sense, as any others. Thus, the particles or atoms of which a mass of stone is made up, are believed on strong grounds-which, however, I cannot explain to you now-to be always in rapid motion among themselves, though these motions are so minute and so quick that even a microscope cannot detect them. In fact, there is nothing in the whole universe that is really at rest. The inconceivably fine grains or particles of which all things consist, are constantly moving among themselves, and you know that the great bodies which bear them, the stars and the earth, are ceaselessly gliding in their paths through the heavens.

7. Thus, a body may be at rest as regards other bodies, and yet its parts may be in active motion among themselves, as in the case of the mass of stone I have supposed; or it may be in motion as regards other bodies besides, as when a railway train passes on its course, or a ship floats down a river. The fields and river-banks remain where they were; the train and the ship, by moving, leave them behind.

8. When any bodies are at rest as respects each other, this state of rest can only be disturbed by force. The arrow on a bow-string would remain there if the force given to the bow-string did not drive it off, and thus change its rest into swift motion; a steam-ship at anchor remains so, till the force of the engine, after the anchor is weighed, sets it moving.

9. Force is thus needed to cause motion, and it is no less so to put an end to it. The arrow stops when it strikes the target or the ground, and thus has its motion resisted and destroyed; and the steam-ship is brought to a stop by reversing the engine, and thus creating an opposite force by which its forward motion is overpowered. It is easy to stop a shuttlecock because it was easy to set it in motion; but a railway-train which took a great force to move it, needs a great force to stop it, as the creaking of the brakes will tell you. The reason is, that it takes just as much force of an opposite kind, to bring a body to rest, as it takes to set it in motion.

10. The forces at work in Nature are known by various names. There is the mysterious principle of LIFE, which is the agent of all activities of animated creatures, and, in a humbler use of the word, of all the wonders of vegetation. GRAVITATION* is the name given to that force which keeps the earth in its orbit round the sun; controls the motions of the stars, and by attracting all things towards the centre of the earth, causes bodies to fall to the ground, and keeps them on it. A third kind of force is that by which the atoms of bodies are kept together. It is known as COHESION.† ELECTRICITY and MAGNETISM

* gravitation. Lat. gravis, heavy. Gravitation is thus the cause of the weight of bodies.

† cohesion. Lat. con, together, and hæreo, hæsum, to stick.

are also forces that cause motion. So is HEAT, and so is CHEMICAL ATTRACTION, which is that force by which atoms of different substances are drawn together so as to form compound bodies. But for this kind of force, there would be only the few elements or simple bodies, instead of the endless combinations to which we owe the existence of Nature as it is.

II." CONSERVATION OF ENERGY."

1. THE word ENERGY is used, in Physics or Natural Science for the FORCE expended or possessed in any case by a body-that is, for the power of doing work which it either has put forth or still has. So much energy, for example, is required to be possessed or expended to walk over the floor; so much more, to ascend a stair.

2. If a stone be thrown into the air, it, of course, must have a great amount of energy to rise as high as it does, in spite of the resistance offered by the air, and by the attraction of the earth. But what becomes of this energy when the stone ceases to rise? Is it lost and wasted? The same force expended in throwing it up, might have been put to some real use. Has it disappeared and ceased to exist? It is only changed, not lost; for the moment it has ceased to be able to move the stone any further upwards, it begins to show itself anew in the force with which the stone begins to descend again, which will be exactly the same as that by which it first rose. In the same way, if water has been made to rise in a pipe to a height, a certain amount of energy was required to raise it; but this energy may be used, without any loss whatever, for any useful purpose for which it is fitted, such as turning a mill-wheel, by letting it fall from the height to the earth again.

3. The greatest discovery of this age, in Physics, is connected with this change or "transmutation of force, or energy, and is known as the doctrine of the CONSERVATION OF ENERGY. We have seen how a falling body acquires, in falling, exactly the same "energy" as was expended in throwing it up; but the question remains: What becomes of this energy when the stone actually strikes the earth? The answer is: It is not lost even then, but passes into the form of heat. Thus, if a weight of one pound be raised to the height of 772 feet, and then be allowed to fall, it will create heat enough, on its striking the ground, to raise the temperature of a pound of water one degree.* It is this

*The latest statement on this point is that of Joule. According to him, if a kilogramme (2.20 pounds) of water be let fall from a height of 424 metres (462 yards), the heat created by its striking the earth-that is, by its motion being suddenly stopped-is sufficient to raise the whole of the water one degree centigrade (1-8 F.) in temperature.

law which causes the creation of heat by friction or rubbing, or by percussion or striking. Sir Humphrey Davy found that he could melt two pieces of ice by rubbing them against each other; the visible motion of his work passing into heat sufficient to cause this result. The heat created by friction of an iron axle in its socket is often sufficient to weld the axle and the socket into one piece, if the two fit tightly. When a railway train is stopped, its motion is changed into heat so great as to fly off in a shower of sparks from the brake-wheel. The Indian kindles two pieces of dry wood by simply rubbing the point of the one in a hole made in the other. A piece of metal beaten with a hammer on a stone, or anvil, soon grows quite hot. Nor is heat the only form of energy into which visible motion is changed. If you rub a piece of amber, the energy you expend is changed into electricity, and it is the same when an electric machine is used. Nor is it lost, though thus changed, for, if the electricity be made to act on a magnetic needle, it will at once reappear as visible motion, by drawing the needle aside. Hence, it may be seen that there is really no such thing as the annihilation of energy. Force expended may change its form, but is never lost-that is, in other words, the amount, or sum of all the Force in the universe, is always the same. This great law is what is known by the name of the CONSERVATION OF ENERGY.

