[ 213 ] 

 XXVI. Intelligence and Miscellaneous Articles. 



ON HYGKOMETRY. BY M. JAMIN. 



]\X MASCART publishes every year, in the Annates du Bureau 

 ±Y1_« meteor ologique de France, observations made in nearly one 

 hundred stations all over France, and the same is the case in all 

 European States. I have taken from them the pressure of the air 

 at the base and on the summit of the Puy de Dome, and I now 

 desire to make a general remark on hygrometry. 



On looking at the tables of relative moisture, it is surprising to 

 see them so uniform. The means are almost the same in each 

 moment and at all stations. Thus we find at Clermont Ferrand, 

 at midday in the year 1880, 



February. June. August. September. October. 



593 599 570 569 622 



It is, however, clear that there are great differences in the hy- 



grometric conditions of the months of February and August, and 



that if these are not brought out in the tables it is, probably, that 



the system adopted in reducing the observations is erroneous. 



f 

 This system consists in expressing the ratio 4 of the elastic 



force observed / to the maximum force F, which air would have 



at the same temperature if the air were saturated ; this is what is 



called the relative moisture. But for any given air of constant 



f 

 composition the quotient ^ varies : 1st, with the proportion of 



vapour ; 2nd, with the height and the barometric pressure, since / is 

 proportional to this pressure ; 3rd, and more particularly, with the 

 temperature which changes the value of F ; it is then a function 

 of three independent variables ; and we may hope that it brings 

 out the variations in the quantity of vapour. We must for this 

 eliminate the disturbing influences of pressure, of height, and of 

 temperature, but that can easily be done. 



When chemists analyze air, they determine the quantities of 

 oxygen, nitrogen, and carbonic acid ; to complete the analysis, it 

 would be logical to add the proportion of aqueous vapour. As 

 this vapour is a gas subject to the same laws of compression and 

 expansion as other gases, there is no reason to measure it other- 

 wise. 



Let / be the tension of vapour, H the total pressure of the 

 atmosphere, H— /that of the dry air, we have 



^, . , , r. v(l-293)(0-622)f 



Weight of vapour P = (1 + crf)760 ■ 



, TT . lx , , . ,., v(l-293)(H-/) 

 We lg ht of dry air F = A__J_^. 



P f 



TT7 = 0-622 



H-/ 



i 



