INTRODUCTION TO MECHANICS. xxvii 



essential part of most machines. They are employed in various ways ; 

 but, when fixed to the axle, their mechanical power is always the same ; 

 that is, as the circumference of the wheel exceeds that of the axle, so 

 much will the energy of the power be increased. In mills and 

 manufactures, you must have admired the immense wheel, the revo- 

 lution of which puts the whole of the machinery into motion ; and though 

 so great an effect is produced by it, a horse or two has sufficient power to 

 turn it ; but a steam-engine is both the most powerful and the most conve- 

 nient mode of turning the wheel. We have the advantage sometimes of a 

 gratuitous force, such as the stream of water to turn a watermill ; and the 

 wind which turns the vanes of a windmill. One of the great benefits 

 resulting from the use of machinery is, that it gives us a sort of empire 

 over the powers of nature, and enables us to make them perform that 

 labour which would otherwise fall to the lot of man. When a current of 

 wind, a stream of water, or the expansive force of steam performs our task, 

 we have only to superintend and regulate their operations. 



The fourth mechanical power is the inclined plane. This is nothing 

 more than a slope, or declivity, frequently used to facilitate the drawing 

 up of weights. It is not difficult to understand, that a weight may with 

 much greater ease be drawn up a slope than it can be raised the same 

 height perpendicularly. But in this, as well as the other mechanical 

 powers, the facility is purchased by a loss of time (Jig. 35) ; for the weight, 

 instead of moving directly from A to C, must move from. B to C, and as 

 the height of the plane is to its length, so much is Ihe resistance of the 

 weight diminished. Thus, if a pulley be fixed at F, so that the string 

 from F to W may be parallel to B C, and a string fixed to the weight 

 W were connected with another weight P ; then if P bear the same pro- 

 portion to W that the line A C does to the line B C, the two weights 

 will balance each other, a considerable portion of the weight W being 

 supported by the plane B C, and only the residue by the power P. 



The wedge, which is the next mechanical power, is composed of two 

 inclined planes (j%. 36) : you may have seen woodcutters use it to 



Fig. 36. 

 Fig. 35. 



cleave wood. The resistance consists in the cohesive attraction of the 

 wood, or any other body which the wedge is employed to separate ; and 

 the advantage gained by this power is in the proportion of half its 

 width to its length. The wedge, however, acts principally by being struck, 

 and not by mere pressure : the proportion stated is that which expresses 

 its power when acting by pressure only. 



All cutting instruments are constructed upon the principle of the in- 

 clined plane, or the wedge. Those that have but one edge sloped, like 

 the chisel, may be referred to the inclined plane ; whilst the axe, the 

 hatchet, and the knife (when used to chop or split asunder), act on the 

 principle of the wedge. But a knife cuts best when drawn across the 

 substance it is to divide, as it is used in cutting meat, for the edge of a 



