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Report S.A.A. Advancement of Science. 



of vapour so introduced is constantly undergoing precipitation, 

 and is thus continually being withdrawn from the total mass, 

 leaving behind it, however, to accumulate, the dry air which 

 accompanied it. Thus, if we regard the total barometric 

 pressure as sub-divided into that of the dry air, and of the 

 aqueous vapour, and denote the former by P, the latter by V, 

 we see that the dry pressure is diminished in the hot, and 

 increased in the cold hemisphere, without any countervailing 

 action, while V is in process of increase from below by 

 evaporation, and of diminution from above by overflow, in the 

 former : and vice versa in the latter. If, then, the observed 

 barometric pressure at every point in either hemisphere be 

 analysed by calculation into its two constituents, by taking 

 account of the hygrometric state of the atmosphere, and 

 subtracting from the total pressure P+V the portion V due to 

 the amount of vapour present, the remainders ought to exhibit, 

 as a general result, an excess of dry pressure P in the winter 

 hemisphere over that in the summer. 



" (i66.) So far as observation has hitherto gone, this result 

 is perfectly corroborated, though, unfortunately, there are not 

 yet accumulated sufficiently numerous and extensive series of 

 observations in which the effects of the aqueous pressure can be 

 duly separated from the dry. As examples, we shall select the 

 series for the Indian stations, Calcutta, Benares, Seringapatam, 

 and Poonah, calculated by Dove from the observations of 

 Prinsep, Sparmann, and Colonel Sykes, as compared with that 

 at Apenrade from those of Neuber, and with the results obtained 

 in the meteorological observatories of Prague, Toronto, and 

 Hobart. 



These differences are large quantities ; but we see that as the 

 maxima of P correspond in point of time with the minima of V, 

 it is only their differences which constitute the total or observed 

 annual fluctuation of barometric pressure. 



