of the Moist-bulb Hygrometer. 279 
pressure, such as occur to the natural atmosphere, the differences of specific heats 
under a constant volume, are proportional to the differences of pressure. And the 
same philosophers have shewn, that for pressures in the ratio of 1 to 1.3583, the cor- 
responding ee are 1 and 1.2396. Hence, as 
3583 : .2396 : & —1 2 —1, ¢ being the specific heat under a constant vo- 
lume at 30, and x that at p,—a proportion from which we deduce 
a = (.0223 p + 8312) c. 
But the specific heats under a constant volume, divided by the densities, give the 
specific heats of equal weights. And as the densities vary as the pressures directly, 
and as the temperatures + 448 inversely, and are, therefore, to each other in the 
30 - 
present case as 77577Gy to ee , we will have 
aan M8 te 
: (.0223 p + .8312)c¢ cb ee 
So that 2’, or the specific heat of air at temperature ¢ and pressure p, = 
ex (.0228 p + .3812) x .267. 
508 
The erie therefore, of f' already given, when subjected to this correction, will 
d A48+¢ | 
become f" — 5, x —5g- x (.0223 p + 3312). 
The equation for this correction, given by the writer in the Edinburgh Philosophi- 
cal Journal, is exclusively a function of p; but if the method here explained be cor- 
rect, and I believe it will be found so, the temperature ¢ of the Hygrometer has a 
still greater influence on its amount. They both, however, affect it in the same 
direction ; i.e. as they rise, it increases; and as they fall, it diminishes: so that if 
the one should augment as the other diminishes, they will counteract, to some 
extent, each other’s effects. When ¢’=50, and p= 29, 
ape =f —£ x.958, 
i.e. the value of the subtractive quantity is diminished by its ath part; but if ¢ being 
still 50, p be supposed 31, 
Ti ar x 1.008, 
or the subtractive term is augmented we by its anth part,—facts from which the ge- 
neral conclusion may be drawn, that when 60—¢ =30—p, the latter difference being 
measured in tenths of an inch, and that they have opposite signs, the correction may 
be altogether neglected. 
The theoretical justness of the fourth objection must also be conceded. The 
medium which is in contact with the bulb of the Hygrometer is not dry air, but air 
