METEOROLOGY AND CLIMATOLOGY. 
19 
otherwise the true reduction of temperature will not be observed. Hence 
special precautions are necessary when observing at sea or in an arid 
country where the ground is covered with incrustations of salt. 
In any form of wet bulb thermometer when the air is much below 
the freezing point, it will usually be found most satisfactory to remove 
the muslin covering and allow the bulb to become covered with a coating 
of ice, by dipping it into water and allowing the water to freeze upon it. 
Evaporation takes place from solid ice sufficiently rapidly to give the 
true wet-bulb readings at least in a sling thermometer. 
When the air is saturated, i.e., relative humidity = 100 per cent., there 
is no difference in the reading of the wet and dry bulb thermometers, 
and the greater the difference between the readings the smaller is the 
relative humidity of the air. 
The wet-bulb thermometer has to be exposed to the air with the same 
precautions as are taken in the case of the dry bulb. The two may be 
hung side by side—but at least six inches apart—in the screen or cage 
described on p. 15; or the wet bulb may be employed as a sling thermo¬ 
meter. One way to do this is to tie a muslin cap on the bulb of the 
sling thermometer with a piece of wet lamp-wick coiled round the upper 
part of the bulb, and then whirl it until the reading becomes constant, 
taking care to moisten the bulb again if it should become dry. Another 
way is simply to twist a piece of filter-paper or blotting-paper round the 
bulb, and dip it in water before swinging. 
Aspiration Psychrometer .—Perhaps the most convenient form of wet 
and dry bulb thermometer for use by a traveller is that known as 
Assmann’s Aspiration Psychrometer. It requires no protecting screen, 
is not subject to the risk attending the use of the sling thermometer, 
and gives an extremely close approximation to the true temperature 
and humidity. The principle of the instrument is very simple. The 
wet and dry bulb thermometers are enclosed separately each in an open 
tube (see Eig. 4) through which a current of air is drawn by means of 
a fan, actuated by clockwork in the upper part of the case. In making 
an observation, all that is' required is to see that the water vessel for 
the wet bulb is filled and the bulb properly moist, and that the dry 
bulb is free from any condensed moisture. The instrument is then 
hung to a branch or other support placed in the open air (or even 
held in the hand), preferably in the shade, although this is not essential, 
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