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THE POPULAR SCIENCE MONTHLY. 



tracted neck at the end connected with the pump, so as to readily 

 admit of being sealed off at any stage of the exhaustion. D is the 

 pum2>gauge, and ^is the barometer. 



a 



IV- 



FlQ. 1. 



The whole being fitted up as here shown, and the apparatus being 

 full of air to begin with, I passed a spirit-flame across the lower pai't 

 of the tube at 5, observing the movement by a low-power micrometer : 

 the pith-ball {a h) descended slightly, and then immediately rose to 



