ii8 



SEVENTEENTH CENTURY. 



PT. III. 



Fig. 15. 



then the column of water c b in the tube will press as heavily 

 on the water at b as the air does on the water outside from 

 w to w, so all the water w b w will again be equally pressed 

 upon, and no further rise will take place in the tube. 



When Torricelli had made this discovery it occurred to 

 him that if it was really the weight of the air which sup- 

 ported the column of water it ought to lift mercury or 



quicksilver, which is fourteen 

 times heavier, to one-four- 

 teenth of the height. So he 

 took some mercury, and filling 

 a tube A, about 34 inches long, 

 with it, he turned the tube 

 upside down into a basin of 

 mercury, which being open was 

 under the pressure of the at- 

 mosphere. The mercury began 

 at once to sink in the tube, and 

 finally settled down at b, about 

 30 inches above that in the 

 basin. From this Torricelli 

 knew that the weight of ordi- 

 nary air is sufficient to keep a 

 column of mercury at a height 

 of 30 inches in vacuum. He 

 had now therefore made an 

 instrument which would mea- 

 sure the weight of the air, 

 and as our atmosphere varies in weight according as the 

 weather is cold or hot, or damp or dry, a column of this 

 kind would be higher when the air was heavy and lower 

 when it was light. He kept this apparatus always in one 



Torricelli's Experiment (Ganot). 



