12 
BINTS TO TRAVELLMS. 
correct. But When the readings are being critically discussed, and 
compared with the observations of other people, the correction is of the 
greatest importance. It cannot be too strongly impressed upon an 
observer that , in reading meteorological instruments , he must read exactly 
what they mark , and record that figure in his observation-book on the 
spot. The corrections can be applied afterwards by the specialist who 
discusses the work. For subsequent reference it is necessary to note 
in the observation-book the registered Kew number of the thermometer 
in use, and if a thermometer should get broken and another be used 
instead, the number of the new instrument must be noted at the date 
where it is first employed. Care should be taken to use the same 
thermometer for one purpose all the time if possible, and only an 
accident to the instrument should necessitate a change being made. 
Thermometers are either direct-reading or self-registering. The former 
are used for obtaining the temperature at any given moment, the latter 
for ascertaining the highest or the lowest temperature in a certain 
interval of time. They are filled either with mercury, or a light fluid 
which freezes less readily, such as alcohol or creosote. 
Thermometer Scales. —The particular system on which the thermometers 
are graduated is of no importance, but merely a matter of convenience. 
The Fahrenheit scale is used for meteorological purposes in English- 
speaking countries; but for all other scientific purposes the Centigrade 
scale is used everywhere. One can be translated into the other very 
simply by calculation *; but it is convenient for a traveller to have all 
his thermometers graduated in accordance with one scale only. 
The graduation, as marked on the stem of the thermometer, is usually 
to single degrees, but anyone can learn to read to tenths of a degree by a 
little practice. Care must be taken to have the eye opposite the top of 
the mercury column. Suppose it to be between 50 and 51 the exact 
number of tenths above 50 is to be estimated thus: If the mercury is 
just visible above the degree mark it is 50° *1, if distinctly above the 
* For comparison of scales see Table XIV., Yol. I., p. 259. 
To convert Centigrade readings into Fahrenheit the rule is “Multiply by 1-8 
and add 32.’ , This can be done mentally in a moment, thus: “ Multiply by 2, 
subtract one-tenth of the product, and add 32.” E.g ., to convert 10° C. to F.: 
10 x 2 = 20; one tenth of 20 = 2; 20 - 2 = 18; 18 + 32 = 50° F. 
