To estimate the force of the screw we compare the length of one turn of the thread around it with the height to which the thread rises in going round. Let _a b_, Fig. 180, represent one turn of the thread, and _b c_ the height to which it goes. It is clear from the figure that the principle which applies to the inclined plane and to the wedge applies here also. As the less is the height of the plane the easier it is for a weight to be drawn up it; and as the less is the depth of the wedge the less is it resisted; so, also, the less the height of the turn of the screw's thread the easier is it to move the screw, and the greater is the force which it exerts. Hence the prodigious power of a screw with a thread which rises very slowly in its spiral turns. Screws are much used when great pressure is required, as in pressing oils and juices from vegetable substances, in compressing cotton into bales, in bringing together with firm grasp the jaws of the vice, etc. In turning the screw a bar is used, so that we have in this instrument the combined advantages of the screw and the lever. That you may have some idea of the power of these two instruments acting together I will suppose a case. Let the weight to be raised by a screw be 10,000 pounds. Let a turn of the screw be 10 inches long, and the rise be but one inch. Then, so far as the screw is concerned, the power requisite to raise the 10,000 pounds will be 1000--the ratio of the height of the thread's turn to its length. But the power of the lever is yet to be estimated. Let the length of the lever, passed through the head of the screw so that it is equal on each side, be 30 inches. The diameter of the screw is about three inches, or one-tenth of the diameter of the circle described by the end of the lever. It will now take but a power of 100 pounds to raise the weight, the ratio of the radius of the screw to half the length of the lever. 244. =Truly but Three Mechanical Powers.=--The Wheel and Axle, you have seen, is merely a modification of the Lever, and the Wedge and the Screw are modifications of the Inclined Plane. The Mechanical Powers are, then, in reality but three--the Lever, Pulley, and Inclined Plane. And these are the elements of all machinery, from the simplest tool that is used for the most common purposes to the most complicated and powerful engine which the ingenuity of man ever designed. The principle upon which a pin is shaped is identical with that of the wedge, by which large masses are cleft in two; and the instrument by which the finest textures are cut by delicate fingers is arranged on the same principle with those varied contrivances by which immense weights are raised by a comparatively small power, viz., the principle of the lever.