one of the five mechanical powers. A screw is a cylinder cut into several concave surfaces, or rather a channel or groove made in a cylinder, by carrying on two spiral planes the whole length of the screw, in such a manner that they may be always equally inclined to the axis of the cylinder in their whole progress, and also inclined to the base of it in the same angle. See Mechanics, n° 32

No. 1. To construct a common, or one-threaded Screw.—Make a parallelogram of paper equal in length to the cylinder which is to be screwed, and equal in breadth to the circumference of that cylinder. Divide the side of the parallelogram, which is equal to the circumference of the cylinder, into two equal parts. Divide the other side of the parallelogram, which is equal in length to the cylinder, into as many parts as the thickness or breadth of the intended thread will run over. Then join the second point on the circumference side to the second point on the length-side of the parallelogram, and so join all the succeeding points as you see in the figure.

No. 2. To make a four-threaded Screw, or that which is commonly used for the letter-press.—Make a parallelogram, as described before; divide that side which is equal to the circumference of the cylinder into eight equal parts, or twice the number of threads. Divide the other side into as many parts as the distance between two threads will run over, then join the points as in no. 1. (fig. 1).

Corollary. To make a left-handed screw.—Make the parallels to the right instead of the left, as expressed by the figures, n° 3.

This is the true and only practicable way of making all kinds of screws that are cut on a cylinder.

Archimedes's Screw. See Hydrostatics, n° 40.

Endless or Perpetual Screw, one so fitted in a compound machine as to turn a dented wheel; so called, because it may be turned for ever without coming to an end.

If in the endless or perpetual screw, AB (n° 4.), whose threads take the teeth of the wheel CD, you take the distance of two threads, according to the length of the axis AB; or the distance of two teeth in the wheel CD, in the direction of the circumference; and if a weight W act at the circumference of the wheel; then, if the power D be to the weight W, as that distance of the teeth or threads, to the length described by the power P in one revolution, the power and weight will be in equilibrium; because in one revolution of P, the wheel DC, with the weight W, has moved only the distance of one tooth.