THE BEGINNINGS OF EXPERIMENTATION 55 



pump the water would never rise over 32 feet. The 

 piston of an ordinary lift pump draws the air out of 

 the pipe and thus creates a vacuum into which the 

 water rushes, as was early recognized. It was Hiero 

 who first attributed this phenomenon to " nature's 

 abhorrence of a vacuum, " and in default of any better 

 explanation this was accepted until Torricelli investi- 

 gated the subject experimentally. 



The reasoning which led Torricelli to his famous 

 experiment is as follows: If air has weight, then on the 

 surface of the water in a well there is the downward 

 pressure of the air. If the air is removed from a part 

 of this surface, as for example that inclosed by the pipe 

 of the pump, the pressure of the air on the rest of the 

 surface of the water should force water up the pipe. 



This, then, was Torricelli' s theory to account for the 

 action of a suction pump. How was he to test it? 

 If the pressure of the air at the surface of the earth was 

 the cause, and not some mysterious effect of a vacuum, 

 then this pressure should support a column of mercury 

 only one thirteenth of 32 feet, since mercury is thirteen 

 times as dense. If this deduction corresponds with 

 observed phenomena, the theory has been corroborated 

 although not necessarily proven. To establish it a 

 further experiment is necessary. 



He, therefore, selected a tube about 4 feet long and 

 sealed one end. This he filled with mercury, closed 

 the open end by his finger, and inverted the tube, 

 immersing the unsealed end in a basin of mercury. 

 When he removed his finger the mercury in the tube 

 sank, thus forming a " Torricelli vacuum." This 

 vacuum was evidently unable to hold the mercury, which 



