237. =Capstan.=--In the capstan, represented in Figs. 170 and 171, the axle is in a vertical position. The top of it is pierced with holes, into which levers are introduced. In Fig. 170 you see the instrument as it is commonly used in moving buildings. Sometimes horse-power is applied at the ends of the levers. Great power is exerted by this instrument; but we have the same fact here as in all cases where a small force produces a great effect--the effect is slow, and the force passes over a great space in producing it. The moving of a building a foot requires many circuits of the horse around the axle. Fig. 171 gives us the capstan as it is commonly on board ship. The head of it is circular, with many holes for levers, so that many men can work together in raising a heavy anchor. [Illustration: Fig. 172.] 238. =Fusee of a Watch.=--In the fusee of a watch we have a wheel and axle of a peculiar construction. When we wind up a watch the chain is wound around the spiral path-way on the fusee, B, Fig. 172, and at the same time the spring is coiled up tightly in the round box, A. The spring, in gradually uncoiling itself, turns this round box around, and thus pulls upon the chain, _c_, making the fusee to revolve, and so give motion to other parts of the machinery. Now the spring, in its effort to uncoil, acts strongest at first; and therefore if the fusee were of uniform size the watch would go fastest when first wound up, and go gradually slower as it run down. This difficulty is obviated by giving the power a small wheel to pull on at first, and gradually enlarging the wheel as the spring uncoils. This is because, in order to produce a certain effect on a given weight by a power, the less the power is the longer must be the arm of the lever on which the power acts. [Illustration: Fig. 173.] [Illustration: Fig. 174. Fig. 175.] 239. =The Pulley.=--The third mechanical power is the Pulley. Pulleys are _fixed_ or _movable_. In Fig. 173 you have a fixed pulley. There is no mechanical advantage in this pulley, for its action may be conceived of as the action of successive levers of equal arms, B F and A F, and therefore equilibrium requires an equality of the power and weight. But this pulley is often a great convenience. For example, a man can raise himself or some weight to any desired elevation, as seen in Fig. 174. It is used also in effecting descents. With two fixed pulleys a horizontal force may be used in raising a weight vertically, as seen in Fig. 175. In using a fixed pulley either one or the other of two objects is attained--applying force where we could not otherwise apply it, and changing the direction of its application. [Illustration: Fig. 176.]