Fortunately, the refractive power and the dispersive
power of lenses are largely independent of each other, so that
one kind of glass may have high refractive power but low dis-
persive power, while another sort has low refractive power but
high dispersive power.
Suppose then we were to put behind a plano-convex lens
(see Fig. 152) of high refractive but low dispersive power a
FIG. 151. — Diagram showing refraction
of light by a prism. The beam entering the
prism is not only refracted but also dispersed
into its component colors, only the extremes
of which are shown, the red (r) and the
violet (v).
DEVICES FOR SEEING
305
7
plano-concave lens of low refractive but high dispersive power,
an image may still be formed that is free from the color fringe
because the second lens will not overcome the refraction of the
first lens completely, while it will undo the dispersive effect
of the first lens. Now to grind
and combine two or more
lenses so as to correct their
defects is a laborious process
that requires great skill, hence
the superior photograph, mi-
croscope, or telescope lens
must be costly.
In the human eye there is
such a combination of lenses.
The aqueous humor in the front of the eye is in the shape of a
convex meniscus; then comes the double convex crystalline
lens; then the vitreous humor making a plano-concave lens,
plane on its posterior side because the retina is imbedded in it
so that no refraction occurs as the light passes from it to the retina
(Fig. 135, p. 288).
According to the still generally accepted theory, light is due
to waves in the ether or in other substances through which it is
passing. The wave form advances, but each molecule moves
in a tiny orbit somewhat as do the particles of water when a
FIG. 152. — Correction of chromatic
aberration of a convex lens by a concave
lens.
w v v-
FIG. 153. — Diagram showing wave motion
water- wave forms. Thus in Figure 153 molecule i is struck by
an impulse that makes it vibrate or revolve in the orbit represented
by the dotted line. It has just completed such a revolution.
It takes an appreciable, though very short, time for the impulse
OUR PHYSICAL WORLD
to travel from i to 2, so that the latter has not completed its
revolution but is at the point indicated in its orbit. The posi-
tions of 3, 4, 5, etc., are also indicated, and are connected by the
solid line i to 9 that outlines the wave form from crest to crest.
The height of the wave is the long diameter of a molecular orbit.
The wave form advances from left to right.
When a light wave enters a glass prism as in Figure 151 the
bottom of the wave encounters the glass and is retarded while
the top continues to move at its initial velocity somewhat as
FIG. 154. — Diagram of marching men to illustrate refraction and dispersion of
light.
happens in the case of a water-wave when it strikes a shelving
shore. The direction of advance is therefore altered, or, as we
say, the light is refracted. On leaving the prism in our diagram,
it is the top of the wave that emerges first and so moves with
increased rapidity, since it is now in a less dense medium, while
the bottom is still retarded, and so the course of the ray of light
is again altered.
Suppose a line of marching men be shown by circles (see Fig.
154). In their path is a wedge-shaped area of deep sand on an
otherwise hard surface. As the line strikes the difficult going
DEVICES FOR SEEING . 307
in the sand, the men entering it are slowed up while the men
still walking on the hard surface can keep their regular pace.
The direction of the march will be changed, the line wheeling
right somewhat. The same thing happens when the line emerges,
since those men at the left of the line get out of the sand while
those at the right are still plodding through it. This rough
analogy may help beginners to clarify the process of refraction.
The stepping of the men corresponds to the vibration of the
particles in the formation of the wave of light.
Now white light is a blend of many-colored lights, each with
its own specific rate of wave-motion. The violet waves are
short waves, the red are long, and the intermediate colors,
indigo, blue, green, yellow, orange, have increasing wave-lengths.
Only the primary colors are here mentioned; there are innumer-
able intergrading shades each of which has its own length of
wave. When a beam of such white light traveling in air passes
through a glass prism, it emerges spread out into a band of
color. The analogy of the marching men may again help to
give some notion of why this occurs. Suppose a company is
marching eight abreast. The first line is made up of short men
who naturally take short steps, the next of taller men who step
less often, the third line of still taller men whose steps are still
longer, and so on. (This is a very unmilitary supposition,
but these men are an illustration, not troops.) Again the
company is tramping through the wedge-shaped area of sand.
The short- stepping men will be retarded in it more than those
who take long steps because they must step in it more frequently.
When the company emerges, therefore, the line of very tall men
will be bent out of its original course least, the line of the very
short men most, and the intermediate lines will fall between
these. The analogy is very crude but it may help to visualize
this process of dispersion of light. The men who take long steps
correspond to the long light waves like those of red light, while
the men who take short steps are analogous to the short waves
of such light, as the violet.
308 OUR PHYSICAL WORLD
When, during a shower, the sun is shining and is fairly near
the horizon, we may see a rainbow or, if in a balloon or on a
mountain peak, a rain circle. The light entering the raindrops
is refracted and dispersed, then totally reflected and further
refracted and dispersed as it leaves the drop. In the accompany-
ing figure (155) two of the raindrops are shown enlarged, so the
course of the light can be traced. The entering light is a heavy
line; the red light a light solid line, the violet light a dotted line;
the intermediate colors are omitted. The color perceived is
FIG. 155. — Diagram showing formation of the rainbow. Drops of water
represented by the small circles are in such position that beams of light entering
them are refracted and totally reflected so as to send to the eye red (solid line)
and violet (dotted line) rays. The eye projects these against the sky in a primary
bow and a dim outer secondary bow. Many thousands of drops are needed in
similar position to complete the bow.
referred back along the line of the light entering the eye, and so
is seen against the sky or clouds. The color band is a bow
(or circle) because the observer is the center of curved rows of
such drops that can refract and reflect the light to his eye.
If you fill a small spherical flask with water and set it on a
support near a window in a darkened room so that a beam of
sunlight entering through a small aperture in the curtain or
shutter will strike it, a circular rainbow will appear on the shutter.
This will be more evident if a sheet of white paper encircles the
opening in the shutter.
CHAPTER XIII
CAMERAS AND PICTURE-MAKING
But who can paint like Nature! — JAMES THOMSON, The Seasons
The pinhole camera described in the preceding chapter is
seldom used because it takes so long to expose the plate that any
moving object produces only a blur. A lens with a large opening
that admits plenty of light is used in place of the pinhole, and
this lens forms an image on the sensitive plate or film. A camera,
then, is a light-tight box
with a lens at the center
of one end and a device
for holding a sensitive
plate or film at the oppo-
site end. The interior
of the box is painted
dull black to absorb any
possible reflections from
the metal mounting of
the lens.
In all box cameras
(Fig. 156), such as the
familiar Brownie No. i
or No. 2, the lens must
be what is known as a universal lens; that is, one which will
give a reasonably distinct image of objects on the plate or film
no matter whether they are distant or quite near. Such a lens
cannot take a picture of a very close object, however. In the
Brownie the near limit is 6 feet.
In all other cameras, the lens is mounted on a movable board
which is connected with the front of the camera box by a bellows.
309
FIG. 156. — A box camera, the Brownie
3io
OUR PHYSICAL WORLD
The lens may be moved nearer to, or farther from, the sensitive
plate as is required to obtain a sharp image of the object. In
plate cameras of this type (Fig. 157), there is a ground-glass
screen covering the opening on the opposite side of the box
from the lens. One throws a black cloth over his head and also
FIG. 157. — A plate camera on its tripod: (a) adjusts time of exposure;
(b) adjusts size of diaphragm opening; (c) raises or lowers the lens; (d) moves front
back and forth; (e) swings back on its vertical axis; (/) moves back of camera
forward or backward; (g) swings back of camera on its horizontal axis; (h) plate
holder.
over the camera box, as in the case of the pinhole camera above,
and then moves the lens back and forth until the image seen on
the ground glass is perfectly sharp. The plate is then inserted
into the camera in a plate holder in the same position that the
ground glass occupied when the camera was focused.
In film cameras of this type a small pointer is attached to
the lens board. Under this pointer lies a fixed scale. If the
CAMERAS AND PICTURE-MAKING 3 1 1
object to be photographed is 10 feet away the operator sets the
pointer over the lo-foot mark on the scale; if it is 100 feet away
or more, over the loo-foot mark. The position of these marks
on the scale has been previously determined by the maker of
the instrument by focusing on a ground glass in the position
later occupied by the film.
In practically every camera, a diaphragm is provided with
openings in it ranging from small to large, so that the photog-
rapher can admit through the lens a small amount of light,
cutting off most of the marginal rays; or he can use a large
opening admitting more light, but using more and more of
the marginal rays as the opening is increased in size. The
size of the diaphragm opening is usually expressed in terms of
the focal length of the lens. Thus when the diaphragm openings
are marked F.i6, F.8, F.4.5, the symbols mean that the openings
in the diaphragm are one- sixteenth, one-eighth, etc., of the
focal length of the lens. This insures that, no matter what the
focal lengths of the lenses may be on several cameras, the same
sized pencil of light is brought to a focus on the plate when their
diaphragms are set for the same opening. In some cameras
the diaphragm openings are marked on the universal system
(U.S.) in which each larger diaphragm is twice the area of the
next smaller size. The U.S.i6 diaphragm is just the same size
as the F.i6. From this it follows that U.S.4 equals F.8, U.S.8
equals F.n approximately, U.S.i6 equals F.i6, U.S.32 equals
F.22 approximately, and U.S. 64 equals F.32.
It is furthermore evident that much more light enters the
camera with a large diaphragm opening than with a small one.
In fact, the amount of light varies as the squares of the diameters
of the diaphragm openings. An F.8 admits four times as much
light as an F.i6.
Since it is the light that acts upon the plate, the length of
time that the plate is exposed must depend on the size of the
diaphragm used, the speed of the plate, and the intensity of
the light at the time of exposure. The exposure on a bright,
312
OUR PHYSICAL WORLD
sunny day will therefore be much shorter with any given dia-
phragm and plate than on a dull, cloudy day. One can learn
by experience to judge the length of exposure under varying
light conditions with different- sized diaphragms and different
plates, but it will be at the expense of spoiling many plates.
It is advisable, therefore, to purchase and use an exposure
meter in order to save both time and material. Cheap ones
can be obtained which will indicate the exposure for any sized
diaphragm under most condi-
tions, such as time of day,
season, cloudiness of the sky,
nature of the object to be
photographed. They are not
as satisfactory under excep-
tional conditions, such as
photographing in deep woods
or indoors, as are the types
in which one exposes a strip
of sensitive paper to find the
light intensity. The method
of operation of one such may
be given as typical. The ex-
posure meter can be opened
as one would take off the
back of a watch, and a strip
or disk of sensitive paper be
laid in, after which the back is closed again. The front of such
an exposure meter is shown in Figure 158. The little opening
through which light gains admission to the sensitive paper
is kept covered by a piece of ruby glass until one is ready to
use the instrument. At one side of this opening is a sample
of dark paper of fixed tint. One holds the exposure meter in
the moderate shadows of the object to be taken, then turns
aside the colored glass so a bit of the sensitive paper is exposed,
and notes in seconds the time required for it to darken sufficiently
FIG. 158. — An exposure meter
CAMERAS AND PICTURE-MAKING 313
to match the dark strip beside it. A circular strip of the face
adjacent to the rim can be turned as the rim is rotated. On
this strip are marked a series of numbers indicating diaphragm
sizes and the sensitiveness of various plates. On the edge of
the central disk, a series of numbers indicates seconds and frac-
tions of a second. Accompanying the exposure meter is a
booklet giving the sensitiveness of various makes of plates.
Suppose we are using Cramer's instantaneous isochromatic
plates. The booklet gives its speed as F.m, which means that
this plate would require an exposure of one second with a dia-
phragm opening of F.m under standard conditions. Suppose
that it has required three seconds for the strip of sensitive paper
to darken. Then set 3 on the central disk opposite F.m on
the circular strip. One may now read the exposure required
for any diaphragm in seconds or fractions of a second. Thus
if one is going to use an F.64 diaphragm opening he will give an
exposure of one second, or if he wishes a short exposure, say one
sixty-fourth of a second, he must use the F. 8 diaphragm opening.
The sensitive paper rotates when the back of the instrument is
turned, thus bringing a fresh bit under the opening for the next trial.
Since the enlargement of the diaphragm opening means the
admission of more of the confusing marginal rays, the rule is
to use as small a diaphragm opening as possible. For motionless
objects one will use say an F.64 stop, and give a long exposure.
But for rapidly moving objects, or even slowly moving ones,
when the light is dim one must use a large stop and give a short
exposure. Under such conditions a well-corrected lens must
be used. The cheaper grades of cameras are therefore not made
with large diaphragm openings.
