THE ATMOSPHERE. 



201 



the atmosphere extends. But the stop-cock D being closed, the effect of the 

 weight of all the air above that point is intercepted ; and, consequently, the 

 surface F can sustain no pressure arising from weight, except the amount of 

 the weight of the small quantity of air included between F and D, which is 

 altogether insignificant. But the air thus included presses on the surface F 

 by its elasticity ; and the amount of this pressure is equal to the force which 

 confined the air within the space F D before the stop-cock was closed : but 

 this force was the weight of the column of atmosphere above D ; and hence it 

 appears, that the elastic force of the air confined in the space D F is equal to 

 the atmospheric pressure. 



Now the other surface, E, the end A of the tube being open, is subject to 

 the atmospheric pressure. Thus the two surfaces, F and E, of the mercury, 

 are each subject to a pressure arising from a different quality of atmosphere ; 

 the one F, being pressed by its elasticity, and the other, E, being pressed by 

 its weight. These pressures being equal, the surfaces F and E continue at the 

 same level. 



Fig. 7. 



Fig. 8. 



The method of ascertaining experimentally, the pressure arising from the 

 weight of the atmosphere, will be fully explained hereafter ; meanwhile, it is 

 necessary for our present purpose to assume this pressure as known. 



Let us suppose, then, that the atmospheric pressure acting upon the surface 

 E is the same as would be produced by a column of mercury 30 inches in 

 height resting on the surface E : the force with which the elasticity of the air 

 confined in D F presses on the surface F is therefore equal to the weight of a 

 a column of thirty inches of mercury. The pressure of the atmosphere acting 

 on the surface E is transmitted by the mercury to the surface F and balances 

 the elastic force just mentioned. Let the position of the surface F be marked 

 upon the tube, and let mercury be poured into the longer leg at A. The in- 

 creased pressure produced by the weight of this mercury will be transmitted 

 to the surface F, and will prevail over the elasticity of the confined air ; this 

 surface will therefore rise toward D, compressing the air into a smaller space. 

 Let the mercury continue to be poured in at A, until the surface F rise to F', 

 fig. 8, the middle point between the end D of the tube, and its first position 

 F. The air included is thus compressed into half its former dimensions, and 

 its elasticity will be measured by the amount of the force with which the sur- 