III.-GRAVITATION.

1. UNIVERSAL GRAVITATION is the name given to that force by which every substance in the universe attracts every other substance according to fixed laws. TERRESTRIAL GRAVITATION, or the gravitation of the earth, is that force by which the earth attracts every particle of every substance towards its centre, according to the same laws. This wonderful force is the grandest we know in Nature, and yet it is the best understood, for its laws have been minutely ascertained. We owe the clear knowledge we possess of them to the genius of Sir Isaac Newton.

It

2. If you were to ask yourself why a body let loose in the air falls to the ground again, you would find it a difficult question to answer. does so by the law of gravitation, which attracts all bodies to itself, with a force proportioned to their mass-that is, their weight-and their distance. It used to be thought, before the days of the great Italian philosopher, Galileo, that a ten-pound weight would fall to the ground ten times as fast as a one-pound weight, but he showed by experiment that this was a mistake. Two weights of ten and one pound respectively, which he let drop from a high tower, reached the earth almost exactly at the same time, the resistance of the air accounting for the slight difference. In the same way, if we exhaust the air from

a high glass jar, by means of an air-pump, and let a feather and a sovereign fall from the top of it together, they fall with equal quickness; but if they be let fall in the open air, the coin will reach the ground at once, while the large surface of the feather will only float downwards, from the air resisting its descent.

3. But Newton was not satisfied with experiments on falling bodies, which are necessarily inexact. The PENDULUM offered him a better means of observing the laws of gravity; for the swinging, or "oscillation," which we have all seen in a pendulum, is just the effort it is making to settle at the lowest position, in obedience to the force of gravitation. The less the gravity, the slower the oscillations: the greater the gravity, the quicker they will be. Thus, the speed of the oscillations supplies a very exact measure of the force of gravity.

4. Acting on this fact, Newton caused a pendulum to be made with a box instead of a bob or weight at its lower end, and into this box he put, one after the other, an equal weight of different substances, and found that the pendulum oscillated exactly alike for the whole. It made no difference whether he put in a pound of lead or a pound of feathers. The mass or weight of a pound being the same in all cases, the pendulum showed in them all, the same results. He thus proved that bodies of the same weight are also of the same mass, or solid substance, and that the force of gravity is always in proportion to the

mass.

5. The explanation of the falling of bodies to the earth, is the greatness of the earth's mass compared to that of other things. The direction in which gravitation acts is seen by hanging up a cord with a weight at its end, as, for instance, a bricklayer's plumb-line. It cannot fall, indeed, because it is held up, but the cord will be stretched in a line which shows the direction in which gravitation draws the weight at its end, and this is found to be what is called vertical, or straight up and down. If it were possible to continue this downwards, it would pass very nearly through the earth's centre, over whatever part of the world the cord might be suspended. Plumb-lines thus do not always hang in the same direction to each other, but if suspended at equal distances round the world, would point to the centre of the earth, as the spokes of a wheel do to the hob. Still, the earth is so large that, if they be near each other, they hang almost exactly parallel, for it takes the distance of a mile between two to make any perceptible difference.

6. As plumb-lines everywhere point to the centre of the earth, it is clear that the attraction of gravitation is, as it were, collected at the centre. Every body on the surface of the earth is, therefore, half the diameter of the earth from this central point, and is drawn towards it

with a falling speed of about ten yards a second; but as solid bodies cannot get down through the earth to the centre, this attraction only serves, when they lie on the surface, to keep them there.

7. Gravitation is not equally powerful, however, at all distances. If it were so, we should be drawn to the sun rather than to the earth, for the mass of the sun is far greater than that of our world. It grows weaker and weaker, according to the distance of a body from the centre of the earth, and this in a fixed proportion which Newton was the first to discover. If the distance of a body be doubled, the attraction is diminished four times; if the distance be tripled, the attraction is diminished nine times.

8. Thus, you have only to multiply the distance by itself, or, as it is called, raise it to its square, to find how many times weaker the attraction has become, using always the distance from the centre to the surface of the earth for your measure. A striking proof of this is seen in the fact that the moon, which is 60 half-diameters of the earth, that is, 60 times the distance from the surface of the earth to its centre, or 60 times 4,000 miles=240,000 miles from the earth's surface, is attracted by the earth with only 3th part as much force as it would be attracted with at the earth's surface; that is, 60 x 60 = 3,600. This law may therefore be expressed thus: Gravitation decreases or lessens, according to the square of the distance of the bodies attracting each other.

1. THE old Greek natural philosophers had already, in their early day, noticed a fact, from the study of which the whole science of electricity may be said to have sprung within the last two hundred years. They saw that amber, then so highly prized, had the strange power, when rubbed, of drawing particles of light bodies to itself—that is, of attracting them-a thing so wonderful, in their opinion, that one of them even fancied it must have some hidden life. You need not think it so foolish, however, to suppose such a thing in those days as it would be now, for men were then just like children, with everything to find out and account for, and the only force or moving power of which children know is that of life. Every child fancies, from this reason, that whatever moves must be alive; and people long ago, when the world was young, reasoned in the same simple way, and thought the air, the seas, the rivers, and the trees so many living creatures. You will remember a notable case of this in Xerxes ordering the Hellespont to be scourged with whips, because it had destroyed his bridges of boats. If he had not thought it a living being, he would never have dreamed of such a command.

2. The power amber has, when rubbed, to attract and drive off small