The procedure in taking the picture, then, is as follows. Set
the camera firmly on its tripod, and point it at the object. Open
the diaphragm wide, and focus so as to get a clear image on the
ground glass, the desired object at about its center. In the
better cameras the lens board may be raised or lowered to facili-
tate such centering without moving the tripod. The back carry-
314
OUR PHYSICAL WORLD
ing the ground glass swings vertically and horizontally so that
one can, with these adjustments, bring all parts of the object
into focus at the same time. If the object is still, diaphragm
down to F.32 or F.64 and find what exposure must be given with
such openings by means of the exposure meter. If the object
is moving, decide how rapid the exposure must be. The nearer
FIG. 159. — Front of camera lens to show device for setting the time (above)
and the diaphragm. Shutter release is at left.
you are to a moving object, the more rapid its apparent move-
ment will be in the image. It might require an exposure of one
one-thousandth of a second to catch an unblurred image of a
running athlete, while a more distant tree whose branches
were swaying in the wind would need only one twenty-fifth
of a second. Having decided on the time of exposure, consult
the exposure meter for the size of diaphragm opening to be
used. Set the diaphragm and the timing device (Fig. 159).
CAMERAS AND PICTURE-MAKING 315
Be sure the diaphragm is closed. Insert the plate holder at
the back of the camera and make certain it clicks into place,
the ridge upon it settling into the slot provided so as to exclude
the light. Draw the slide that covers the plate straight out.
If it is tilted so that one corner is withdrawn before the other, light
may leak in at the corner first withdrawn because the other
corner prevents the little clip, operated by a spring, from closing
along its entire length. Now make the exposure by pressing the
release or bulb. In some cameras the release that opens and
closes the shutter must be lifted to set the spring that operates
it before it will work. Attend to this, if necessary, before making
the exposure. Return the slide that covers the plate in the
same careful way it was withdrawn.
The operation will be the same for film cameras, except that
one judges the distance of the object and sets the pointer on
the scale accordingly. In roll film cameras there is no slide
over the film to withdraw. In reflecting cameras like the Graflex
and Reflex, the image is thrown by a mirror on to a ground
glass, the mirror serving to protect the film or plate from the
light. One sees the image of the object up to the moment the
trigger is pressed that swings the mirror out of the way and
immediately releases the shutter to make the exposure (Fig. 160).
The plate or film must be taken out of the camera (except
in those provided with daylight-loading devices), and developed
in the darkroom. The glass plate or film used in the camera
has one face covered with a thin layer of gelatine so treated that
it does not dissolve readily ; in this film there are imbedded mi-
nute particles of certain silver salts, usually the bromide and iodide
Wherever light strikes this film, the silver salts are so affected
that, in developing, the metallic silver is deposited in tiny grains,
giving the area a black appearance. If you will take a plate out
of its box in the darkroom, you will see that one side of it is shiny,
the other dull. The shiny side is the uncovered glass, the dull
side that upon which the gelatine is spread. Cover one-half
of such a plate with a piece of cardboard, then bring the plate,
316 OUR PHYSICAL WORLD
the half still covered, out of the darkroom and put it in strong
sunlight. Very shortly the uncovered portion turns dark, in
time black, but the covered portion remains yellowish white.
When the plate or film is exposed in the camera the light
areas of the image, such as those of the cuffs or shirt bosom
in the image of a man, are affected by the light while the dark
areas, such as the image of a black coat, remain largely unaffected.
No image is visible on the plate, however, as the exposure is so
FIG. 1 60. — Diagram of a reflecting camera
very brief. The latent image is brought out only when the plate
is chemically treated by the developer.
The sensitive plates, films, and sensitized paper that the
photographer handles in order to make his negatives and print
his pictures must be handled in light that will not affect these
objects. As a matter of fact, all light does not affect them
equally, but the rays that are at the violet end of the spectrum
are the most active ones. By covering the ordinary sources
of light, the window or electric light, with screens of orange and
CAMERAS AND PICTURE-MAKING 317
ruby glass or paper, these rays may be kept out of the darkroom,
and yet there will be left light enough for the photographer to see.
One can purchase a darkroom lantern or use a ruby bulb on the
electric light, or one can cover the window, the electric light,
or the front of a starch box in which there is a candle, with
orange and red tissue paper, or, better still, with the tough
orange and ruby paper purchased from a photographic supply
house. So one may use the kitchen sink or bathroom washbowl
for photographic work, if one can work at night, or can shut
out all light by opaque curtains during the day. The photog-
rapher has a room fitted with a sink with running water,
shelves on which he can keep his apparatus, and other conven-
iences. This room is light-tight and is illuminated by a safe
source of light.
One needs for darkroom appliances, in addition to the dark-
room lamp, a hard-rubber or glass tray in which to develop
plates and prints, an 8-ounce graduate and some stirring rods,
a glass tank to hold the plates while they are being fixed, and a
similar tank for washing them, one or more print frames of the
same size as your plates, and a couple of good-sized trays for
washing and fixing prints. These latter may be used in place
of the glass tanks in washing and fixing plates. One may
appropriate the galvanized kitchen ware, but it is well to have
these usual appliances if one is going to do much developing.
There are also required a bottle or large fruit jar holding two
quarts or more for the fixer, as it can be used repeatedly, a small
roll of absorbent cotton, and a towel that you are not afraid of
staining. A small pair of scales with gram weights is needed
if one is going to make his own developer and other solutions,
but the beginner will prefer, probably, to buy these all ready for
use (Fig. 161).
There are a number of developers used by photographers
and each man has his favorite. It is well to select some one and
use it persistently until you have mastered the technique of
handling it. Suppose we select hydrochinone, which comes
OUR PHYSICAL WORLD
in small tubes, five to a box. Also purchase one half-pound
box of acid fixer. Be sure that the tray, fixing bath, and all
apparatus to be used are washed clean. Dissolve the fixer in
32 ounces of water, and fill the fixing bath. Now dissolve the
contents of a tube of the developer in 4 ounces of water in the
FIG. 161. — Some darkroom equipment. At rear a large tray for fixing or
washing. At right, graduate. At its left trays for developing, the box of develop-
ing powders, with two tubes on table still farther to the left. At extreme left a
plate holder with slide partly removed. At its right are print frames, one showing
its back, the other with negative in place ready to print.
graduate. The tube contains at one end the developer and at
the other some chemicals that speed up the rate of development.
As you hold the tube in hand to read the label, the developer is
at the right-hand end. Open this end first and pour the powder
into the water as you stir with a glass rod. Then open the other
end of the tube and pour in the chemicals while stirring. The
CAMERAS AND PICTURE-MAKING 319
stirring helps to prevent the formation of lumps that will require
a long time to dissolve. When the chemicals are completely
dissolved, pour the developer into the tray and put in a small
wad of absorbent cotton as large as a walnut. Be sure that
all light is excluded except that from the darkroom light. Take
the plate from its holder, handling it only by its edges. If oil
from the fingers makes a finger mark on the gelatine surface,
the developer will not get at the contained silver salts at this
point and your negative will show the finger mark. Immerse
the plate in cold water, then in the developer, which should have
a temperature of about 70° Fahrenheit. The film side of the plate
is to be kept up. Wipe off this side quickly but gently with the
absorbent cotton wet in the developer so as to remove any
adherent air bubbles. If this is not done the air bubbles may
prevent the developer from reaching the silver salts, and the
plate when developed will look as if dotted with pin pricks.
Rock the tray to keep the developer moving over the plate.
Lights and shadows should begin to appear in four or five seconds,
and the clear outlines of the object in ten seconds or so. If the
picture flashes up and the whole plate begins to darken at once
when it is put in the developer, it has been overexposed. If
the image comes very slowly and is weak, it has been under-
exposed. When the process of development is sufficiently
advanced so that the picture begins to show clearly on the back
of the plate, immerse it in water to wash off the developer and
put it in the fixer. This is a solution of sodium hyposulphite
together with other chemicals which tend to harden the gelatine
that has been more or less softened by immersion in the developer.
This "hypo" dissolves out of the gelatine film all the silver
compounds that were not reduced to metallic silver in the
process of development. The plate is left in the fixer until
all the yellowish white has disappeared ; this will take from three
to ten minutes. The plate is then washed in running water
for a half-hour to remove the fixer, and is stood on edge to
dry. Such a plate will display dark areas corresponding to
320 OUR PHYSICAL WORLD
the light areas of the object and transparent areas correspond-
ing to the dark areas of the object; it is therefore known as
the negative (Fig. 1620). When thoroughly dry, it is to be
used to make the print or picture (Fig. 1626). If one is
developing several plates, one after another, he should be sure
to wash off from his fingers all traces of the fixer before handling
the next plate, for the fixer readily spoils the developer. When
through developing, put the fixer into the large- stoppered bottle
to save for the next lot of plates. It will fix six dozen 4X5
plates. The developer is to be made up fresh for each new
batch of plates. One tube of developer is sufficient for a dozen
such plates.
The roll of films is handled in the same manner except that
one holds an end of the roll in each hand and runs it through
first the water and then the developer (Fig. 163). It is not
necessary to wipe its surface with the cotton as the movement
takes off the air bubbles. If the exposures are not accurate
in the several films so that some images develop rapidly and
others slowly, it will be wise to wash off the developer in the
water when this fact is apparent, cut the roll into its separate
films, and develop each separately. When fixed, films are pinned
up to dry on a taut string like clothes on a line.
Many photographers now prefer to use the tank method of
developing. A tank developer is then used, which can also be
purchased in tubes. The tank is filled with the developer at
proper temperature, the plates (or film) are put in and left for
the time specified on the directions, when the developer is poured
off and the fixer is added.
To make a print, remove the back from a print frame and
lay the negative in, its uncovered side toward the light. A
film must be laid on a piece of clean glass that fits the print
frame. In the darkroom, take a sheet of print paper from its
box or envelope and lay it on the negative, film side of the paper
against that of the plate. The film side of the paper is told in
the same way as in the case of the plate, though the difference in
CAMERAS AND PICTURE-MAKING
321
JMI
PPT
m
• • • RBI
FIG. 1 6 20. — A negative
FIG. 1626. — A print from the negative shown above
322
OUR PHYSICAL WORLD
the two sides is not as marked as in the plate. Put the back in
the print frame, and fasten it in by the spring clips so it w$l hold
the print paper firmly against the negative. Expose to the light
of the electric lamp so that it will fall on the face of the negative
and through it on the paper. The paper is then removed, devel-
oped, and fixed in the same way as a plate would be handled,
except that it is not necessary to wipe its face. The print should
be developed until it is a trifle darker than really desired as it pales
a little in the fixing bath.
Slide the paper into the de-
veloper rapidly and see that it
is covered by the developer
at once. One uses a different
developer for prints than for
plates usually, one that gives
less contrasty results. Elon-
quinol, purchased in tubes, is
a good one to begin with.
One tube makes up 8 ounces
of developer, enough for a
dozen 5X7 prints.
Just how long an exposure
is to be given to make a good
print depends on the brand of
FIG. 163.— Handling the film
paper, the thickness or density
of the negative, and the intensity of the light used. The print
frame may be held a foot or so from a fifty-candle-power -electric
light with a frosted globe. Cut a sheet of point paper into
several strips, and try one strip with an exposure of five seconds.
If the picture comes up in the developer clearly in ten seconds
or so, that exposure is about correct. If it flashes up suddenly
and the strip darkens all over, the exposure is too long. If it
comes up slowly and weakly, the exposure is too short. Try
other strips until the exposure is correctly timed, then print
the picture on the full-sized sheet. After some experience one
CAMERAS AND PICTURE-MAKING
323
will judge the length of exposure needed quite accurately,
without preliminary trials.
The prints are to be left in the fixer for ten minutes, then
washed for twenty minutes in running water. Dry the prints,
face down, on cheesecloth stretched on a wooden frame, or if
glossy prints are desired, dry on a clean glass or porcelain surface.
Print papers come in a variety of grades. The surface may be
dull, matte, glossy, etc. The paper may be soft, normal, con-
trast, portrait, etc., according to the effect desired. Contrast
papers are needed to give proper values in prints of weak nega-
tives, soft papers for
contrasty negatives,
those in which the
high lights and shad-
ows are very strong.
Lantern slides and
transparencies are
printed in the same
fashion as paper
prints, using a lantern-
slide negative or trans-
parency negative in
place of the print
paper. The exposure will be about one-half second at a foot from
the light. Such negatives are developed in the same way as are
plates. The image should be allowed barely to begin to come
through on the back of the plate before it is placed in the fixing
bath, as the plate needs to be thin to let the light through it readily.
A mat is laid on the film face when the plate is dry, with an open-
ing in it large enough to show the picture. This is covered with a
cover glass the same size as the plate, and plate and cover glass
are bound together with adhesive paper strips applied to the edges
(Fig. 164). Lantern slides, transparencies, and prints may be
tinted by applying to the film side by means of camel's hair
brushes transparent water colors purchased for the purpose.
FIG. 164. — A lantern slide
324 OUR PHYSICAL WORLD
Sometimes a negative or a lantern slide is too thick or too thin
when finished to give satisfactory results. Such may be improved
by reducing or intensifying. To reduce, add a teaspoonful of a
saturated cold-water solution of potassium ferricyanide to a
solution of hyposulphite of soda made by adding a tablespoonful
of this salt to 4 ounces of water. These proportions do not
need to be exact. Put this in a tray and lay the negative in it,
rocking the tray to cover all parts promptly. The operation
is carried on in daylight. The more of the ferricyanide used,
the more rapid the reduction. The negative is taken out when
sufficiently thin, washed in running water twenty minutes, and
set up to dry.
