of itself begin to move, will be at once understood and agreed to. But the truth of the second is not so obvious. It does seem, at first sight, that matter is more inclined to rest than to motion; for, if we roll a stone along the surface of the ground, it gradually moves more and more slowly, till at last it stops altogether. It would be a great mistake, however, to suppose that the stone stops in virtue of any power of its own. If the ground is made level, and the stone round and smooth, it will roll longer and farther, and on a sheet of ice longer and farther still. It appears, then, that the friction, as it is called, arising from the roughness of the stone itself, and of the surface on which it moves, tends to stop its motion. Besides, even if there were no friction, the stone has to move through the air, making way for itself as it goes by the displacement of that fluid; this also retards its progress, and would at length bring it to a state of rest. Friction, then, and the resistance of the air, are among the influences which oppose the continuance of motion in all bodies near the earth's surface, and the effect of their operation is so constant and so gradual, that we are apt to think moving bodies stop of their own accord. But it is only living bodies which have any such power. Nay more, it is not easy nor safe for even a living body, when in rapid motion, to stop suddenly. When a man leaps from a railway carriage in motion, his body retains, in virtue of its inertia, all the progressive motion which it had in common with the carriage. But no sooner do his feet touch the ground, than their motion is suddenly arrested, and the rest of his body, having still a tendency to go on, falls in the direction of the train's motion, and with a violence proportional to its speed. The same effect will be felt, though in a less degree, in leaping from an ordinary gig or cart.

We cannot, it is true, prove by actual experiment that a moving body has no inherent tendency to stop, because we cannot get rid of disturbing influences. But we have the strongest reasons for believing that, if these were removed, a motion once begun would continue for ever in the same

direction, and at the same rate. In the heavens we have a vast mechanism whose movements cannot be explained on any other supposition. The planets, and other celestial bodies, removed from all the casual obstructions and resistances which are unavoidable on the surface of the earth, roll on in their appointed paths with unerring regularity, preserving undiminished all that motion which they received at their creation from the Hand that launched them into space.

NATURE AND EFFECTS OF FORCE.

A BODY at rest, if not disturbed, will remain at rest for ever; a body in motion, if not disturbed, will move on for ever at the same rate, and in the same straight line. It has been explained that this passive indifference to rest or motion is what we mean by inertia. But, in the bodies around us, we see no such unbroken rest, and no such uniform motion. Many of them are observed beginning to move from a state of apparent rest, and the motion of others is ever changing both in direction and rapidity. A stone projected from the hand along a level surface is gradually stopped, as we have seen, by friction and the resistance of the air which it must displace. If it be thrown upwards, it very soon ceases to rise, and then falls again. But why does it fall? Or, why does it not fall upwards, instead of downwards? If, owing to its inertia, it has no power to change its motion, how is that motion changed into an opposite motion? The planets, as was stated in last lesson, roll on unceasingly, yet even they do not move always in the same direction. Their speed remains on the whole undiminished, yet it too is ever changing. How, we may well ask, are such facts to be explained?

As a first step towards the answer to these questions, and to many other questions such as these, it is obvious, that there must be, in all such cases, some power or influence

at work, by whose operation the inertia of the moving bodies is counteracted. As the rolling stone is retarded by friction, so the ascending stone must be stopped and pulled down again by some power acting upon it. It could not fall of its own accord. So also, the planets must be drawn, by some other influence, out of the straight line in which inertia would make them move. Now, every such influence, everything which counteracts the inertia of matter, which sets a body in motion, or has a tendency to set a body in motion, which stops a moving body, or alters its motion, or tends to alter it, either in direction or velocity— every cause which produces any of these effects, is called a FORCE. Force, then, is intimately connected with motion, for, whenever a force acts, motion is the natural result. It does not follow, however, that every force really does produce motion, because some other force or forces may counterbalance it. Forces which thus counterbalance, and, as it were, destroy each other, are said to be in equilibrium. But, it is to be particularly observed, that every force would produce motion, if nothing else interfered. When forces are not in equilibrium, motion must necessarily

ensue.

