THE CONQUEST OF NATURE 



title. What he did know, from practical experience, 

 was that with the aid of a stick he could pry up stones 

 or logs that were much too heavy to be lifted without 

 this aid. 



This practical knowledge no doubt sufficed for a 

 vast number of generations of men who used the lever 

 habitually, without making specific study of the rela- 

 tions between the force expended, the lengths of the 

 two ends of the lever, and the weight raised. Such 

 specific experiments were made, however, more than 

 two thousand years ago by the famous Syracusan, Archi- 

 medes. He discovered or if some one else had dis- 

 covered it before him, he at least recorded and so gains 

 the credit of discovery the specific laws of the lever, 

 and he also pointed out that levers, all acting on the 

 same principle, may be different as to their practical 

 mechanism in three ways. 



First, the fulcrum may lie between the power and the 

 weight, as in the case of the balance with which we 

 were just experimenting. This is called a lever of 

 the first class, and familiar illustrations of it are fur- 

 nished by the poker, steelyard, or a pair of scissors. 

 The so-called extensor muscles of the body those 

 for example, that cause the arm to extend act on the 

 bones in such a way as to make them levers of this 

 first class. 



The second type of lever is that in which the 

 weight lies between the force and the fulcrum, as 

 illustrated by the wheelbarrow, or by an ordinary door. 



In the third class of levers the power is applied be- 

 tween weight and fulcrum, as illustrated by a pair of 



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