VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 247 



instrument suggested with regard to the different densities of the atmosphere in 

 the different zones of the earth. But since the experiments on the expansion of 

 moist air have shown its elasticity to be so much greater than that which is dry, 

 the simple principle of heat and moisture may suffice to account for all the phe- 

 nomena. Thus it is universally admitted, that there is a greater degree of 

 humidity and heat in the air near the earth's surface, than there is in the higher 

 regions of the atmosphere. The elasticity or expansion of the lowermost 

 section of every column of air, whether long or short, will consequently be 

 greater than the uppermost section of it; for the heat, by dissolving the mois- 

 ture, produces a vapour lighter than air, which mixing with its particles, 

 removes them farther from each other, increases the elasticity of the general 

 mass, and diminishes its specific gravity comparatively more than it does that 

 of the section immediately above it, where there is less heat and less moisture. 

 Hence it is inferred, that the equation for the air, in any assigned vertical, will 

 gradually diminish as the elevation of the place above the sea increases, and that 

 it will vanish at the top of the atmosphere. This is in some respect confirmed 

 by the experiments on the expansion of rare air; for from them it appears, 

 when the particles are very far removed from each other, by a great diminution 

 of pressure, as is undoubtedly the case in the higher regions of the atmosphere, 

 they lose a great part of their elastic force. Thus the equation, answering to 

 any particular temperature, above or below the zero of the scale, at different 

 heights above the surface, will be expressed by the ordinates to a curve of the 

 hyperbolic order, whose curvature may be supposed to change fast near the 

 surface of the earth, and differ insensibly from a straight line at great heights 

 above it. 



With regard to the latitudinal equation, the same principal of heat and mois- 

 ture seems to make it probable, that such will become necessary in operating 

 with the barometer; for it is well known, that there is a great degree of humi- 

 dity in the air between the tropics ; and, on the contrary, that the polar atmos- 

 pheres are very dry. The heat and moisture being greatest at the equator, there 

 the elasticity or equation will also be the greatest at the level of the sea; and the 

 zero of the scale will necessarily descend to a lower point of the thermometer, 

 than that to which it corresponds in middle latitudes. As the elasticity of the 

 air at the level of the sea, or equal heights above it, with the same 

 degree of heat, will always be proportionable to the quantity of moisture 

 dissolved in it, therefore it will gradually diminish from the equator towards the 

 poles, that is to say, the zero of the scale will ascend in the thermometer, 

 coincide with the 32d degree in the middle latitudes, and in its motion upwards 

 will give the equation to be applied with the contrary sign in high latitudes. 

 Hence it is inferred that every latitude, climate, or zone, will not only have its 



