THE BAROMETER. 91 



72) ; and if we suppose a cylindrical column extending 

 above any horizontal area to the surface of the atmosphere, 

 the weight of the column of air must be entirely supported 

 by the horizontal area upon which it rests, and the pressure 

 upon the area is therefore equal to the weight of the column 

 of air. The pressure of the air must then diminish as the 

 height above the earth's surface increases ; and from exper- 

 iments in balloons and in mountain ascents, this is found to 

 be the case. 



The action of gravity is equivalent to the effect of a compression of 

 the gas, and it is thus seen that the pressure of a gas is in fact caused 

 by its weight, as in the case of a liquid. 



Taking TT for the pressure of the air at any given place 

 (Art. 11, Cor. 2), and assuming that the density of the air 

 throughout the height z is constant and equal to p, the 

 pressure at the height z will be 



TT gpz. (I) 



COR. It may be shown, in the same manner as for air, 

 that any other gas has weight, and that the intrinsic weight 

 is in general different for different gases. Carbonic acid 

 gas, for instance, is heavier than air, and this is illustrated 

 by the fact that it can be poured, as if it were 

 liquid, from one jar to another. 



43. The Barometer. This instrument, 

 which is employed for measuring the pressure 

 of the atmosphere, is, in its simplest form, a 

 straight glass tube AB, about 32 or 33 inches 

 long, containing mercury, and having its lower 

 end immersed in a small cistern of mercury ; the _. 

 end A is hermetically sealed, and there is no air 

 in the branch AB. Since the pressure of a fluid at rest is 

 the same at all points of the same horizontal plane (Art. 10), 

 the pressure at B, in the interior of the tube, is equal to 



