38 Meteorological Observations on Parisnath Hill. [No. 1. 



which the first gas, g, would have shown, if the other gas had never 

 been introduced, but a smaller one. 



The space between the marks a and b, which was at first only 

 filled with the gas g contains now two gases, the quantity of the 

 first having been diminished and replaced by a corresponding volume 

 of the other. To express the process in one word, it may be said, 

 that in the original space the second gas has displaced a part of the 

 first, the pressure remaining the same. 



Returning again to the original figure, if instead of the gas g', a 

 quantity of another gas, g", be introduced, which, occupying the 

 same volume, 2 V, as the gas g, exerts an elastic pressure, p', which 

 is smaller than p, then both gases, the volume remaining the same, 

 would exert the pressure p + p\ But the weight they have to 

 support being smaller, they will raise it until the volume they 

 occupy will be to the original volume, 2 V, as p + p' • p> when equili- 

 brium will again have been restored, the sum of their elastic pressures 

 not exceeding p. The individual pressure of each gas would again 

 be diminished in proportion with the alteration of its density. 



Any circumscribed space in the lower part of the atmosphere, in 

 which the circulation of the watery vapour principally takes place 

 may be compared to the space filled by the first gas g, in the cj'linder, 

 and the moveable weight would then represent the weight of that 

 height of the atmosphere which is pressing upon the circumscribing 

 surface and which is balanced by the elastic pressure of the air. The 

 third gas g", of the same volume and smaller elastic pressure 

 would be the watery vapour, which during the day is continually be- 

 ing supplied wherever water is present on the surface of the ground, 

 or by currents of moist air. 



The weight of the watery vapour itself must increase the weight 

 of the whole atmosphere, but the variation in this addition in weight 

 must be comparatively small, and may be neglected in applying the 

 illustration, the greater part of the absolute quantity of watery vapour 

 contained in the air being permanent and not subject to changes. 



So long as the supply of watery vapour is equably and continu- 

 ously kept up from a large surface, a state nearly approaching equi- 

 librium must soon be established between the stratum containing 

 the vapour and the remaining part of the atmosphere which presses 



