METEOROLOGY AND CLIMATOLOGY. 
11 
impression that the readings of instruments are of little value. It is, in 
ordinary circumstances, only by the readings of instruments that the 
climate of one place can be compared with that of another, and only the 
best results of instrumental work are precise enough to form a basis for 
climatological maps. 
Instrumental Observations. 
The minimum requirement of instrumental observations by a traveller 
is the reading twice daily of the dry and wet bulb thermometers, to 
ascertain the temperature and humidity of the air, and of the barometer; 
also the reading once daily, in the morning, of the minimum thermometer 
which has been exposed all night, and on days in camp of the maximum 
thermometer also. It is very desirable to expose a rain-gauge when¬ 
ever it is possible to do so. Unless special meteorological researches are 
to be carried out, nothing farther in the way of observations need be 
attempted. A very useful supplement to the necessary observations is 
the use of a self-recording barograph or thermograph; but these are 
delicate instruments, liable to get out of order unless very carefully 
handled, and it will not always be possible to make use of them. 
The observer must understand what his instruments are intended to 
measure, how they act, and how they should be exposed, read, and the 
reading recorded. He must know enough about all these things to be 
able to dispense with unnecessary precautions only possible at fixed 
observatories, and, at the same time, to neglect nothing that is necessary 
to secure accuracy in the results. 
Thermometer Corrections .—All thermometers, without exception, should 
have the degree marks engraved on the stem, or on a slip of enamel 
within the outer tube, and be supplied with a certificate from the 
National Physical Observatory showing what the error of the scale is at 
different points. This certificate should be in duplicate, and a copy 
ought to be left in a safe place at home. After a long journey, the 
thermometers which have been in use should be sent to have their 
errors re-determined. The corrections are not, however, to be applied by 
the observer unless he is working out his observations for some special 
purpose. No thermometer is passed at the Kew Observatory if its error 
approaches one degree in amount, so that for all ordinary purposes of 
description, a Kew certificated thermometer may be looked on as 
