HOW WORK IS DONE 



and in cogged and bevelled gearings. A moment's 

 reflection will make it clear that the wheel is a lever 

 of the first class, of which the axle constitutes the ful- 

 crum. The spokes of the wheel being of equal length, 

 weights and forces applied to opposite ends of any diame- 

 ter are, of course, in equilibrium. It follows that when 

 a wheel is adjusted so that a rope may be run about it, 

 constituting a simple pulley, a mechanism is developed 

 which gives no gain in power, but only enables the 

 operator to change the direction of application of 

 power. In other words, pound weights at either end 

 of a rope passed about a simple pulley are in equilibrium 

 and will balance each other, and move through equal 

 distances in opposite directions. 



If, however, two or more pulley wheels are connected, 

 to make the familiar apparatus of a compound pulley, 

 we have accomplished by an interesting mechanism a 

 virtual application of the principle of the long and 

 short arm of the lever, and the relations between the 

 weight at the loose end of the rope and the weight at- 

 tached to the block which constitutes virtually the short 

 end of the lever, may be varied indefinitely, according 

 to the number of pulley-wheels that are used. A 

 pound weight may be made to balance a thousand- 

 pound weight; but, of course, our familiar principle 

 still holding, the pound weight must move through 

 a distance of a thousand feet in order to move a thousand- 

 pound weight through a distance of one foot. Familiar 

 illustrations of the application of this principle -may be 

 seen on every hand; as when, for example, a piano or a 

 safe is raised to the upper window of a building by the 



voL.vn.-s [33] 



