34 
HINTS TO TRAVELLERS. 
increase in force, and when the barometer reaches its lowest point it 
will either fall calm (if the centre of the cyclone is passing over the 
observer) or suddenly change in direction. The rapid rise of the baro¬ 
meter after a great depression is also always accompanied by strong 
wind, though not so frequently by rain. 
It must be clearly understood that these remarks refer only to 
observations at a fixed station. If a fall of barometer is observed in 
travelling, it may be due either to a change in the state of the atmosphere 
or to a change in the traveller’s height above sea-level. This is the 
reason why it is absolutely essential, in making barometric estimates of 
height (or boiling-point determinations), to have simultaneous observa¬ 
tions going on at a base-station, or preferably at a series of intermediate 
stations. 
The ordinary prognostics of the approach of rain or bad weather differ 
in different localities, and require a considerable amount of local know¬ 
ledge before they can be utilised. The peculiar absorption band in the 
solar spectrum due to the water-vapour of the Earth’s atmosphere, and 
called the rain-band, is a valuable guide to an experienced observer with 
a spectroscope in predicting rain. The only instrument, however, likely 
to be useful to the ordinary traveller is the wet and dry bulb thermometer. 
When the two thermometers have the same reading, indicating satura¬ 
tion of water vapour, or when they approach at temperatures above 
60° F. within two degrees or so, rain may be expected, or possibly mist. 
The appearance of low clouds clinging to the hillsides is an indication that 
the temperature at the place where they are is below the dew-point. 
The appearance of the upper clouds, taken in conjunction with the 
readings of the barometer, is a valuable indication of forthcoming 
weather changes. The increase of cirrus clouds in a clear sky with 
a falling barometer, or the appearance of a solar or lunar halo, 
may be taken as a sure sign of an approaching cyclone, the intensity 
of which may be foreseen by the rate at which the barometer is 
falling. 
While the weather of places on the west coasts of temperate continents 
exposed to the prevailing sea-wind is usually made up of a succession of 
cyclones of different degrees of intensity, and of the anticyclonic intervals 
between them, over the greater part of the Earth’s surface the climate is 
much more uniform, and the seasonal changes are the principal cause of 
