GALILEO AND THE NEW PHYSICS 



planation of the principle. Torricelli was able to de- 

 monstrate that the height at which the water stood de- 

 pended upon nothing but its weight as compared with 

 the weight of air. If this be true, it is evident that 

 any fluid will be supported at a definite height, accord- 

 ing to its relative weight as compared with air. Thus 

 mercury, which is about thirteen times more dense 

 than water, should only rise to one - thirteenth the 

 height of a column of water that is, about thirty 

 inches. Reasoning in this way, Torricelli proceeded to 

 prove that his theory was correct. Filling a long 

 tube, closed at one end, with mercury, he inverted the 

 tube with its open orifice in a vessel of mercury. The 

 column of mercury fell at once, but at a height of 

 about thirty inches it stopped and remained station- 

 ary, the pressure of the air on the mercury in the ves- 

 sel maintaining it at that height. This discovery was 

 a shattering blow to the old theory that had dominated 

 that field of physics for so many centuries. It was 

 completely revolutionary to prove that, instead of a 

 mysterious something within the tube bein respon- 

 sible for the suspension of liquids at certain heights, 

 it was simply the ordinary atmospheric pressure 

 mysterious enough, it is true pushing upon them from 

 without. The pressure exerted by the atmosphere 

 was but little understood at that time, but Torricelli 's 

 discovery aided materially in solving the mystery. 

 The whole class of similar phenomena of air pressure, 

 which had been held in the trammel of long-estab- 

 lished but false doctrines, was now reduced to one 

 simple law, and the door to a solution of a host of un- 

 solved problems thrown open. 



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