OBSERVATIONS TAKEN IN INDIA. 
347 
With assumed values for y, ^ and X , as the real values are not accurately known, 
and neglecting small quantities, and supposing y^='y, August’s formula became 
j .. 0 - 428 { t - t ') , 
Z77, — h, 
640 — ^ ’ 
for the determination of the elastic force and consequently the dew-point, and after 
certain comparisons of the dew-point from his formula with the direct dew-point 
from Daniell’s hygrometer, he found what he considered a sufficient agreement be- 
tween them. In a comparative table* of the results by his formula and the results 
by Daniell’s hygrometer, they appear to go pretty well together, while the variations 
of temperature and of the depression of the wet bulb are small ; but the moment the 
temperature exceeds 20° Centigrade (68° Fahr.), and the depression exceeds 5° 
(9° Fahr.), the discrepancy is very considerable ; for instance, at bar. 755'3 millims. 
(29'736 inches), dry 28°‘5 (83°'3 Fahr.), wet 21°'l (69°‘98 Fahr.), depression 7°'4 
(13°'32 Fahr.), the tension of vapour by the formula is 14' 181 millims. ('558 in. 
=:dew-point 62°), and by Daniell’s hygrometer 12’087 millims. ("475 in. = dew-point 
57°'15), the difference 2"094 millims ("083 in.). Even with a depression of only 0"6 
(1°’08 Fahr.) the tension of vapour by the formula and by Daniell is respectively 
8"612 millims. ("339 in.), and 8"534 millims. ("335 in.), difference 0"078. These 
discrepancies induced M. Regnault to modify, in 1845, nearly twenty years 
afterwards, August’s formula, and he in common with August assumed y=:0"2669, 
^=0"622 and «= 7 , but X=610— Substituting these numbers and neglecting 
0’429 t '\ 
small quantities, the formula became x=f’— — QiQ—fi ^5 formula that 
M. Regnault tests by his various experiments ; and at the close of his able and ela- 
borate paper he says, “Jene pense pas que I’on puisse admettre comme base du 
calcul du psychrometre I’hypothese fondamentale adoptee par August : a savoir, 
que tout fair qui fournit de la chaleur au thermometre mouille descend jusqu’a 
la temperature t' indiquee par celui-ci, et se sature completement d’humidite. 
II me parait probable que la portion de I’air que se refroidit ne descend pas jusqu’a 
i , et qu’elle ne se sature pas d’humidite. Le rapport de la quantite de chaleur 
que fair enleve a la boule par vaporisation de I’eau, a la quantite de chaleur 
qu’il perd en se refroidissant, est probablement d’autant plus grand que cet air est 
plus sec, parceque dans cet etat, il est beaucoup plus avide d’humidite que quand il 
approche de son etat de saturation. 
Enfin, la temperature de la boule mouillee est influencee encore autrement que par 
fair immediatement ambient, elle est soumise au rayonneinent de I’enceinte dont 
I’influence sera variable suivant I’^tat d’agitation de fair. 
“ 1 1 me parait impossible de faire entrer toutes ces circonstances dans le calcul thdo- 
retique de Vmstrument ; est je crois qu’il est plus sage de ne faire servir les consi- 
derations th^oretiques qu’a la recherche de la forme de la fonction, et a determiner 
ensuite les constantes par des experiences dans les conditions determinees. Cette 
* Annalen der Physik, B. v. p. 87. 
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