of time taken for each beat depends on the length of the pendulum, a shorter one making more in a given time than a longer one, so that to make just one beat exactly, in a second, the pendulum must be of a certain fixed length.

2. It is found that a pendulum of a given length does not make the same number of beats at different parts of the earth's surface. At the equator it makes fewer, and at the Polar regions more. By means of this fact the flattened shape of the earth may be proved, for, as the rate of the motion of the pendulum depends on that attraction of the earth which we call the "force of gravity," and as this attraction grows less the further anything is from the earth's centre, it becomes evident that the greater speed of the pendulum near the Poles, rising as it does from the greater attraction there, will show that the Poles are nearer the centre of the earth than the equator is. But though the shape of the earth may be thus ascertained by the varying motions of the pendulum, the more exact method of measuring degrees of longitude on different parallels of latitude is that actually adopted by scientific

men.

3. The Spring is the other measure of time now in use. The regulation of the elasticity which a spring of steel possesses, that is, of the tendency to spring back to its original form or position, is the principle on which this use of it rests. By the aid of a balance-wheel, and other ingenious contrivances, a spring placed in clocks and watches, when tightly wound together by a key, will press so steadily outwards again, by its elasticity, that time may be accurately measured by its thus expanding. When the watch is wound up, the spring fills only a small part of the centre of the little round box you see in a watch; but when it has run down, the spring has pressed outwards till it has quite filled the box, and thus cannot any longer keep the wheels in motion, that is, cannot keep the clock or watch going any longer.

7. MOTION.

1. There are many degrees of velocity or speed of motion. The second-hand of a watch goes round its circle while the minutehand moves a single point, and while the short hand passes from one hour to the next, the minute-hand goes round the whole dial. A snail scarcely advances at all in a second, but a swift runner passes over 20 feet, a greyhound over 50 or 60, a hurricane over

180, sound over 1,125 feet, and light over about 190,000 miles a second.

2. The velocity or swiftness of a moving body is either increasing or retarded motion. In the latter, some force steadily resists the body that is in motion, so as to make it move more slowly each moment, as the air resists and thus retards the flight of a ball through it. The velocity is uniform when an equal space is always passed over in the same time, and it is increased or accelerated when it grows steadily greater each moment.

3. The power which puts a body in motion is called force, and we speak of its exerting a pressure when brought to bear on anything at rest. Force and pressure thus, in effect, mean the same thing, and as weight is the kind of force with which we are most familiar, we usually speak of the amount of a force or pressure as so much weight of ounces, pounds, etc.

4. When a body is in motion, it gains a power of putting other bodies in motion also; which is the same as saying that it gains force, and to this force of a moving body the name momentum, or quantity of motion, is given. But momentum depends on the mass or weight of a body in motion, and not merely on its velocity, that is, on the swiftness with which it moves. Thus, if two cannon-balls rush forward at the same rate, the one weighing twelve pounds, the other weighing twenty-four, we say that the heavier one has twice the force or momentum of the lighter one. When it is desired to express the different momenta of two moving bodies, the weight of each has, therefore, to be multiplied by its velocity, for the force with which either would act on another body, or the force that has been needed to put it, itself, in motion, is necessarily to be measured by the weight that had to be moved at first, and by the swiftness of the motion it acquires. In the case of a cannon-ball of 12 pounds, which moves 80 feet in a second, the momentum is found by multiplying 12 by 80. The momentum is thus 960. If another ball, weighing 24 pounds, fly 80 feet in a second, its momentum will be 24 x 80 = 1,920, or double that of the other.

PHYSICS.-LIGHT AND HEAT.

LIGHT.
I.

1. (1.) The great source of light, to man, is the sun, but there are inferior sources which also yield more or less. Thus (2), the

fixed stars are suns to their systems, though only specks of brightness to us. (3.) Heat causes light, for all bodies, as soon as they are brought to a certain heat, become luminous. (4.) So does electricity. (5.) Many of the lower animals emit light, as fish, the firefly, or the glow-worm; and (6) so do many plants, especially the Rhizomorpha, which grow in mines. (6.) There is also a light from the decay of the bodies of various creatures, as is seen in the blue light often rising from recent graves, and the light from decaying fish.. (7.) Wood, when it is rotting, is likewise often luminous.

The light of the moon is derived from the sun, the moon, like the earth, having no light of its own--that is, being nonluminous.

2. Though heat and light almost always go together, the one is sometimes found separate from the other, as in the light of the moon, and in that of various insects, which have no heat, or, at any rate, so little that the amount is not perceptible.

