306 Proceedings of the Boy at Society 
equal pressure over the same region. There are two other regions, 
though of comparatively small extent, where the mean pressure 
exceeds 30 inches, — one in South Atlantic, and the other between 
South America and Australia. 
There occurs in the North Atlantic an extensive diminution of 
pressure, which deepens northwards till the greatest depression 
(29*5 inches) is reached in Iceland.* It is this low pressure over 
the North Atlantic, together with the high pressures in North 
America, in the Atlantic south of lat. 40°, and in Asia, which fur- 
nishes the key to the winter climates of North America and Europe. 
Another and equally remarkable depression occurs in the North 
Pacific, having its course of greatest depression (29'6 inches) in the 
ocean between Kamscbatka and Sitka in the north-west of America. 
The pressure is also under the average in the south of Africa and 
in South America. 
The equatorial depression stretches quite across the globe in an 
irregular belt, which attains its greatest breadth in Africa and its 
least in the Pacific. In crossing the Indian Ocean it does not 
lie parallel to the equator, but slants from Tamatave in Madagascar 
18° lat. S. to the coast of Sumatra, in 5° lat. S. It is in this 
trough that nearly all the tropical storms of the Indian Ocean 
have their origin. 
Mean Atmospheric Pressure for the Year . — There are two broad 
belts of high pressure passing completely round the globe, — the one 
north, the other south of the equator, — enclosing between them the 
low pressure of the tropics, through the centre of which runs a 
narrow belt of still lower pressure, towards which the trade winds 
blow. The southern belt of high pressure lies nearly parallel to 
the equator, and is generally of uniform breadth throughout. But 
the belt north of the equator has a very irregular outline and great 
differences in its breadth and in its inclination to the equator, — 
these irregularities being due to the unequal distribution of land 
and water in the northern hemisphere. 
Considered in a broad sense, there are only two regions of low 
pressure, one round each pole, bounded by or contained within the 
belts of high pressure just referred to. The most remarkable of these 
* The diminution of pressure in this region is even greater than this in 
February. 
