264? Dr. Hudson's Method for 'inferring the Dew- point 



P.S. Since the addition of the last page, as well as of 

 the note on page 259, it has occurred to me that the for- 

 mula V:V—di : moisture of saturation at hygrometer tem- 

 perature : moisture of saturation at dew-point, ought to be 

 altered to V: V — d: :f ! (tension of vapour at hygrometer 

 temperature) :f n (tension of vapour at dew-point). The former 

 is true of a given volume of vapour cooled (with diminution 

 of elasticity) without alteration of density, as in the case of 

 vapour cooled in a tube hermetically sealed. The latter re- 

 fers to the case where a given volume of vapour (cooled) in- 

 creases in density (preserving the same elasticity), and is more 

 applicable to the actual state of the atmosphere. In this re- 

 spect, therefore, Dr. Apjohn's point of view was more correct. 

 And it would appear to me that our methods would in this 

 case be perfectly alike, the difference only consisting in Dr. 

 Apjohn's obtaining his value of V from theoretical views on 

 which I have no reliance. It would appear, however, that 

 Dr. Apjohn does not view the matter in this light, as he has 

 had the kindness recently to communicate to me the following 

 proportion as derived from his views, viz. If V be the de- 

 pression in dry, and d in moist air, f~ the elastic force of 

 vapour at stationary temperature of hygrometer in dry, and 

 f f in moist air, and f" the elastic force at dew-point of the 

 latter, then, he states that 



instead of being, as J conceive it should be, 



V:d:: /':/'-/"; 



in fact, his proportion would make d greater than V. Of 

 course, the termsy and/*" should be substituted for m' and 

 m" (i. e. the tension of vapour at the different temperatures 

 for the density of vapour at the same temperatures,) the various 

 places where V or f (the temperature of dew-point) is to be 

 found by proportion in the preceding pages. 



In concluding, I beg to state that I hope very soon to bring 

 forward experiments to prove that radiation can have no ef- 

 fect on the indications of the wet-bulb hygrometer, unless sur- 

 rounding bodies are of a different temperature from the at- 

 mosphere. 



