MAGNETISM.
397. =Loadstones.=--It was discovered many centuries ago that a
certain ore of iron has the property of attracting pieces of common
iron or of steel. The fact was probably considered at first as a
mere curiosity, and the world were slow to find out its value. It
is not till quite recently that it has been discovered that in
magnetism we have one of the great forces of the earth; and even
now we know but little probably of the real extent and variety of
its action. New and important discoveries are yet undoubtedly to be
made in regard to the agency and the laws of this mysterious power,
and its connections with the other grand forces of nature. The
terms magnet and magnetism come from the fact that the loadstone
was first found near Magnesia, an ancient city in Asia Minor. This
ore appears in considerable masses in the iron mines of Sweden and
Norway, and also in different parts of Arabia, China, and Siam. It
has occasionally been found in small quantities in England and in
this country.
398. =Attraction of Magnetism.=--The attraction of the magnet
and iron for each other is exhibited in many different ways. If a
magnet be brought near to a heap of iron-filings or needles, it
will have a quantity of them adhering to it as you raise it up. In
the toy fishes of children there is fastened in the head a bit of
iron, which occasions the following of the fishes after the magnet.
In this case you can see very plainly that the nearer the magnet
and the iron are to each other the stronger is the attraction.
Indeed, the attractive influence is governed by the same law in
regard to distance as the common attraction of matter is, viz., it
is inversely as the square of the distance. The attraction also is
mutual here, the iron attracting the magnet as much as the magnet
does the iron.
[Illustration: Fig. 279.]
[Illustration: Fig. 280.]
399. =Poles of the Magnet.=--Every magnet has two poles. It is
about these poles where the chief power resides. For this reason,
if a magnet be rolled in iron-filings, these are collected about
the ends, as represented in Fig. 279. There is a diminution of
attraction from the ends to the middle line, which is called the
_neutral line_. These poles are called north and south poles,
because if a magnet be suspended, or be supported upon a pivot, so
that it can revolve, it will take a north and south direction, one
of its ends invariably pointing toward the north. In Fig. 280 is
represented a magnet supported upon a pivot, C.
[Illustration: Fig. 281.]
400. =Magnetism by Induction.=--The magnet in exerting its
attraction really temporarily makes a magnet of what it attracts.
Actual contact is not necessary to this result. Thus if a large
key be only brought very near to a powerful magnet it will support
small keys, as represented in Fig. 281. When the key is removed
away from the magnet the keys attached to it fall. You see the
analogy to the induction of electricity noticed in § 379. As in
the induction of electricity, so here the two ends of the body in
which the influence is induced are in opposite states. If the end
of the magnet, to which the first key is near or attached, be the
north pole, the end of the key next to the magnet is the south
pole, and its farther end is the north pole. The same is the case
with the small key attached to the end of the large one. And so
if a nail should hang from the small key, and a needle from that,
both of these would have the same polarities. But all this would be
reversed if the large key were attached to the south pole of the
magnet. In this case the upper end of each of these articles would
be the north pole, and its lower end the south pole.
401. =Attraction and Repulsion in Magnets.=--You have seen in
induction that in magnets _like poles repel while unlike attract_.
But this law can be more strikingly illustrated. If a magnet be
placed on a pivot, as in Fig. 280, and another magnet be brought
near it, attraction or repulsion will be manifested according to
the mode of presentation. If a north pole be presented to a north
pole, or a south to a south, repulsion will be the result. But if
a north pole be presented to a south, or a south to a north, then
attraction will be manifested.
[Illustration: Fig. 282.]
402. =Magnetic Curves.=--The polarity of magnetism causes a very
singular arrangement of iron filings when gently agitated upon a
sheet of paper over a magnet, as represented in Figure 282. The
curves which you see have been supposed by some to be occasioned
by the escape of some fluid or influence from the magnet in these
particular directions. But they are owing entirely to the fact that
each bit of filing is polarized by the bit next preceding it in the
row reckoning from the magnet outward, the nearest one in each row
deriving its magnetic state from the magnet itself. This being so,
as the chief power resides in the ends of the magnet, it is easy
to see how such a disposition of the lines of magnetic filings is
effected. These curves may be beautifully and curiously varied by
having several magnets variously arranged under the paper.
403. =Artificial Magnets.=--The power residing in the loadstone
can be communicated readily, as you have seen, to iron and steel.
Though soft iron takes the magnetic influence more readily than
steel, it does not retain it as steel does, and the latter is
therefore used in making artificial magnets. When a magnet imparts
its magnetic influence it loses none of its own power, whether it
be an original loadstone or an artificial magnet. There, are many
ways of imparting magnetism permanently to steel, but I will notice
only two of them. If you wish to magnetize a bar or needle pass one
pole of a magnet from one end of it to the other a considerable
number of times, always in the same direction. A more effectual
way is to take two magnets, and, placing the south pole of one and
the north pole of the other in contact over the middle of the bar
or needle, draw them slowly and steadily apart toward the opposite
ends. This process must be repeated several times.
[Illustration: Fig. 283.]
[Illustration: Fig. 284.]
404. =Horseshoe Magnets.=--One of the most common forms of the
magnet is the horseshoe magnet, Fig. 283. There is a piece of soft
iron attached to the end of this, held there by attraction. This is
called the _armature_. So long as it is suffered to remain there
it is a magnet having its _two_ poles, the north pole + being
attached to the south pole - of the magnet which holds it, while
the reverse is the case with its south pole. The object of the
armature is to preserve the power of the instrument. Indeed it is
found that the exertion of the magnet's power not only preserves
but actually increases it. If you attach, therefore, to a magnet
an armature having a hook, as seen in Fig. 284, you can add to the
weight gradually from day to day, and so considerably augment the
power of the magnet.
405. =Magnetic Needle.=--The magnetic needle is a very small magnet
fixed upon a pivot. As it points north and south it is of great use
to the mariner. The mariner's compass is a round box with such a
needle balanced in it, and having a card on which is drawn a circle
divided into thirty-two parts, as seen in Fig. 285. The original
compass was a rude affair, consisting of a slip of loadstone laid
upon a piece of cork floating in water. The date and place of its
first use are unknown.
[Illustration: Fig. 285.]
406. =Declination of the Needle.=--The declination of the needle is
its deviation from a north and south line. It is in comparatively
few parts of the earth's surface that there is no deviation from
this line to the east or the west. "True as needle to the pole" has
become a proverb, and when it was first uttered it was supposed
to be founded in strict truth; but modern investigation has shown
not only that the needle varies in its pointing in different
localities, but that it varies to some little degree in its
variations. The declination of the needle was first observed by
Columbus in his first voyage of discovery, and it occasioned great
alarm among the sailors, who, as Irving states, "thought the laws
of nature were changing, and that the compass was about to lose its
mysterious power." Notwithstanding these and other observations of
a similar character, no great account was made of the declination
of the needle till the middle of the seventeenth century. But
since that time extensive records of its declinations at different
localities have been made, and tables and charts have been
constructed exhibiting them. These declinations are not constant,
but vary somewhat every day, from the influence, it is supposed, of
the sun upon the earth.
407. =Dip of the Needle.=--It is found that in most parts of the
earth, if a needle be balanced before it is magnetized, and then
be suspended from the same point, it will not be balanced, but
one end will dip downward. This fact was discovered by Norman,
a London optician, in 1576. He found that the dip at London was
toward the north at an angle of 72°. In pursuing the investigation
of this phenomenon it was found that going from the north toward
the equator the dip constantly lessened, until a point was reached
where the needle was horizontal. Then, on going south of this, a
reverse dip occurred, that of the south pole, and the farther
south the needle was carried the greater was the dip. In the north,
Captain Ross in 1832 came to a locality north of Hudson's Bay, in
lat. 70° 5′ N., long. 96° 45′ W., where the magnetic needle, freely
suspended, was in a vertical line. No such locality has yet been
discovered toward the south pole.
408. =The Earth a Magnet.=--You can readily see, from all that has
been stated in regard to the magnetic needle, that the earth is
a magnet, or has that covered up in it which in some way acts as
such. The dip of the needle shows that the two poles of this magnet
are somewhere near the north and south poles of the earth. The
locality which Captain Ross found must be near the north pole of
the magnet in that quarter of the world. The vertical position of
the needle there is analogous to the straight lines of iron filings
which you see in Fig. 282, near the poles of the magnet; and it is
easy also to trace the analogy between the dip of the needle at
different distances from what is called the magnetic equator of
the earth, where the needle is horizontal, and the curves which
you see extending from pole to pole. The different declinations
of the needle and the different intensities of the magnetic force
in different localities corresponding in latitude show that the
magnet in the earth, if there be one, is irregular in shape, or in
some way has its power varied much in differed parts of the earth's
crust.
