138 Introduction to Boileaus [No. 147. 



being obliged to go through the labour of computing the value of J for 

 each observation, I have, for the information of those who may not 

 have had an opportunity of perusing his papers on the Dew-point, 

 given in the above a brief sketch of the steps by which the Hygro- 

 metric formula has been obtained, and shall now proceed to explain 

 the manner in which the same has been applied to the computa- 

 tion of the accompanying tables. 



The equation which I have adopted is 



/"=/'_.oii47U-Ox^ 



in which as the co-efficient employed is the arithmetical mean of the 

 three values of m given above, and not the most probable value, as 

 computed by Professor Lloyd, a reason will be expected for the adop- 

 tion of this number. 



The Table of the elastic force of vapour which I have used for 

 giving the values of/' that enter into the computation of the second 

 term in the right hand member of the equation, has been computed 

 specially for this purpose by Biot's formula, " Traite de Physique, 

 1816, Tome 1, p. 278."* 



This Table differs so little from that employed by Dr. Apjohn, com- 

 puted by Anderson from the experiments of Dalton and Ure, that 

 as this latter has been shewn by Professor Lloyd to be more probably 

 accurate, within the ordinary limits of observation, than either the 

 table of Kaintz, or that adopted by the Royal Society in the report of 

 their Physical Committee, the employment of the Table which I have 

 computed, will not materially affect the resulting values of the Dew- 

 point tension or temperature. 



By means of this Table, and with the three series of experiments 



* This formula, which is deduced from experiments by Dalton, is as follows : — 



LogF/=Log30 + a/+6/« + c/3 



The numerical values of the co-efficients are 



a= — -00854121972 Log. £9315199 



& = —.00002081091 „ 573182910 



c= +.00000000580 „ 9.7634280 



/ being the number of degrees of Fahrenheit reckoned from 212° positively below, and 

 negatively above that point. N 



