(315.) By the wheel and axle a continued rectilinear motion may be made to produce a continued rotatory motion, or _vice versâ_. If the power be applied by a rope coiled upon the wheel, the continued motion of the power in a straight line will cause the machine to have a rotatory motion. Again, if the weight be applied by a rope coiled upon the axle, a power having a rotatory motion applied to the wheel will cause the continued ascent of the weight in a straight line. Continued rectilinear and rotatory motions may be made to produce each other, by causing a toothed wheel to work in a straight bar, called a _rack_, carrying teeth upon its edge. Such an apparatus is represented in _fig. 159._ In some cases the teeth of the wheel work in the links of a chain. The wheel is then called a _rag-wheel_, _fig. 160._ Straps, bands, or ropes, may communicate rotation to a wheel, by their friction in a groove upon its edge. A continued rectilinear motion is produced by a continued circular motion in the case of a screw. The lever which turns the screw has a continued circular motion, while the screw itself advances with a continued rectilinear motion. The continued rectilinear motion of a stream of water acting upon a wheel produces continued circular motion in the wheel, _fig. 93_, _94_, _95_. In like manner the continued rectilinear motion of the wind produces a continued circular motion in the arms of a windmill. Cranes for raising and lowering heavy weights convert a circular motion of the power into a continued rectilinear motion of the weight. (316.) Continued circular motion may produce reciprocating rectilinear motion, by a great variety of ingenious contrivances. Reciprocating rectilinear motion is used when heavy stampers are to be raised to a certain height, and allowed to fall upon some object placed beneath them. This may be accomplished by a wheel bearing on its edge curved teeth, called _wipers_. The stamper is furnished with a projecting arm or peg, beneath which the wipers are successively brought by the revolution of the wheel. As the wheel revolves the wiper raises the stamper, until its extremity passes the extremity of the projecting arm of the stamper, when the latter immediately falls by its own weight. It is then taken up by the next wiper, and so the process is continued. A similar effect is produced if the wheel be partially furnished with teeth, and the stamper carry a rack in which these teeth work. Such an apparatus is represented in _fig. 161._