409. =The Earth as a Magnetizer.=--As the earth is really a magnet,
it might be expected to impart magnetism by induction as other
magnets do. And this is found to be the fact. If you hold a bar
of soft iron in the direction of the dip of the needle it becomes
a magnet, its lower end being the north pole, and its upper the
south. That this is so can be ascertained by bringing a small
magnetic needle near each end. No effect of this kind is produced
when the bar is held horizontally east and west. Lightning-rods,
pokers, upright iron bars in fences, etc., are often found to
be magnetized because they have continued so long nearly in the
required position for magnetization. When a bar of iron has been
magnetized in the manner indicated, its magnetism may sometimes
be fixed by giving it a stroke with a hammer. It is a curious but
inexplicable fact that this vibration of the particles of the iron
should have this effect. But though such vibration helps to impart
magnetism, it is not at all favorable to its retention, for magnets
are always injured by blows or falls, or indeed any rude treatment.
For this reason care is requisite in removing an armature from a
magnet. If pulled off abruptly the power of the magnet is lessened.
410. =Magnetism in Other Substances besides Iron.=--It was formerly
supposed that magnetism was confined to ferruginous substances, but
this has been found not to be true. Various minerals are magnetic,
especially when they have been heated, also some of the precious
stones, and even silica, which enters so largely into some of
the rocks of the earth. And it is supposed by some that future
investigations will show that the influence of magnetism is as
extensive in the earth as that of electricity.
411. =In what Magnetism is Like Electricity.=--Magnetism is like
electricity in several particulars: 1. Its power is on the surface
of bodies. 2. It is of two kinds, north and south, or boreal and
austral, comparing with the positive and negative electricities.
3. The same rule of attraction and repulsion applies to both;
viz., like repel and unlike attract. 4. As electricity can be
communicated by induction, so can magnetism.
412. =In what Magnetism is Unlike Electricity.=--The circumstances
in which magnetism is unlike electricity are chiefly these: 1.
The obvious manifestations of magnetism are to a great extent
confined to one class of substances, the ferruginous, and to but
a portion of them; while electricity makes its manifestations in
connection with all kinds of substances. 2. Magnetism is never
transferred as electricity is from one body to another, but a
body gains rather than loses in imparting magnetic power to other
bodies. 3. The two magnetisms, the boreal and austral, can not be
obtained separately as the two electricities can. If a magnet be
broken in two, each piece will have in it the two magnetisms and
the two poles as the whole did. This is in entire contrast with
the electrical experiment noticed in the last of § 379. 4. There
are no non-conductors to interrupt magnetic influence. If in the
experiments in § 379 a plate of glass or resin were interposed
between A and B, the influence would cease, but it would have no
effect on the induction of magnetism if interposed between a magnet
and a bit of steel or iron.
413. =Electro-Magnetism.=--Though electricity and magnetism differ
so much from each other, yet they have intimate relations, and
it is now the general opinion among scientific men that they are
merely different modes of the same power. Magnetism can produce
electricity, and electricity can produce magnetism. The first
discovery of facts revealing this connection was made by Professor
Oersted of Copenhagen in 1819. Since that time electro-magnetism,
or the production of magnetism by electricity, has been a prominent
subject of observation and experiment. Oersted's first observation
was that a current of electricity passing over a wire near a
magnetic needle affected the position of the needle. He found also
that iron filings would adhere to a wire over which a current of
electricity is passing, just as they do to a magnet, dropping off,
however, as soon as the current ceases to pass. Such facts led to
a great variety of investigations and arrangements of apparatus by
Oersted and others.
[Illustration: Fig. 286.]
414. =Electro-Magnets.=--The most powerful electro-magnets are made
by bending a thick cylindrical bar of soft iron into the form of
a horseshoe, A B, Fig. 286, and coiling around it a copper wire.
The wire must be insulated by being wound with some non-conducting
material, as silk, so that the electric current may pass through
the whole length of the wire. With the instrument thus prepared, if
the two ends of the wire be connected with the poles of a voltaic
battery which is in action, the bar will be magnetized, and will
hold up a heavy weight so long as the electric current is passing
through the wire. Whenever the current is cut off by disconnecting
the wires the weight will fall.
[Illustration: Fig. 287.]
Electro-magnets have been made in this way having such power as
to sustain a weight of four thousand pounds. In Fig. 287 we have
represented an apparatus which exhibits electro-magnetism very
prettily. The soft iron, you see, is in two pieces which when
put together form a ring, _d_, and each piece has a handle. If
the pieces be put together with the coil, _c_, in the position
represented, on connecting the wires P and N with a battery in
action, the adhesion is so strong as to resist a great force; but
as soon as the connection is broken the pieces come apart at once.
[Illustration: Fig. 288.]
415. =Electric Telegraph.=--The most remarkable and useful
application of electro-magnetism we have in the electric telegraph.
As before stated, voltaic electricity is used. This is generated at
the place from which the message is sent, and passes over the wire
to the place where the message is received. There it acts upon soft
iron by passing through coiled wire, producing the modified power
called electro-magnetism. I will make all this plain to you by
describing the machine used in Morse's Telegraph, Fig. 288. W W are
the wires which connect with the station from which the message is
to be received, and these connect with the copper wire coiled round
the horseshoe of soft iron, _m m_. Above the magnet is a lever,
_a l_, which works on a fulcrum at _d_. One end of this lever has
a steel point, _s_, attached to it. At _c_ is an arrangement of
wheel-work, the object of which is to pass along regularly a slip
of paper, _p_, in the direction of the arrows. Observe now how
the apparatus works. When the electric current passes through the
coiled copper wire it makes a magnet of the iron, _m m_. The lever,
_a l_, is therefore attracted at the end, _a_, downward. Of course
the end, _l_, moves upward, bringing the steel point, _s_, against
the paper, where it makes a mark. The length of this mark depends
upon the length of time the electricity is allowed to pass along
the coiled wire, for the moment that it is shut off _m m_ ceases to
be magnetic, the "keeper," _a_, being no longer attracted, moves
upward, and the other end, _l_, of the lever moves downward, taking
the point, _s_, from the paper.
[Illustration: Fig. 289.]
In order to make the marks on the paper of different lengths,
there is a contrivance for regulating the length of time that the
current shall pass through the coiled wire. This contrivance,
called the _signal key_ is represented in Fig. 289. N and P are two
strips of brass connected with the two wires R and M, of which M
comes from the battery. The end of the strip N is raised a little
above the end of P. So long as they do not touch the circuit is not
complete, and no electricity passes. But if the operator press N
down upon P, the circuit is established, and the electricity passes
to the station with which he is in communication, and there acts
upon the apparatus seen in Fig. 287. Now the longer the finger
presses down N upon P the longer will be the mark on the paper at
the distant station. An operator then at New York, for example,
controls by this key the length of the marks made on the paper in
New Haven or any other place with which he is communicating.
You can see then very readily how a telegraphic alphabet can be
constructed by combinations of marks of different lengths agreed
upon to represent different letters and numerals. I give the
alphabet used in connection with Morse's Telegraph:
A -- ----
B ---- -- -- --
C -- -- --
D ---- -- --
E --
F -- ---- --
G ---- ---- --
H -- -- -- --
I -- --
J ---- -- ---- --
K ---- -- ----
L --------
M ---- ----
N ---- --
O -- --
P -- -- -- -- --
Q -- -- ---- --
R -- -- --
S -- -- --
T ----
U -- -- ----
V -- -- -- ----
W -- ---- ----
X -- ---- -- --
Y -- -- -- --
Z -- -- -- --
Numerals.
1 -- ---- ---- --
2 -- -- ---- -- --
3 -- -- -- ---- --
4 -- -- -- -- ----
5 ---- ---- ----
6 -- -- -- -- -- --
7 ---- ---- -- --
8 ---- -- -- -- --
9 ---- -- -- ----
0 ---------------
One of the most singular and interesting things in the operation
of the telegraph remains to be noticed. In order to have the
electricity work it is necessary to have the connection between
the poles of the battery at the point where the effect is to be
produced. You see this in the experiments represented in Figs. 286
and 287. The same is true of the electro-magnet of the telegraph.
This being so, it was thought at first that it was necessary to
have two wires connecting two communicating stations; but it was
found that only one wire was needed, the earth itself answering the
same purpose as another wire. To make the communication through the
earth effectual, there is at each station a plate of metal, having
a surface of several square feet, buried in the ground, with a wire
running up to the machine.
QUESTIONS.
[Teachers differ much in their plans of conducting recitations.
Some are very minute in their questions; while others go to the
other extreme, and merely name the topics, the pupils being
expected to give in full what is said upon them. Neither of these
plans should be adopted exclusively, but the mode of recitation
should be much varied from time to time. This variety is somewhat
aimed at in the questions which I have prepared, though in no
case are the questions as minute as they should occasionally be
made by the teacher. The numbers refer to the pages.
It would be well to have the pupils draw many of the figures
upon the blackboard, and then recite from them. By drawing the
simplest figures first sufficient skill may be acquired to enable
the pupil to draw those which are quite difficult.]
CHAPTER I.
13. What is said of the distinction between matter and spirit? What
of Bishop Berkeley's ideas? What of Hume's?
14. What is the origin of the word spirit? What is the relation of
the senses to the spirit? What is said of the effects of matter on
the senses? What are the forms of matter?
15. Illustrate the difference between elastic and non-elastic
fluids. What is said of the union of the particles of a solid? Give
the difference noted between different solids. How does a liquid
differ from a solid? Give particularly what is said of water.
