OBSERVATIONS TAKEN IN INDIA, 
343 
France, in a closed room in the College, and in the theatre of the College, opening 
the windows. In the open air, with a temperature ranging from 7°' 16 to 17°*88 Cen- 
tigrade, and a depression ranging from 1°'86 to 9°'60, the results, by observation and 
by M. Regnault’s tentative formula^ were sufficiently satisfactory ; but in the closed 
chamber he says, “Les fractions de saturation calculees avec la formule-f-, sont 
ici beaucoup plus fortes que celles que Ton deduit des pesees directes de I’eau ren- 
fermee dans I’air ; en d’autres terrnes la temperature t' marquee par le thermometre 
mouille n’est pas assez abaissee par la vaporisation de I’eau que se fait a sa surface 
pour donner dans la formule la veritable force elastique x de la vapeur. Cette cir- 
constanee tient evidemment a ce que I’air se trouve beaucoup moins agite qu’a I’ex- 
terieur.” — Page 219. At page 220 M. Regnault adds, “ Ces experiences demontrent 
de la maniere la plus evidente que la formule ne peut pas rester la meme pour les 
divers etats d’agitation de I’air.” 
Here then is a second source of error ; and it is somewhat curious that Professor 
Orlebar, in guarding against another grave source of error, which will be adverted 
to, himself contributes to an error of observation. He had observed the effect of 
wind blowing upon the wet bulb in unduly depressing the temperature, and to guard 
against this he says, “ As it was equally essential that the bulb of this thermometer 
should not be exposed to the wind, and that it should be in the same body of air as 
the air-thermometer when the latter was exposed to the wind, a small mirror, about 
an inch square, was put on a little stand, and this being placed upon the tin board 
could be moved about by the observer into such a position that it might always cut 
off thewind from the hulh of the wet thermometer only” (page Ixiii.). Now the wet bulb 
being thus screened, would be buried in its own vapour and the reading would neces- 
sarily be too high. When the air is perfectly calm the same would be the result 
without the screen, for there would be a shell or coat of saturated air round the bulb. 
I had occasion to notice this local character of aqueous vapour in my Meteorology of 
the Deccan, where I constantly witnessed it, as regulated in its distribution by nature. 
Speaking of dew, I said in the year 1828, ‘‘At Marheh in the Pergunnah of Mohol, 
garden produce (which is usually irrigated during the day-time) was covered with a 
copious dew every morning ; the lands bordering the gardens for forty or fifty yards 
around were slightly sprinkled with it, hut there was not a vestige of it in the fields 
constituting the rising ground north and south of the tract of garden land.” Hence 
I inferred that “ aqueous vapour had been taken up by the action of the sun during 
the day, suspended over the spot, and deposited by the lower temperature at night as 
dew upon the land in proportion to the supply obtained by day.” My tents were 
within 200 yards of the fields where I observed these phenomena, but from the 1 1th 
to the 30th of January 1828, there was not any deposition of dew about them, ex- 
cepting on the 13th of January. In consequence of these observations I was induced 
T 610-i' 
H. 
* Annales de Chimie, tom. xv. p. 218. 
