METEOBOLOGY AND CLIMATOLOGY. 
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bottle may be used, or in default of a bottle, the copper case itself will 
act as a receiver, although the risk of loss by evaporation, and by 
the wetting of a large surface in pouring out the water, is considerably 
increased. 
At a fixed station the rain-gauge should be read every morning. The 
traveller who only exposes his rain-gauge during a halt should be 
careful to state the hours when it was exposed and when it was read. 
Barometers .—The barometer is the most delicate, and at the same time 
the most important, instrument which a meteorologist has to employ. 
It requires particular care in transport, and must be very carefully 
mounted and read, while several accessory observations have to be made 
at each reading in order to ascertain the corrections required for the 
subsequent calculation of the results. The function of the barometer is 
to measure the pressure of the air at the time of observation, and this 
purpose may be carried out by the use of two different principles. The 
oldest and best method is to measure the height at which a column of 
heavy fluid is maintained in a tube entirely free from air. The weight of 
this column is equal to the weight of a column of the atmosphere of the 
same sectional area. Mercury being the densest fluid is the only one 
usually employed, because the column balancing a column of the atmos¬ 
phere of equal sectional area is the shortest that can be obtained, and, 
consequently, a mercurial barometer is the most portable that can be 
constructed on this principle. The mercurial barometer has come to 
be recognised as the standard in all parts of the world. 
The average height of the column of mercury in a barometer is about 
thirty inches, and, consequently, the whole instrument cannot well be 
made less than three feet long, so that when account is taken of the 
glass tube, and the amount of mercury it contains, it is long, fragile and 
heavy. To avoid the disadvantages inherent in such an instrument, the 
method of measuring the pressure of the air by the compression of a 
spring holding apart the sides of an air-free flexible metallic box was 
devised, and the aneroid barometer invented. The aneroid is graduated 
on the dial in “ inches,” i.e., divisions each of which corresponds to a 
change of atmospheric pressure, equal to that measured by one inch 
of mercury in a standard barometer. Although a carefully constructed 
aneroid is a very useful instrument indeed, it is not to be trusted like 
a mercurial barometer kept in a proper place. But a good aneroid 
