644 REGNAULT’S HYGROMETRICAL RESEARCHES. 
some ordinary alcohol, and I have very easily obtained the de- 
posit of dew. The aspiration of the air must in this case be 
more rapid than with the ether, and the thermometer sinks more 
slowly. There is no inconvenience in this, because fewer ex- 
periments need be made in order to observe exactly the tempe- 
rature of the dew-point. The greatest inconvenience in the 
apparatus is the bulk of the aspirator, and the necessity of 
filling it with water, which is not always easy on ajourney. I 
will remark first, that the vessel I employ is much too large when 
a single determination is only required; in this case a vessel 
capable of holding a quart is sufficient; but I am convinced 
that, with a little practice, the aspirator may be wholly dis- 
pensed with. 
I terminate the leaden tube with a small mouthpiece similar 
to that of an ordinary blowpipe; and before this mouthpiece 
I place a stopcock; the observer blows at first quickly through 
the ether, so as to bring the liquid to the dew-point; he then 
stops, lets the dew disappear, then blows more gently, turning 
the stopcock as required. It is easy by this means to keep the 
thermometer of the condenser in an almost stationary condition. 
A practised observer might even dispense with the regulating 
stopcock, but the experiment is far more easily made with it. 
4. Of the Psychrometer. 
M. Gay-Lussac was the first to propose to determine the hy- 
grometric state of the air by observing the temperatures indi- 
cated by a dry thermometer and by a thermometer whose bulb 
is kept constantly moist*; but he is of opinion that, in order 
to deduce from these observations the quantity of moisture ex- 
isting in the air, it will be necessary to construct tables, the 
elements of which will require numerous experiments. 
Since that period, a German physicist, M. August, has inves- 
tigated this question, and he has published several interesting 
memoirs, in which he has sought to establish upon theoretical 
grounds the formule according to which may be calculated the 
elastic force of aqueous vapour existing in the air, from the 
temperatures indicated by a dry and a moist bulb thermometer 
in this air. The apparatus, composed of these two thermome- 
ters, has been named the Psychrometer. 
The following are the grounds on which M. August esta- 
* Annales de Chimie et de Physique, vol. xxi. p. 91. 