16. What is said of the particles of gaseous substances? What
of the atmosphere? What of the vapor in it? What is said of the
entering of liquids and gases into interstices? What of the
mingling of gases with liquids? Give the illustration in regard to
fishes.
17. What is said of the solution of solids in liquids? What of the
evaporation of water in the air? Illustrate the influence of heat
on the forms of matter. What is said of the thermometer? What of
mercury, water, and iron in relation to the liquid state? What is
said of our knowledge of matter?
18. What was the supposition of Newton about the composition
of matter? What is said of the changes of matter? What are the
imponderable agents, and why are they so called?
CHAPTER II.
19. What is said of variety in the properties of matter? What of
the divisibility of matter?
20. What is said of gold-leaf, and of the wire of gold-lace? What
of the soap-bubble? What of the thread of the silk-worm, and of the
web of the spider? What of solution of blue vitriol? What of odors?
21. What is said of the dust of the puff-ball? What of pollen? What
of the dust rubbed from a moth's wing? What of guano? What of the
glazing of visiting-cards? What of the minuteness of some animals?
22. What is said of the Deity in relation to minute animals? What
is said of substances called porous? What of those in which there
are no pores apparent? What proof is there that all substances have
spaces in them?
23. What is said of the amount of space in gases and vapors? Give
the statement in regard to steam. What is said of solutions of
solids in fluids? What of evaporation? What of the diffusion of
odors in the air?
24. What is the relation of heat to space in matter? Upon what do
density and rarity depend? Explain tenacity. What is said of this
property in gases and liquids?
25. How is the comparative tenacity of substances ascertained?
Give the comparative tenacity of various substances. What animal
substances have great tenacity? What is said of the value of
tenacious substances?
26. What is said of hardness? What of flexibility and brittleness?
27. Give examples of flexible and brittle steel. Explain the actual
difference between them. Explain the tempering of steel.
28. What is said of the annealing of glass? What of Prince Rupert's
drops? What metals are the most malleable? What the most ductile?
29. What is said of the ductility of melted glass? What of the
change of position of particles in making plates and wires of
metals? What of welding? What is said of compressibility? What of
the incompressiblity of liquids?
30. What influence has heat on the bulk of liquids? Illustrate by
the thermometer. What is said of the compressibility of gases? How
does elasticity operate in the case of the India-rubber ball?
31. Give the illustration in regard to jumping. State the
experiment with the ivory ball. What is said of the movements of
particles on each other in elastic substances? What of the degrees
of elasticity in different substances?
32. What is the definition of elasticity? What is said of the
usefulness of the variety of properties in matter?
CHAPTER III.
33. What is meant by extension as a property of matter? Illustrate
the fact that it is an _essential_ quality. What is said of it in
reference to air?
34. Does matter ever penetrate matter? Give the illustration
represented in Fig. 6. Give that represented in Fig. 7.
35. State the arrangement of the diving-bell. Give the comparison
between bullets and needles in relation to penetration. What is
said of solution? What of odors?
36. What is the property of matter called inertia? Give
illustrations of it. Illustrate the fact that matter has no power
to stop its own motion. What is the reason that the popular notion
is that matter is more inclined to rest than to motion?
37. What is said of perpetual motion? Why is it not true of
divisibility that it is an essential property of matter? What is
said of weight?
CHAPTER IV.
38. What is said of the nature of attraction? What was Newton's
idea of it?
39. What is said of attraction in solids? What of its different
modes of action? What is the difference in attraction in the case
of steel and of water? What is said of the freeness with which
particles of a fluid move among each other?
40. Explain Fig. 9. Explain Fig. 10.
41. Give the difference between mercury and water in regard to the
globular form. What if said of drops of water on leaves?
42. What is said of oil in reference to attraction? Describe and
explain the manufacture of shot. What is said of the globular form
of the earth and the heavenly bodies?
43. What is said of crystallization? State the examples cited.
What is said of the crystallization of water? Give and explain the
example of sudden crystallization.
44. What is said of frost-work? What of snow?
45. What is stated in regard to the snow-crystals of the arctic
regions? What is said of order in nature? Why can you not make the
surfaces of broken glass adhere?
46. Explain the cementing of glass. What is said of the adhesion of
pieces of India-rubber? Describe and explain the experiment with
bullets and with balls of lead. How may silver and gold be made to
adhere to iron? What is said of the adhesion of tin and lead? What
of the adhesion of panes of glass?
47. Upon what does the strength of adhesion depend? Illustrate
the agency of heat in promoting adhesion. Give familiar examples
of attraction between solids and liquids. Explain the experiment
represented in Fig. 15.
48. What is said of stems in stagnant water? Explain Figs. 16, 17,
and 18.
49. Explain Fig. 19. Explain the rise of fluids in tubes by Fig.
20.
50. What is meant by _capillary_ attraction? Give familiar examples
of the rising of liquids in interstices.
51. Describe and explain the process of getting out millstones. How
does a blotter differ from writing-paper?
CHAPTER V.
51. What is the attraction of _cohesion_? Give examples of
attraction between masses or portions of matter.
52. Explain the falling of a stone to the ground. Illustrate the
fact that attraction is mutual. Give the illustration of the ship
and boat in full.
53. Illustrate the proportion between the mutual motions of the
attracting bodies. Give the calculation in regard to the motion of
the earth in attracting smaller bodies.
54. What is said of the universality of attraction? Explain the
tides. What is said of the attraction of the moon for the land?
What is the difference between the attraction of cohesion and the
attraction of gravitation? Why is the word gravitation thus used?
What is terrestrial gravitation?
55. Explain Fig. 22. Explain Fig. 23. What is said of substances
suspended in different parts of the earth?
56. Explain Fig. 24. What is said of plumb-lines?
57. What is weight? Give the comparison in regard to muscular
force. What is said of scales and weights? What of using springs in
weighing?
58. What would be the effect on weight if the density of the earth
were increased? In what ways would this be perceived? What is said
of the variation of weight with distance?
59. What is said of the difference of weight on mountains and in
valleys? What of weight in the moon? What of it in the sun? What is
said of the different modes of attraction?
60. Show why attraction of cohesion _seems_ to be different from
gravitation. Show now that it is really not different. What is said
of the experiment with the two bullets mentioned in § 66? What of
the adhesion of liquids to solid substances?
61. What is said of the various results of attraction? Explain
fully why you can pour water from a pitcher easier than from a
tumbler.
62. Explain the operation of the quick movement by which you
prevent water from running down the side of a tumbler in pouring it
out. What is said of dropping from a vial? How is the size of drops
limited? What is said of the movements of drops on window panes?
63. Why are the drops of different liquids different in size? Give
the illustration about chalk. Give that about dust. Explain Figs.
27 and 28.
64. Explain Fig. 29. What is said of the difference in size between
water and land animals?
65. Give the illustration in regard to trees. Give that in regard
to mountains. What is said of the mountains of the moon? What of
those of Jupiter? Give the illustrations in § 93 of transgression
of the principles which have been elucidated.
66. What is the difference between the attraction treated of in
natural philosophy and chemical attraction?
CHAPTER VI.
67. Show what we mean by the centre of gravity by Figs. 30, 31, and
32.
68. Give the definition of centre of gravity, and explain it. What
is shown by Fig. 33?
69. How can we find the centre of gravity of a body? What is said
of scales and steelyards?
70. State what is represented by Fig. 38. Illustrate the fact that
the centre of gravity seeks always the lowest point.
71. Give the illustrations of the rocking-horse, the swing, etc.
What is said of the Laggan stones? Why does an egg lie on its side?
72. Give the illustrations from toys in § 101. Give the
illustrations in § 102.
73. Upon what two things does the stability of a body depend?
74. What is said of the stability of bodies whose shapes are
represented in Figs. 48, 49, and 50? What of that of a round ball?
Why is the pyramid the firmest of all structures?
75. What is the relation of upright position to stability? What is
stated of the tower of Pisa?
76. Give the familiar illustrations in § 105. What is said of the
support of the centre of gravity in animals?
77. What is said of the skill exercised in walking? What of the
mode of walking in a child? What of the motions of the centre of
gravity in walking?
78. What is said of the walking of a man with wooden legs?
Illustrate the management of the centre of gravity in different
attitudes. Describe and explain the way in which one rises from a
chair.
79. State and explain the wager case. What is said of unstable
equilibrium? Give illustrations.
CHAPTER VII.
80. What is said of the phenomena treated of in Hydrostatics? What
are the two characteristics of liquids? What makes a liquid have
a level surface? Give the explanation. Give the comparison of the
shot.
81. What is said of water as a mirror? Show that the surface of a
liquid is not strictly level. If the earth had no elevations of
land why would it have a perfectly globular covering of water?
82. What is a so-called perfectly level surface? What is the
variation per mile from a real level? Describe the spirit-level.
Give the comparison between a trough and a river.
83. What is said of the declivity of rivers? How have some rivers
been made? What is stated in regard to the River Danube?
84. What is stated about the Lake of Geneva? Describe the
arrangement of canal locks.
