SE VENTEENTH CENTUR Y. 



FT. III. 



When Torricelli had come to this conclusion 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 atmo- 

 sphere. The mercury began 

 at once to sink in the tube, 

 and finally settled down at B, 

 about 30 inches above that 

 in the basin. This was a 

 beautiful experiment, and pro- 

 ved almost to demonstration 

 that the weight of ordinary air 

 is sufficient to keep a column 

 of mercury at a height of 30 

 inches in vacuum. He had 

 now therefore made an instru- 

 ment which would measure the 

 weight of the air, and as our 

 atmosphere varies in weight ac- 

 cording 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 place, and when the mercury rose above the 

 30 inches he concluded that the air outside was heavy; 

 when, on the contrary, it sank, this showed him that the air 

 was light. When once this was discovered it was easy to 



FIG. 15. 

 Torricelli's Experiment (Ganot). 



