62 MECHANICS. 



as great for four feet, and so on. This holds true, no 

 matter by what kind of machinery it is accomphshed. 

 Now this may all seem very simple, hut it serves to ex- 

 plain -many difficult questions in relation to the real 

 power possessed by all machines. 



Take another example. Suppose that one wishes to 

 raise a weight of 1000 pounds to a height of one foot. 

 If his strength is only equal to 100 pounds, the weight 

 would he ten times too heavy for him. He might, 

 therefore, divide it into ten equal parts of 100 pounds 

 each. Raising each part separately the required height 

 of one foot, would he the same as raising one of them 

 ten feet high. The weight is lessened ten times, hut 

 the distance is increased ten times. But there are 

 some bodies, as, for example, blocks of stone or sticks 

 of timber, which can not well be divided into parts in 

 actual practice. He therefore resorts to a machine or 

 mechanical power, through which the same result is 

 accomplished by raising the whole weight in one mass 

 with his single strength ; but in this case as well as 

 the other, the moving force which he applies must pass 

 through ten times the space of the weight. We arrive, 

 therefore, at the general rule, that the distance moved 

 by the weight is as much less than that moved by the 

 power as the power is less than the weight. This rule 

 is termed by some writers the " rule of virtual veloc- 

 ities,'''' virtual meaning not apparent or actual, but 

 according to the real effect, because the increase in 

 the velocity of the power makes up for increase in the 

 size of weight. This rule will be better understood 

 after considering its application to the different simple 

 machines. 