To intensify, place enough saturated cold-water solution of
bichloride of mercury (poison) to cover the plate in one tray,
and a similar amount of water to which ten drops of concentrated
ammonia are added in a second tray. Immerse the plate in the
first and leave until its surface whitens a bit. Then put it in
the second tray where it will darken, especially the more opaque
areas. Wash it in water for two or three minutes, and repeat
the process until it is sufficiently intense. Then wash twenty
minutes in running water and dry. There are many other
methods of reduction and intensification that use other chemicals;
these methods will be found in the books given in the Book List.
The sensitive film or plate or paper is produced by spreading
evenly on these objects a thin layer of gelatine all through which
there are suspended tiny particles of silver bromide and silver
iodide, put there by apparently dissolving these salts in the
gelatine. If the preparation is made up hot and allowed to
stand and ripen before it is spread, the particles of silver salt
aggregate somewhat, and the plate is coarse grained, but rapid.
If it is made up cold and spread at once, the particles do not
cohere, and the plate is slow, but fine grained. The slow plate,
therefore, will give finer detail than a rapid one.
CHAPTER XIV
THE HOMEMADE ORCHESTRA
The man that hath no music in himself
Nor is not moved with concord of sweet sounds
Is fit for treasons, stratagems, and spoils.
— SHAKESPEARE, Merchant of Venice.
A modern orchestra is a very wonderful thing, with its
aggregation of varied instruments gathered from the four quarters
of the globe. I half close my eyes, sometimes, as I sit listening,
and let my imagination change the stage setting. The immacu-
late gentleman who is rolling sonorous sounds from his kettle
drum becomes a painted savage, his instrument a skin stretched
over a hollow log; and as he pounds his war drum, his fellows
brandish their cruel spears and leap in a frenzy of ecstasy in
anticipation of the coming battle. The gentleman in evening
attire who presides at the great organ changes to a Greek shep-
herd, clothed in a draped skin, who blows on his pipes, the primi-
tive ancestor of the organ, while his sheep graze on the sun-flecked
hills about him. The clarinet player I see as a squatting Indian
snake-charmer who, in his gaudy robes, sways in unison with the
hooded serpent before him, as he draws strange melody from
his reed, the precursor of the present instrument. The French
horn is the horn of a hunter who goes dashing by on his splendid
horse, after a pack of dogs that are close on the heels of the
fox. What a strange history each of the orchestral instruments
has had ! They have come down to us from the inventive genius
of peoples scattered from pole to pole. Yet, while they are
so very different in present form and in their evolution from many
primitive types, the principles of sound on which their perform-
ance depends are few and simple. Vibrating strings or vibrating
columns of air originate all the notes that are strengthened and
325
326 OUR PHYSICAL WORLD
modified by the resonance of the body of the instrument and the
air in its chambers.
Sound is due to the vibration of the body that initiates it,
and these vibrations pass out as pulses into the surrounding
medium. Strike a gong or bell and hold against its edge a ball
made of the pith of the elderberry stem, or a tissue-paper wad
suspended by a string, and the ball flies off from the bell
repeatedly, impelled by the push of the oscillating particles.
One can see the vibrations of a taut string, for, when plucked,
it is a blur, widest usually at its central region, where it is swing-
ing back and forth with the greatest amplitude (Fig. 165).
Sound, like light, is a form of wave-motion. The vibrating
particles of the substance that carries the sound move back
and forth in the same direction the sound is traveling; while,
FIG. 165. — Vibration of a taut string
in the case of light, this oscillation is transverse to the line of
propagation. The sound waves move out in all directions in
air, for instance, from the sounding body as concentric spheres
that are alternately dense and rare (Fig. 166). Sound, therefore,
like light travels from point to point in straight lines, the radii
of these concentric spheres.
Its rate of propagation is relatively slow. In air it goes
about 1,100 feet per second, while light in the same time travels
186,300 miles. In general this discrepancy in the rates of move-
ment of sound and light is familiar from commonplace experi-
ences, even if the exact difference is unknown. You see the puff
of steam from a distant locomotive whistle long before you
hear the toot. You see a distant woodchopper, or a section
hand driving a spike into the ties, deliver a stroke and straighten
up ready for the next one before you hear the sound of his blow.
THE HOMEMADE ORCHESTRA
327
The rate of propagation varies according as the substance
through which the sound is traveling is more or less elastic.
Sound travels through water about four times as fast as through
air. It travels farther, also, the more elastic the conductor
is. One can hear an approaching train or wagon when the
ear is held on the rail or on the ground long before the rattle of
its approach can be heard through the air. The taut string or
the wire of the simple tin-can telephone (directions for making
given on p. 95 of the Field and Laboratory Guide in Physical
Nature-Study) carries the sound of the voice much farther
than it could be heard through the air.
FIG. 1 66. — Sound waves radiating from a bell
Since sound travels in straight lines, there are sound shadows
just as there are light shadows; or, in other words, an object
shuts off the sound as it does the light. A block away from a
noisy thoroughfare, with its clanging street cars, automobile
horns, and rattling vehicles, one hears little of the hubbub,
for the intervening buildings shut off the sound waves. It
is true, however, that sound waves swing around the edges of an
obstruction much more readily than light does, for light waves are
very much smaller than are sound waves. The larger waves of deep
tones can do this more readily than do the smaller waves of shrill
sounds. Therefore the roar of the distant street is a hoarse roar.
Sound, too, like light, is reflected from a surface. One may
focus sound with a concave mirror quite as readily as light.
(See Field and Laboratory Guide in Physical Nature-Study,
328 OUR PHYSICAL WORLD
p. 83.) When some building, or the face of a cliff, serves as a
reflecting surface, the sound of the voice is sent back, as an echo,
to a person listening. Do you recall the incident in Treasure
Island in which, when the pirate crew is hunting for the buried
treasure, Ben Gunn scares them away by imitating the call of old
Flint, their dead but still dreaded captain ? Silver, hearing the
echo of Ben's voice, reassures himself and his companions by the
comment that if a " spirit" does not make a shadow it stands to
reason it cannot make an echo.
The violinist throws the strings of his instrument into
vibration by drawing over them the bow, which takes hold of
the strings because it is rosined. The banjo player or the harpist
plucks the strings to cause them to vibrate. In the piano,
FIG. 167. — Strings stretched across a table
the string is struck by a hammer operated by pressing a key.
You will notice that in harp and piano there is a string for
every note emitted, and these strings vary in length, caliber,
and tension. On the violin and banjo, however, there are only
a few strings, but the player varies their length by pressing them
down with his finger tips; only the portion between finger and
bridge vibrates.
Tie one end of a string or thread to the leg of a table. Hold
the free end in your left hand, pull on it, and pluck the string
with your right hand so it will give out a note. Pull harder
still and again pluck the string, and you will notice that the
pitch of the note emitted is higher, the harder you pull. Lay
the string across the table, and fasten to the free end a heavy
weight like a flatiron. Support the string by a couple of strips
of wood laid on edge under it near opposite sides of the table
(Fig. 167). Pluck the string to get a sound and note its pitch.
THE HOMEMADE ORCHESTRA
329
Then slide one of the wooden strips nearer the other, pluck the
string again, and you will find that the shorter the string, the
higher the pitch of the
note it gives. If now
you lay a second heavier
string from the table leg
across the wooden strips
and stretch it by another
weight equal to that on
the first string, you will
find that the pitch of
the note emitted by the
heavier string is lower
than that from the lighter
one. Thus we learn that
when a string vibrates,
the note it emits is higher
in proportion as the
string is short, taut, and
of small caliber. There-
fore the strings on the
bass viol are long and
heavy, those on the cello
FIG. i68.-A cello and a violin. (Photo are of medium length
by Lyon and Healy.) and caliber, those on the
violin are short and of small diameter. In each instrument the
330 OUR PHYSICAL WORLD
strings can be made more or less taut, and so tuned to play in any
desired key (Fig. 168).
The sound produced by a vibrating string is weak. It does
not hit enough air particles to start vigorous waves. If, however,
it is mounted on a thin-walled box so that the vibrations of the
string are imparted to the box, which presents a broad area
to the air and in turn imparts its vibrations to many air particles,
then the volume of sound given out is greatly increased. A
watch held in the hand is scarcely heard, but place it on an
empty cigar box and it sounds quite loudly. Strike an ordinary
table fork on the edge of the table so as to set its tines in vibration,
and the sound it gives out is scarcely audible; but press the end
•
FIG. 169. — Mouth end of a clarinet, showing reed
of its handle on the table, and its note is loud and clear. Not
only does the wood vibrate in the violin and similar instruments,
but the contained air is thrown into vibration and contributes
to the volume and character of the sound. The shape of the
instrument affects the quality of its notes and hence it must
be skilfully made. That is one reason why the "old masters"
are such costly instruments; they were made with rare skill
and some luck, which even their skilful makers could seldom
duplicate.
In wind instruments, it is the contained column of air that
is thrown into vibration, and, pulsing back and forth, imparts
its motion to the surrounding air to start the sound we hear.
This column of air may be thrown into vibration by blowing
across a hole in the instrument as in the flute, or by a vibrating
THE HOMEMADE ORCHESTRA 331
reed or membrane. In the clarinet, the player blows upon a
thin elastic strip that lies over a slot (Fig. 169). The air pressure
depresses this strip and closes the opening. But the moment
the air current stops because the slot is closed, the springy
tongue flies up again, opens the slot, and the current flows
once more. This process is rapidly repeated, so the successive
puffs of air caused by the rapidly vibrating tongue set in corre-
sponding motion the air column within the body of the instrument.
Nearly every country lad has made a similar reed instrument.
He takes a hollow stalk like an oat straw or the leaf stalk of
squash or pumpkin, and cuts a slanting slash in it near the node
or closed end. He cuts off the other end so as to leave the crude
instrument 6 or 8 inches long (see Fig. 170). Then he sticks the
slot end in his mouth, covering the reed entirely, and blows to
produce the note, which may
be a squawk rather than a
musical sound. It may be
necessary to shorten the FlG- 170— A squawker made from an
•i ., , . oat straw,
instrument a bit at a time
until just the proper length is found that will give the best
result.
Just as with the string, other things being equal, the shorter
the string, the higher the pitch of the note emitted, so with
the vibrating air column, the shorter it is, the higher the note.
Blow across the mouths of two bottles, or tubes closed at one
end, one after the other, and you will note that the long bottle,
or tube, gives out the lower note. This principle is well illus-
trated by Pan's pipes, the flute, or the whistle with movable
bottom, directions for making all of which are given in the
Field and Laboratory Guide in Physical Nature-Study. When
the fife-player holds his finger tips over all the holes, the length
of the column of air coincides with the length of the instrument;
but when he takes his finger off one hole, the vibrating column
ends at this point, reaching in the other direction to the closed
end of the instrument (Fig. 171). It is true also that, of two
332
OUR PHYSICAL WORLD
tubes of equal length each closed at one end, the one that has
the greater diameter will give out the lower note when one blows
across the open end. The pipes on the organ that produce the
bass notes are long and of large diameter, while those for the
high notes are short and have a small bore. So the wind blow-
ing over the opening at the top of the chimney produces a deep
note, and we say the chimney roars. This is due to the fact
that the pitch of the note emitted by a vibrating string or air
column depends on the rate of vibration. The shorter the
string or column of air, other things being equal, the more
rapidly it vibrates and the higher the note emitted; the greater
FIG. 171. — A fife, showing change of length of air column
the caliber of the string or tube, the less rapid the vibration. A
long or thick string or column of air means a greater mass, and
the greater the mass, the less rapidly it swings into motion.
More than that, a note of a given pitch is always produced
by exactly the same number of vibrations. Thus the piano is
tuned so that middle C is given off by a string vibrating at the
rate of 256 vibrations per second. The C note one octave higher
is produced by double the number of vibrations, and the one
an octave lower by half as many. Match on the piano the pitch
of a mosquito's or bee's hum and you can tell how many times
per second the insect's wings are beating the air, for you can calcu-
late the number of vibrations for any musical note.
THE HOMEMADE ORCHESTRA 333
We have chosen a musical scale in which the rate of vibration
starting from middle C is as follows :
CDEFGABC
256 288 320 34i£ 384 426! 480 512
The intervals between the notes corresponding to these
numbers of vibrations are pleasing to our ears. These numbers
are in the ratio of
24 27 30 32 36 40 45 48
Or we may say that D has nine-eighths of the number of vibra-
tions of C, E five-fourths as many, and so on, the series of fractions
being
CDEFGABC
I I \ - i I I V 2
So in making Pan's pipes or the flute (see Field and Laboratory
Guide in Physical Nature-Study, p. 95), these relations must
be maintained between the lengths of the pipes used or the dis-
tances of successive holes from the mouth opening of the flute.
In making an instrument like the piano the manufacturer
is confronted with a difficulty, for one may want to play on other
keys besides C. Suppose, for instance, it is desired to start the
scale with D or with E. Now the number of vibrations of the
successive notes in the scale must bear to those of D or E the
same ratio which the number of vibrations producing the notes
of the C scale bear to the number of the C string. The num-
ber of vibrations needed for the notes of these new scales as
compared with the number needed for the notes of the C scale
is indicated below.
