of the Moist-bulb Hygrometer. 287 
If the mean of all these values of m be taken, it will be found to be .01122, or the 
1 
equivalent vulgar fraction = an approximation to the coefficient = employed in the 
formula, which, under all the circumstances, cannot but be considered as remarkably 
close. Indeed the difference, which is less than 3 in the fourth place of decimals, is 
so small, that°they may be substituted indiscriminately for each other without the 
occurrence, at least in ordinary cases, of sensible error. Had values of m been cal- 
culated from the comparison alone of the first series of observations in each table with 
the subsequent ones, the mean, it is worthy of remark, would be .01156, or almost 
exactly a3 and as, for such observations, F” —f’ and D—d are necessarily greatest, 
they are best calculated to afford correct results, since any error of experiment would 
obviously, in their case, exercise the least influence. 
The next test experiments performed were suggested by the formula itself. If 
f'=f'-Z x = and f” be supposed equal to 0, a condition which can only be 
fulfilled in perfectly dry air, f’= a a an equation from which we deduce 
a 37 fx rh Hence, by determining experimentally the depression of the hygro- 
meter in perfectly dry air, we will be able to pronounce upon the validity of the 
general method under discussion. 
The first attempts for determining values of d experimentally, consisted in suspend- 
ing a pair of thermometers, one of which had its bulb moistened, in a close corked 
bottle, the bottom of which was covered with a stratum of oil of vitriol; but this 
method was soon abandoned, as the depressions it afforded were, on an average, one- 
fifth less than they should be according to the formula. In fact the extreme depres- 
sion could not be expected here, for it is obvious that the air, in contact with the bulb 
of the moist thermometer, is never perfectly dry except at the very commencement of 
the experiment. 
The next contrivance to which I resorted was as follows. A bag of India rubber 
cloth, furnished with a cap and stop-cock, was inflated by a bellows, and then con- 
nected, by means of a caoutchane collar, to a glass tube, traversing a cork fitted to 
the tubulure of the lower bottle of a Noothe’s apparatus. The middle bottle of the 
apparatus was next filled, 2 rds, with oil of vitriol, and the pair of thermometers last 
described being introduced into the axis of a small tube, perforating a cgrk fitted to 
the upper opening of this bottle, a stream of air was forced by pressing on the caout- 
chouc bag, through the oil of vitriol, and, of course, over the thermometers ; and as 
soon as the instrument with moistened bulb ceased to fall, the temperatures of both 
were noted. ‘The following table comprehends the results of five experiments thus 
performed :— 
