398 Experimental Researches on the Depressions [July, 



represented by 100° ; while extreme dryness, or entire absence of 

 aqueous vapour, is expressed by 0°. The intervening degrees com- 

 prehend every intermediate state of moisture that can possibly occur, 

 and conveniently express the percentage of actual moisture present, or 

 as it is more scientifically termed, the centesimal tension of the vapour. 

 The point of saturation on the wet-bulb instrument (100) is indicated 

 by 0°, because evaporation, and the cold consequent on it, then ceases. 

 The questions to be solved then are, 1st, What is the maximum depres- 

 sion, which corresponds to perfect dryness (0) in the assumed scale, for 

 everv temperature ? — and 2nd, What is the value of each intermediate 

 degree (Fahrenheit) of depression of the wetted thermometer in terms 

 of the centesimal tension or 100 hygrometric degrees above alluded to ? 

 I. There is one very easy method of attaining the first object : viz., 

 by exposing a wet-bulb thermometer to a current of perfectly dry 

 air of various temperatures. This was the mode pursued by Gat 

 Lussac between the temperatures- 32° and 70°, in 1827: by myself 

 in 1829, between 70° and 140°, and recently continued up to 700° 

 Fahrenheit : it is the plan proposed to be pursued by Dr. Hudson*, 

 and employed in the test experiments of Professor Apjohn in 1835f. 

 In fact, this is the only accurate plan of testing the maximum depres- 

 sion, which is to represent 0° on the hygrometric scale : for the 

 exposure of a wet-bulb thermometer in still air dried to the utmost, 

 fails to produce a maximum, the instrument being necessarily sur- 

 rounded with a medium not perfectly dry. Dr. Apjohn makes the 

 error by this method gth ; I have found it about T 'oth. 



II. The second question, as to the value of intermediate depres- 

 sions ? may be ascertained by drying the air to various points, as 20, 

 30, 40 per cent, which can be done by exposing it to various saline 

 liquids, or more conveniently to sulphuric acid of different strength, 

 and then submitting the thermometer to a current of it as before. 

 This mode was used long since by M. Gat Lussac in testing the 

 value of the degrees of Saussure's hair hygrometer, and it was fol- 

 lowed by myself in a repetition of the same train of experiments in 

 1825 J. To obtain, however, an equable current of wholly or of 

 partially dried air for a sufficient duration of time, is by no means 

 easy ; nor do I think that air merely passed through a tube containing 

 sulphuric acid or chloride of lime, without remaining in protracted 

 contact with it, would be thoroughly deprived of moisture. At any 

 rate, to ensure confidence, there should be the means at hand of record- 



* Phil. Mag. Vol. VII. p. 260. 



t Ditto, p. 271. 



X See Brande's Journal of the Royal Institution, XXII. 28. 