When C begins the scale:
CDEFGAB CD E
256 288 320 341^ 384 426! 480 512 — 576 640
When D begins the scale :
288 324 360 384 432 480 540 576— 648
When E begins the scale:
320 360 400 426! 480 53 1 £ 600 640
334 OUR PHYSICAL WORLD
It is evident that there is a discrepancy between the rates of
vibration needed for the notes of the C scale and the numbers
needed for the D and E scales. The E note will serve for the
third note of the C scale and reasonably well for the second note
of the D scale, but F will not do both for the fourth note of the
C scale and the third note of the D settle, so an additional key
has been put into the piano at this point as a black key, which
we call F sharp. Similarly, the upper C will not do for the
seventh note of the D scale, and so another black key is intro-
duced as C sharp. So it will be evident from the requirements
of the E scale and others that additional black keys are required,
and it has been found that we can reasonably well meet all
requirements by putting in five additional black keys in each
octave, and, of course, the corresponding strings. Even then
you will find we have to put up with notes that do not exactly
meet requirements.
The same thing is true in all instruments in which the number
of vibrations is fixed by the mechanical limitations of its manu-
facturers. That is not true for the violin, for the length of the
string and consequently the pitch of the note is determined by
the pressure of the finger of the player, and this can be applied
to the string anywhere. So the skilful violinist can render his
notes exactly true where the pianist must be satisfied with
approximately correct ones. A hundred years ago the piano
used to be tuned so that certain of the notes met the requirements
exactly, others only approximately. For instance, we may
tune the E above middle C so it will vibrate at the rate of 320
per second and meet exactly the requirements of the C scale.
Then, however, it will not meet the requirements of the second
note of the D scale. Or we may tune E to 322 vibrations,
when it will meet the requirements for both these scales more
nearly, but for neither exactly. In the old method of tuning,
certain notes sounded just right, others were distinctly unharmoni-
ous and were known as "wolves" because they howled so badly.
Now the piano is tuned so that the twelve intervals in the octave
THE HOMEMADE ORCHESTRA 335
from C to C are all equal, and we have become more or less accus-
tomed to the little discrepancies this involves so that we scarcely
notice them.
When a taut string is made to vibrate by bowing it and at the
same time it is lightly touched at its mid-point, it may then
vibrate not only as a whole, but in each half also (Fig. 172).
The note emitted by the vibrating halves is of course an octave
higher than the note emitted by the whole string. Again, if
the string is similarly touched at a point one-third of the distance
from one end to the other, it vibrates in segments as well as in
its entirety and other notes are emitted in addition to the funda-
mental one. Such tones are known as overtones, and in most
musical instruments the quality of the sound emitted is due
quite as much to the number and character of the overtones
FIG. 172. — String vibrating as a whole and in halves
and to the resonance as to the vibration of the string or air
column that sets the sound going.
So the pitch of a note is determined by the rate of vibration
of the body that originates it or by the wave-length, since this
is determined by the former factor. The intensity of the note
is determined by the amplitude of vibration of the particles
of the body from which the sound comes. The greater the
amplitude, the louder the sound. The quality of the sound
depends on the overtones.
The human voice is produced by the vibration of two mem-
branous flaps that lie on either side of the larynx or voice box,
a cartilaginous structure at the top of the windpipe, felt in the
neck as the Adam's apple (Fig. 173). In ordinary respiration
these flaps are drawn to one side and lie loose. When one desires
to speak, they are drawn nearly together and rendered taut, so
336
OUR PHYSICAL WORLD
FIG. 173. — The larynx. At left, outside view;
at right, sectional view of inside showing vocal
cord at V.
that their cordlike, nearly parallel edges form a slot through
which the air rushes, when expelled from the lungs, and throws
them into vibration. The sound thus originated passes out
through the open mouth and is modified by the resonance of
the air masses in mouth,
nose, and throat. One
can sing a note, and
then by changing the
tension of the cords
sing another higher or
lower one. By forcing
the air more rapidly
past the cords, they are
made to vibrate more
vigorously, and the note
sung is made louder.
If, while singing or
speaking, the nose is pinched shut by the fingers so as to cut off
some of the air masses that are customarily thrown into sympa-
thetic vibration, the qual-
ity of the tone is altered.
Similar changes are pro-
duced by varying positions
of teeth, lips, and tongue,
thus changing the shape
of the resonance cavities.
One of the marvelous
inventions of our own
times is the phonograph,
which reproduces with
such remarkable fidelity FIG. i74.-A phonograph
the human voice, the music of the orchestra, and other sounds.
Directions for making the instrument are given in the Field and
Laboratory Guide in Physical Nature-Study. A hard-rubber
disk rotates horizontally on a turntable (Fig. 174). The point
THE HOMEMADE ORCHESTRA
337
of a needle is placed in a spiral groove on the face of this disk.
The base of the needle attaches to a diaphragm that closes the
mouth of a small funnel. A tube leads from the stem of the
funnel to the small end of a horn. When the disk is used to
make a record, a disk of impressionable material is used in place
of the hard-rubber disk. The voice, or other sound, is caught by
the horn, travels down the tube, sets the membrane in vibration,
and that in turn the needle. As the point presses on the disk and
moves by appropriate mechanism in a spiral path, it engraves
on the disk a series of tiny hills and valleys. Now, when from
this disk a duplicate hard-rubber record is made and is set
rotating on the turntable of the instrument, the needle, as it
traverses the groove with its inequalities, is made to move
exactly as it did when the voice was making the impression on
the soft disk. That naturally makes the membrane vibrate,
which vibration is imparted to the air and reinforced by the
horn; so the sound is reproduced.
When one talks into the telephone, his voice strikes a metallic
membrane and sets it in vibration. These vibrations constantly
alter the intensity of an electric current that is passing through
the instrument. The current of varying intensity passes through
the wire to the receiver, and produces corresponding changes in
the force of an electromagnet by means of which another metal
disk is set to vibrating exactly in unison with that of the sending
instrument. Thus the voice is reproduced so that the person at
the distant end of the line hears the speaker. The method of
operation of the electrical device in the instrument has been
already explained.
During the war an exceedingly interesting method of locating
the position of an enemy gun was employed, dependent upon the
velocity of sound. Suppose that in the accompanying figure
(Fig. 175), observers with accurate recording apparatus are
stationed at points a, b, and c. Each notes the exact time at
which his instrument records the arrival of the boom of the
gun, and promptly telephones this time to a central station. The
338
OUR PHYSICAL WORLD
officer stationed here notes these times. Suppose that the instru-
ment at b registers the reception of the sound a half-second
after it is received at a
and the instrument at c
one second after it is
received at a. Suppose,
further, that the atmos-
pheric conditions are such
that sound is traveling at
the rate of 1,100 feet per
second. Then evidently
b is 550 feet and c 1,100
feet farther from the gun
than is a. The officer at
the central station has
a diagram showing the
relative positions of a, b,
and c, and their distances
from each other laid out
to a scale. On this same
scale he draws about b a
circle with a radius of 550
feet and about c a circle
with a radius of 1,100
feet. The gun is located at the center of a circle which passes
through a and is tangent to the circles about b and c. The
mathematics involved in the determination of this center is too
complicated to be briefly explained. This method was found so
efficient that a gun miles away could be located within 50 feet
of its exact position.
FIG. 175. — Diagram to show method of
locating a gun g, by sound.
CHAPTER XV
SOME SIMPLE MACHINES
Give me a fulcrum on which to rest and I will move the earth. — ARCHI-
MEDES.
This has been aptly called an age of machinery. The food
we eat, the clothing we wear, the houses we live in, the furniture
that contributes to our comfort, are all largely prepared for us
by machinery. We ride to school or to work in a machine,
we travel by machinery, our work is largely done by the machines
we direct. We farm by machinery, and machines mine our
coal, furnish our light, load our ships, sweep the floor, wash the
clothes, pump the water. They are our omnipresent servants;
at every turn we see them at work. Yet they are all applications
and combinations of three simple types of machines — the lever,
the pulley, and the inclined plane — that have been in use ever
since the earliest glimmerings of civilization. We think of the
invention of the steam engine as a revolutionary event. Yet the
savage who first discovered the use of the lever made even a greater
contribution to man's advancement. It will be worth while to
understand the principle of operation of these simple machines
and see some of their commonplace applications.
There are one thousand and one applications of the lever
about us in the home, in industrial life, and in our own bodies.
Nearly every child has had experience with the teeter. A board
is put over a log or saw horse, so it about balances at the middle
point; then a child sits astride on either end, and they go up and
down alternately as first one, then the other, gives a little shove
as his feet strike the ground. This is a simple lever with its
arms of equal length, and the point on which it rests, the fulcrum,
at its center. If, now, one child is considerably heavier than
the other, the board must be moved along on its support so that
339
340
OUR PHYSICAL WORLD
the length of board from the heavier child to the fulcrum is
shorter than that on which the lighter child sits. This same
type of lever is seen in the scales or balances which the storekeeper
uses to weigh his wares (Fig. 176). If you put a weight of exactly
FIG. 176. — A pair of scales
one pound in one scalepan on the end of one arm of this lever,
you know you have a pound of candy on the other scalepan
at the end of the other equal arm when the two just balance.
FIG. 177. — The crowbar in use
If one wants to use such a lever to raise a heavy weight, say
a crowbar (Fig. 177), he places the fulcrum so that the arm of
the lever that is under the weight is short and the arm on the
end of which he is pressing is long.
SIMPLE MACHINES 341
Notice, however, that he must move his end of the lever a
long way down to lift up the weight a short distance. That
is because one can never get more energy out of a machine than
he puts into it. The weight raised, multiplied by the distance
it moves, must equal the power applied, multiplied by the
distance it moves. This is a law that will apply to all the
machines described below. Now, in the case of the crowbar,
both weight and power move through the arcs of circles whose cen-
ters are at the fulcrum, and whose radii are the weight arm and
the power arm of the lever. The lengths of the weight arm
and the power arm are the distances of the ends of these arms
respectively from the fulcrum, but these arms are the radii.
So we may say that the weight arm multiplied by the weight
always equals the power arm multiplied by the power. Suppose
that the board of the teeter is 1 1 feet long, and that it weighs
2 2 pounds, while the smaller boy weighs 66 pounds and the larger
boy 100 pounds, and each sits one-half foot from the end of the
plank. Then the fulcrum would have to be 6j feet from the end
the smaller boy sits on, for
396+13=400+9
409=409
If the man in Figure 177 were pressing down on his end of
the crowbar with all his weight, say 160 pounds, and this
power arm on which he presses were 4 feet long, while the
weight arm were 6 inches long, leaving out of consideration the
weight of the bar, which may be considered as approximately
balancing the element of friction, he could raise a weight of
i, 080 pounds.
Sometimes it is desirable to gain speed of motion in using
a lever and sacrifice mechanical advantage. Thus, in striking
a blow with the fist in boxing, when the fist is suddenly shot out
from the elbow, as the arm is straightened, the fist is the weight.
342
OUR PHYSICAL WORLD
The bone of the forearm hinges near one end on the bone of
the upper arm, the bearing serving as a fulcrum (Fig. 178). The
big muscle at the back
of the upper arm, at-
taching to the short
end or power arm that
projects back from
the elbow joint fur-
nishes power. When
the muscle contracts, it
straightens the arm,
FIG. 178. — The arm showing the triceps muscle
and the hand moves
very rapidly. It weighs
much less, however, than the equivalent of the energy that is
applied by the muscle.
Levers are of three sorts. Levers of the first class are those
in which the fulcrum lies between the power and the weight.
Levers of the second class are those in which the fulcrum is at one
/VfUcrum
First Claw
/Xfukrum J\veigtt- power
Second Class
•fulcrum
pcmr
Third Class
FIG. 179. — Levers of three classes
FIG. 1 80.— A hammer as
a bent lever.
end, the power at the other, and the weight between. Levers
of the third class have the fulcrum at one end, the weight at
the other, and the power between. (See Fig. 179.) But in
all cases the weight times the weight arm will equal the power
SIMPLE MACHINES
343
times the power arm. The law applies just as well in the case
of bent levers as in those in which the weight arm and the power
arm form a straight line. The hammer is a good illustration
of the bent lever when it is used to pull a nail (Fig. 180). It
FIG. 181. — A wheelbarrow as a lever
will be interesting to place the various levers seen in common
mechanical devices in one or the other of these classes and to
calculate whether one needs little or much power, as compared
with the resistance
overcome, to operate
such devices. A few
such contrivances may
be mentioned; pupils
will think of niany
more: the lemon
squeezer, wheelbarrow
FIG. 182. — Wheel and axle used in steering
a boat.
(Fig. 181), scissors,
nutcracker, crank of a
wringer or coffee mill, the forearm when the fist is brought up to
the shoulder, the pump handle, etc.
