The screw is one of the most powerful and useful of the simple machines or mechanic powers. It is a modification of the inclined plane, as will easily appear to any one who reflects a little on its construction. If a triangular piece of paper be rolled round a cylinder, it will form a spiral inclined line round it, which will be not an inapt representation of the nature of the screw. The screw with the projecting thread moves within a concave spiral groove cut in the interior of a hollow cylinder, which is termed the female screw or nut. The screw is generally turned by means of a lever, as represented in the annexed cut at a b; and the power obtained by the instrument is calculated by dividing the circumference of the circle described by a b by the distance between two successive threads of the screw. Thus, if the lever a b be thirty inches long, and the distance between two threads of the screw be half an inch, the circumference described will be 94 inches; which, divided by half an inch, gives 118 as the increase of power obtained by this machine. In this case, a man who could exert a force of l00lbs. would be enabled to produce a pressure equal to that of 18,800 lbs.