278 Observations an the Hygrometer. 



the vapour rising from the wet surface carried off in equal suc- 

 cessive instants of time, and that in each of the same times, the 

 surrounding bodies restored to the evaporating surface a portion 

 of heat =1 {t — t') where f is variable, then it follows that, du- 

 ring the 1st, 2d, 3d, ... nth. instants of time, the actual decre- 

 ments of temperature would be (1 — l)d^{\ — ly d, (1 — lY d, 

 ... (1 — lY d; and that the temperature of the evaporating 

 surface would constantly approach, while it never attained, the 

 limit t — (9 — 1) d. It therefore appears, even on the supposi- 

 tion that d suffered no diminution, that t — t^ would not reach 

 its maximum suddenly. 



Immediately after the passage to which the above remarks re- 

 fer, Dr Anderson proceeds to apply the proposed correction to 



the equation D = tw (^ — jf') by substituting (P -\ for m, 



P and r being constant quantities to be determined by experi- 

 ment. Then, from two experiments, conducted apparently with 

 admirable care and ingenuity, he deduces the following equations: 



P = 3466 + — 



r 



P = 35.34 -f — 



r 



Now Dr Anderson's theory evidently requires, that the value 

 of r, deduced from experiment, should be positive ; but had 

 it not been for one oversight in calculating the first of the above 

 values of P, it would have appeared that r was negative, and 

 therefore that the theory of his correction was, in some way, 

 defective. Upon repeating the calculation, it will be found, 

 that, in the first equation, the quantity .22723 =r^/ had inad- 

 vertently been used for .24223 =y, and that by correcting 

 this, the equations became 



P = 35.89 + — - 



T 



P=: 35.34 -I- ^' 



where'r is evidently negative. 



It was before remarked, that the relation between the calcu- 

 lated and observed values of^r, suggested a modification of 

 Professor Leslie's theory. This consists simply in adding one 

 limitation to his former enunciation of that theory, and may be 

 thus expressed. The degree of cold is proportional to the dry- 

 ness of the air at the temperature of the evaporating surface. 



