276 Observations on the Hygrometer. 



^' I _ ( The weight of moisture sufficient to saturate a given 

 Jt" J *• ^olunie of air at the temperatures t, H and f . 



B = The height of the barometer in inches. 



It has been stated, that, according to Professor Leslie, L is 

 proportional, under the same pressure, to the dryness of the air 

 at the temperature t ; that is, when B is constant, L is propor- 

 tional to^f, — gtn. From this theory Professor Leslie deduces 

 the following formula for calculating the weight of moisture 

 which a given volume of air contains ; 



L 



the value of a being constant for the same pressure. In order, 

 however, to simplify this formula, the volume of air which gi> is 

 sufficient to saturate is so chosen as to render « = 1, under the 

 presure of 30 inches, and 



gif—^-gn' (1) 



But the dryness of the air, ^when the temperature is reduced 

 to ^5 being also proportional to fi — fl'-i the elastic force of va- 

 pour at the dew-point, may, in like manner, be found by means 

 of the formula 



/.-§=/.' (2) 



Mr H. Meikle * has deduced from the same theory the follow- 

 ing approximate formula for finding the value of t"-. 



which is remarkable both for its convenience in calculation, and 

 the ease with which it may be remembered. 



These formulas agree well, in general, with observation when 

 t does not differ much from the value it had in the experiment 

 by means of which the coefficient a or h was determined. For 

 example, if the value of 6 in eq. (2), have been ascertained when 

 t = 67.2, the formula will give the dew-point with considerable 

 correctness when the temperature of the air is neither much 

 above nor below 67.2 ; but when it greatly exceeds or falls 

 short of that temperature, the theoretical dew-point will, in the 

 former case, be too low, and in the latter too high. This obser- 



• Edinburgh Phil. Journal. ^ 



