Meteorology and climatology. 
17 
experiment that the true temperature of the air is obtainable in this way 
whether the operation is performed in sunshine or in shade; but it is 
preferable to do so in the shade. 
Sling Thermometer .—The sling thermometer is the most simple and 
convenient of all instruments for ascertaining the temperature of the air* 
It is an unmounted thermometer with a cylindrical bulb, and the 
degree-marks engraved on the glass stem. The upper end terminates 
in a ring to which a silk cord about two feet long is attached. As a 
precaution it is as well to secure the cord by a couple of clove hitches 
round the top of the thermometer stem as well as to the ring, as the 
thermometer would then be held securely even if the ring broke. 
The thermometer is used by whirling it in a vertical circle about a 
dozen times, taking care, by having a loop of the string round the 
wrist or finger, that it is not allowed to fly off. Then the thermometer 
is read, swung once more and read again. This process is repeated 
until two consecutive readings are identical; when this is the case the 
instrument shows the true temperature of the air. It is sufficient to 
note the final temperature in the observing book. 
The risk of breaking a sling thermometer is the only drawback to its 
use. Only a silk cord should be used, and it should be examined frequently 
to see that it has not got chafed. In swinging the thermometer, an open 
place must be selected where it is not likely to come in contact w T ith a 
branch or any other object. 
Hygrometers .—As the humidity or degree of moisture in the atmos¬ 
phere is a very important climatic factor it is necessary to measure it as 
carefully and as frequently as the temperature of the air. There are 
many instruments, called psychrometers or hygrometers, for doing this; 
but few of them are simple enough for .the use of a traveller. The 
proportion of water-vapour in air is a little difficult to understand at 
first, because it is not a constant quantity as in the case of the other 
constituents of air, but varies according to the amount of water-surface 
exposed to the air and according to the temperature. The maximum 
amount of water-vapour which can be present in air varies with the 
temperature, being greater as the temperature is higher and less as the 
temperature is lower. Thus, if air at 50° F. contains the maximum 
amount of water-vapour which it can contain at that temperature, it 
is said to be saturated , for it will take up no more and evaporation stops; 
VOL. 11. c 
