188 Dr. Apjolui on ceriahi Statements relative to 



ill a few days audressed to me a letter, which was followed by 

 several others, with the professed object of having my opinion 

 upon another methotl of inferring the dew-point from the in- 

 dications of a wet and dry thermometer. This method is 

 founded on the following proportion : 



D:D-rf ::/':/", 

 in which f •>/'-, and d have the same significations already 

 assigned to them, while D represents the depression experi- 

 enced by the wet thermometer in dry air of such a tempera- 

 ture that, both for it and the moist air,y' has the same value. 

 To render this proportion applicable in practice to the deter- 

 mination ofy", — the elastic force of vapour at the dew-point, — 

 D must obviously be known as well as d andy"'; but Dr. Hud- 

 son shows that if D be determined experimentally for any 

 one value ofy"', it may be deduced for any other value ofy 

 by calculation*. 



Upon this method of Dr. Hudson I wish here, in limine^ to 

 make two remarks. 1. It was proposed after a public exposi- 

 tion of my formula, and by one who was present when my 

 paper was read. 2. It is unsusceptible of practical applica- 

 tion without the experimental determination of some one value 

 of D, a thing which has not been attempted by Dr. Hud- 

 son. 



I shall now, however, go further, and demonstrate that the 

 method proposed by Dr. Hudson is but my method in 

 disguise, or rather but a particular case of my general ex- 



d p 



pression. Thus, since, as I have shown,y" ^f — — x ^» 



30 

 d = 87 if'—/") X — . And, if the air be perfectly dry, in 



30 

 which casey" = 0, d = 87/' x — Hence, calling the de- 

 pression in the latter case D, we shall have 



B:d::S7fxj:SUr-r)Xy 

 oi', when p is constant, 



T>:d.:/' :/'-/", and dividendo, 



T):Y) — d::f' :/", the proportion of Dr. Hudson. 

 This result I communicated to Dr. Hudson in my reply to 

 one of his letters; but perceiving that it was true only on the 

 hypothesis of/' having the same value in the dry and moist 



* \\\i proportion for tliis purpose is only exact on the hypothesis of the 

 pressure being constant. The same may be said of his fundamental pro- 

 portion, D : D-rf: :/' :/"• 



