METEOROLOGY AND CLIMATOLOGY. 
31 
a horizontal position. Every opportunity of comparing the aneroid with 
a standard mercurial barometer should be taken, and a note made of the 
readings of both. The mercurial barometer will require to be corrected 
for temperature before its indications can be used for correcting the 
aneroid, as all good aneroids are compensated for changes of temperature. 
The readings of an aneroid give a very fair idea of the changes of atmo¬ 
spheric pressure, and are very much better than none at all, although 
they cannot in any case be accepted as of the highest order of accuracy. 
The new Watkin mountain aneroid, which is so constructed as to be 
thrown into gear at the moment when it is read, appears to be free from 
the worst errors of the ordinary aneroid. 
For climatological purposes, it is impossible to make barometric 
observations of value while travelling unless the altitude of each 
camping-place is accurately known. This is practically never the case 
except when travelling along the sea-shore or the margin of a great lake 
the elevation of which has been, determined. But, meteorology apart, 
barometric readings in any little known country are of value, because by 
comparing them with simultaneous readings taken at a neighbouring 
fixed station, new data as to the altitude of the country may be obtained. 
While in camp, it would be an extremely useful thing to make barometer 
readings, even with an aneroid, every two hours, in order to get some 
information as to the normal daily range of atmospheric pressure. 
The Boiling-point Thermometer .—The temperature at which water boils 
depends on the pressure of the atmosphere, so that an accurate observa¬ 
tion of the boiling-point of water enables the pressure of the atmosphere 
at the moment of observation to be determined with the utmost accuracy 
This method of determining atmospheric pressure having been used 
hitherto almost solely for the purpose of measuring altitudes, the boiling- 
point thermometer is usually known as the Hypsometer, but its records 
are quite as valuable for use at fixed stations as in mountain climbing. 
Mr. J. Yi Buchanan recommends the use of a boiling-point thermometer 
with a very open scale graduated to fiftieths of a degree Centigrade and 
entirely enclosed in a wide glass tube through which steam from water 
boiling in a copper vessel is passing. On a thermometer of this kind 
change of pressure can be measured by the change of boiling-point more 
accurately than with the aid of a mercurial barometer. See Table, 
Yol. I., p. 210. 
