27.0 WRITINGS OF JOSEPH HENRY. [1855- 



creased by the addition of every grain of water which rises 

 in the form of vapor from the surface of the earth or ocean, 

 but when the evaporation is copious in a limited space, as for 

 example, over the surface of a pond of water, or a portion of 

 the earth subject to sunshine while the regions around are ob- 

 scured by clouds, the elastic force of the vapor tends to dimin- 

 ish the specific gravity of the aerial column and to produce a 

 fall rather than a rise of the barometer. This is always the 

 case while the vapor is in the act of dififusion ; for the resist- 

 ance of the atmosphere at the surface of the expanded volume 

 of vapor may be considered as an elastic envelope against 

 which, as in the case of the India-rubber bag to which we have 

 previously alluded, the aqueous atoms press by their repul- 

 sion and tend to expand it, and therefore to increase their 

 own volume as well as that of the inclosed atmosphere. 



If the vapor ascended into the air without resistance, (as 

 in a vacuum,) it would in all cases increase the weight of the 

 latter, but on account of the resistance under the conditions 

 we have just mentioned, the ascending vapor by its elasticity 

 would lift up the atmosphere, tend to lessen its pressure, and 

 thus temporarily to expand the air in the space included 

 within the surface of the aqueous volume. It is therefore a 

 difficult point to ascertain in the explanation of these phe- 

 nomena, when we must consider the weight of the atmos- 

 phere increased, or when diminished — by the pressure of 

 vapor. 



It is evident from the experiments which have been made 

 on evaporation under diminution of pressure of air, that the 

 resistance to diffusion spoken of diminishes in proportion to 

 the rarity of the atmosphere; and hence the vapor which 

 exists at great elevations would be in a state of entire diffu- 

 sion, and its presence would increase the specific gravity of 

 a portion of air through which it is disseminated, instead 

 of diminishing it. 



We think erroneous conclusions have frequently been 

 arrived at on account of a want of a proper consideration of 

 this subject, and from too exclusive an attention to the ex- 

 pansive influence of the aqueous vapor in a confined space 



