186 Dr. Apjobn*s Formula for inferring the Dew-point 



multiply by the ratio of these the subtractive terms in the value 

 ofy" aheady obtained. 



To determine the specific heat of a mixture, the simple 

 rule is to multiply the relative weights of its constituents by 

 their respective capacities, and to divide the sum of the pro- 

 ducts by the sum of the weights. But in the present instance 

 the weights, being obviously as the specific gravities, are to 



each other as 1 : -625 •^^. Also the specific heat of air being 



•267, and that of vapour '847, the former is to the latter as 

 1 : 3*172. Hence, according to the rule given above, we shall 



l+-625-^" X 3-172. , .^ ^ P u • 



nave J= tor the specific heat of the mixture 



fn * 



H--625^^— 

 P 



of air and vapour referred to that of dry air taken as unity ; 

 and, applying the correction as already explained, we shall 

 have an equation in which y" is the only unknown quantity, 

 and from which, therefore, its value may be found. This 

 equation, however, being a quadratic, and the unknown quan- 

 tity in its first elimination having a coefficient of three terms, 

 its solution would involve tedious arithmetical operations, 

 and cannot, therefore, be recommended to the practical me- 

 teorologist as a means of making the correction in question. 

 Nor is this course at all necessary ; for the same object may 

 be achieved, according to a simpler, though less rigorous 

 method, by either assigning to^" an average value, or by de- 

 ducing approximately the tension of vapour at the dew-point 

 by the formula 



/" =/' - ^x ^^'x(-0223;> + -3312), 



and using the value ofy" thus obtained, in order to deter- 



H- -625— X 3-172 

 mine that of — ^ , the specific heat of the mix- 



l+-625^ 



ture of air and vapour. The latter method is decidedly the 

 best, and though not mathematically accurate, will not, I be- 

 lieve, exhibit a deviation from the truth until the calculation 

 be pushed to the seventh or eighth decimal place. 



I have now to notice the last circumstance which, as far as 

 I understand the subject, can have any influence upon the 

 accuracy of my determination of the dew-point. 



When the wet-bulb hygrometer has attained its stationary 