3. Light is supposed by some to be caused by the luminous body from which it proceeds scattering forth in all directions inconceivably minute shining particles, which strike on the eye, and thus produce vision. By others it is supposed that a wonderfully subtle fluid, called ether, pervades all space, and causes light by being set in motion, in waves, which reach the eye; just as sound is caused by vibrations, or waves of air set in motion by the body from which the sound proceeds, and travelling from it to the ear. This theory is called the undulatory-from unda (L.), a wave; and the other the material theory, because it supposes light not to be mere waves or undulations, but a material substance. It is also called the theory of emission, or sending forth, from the idea that light is something emitted, or sent forth, from the luminous body.

4. Light always moves in straight lines, as is proved by the fact that while you can see an object through a narrow tube if straight, you cannot see it if the tube be at all bent. It travels with a wonderful swiftness, passing, in a single second, through a distance variously estimated at from 168,000 to 191,000 miles, and, therefore, taking only from eight to nine minutes in travelling from the sun to the earth, a distance of about 91,500,000 miles.

5. Rays of light, when they come in contact with bodies, are either absorbed,* that is drunk in, and pass no farther, or they pass through it, or, finally, they are reflected,† or bent back.

* From (L.) absorbeo, to drink in.
+ From (L.) reflecto, to bend back.

6. When all the rays of light which fall on a body are absorbed, they disappear entirely, and it appears perfectly black, as in the case of lamp-black, which is the substance that most perfectly absorbs light. But the far greater number of bodies absorb only some part and reflect the rest; metals, if bright, reflect light better than any thing else.

D

7. It is by the reflection of light in every direction, from every visible point of a body, that the body becomes visible, and it is by these reflected rays entering our eye that the body becomes visible to us. We see things at all only by their reflectMing the rays of light that fall on them. 8. If a ray of light fall at a certain slant on a flat mirror which lies flat on S M here represents a mirror laid flat; D B a perpendicular a table, it is reflected from the mirror line. Now, if a ray of light fall in exactly the same slant in the opon the mirror in the direction A B, it will be reflected in the posite direction, as in Fig. 4. line B C, which is exactly on the same slant with the perpendicular line, or forms the

same angle.

B
Fig. 4.

Through this law it follows that the rays slant from a mirror as if they issued from a point as far behind the surface of the mirror as the object is from which they proceeded, and thus the image seen appears to be as far behind the mirror as that which throws it is before it.

9. If two or more mirrors be placed in such positions that the light from each shall be reflected on the others, the images formed will be as many times repeated as they are thus reflected. This is the principle of the Kaleidoscope, which is just an arrangement of mirrors so as to form a number of images of the same objects.

LIGHT.
II.

REFLECTION-SHADOWS.

1. To explain more clearly the formation of an image of any thing in a flat mirror, let us study the following illustration, Fig. 5.

S M is, here, a plane or level mirror, lying flat. F G is the eye of one looking into the mirror. L N (above the mirror) is an arrow reflected into it, at the image of which the eye is looking.

Now, notice that though rays are passing from every point of

the arrow to the mirror, comparatively few of them can enter the eye. Only those, in fact, which are reflected from the portions of the mirror from A to B, and from C to D can do so. As the slant or angle at which the rays are reflected must be exactly the same as that at which they fall on the mirror, the ray L A must be reflected to the eye along the line A F, and the ray L B must be reflected to it in the direction B G. In the same way the rays N C and N D must be reflected

into the eye along the lines C F and D G.

n

Fig. 5.

M

If, now, you look, you will see that the reflections of the first two rays A F and B G (the reflections of the feather end of the arrow) enter the eye as if they came from the point 7, where the dotted lines meet, which is as far behind the mirror as L (the arrow feather) is before it.

In the same way the rays from the tip of the arrow, which enter the eye by the lines C F and D G, do so as if they came from the point n, at the end of the other dotted lines—which, again, is just as far behind the mirror as the tip of the arrow is before it.

Turn the illustration sideways and you will see this clearly, for you will notice that it is just as far from E to L as it is from E to 1, and just as far from H to N, as it is from H to n, and just as far from 1 to n, inside the mirror, as it is from L to N on the arrow before it.

Thus the image must have the same position in the mirror as the arrow has before it, and it must be of the same size.

2. Any object that does not let the light pass through it, must, of course, stop those rays which shine on it, and, by preventing their shining beyond, cause darkness on the other side. This dark space is called the shadow of the object. We all know how, in the evening, when the gas is lit, if any one sit between us and it, he intercepts the rays of light and so throws the space next us, on the side of his body, which is away from the brightness, into deep shade. Our shadows, as we walk, are caused by our bodies coming in the way of the sunbeams which fall on the side of it next the sun, and preventing them from shining any farther. The darkness of night is only the shadow cast on the sky by the earth, when the sun shines on the side of it that is away from us.

K