The windlass, wheel and axle, and capstan are familiar
applications of the lever with which astonishing results may be
accomplished. Recently in Chicago, a large brick building,
344
OUR PHYSICAL WORLD
estimated to weigh 15,000 tons, was moved to its new location
by two teams of horses operating capstans. The wheel and axle
is commonly used in moving a rudder to steer a boat (Fig. 182).
The city child who watches
the construction of a building
or the country lad who sees a
well dug will likely see the
windlass used.
In this last contrivance, a
crank is firmly fixed to a hori-
zontal cylinder of wood or
metal, the axis of which is sup-
ported on uprights (Fig. 183).
A rope winds about this cyl-
inder, bearing at its free end
the bucket of earth, water, or
other substances it is desired
to raise. Water was drawn out of the old-fashioned well by
such a windlass. A man turning the crank is applying power
to one end of a lever of the first class. The fulcrum is the
center of the axle, and the weight is the rope and bucket. Sup-
pose the distance from the center of
the axle to the end of the crank is 2
feet and the radius of the cylinder is
3 inches. Evidently a pressure of 10
pounds exerted to turn the crank will
lift a weight of 80 pounds, leaving
friction out of consideration.
The capstan (Fig. 184) is like
the windlass except that the cyl-
FIG. 184. — A capstan
inder is set vertically, and the capstan has a bar or bars which
turn in a horizontal plane, the equivalent of the crank on the
windlass. When a horse attached to the end of this bar is
driven around in a circle, the rope is wound on the cylinders,
and the power of the horse is tremendously multiplied. Suppose
SIMPLE MACHINES
345
that the capstan bar is 10 feet long and the horse at its end is
exerting a pull of a ton and a half; suppose, further, that the radius
of the cylinder is 6 inches: then the rope winding on the cylinder
is exerting a pull of 30 tons minus whatever power is used in over-
coming the friction of the machine.
The form of capstan in Figure 184 is
much used on shipboard for raising the
anchor or for similar heavy tasks.
The wheel and axle is evidently like
the windlass except that the crank at-
taching to the cylinder is replaced by a
wheel. Several such simple machines may be combined in the
train of gear wheels so as to develop immense mechanical advan-
tage. Suppose in Figure 185 the power is applied as a weight on
FIG. 185. — A train of gear
wheels.
FIG. 1 86. — A hand derrick
a rope that winds on the axle of the right-hand wheel. This
wheel has cogs that play into those of the small middle
wheel which is firmly fixed to the large wheel on the same
axis. The cogs of this play into those of the small left-hand
wheel, which turns the large left-hand cylinder. As the weight
346 OUR PHYSICAL WORLD
drops, it unwinds the rope, causing the wheels to revolve, and so
winds up the rope on the large cylinder and raises the weight.
Since the number of cogs on the wheels will be in proportion to
their size, the mechanical advantage may be found by dividing
the number of cogs on the large wheel by the number on the small.
If power and weight were interchanged, then the weight would be
moved rapidly, but at the expense of power applied. On a hand
derrick, which combines the advantage gained from the use of a
crank with that of the train of wheels attached to the crank
(Fig. 1 86), one man may lift a weight of several tons, but his
hand on the crank handle must move through a distance of
many feet to raise the weight a few inches.
FIG. 187. — The sprocket wheel and chain on a bicycle
The sprocket wheel on the bicycle is a familiar illustration
of the use of such gears (Fig. 187). The pedal shaft and axle
form a windlass which increases the power applied by the pedal
to the sprocket wheel. Power is lost as this plays into the small
gear wheel on the hind axle with which it is connected by the
chain, but speed is gained and this is desired.
The pulley is another simple machine. In its simplest form
it consists of a single wheel over which a rope passes. The weight
is on one end of the rope, and the power is applied to the other
end. The pulley simply serves to change the direction of the
application of the power, but this is often convenient. Thus,
in hoisting hay into the barn loft, one can stand on the ground,
put his whole weight on to the rope that passes over the pulley
fastened above the window, and pull the hay up. In raising
a flag on a flagpole, it is much easier to tie it to a rope that runs
SIMPLE MACHINES 347
through a pulley at the top of the pole, and so run it up into posi-
tion, than it would be to shin up the pole and fasten it in place.
When we use two pulleys in combination, especially if each
has several wheels over which the rope may run, we gain a
mechanical advantage. There is shown in Figure 189 a combina-
tion of two pulleys, each with two wheels. It is evident now
that the weight to be raised is supported by four strands of rope,
while the one you pull on in passing over the pulley merely gives,
as before, the advantage of a change in direction of the power
FIG. 1 88 FIG. 189
FIGS. 188-89: FIG. 1 88. — A single-wheeled pulley. FIG. 189. — Double pulleys
applied. A fourth of the weight is borne by each strand of rope.
To raise the weight a given distance, the power must move through
four times that distance. Therefore, the power applied will be
only one-fourth as great as the weight plus whatever is required
to overcome the friction of the system. Divide, then, the weight
to be raised by the number of strands of rope between the pulleys
excepting the one to which the power is applied to obtain the
power required to raise the weight. If there is one wheel in
each pulley of such a block and tackle (as a combination of
pulleys is called) there will be two strands of rope not counting
the one on which the pull is exerted, and the weight raised will
34*
OUR PHYSICAL WORLD
be approximately twice the power applied. The power now will
move two times as far as the weight.
The third simple machine found in many common appliances,
either by itself or in combination with one of the foregoing, is the
inclined plane. When the truck man wants to load a heavy
barrel into his wagon, he often lays a plank from the rear end
of the wagon to the ground and rolls the barrel up this plank
instead of trying to lift it, because he can roll it up the plank so
much more easily (Fig. 190). Suppose that the bed of the wagon
is 3 feet above the ground and the plank 1 2 feet long. Suppose
FIG. 190. — Loading a barrel on to a wagon with the inclined plane
that it is a barrel of flour weighing 196 pounds that is to be loaded.
This is to be raised 3 feet from the ground, but to do this the
truckman applies force to it as it rolls a distance of 12 feet.
Remembering now that the weight multiplied by the distance it
is raised equals the power applied multiplied by the distance
through which it acts, it is evident that a push of 49 pounds is
sufficient to roll the barrel:
196X3=49X12.
The truckman, then, by applying power of 49 pounds plus
what is needed to overcome friction, can get the barrel weighing
196 pounds into his wagon.
SIMPLE MACHINES
349
FIG. 191. — The chisel as an inclined
plane.
Suppose one is cutting a shaving from a stick of wood with
a knife whose blade is six-sixteenths of an inch wide and one-
sixteenth of an inch thick on the side opposite its edge ; then this
wedge-shaped blade is really an
inclined plane. If he is bearing
down on the handle of the knife
with a pressure of 10 pounds,
the blade is exerting a force of 60
pounds, less friction, to overcome
the cohesion of the wood. So in
a chisel (Fig. 191), plane blade, axe, and other cutting tools, we
constantly use this simple machine.
The screw, as we use it on bolts, ordinary wood screws,
on the carpenter's bench vise, the screw jack (Fig. 192), and in
many other places, is really an application of the inclined plane
combined with the lever. Cut a right-angled triangle out of
paper, making its base 6 inches long, its altitude i inch. Apply
the i -inch side to a pencil and then wrap the paper about the
pencil. The hypotenuse of the triangle will make a line like
the thread of the screw, but this line in the
triangle is a section of an inclined plane. Sup-
pose we are turning a nut on a bolt with a
wrench (see Fig. 193); the power applied on
the handle moves in a circle whose radius we
will say is 4 inches. Meantime the head of
the bolt has moved 'toward the nut, a distance
equal to the space between two turns of the
thread. Suppose there are twenty turns of
the thread per inch. The distance between
threads is then one-twentieth of an inch, which
is known as the pitch of the screw. The
weight, therefore, has moved one-twentieth of an inch while the
power has moved through the circumference of the circle with a
radius of 4. The circumference of this circle is twice the radius
times 3.1416, or slightly over 25 inches. The power is therefore
FIG. 192. — A screw
jack.
350
OUR PHYSICAL WORLD
multiplied 500 times, ignoring friction. If one were pressing,
therefore, on the handle with a pressure of 20 pounds to turn the
nut, the bolt head would be drawn toward the nut with a pull of
5 tons.
FIG. 193. — A wrench used to turn the nut on a bolt
Examine the machines that are commonly seen, the sewing
machine, locomotive, automobile, typewriter, etc., and you will
find they are made up of ingenious applications and combina-
tions of these three simple machines so arranged as to accomplish
the desired end. The elements that enter into any mechanical
invention are few and simple, but the possible combinations
and variations in the form of these elements are bewilderingly
numerous.
BOOK LIST
Adams, Joseph H. Harper's Electricity Book for Boys. New York:
Harper Bros., 1907. $1.75.
Backert, A. O. The A. B.C. of Iron and Steel. Cleveland: Penton Pub-
lishing Co. $5.00.
Ball, Sir Robert. Great Astronomers. Philadelphia: J. B. Lippincott Co.,
1907. $1.50.
Ballantine, Stuart. Radio Telephony for Amateurs. Philadelphia: David
McKay Co. $2.00.
Bargg, William. The World of Sound. New York: E. P. Button. $2.00.
Bayley, W. S. Minerals and Rocks. New York: D. Appleton & Co., 1915.
$2.00.
Beard, Dan C. The American Boy's Handybook. New York: Charles
Scribner & Sons, 1914. $1.50.
. Boat-building and Boating. New York: Grosset, Dunlap & Co.,
1914. $0.50.
— . Handicraft for Outdoor Boys. New York: Grosset, Dunlap & Co.,
1915. $0.50.
Bond, Alexander R. The Scientific American Boy. New York: Munn &
Co., 1905. $1.50.
Brechner, C. H. Household Physics. Boston: Allyn & Bacon, 1919. $1.12.
Buckley, Arabella. A Short History of Natural Science. New York:
D. Appleton & Co. $2.00.
Burns, E. E. The Story of Great Inventions. New York: Harper Bros.
$1.25.
Butler, Joseph Green. Fifty Years of Iron and Steel. Youngstown, Ohio:
Printed by Author, 1920.
Chadwick, M. L. Pratt. Storyland of Stars. Chicago: Educational
Publishing Co., 1906. $0.50.
Cohn-Lassar, Dr. Chemistry in Daily Life. Philadelphia: J. B. Lippin-
cott Co., 1909. $1.50.
Collins, Archie F. Easy Lessons in Wireless. New York: Theo. Audel &
Co., 1915. $0.50.
Collins, Francis A. The Boy's Book of Model Aeroplanes. New York:
The Century Co., 1910. $1.20.
Crosby, W. O. Common Minerals and Rocks. Boston: D. C, Heath &
Co., 1881. $0.64.
352 OUR PHYSICAL WORLD
Desmond, Charles. Naval Architecture Simplified. New York: Rudder
Publishing Co., 1918. $5.00.
Duncan, R. K. Chemistry of Commerce. New York: Harper Bros., 1907.
$2.00.
. The New Knowledge. New York: Barnes, 1905. $2.00.
Estep, H. C. How Wooden Ships Are Built. Cleveland: Penton Pub-
lishing Co. $2.00.
Fairbanks, H. W. Stories of Rocks and Minerals. Chicago: Educational
Publishing Co., 1903. $0.60.
Fire-making Apparatus in the United States National Museum. Smith-
sonian Report, 1888.
Fisher, Sydney George. The True Benjamin Franklin. Philadelphia:
J. B. Lippincott Co., 1899. $2.50.
Forsythe, Robert. The Blast Furnace. New York: U.P.C. Book Co.
$4.00.
Fraprie, R. E. The Elements of Photography. Boston: American Pho-
tography Publishing Co. $1.00,
— . How to Make Lantern Slides. Boston : American Photography
Publishing Co. $1.00.
Geikie, Sir Archibald. Founders of Geology. 2d ed. New York: The
Macmillan Co., 1906. $4.00.
Gibson, Charles K. The Romance of Electricity. Philadelphia: J. B.
Lippincott & Co. $1.50.
Grant, Robert. History of Physical Astronomy. L. R. Baldwin, 1852;
L. H. G. Bohn.
Griffith, Alice M. The Stars and Their Stories. New York: Henry Holt
& Co., 1913. $1.25.
Hall, A. N. Handicraft for Handy Boys. Boston: Lothrop, Lee &
Shepherd, 1911. $2.00.
. Homemade Toys for Boys and Girls. Boston: Lothrop, Lee &
Shepherd, 1915. $1.35.
Henderson, W. J. Elements of Navigation. New York: Harper Bros.
$1.50.
Hendrick, Ellwood. Everyman's Chemistry. New York: Harper Bros.,
1917. $2.00.
Hobbs, W. H. Simple Directions for the Determination of the Common Min-
erals and Rocks. New York: The Macmillan Co., 1914. $0.25.
Hood, Christopher. Iron and Steel: Their Production and Manufacture.
("Pitman's Common Commodities of Commerce.") New York: Pit-
man, 1911. $0.75.
Hopkins, George M. Experimental Science. New York: Munn & Co.,
1906. $7.00.
BOOK LIST 353
Hubbard and Turner. The Boys1 Book of Aeroplanes. New York: F. A.
