OBSERVATIONS TAKEN IN INDIA. 
351 
tions to Daniell’s hygrometer, and to which it is no doubt subject, but which are 
applicable to all temperatures and all depressions, objections which were urged by 
M. Regnault, and which induced him to invent his “ hygrometre condenseur,” an 
instrument which he pronounces to be free from the errors of Daniell’s instrument, 
but the use of which I have not heard of in England. 
I have thus reviewed, in extenso, the possible, indeed the probable sources of error 
in the very high degree of humidity constantly in the air, as represented by the wet 
bulb observations made in India, and I have no hesitation in expressing my belief 
that the results, which I have obtained with the labour of some months, do not re- 
present the real fractions of saturation of the air at the several places where the wet 
bulb was observed*; and I am the more confirmed in this opinion, with respect to 
the Neelgherries, by an ofiicer now in London, who resided some time at Ootacamund 
and kept a meteorological register, who says that if the mean annual moisture in the 
air had amounted to anything like 90 per cent, it must have been most inconveniently 
felt in the clothes, hats, bedding and furniture of the residents ; but so far from such 
being the case, that, with the exception of occasional fogs, the hills were looked upon 
as rather dry than otherwise. From my doubts respecting the correctness of the de- 
duced fraction of saturation of the air at places in India, with the formulse I have 
passed in review, I may possibly except the observations made in the Deccan with 
Daniell’s hygrometer, which observations have a semblance of truth, although the 
instrument may be imperfect. Even with a comparatively low mean annual per- 
centage of 55 of moisture in the air, I would say such fraction of saturation was 
rather higher than the truth, for during some months of the year in the Deccan the 
air is so dry that it is difficult to prevent the disposition of the leaves of tables to 
curl up into hollow cylinders, and the nib of a quill pen is always most provokingly 
straddling out into the form of a pair of open compasses. 
By reason of the above-noticed sources of error in the wet bulb itself, and of the 
inadequacy of the formulse to give a satisfactory value to its readings, supposing the 
indications to be correct, I have deliberately not applied any corrections to the read- 
ings of the barometer on account of moisture. But even had the instrument been 
free from error and the formulse exact, I still should have deemed it ineffectual to 
attempt to measure the moisture in a whole column of the atmosphere /rom a local 
observation^ with a view to apply to the barometer, which represents the pressure of 
an entire column of the atmosphere, a correction for the tension of vapour:|:. Fur- 
ther, I have doubts of the propriety of applying to the barometer a correction for 
* Dr. M'Clelland says, “ The wet bulb thermometer would add greatly to the value of these results, but 
there are discrepancies in the register of the instrument in use (at Calcutta) which prove it to be imperfect." 
— Calcutta Journal of Nat. Hist., vol. v. p. 554. 
t “This instrument [wet bulb] simply indicates the conditions of the air of the place where it is situated : 
at 100 feet above it the conditions may be very different.” — Glaisher’s Hygrometrical Tables, p. 16, edition 
of 1847. 
J The capacity of air to hold water in solution, or in a state of vapour, diminishes with the tempe- 
rature. 
