CHAP. XII.-Friction.*

WE shall not pursue this argument of the last chapter, by considering the other laws of motion in the same manner as we have there considered the first, which might be done. But the facts which form exceptions and apparent contradictions to the first law of which we have been treating, and which are very numerous, offer, we conceive, an additional exemplification of the same argument; and this we shall endeavour to illustrate.

The rule that a body naturally moves for ever with an undiminished speed, is so far from being obviously true, that it appears on a first examination to be manifestly false. The hoop of the school-boy, left to itself, runs on a short distance, and then stops; his top spins a little while, but finally. flags and falls; all motion on the earth appears to decay by its own nature; all matter which we move appears to have a perpetual tendency to divest itself of the velocity which we communicate to it. How is this reconcilable with the first law of motion on which we have been insisting?

It is reconciled principally by considering the effect of Friction. Among terrestrial objects friction exerts an agency almost as universal and constant as the laws

* Though Friction is not obviously concerned in any cosmical phenomena, we have thought this the proper place to introduce the consideration of it; since the contrast between the cases in which it does act, and those in which it does not, is best illustrated by a comparison of cosmical with terrestrial motions.

of motion themselves; an agency which completely changes and disguises the results of those laws. We shall consider some of these effects.

It is probably not necessary to explain at any length the nature and operation of friction. When a body cannot move without causing two surfaces to rub together, this rubbing has a tendency to diminish the body's motion or to prevent it entirely. If the body of a carriage be placed on the earth without the wheels, a considerable force will be requisite in order to move it at all: it is here the friction against the ground which obstructs the motion. If the carriage be placed on its wheels, a much less force will move it, but if moved it will soon stop: it is the friction at the ground and at the axles which stops it: placed on a level railroad, with well made and well oiled wheels, and once put in motion, it might run a considerable distance alone, for the friction is here much less; but there is friction, and therefore the motion would after a time

cease.

The same kind of action between the surfaces of two bodies which retards and stops their motions when they move touching each other, will also prevent their moving at all, if the force which urges them into motion be insufficient to overcome the resistance which the contact of the surfaces produces. Friction, as writers on mechanics use the term, exists not only when the surfaces rub against each other, but also when the state of things is such that they would rub if they did move. It is a force which is called into action by a tendency to move, and which forbids.

motion; it may be likened to a chain of a certain force which binds bodies in their places; and we may push or pull the bodies without moving them, except we exert a sufficient force to break this imaginary chain.

I. The friction which we shall principally consider is the friction which prevents motion. So employed, friction is one of the most universal and important agents in the mechanism of our daily comforts and occupations. It is a force which is called into play to an extent incomparably greater than all the other forces with which we are concerned in the course of our daily life. We are dependent upon it at every instant and in every action and it is not possible to enumerate all the ways in which it serves us; scarcely even to suggest a sufficient number of them to give us a true notion of its functions.

What can appear more simple operations than standing and walking? yet it is easy to see that without the aid of friction these simple actions would scarcely be possible. Every one knows how difficult and dangerous they are when performed on smooth ice. In such a situation we cannot always succeed in standing: if the ice be very smooth, it is by no means easy to walk, even when the surface is perfectly level; and if it were ever so little inclined, no one would make the attempt. Yet walking on the ice and on the ground differ only in our experiencing more friction in the latter case. We say more, for there is a considerable friction even in the case of ice, as we see by the small distance which a stone slides when thrown along the surface. It is

this friction of the earth which, at every step we take, prevents the foot from sliding back; and thus allows us to push the body and the other foot forwards. And when we come to violent bodily motions, to running, leaping, pulling or pushing objects, it is easily seen how entirely we depend upon the friction of the ground for our strength and force. Every one knows how completely powerless we become in any of these actions by the foot slipping.

In the same manner it is the friction of objects to which the hand is applied, which enables us to hold them with any degree of firmness. In some contests it was formerly the custom for the combatants to rub their bodies with oil, that the adversary might not be able to keep his grasp. If the pole of the boatman, the rope of the sailor, were thus smooth and lubricated, how weak would be the thrust and the pull! Yet this would only be the removal of friction.

Our buildings are no less dependent on this force for their stability. Some edifices are erected without the aid of cement: and if the stones be large and well squared, such structures may be highly substantial and durable; even when rude and slight, houses so built answer the purposes of life. These are entirely upheld by friction, and without the support of that agent they would be thrown down by the zephyr, far more easily than if all the stones were lumps of ice with a thawing surface. But even in cases where cement binds the masonry, it does not take the duty of holding it together. In consequence of the existence of friction, there is no constant tendency of the stones to separate; they

are in a state of repose. If this were not so, if every shock and every breeze required to be counteracted by the cement, no composition exists which would long sustain such a wear and tear. The cement excludes the corroding elements, and helps to resist extraordinary violence; but it is friction which gives the habitual state of rest.

We are not to consider friction as a small force, slightly modifying the effects of other agencies. On the contrary its amount is in most cases very great. When a body lies loose on the ground, the friction is equal to one third or one half, or in some cases the whole, of its weight. But in cases of bodies supported by oblique pressure, the amount is far more enormous. In the arch of a bridge, the friction which is called into play between two of the vaulting stones, may be equal to the whole weight of the bridge. In such cases this conservative force is so great, that the common theory, which neglects it, does not help us even to guess what will take place. According to the theory, certain forms of arches only will stand; but in practice almost any form will stand, and it is not easy to construct a model of a bridge which will fall.

We may see the great force of friction in the brake, by which a large weight running down a long inclined plane has its motion moderated and stopped; in the windlass, where a few coils of the rope round a cylinder sustain the stress and weight of a large iron anchor; in the nail or screw which holds together large beams; in the mode of raising large blocks of granite by an iron rod driven into a hole in the stone. Probably

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