from the Indications of the Wet-bulb Hygrometer. 261 



foot of air requires to deposit its moisture by successive con- 

 tacts becomes considerably increased), I doubt if our senses 

 could appreciate such an amount of dew at all ; and on the 

 supposition of a considerable time being necessary for the de- 

 position of a sensible amount of moisture (from air at 57°, the 

 point of saturation being 58°), the chances that the tempera- 

 ture will remain steadily even at 57° (long enough for us to 

 observe the process) are with such an instrument very slight. 

 Of course, the lower the dew-point, the greater the error from 

 this cause is likely to be. This mode of inquiring into the 

 value of the numbers (given by formula B.) has nevertheless 

 its advantages, as whenever the hygrometer stands at the 

 same point, observation of the dew-point gives us a new value 

 of V (the depression in dry air) at that temperature of hygro- 

 meter ; and by taking a mean of various values, and making al- 

 lowances for the probable amount of error in different cases, 

 we may approximate to the true value of V. 



The best method of ascertaining the value of V on this 

 principle is obviously similar to that which I have pointed out 

 (p. 260.) for ascertaining V directly, and is much more easily 

 executed, as the thermometer and wet-ball need only be ex- 

 posed equally to a current of the atmospheric air (previously 

 heated), for in this case the mixture of the external air with 

 the heated air can produce no error, as they have the same 

 dew-point. The experiment would be best tried when the 

 atmosphere was perfectly damp (i. e. when temperature of 

 thermometer and wet-ball were the same), as then the know- 

 ledge of the dew-point would be perfect. If not so, however, 

 the dew-point should be ascertained by Dalton's method, 

 and then expose thermometer and hygrometer to same air 

 heated, marking their cotemporaneous indications in a table. 

 Then calling t temperature of thermometer, and t' tempera- 

 ture of hygrometer ; also m' = moisture of saturation at t 1 , and 

 m" = moisture of saturation at /" (the dew-point), we have 



V = t — t x —, r, for the temperature of the hygrometer 



m' — m" r " & 



in each case*. 



Now, supposing the atmospheric pressure to be = 30 at the 



time, and to continue the same during the experiment, and 



* We might, perhaps, use a method somewhat similar to the one alluded 

 to above for ascertaining the dew-point, viz. by directing a current of cold 

 air (equally) in thermometer and wet-bulb, and marking the exact tempe- 

 rature at which thermometer and hygrometer became of the same tem- 

 perature. This would probably, however, be of more difficult execution. 

 It may not be improper to state here, that I communicated the views 



