VOL. XV. (2) NOTES ON A DAILY WEATHER CHART I51 
comes from the north, the smoke will trail off to the 
south-west, and, if the passengers on board are uncon- 
scious of any movement, the wind on board will be 
north-east. 
The circulation of atmosphere thus existing gives rise, 
in parts of the globe, to more or less constant, or regularly 
recurring conditions, such as the trade winds, monsoons, 
etc., but, for our purposes, we may confine ourselves to 
the more complex, though less violent, manifestations (in- 
creases and decreases in the pressure of the atmosphere) 
which, in broad features, are recognised as cyclones, anti- 
cyclones, V depressions between two high pressures, 
colls, or necks of high pressure, between two cyclones. 
Western Europe lies north of the calms of Cancer, and 
the prevalent winds are south-west (hence the “ west end” 
of a town is the best end.) The main tendency, therefore, 
of circulation is to give westerly currents or winds, and 
these, coming over the Atlantic and the Gulf Stream, 
bring all our heat and moisture. 
Synoptic readings over a sufficiently large area enable a 
forecaster to plot on a map the varying pressures existing 
over the whole area at a certain fixed hour, and by giving 
all the lines of equal pressure (Isobars)—which are cal- 
culated at the rate of fifteen nautical miles for one- 
hundredth of an inch shown by the barometer—the 
existence of areas of low pressure (cyclones), and of high 
pressure (anti-cyclones), in all their varying types become 
evident. 
Of these areas of pressure, with their consequent 
circulation of winds, various types are recognised as more 
or less persistent during the various seasons, and depend, 
in the main, on the varying seasonal positions of large 
areas of higher and lower pressure over the Atlantic and 
North-Eastern Europe. 
