20 ON THE SEPARATE PRESSURES OF THE AQUEOUS AND 



penetrated each other, and each rests on the earth, each 

 exists independently of the other, and is in the state that it 

 would be if it were in the atmospheric space alone; and so 

 it is with the atmospheric vapour and the gases. It may, and 

 we have presumed that it does, at first and temporarily act 

 against the gases. It has, indeed, been shewn that the actual 

 tension of the vapour which is ordinarily found in the middle 

 of the day near the surface of the earth, is partly due to this 

 cause, the gases obstructing temporarily its expansion into 

 the atmospheric space. But when the vapour has fully 

 diflfused itself through the gases, and rests independently on 

 the earth, it exists within them without affecting them. 

 This is a consequence of the law, discovered by Dalton, of 

 mechanical diffusion of aeriform substances through each 

 other, and of their independence of each other when so 

 diffused. 



Globules of water, on the contrary, when floating in the 

 gases, rest upon them, because those globules are not aeriform, 

 but liquid, and the gases sustain them for the time because 

 they interpose an obstruction to their descent, just as they 

 would to the descent of any other light substance. But the 

 vapour being an aeriform body, presses with a weight of its 

 own on the surface, and although for the reasons given we 

 cannot admit the tension of vapour to be always a correct 

 measure of its own weight, yet there is no doubt that it has 

 a certain amount of pressure, and that amount is so much to 

 be added to the pressure of the gases, — sustaining, as they 

 often do, at the same time, the weight of the particles of water 

 that may be floating in them. 



There is, then, no rsason to doubt, that the greater changes 

 of total average atmospheric pressure, as measured by the 

 barometer, which take place in different parts of the world, 

 are due to the same causes as have been traced in the smaller 

 ones. But we know that such very great and sudden re- 

 ductions of pressure sometimes occur, as to cause the 



