HOW TO STUDY METEOROLOGY. 
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careful regulation as to moisture. In all such cases the 
hygrometer is useful, and if used in conjunction with the baro- 
meter, storms of rain or wind, whether on shipboard or on 
land, may be predicted.” 
Of hygrometers, Daniell’s is an elegant instrument, Regnault’s 
elegant and manageable, but Mason’s wet and dry bulb (called, 
by the Germans, the psychrometer) is the one for business 
purposes. It consists (Fig. 4) of two thermometers, as nearly 
similar as possible, placed vertically parallel on a stand, and 
about four inches apart. The bulb of one is covered with thin 
muslin from which a few threads of lamp or darning cotton 
hang down ; these, passing into a small vessel of water below, 
keep the muslin and bulb continually moist, and cause this 
thermometer to indicate a lower temperature than the other 
one, in proportion to the humidity of the atmosphere, the 
tendency of objects to dry, or the evaporation going on from 
surrounding substances. A small difference in the readings 
indicates the presence of much moisture ; but a large diffe- 
rence proves the air to be very dry. The temperature of the 
dew point can also be arrived at with the aid of this instrument, 
this temperature being that to which the air must be reduced 
before the deposition of dew will commence. 
In taking an observation, begin with the dry bulb, and be 
careful always to avoid breathing on or otherwise communicating 
heat to the instrument. If the temperature of the air be below 
32° the wet bulb may read higher than the dry one. Such a 
reading must not be recorded, but the wet bulb must be 
moistened, when a coating of ice will form on it, from 
which evaporation will take place, though an hour or more 
may elapse before the temperature of the wet bulb has fallen 
below that of the dry one. If the temperature of the air 
should rise above 32°, the wet bulb should be carefully im- 
mersed in warm water to melt away any ice which may remain 
on it. The temperature of the water must be less than that up 
to which the tube is graduated, for if it be greater the. expan- 
sion of the mercury will burst the tube at the top, for w T ant of 
a vent. 
To discover the dew point we must employ a table framed 
from an extensive series of comparisons executed at the Royal 
Observatory, Greenwich, by Mr. Grlaisher, who has concluded 
that this temperature may be ascertained by multiplying the 
difference between the temperatures of the dry and wet bulb 
by certain multipliers, and subtracting the product from the 
temperature of the dry bulb. 
