THE ATMOSPHERE. - 
fall in the southern, and this is just about the proportion 124) 
that observation gives. 
In like manner, the southeast trade-winds take up the vapors 
which make our rivers, and as they prevail to a much greater ex- 
tent at sea, and have exposed to their action about three times as 
much ocean as the northeast trade-winds have, we might expect, 
according to this hypothesis, more rains in the northern — and, con- 
sequently, more and larger rivers— than in the southern hemi- 
sphere. A glance at Plate VIII. will show how very much larger 
that part of the ocean over which the southeast trades prevail is 
than that where the northeast trade-winds blow. 
This estimate as to the quantity of rain in the two hemispheres 
is one which is not capable of verification by any more than the 
rudest approximations ; for the greater extent of southeast trades 
on one side, and of high mountains on the other, must each of ne- 
cessity, and independent of other agents, have their effects. Nev- 
ertheless, this estimate gives as close an approximation as we can 
make out from any other data. 
129. The rainy seasons, how caused. — The calm and trade-wind 
regions or belts move up and down the earth, annually, in latitude 
nearly a thousand miles. In July and August the zone of equato- 
rial calms is found between 7° north and 12° north; sometimes 
higher ; in March and April, between latitude 5° south and 2° north. 
With this fact and these points of view before us, it is easy to 
perceive why it is that we have a rainy season in Oregon, a rainy 
and dry season in California, another at Panama, two at Bogota, 
none in Peru, and one in Chili. 
In Oregon it rains every month, but more in the winter months. 
The winter there is the summer of the southern hemisphere, 
when this steam-engine is working with the greatest pressure. 
The vapor that is taken up by the southeast trades is borne along 
over the region of northeast trades to latitude 35° or 40° north 
(§ 124), where it descends and appears on the surface with the 
southwest winds of those latitudes. Driving upon the highlands 
of the continent, this vapor is condensed and precipitated, during 
this part of the year, almost in constant showers. 
130. In the winter, the calm belt of Cancer approaches the 
equator. This whole system of zones, viz., of trades, calms, and 
westerly winds, follows the sun ; and they of our hemisphere are 
