THE LEVER AND WHEELWORK. 



247 



tions. Optics and all other branches of natural science are liable to the same 

 observations. 



THE LEVER. 



An inflexible, straight bar, turning on an axis, is commonly called a lever. 

 The arms of the lever are those parts of the bar which extend on each side 

 of the axis. 



The axis is called the fulcrum or prop. 



Levers are commonly divided into three kinds, according to the relative po- 

 sitions of the power, the weight, and the fulcrum. 



In a lever of the first kind, as in fig. 1, the fulcrum is between the power 

 and weight. 



Fig. l. 



T? <* F aw 



In a lever of the second kind, as in fig. 2, the weight is between the fulcrum 

 and power. 



In a lever of the third kind, as in fig. 3, the power is between the fulcrum 

 and weight. 



Fig. 2. Fig. 3. 



In all these .cases the power will sustain the weight in equilibrium, provided 

 its moment be equal to that of the weight. But the momen't of the power is, 

 in this case, equal to the product obtained by multiplying the power by its dis- 

 tance from the fulcrum, and the moment of the weight, by multiplying the 

 weight by its distance from the fulcrum. Thus, if the number of ounces in P, 

 being multiplied by the number of inches in P F, be equal to the number of 

 ounces in W, multiplied by the number of inches in W F, equilibrium will be 

 established. It is evident from this, that, as the distance of the power from 

 the fulcrum increases in comparison to the distance of the weight from the 

 fulcrum, in the same degree exactly will the proportion of the power to the 

 weight diminish. In other words, the proportion of the power to the weight 

 will be always the same as that of their distances from the fulcrum taken in a 

 reverse order. 



In cases where a small power is required to sustain or elevate a great weight, 

 it will therefore be necessary either to remove the power to a great distance 

 from the fulcrum, or to bring the weight very near it. 



Numerous examples of levers of the first kind may be given. A crowbar, 

 applied to elevate a stone or other weight, is an instance. The fulcrum is an- 

 other stone placed near that which is to be raised, and the power is the hand 

 placed at the other end of the bar. 



A handspike is a similar example. 



A poker applied to raise fuel is a lever of the first kind, the fulcrum being 

 the bar of the grate. 