85. How are canals used for working machinery? Give various
illustrations of the tendency of water to be on a level.
86. Describe the arrangement represented in Fig. 71, and give the
explanation.
87. Describe a foolish man's plan for perpetual motion, and give
the reason of its failure. What is said of ancient and modern
aqueducts?
88. Explain the operation of springs and Artesian wells.
89. Whence comes the name Artesian? What is stated of a well in
Paris? What of the situation of London? Why is the pressure of a
liquid in proportion to its depth? Give the illustrations of this
mentioned in § 122.
90. Explain Fig. 75.
91. What is said about the construction of dams and brewers' vats?
Explain the lateral pressure of liquids. Show the difference
between a liquid and a solid in this respect.
92. Show how the earth's attraction causes the lateral pressure by
Figs. 77 and 78. Give the view presented in § 124.
93. What is said of the proposed ship canal between the
Mediterranean and the Red Sea? Show that pressure in liquids is
equal in all directions.
94. Give the illustrations in § 126. Show that the upward pressure
in a liquid is as the depth, and that this is produced by
gravitation.
95. State the experiment represented in Fig. 82. Give the
experiment with the tube and India-rubber.
96. State the examples given of great effects produced by small
quantities of a fluid. Explain these effects by Fig. 83.
97. Explain Fig. 84.
98. What is the Hydrostatic Paradox, and why is it so called?
Describe and explain the Hydrostatic Bellows.
99. Describe and explain Bramah's Hydrostatic Press.
CHAPTER VIII.
100. Define specific gravity.
101. What is the most obvious way of ascertaining the specific
gravities of different liquids? Explain the sinking of heavy
substances in water. Explain the rising of light substances in
water. Explain what is illustrated in Fig. 87.
102. Explain Fig. 88. Explain Fig. 89.
103. Give the illustrations in § 138: lifting a stone; raising
a bucket; and raising the arm in a bath. Relate the anecdote
of Archimedes. What is said of boats and life-boats? What of
estimating the weight of the load in a canal-boat?
104. What is said of the specific gravity of birds? Of insects? Of
fishes? What of the specific gravity of the human body, and of the
prevention of drowning?
105. Give the reasons why so many are drowned that might easily be
saved?
106. What is stated about children in China? Why does the body of a
drowned person sink? Why does it after a while rise? What is said
about wading in rivers?
107. Explain the manner in which the specific gravity of a solid
may be ascertained? Give the experiment of weighing water. What is
stated of Archimedes and the crown?
108. Describe and explain the hydrometer. Relate the anecdote of
the Chinese. What is said of the selling of milk in Switzerland?
109. What is said of the centre of gravity in floating bodies? Give
the illustrations.
CHAPTER IX.
110. What does pneumatics teach? How can you show that air is
material? How that it has weight? What is its weight compared with
that of water?
111. What is said of the air's being attracted by the earth?
Explain why some things rise and others fall in air. How thick is
the earth's air-covering?
112. How is the height of the atmosphere ascertained? At what rate
does the earth move round the sun? How does it carry along the air
with it? State the influence which gravitation has upon the density
of the air at different heights.
113. Give the comparison of air to wool. What is said of hydrogen
and balloons? In what are gases and liquids alike, and what are the
results of the similarity? What is the amount of pressure of the
atmosphere on each square inch of surface? Give the calculations in
regard to this pressure.
114. Show why the great pressure of the air does not produce
destructive effects. Describe the air-pump.
115. Explain by Fig. 95 the plan and working of the air-pump.
116. State some of the experiments with the air-pump. How can you
prove that air, like water, presses equally in all directions?
State the comparison about the fish.
117. What is said of the Magdeburg hemispheres? Give the experiment
with mercury. Explain the operation of the boy's sucker.
118. Give the statements about sucker-like arrangements in animals.
State the experiment of the bladder and weight. Give the experiment
with the India-rubber bag.
119. State the experiment with the egg. Explain the operation of
the hydrostatic balloon.
120. Explain the operation of the Cartesian image. What is said of
the presence of air in various substances?
121. What is said of the elasticity of air? Describe and explain
the condenser.
122. Describe and explain the gasometer. Show how the air-gun
operates. Explain the pop-gun.
123. Explain the operation of gunpowder. Explain that of steam.
What is said of retardation by condensed air in gunnery?
124. Describe and explain what is represented in Fig. 113. Explain
the collection of gases in the pneumatic trough.
125. Explain the experiment represented in Fig. 115. What is said
of tapping a barrel? What causes the gurgling sound when a liquid
is poured from a bottle? How high a column of water will the
pressure of the atmosphere sustain? How do you find from this the
pressure of the air on every square inch of surface? How high a
column of mercury will the atmosphere sustain?
126. Explain the barometer. Relate the incident given by Dr. Arnot.
127. Why would not a water-barometer answer? What is said of the
barometer as a measurer of heights? How is the boiling point
influenced by the amount of the air's pressure? Give the experiment
with ether.
128. State the experiment with the flask. What would happen to
liquids if the atmosphere were removed from the earth? Explain the
operation of the syphon by Fig. 117.
129. Explain what is represented by Fig. 118.
130. Explain the uses of the syphon. Explain the operation of the
Cup of Tantalus.
131. How are intermitting springs accounted for? Explain the
operation of the common pump.
132. Why does the water rise in the pump? How is sucking done?
Explain the forcing pump.
133. Explain the fire-engine.
CHAPTER X.
134. What is said of the universality of motion? What of attraction
as a cause of motion? What of heat? What of chemical agencies? What
of life?
135. What is meant by saying that action and reaction are equal?
State the illustrations of this truth which are given. Describe
Barker's mill.
136. Give the comparisons to the operation of a spring, of firing
of a cannon, and the throwing of stones from the crater of a
volcano. What is said of the jumping of a man from the ground?
137. What is said of the reaction in the case of a hopping bird?
Illustrate the inertia of matter as shown in the communication of
motion. Give the illustrations of the fact that time is required to
communicate motion to bodies.
138. Give illustrations of inertia as shown in the disposition of
motion to continue.
139. Describe and explain the equestrian feat represented in Fig.
127. What is said of skill in jumping from a moving carriage?
Relate the case in court which is stated.
140. What is said of the course of bodies thrown into the air? What
of a man falling from a mast-head?
141. What is said of the atmosphere as revolving with the earth?
What rapid motions are we subjected to when we speak of ourselves
as at rest? Why are we insensible to these motions?
142. Follow out in full the comparison of the steamboat. What is
the difference between absolute and relative motion? What is said
of absolute rest?
143. Illustrate the truth that all the motions which are apparent
to the eye are slight differences in the common absolute motions.
What are the obstacles to motion? How is the motion of a stone
thrown upward destroyed? What causes and what opposes its descent?
144. State and explain the experiment with the lead and feather.
Explain the operation of the water-hammer. Show the relation of
bulk to the resistance of liquids and gases.
145. Illustrate the relation of bulk to the motion of solids,
produced by moving gases and liquids.
146. What is said of the opposition of gravitation to water and air
in moving solids? What difference does the presence of obstacles
make in the relation of force to velocity?
147. State the law of the relation of force to velocity, and
illustrate by Fig. 136. What are some of the practical applications
of this law? What is said of the relation of shape to velocity?
What is said of the shape of fishes?
148. What is said of the shapes of boats? What of the management of
the webbed feet of water-fowls? What of the wings of birds? What is
said of friction as an obstacle to motion? What of it as a cause to
motion? Illustrate fully in the case of the wheel.
149. What is said of the friction of liquids in tubes? What is
the effect of sudden turns in pipes? What is the arrangement of
arteries in the heads of grazing animals? Illustrate the difference
of friction in small and large pipes by Fig. 131.
150. What is said of the effect of friction in brooks and rivers?
In what part of a stream does the water move most rapidly? Explain
the formation and breaking of the crest of waves rolling over
a beach. What is said of the velocity of rivers as affected by
friction? Explain the formation of waves.
151. What is it that really advances in the forward movement of a
wave? Give the comparison mentioned. What is said of the height of
waves?
152. What is momentum? Upon what two things does it depend?
Illustrate this dependence. Explain Fig. 133.
153. Give the illustration of the musket-ball and cannon-ball. Give
that of the plank. That of the candle. That of the air.
154. What is said of the expression, quantity of motion? Under what
circumstances may a single impulse produce a great velocity? What
examples have we of this? How is it with the motions that we see
around us? What is said of the fall of bodies to the earth?
155. Give examples from muscular action. Give that of the arrow.
Give that of gunpowder. What is said of the arrest of great
velocities? Give the illustrations in regard to cannon-balls.
156. State and explain the feat of the anvil. Give examples from
common efforts and labors.
157. Explain the communication of motion in the case of elastic
bodies by Figs. 133 and 134. What is said of the reflection of
motion?
158. What is said of the uniformity of motion? What of its
uniformity in velocity? State by what means we calculate as to time.
159. What is said of the sun-dial? What of the hour-glass? What of
Galileo and the pendulums? Explain the operation of the pendulum.
160. Explain Fig. 137. Explain the operation of the gridiron
pendulum by Fig. 138.
