of the Moist-bulb Hygrometer. 281 
I have now to notice the last circumstance which, as far as I understand the sub- 
ject, can have any influence upon the accuracy of my determination of the Dew- 
point. 
When the wet-bulb Hygrometer has attained its stationary temperature, the caloric 
which it loses and gains in a given time are perfectly equal. This requires no demon- 
stration. ‘The caloric lost also is entirely employed in converting the water into 
vapour ; but the whole of the acquired caloric is not necessarily derived, although such is 
assumed to be the case, from the air cooled by contact with the bulb of the instrument. 
In fact, the Hygrometer is in the predicament of a cool body placed in a warm 
medium, and it must consequently receive from surrounding bodies, by radiation, a 
greater amount of caloric than it imparts to them in virtue of the same process. ‘To 
the d grains, therefore, of moisture converted into vapour by the heat given out by 
4195 grains of air, in cooling through d degrees, we should add the additional quan- 
tity vaporized by the heat which the bulb has in the same time received by radiation. 
When ¢—? is small, this quantity may probably be safely neglected; but it will 
sometimes, I make no doubt, be of sufficient magnitude to exercise an appreciable 
influence. I regret my inability to assign any means of determining its amount ; 
and shall merely add, that the neglect of this correction will always tend to render the 
calculated Dew-point somewhat higher than the true. 
Having disposed of the theory of my method, I shall now conclude by subjecting 
the results which it affords to the test of experiment. I shall not at present refer to 
my own observations, though I have of late amassed a considerable number on the 
Hygrometer and Dew-point. As a more unimpeachable criterion, I shall compare 
my formula with the observations of others, and shall select for this purpose, it being 
the nearest at hand, a table published in the last number of the Edinburgh Journal. 
The differences, it will be seen, between the corresponding numbers of the fourth and 
fifth* columns of this table are so small, that we may consider them as almost entirely 
due to errors of observation. I may add, that as in the original table there is no 
notice taken of the barometer, the formula, in its most complete form, could not be 
applied; so that a perfect coincidence between calculation and observation was not in 
this instance to be expected. 
* The numbers in the fourth column are the observed, and in the fifth the calculated dew-points. 
