MECHANICAL POWERS. 69 



or circumference of the axle : thus, if the diameter of 

 the axle be one foot, and that of the wheel be ten feet, a 

 man with a force of one hundred weight will be able to 

 raise ten hundred weight. 



The Pulley is a small wheel turning on an axis with 

 a rope passing over it. In a singleyfaed pulley there is 

 no advantage except convenience ; for, as the weight is 

 at one end of the rope and the power at the other, a 

 person will be able to use only his weight in raising a 

 body, and not his strength. As no advantage is ob- 

 tained, this cannot be correctly said to be a mechanical 

 power. In a single moveable pulley the advantage is as 

 tu-o to one. In a system of pulleys the advantage is found 

 by multiplying the number of moveable pulleys by two. 



The Inclined Plane is usually made by a plank laid 

 aslope, by means of which a heavy body may be easily 

 raised or lowered. The advantage obtained in the in- 

 clined plane is as the length of the plane is to its height : 

 thus, if a person who can use a force of one hundred 

 weight has to raise a weight of ten hundred weight one 

 foot, he can effect it by means of a plank ten feet long, 

 but the body will have to travel over ten times the space. 

 A body moving down an inclined plane acquires the same 

 velocity as in falling perpendicularly through the height 

 of the plane, and its rate of moving is in proportion to 

 the square of the time : thus, it moves/our times as far 

 in two seconds as in one. The Chisel, the Adze, and 

 similar instruments, are referred to this power. 



The fl'edge may be considered a double inclined plane, 

 and is useful in cleaving blocks of wood and stone. The 

 advantage gained by the wedge is as the thickness of 

 the back is to the length of the two sides. The \\edge 

 possesses a great superiority over the other mechanical 

 powers, as it is capable of being impelled by percussion, 

 so that by the simple blow of a heavy mallet an immense 

 resistance may be overcome. To the wedge may be 

 referred the Axe, the Spade, Knives, Needles, and all 

 sorts of instruments which begin from edges or points, 

 and become gradually thicker as they lengthen. Birds 

 flying are of a wedge-like shape, and so are Fish swim- 

 ming, for the purpose of cleaving their respective ele- 



