MATHEMATICAL SCIENCES SIXTEENTH CENT. 171 



FIG. 72. VON GUERICKE. 



continuing his patient observation of the instrument they had left. 

 He assured them that during their absence no variation had occurred 

 in the height of the mercury in the glass tube. Perier then once 

 more repeated the Torricellian experiment with the apparatus which 

 had been carried up the mountain. The mercury now stood in this 

 tube at the same height as in the morning. The next day a sugges- 

 tion was made that measurement should be taken of the mercurial 

 column at the top and at the base respectively of the loftiest tower 

 of the Cathedral of Clermont. This was done, and a difference of 

 one-sixth of an inch was observed. Pascal himself, subsequently re- 

 peating similar experiments at Paris, was able to observe a difference 

 when the instrument was carried from the cellar to the attic of a 

 private house. 



The question as to the pressure of the atmosphere being the cause 

 of the suspension of the mercury in the Torricellian tube was now vir- 

 tually settled, and thus there was acquired to science an instrument 

 of the greatest value the barometer. The space above the mercury 

 attracted a great share of the attention of philosophers, and much dis- 



