82 
HINTS TO TRAVELLER^. 
2. Observations for Forecasting the Weather. —The familiar 
name of “ weather-glass ” is appropriately applied to the barometer, for it 
is the surest indicator of any approaching storm. The scientific prediction 
of the weather by means of the barometer involves the comparison of the 
simnltaneons readings of barometers over as wide an area as possible, and 
can only be carried ont where there is a complete telegraph system and 
a public department charged with the work. The storms of wind and 
rain which break the more usual steady weather are always associated 
with the formation of centres of low atmospheric pressure towards which 
wind blows in from every side. These atmospheric depressions move, as 
a rule, in fairly regular tracks, the rate of movement of the centre of the 
depression having no relation to the rate at which the wind blows or to 
the direction of the wind. The term cyclone is usually applied to such a 
moving depression, because of the rotating winds round the centre; but 
the size of a cyclone may vary from a vast atmospheric eddy extending 
across the whole breadth of the Atlantic to one only a few miles in 
diameter. The strength of the wind in a cyclone depends on the baro¬ 
metric gradient; in other words, the greater the difference in atmospheric 
pressure between two neighbouring points the stronger is the wind that 
blows between them. Or, when a cyclone is passing over an observer, 
the more rapidly the barometer falls or rises the stronger may the wind 
be expected to blow. 
In direct contrast to the cyclone or depression is the system of high 
pressure rising to a centre from which the wind blows out on every side. 
This is called an anticyclone, and is a condition which, once established, 
may last for many days, or even weeks, without change. It is the typical 
condition for dry calm weather in all parts of the world. 
The direction of movement of the centres of cyclones in the northern 
hemisphere is usually westward and northward near the equator, the 
path of the centre bending to the right as it proceeds, and becoming 
ultimately eastward and southward. In the southern hemisphere the 
direction of the centre near the equator is westward and southward, 
turning towards the left as it proceeds. The rotation of the wind about 
the centre of a cyclone in the northern hemisphere is inwards towards 
the centre in the direction opposite to the hands of a watch, and in the 
southern hemisphere it is in the direction in which the hands of 
a watch move (Fig. 9). In the ^centre of a cyclone there is a calm, a 
