642 REGNAULT’S HYGROMETRICAL RESEARCHES. 
to cause the thermometer to remain stationary at 12°9 as long 
as is desired. If no dew is formed, after a few instants it is clear 
that 12°°9 is higher than the dew-point. I descend now to 
12°8, and keep the thermometer at that point, regulating the 
escape of the water suitably. I will suppose that the metallic 
surface tarnishes after a few instants; I conclude that 12°8 is 
lower, and that 12°'9 is higher than the temperature to which 
the saturation corresponds. I can obtain a considerable ap- 
proximation by seeking whether 12°°85 is above or below this 
point. For this purpose I turn the stopcock 7 a very little, so 
that the thermometer ascends very slowly, notwithstanding the 
passage of the bubbles through the zther, and I observe whether 
the dew disappears or whether it remains at 12°85, at which 
temperature I keep the thermometer stationary for some in- 
stants. 
All these operations take longer to describe than to perform ; 
with a little practice, three or four minutes are enough to deter- 
mine the dew-point to within jth of a degree. 
The aspirator has a much greater capacity than is necessary 
for a single determination. The one of the capacity I have 
stated suffices to maintain the condenser for above an hour near 
the dew-point, so that more than ten consecutive determinations 
may be made. 
I made a great number of determinations by means of this 
instrument in a large amphitheatre, the temperature and hygro- 
metric state of which changed but very slowly, and I always 
found perfectly identical results in successive determinations. 
When observations are made in the open air, it is seen how 
much the hygrometric state varies from one minute to another, 
owing to the incessant changes of temperature. When the 
thermometer of the stationary condenser is kept near the dew- 
point, the metal is seen to tarnish or resume its lustre, accord- 
ing as the lightest breath comes from either side. The ordinary 
hygrometers and the psychrometers are by no means sufficiently 
delicate to indicate these momentary variations. 
It is necessary to ascertain very exactly the temperature 
of the dry air in the point of space the hygrometric state of 
which is determined. For this purpose, I place a very delicate 
thermometer in a second small apparatus similar to the first, at 
least with the difference only that it contains no liquid. This 
second apparatus is arranged immediately beside the first; it is 
