the Dew-point by the Wet-bulb Hygrometer, 



273 



Of the nineteen observations of depression in dry air re- 

 gistered in the preceding table, eleven are greater and eight 

 less than the calculated results. The mean of the plus errors 

 of the formula is # 28, and of the minus errors 4, of a de- 

 gree; so that *28 — # 40 = — *12 of a degree is the mean dif- 

 ference deducible from the whole between experiment and 

 calculation. A closer approximation between them than this, 

 could not, I think, be anticipated even upon the hypothesis of 

 the strict accuracy of the formula. I may also observe that 



d 7) 

 if by means of the equation f = — X-|^, which, as we have 



already seen, belongs to perfectly dry air, we deduce from 

 the precedng table 19 values of m, the mean of all will be 

 found almost accurately equal to ^ 7 , a result the more entitled 

 to confidence in as much as the mean pressure for the 19 ex- 

 periments being but very little over 30, and the air being 

 perfectly dry, neither of the corrections which I investigated 

 in my former paper requires to be applied. 



If from the experiments already detailed I were to draw the 



d j) 

 conclusion that the equation f* =f ] — — x -^r will afford 



the dew-point with a degree of accuracy far surpassing ordi- 

 ninary hygrometrical observations, I should probably have the 

 concurrence of most of my readers. The evidence adduced 

 in support of the formula appears, at least to me, ample and 

 Third Series, Vol. 7. No. 40. Oct. 1835. 2 N 



