hill were a perfect hemisphere, a road over it would be just equal in length to a road around its base to the opposite point. [Illustration: Fig. 179.] 242. =The Wedge.=--This is the fifth of the mechanical powers. It may be considered as two inclined planes placed with their bases together, as seen in Fig. 179. Indeed, sometimes the wedge has one side only inclined, it being only half of the ordinary wedge. The difference between the inclined plane and the wedge in operation is, that in the first the inclined plane is fixed, and the weight is made to move up along its surface, while in the latter the weight, that is, the resistance, is stationary, and the surface of the plane is made to move along upon it. The power of the wedge is estimated just as the power of the inclined plane is, that is, by comparing the thickness of the wedge with the length of its side. The less the thickness of the wedge compared with its length, obviously the more powerful is the wedge as a penetrating instrument. The wedge is used for splitting blocks of wood and stone, for producing great pressures, for raising heavy bodies, etc. All cutting and piercing instruments, knives, razors, axes, needles, pins, nails, etc., act on the principle of the wedge. 243. =The Screw.=--This is the sixth mechanical power. The principle of it is essentially that of the inclined plane. The "thread" running around the screw is an inclined plane which is spiral instead of straight, and so is also the corresponding part in the nut an inclined plane running in the opposite direction. In the common screw the nut is fixed, and the screw is made to play up and down in it; but sometimes the screw is fixed, and the nut is made to play around it. The screw acts like a wedge, and has the same relation to a straight wedge that a road winding up a hill has to a straight road of the same length and rise. Especially does the comparison hold when the screw is forced into wood; the wedge goes straight into the wood, but the edge of the screw's thread enters the wood spirally. [Illustration: Fig. 180.]