THE GASEOUS PORTIONS OF THE ATMOSPHEBE. 



11 



therefore cannot be attributed to any diniinution in the 

 quantity of the aqueous constituent of the atmosphere, but is 

 clearly traceable to the warming and lightening effect of 

 condensation of vapour on the gases. A local stratum of the 

 atmosphere of some certain depth or thickness is evidently 

 thereby rendered lighter, when it presses with reduced weight 

 on the mercury of the barometer. And this reduction in the 

 weight of the gases must be really greater than is indicated 

 by the fall of the barometer, to the extent of the known 

 increase in the weight of the aqueous element of pressure. 

 The increase of that element would have made the barometer 

 rise, had there been no heating, and it must consequently 

 counteract the fall which is taking place from the heating to a 

 corresponding extent. 



We have then a barometer falling from say ten o'clock in 

 the morning, when we know that additional quantities of 

 aqueous matter are directly or indirectly pressing upon it; and 

 as these additional quantities remain in the air until say about 

 four o'clock in the afternoon, during which time the barometer 

 continues to fall, that fall can be attributed only to the 

 lightening effect of condensation. This condensation is at the 

 same time seen to take place in the formation of cloud, whilst 

 the generally moderate increase of the tension of vapour 

 below, or its stationary condition, or its occasional positive 

 decrease, shews that that vapour is expanding into and 

 penetrating the air with proportionate facility, and is freely 

 ascending to form cloud. During this period, therefore, that 

 is to say, from ten in the morning to four in the afternoon, 

 the tension of vapour near the surface is not a true indicator 

 of the total quantity of aqueous matter that is in the atmo- 

 spheric space ; and the whole evidence presented obliges us 

 to conclude that much of that matter then exists in the form 

 of minute globules of water, which float in and rest upon the 

 gases, leaving them to sustain the weight of the globules, 

 which weight must increase the pressure of the gases on the 