161. What is said of the disposition of motion to be straight? Why
is motion never straight, so far as we know? How can we make motion
very nearly straight? Give the illustration of the bullet in full.
162. Give the illustration represented in Fig. 141. What is
compound motion? Illustrate straight compound motion.
163. Explain Fig. 143.
164. Explain what is represented Figs. 144, 145, and 146.
165. Explain Fig. 147. How is curved motion produced? Give the
illustration of the ball and string. What are centrifugal and
centripetal forces?
166. What are these two forces in the revolution of the earth
around the sun? Give various illustrations of the operation of
centrifugal force.
167. What is said of the formation of bends in rivers?
168. Show how eddies and whirlpools are formed. How is the
centrifugal force used in the art of pottery. How in making
window-glass?
169. Describe and explain the operation of the steam-governor.
170. What is said of the agency of the centrifugal force in shaping
the earth?
171. Explain the operation of the apparatus represented in Fig.
154. What are the forces which act on a projectile? What is said of
balls thrown horizontally from cannon with different velocities?
172. Show by Fig. 155 why a ball dropped from the mouth of a cannon
will fall to the ground in the same time that one fired from it
will. By What two forces is a falling body acted upon?
173. Explain Fig. 156. What is the course of a ball dropped from a
railway car or from a mast-head? Give the comparison between the
cannon-ball and the moon.
174. What is said of the velocities of the heavenly bodies?
CHAPTER XI.
174. What are the Mechanical Powers? Why is the term power not
strictly proper?
175. Explain the terms power, weight, and fulcrum. What is said of
the use of the lever? What is the lever of the first kind? What is
said of its force?
176. What is said of scales? What of steelyards?
177. Give examples of the first kind of lever. Show by Fig. 159
that there is no gain of power in this lever.
178. Give the illustration of the see-saw. What is said of
Archimedes's lever?
179. State the analogy between this lever and the Hydrostatic
Bellows, Bramah's Press, etc. What is the lever of the second
kind? Apply the rule of equilibrium to it. Show how the common
wheel-barrow is a lever of this kind.
180. Give other examples of the second kind of lever. What is lever
of the third kind? How does this differ from the other two kinds?
Apply the rule of equilibrium to it.
181. Give examples of the third kind of lever. Show how it acts
at a mechanical disadvantage in the different examples mentioned.
State in full what is said of muscular action.
182. Explain by the figures the operation of compound levers.
183. State the comparison between the lever and the wheel and axle.
What is said of the common windlass?
184. Describe and explain the capstan. What are its chief uses?
What is said of the fusee of a watch?
185. Describe the arrangement of the fixed pulley. What are its
uses?
186. Describe the arrangement of the movable pulley. Show how the
relation of the power to the weight is estimated in the case of
compound pulleys.
187. Explain the mechanical advantage of the inclined plane. Give
examples of it.
188. What is said of roads? Give the comparison of the wedge to
the inclined plane. How is the power of the wedge estimated? Give
examples of the wedge.
189. What is said of the screw? Show by Fig. 180 how the force of
the screw is estimated. What are some of the uses of the screw?
190. Give the estimate of the power of the screw and lever as
used together. How can you show that there are really but three
mechanical powers? What is said of these as composing tools and
machinery? What is said of friction in machinery?
191. What is the first advantage of the mechanical powers which is
mentioned? Give the illustrations. What is the second advantage?
Give the illustrations.
192. What is the third advantage? Give examples. How is the
velocity of motion in machinery usually varied? What is the fourth
advantage? Mention examples. Describe the instrument called a Lewis.
193. What is said of the title by which Aristotle distinguished man
from other animals?
CHAPTER XII.
194. What is sound? What relation has sound to rapidity of
vibration? Mention cases in which the vibration of sounding
bodies is manifest to the sight and touch. What is said of wind
instruments?
195. State the analogy of a sounding body to a pendulum. Describe
the process by which the sensation of sound is produced. Where does
the vibration caused by the sounding body stop in the ear? What is
transmitted from thence to the brain?
196. Give examples of the transmission of sound through various
substances? State the experiment by which it is shown that sound is
not transmitted through a vacuum. What is said of sound at great
heights?
197. How far has the sound of a volcano been heard? If the same
sound were made in space at that distance from the earth why could
not the inhabitants hear it? What is the cause of the noise of
bodies passing through the air? Why do the heavenly bodies, moving
so rapidly, produce no sound? Cite examples showing the different
velocities of sound in different media.
198. What is said of the uniformity of the velocity of sound? Show
how we can measure distances by sound as compared with light in
velocity. Upon what does the loudness of sound depend? Illustrate
this point.
199. What is said of the diffusion of sound? What of its
reflection? What of echoes?
200. What is said of multiplied and mingled reflections of sound?
Explain the operation of whispering galleries by Fig. 187.
201. Explain the operation of the speaking-trumpet. Give other
examples of the concentration of sonorous vibrations.
202. What is the difference between a musical sound and a noise?
What is said of the exact regularity of musical vibrations? How are
different notes produced in stringed instruments? Upon what does
the note depend in wind-instruments?
203. Explain the operation of the organ-pipe represented in Fig.
190. What is said of the notes of bells and of musical glasses?
Explain the mechanism of the human voice.
204. What is harmony? Upon what does it depend? Between what two
notes of the scale is there the greatest harmony? What note next to
the octave harmonizes best with the fundamental note? And what note
next? Show why the second note, in contrast with the octave, is so
discordant with the fundamental note.
205. State the proportions between the numbers of the vibrations in
the different notes. If you know the number of vibrations of the
fundamental note in a second, how may you determine the number of
vibrations in the other notes? What is said of the number of notes
in the diatonic scale? What of the proportionate lengths of strings
for different notes? What is said of tuning instruments?
206. What is meant by saying that a note is too sharp or too flat?
State in full what is said about the mysteries of sound and hearing.
CHAPTER XII.
207. State the experiment of the three vessels, and the inference
from it.
208. What other facts sustain this inference? How did Sir Humphrey
Davy prove that there is heat in ice? What are the two theories of
heat? What is the chief source of heat for the earth? What is said
of the heat of the sun itself?
209. What is said of the universal influence of the heat of the
sun in the earth? What of the heat supplied from within the earth
itself? What of electricity as a source of heat?
210. What is said of chemical action as a source of heat? Give
examples of the production of heat by mechanical action. What is
said of the relations of heat and light?
211. Show the expansive influence of heat by describing the
experiment represented in Fig. 192. Give familiar examples of this
expansion.
212. How can you loosen a stopper stuck fast in a bottle? Give the
anecdote about the _Persia_. Give the statement about the building
in Paris.
213. What is said of the expansion of liquids by heat? How may the
influence of this expansion upon specific gravity be shown?
214. What is said of thermometers? What of the invention of the
thermometer?
215. State the plan of Fahrenheit's thermometer. Give the plans of
other thermometers.
216. Why is Fahrenheit's thermometer, on the whole, the best? What
is said of the expansion of gases by heat? State experiments in
illustration.
217. What is said of balloons? What of the influence of heat on the
atmosphere? Give examples of this influence.
218. Why, in heating apartments, do we have the heat created or
introduced at as low a place as possible? Explain the _draught_
of a chimney. Why does a stove-pipe generally draw better than a
chimney?
219. State the experiment with the candle and the door. What is the
explanation of the occurrence of wind? Explain the land-breeze.
220. Explain the sea-breeze. How are winds affected by the rotation
of the earth?
221. Show by Fig. 201 why the prevailing winds at the equator are
northeast and southeast.
222. Mention the melting points of various substances. What is said
about the natural state of water and other substances? What are the
two modes of changing a liquid into vapor?
223. What is said of the rapidity of evaporation? What of the
solution of water in air? What influence has heat upon the
capability of air to dissolve water? What phenomena illustrate
this? How is it supposed that water rises in air? What fact is in
opposition to this supposition?
224. What becomes of the water that rises in the air? What is said
of the formation of fog and of clouds? Mention the different shapes
of clouds and their names.
226. What is said of the influences that give shape to clouds?
228. State how rain is produced, and explain Fig. 208. How are snow
and hail formed? What is said of vaporization?
229. What influence has pressure upon the formation of vapor? Give
the experiment with ether in illustration. Describe the experiment
represented in Fig. 209. What is said of Papin's digester?
230. What is said of steam? In what consists the power of the
steam-engine? How is the expansive force of the steam in the boiler
estimated? Describe the working of the engine by Fig. 210.
231. What is the difference between high and low pressure engines?
What is said of the communication of heat? How many modes of
communication are there, and what are they? What is the mode called
convection?
232. Give examples of convection. What is the _conduction_ of heat?
233. State the experiment represented in Fig. 211. State the
experiment represented in Fig. 212. What is said of non-conductors
of heat? Give the examples cited.
234. Explain Davy's safety-lamp.
235. Give what is stated in the note about Stephenson and Davy.
What is said of the influence of density on the conduction of heat?
Give the illustration about melting snow.
236. State the experiments which show that liquids are poor
conductors of heat. What is said of air as a non-conductor of heat?
What is said of double windows?
