108 Aqueous Vapour Atmosphere. 
then to take a cup of water from it and pour 
. into a clear dry tumbler glass; if dew was 
produced immediately on the outside of the 
glass, the water was returned into the cup 
and the glass was again carefully dried out- 
side. During this time, the water in the cup 
was acquiring temperature from the air. It 
was then returned into the tumbler and held 
out exposed to the current of air. This pro- 
cess was repeated till no dew was found to be 
formed on the glass. The temperature of 
the water each time it was put into the tum- 
bler was found by a small pocket thermometer ; 
and that when it last produced dew on the 
glass was marked down as the dew-point. 
At the same time the barometer was noted, 
to find the height of the place of observation, 
the thermometer to find the temperature of 
the air; and the temperature of the springs 
was an object not wholly devoid of interest. 
Difficulties however sometimes occurred. 
Springs were not always to be found where 
they were wanted; and many times when 
found the water was nos cold enough to pro- 
duce dew. In such cases a tea-spoonful or 
two of pounded nitre and sal-ammoniac were 
thrown into the water and stirred about till 
dissolved; this generally succeeded. On two 
or three occasions large snom-drifis were 
