6 
METEOROLOGY OF i8Q/. 
ture falls, the constriction prevents the passage of the col- 
umn of mercury, and the upper end of the column indicates 
the highest temperature since the last setting of the instru- 
ment. The minimum thermometer is the ordinary spirit 
thermometer with a sliding index in the tube. As the 
temperature lowers and the column shrinks, the index is 
pulled along by the surface of the liquid. When the tem- 
perature rises, the liquid flows past the index without carry- 
ing it along, and the upper end of the index thus indicates 
the lowest temperature since the previous observation, 
WET AND DRY BULB THEMOMETERS OR PSYCHROM- 
ETER. 
§io. Fora portion of the time 
the ordinary pattern of stationary 
wet and dry bulb thermometers has 
been used. Since 1889 the sling 
thermometers have been used. In 
both cases the set consists of two 
thermometers as near exactly alike 
as possible, one of which is observed 
with its bulb dr}% and the other, 
which has its bulb covered with thin 
muslin, is moistened with water. 
The ordinary set of wet and dry 
bulb thermometers is fixed in posi- 
tion, and the moistening of the bulb 
is accomplished by candle wicking 
which connects with a little water in 
a small vessel. d'he evaporation 
from the muslin cools the bulb so 
that the thermometer indicates the 
temperature of evaporation, which 
depends upon the amount of moist- 
ure that is contained in the air. 
I'he air next the bulb tends to 
in which case the evaporation proceeds 
the wet bulb does not fall so low as it 
would if a brisk wind were blowing. Hence the stationary 
psychrometer, where the air is more or less stagnant, is not 
as accurate as the sling psychrometer, which is essentially 
the same instrument, but is swung in the open air to obtain 
complete ventilation. In this case, the bulb is covered with 
become saturated, 
less rapidly, and 
