CONVERSATION XI.

On the Laws of Motion.

Charles. Are you now going, papa, to describe those machines, which you call mechanical powers?

Father. We must, I believe, defer that a day or two longer, as I have a few more general principles with which I wish you previously to be acquainted.

Emma. What are these, papa

Father. In the first place, you must well understand what are denominated the three general laws of motion: the first of which is, "that every body will continue in its state of rest, or of uniform motion, until it is compelled by some force to change its state."*

Charles. There is no difficulty of conceiving that a body, as this inkstand, in a state of rest,

*The author is aware that this Law of Motion is not admitted by some modern philosophers of high name; to him, however, their reasonings appear inconclusive. At any rate, in a work intended for very young minds, he thinks it a duty to avoid metaphysical distinctions: preferring, at all times, rather to guide them by matters of fact than to load their tender memories with curious and subtile theories.

must always remain so, if no external force be impressed upon it to give it motion. But I know of no example which will lead me to suppose, that a body once put into motion would of itself continue so.

Father. You will, I think, presently admit the latter part of the assertion, as well as the former, although it cannot be established by experiment.

Emma. I shall be glad to hear how this is.

Father. You will not deny that the ball which you strike from the trap, has no more power either to destroy its motion, or cause any change in its velocity, than it has to change its shape.

Charles. Certainly; nevertheless, in a few seconds after I have struck the ball with all my force, it falls to the ground, and then stops.

Father. Do you find no difference in the time that is taken up before it comes to rest, even supposing your blow the same?

Charles. Yes, if I am playing on the grass, it rolls to a less distance, than when I play on the smooth gravel.

Father. You find a like difference when you are playing at marbles, if you play in the gravel court, or on the even pavement in the arcade.

Charles. The marbles run so easily on the smooth stones in the arcade, that we can scarcely shoot with a force small enough.

Emma. And I remember Charles and my cousin were, last winter, trying how far they could shoot their marbles along the ice in the canal; and they went a prodigious distance, in comparison of that which they would have gone on the gravel, or even on the pavement in the arcade.

Father. Now these instances properly applied will convince you, that a body once put into motion, would go on for ever, if it were not compelled by some external force to change its

state.

Charles. I perceive what you are going to say it is the rubbing or friction of the marbles against the ground which does the business. For on the pavement there are fewer obstacles than on the gravel, and fewer on the ice than on the pavement; and hence you would lead us to conclude, that if all obstacles were removed, they might proceed on for ever. But what are we to say of the ball, what stops that?

Father. Besides friction, there is another and still more important circumstance to be taken into consideration, which affects the ball, marbles, and every body in motion.

Charles. I understand you: that is the attraction of gravitation.

Father. It is for from what we said when we conversed on that subject, it appeared that gravity has a tendency to bring every body in motion to the earth; consequently, in a few seconds,

your ball must come to the ground by that cause alone; but besides the attraction of gravitation, there is a resistance which the air, through which the ball moves, makes to its passage. Emma. That cannot be much, I think.

Father. Perhaps, with regard to the ball struck from your brother's trap, it is of no great consideration, because the velocity is but small; but in all great velocities, as that of a ball from a musket or cannon, there will be a material difference between the theory and practice, if it be neglected in the calculation. Move your

mamma's riding-whip through the air slowly, and you observe nothing to remind you that there is this resisting medium; but if you swing it with considerable swiftness, the noise which it occasions will inform you of the resistance it meets with from something, which is the atmosphere.

Charles. If I now understand you, the force which compels a body in motion to stop, is of three kinds; (1.) the attraction of gravitation; -(2.) the resistance of the air;-and (3.) the resistance it meets with from friction.

Father. You are quite right.

Charles. I have no difficulty of conceiving, that a body in motion, will not come to a state of rest, till it is brought to it by an external force, acting upon it in some way or other. I have seen a gentleman, when skating on very slippery ice, go a great way without any exertion to

himself, but where the ice was rough, he could not go half the distance without making fresh efforts.

Father. I will mention another instance or two on this law of motion. Put a basin of water into your little sister's wagon, and when the water is perfectly still, move the wagon, and the water, resisting the motion of the vessel, will at first rise up in the direction contrary to that in which the vessel moves. If, when the motion of the vessel is communicated to the water, you suddenly stop the wagon, the water, in endeavouring to continue the state of motion, rises up on the opposite side.

In like manner, if while you are sitting quietly on your horse, the animal starts forward, you will be in danger of falling off backward; but if while you are galloping along, the animal stops on a sudden, you will be liable to be thrown forward.

Charles. This I know by experience, but I was not aware of the reason of it till to-day.

Father. One of the first, and not least important uses of the principles of natural philosophy is, that they may be applied to, and will explain many of the common concerns of life.

We now come to the second law of motion, which is that the change of motion is proportional to the force impressed, and in the direction of that force."

Charles. There is no difficulty in this, for if