237. What is said of arrangements of the walls of buildings? What
of an arrangements for preventing the spreading of fires in blocks?
238. How are animals in very cold regions protected from the cold?
What is it in their coverings that affords the protection? What
is said of the coverings of quadrupeds that are natives of warm
climates? What of the elephants whose remains are found in Siberia?
239. What changes take place in the coverings of animals carried
from a cold to a warm climate, and the reverse? Why has man no
covering against the cold? Explain the object of clothing. What
is said of articles of clothing? What of loose clothing? What of
coatings of straw put on trees? What of bricks compared with stones?
240. What is said of cocoons? What of buds of plants in winter?
What of snow as a protection of plants?
241. State the arrangement of snow observed in the arctic regions.
State in full what is said of the influence of the conduction of
heat upon sensation.
242. What is meant by the radiation of heat? Give examples of it.
What is said of the connection of heat and light in the rays of the
sun? What is said of heat from a common fire?
243. What is said of the relation between absorption and radiation?
What of the reflection of heat? State the experiment with the
mirrors and the thermometer and flask.
244. Explain the experiment with the ice. Give the experiment with
phosphorus. Give the experiment represented in Fig. 218.
245. Explain the formation of dew. State the analogy of the
tumbler. What is said of the circumstances that influence the
deposition of dew and frost?
246. What is said of different substances in regard to the
deposition of dew? What about Gideon's fleece? What is the
dew-point? How can you ascertain in it?
247. What is said of the freezing of mercury? Explain the
difference between sensible and latent heat. What is said of
capacity for heat? State the experiment represented in Fig. 219.
248. What is the relation of heat to density? Give the
illustrations.
249. What is the reason that the air is so cold on great heights?
What is the relation of heat to the forms of substances? What is
said of the melting of ice? What of the vaporization of water?
State the general conclusion in regard to latent and sensible heat.
250. State in full what is said of latent heat in reference to
clouds. Explain the operation of freezing mixtures.
251. State the examples of the production of cold by evaporation.
252. State and explain the experiment represented in Fig. 221.
253. Give the facts stated in regard to the degree of heat which
man can endure. Give the reasons why the heat did not produce a
greater effect in these cases.
254. What effect does heat produce upon the bulk of substances?
What is said of water as an exception? Describe the process of
freezing as illustrated by the diagram.
256. What would be the process if the exception did not exist?
State what would be the results.
257. What would be the consequence if the freezing point were
above 32°? What if it were below? What is said of the force of
expansion in ice? What are some of the benefits which come from
this expansion?
CHAPTER XIV.
258. What is Newton's theory of light? What is the undulatory
theory? State the analogies to sound and heat. When is a body
luminous? What are the sources of light?
259. How may you see that light moves in straight lines? State
various familiar recognitions of this fact. Illustrate the fact
that the intensity of light is inversely as the square of the
distance.
260. What is said of the velocity of light in regard to ordinary
distances? How long is light coming from the sun to the earth? What
is said of the light coming to us from certain stars?
261. Give the observation of Roemer represented in Fig. 226.
262. What is said of the reflection of light? What of its
reflection in relation to seeing? What of the images formed in
mirrors?
263. Show by Fig. 228 why the image in a mirror seems to be as the
same distance behind it that the object is before it. Explain by
Fig. 229 the operation of the kaleidoscope.
264. Explain the operation of a concave mirror by Fig. 230. Explain
that of a convex mirror by Fig. 231.
265. What is meant by the refraction of light? Illustrate its
reflection in passing from a denser into a rarer medium. Then from
a rarer into a denser.
266. How is the refraction in regard to a perpendicular in the two
cases? Explain dawn and twilight. Explain what is represented in
Fig. 234.
267. What are mirages? Describe the mirage which occurred at
Ramsgate. Describe that seen by Captain Scoresby. Relate the
incident which occurred at New Haven.
268. What is said of mirages in deserts?
269. Describe the mirage of the French coast. Explain what is meant
by the visual angle as illustrated by Fig. 236.
270. Explain Fig. 237. What are lenses? What are the different
kinds?
271. What is the difference of effect in convex and concave lenses?
Explain the effect of a convex lens on the visual angle. What is
said of microscopes and telescopes?
272. Describe and explain the magic lantern. Describe and explain
the camera obscura.
273. Describe the arrangement of a camera for sketching. How is the
eye like a camera?
274. Describe the arrangement of the parts of the eye as mapped in
Fig. 244.
275. Show now particularly how the eye is like a camera. What is
said of the influence of the cornea on the light? Show what is
required for distinct vision, as illustrated in Fig. 245. Show
why it is that objects brought very near the eye are not seen
distinctly.
276. What is said of the microscope? Explain the difficulty in the
near-sighted. In the far-sighted.
277. How can you show that the images of objects in the retina are
inverted? Give in full what is said of explanations of the fact
that we see objects erect notwithstanding this inversion. Explain
single vision.
278. By what simple experiment can you show the explanation of
single vision to be correct? What is said of squinting? Explain the
stereoscope.
279. What is said of distinct impressions on the retina? Explain
the thaumatrope.
280. State in full what is said of the compound nature of light.
Give the proportions of the colors in it. What is said about there
being only three colors?
281. What is said of the recomposition of decomposed light? Give
the illustration of the powder--the circular board--the top.
282. What is said of the colors of substances? What of the
variations of these colors in different lights? What of variations
with varying positions? What of the colors of clouds?
283. Explain the formation of the first rainbow by Fig. 253.
Explain the formation of the second bow by Fig. 254.
284. What is said of the circumstances under which rainbows are
seen?
285. Explain in full the formation of the two bows as illustrated
by Fig. 255. What is said of the bow as seen by different persons,
and at different moments by the same person? What of rainbow hues
in dew-drops and ice-crystals?
286. Give the dissection of light as represented in Fig. 256. What
is said of Daguerreotyping?
CHAPTER XV.
287. What is the origin of the term electricity? What is said of
attraction and repulsion in electricity?
288. What is the supposed explanation of electrical repulsion?
Explain the difference between resinous and vitreous electricity.
What is said of the two supposed electrical fluids? Detail the
illustrations of attraction which are stated.
289. State the theory of Franklin. Explain the use of the terms
positive and negative. Illustrate the fact that the kind of
electricity excited depends on what a substance is rubbed with.
290. What is said of the incorrectness of the terms vitreous and
resinous? What is said of conductors and non-conductors? Why are
non-conductors called insulators?
291. What marked difference is there between heat and electricity?
State the experiment represented in Fig. 258. What is said of
electrics and non-electrics?
292. What is said of equilibrium in electricity, and of its
disturbance? Give in full what is said of the universality of
electricity.
293. State what is said of induction, as illustrated by the
experiment represented in Fig. 259.
294. State the experiment represented in Fig. 260. Describe the
arrangement and operation of the electrical machine represented in
Fig. 261.
295. Describe the cylinder machine. State the experiment with the
pith balls. State that with the head of hair.
296. State the experiment with the tissue-paper. State that with
the dancing figures. State that with the bells.
297. Describe the experiment with the tin-foil. Describe the
insulating stool and the operation of it.
298. What is said of the escape of electricity from points?
Describe the apparatus represented in Fig. 269, and the operation
of it. What is said of the discharge of electricity from a point in
a dark room? Describe the Leyden jar.
299. Explain the operation of the Leyden jar. What would be the
effect of connecting the inside foil with the outer by a strip of
foil? Give in full the experiment represented in Fig. 272, and the
explanation.
300. What is said of the discharge of the jar? How can a large
number of persons take a shock from it together?
301. Explain the effect of moisture upon the charged jar. State the
experiment represented in Fig. 274.
302. What is the electrical battery? What is said of the light
produced by electricity?
303. To what is the report of electricity owing? What is said of
mechanical injuries caused by electricity? What of the heat caused
by it? What effects may be produced by this heat?
304. What was the discovery of Franklin, and how did he make it?
305. Relate the accident which occurred at St. Petersburg. What is
said of lightning-rods?
306. What was Sulzer's experiment? What were Galvani's observations?
307. What is said of the pile of Volta? What of his cup battery and
of other batteries?
308. What difference is there between frictional and voltaic
electricity?
CHAPTER XVI.
308. What are loadstones? Where do they abound? What is said of
discoveries in magnetism? Whence come the terms magnetism and
magnet?
309. What is said of the attraction of magnetism? What law is there
in regard to it? What is said of the poles of a magnet? What of
magnetism by induction?
310. What is said of attraction and repulsion in magnets? Explain
the formation of the curves of iron filings in the experiment
represented in Fig. 282.
311. How may artificial magnets be made? What is said of the
horseshoe magnet and its armature?
312. What is said of the magnetic needle and the mariner's compass?
313. What is the declination of the needle? When was it first
observed? What is said of observations after this? What is said of
the dip of the needle?
314. What is said of the earth as a magnet? What of it as a
magnetizer?
315. What is said of fixing magnetism? What of impairing it? In
what other substances besides iron does magnetism exist? In what is
magnetism like electricity? In what is it unlike it?
316. What relation has magnetism to electricity? What were the
original observations of Oersted in regard to it?
