48 THE ENGINES OF THE HUMAN BODY 



to lay emphasis on that fact because I did not insist 

 enough on the living nature of muscular engines. 



We are all well acquainted with levers. We apply 

 them every day. A box arrives with its lid nailed down ; 

 we take a chisel, use it as a lever, prise the lid open 

 and see no marvel in what we have done (fig. 9). And 

 yet we thereby did with ease what would have been 

 impossible for us even if we had put out the whole of 

 our unaided strength. The use of levers is an old dis- 

 covery ; more than 1 500 years before Christ, English- 

 men, living on Salisbury Plain, applied the invention 



Fig. 9. — Showing a chisel 10 inches long used as a lever of the first order. 



when they raised the great stones at Stonehenge and at 

 Avebury ; more than 2000 years earlier still, Egyptians 

 employed it in raising the pyramids. Even at that time 

 men had made great progress ; they were already reaping 

 the rewards of discoveries and inventions. But none, I 

 am sure, surprised them more than the discovery of the 

 lever ; by its use one man could exert the strength of a 

 hundred men. They soon observed that levers could be 

 used in three different ways. The instance already given, 

 the prising open of a lid by using a chisel as a lever, is an 

 example of one way (fig. 9) ; it is then used as a lever 

 of the first order. Now in the first order, one end of the 

 lever is applied to the point of resistance, which in the 

 case just mentioned was the lid of the box. At the other 

 end we apply our strength, force, or power. The edge of 

 the box, against which the chisel is worked, serves as a 

 fulcrum and lies between the handle where the power is 

 applied and the bevelled edge which moves the resistance 



