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. Petersburgh 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,t was not aware of the results that 
had been already obtained. He thus arrived at a newand 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: Trajectiad Rhenum, 1836. An excellent work, to which 
every student of this subject must refer. 
+ It appears that another Member of the Academy, Dr. Henry Hudson, was also 
led, by this recommendation, to consider this interesting subject. 