Stokes & Co., 1913. $1.75.
lies, George. Flame, Electricity, and the Camera. New York: Doubleday,
Page & Co., 1900. $2.00.
Jackson, Douglas C., and Jackson, John Price. An Elementary Book on
Electricity and Magnetism. New York: The Macmillan Co., 1919.
$1.90.
Johnson, G. F. Toys and Toy Making. New York: Longmans, Green
& Co., 1912. $1.00.
Johnson, J. E. Principles, Operation and Products of the Blast Furnace.
New York: McGraw-Hill Book Co. $5.00.
Johnson, V. E. Modern Inventions. New York: F. A. Stokes & Co.
$2-75-
Jones and Oberg. Iron and Steel. New York: Industrial Press. $2.50.
Kahjenberg, Louis, and Hart, Edwin B. Chemistry and Its Relations to
Daily Life. New York: The Macmillan Co., 1916. $0.93.
Kendall, L. F., and Kochler, R. P. Radio Simplified. Philadelphia: John
C. Winston Co. $1.00.
Lodge, Sir Oliver. Pioneers of Science. New York: The Macmillan Co.,
1904. $2.50.
Lynde, Carleton John. Physics of the Household. New York: The Mac-
millan Co., 1915. $1.00.
Markham, R. E. Steel, Its Selection, Annealing, Hardening, and Tempering.
New York: Norman W. Henley Publishing Co. $3.00.
Marvin, W. L. The American Merchant Marine. New York: Chas.
Scribner & Sons. $2.00.
Mason, Flora. Robert Boyle, A Biography. New York: E. P. Button & Co.
Mason, Otis T. The Origins of Inventions. New York: Chas. Scribner &
Sons, 1915.
Mayer, A. M. Sound. New York: D. Appleton & Co. $1.00.
Mayer and Barnard. Light. New York: D. Appleton & Co. $1.00.
Meloda, R. Chemistry of Photography. New York: The Macmillan Co.
$2.00.
Mills, John. Within the Atom. D. Van Nostrand & Co., 1921. $2.00.
Moldenke, G. G. Principles of Iron Founding. New York: McGraw-Hill
Co. $4.00.
. Production of Malleable Castings. Cleveland: Penton Publishing
Co. $3.00.
Newbigin, Marion I. Man and His Conquest of Nature. New York: The
Macmillan Co., 1912. $0.75.
Oberg, E. V., and Jones, F. D. Iron and Steel. New York: Industrial
Press, 1918. $2.50.
354 OUR PHYSICAL WORLD
Olcott, William T. A Field Book of the Stars. New York: G. P. Put-
nam's Sons, 1907. $1.00.
— . The Book of the Stars. New York: G. P. Putnam's Sons, 1923.
— . Star Lore of All the Ages. New York: G. P. Putnam's Sons,
1911. $3.50.
Philips, James C. The Romance of Modern Chemistry. London: Seeley Serv-
ice & Co. $1.25.
Pirrson, L. V. Rocks and Rock Minerals. New York: John Wiley & Sons,
1908. $2.50.
Porter, J. G. The Stars in Song and Legend. Boston: Ginn & Co., 1901.
$0.50.
Proctor, Richard A. Myths and Marvels of Astronomy. New York:
Longmans, Green & Co. $1.75.
. Stars in Their Season. New York: Longmans, Green & Co.,
1907. $2.00.
Roscoe, Sir H. E. John Dalton. New York: The Macmillan Co. $1.25.
Routledge, Robert. Discoveries and Inventions of the Nineteenth Century.
London: George Routledge & Sons, 1900.
Rowe, J. P. Practical Mineralogy, Simplified. New York: John Wiley &
Sons, 1911. $1.25.
St. John, Thomas M. Real Electric Toy Making for Boys. New York:
Thomas M. St. John, 1911. $1.00.
Serviss, Garrett P. Astronomy with the Naked Eye. New York: Harper
Bros., 1908. $1.40.
. Round the Year with the Stars. New York: Harper Bros., 1910.
$1.00.
Sloane, Thomas O. Electric Toy Making for Amateurs. New York:
Norman W. Henley Publishing Co., 1914. $1.00.
Slosson, Edwin E. Creative Chemistry. New York: The Century Co.,
1921. $2.00.
Snell, John Ferguson. Elementary Household Chemistry. New York:
The Macmillan Co., 1914. $1.25.
Spencer, L. J. World's Minerals. New York: F. A. Stokes & Co., 1916.
$2.75- '
Spring, L. W. Non-technical Chats on Iron and Steel. New York: F. A.
Stokes & Co. $2.50.
Thompson, S. P. Michael Faraday, His Life and Work. New York:
The Macmillan Co. $1.25.
Thorpe, T. E. Joseph Priestley. New York: E. P. Dutton & Co., 1906.
$1.00.
. Humphrey Davy, Poet and Philosopher. New York: The Mac-
millan Co. $1.25.
BOOK LIST 355
Thurston, Robert Henry. Century's Progress of the Steam Engine, 1799-
1901. Smithsonian.
Tilden, Sir William A. Progress of Scientific Chemistry in Our Own Times.
New York: Longmans, Green & Co., 1899. $1.50.
Venable, C. S. A Short History of Chemistry. Boston: D. C. Heath &
Co. $1.00.
Verrill, A. Hyatt. Harper's Wireless Book. New York: Harper Bros.,
1913. $1.50.
Vivian, Alfred. Everyday Chemistry. New York: American Book Co.,
1920. $1.64.
Walton, Thomas. Steel Ships: Their Construction and Maintenance.
Philadelphia: J. B. Lippincott Co. $7.00.
Weed, Henry T. Chemistry in the Home. New York: American Book Co.,
1915. $0.96.
White, Claude Grahame, and Harper, Harry. The Aeroplane. New York :
F. A. Stokes & Co., 1914. $2.75.
Williams, Archibald. The Romance of Modern Engineering. London:
Seeley, Service & Co., 1913. $2.50.
FIG. 194.— The planisphere (Part I)
MIDNIGHT
—
FIG. 195 —The planisphere (Part II)
To put the planisphere together paste Fig. 194 smoothly on a thin card and cut it
out. Do the same for Fig. 195, but after it is pasted on the card, with a sharp pen-
knife, cut out the ellipse from the card. Cut a second card circle the size of the circle
of Fig. 195 and mark its center. Run a pin through the center of Fig. 194 and through
the center of this circular card placed below Fig. 194. Lay Fig. 195 on Fig. 194, its
circular edge just inside the strip bearing the names of the months. Bend the four flaps
on Fig. 195 over the edge of Fig. 194 and paste them to the circular card below. Bend
the pin so that its end will lie down against the circular card back and hold it in place
by a piece of paper pasted over it.
INDEX
Aberration: chromatic, 301, 304, 305;
correction of, 303, 305; spherical, 301,
303
Accumulator, electric, 231
Acid, 176
Acids, naming of, 177
Ader, Clement, 91
Aeroplane, 78, 85, 90; balancing the,
93, 94; early history of, 85, 90, 94;
flights, 91, 94; height record, 95;
international meet, 95; mail service,
96; making model, 96; propeller, 99;
speed record, 96; transatlantic flight,
96
Agate, 54
Air: compression of, 144; conquest of,
77; moisture in, 155, 156; movements
of, 156; weight of, 112
Air column, vibration of, 330, 331
Air compressor, 144
Air pressure, demonstrating, in, 112
Alabastine, 56
Albite, 56
Alcohol, wood, 162
Alcor, 20, 31
Alcyone, 31
Aldebaran, 31, 34, 35
Algol, 25
Alpha rays, 173
Aluminium, 167, 170
Amalfi, 2 op
Amethyst, 54
Ammeter, 195, 227
Amperage, 227
Ampere, 227
Ampere, Andre, 207, 233
Ampere's law, 207
Amphibole, 50, 58, 61; characteristics
of, 58, 61
Amygdaloid, 65
Andesite, 66, 68, 70
Andromeda, 25, 26, 27
Anorthite, 56
Antares, 38, 39
Antenna of wireless, 252, 253, 256;
making, 265
Anthracite, 74
Antitrade winds, 156
Apatite, 50
Aquamarine, 51
Aquarius, 39, 41
Arc light, 247
Archer, 39
Archers, Royal Scottish, 136
Archery, 136
Archimedes, 115
Archytas, 85
Arcos, 21
Arcturus, 20
Argo Navis, 40, 41
Argon, 167, 170
Aries, 36
Arm of man, 342, 343
"Armada," Spanish, 118
Armature, 236
Arrow-maker, Indian, 135
Artemis, n
Asbestos, 59, 153
Ashtaroth, n
Asteroid, 2
Astrologer, 2, 39
Astrology, 39, 40
Astronomy, 40
Atlas, 31
Atmosphere: moisture in, 155, 156;
movements of, 156; pressure of, in,
112
Atom, 148, 149, 163, 165; nucleus of,
165-67
Atomic theory, 165, 168
Atoms, structure of, 165-71
Audio frequency, 255
Augite, characteristics of, 58, 61
Auriga, 29, 30
357
358
OUR PHYSICAL WORLD
Automobile, 191; tires, 145
Avion, 91, 92
Axe, 349
Azurite, 50
Bacquerel, 173
Balance, 340
Balloon, 78, 104, 105, 107, 109; dirigible,
108-10; first ascension, 104, 105;
history of, 104; hot-air, 104; hydro-
gen, 106; kite, 107; military, 106;
transatlantic flight of, 109; why it
rises, 109-16
Banjo, 328
Barometer, 112
Basalt, 63, 66, 69, 70
Base, 176
Battery: bichromate, 224, 230; Bunsen,
224, 230; current of, 222, 224, 226;
DanieU, 224; dry, 230; electric,
207, 222, 229; gravity, 224; opera-
tion of, 222, 224; polarization of,
223; poles of, 223, 224; storage,
192, 231, 232
Batteries: in series, 226; parallel, 226
Bear, Big, 20; legends of, 21
Bear, Little, 20, 21, 22
Bell, Alexander Graham, 218
Bell: electric, 221; vibrations of, 326,
327
Beryl, 51
Beryllium, 166, 167, 170
Besnier, 86
Bessemer converter, 162
Beta rays, 173
Betelgeuse, 18, 32, 34
Bichromate battery, 224, 230
Bicycle, 346
Biotite, 59
Biplane, 90
Bleriot, Louis, 95
Boat, 78, 117, 119, 133; floating of, 117;
history of, 119; motor, 119; records
of, 118, 119; sail, 117, 1 18; sailing of,
118
Boats, various kinds of, 119, 120, 121
Boiling-point, 172
Bootes, 20
Borax, 50
Bornite, 50
Boron, 166, 167, 170
Bow and arrow, 133, 134
Bow: ^how to shoot, 137; long, 134;
making, 137; of Eskimos, 134
Bowmen, 134; organizations of, 136
Breccia, 73
Bromine, 172
Bronze, 158
Bronze Age, 158
Bull, the, 30, 31, 32
Bullet, 141, 142
Bunsen battery, 224, 230
Burning, nature of, 149
Buzzer, electric, 222
Cable, transatlantic; 213, 215; com-
• pletion of, 215
Calcite, 45, 47, 48, 50, 61; character-
istics of, 55, 6 1
Calcium hydroxide, 176
Callisto, 20
Calms, belt of, 156
Calorie, 228, 229
Calumet and Hecla mine, 47, 127
Cam, 193, 194
Camera, 309, 310; back swing of, 310;
box, 309; Brownie, 309; film, 310,
315; focusing the, 310; Graflex,
315, 316; method of handling, 313;
obscura, 284, 285; pin hole, 283, 284,
309; reflecting, 315; timing device
of, 3H
Cancer, 174
Candle power, 283
Canis Major, 33, 34, 40
Cams Minor, 33, 34
Cannon: early, 143; location of, by
sound, 337
Canoe, 120
Capacity, 255, 258
Capella, 29
Capricornus, 40
Capstan, 343, 344
Carbon, 166, 167, 170
Carbon disulphide, 292
Carbon monoxide, 191
Carburetor, 192, 194, 195
INDEX
359
Carnotite, 174
Cassiopeia, 22, 23, 24, 27
Cassiterite, 50
Castor, 34, 35
Catapult, 138, 139
Cavello, 1 06
Cayley, Sir George, 87
Cello, 329
Centaur, 41, 42
Centrifugal force, 132, 133
Cepheus, 24, 25, 27
Cetus, 40, 41
Ceyx, 31
Chalcedony, 54
Chalcopyrite, 50, 61
Chalk, 56, 60
Chanute, 88
Charcoal, 159
Charioteer, 29, 30
Charlemagne's cart, 22
Chemical: change, 176; equation, 176,
177
Chimney, 151; why it draws, 152; why
it roars, 332
Chisel, 349
Chlorine, in, 167, 170, 172
