Barometer. 353 



With regard to the facility and rapidity of their motions, as the 

 incumbent weight acts equally in all directions, it neutralizes 

 itself, by aiding just as much as it obstructs their efforts to move 

 and turn themselves. The case is precisely similar with respect 

 to land animals. The vessels of the animal, together with the 

 bones, are filled with air or some other fluid capable of support* 

 ing any weight, and whose elasticity being equal to that from 

 without, proves an exact counterbalance to it. 



467. In the barometer there is an equilibrium between the 

 pressure of the mercury and that of the atmosphere. Now we 

 have seen that when two fluids thus counterbalance each other, 

 the altitudes must be inversely as the specific gravities. Accord- 

 ingly, as the specific gravity of mercury is to that of air at the 

 surface of the earth as 13,57 to 0,00122, we shall have 



1 3 ^7 V 30 



0,00122 : 13,57 : : 30 : ^ * g = 333688. 



We infer, therefore, that the height of the atmosphere, on the 

 supposition of a uniform density throughout, is 333688 inches, 

 or a little more than 5 miles. But the air being eminently elas- 

 tic, the lower strata are compressed by the incumbent weight of 

 those above, so that the density becomes less and less continu- 

 ally as we ascend. Let the weight of the column of mercury 

 which measures the pressure of the atmosphere, exerted upon a 

 unit of surface, be denoted by g A h, g being the force of gravity, 

 A the density of the mercury, and h the perpendicular height of 

 the column above the level of the surface in the basin, and let 

 the weight of the atmosphere upon the same surface be denoted 

 by w, we shall have 



g A h = w. 



As we ascend into the atmosphere, the weight w and the height 

 h diminish continually, and these diminutions depend upon the 

 elevation attained, and the law according to which the densities 

 of the atmospheric strata decrease. If this law were known, it 

 might be made use of for the purpose of determining the differ- 

 ence in the altitudes of two points above a common level, as the 

 sea, or any assumed level. But in order to discover this law, it 

 Mech. 45 