317. Describe the manner of making the most powerful
electro-magnets. Describe the experiment represented in Fig. 287.
318. Show the application of electro-magnetism in the electric
telegraph.
319. What is the contrivance called the signal key? How is the
alphabet of Morse's Telegraph constructed?
320. What is said of the communication through the earth in
telegraphing?
INDEX.
[The numbers refer to the paragraphs]
Action and reaction equal, 182
Air, compressibility of, 152
Air, elasticity of, 161
Air, density of, dependent on pressure, 158
Air, pressure of, as affecting the boiling point, 171, 327
Air, currents in, from heat, 281
Air, solution of water in, 287
Air attracted by the earth, 148
Air in interstices of substances, 160
Air a non-conductor of heat, 302, 304
Air-pump, 155
Air-guns, 164
Animals, coverings suited to climate, 305
Archimedes, his lever, 229
Archimedes, his discovery in the bath, 138
Archimedes and the crown, 143
Artesian wells, 120
Aqueducts, 119
Atmosphere, pressure of, 154, 168
Atmosphere, how it moves with the earth, 188
Atomic theory, 15
Attraction, nature of, 53
Attraction, capillary, 72
Attraction between masses, 73
Attraction between solids and liquids, 69
Attraction, Newton's idea of, 54
Attraction, universality of, 77
Attraction, variety in results of, 87
Attraction, opposition between modes of, 88
Attraction, proportion of the mutual motions of, 76
Balloons, 149, 280
Balloon, hydrostatic, 159
Barometer, 169
Barker's mill, 182
Battery, electrical, 387
Berkeley, his ideas of matter, 2
Boats, why they float, 139
Boiling point as affected by the pressure of the air, 171
Brittleness, 29
Buds in winter, 308
Bulk, relation of, to resistance of liquids and gases, 193
Camera obscura, 352
Canals, 117
Capillary attraction, 72, 86
Cartesian image, 159
Centre of gravity defined, 96
Centre of gravity, support of, in animals, 106
Centre of gravity, movements of, in walking, 107
Centre of gravity in relation to attitudes, 108
Centre of gravity in floating bodies, 145
Centrifugal force, 213
Centripetal force, 213
Chimneys, draught of, 282
Clothing, object of, 306
Clouds, how formed, 288
Clouds, colors of, 364
Clouds, shapes of, 289
Clouds, latent heat in, 324
Cocoons, constructed for winter, 307
Colors of objects, 363
Colors of the clouds, 364
Cohesion, strength of, 68
Cohesion, what is essential to it, 66
Cold, absence of heat, 270
Cold at great heights, cause of, 322
Cold from evaporation, 326
Compressibility of substances, 35
Compressibility of air, 152
Condenser, 162
Conductors of heat, 298
Conductors of electricity, 375
Conduction of heat, 297
Conduction, relation of density to, 300
Conduction in liquids, 301
Conduction, influence of, on sensation, 310
Convection of heat, 296
Coverings of animals suited to climate, 305
Crystallization, 62
Daguerreotype, 369
Dams, 122
Davy's safety-lamp, 299
Dawn explained, 346
Density, 24
Dew, formation of, 315
Dew-drops, colors in, 367
Dew-point, 317
Diatonic scale, 267
Draught of chimneys, 282
Drowning, how it can often be prevented, 141
Earth, globular form of, 61
Earth, roundness of, shown, 113
Earth, attraction toward centre of, 80
Earth, thickness of its air-covering, 150
Earth, shape of, influenced by centrifugal force, 218
Earth, rotation of, affecting winds, 284
Elasticity, 39
Elasticity, definition of, 42
Electricity, origin of the term, 370
Electricity, attraction and repulsion in, 371
Electricity, vitreous and resinous, 372
Electricity, galvanic, 394
Electricity, universality of, 378
Electricity, discharge of, from points, 383
Electricity, sound of, 389
Electricity, heat produced by, 391
Electricity, mechanical injuries by, 390
Electrics and non-electrics, 377
Electrical battery, 387
Electro-magnetism, 413
Electro-magnets, 414
Electric telegraph, 415
Equestrian feat, 185
Equilibrium, stable and unstable, 109
Evaporation, 286
Evaporation, cold produced by, 326
Expansion by heat, 274
Expansion, exception to it, 330, 331
Extension, a property of matter, 44
Eye, description of, 353
Eye, images in, inverted, 356
Feathers, how a non-conductor of heat, 305
Fire-engine, 179
Flexibility, 29
Flies, feet of, 157
Force, relation of, to velocity, 194
Forcing-pumps, 178
Franklin, his theory of electricity, 373
Franklin, his discovery, 392
Freezing, process of, described, 330
Freezing point, why at 32°, 332
Freezing in the midst of boiling, 327
Freezing mercury, 318
Freezing mixtures, 325
Friction, 196-198
Friction in machinery, 245
Frost, 44
Fur, how a non-conductor of heat, 305
Fur, how altered by climate, 305
Fusee of a watch, 238
Galvanism, 394
Gases, movements of the particles in, 10
Gases, space in, 21
Gases, compressibility of, 38
Gasometer, 163
Glass, annealing of, 32
Glass-making, centrifugal force used in, 216
Governor, steam, 217
Gravitation, 79, 86
Gravitation, limiting size, 90
Gravitation, action of, on solids in a liquid, 136
Gravity, specific, defined, 135
Gridiron pendulum, 210
Gunnery, retardation by condensed air in, 166
Hail, how produced, 290
Hardness, 28
Harmony, 266
Hearing, mysteries of, 269
Hearing, trumpet, 262
Heat, nature of, 271
Heat, sources of, 272
Heat, relations of, to forms of matter, 13
Heat, relation of, to bulk of liquids, 37
Heat, relation of, to light, 273
Heat producing expansion, 274, 275, 279
Heat, communication of, 295
Heat, convection of, 296
Heat, conduction of, 297
Heat, radiation of, 311
Heat, reflection of, 314
Heat, connection of, with light, 312
Heat, production of, by electricity, 391
Heat, degree of, endurable by man, 328
Heat, latent, 319
Heat, capacity of different substances for, 320
Heat, relation of, to density, 321
Heat, relation of, to forms of substances, 323
Heat and cold, 270
Heights measured by barometer, 170
Hume, his ideas of matter, 3
Hydrometer, 144
Hydrostatics, what it teaches, 110
Hydrostatic balloon, 159
Hydrostatic bellows, 132
Hydrostatic paradox, 131
Hydrostatic press, 133
Ice, formation of, 329
Ice, force of its expansion, 333
Ice-crystals, colors in, 367
Icebergs, centre of gravity in, 145
Impenetrability, 45
Imponderable agents, 16
Inclined plane, 241
Induction in electricity, 379
Induction in magnetism, 400
Inertia, 48
Inertia shown in the communication of motion, 183
Inertia shown in disposition of motion to continue, 184
Insulating stool, 382
Jupiter, mountains in, 92
Kaleidoscope, 343
Laggan stones, 100
Land-breeze, 283
Lenses, 349
Lever of first kind, 224
Lever, no gain of power in it, 227
Lever of second kind, 231
Lever of third kind, 232
Leyden jar, 384, 385
Life-boats, 139
Light, nature of, 334
Light, sources of, 335
Light, diffusion of, 337
Light, velocity of, 338
Light, reflection of, 340
Light, refraction of, 345
Light moves in straight lines, 336
Light shown to be compound, 360
Light, colors in, 360
Light, recomposition of, 362
Light, chemistry of, 369
Lightning-rods, 393
Liquids, movability of their particles, 9, 57
Liquids, incompressibility of, 36
Liquids, attraction in, 56
Liquids, globular shape of drops of, 58
Liquids, cause of level surface of, 111
Liquids, pressure of, in proportion to depth, 121
Liquids, lateral pressure of, 123
Liquids, pressure of, equal in all directions, 125
Liquids, upward pressure as depth, 127
Liquids, in what like gases, 153
Liquids, friction of, in tubes, 197
Liquids, friction of, in streams, 198
Liquids, expansion of, by heat, 275
Loadstones, 397
Machines not sources of power, 222
Machinery, friction in, 245
Magic lantern, 351
Magdeburg hemispheres, 156
Magnetism, origin of term, 397
Magnetism, attraction of, 398
Magnetism, repulsion of, 401
Magnet, poles of, 399
Magnets, artificial, 403
Magnets, horseshoe, 404
Magnetic needle, 405
Malleability, 28
Man a tool-making animal, 247
Mariner's compass, 405
Matter distinguished from spirit, 1
Matter, effects of, on senses, 6
Matter, forms of, 7
Matter, nature of, 14
Matter, constitution of, 14
Matter, variety in properties of, 17
Matter, divisibility of, 18, 51
Matter, minute division of, 19
Matter, pores and spaces in, 22
Matter, relation of heat to spaces of, 23
Matter, inertia of, 48
Matter, impenetrability of, 45
Matter equally inclined to rest and motion, 50
Mechanical powers, real advantages of, 246
Mercury, freezing of, 318
Microscopes, 350
Microscopical animals, 19
Mirages, 347
Mirrors, 342
Mirrors, curved, 344
Moth's wing, 19
Motion, causes of, 181
Motion, universality of, 180
Motion, absolute and relative, 191
Motion, obstacles to, 192
Motion, reflection of, 206
Motion, compound, 212
Motion, curved, 213
Motion in orbits, 221
Motion disposed to be straight, 211
Motion, that of falling bodies projectile, 220
Motion and rest, 189
Momentum, 201
Mountains, size limited, 92
Nature, order in, 65
Near-sightedness, 355
Needle, magnetic, 399
Needle, declination of, 406
Needle, dip of, 407
Noise, how it differs from musical sound, 263
Non-conductors of heat, 298
Non-conductors of electricity, 375
