his Hygromelrical Researches made by Dr. Hudson. 191 



so that if a value of d in perfectly dry air be determined by 

 observation, theory and experiment can be compared. 



2. Though f" be supposed constant, f and d will vary if 

 the temperature varies. Hence if air having a constant hy- 

 grometrical slate be raised to different temperatures, and a 

 series of corresponding values of^ and d be determined by 

 observation, the formula, if correct, when applied to each pair, 

 should obviously give the same result, or value ofy". 



3. If air be saturated with moisture at a known temperature, 

 such temperature is necessarily its dew-point. Let it now be 

 heated, and values of y and d then observed in it by means 

 of a v/et and dry thermometer. The formula, if correct, when 

 applied to these, should give for the dew-point the temperature 

 at which the air was in the first instance saturated with mois- 

 ture. 



Such are the methods which I employed. They flow imme- 

 diately from my formula, and I beg to state in the most posi- 

 tive manner that I had resolved to employ them all before 

 Dr. Hudson commenced his correspondence widi me. Dr. 

 Hudson, however, states that the first and third are his ex- 

 clusive property, and that I derived them from him. Now 

 what is the evidence he adduces in support of this assertion ? 

 Not a tittle, that I can perceive, save a passage in one of my 

 letters in which I i-ecommend him before publishing his pro- 

 portions relative to the dew-point, to investigate the value of 

 D by some of the experimental methods mentioned by him 

 in a previous letter. From this passage Dr. Hudson infers, 

 by some process of reasoning with which I am unacquainted, 

 but which certainly would not appear to be characterized by 

 much of the vis con^eque7itice, that I must have been ignorant 

 of these methods before they were mentioned by him ; that is, 

 that I must have overlooked the most simple consequences of 

 the expression I had investigated, and felt so anxious to verify 

 by experiment. But it is not necessary to argue the matter 

 further. The following letter from Professor Lloyd has, I 

 presume, satisfied Dr. Hudson himself that he is in error, and 

 that I contemplated my experiments on dry air before I had 

 ever any intercourse with him, direct or indirect, on this or any 

 other subject. 



" My dear Apjohn. 



" I very well recollect your mentioning on the evening on 

 which you read your first paper on the moist-bulb thermome- 

 ter to the Academy, that your hygrometric formula suggested 

 an easy method of determining by experiment the specific 

 heats of the gases, viz. by observing the temperatures indicated 

 by the two thermometers in gases deprived of their moisture. 

 I think you stated at the same time that it was your intention 



