280 



few years ago by Dr. Apjohn here, and elsewhere by Dr. Suerman ;* 

 and which may be said to consist in determining, (indirectly,) through 

 the help of a thermometer with moistened bulb, the weight of gas 

 which is required for the conversion (at a known temperature and 

 under a known pressure) of a known weight of water into vapour, 

 by cooling through a number of degrees which is known from obser- 

 vation of another thermometer. 



The general theory of the evaporation hygrometer, or the manner 

 of employing a thermometer with moistened bulb, to discover the 

 amount of moisture which is contained at any given time in the 

 atmosphere, was very well and clearly set forth by Mr. (now Sir 

 James) Ivory, in Tilloch's Philosophical Magazine for August, 1822. 

 The same theory was also discovered by M. August of Berlin, with the 

 date of whose work upon the subject I am unacquainted, having only 

 seen the extracts made from it in M. Kupffer's Meteorological and 

 Magnetical Observations, (published at St. Petersburg!! in 1837,) and 

 in a recent volume of M. Quetelet's Correspondence. It appears, 

 indeed, that M. Gay Lussac had prepared the way for this discovery, 

 by his researches on the cold of evaporation ; and the laws of the 

 elastic force of vapour, and of its mixture with the gases, without 

 which the theory could not have been constructed, are due to the 

 venerable Dalton. Notwithstanding all that had thus been done, the 

 subject seems to have attracted little general notice in these coun- 

 tries, until it was recommended to the attention of scientific men at 

 the first meeting of the British Association ; and Dr. Apjohn, who 

 was thus led to examine it anew,| was not aware of the results that 

 had been already obtained. He thus arrived at a new and indepen- 

 dent solution, of which he had the satisfaction of testing the correct- 

 ness, by several different series of experiments ; and this success 

 encouraged him to extend the research, and to apply the same prin- 

 ciples and methods to other gases, and not to atmospheric air alone. 



* Dissertatio Physica Inauguralis de Calore Fluidorum Elasticorum Specifico ; 

 auctore A. C. G. Suerman : Trajecti ad Rhenum, 1836. An excellent work, to which 

 every student of this subject must refer. 



t It appears that another Member of the Academy, Dr. Henry Hudson, was also 

 led, by this recommendation, to consider this interesting subject. 