Notes, musical, how produced, 264
Odors, minuteness of their particles, 19
Order in nature, 65
Papin's digester, 292
Pendulum, 208-210
Perpetual motion attempted, 118
Persia, accident to, 274
Pisa, tower of, 104
Pitchers, why they have lips, 89
Pneumatics, what it teaches, 146
Pneumatic trough, 167
Pop-guns, 164
Pottery, centrifugal force used in, 216
Powder, explosive force of, 165
Prince Rupert's drops, 33
Projectiles, 219, 220, 221
Pulleys, 239
Pumps, 176
Pumps, forcing, 178
Radiation of heat, 311
Radiation, its relation to absorption, 313
Rain, how it is caused, 290
Rainbow, 365, 366
Rarity, 24
Reaction equal to action, 182
Reflection of heat, 314
Reflection of light, 340
Reflection of motion, 206
Reflection of sound, 260
Remora, 157
Rest, merely relative, 189
Rivers, 115, 116
Rivers, bends in, 215
Roemer, observations of, in regard to the velocity of light, 339
Rope-dancers, skill of, in managing centre of gravity, 109
Safety-lamp, 299
Scales, 98, 225
Screw, 243
Sea-breeze, 283
See-saw, a lever, 228
Sensation, influence of the conduction of heat on, 310
Shape, relation of, to velocity, 195
Shot, manufacture of, 60
Signal key, 415
Silk-worm, thread of, 19
Size limited by gravity, 90
Sluice-gates, 122
Snow, how produced, 290
Snow, crystals of, 64
Snow a protection to plants, 309
Solids, constitution of, 8
Solids, attraction in, 39
Solution, 12, 22
Sound, what it is, 248
Sound, sensation of, how produced, 252
Sound, velocity of, 256
Sound, transmission of, 253
Sound, loudness of, 258
Sound, diffusion of, 259
Sound, reflection of, 260
Sound, concentration of, 262
Sound, none produced by the heavenly bodies, 255
Speaking-trumpet, 262
Specific gravity defined, 135
Specific gravity of animals, 140
Specific gravity of the human body, 141
Specific gravity of solids, how ascertained, 142
Specific gravity of liquids, how ascertained, 144
Spectrum, 360
Spider, web of, 19
Spirit distinguished from matter, 1
Spirit, origin of word, 4
Spirit-level, 114
Springs, 120
Stability of bodies, 103-105
Steam transparent and invisible, 293
Steam, expansive force of, 165
Steam-engine, 294
Steel, flexible and brittle, 30
Steel, tempering of, 31
Steelyards, 98, 225
Stereoscope, 358
Suckers, 157
Sucking, explanation of, 167
Suction, 177
Sulzer's experiment, 394
Sun as a source of heat, 272
Surface, relation of, to movability, 193
Syphon, 172
Tantalus, cup of, 174
Telegraph, 415
Telescopes, 350
Tenacity, 25
Tenacity, comparative, of substances, 26
Tenacious substances, value of, 27
Thermometer, 37, 276
Thermometer, Fahrenheit's, 277
Tides, 78
Tubes, friction of liquids in, 197
Twilight explained, 346
Unison, 268
Up and down, explanation of, 81
Vapor, influence of pressure upon the formation of, 292
Vaporization, 291
Velocity, relation of, to force, 194
Velocity, relation of, to shape, 195
Velocities, great, how produced, 203
Velocities, great, how arrested, 204
Vibration of sounding bodies, 249
Vision, distinct, what necessary to, 354
Vision, why single, 357
Vision, why erect, 356
Visual angle, 348
Voice, how produced, 265
Voltaic electricity, 394
Volta's pile, 395
Walrus, feet of, 157
Water, crystallization of, 63
Water man's first mirror, 112
Waves, how formed, 199
Waves, height of, 200
Wedge, 242
Weight, 52, 82-85
Wheel and Axle, 235
Wheel-barrow a lever, 231
Whispering galleries, 261
Windlass, 236
Windows, double, 303
Winds, 283
Winds as affected by the rotation of the earth, 284
THE END.
FOOTNOTES:
[1] I take the following from Dr. Arnot:
The reasons that so many people are drowned in ordinary cases, who
might easily he saved, are the following:
1. Their believing that continued exertion is necessary to keep the
body from sinking, and hence their generally assuming the position
of a swimmer, in which the face is downward, and the whole head
must be kept out of the water to allow of breathing. Now as a man
can not retain this position without continued exertion, he is soon
exhausted, even if a swimmer, and if not, the unskillful attempt
will scarcely secure for him even a few respirations. The body
raised for a moment by exertion above the natural level, sinks as
far below when the exertion ceases; and the plunge, by appearing
the commencement of a permanent sinking, terrifies the unpracticed
individual, and renders him an easier victim to his fate.
2. From a fear that water entering by the ears may drown as if
it entered by the nose or mouth, a wasteful exertion is made to
prevent it; the truth being, however, that it can only fill the
outer ear, or as far as the membrane of the drum, and is therefore
of no consequence.
3. Persons unaccustomed to the water, and in danger of being
drowned, generally attempt in their struggle to keep their hands
above the surface, from feeling as if their hands were tied while
held below; but this act is most hurtful, because any part of the
body kept out of the water, in addition to the face, which must
be so, requires an effort to support it which the individual is
supposed at the time incompetent to afford.
4. Not having reflected that when a log of wood or a human body
is floating upright, with only a small portion above the surface,
in rough water, as at sea, every wave in passing must cover the
head for a little time, but will again leave it projecting in the
interval. The practiced swimmer chooses this interval for breathing.
5. Not knowing the importance of keeping the chest as full of air
as possible; the doing which has nearly the same effect as tying
a bladder of air to the neck, and without other effort will cause
nearly the whole head to remain above the water. If the chest be
once emptied, and if from the face being under water the person can
not inhale again, the body is then specifically heavier than water,
and will sink.
[2] This is true except when the tube is so small that capillary
attraction exerts considerable influence.
[3] I was once consulted in regard to a smoking stove. It was an
open Franklin stove, the pipe of which went through a fire-board
into a monstrous chimney. I recommended that a pipe with a knee
should extend from the pipe of the stove a little way up the
chimney. The expedient was successful, because but a small body of
air, that in the pipe, needed to be heated to establish an upward
current.
[4] As in the case of many other inventions, so here the same
idea was originated and put to practical use by more minds than
one. George Stephenson, who from being a common engine-wright in
a colliery rose step by step till he invented the locomotive,
constructed a lamp which illustrated in another way the same
principle as the lamp of Davy does--in other words, he invented
another safety-lamp. But this does not in the least detract from
the glory which the invention has given to the name of Davy, for
each acted independently of the other. In Davy's case, it is to
be remarked, there was a long course of scientific reasoning and
investigation which led him at length to the invention, the record
of which is exceedingly interesting. No invention or discovery is
made without thought, though accident may suggest the thought; but
here is an invention which, without any suggestion by accident, was
evolved by laborious and long-continued thought, proceeding step by
step to its conclusion.
[5] I will mention here a contrivance that I once adopted for a
small conservatory, which I wished to keep warm from the heat of a
room to which it was adjoining. In each space of the window-frames
were put two panes of glass, there being nearly half an inch of
space between them. In this way I secured all the benefit of double
windows with less expense and a less cumbrous arrangement. In
mentioning this contrivance now and then, casually, I have found
that a few others have thought of it, and adopted it with the same
success that I did.
[6] We are in entire ignorance of the nature of electricity, and we
use the term _fluid_ merely as a matter of convenience.
TRANSCRIBER'S NOTE
Italic text is denoted by _underscores_.
Bold text is denoted by =equal signs=.
Obvious typographical errors and punctuation errors have been
corrected after careful comparison with other occurrences within
the text and consultation of external sources.
Except for those changes noted below, all misspellings in the text,
and inconsistent or archaic usage, have been retained. For example:
farther, further; India rubber, India-rubber; every where;
catechetical; incloses; tricksy; pervious; motal; enrobes; subtile.
Pg 79, 'gravity made made him' replaced by 'gravity made him'.
Pg 157, 'if non elastic' replaced by 'if non-elastic'.
Pg 264, 'the divergency when' replaced by 'the divergence when'.
Pg 297, '283. Electricity' replaced by '383. Electricity'.
Pg 321, the Questions begin with number '13' and has not been changed.
Pg 331, the number '174' is used twice and has not been changed.
Pg 335, 'with phosporus' replaced by 'with phosphorus'.