Mechanic Powers. 



the mechanic powers, and was probably the first 

 which was brought into use. It is a bar of iron 

 or wood, one part of which is supported by a 

 prop, and upon that prop all the other parts turn 

 as on their centre of motion. You see the lever 

 made use of in one form or other every day when 

 a labourer takes a hand-spike, or large stake, 

 and placing a stone under some part near the 

 end, by putting the extremity under a cask, a 

 piece of timber, or any other body, and attempts 

 to move it, by pulling at the other end, he makes 

 use of a lever. The handle of a pump is a lever 

 also ; even the poker with which I raise the fire 

 is a lever, the bar of the grate is the prop, and 

 at the end which I hold in my hand is applied 

 the strength or power. This is, however, not 

 the only kind of lever, for in fact there are three 

 different sorts or orders of these instruments. 

 The first is that which I have been describing, 

 viz. when the prop is placed between the weight 

 to be raised and the power (see fig. 95.) In this 

 figure ABC is the lever; D is the fulcrum or 

 prop; and the part AB and BC, on different 

 sides of the prop, are called the arms of the lever. 

 It is demonstrable that in this instrument the 

 nearer the prop is to the end A, and the longer 

 the arm BC is, the less force will be required to 

 effect any given purpose. This is, indeed, re- 

 duced to a matter of experiment. For let P repre- 

 sent a power, whose gravity is equal to one ounce; 

 and W a weight, whose gravity is equal to twelve 



