472 Prof. Apjohn on a Formula for ascertaining 



but -35, or about one third of a degree Fahrenheit. It will 

 also be noted that the difference is negative, or that the mean 

 calculated dew-point is lower than the observed, and not vice 

 versa. If we were justified in considering this latter result as 

 anything more than accidental, it might certainly be urged as 

 an argument against the strict accuracy either of our experi- 

 ments or of our theoretical views ; for the corrections for the in- 

 fluence of pressure and aqueous vapour on the specific heat of 

 air being neglected in the preceding calculations, the calcu- 

 lated dew-points instead of being lower should be higher than 

 the truth. In order, in fact, to account for the discrepancy in 

 question, supposing it to be well established, it would be ne- 

 cessary to conclude either that m, the coefficient of our hygro- 

 metric formula, is assumed somewhat too great, or that the 

 observed depressions are a little too small. The first I believe 

 to be the true solution, and I am at present disposed to con- 

 sider in as more correctly represented by the fraction ■£■% than 

 ■£ T . This point, however, I have not as yet been able fully 

 to satisfy myself upon, nor can the more exact determination 

 of the value of the constant be considered a matter of much 

 practical importance, since the formula in its present state 

 conducts, as we have seen, to results which harmonize admi- 

 rably with each other and with observation. 



I shall conclude by subjoining a couple of tables, by the aid 



of which the application of my formula f" = t / v x — 



to the determination of the dew-point is greatly facilitated. 

 Table A, which I have taken from the Edinburgh Encyclopae- 

 dia, article Hygrometry, gives the elastic force of the vapour 

 of water for every degree Fahrenheit between 0° and 100° 



inclusive. Table B gives — — — for every value of d between 



o / X 9v 



•] and 10. This quotient, as is obvious from a glance at the 

 formula, is, in calculating an observation, to be multiplied by 

 p, the existing pressure, and the product when deducted from 

 f\ as given by table A, will afford/ 7 ', or the tension of vapour 

 at the dew-point. Should the depression exceed 10°, the 



value of -~ x30 may still be got from table B by addition. 



Thus, ifd= 13°,^ — - ='00383 + '00114 = -00497. 

 o7 X o\j 



