evidently an inclined 
/ 
_ ga’ (fig. 15); but it has actually passed along the 
THE MECHANICAL POWERS. 15 
to cf as BD is to AB; therefore, the force necessary to balance a weight on 
an inclined plane is proportioned to that weight, as the height of the 
plane is to its length. The degree of slope in an incline is expressed by 
saying that there is a rise of one foot. in so many feet ; thus, in fig. 14, 
where AB is ten times BC, the horse is pulling a load up an incline 
of one in ten. And, according to what has been said, the draught 
necessary to be exerted by the horse, if the plane be perfectly 
smooth, is only one-tenth of the weight of the load. More correctly, 
a tension of one-tenth the weight of the load would keep the cart from 
rolling backward ; a little more exertion on the part of the horse would 
be necessary to pull it up. 
V.—The Screw. ' 
The Screw is an application of the inclined plane. It is simply 
an inclined plane passing round a cylinder, as can be shewn by 
a very simple experiment. If a triangular piece of paper, such as 
abf in fig. 15, be wound round a pencil or ruler, the upper edge of 
the paper, which is , 
plane, will form a 
spiral exactly like a 
screw, except that a °°7 
screw is formed by a Fig. 15. 
spiral ridge. This spiral ; 
ridge is called the thread of the screw, and it works in a nut (M, fig. 16), 
which has a spiral groove to receive the thread, and is turned by means 
of a lever, L. When the screw is turned once 
round, it is carried forward in a fixed nut, or it 
draws forward a movable nut upon it, through the 
space between two of its threads. The resistance, 
therefore, has been pulled forward a distance, eg or 


inclined plane de, supposing de to be the part that SS, 
would go round the pencil. Therefore, the resistance Rs 
overcome by the screw is to the force exerted in Fig. 16. 
turning the screw as de is to ec. A screw, however, 
seldom acts by itself, being generally turned by a lever, so that, 
by means of the lever and the screw, a man is often able to overcome a 
resistance more than a hundred times greater than the force he applies 
to the lever. 
Vi.—The Wedge. 
The Wedge (fig. 17) is a contrivance for separating or overcoming 
resistances by being forced in between them, and is really a combination 