Chlorite, 50, 58, 59, 60; characteristics
of, 59, 60
Christ, star at birth of, 18
Chromatic aberration, 301, 304, 305
Cinnibar, 50
Circumpolar stars, 19
Clarinet, 325, 330, 331
Clay, 59, 73
Cleavage, 48, 49
Clippers, American, 118
Clutch of automobile, 196, 197
Coal, 3, 73, 75; anthracite, 74; bitumi-
nous, 74; brown, 73; soft, 73; supply
of, 75; wastage of, 76
Cochina limestone, 71
Coffee mill, 343
Colors, primary, 307
Commutator, 236
Compass, 200; deviation of, 202; inven-
tion of, 200; needle and the electric
current, 207
Composition of forces, 80
Compounds, chemical, 166, 168
Compressed air, 138
Conchoidal fracture, 49
Condenser, 253; variable, 267
Conductors, 153; electric, 225, 226;
of heat, 153
Conglomerate, 73
Conjugate foci, 296
Constellations: circumpolar, 19, 24;
zodiacal, 35, 37, 39
Copper, 45, 47; conductor of heat, 153;
mines, 47
Coracle, 120
Coral beds, 71
Cornucopia, 30
Corona of sun, 5
Corundum, 50, 51
Corvus, 41
Crab, the, 36
Crank shaft, 193, 194
Cream separator, 133
Creosote, 162
Cross, northern, 28
Crossbow, 136
Crowbar, 340
Crystal detector, 257
Crystalline, 48
Crystals, 48
Curie, Madam, 173
Curtis, Glenn, 95
Cygnus, 17, 28, 29
Damien, Albert, 85
Damped waves, 255, 256
Darkroom, 316, 317; appliances, 317;
lamp, 317
David and Goliath, 131
Days: length of, 1 1 ; of the week, names
of, 13
De Bacqueville, 86
Decomposition of forces, 80
Definite proportion, law of, 168
Deneb, 28
Denebola, 36
Density: optical, 292, physical, 292
De Rozier, Pilatre, 105; death of, 106
360
OUR PHYSICAL WORLD
Derrick, 346
Detector, 256, 257, 264; crystal, 257;
vacuum tube, 268, 269
Developers, 316, 317; making up, 318
Developing, directions for, 319
Devil's Pile Quarry, 69
Diabase, 66, 69
Diamond, 50, 51, 247
Diana, n, 33, 34
Diaphragm, 303, 311, 314; openings,
sizes of, 311
Diorite, 66, 68; porphyry, 66, 69
Dipper, Big, 19, 20, 38, 39; Little, 22
Dispersion of light, 304
Distances, judging, 288
Distributor, 196
Dog days, 18
Dog star, 18
Dogs, Greater and Lesser, 33, 34
Dolerite, 66
Dolomite, 50, 56, 61
Dolphin, the, 40
Draco, 25, 26, 28, 29
Dragon, the, 25, 26, 28, 29
Drill: compressed air, 144; dentist's,
240; fire, 147
Dry battery, 230
Dugouts, 1 20
Du Moncel, 219
Dynamo, 195, 240; method of opera-
tion, 241, 242
Earth: axis of, 10, 15; crust of, 63;
equatorial bulge of, 15; North Pole
of, 10, 16; orbit of, 10; size of, 4, 7
Eccentric, 184
Echo, 328
Ecliptic, plane of, 9, 10
Edison, Thomas, 219, 246
Electric appliances: bell, 221; buzzer,
222; cream separator, 133; dynamo,
195, 240; flatiron, 228, 249; heater,
228, 249; light, arc, 247; light,
incandescent, 246; meter, 228; per-
colator, 247, 249; sewing machine,
239; telegraph, 211; toaster, 247,
249; transformer, 245; vacuum
cleaner, 239
Electric current: alternating, 243, 245;
cause of, 223, 224, 226; direct, 242;
direction of flow of, 223, 242; heat
equivalent, horse-power, equivalent
of, 228; long-distance transmission
of, 243; produced by moving magnet,
210
Electric motor, 233; commercial, 234;
directions for making, 233; explana-
tion of action of, 234, 237; simple,
233; toy, 236, 237
Electric: pressure, 225, 227; repulsion,
203, 208; resistance, 225, 229
Electric wiring of house, 247, 248
Electrical attraction and repulsion,
203, 208
Electricity: early knowledge of, 202;
frictional, 203, 204; galvanic, 205,
206; positive and negative, 204;
resinous, 204, vitreous, 204
Electromagnet, 209, 236; winding of,
238
Electron, 165
Elements: chemical, 145, 165; dis-
covery of, 173; names of, 170, 171,
176; nature of, 166; negative, 168,
169; positive, 168, 169; table of, 170;
transmutation of, 173
Elemus, 85
Ellipse, how to draw, 9
Elon-quinol, 317
Emerald, 51
Engines, 178
Equinoxes, 10, n; precession of, 15
Eridanus, 28, 40
Erosion, 52, 74
Ether, 250
Europa, 32
Expansion by heat, 116
Exposure, length of, 311, 313, 314, 322
Exposure meter, method of using, 312
Eye, structure of, 188, 305
Faraday, Michael, 210, 240
Farman, Henri, 94
Feldspar, 48, 50, 56, 62; characteristics
of, 56, 57, 62
Field, Cyrus W., 215
Fife, 331, 332
Film, photographic, 320, 322
Fish, the Southern, 40, 41
Fishes, the, 39
INDEX
361
Fire, 146
Fire drill, 147
Fire engine, 181
Fireplace, 151, 152
Fixer, acid, 318, 319
Fixing bath, 317, 318, 323
Fletcher of Rye, 119
Flint, 49; and steel, 144
Floating, explanation of, 116
Flood, the, 40, 42
Florida, coast of, 71
Fluid pressure, 113, 114; law of, 114
Fluorine, 166, 167, 169, 170, 172
Fluorite, 50
Flute, 333
"Flying Cloud," 118
Flying machines, early, 85
Flywheel, 183
Focal length of lens, 295, 311
Foci, conjugate, 295
Focus: of lens, 295, 310; of mirror, 291
Formalhaut, 40
Fossils, 71, 72
Fracture, 49
Franklin, Benjamin, 151, 204
Friction, 341, 344
Fulcrum, 340
Fulton, Robert, 189
Furnace, hot-air, 154, 155
Furnace, puddling, 162
Fuse box, 247
Gabbro, 66, 69
Galena, 48, 49, 50, 61
Galvani, 205
Galvanoscope, 208
Gamma rays, 173
Garnet, 51
Gas: elasticity of, 138; natural, 76;
nature of, in
Gasoline engine, 191, 192; parts of,
192, 193; working of, 192
Gear shift, 197
Gear wheels, 345
Gemini, 34, 35
Geode, 54
Giants' Causeway, 69
Gioja, Flavio, 200
Glacial bowlders, 70
Glider, 87-90
Gneiss, 74
Gnome engine, 95 »
Goat, the, 39
Gold, 45, 47
Governor, 183, 185
Graflex camera, 315, 316
Granite, 45, 50, 66, 67, 74; pegmatite,
66, 67; porphyritic, 66, 67
Gravity battery, 224
Gray, Elisha, 218
" Great Eastern," 69
Greenstone, 69
Grid, 269, 270
Ground glass, 310
Ground wire, 213, 256, 264
Guericke, Otto, 203
Gun: breech-loading, 140, 142; flint-
lock, 140, 141; locating by sound, 337
Gun barrel, grooving, 142
Gunpowder, 138, 139, 191; making, 139
Gypsum, 2, 51, 56, 60
Gyroscope, 133
Halcyone, 31
Halite, 50
Halogens, 172
Hammer, 342
Hardness, 50; scale of, 50
Hargrave, Lawrence, 79
Harp, 328
Harvester, 187
Head set, 258
Heat: conductivity of, 153; expansion
by, 116; latent, 164; sensible, 164
Heater, electric, 247, 249
Heating plant: hot water, 154; steam,
iS4
Helen of Troy, 35
Helicopter, 90
Helium, 166, 167
Hematite, 48, 49, 50
Henry, Joseph, 218
Herschel, Sir William, 2
Heterodyne, 272
High-school attendance, increase of, 188
362
OUR PHYSICAL WORLD
Horn, French, 325
Hornblende, 50, 58, 65; characteristics
of, 58, 61
Horse-power, 183, 191, 228
Hot- water heating, 154
House, wiring of, 247, 248
Humidity, 155
Humor: aqueous, 305; vitreous, 305
Hyades, 31
Hydra, 42
Hydrochinone, 317
Hydro-electric plants, 125, 126
Hydrogen: atom of, 165; discovery of,
1 06; molecule of, 163, 165
Hygrometer, 155
"Hypo," 318, 319
Hyposulphite of soda, 318, 319
Iceland spar, 55
Illinois, bed rock of, 71
Illumination: intensity of, 282; meas-
uring, 282
Image: in curved mirrors, 289, 290;
in plane mirror, 286, 287; with a lens,
296
Images, multiple, 289
Inca, 12
Incandescent lamp, invention of, 246
Inclined plane, 348
Inductance, 255, 258, 260
Induction, 210, 242
Induction coil, 195, 252
Inertia, 83, 132.
Injector, 186
Intensifying, directions for, 324
Interrupter, 245
Iodine, in, 172
Iron, 45, 159; burning of, 176; pig,
1 60
Iron furnace, 159
Iron, oxide, 176
Iris of eye, 303
Isis, ii
Joule, 228
Juno, 21
Jupiter, 2, 7,9, 11, 13,39
Kaleidoscope, 289
Kaolin, 50, 58, 59, 60; characteristics
of, 59, 60
Kettle drum, 325
Kids, the, 30
Kilns, charcoal, 159
Kilowatt-hour, 228, 229
Kite, 78; bird, directions for making,
82; bow, 84; box, 79, 84; bridle for,
81; invention of, 78; Franklin's,
204; tail of, 83; tetrahedral, 83, 84
Kites: explanation of flight, 79; flying,
78, 79, 84; games with, 84; map-
ping with, 79; weather observations
with, 78
Knife, 349
Krypton, 167, 170
Kyak, 120
Labradorite, 57
Langley, S. P., 92, 93
Lantern slides, directions for making, 323
Larynx, 336
Latham, Herbert, 95
Lavoisier, 150
Law of: Ampere, 207; Archimedes,
115; definite proportions, 168; elec-
tric pressure, 226; fluid pressure, 114;
induced electric current, 210; inten-
sity of illumination, 282; lever, 341,
342; light propagation, 281; light
reflection, 286; light refraction, 293,
294; machines, 340; Mendeleeff
(periodic), 168, 170; Oersted, 207;
pulleys, 347; screw, 349; vibrating air
columns, 331; vibrating strings, 329
Laws of nature, 281
Lead, 173
Leda, 35
Legends, Greek, 18, 19, 21, 27, 29, 31, 32,
33,34
Lemon squeezer, 343
Lens, 292, 295, 296; crystalline, 288,
305; focal length of, 257; focus of,
295; image formed by, 296; uni-
versal, 309
Lenses: grinding, 303; making, 298;
shapes of, 297
Leo, 36, 37
Levers, 339-42; kinds of, 343; law of,
34i, 342
INDEX
363
Leyden jar, 267
Libra, 38
Light: arc, 247; ^direction of propaga-
tion of, 281; dispersion of, 304, 306,
307; incandescent, 246; nature of,
305; reflection of, 285, 287, 291;
reflection, total, 295; refraction of,
285, 292, 306, experiment to show
292, laws of, 293, 294; speed of, 17,
travels in straight lines, 281; wave
theory of, 305
Lightning, 204, 205
"Lightning," record maker, 118
Lignite, 73
Lillienthal, 88, 93
Limestone, 45, 71, 74; characteristics
of, 71
Limonite, 49, 50, 61
Lion, the, 36, 37
Lithium, 166, 167, 170
Locomotive, 189
Lodestone, 199, 200
Loom, power, 186, 188
Luna, ii
Luster, 49
Lyre, 29
Machinery, labor saving, 186, 339
Machines, 339; law of, 340
MacReady, J. A., 95
Magdeburg spheres, in, 203
Magic lantern, 301
Magnesium, 167, 170, 172
Magnesium chloride, 168
Magnesium fluoride, 172
Magnesium oxide, 172
Magnet, 199-201, 209; electro-, 209,
238; lines of force of, 201, 202
Magnetic field, 202
Magnetic pole, 200, 202
Magnetism, 199, 209
Magnetite, 50, 200
Magneto, 192, 241
Magnification by concave mirror, 291,
292
Magnifying glass, 297
Malachite, 50
Man, primitive, 130, 131
Marble, 56, 74, 75
Mars, 2, 7, 8, n, 13, 39; inhabitants of,
8; polar regions of, 8
Match, invention of, 147
Matter, nature of, 148, 163
Maxim, Sir Henry, 91
Melting-point, 172
Mendeleeff, 168
Mercury, the metal, 45
Mercury, the planet, 2, 7, 8, 11, 13, 39
Metals, 172
Metamorphism, 74
Meztli, 12
Mica, 50, 58-60; characteristics of, 59, 60
Microphone transmitter, 219, 276