The different kinds of force, the laws which regulate their action, and the effects which they produce, form the subject of a group of sciences, which are all included under the general name of Natural Philosophy.* It is to these sciences that we must look for an explanation of the various motions which we see around us. They tell us why a stone falls while a balloon rises; why the planets move in orbits nearly circular; why the thunder rolls; why the tide ebbs and flows; and (to come to the works of man) how the steam-engine is able, like a living creature, to pull its enormous load, or the electric telegraph to convey the thoughts and commands of men, with lightning speed, to the remotest corners of the earth.

* Or Mechanical Philosophy.

THE VARIOUS KINDS OF FORCE.

FORCES are of different kinds. That which seems to require least explanation, and from which, probably, our first ideas of force are derived, is the muscular power of living animals. A man can produce a certain mechanical effect, varying according to his strength; a horse produces a greater effect, and an elephant a greater still. This will be at once and easily understood.

One kind of force has been already mentioned, which has rather a passive influence in retarding and destroying motion, than any active power to produce it. This is friction. It ceases to act where there is equilibrium, but is of very great importance in all mechanical problems in which motion is involved. Without it a railway train could not move forward at all; the engine wheels, in their revolution, would slip on the rails. Without it even walking would be impossible. Indeed, we find it difficult and unsafe to walk wherever there is much less than the usual amount of friction, as, for example, on a sheet of ice.

Heat, magnetism, and electricity also produce motion, and so far fall within the domain of mechanical science. But by far the most important of all the forces with which that science has to deal, is one which, manifesting itself in various forms, receives in all the general name of attraction. There is a mutual attraction between particles in close contact, which causes them to cohere into a lump or mass of greater or less consistency. This is sometimes called cohesion, or the attraction of cohesion. It is obviously a force of the greatest consequence, for, if it ceased to operate, every material object would crumble to atoms. But it does not produce perceptible motion, for it acts only when the particles are, or appear to be in contact. On the other hand, the attraction of gravitation, a force which binds together bodies far distant from each other, is the cause of all the most stupendous motions with which we are acquainted. It guides the moon in her course round the earth, and the earth itself in its annual circuit round the sun. Its influence extends to

all the other worlds that compose the universe, and gives to each its proper place and motion. And yet, how strange to think that it is the very same force which makes a stone fall to the ground!

The effects of this great universal principle must be more minutely examined. Every particle of matter in the universe attracts every other with a certain force. In short, every particle of matter, though utterly destitute of any power to set itself in motion, is endowed with a power, the tendency of which is to set in motion every other particle. The farther removed two particles are from each other, the less is this attraction between them. And here is a point which it is absolutely necessary to understand. If at a distance of one foot two particles attract each other with a certain force, then, at a distance of two feet, their attraction will be, not one-half, but only one-fourth of what it was before. At three feet, it will be only one-ninth, at four feet, one-sixteenth, and so on. This relation between the distance of two bodies and the force of their attraction, is expressed as follows:“Attraction varies inversely as the square of the distance between the attracting bodies." Such is one great law of attraction, and there is another no less important. Since every particle of matter in a body exerts an attraction independently of the rest, it follows that the total attraction which any body exerts, being inade up of the separate attractions of its particles, must be proportional to its mass, that is, to the quantity of matter which it contains. The earth, for example, attracts the moon with a certain force, which would be twice as great if the quantity of matter in the earth were doubled. But farther, the earth attracts every particle of the moon independently of the rest, so that the total force which it exerts upon the moon must be proportional to the moon's mass, as well as to its own. The moon, in its turn, attracts the earth with a force also proportional to their respective masses, and, therefore, exactly equal to the earth's attraction upon it. Hence we have the second law of attraction-" The mutual attraction of two bodies is proportional to their

masses.