284 Professor Arsoun on the Theory 
The equation which, as I believe, comprehends the theory of the wet-bulb hygro- 
meter, is as follows :-— 
f=. —mdx 
in which f” is the tension of steam at the dew-point, /” its tension at the temperature 
of the hygrometer, d the depression or difference between the temperature of the hy- 
grometer and air, p the existing, and 30 the mean pressure, and m a coefficient de- 
pending upon the specific heat of air and the caloric of elasticity of its peeuded 
vapour, its arithmetical value being .01149, or the equivalent vulgar fr action 2 
In the paper to which I have already referred, corrections are given for the reflnesec 
on the specific heat of air, of the fluctuations of the barometer, and the moisture 
present in the atmosphere. These corrections are, I believe, deduced from correct 
principles, and should be resorted to when extreme precision is desirable. Expe- 
rience, however, and a careful consideration of the subject, have satisfied me that they 
are, generally speaking, in their effects, much too insignificant to be objects of atten- 
tion to the practical meteorologist. 
The first and most obvious method of verification which presented itself to my 
mind, was the comparison of my formula with recorded cotemporaneous observations 
on the temperature of air, that shown by a moist-bulb hygrometer, and the actual 
dew-point. I have, however, unfortunately been able to meet but few at all suited 
to my purpose. Those in which ¢—?' is small, and this is generally the case in the 
few registers to which I have had access, cannot, as we have already seen, serve for 
deciding the value of any formula. In the first report, indeed, of the British Asso- 
ciation for the Promotion of Science, page 50, mention is made of a register of obser- 
vations kept in the East Indies, which, as belonging to high temperatures, would 
necessarily exhibit great depressions, and would therefore be valuable as a standard 
of comparison ; but I have in vain searched for the Calcutta Journal, ‘* Gleanings in 
Science,” in which they are said to be contained. In fact, the only observations I 
have been able to procure, adapted to my purpose, and made, apparently at least, 
with the necessary precision, are those adduced in the article ‘ Hygrometry’ of Sir 
_ David Brewster’s Encyclopedia, and there made, by the author of the article, the 
basis of a calculation for investigating the constants of a tentative formula for con- 
necting the indications of the wet-bulb hygrometer with the dew-point. They are but 
two in number, and are comprehended in the following table, in which the numbers 
in the first column represent the temperatures of air, those in the second the corres- 
ponding indications of the hygrometer, those in the third the depressions, those in the 
