Father. They do; for if I lay a little paint on a, and let it touch b, it will make but a very small speck upon it: but if it fall upon b, the speck will be much larger; which proves that the balls are elastic, and that a little hollow, or dent, was made in each by collision. If now two equal soft balls of clay, or glazier's putty, which are non-elastic, meet each other with equal velocities, they would stop and stick together at the place of their meeting, as their mutual actions destroy each other.

Charles. I have sometimes shot my white alley against another marble so plumply, that the marble has gone off as swiftly as the alley approached it, and that remained in the place of the marble. Are marbles, therefore, as well as

ivory, elastic?

Father. They are.-If three elastic balls, a, b, c, (Plate III. Fig. 16.) be hung from adjoining centres, and c be drawn a little out of the perpendicular, and let fall upon b, then will c and b become stationary, and a will be driven to o, the distance through which c fell upon b.

If you hang any number of balls, as six, eight, &c. so as to touch each other, and if you draw the outside one away to a little distance and then let it fall upon the others, the ball on the opposite side will be driven off, while the rest remain stationary, so equally is the action and reaction of the stationary balls divided among them. In the same manner, if two are drawn

aside and suffered to fall on the rest, the opposite two will fly off, and the others remain stationary.

There is one other circumstance depending upon the action, and re-action of bodies, and also upon the vis inertiæ of matter, worth noticing: by some authors you will find it largely treated upon.

If I strike a blacksmith's anvil with a hammer, action and re-action being equal, the anvil strikes the hammer as forcibly as the hammer strikes the anvil.

If the anvil be large enough, I might lay it on my breast and suffer you to strike it with a sledge hammer with all your strength, without pain or risque, for the vis inertia of the anvil resists the force of the blow. But if the anvil were but a pound or two in weight, your blow would probably kill me.

CONVERSATION XIV.

On the Mechanical Powers.

Charles. Will you now, papa, explain the mechanical powers?

Father. I will, and I hope you have not for gotten what the momentum of a body is.

Charles. No; it is the force of a moving body, which force is to be estimated by the weight, multiplied into its velocity.

Father. Then a small body may have an equal momentum with one much larger?

Charles. Yes, provided the smaller body moves as much swifter than the larger one, as the weight of the latter is greater than that of the former.

Father. What do you mean when you say that one body moves swifter, or has a greater velocity than another?

Charles. That it passes over a greater space in the same time. Your watch will explain my meaning; the minute-hand travels round the dial plate in an hour, but the hour-hand takes twelve hours to perform its course in, consequently the velocity of the minute-hand is twelve times greater than that of the hour-hand; bes

cause, in the same time, viz, twelve hours, it travels twelve times the space that is gone through by the hour-hand.

Futher. But this can be only true on the supposition, that the two circles are equal. In my watch, the minute-hand is longer than the other, and, consequently, the circle described by it is larger than that described by the hour-hand.

Charles. I see at once, that my reasoning holds good only in the case where the hands are equal.

Father. There is, however, a particular point of the longer hand, of which it may be said, with the strictest truth, that it has exactly twelve times the velocity of the extremity of the shorter.

Charles. That is the point, at which, if the remainder were cut off, the two hands would be equal. And, in fact, every different point of the hand describes different spaces in the same time.

Father. The little pivot on which the two hands seem to move (for they are really moved by different pivots, one within another) may be called the centre of motion, which is a fixed point; and the longer the hand is, the greater is the space described.

Charles. The extremities of the vanes of a wind-mill, when they are going very fast, are scarcely distinguishable, though the separate parts, nearer the mill, are easily discerned; this

is owing to the velocity of the extremities being so much greater than that of the other parts.

Emma. Did not the swiftness of the roundabouts, which we saw at the fair, depend on the same principle, viz. the length of the poles upon which the seats were fixed?

Father. Yes, the greater the distance at which these seats were placed from the centre of motion, the greater was the space which the little boys and girls travelled for their half-penny.

Emma. Then those in the second row had a shorter ride for their money, than those at the end of the poles.

Father. Yes, shorter as to space, but the same as to time. In the same way, when you and Charles go round the gravel-walk for half an hour's exercise, if he run, while you walk, he will, perhaps, have gone six or eight times round, in the same time that you have been hut three or four times; now, as to time, your exercise has been equal, but he may have passed over double the space in the same time.

Charles. How does this apply to the explanation of the mechanical powers?

Father. You will find the application very easy without clear ideas of what is meant by time and space, it were in vain to expect you to comprehend the principles of mechanics.

There are six mechanical powers. The lever; the wheel and axle; the pulley; the inclined plane; the wedge; and the screw.