SCIENCE AS MEASUREMENT 95 



sealing the top of the tube after driving out the air 

 by heat. Spirits of wine were used in place of water. 

 Mercury was not employed till 1670. 



Descartes and Galileo had brought under criticism 

 the ancient idea that nature abhors a vacuum. They 

 knew that the horror vacui was not sufficient to raise 

 water in a pump more than about thirty-three feet. 

 They had also known that air has weight, a fact 

 which soon served to explain the so-called force of 

 suction. Galileo's associate Torricelli reasoned that if 

 the pressure of the air was sufficient to support a 

 column of water thirty-three feet in height, it would 

 support a column of mercury of equal weight. Ac- 

 cordingly in 1643 he made the experiment of filling 

 with mercury a glass tube four feet long closed at 

 the upper end, and then opening the lower end in a 

 basin of mercury. The mercury in the tube sank until 

 its level was about thirty inches above that of the 

 mercury in the basin, leaving a vacuum in the upper 

 part of the tube. As the specific gravity of mercury 

 is 13, Torricelli knew that his supposition had been 

 correct and that the column of mercury in the tube 

 and the column of water in the pump were owing to 

 the pressure or weight of the air. 



Pascal thought that this pressure would be less 

 at a high altitude. His supposition was tested on a 

 church steeple at Paris, and, later, on the Puy de 

 Dome, a mountain in Auvergne. In the latter case a 

 difference of three inches in the column of mercury 

 was shown at the summit and base of the ascent. 

 Later Pascal experimented with the siphon and suc- 

 ceeded in explaining it on the principle of atmos- 

 pheric pressure. 



