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 but 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 easywithout 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.

Emma. Why are they called mechanical powers?

Father. Because, by their means we are enabled mechanically to raise weights, move heavy bodies, and overcome resistances, which, without their assistance, could not be done.

Charles. But is there no limit to the assistance gained by these powers? for I remember reading of Archimedes, who said, that with a place for his fulcrum he would move the earth itself.

Father. Human power, with all the assistance which art can give, is very soon limited, and upon this principle, that what we gain in power, we lose in time. That is, if by your own unassisted strength, you are able to raise fifty pounds to a certain distance in one minute, and if by the help of machinery, you wish to raise 500 pounds to the same height, you will require ten minutes to perform it in; thus you increase your power ten-fold, but it is at the expense of time. Or, in other words, you are enabled to do that with one effort in ten minutes, which you could have done in ten separate efforts in the same time.

Emma. The importance of mechanics, then, is not so very considerable as one, at first sight, would imagine; since there is no real gain of force acquired by the mechanical powers.

Father. Though there be not any actual increase of force gained by these powers; yet, the

advantages which men derive from them are inestimable. If there are several small weights, manageable by human strength, to be raised to a certain height, it may be full as convenient to elevate them one by one, as to take the advantage of the mechanical powers in raising them all at once. Because, as we have shown, the same time will be necessary in both cases. But suppose you have a large block of stone of a ton weight to carry away, or a weight still greater, what is to be done?

Emma. I did not think of that.

Father. Bodies of this kind cannot be separated into parts proportionable to the human strength without immense labour, nor, perhaps, without rendering them unfit for those purposes to which they are to be applied. Hence then you perceive the great importance of the mechanical powers, by the use of which a man is able with ease to manage a weight many times greater than himself.

Charles. I have, indeed, seen a few men, by means of pulleys, and seemingly with no very great exertion, raise an enormous oak into a timber-carriage, in order to convey it to the dock-yard.

Father. A very excellent instance; for if the tree had been cut into such pieces as could have been managed by the natural strength of these men, it would not have been worth carrying to

Deptford or Chatham for the purpose of shipbuilding.

Emma. I acknowledge my error;-what is a fulcrum, papa ?

Father. It is a fixed point, or prop, round which the other parts of a machine move.

Charles. The pivot, upon which the hands of your watch move, is a fulcrum then.

Father. It is, and you remember we called it also the centre of motion; the rivet of these scissors is also a fulcrum.

Emma. Is that a fixed point or prop?

Father. Certainly it is a fixed point, as it regards the two parts of the scissors; for that always remains in the same position, while the other parts move about it. Take the poker and stir the fire; now that part of the bar on which the poker rests is a fulcrum, for the poker moves upon it as a centre.

CONVERSATION XV.

Of the Lever.

Father. We will now consider the Lever, which is generally called the first mechanical power.

A

The Lever is any inflexible bar of wood, iron, &c. which serves to raise weights, while it is supported at a point by a prop or fulcrum, on which, as the centre of motion, all the other parts turn. A B (Plate III. Fig. 17.) will represent a lever, and the point c the fulcrum or centre of motion. Now, it is evident, if the lever turn on its centre of motion c, so that a comes into the position a ; B at the same time must come into the position b. If both the arms of the lever be equal, that is, if A c is equal to B c, there is no advantage gained by it, for they pass over equal spaces in the same time; and according to the fundamental principle already laid down (p. 76)" as advantage or power is gained, time must be lost:" therefore, no time being lost by a lever of this kind, there can be no power gained.

Charles. Why then is it called a mechanical power?

Father. Strictly speaking perhaps it ought not to be numbered as one. But it is usually reckoned