On the Hygromtier. 331 



the temperature of the air, of the wet thermometer, and of the 

 dew-point. This ingenious method seems to correspond re- 

 markably with the results of the formula, except when the air 

 is supposed to approach to dryness. The error then rapidly in- 

 creases, as will be apparent from the following example. Sup- 

 pose that in air absolutely dry, the wet thermometer indicated 

 54}°; the temperature of the dew-point would then be infinitely 

 low, and the line DE in the diagram would coincide with the 

 asymptote. But this would give to the air a temperature infi- 

 nitely high ; whereas, by the formula, it ought only to be about 

 114°. 



The author regrets, that, before writing his former observa- 

 tions, he had not seen Sir James Ivory's paper " On the Hy- 

 grometer by evaporation,'' contained in vol. Ix. of the Philoso- 

 phical Magazine. That investigation seems to be so compre- 

 hensive, that, if the result does not entirely correspond with ex- 

 periment, the cause of the non-accordance must be sought for 

 in the inaccuracy of some of the data. But the only two data 

 not derived from experiment are, that the air in contact with 

 the wet bulb is fully saturated ; and, that the heat by which the 

 water is converted into vapour is entirely derived from the air. 

 The writer had at one time some difficulty in reconciling the 

 latter supposition with the phenomena, but now he sees that 

 they are strictly consistent ; and that, while, at the commence- 

 ment of the evaporation, the bulb supplies nearly the whole 

 heat, and the air scarcely any, the case is perfectly reversed by 

 the time the maximum cold is established. When the maximum 

 has been once attained, the film of water may be conceived to act 

 asa perfect non-conductor between the air and the bulb of the ther- 

 mometer; for, instead of transmitting to the latter the heat which 

 it receives from the former, a portion of it combines with that 

 heat, and both are dissipated in the form of vapour. With re- 

 spect to the other supposition, namely, that the air in contact 

 with the wet bulb is fully saturated, it will be considered at a 

 subsequent part of the paper. In the mean time, an attempt 

 will be made to trace the nature of Sir James Ivory's investiga- 

 tion. 



It commences with the supposition, that, in a given volume 

 of air, of given temperature, and under a given pressure, va- 



