time support the weight in case of an accident. Sometimes, instead of men walking within the great wheel, cogs are set round it on the outside, and a small trundle wheel made to work in the cogs, and to be turned by a winch.

Charles. Are there not other sorts of cranes in which all danger is avoided?

Father. The crane is a machine of such importance to the commercial concerns of this country, that new inventions of it are continually offered to the public: I will, when we go to the library, show you in the 10th vol. of the Transactions of the Society for the Encouragement of Arts and Sciences, an engraving of a safe, and, I believe, truly excellent crane; it was invented by a friend of mine, Mr. James White, who possessed a most extraordinary genius for mechanics, and who formerly offered his services to a noble Duke, then at the head of the Board of Ordnance, but they being rejected, he went to the Continent, where he is very profitably exercising his talents.

Charles. But you said that this mechanical power might be considered as a lever of the first kind.

Father. I did; and if you conceive the wheel and axis (Fig. 22.) to be cut through the middle in the direction A B; F G B (Plate 111. Fig. 23.) will represent a section of it. A B is a lever, whose centre of motion is c; the weight w, sustained by the rope a w, is applied at the distance

CA, the radius of the axis; and the power P, acting in the direction B P, is applied at the distance C B, the radius of the wheel: therefore, according to the principle of the lever, the power will balance the weight, when it is as much less than the weight, as the distance c B is greater than the distance of the weight a C.

CONVERSATION XVIII.

Of the Pulley.

Father. The third mechanical power, the pulley, may be likewise explained on the principle of the lever. The line A B (Plate Iv. Fig 24.) may be conceived to be a lever, whose arms A C and B c are equal, and c the fulcrum, or centre of motion. If now two equal weights w and P, be hung on the cord passing over the pulley, they will balance one another, and the fulcrum will sustain both.

Charles. This pulley then, like the common balance, gives no advantage.

Father. From the single fixed pulley no mechanical advantage is derived; it is, nevertheless

of great importance in changing the direction of a power, and is very much used in buildings for drawing up small weights, it being much easier for a man to raise such burdens by means of a single pulley, than to carry them up a long ladder.

Emma. Why is it called a mechanical power?

Father. Though a single fixed pulley gives no advantage, yet when it is not fixed, or when two more are combined into what is called a system of pulleys, they then possess all the properties of the other mechanical powers. Thus in C D B (Plate 1v. Fig. 25.) c is the fulcrum, therefore a power P, acting at B, will sustain a double weight w, acting at A, for B C is double the distance of A c from the fulcrum.

Again it is evident, in the present case, that the whole weight is sustained by the cord E D P, and whatever sustains half the cord, sustains also half the weight; but one half is sustained by the fixed hook E, consequently the power at P has only the other half to sustain, or in other words, any given power at P will keep in equilibrio a double weight at w.

Charles. Is the velocity of P double that of w? Father. Undoubtedly; if you compare the space passed through by the hand at P with that passed by w, you will find that the former is just double the latter, and therefore the momenta of the power and weight, as in the lever, are equal.

VOL. I.-I