Observational on the Hygrometer. 281 



with B ; t and f being supposed constant : but if, 'as Mr 

 Meikle allows^ 



when X varies only with B, then surely the true object of en- 

 quiry is not how D, but how . — —^ varies with B; and the cor- 

 rection for pressure ought not to affect the value of f, but only 

 of (^ — ^). 



The practical determination of the correction for pressure, 

 must be attended with considerable difficulty, whether the ex- 

 periments be conducted in the open air at different altitudes, 

 or, like those of Dr Anderson and Mr Meikle, in the receiver of 

 an air-pump. In the latter case the air is exposed to the ac- 

 tion of sulphuric acid, and ff reckoned =: o, or, at least, it is 

 supposed to be constant ; but this supposition cannot be veri- 

 fied beyond a certain temperature, even by means of Mr Da- 

 niell's hygrometer. When the former method of ascertaining 

 the influence of pressure is adopted, it is necessary to assume 

 that the formula for finding the dew-point is correct under a 

 pressure of 30 inches. On this principle, the writer compared, 

 in a variety of instances, the indications of Leslie's and DanielPs 

 hygrometers at different altitudes, and found that when D was 



multiplied by — — — , as suggested by Mr Meikle, the calculat- 

 ed agreed better with the observed dew-point than any other 

 hypothesis. He therefore proposes to adopt this as the correc- 

 tion for pressure in equation (6), which, accordingly, after the 

 substitution of 87.719 for b, and 132.17 for c, becomes 



^ (ft + .66372) (B + 27) D _ ^ .gy 



^^ 10000/f ~*^*" ^ ^ 



But, as before stated, there is a diversity of opinion respect- 

 ing the cause as well as the degree of the variation of D with 

 the pressure. It has occurred to the writer that some light 

 may be thrown upon this subject, by considering whence the 

 heat is derived which is dissipated during evaporation. 



Professor Leslie regards the heat in question as contributed 

 entirely by the portion of air which dissolves and carries off the 

 moisture; while Dr Anderson thinks it is exclusively derived 

 from the moist bulb. 