Microscope, 298, 300; construction of,
299; parts of, 300
Milky Way, 17
Mineral, 45, 47
Minerals: accessory, 50; anhydrous,
58; essential, 50; hydrous, 58;
primary, 58; secondary, 58; table of
distinguishing characters, 60
Mirror: concave, 286, 291; convex, 286,
290; cylindrical, 289, 291; focus of,
291; maze, 291; plane, 286
"Miss America II," 119
Mizar, 20
Modulator, 276
Molecular collisions, 164
Molecule, 148, 163, 164; movements of,
164; size of, 163; structure of, 165;
temperature and the, 164
Montgolfier brothers, 104
Month, 12
Moon, i, 12, 13; diameter of, 12; dis-
tance of , 12; light of, 12; man in, 12;
orbit of, 15; phases of, 12; woman in,
12; worship of, n
Moons, ii
Morse code, 214
Morse's telegraph, 212
Motor boat records, 119
Motor boats, 119
Motor car, 191
Motor, electric, 233, 234, 237
Muffler, 196
Muscovite, 59
364
OUR PHYSICAL WORLD
Musical instruments, 325
Musical scale, 333
Negative, photographic, 320, 321
Neon, 167, 170
Neptune, 2, 7, n
Newcomen's engine, 179, 180
Nights, length of, n
Niter, 51
Niton, 167, 171, 173
Nitrogen, 166, 167, 170
Noah, 40, 41
Non-metals, 172
North Pole: of earth, 10, 16; of magnet,
200
Nutcracker, 343
Obsidian, 64, 66, 67
Octaves, law of, 168
Oersted, Hans, 207
Oersted's law, 207
Ohm, 227
Oil, 73, 76; consumption of, 76; supply
of, 76
Oil gauge, 198
Oil shale, 73
Olivine, 50, 58, 59, 62; characteristics
of, 59
Onyx, 54
Opal, 54
Opera glasses, 302
Orchestra, homemade, 325
Ores, 50
Ores, iron, 50, 159
Organ, 325; pipes, 332
Orion, 1 8, 31, 33, 34
Orthoclase, 50, 56, 57, 65; characteris-
tics of, 57
Osage orange, 134
Oscillion, 270
Overtones, 335
Oxidation, 149, 176
Oxygen, 45, 47, 149, 164, 166, 167, 170;
discovery of, 150; generation of, 149;
molecules of, 164; properties of, 149
Pan's Pipes, 333
Paper, print, 320, 323
Papin, Denis, 178
Peat, 73
Pegasus, 25, 27, 39, 40
Penaud's toy bird, 90
Percolator, 247, 249
Percussion cap, 140
Peregrinus, 200
Periodic law, 168, 170
Peridotite, 66, 70
Perseus, 24, 25, 27
Phaethon, 28
Phillips, Horatio, 91
Phlogiston, 150
Phoebe, n
Phonograph, 336
Phosphorus, 167, 170
Photographic plate, 3, 310, 311, 315
Physical change, 176
Piano, tuning of, 333, 334
Picture, taking the, 313
Pig iron, 160
Pitch, musical, 332, 335; of a screw, 349
Plagioclase, 57, 65; characteristics of, 57
Plane, inclined, 348
Planetoid, 2
Planet, 2, 7, 13
Planets, orbits of, 8, 9; sizes of, 7
Planisphere, facing 356 and 357
Plate: holder, 310, 316; photographic,
311, 315; sensitivity of, 313, 324
Pleiades, 30, 31, 33, 35
Pliotron, 270
Pointers, pole star, 19, 20, 22, 24
Pole, North, 10, 16; star, 16, 20, 22
Polignac, Cardinal, 151
Pollux, 34, 35
Polonium, 173
Porphyry, 65
Potter, Humphrey, 101
Powder, 138, 139
Power arm, 341
Power plants, 125, 126
Pressure, electric, 225, 227
Pressure of air, in, 112
Pressure of water, 113, 124, 125
Priestly, Joseph, 150
Print, photographic, 321, 322
Print paper, 320, 323
INDEX
365
Printing, directions for, 320, 322
Prism, 304
Procyon, 34, 41
Proton, 165; size of, 165
Pulley, 346
Pulleys, law of, 347
Pumice, 64, 66, 67
Pump, 127, 128; air, 144; force, 128;
lift, 127, 129; making, 128
Pupil of eye, 303
Pyramid of Cheops, 16
Pyrite, 48, 50, 62
Pyrolusite, 50
Pyroxene, 50, 58, 6 1, 65; characteristics
of, 58, 61
Quartz, 44, 45, 47, 51, 53, 62; character-
istics of, 52, 62; rose, 54; smoky, 54;
solution of, 53; veins, 52
Quartzite, 51, 74
Radiator, 194
Radio-active substances, discovery of,
173
Radio: broadcasting, 251, 252, 254, 278;
frequency, 255, 268, 274; nature of,
250; receiving, 251, 256, 265; tele-
phone, 274, 276; transmitting station,
251,252,254; tuning coil, 260; waves:
continuous, 267, 275, speed of, 255;
wave-trains, 255
Radium, 5, 171, 173, 174
Radium paint, 174
Railroad, 189, 190
Rain, 157
Rainbow, 308
Ram, the, 35, 36
Raven, the, 41
Rays: alpha, 173; beta, 173, gamma,
173
Rectification, 257
Reducing, directions for, 324
Redwood, 127
Reflection, 285, 291; law of, 286, 287;
total, 295
Refraction, 285, 292; amount of, 293;
index of, 293; law of, 293, 294
Regulus, 36
Relays, 213
Resistance, 227
Resonance, 330, 336
Rhyolite, 66
Rigel, 31, 40
Rock, 45, 47
Rocks : crystalline, 64; formation of,
62; igneous, 62, 63, 64, 70; igneous,
table of, 66; metamorphic, 59, 74;
plutonic, 64; sedimentary, 59, 63, 70;
volcanic, 64
Ruby, 51, 54
Rusting, 149
Sailboat, 118
Sailboats, American, 118; ancient, 118;
records of, 118
St. Elmo's fire, 35
Sagittarius, 39
Salt, 165
Saltpeter, 50
Salts: color of, 172; naming of, 177;
solubility of, 172
Sand, formation of, 52; torpedo, 53
Sandstone, 45, 51, 63, 74; character-
istics of, 73
Santos-Dumont, 88, 94
Sapphire, 51, 54
Saturn, 2, 7, n, 13, 39; rings of, 9
Savery's engine, 179
Savery's steam pump, 179
Scale: intervals of, 333, 334; musical,
333
Scales, 340
Scales, the, 38
Scandium, discovery of, 173
Schist, 74
Schooling, days of, 188
Scissors, 343
Scorpion, 38, 39
Screw, 349; pitch of, 349
Seasons, change of, 1 1
Selene, n
Selenite, 49, 56, 217
Serpentine, 50, 58, 60, 61; characteris-
tics of, 60, 6 1
Seven Sisters, 30
Sewing-machine motor, 239
Shale, 73, 74
Siderite, 50
Silica, 65
366
OUR PHYSICAL WORLD
Silicon, 47, 167, 170
Silver, 45
Simon, the magician, 85
Sirius, 1 8, 34
Slag, 1 60
Slate, 74
Sling, 131; making, 131
Squawker, 331
Squirt gun, 128
Soapbubble, 109, 116
Sodium, 167, 169, 170
Sodium chloride, 168
Sodium fluoride, 169
Soil, 46
Solstices, summer and winter, 1 1
Sound, intensity of, 335; moves in
straight line, 326; nature of, 326;
pitch of, 332, 333; quality of, 330,
335; rate of propagation, 326, 327;
reflection of, 327, 328; waves of. 326
"Sovereign of the Seas," 118
Spar, Iceland, 55
Spark gap, 252, 253
Spark plug, 192, 193, 195
Spearheads, 133
Speed boats, 119
Sphalerite, 50, 61
Spherical aberration, 301, 303
Spica, 38
Spinning wheel, 186
Springs, hot, 53
Sprocket wheel, 346
Stars, i, 17; distance of, 17; magnitude
of, 1 6, 17; nature of, 17; nearest, 17;
number of, 16, 17; size of, 18
Star in the East, 18
Starter, electric, 198
Steam, pressure of, 186
Steam engine, effect of on industry,
186; governor of, 183, 185; history
of, 178; operation of, 184; and
schools, 1 88
Steam injector, 186
Steel, 162
Stephenson, George, 190
Stereopticon, 301, 303
Stone Age, 158
Storage battery, 192
Stove: early, 151; improvements in,
iSi
Stratification, 71, 74
Streak, 49
Stringfellow, 90
Strings: vibrating, 325, 328, laws of, 329
Sulphur, 45, 47, 51, 167, 170
Summer, n
Sun, i, 3, 13; corona of, 5; energy of, 3,
4; size of, 3; source of heat, 4;
storms on, 5; temperature of, 4
Swingback of camera, 314
Sympathetic vibrations, 251
Talc, 50, 58, 60; characteristics of, 60
Tanks, for developing, 320
Taurus, 31, 32
Teeter, 339, 341
Telegraph, 211, 212; photograph trans-
mitted by, 216; signatures trans-
mitted by, 216; wireless, 252
Telegraph Code, 213, 214
Telegraph of Morse, 212
Telegraph receiver, 212, 213
Telegraph sender, 212, 213
Telegraph of Wheatstone and Cook, 211
Telephone, 217, 337; construction of,
217; Edison's transmitter, 219; inven-
tion of, 217; radio, 274; receiver, 218;
switchboard, 220, 221; transmitter,
218, 219
Telescope, 300, 302; method of opera-
tion, 300
Temperature and molecular movements,
164
Thermos bottle, 153
Thunder, 205; storm, 165
Tides, causes of, 13
Timer of gas engine, 196
Top, 133
Topaz, 50, 51, 54
Trachyte, 66, 68, 70
Trade winds, 156
Transformer, 245, 267
Transmountain, 22
Transparencies, 323
Triangulum, 36
Triceps muscle, 342
Tuff, 66, 69
INDEX
367
Tuning coil, 260
Tuning in, 251, 259, 260
Turbine, 124
Turquoise, 51
Twins, the, 34, 35
Uranium, 168, 171, 173
Uranus, 2, 7, n; discovery of, 2
Ursa Major, 19, 20, 21, 22, 38
Ursa Minor, 22
Vacuum cleaner, 239
Vacuum tubes, 174, 268, 270, 271
Vacuum valve, 268
Valence, 65, 168, 169, 172, 176
Vega, 29, 39
Veins in rocks, 52
Vibrating column of air, 325, 330; laws
of, 33i
Vibrating strings, laws of, 329
Violin, 328, 329, 334
Virgin, 38
Virgo, 38
Vocal cords, 335
Voice modulations, 335
Voisin, 94
Volt, 227
Volta, Alessandro, 206
Voltage, 227, 244
Voltaic pile, 206
Voltammeter, 228
Volta's crown of cups, 206
Voltmeter, 228
Water-Bearer, 39, 40, 41
"Water, displacement of, 114, 155
Water power, 125; in the United States,
125-27
Water pressure, 113, 124, 125
Water wheel, 125
Watt, 228
Watt, James, 181
Watt's engine, 181, 182
Wave: compensating, 274; formation
of, 305
Wave-length, 255, 279
Wave motion, 250
Wave- train, 255
Waves, damped, 255, 256
Wealth, increase of, 186
Weapons, early, 131
Weather, prediction of, 157
Weather bureau, 157, 158
Weather map, 158
Weathering, 63
Week, 12
Weight arm, 341
Well, the deepest, 45
Wenham, Herbert, 90
Westerlies, 156
Whale, the, 40
Wheatstone and Cook's Telegraph, 211
Wheel and axle, 343, 344, 345
Wheelbarrow, 343
Wheel, sprocket, 346
Wheels, gear, 345
Williams, J. A., 96
Wind instruments, 330
Windlass, 343, 345
Windmill, 78, 121; directions for mak-
ing, 121, 122; paper, 121; wooden,
122
Windmills with sails, 123
Winds, 155; cause of, 155; local, 157;
trade, 156; westerlies, 156
Winter, n
Wireless, 250, 252, 254; making, 261;
receiving outfit, 256, 258; secondary
circuit, 260; transmission, 252, 254
Wiring of house, 247, 248
Woodsmen of Arden, 136
Wrench, 350
Wright brothers, 88, 92, 93, 95
Wringer, centrifugal, 133
Xenon, 167, 171, 239
X-ray, 173, 175
Zeppelin, Count von, 107
Zeppelins, see Balloon
Zero, absolute, 164
Zinc mine, 51
Zodiac, 35
Zodiacal constellations, 35, 37, 39
PRINTED IN THE U.S.A.
The University of Chicago
School Science Series
By ELLIOT R. DOWNING
Our Living World
A Source Book of Biological Nature-Study
A Field and Laboratory Quide in Biological Mature-
Study
Our Physical World
A Source Book of Physical Nature-Study
A Field and Laboratory Quide in Physical Nature-
Study
A Naturalist in the Qreat Lakes Region
Teaching Science in the Schools
By W. L. EIKENBERRY
The Teaching of Qeneral Science
THE UNIVERSITY OF CHICAGO PRESS
CHICAGO , ILLINOIS
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