6 On the Physical Geology of the United States, &§c. 
and tends to flow off towards the polar regions, while an under- 
flow of colder and heavier water restores the equilibrium. 
The real effects of this cause may be deemed theoretically 
true without having any sensible influence; but the geological 
effects in the development of organic life, when considered in 
connection with the flow produced by other causes, are not unim- 
portant, and will be considered in another place. 
The effect of the solar rays upon the atmosphere, panbiGalaly 
under the tropics, is to produce a rarefaction of the air, and 
ascending currents that flow off toward the polar regions, and a 
counter flow from the polar towards the tropical regions, restores 
the equilibrium.* 
The northwardly compensating current of the atmosphere over 
the eastern part of the Atlantic Ocean, as it reaches successively 
lower latitudes, bends more and more to the westward until un- 
der the tropics it forms the trade wind.t 'This sweeps to the 
westward and by its constancy and moderate limits of variation 
in direction, gives great aid to the equatorial current of the ocean, 
and is perhaps more effective in producing and maintaining this 
current than any of the other causes. 
Another cause may be adduced for the westward flow of the 
equatorial current, viz. the current that flows southwardly along 
the eastern part of the north Atlantic, and that flowing north- 
wardly from the Lagullas banks along the coast of the southwest 
* This is the commonly received theory. Many facts are in opposition to this 
theory and seem irreconcilable with it; but of the great circular flow of the cur- 
rents of the atmosphere and smaller secondary gyrating currents, there is no doubt, 
and change of temperature and the rotation of the earth are the main causes. 
Vide Mr. Redfield’s papers, Am. Jour. Science, Vols. xxv and xuy. 
t Mr. Espy accounts for the trade winds on another principle, viz. that the up- 
ward ascending currents as they reach higher elevations and are more remote 
from the axis of rotation of the earth, have by their inertia a different linear velocity 
from that due to a point rotating at that distance from the axis with the same an- 
gular velocity, and as the earth rotates from W. to E., these uprising columns of 
rarefied air have a relative retrograde motion with regard to the surface of the 
earth, giving rise to a general westwardly motion of the air under the tropics, 
This cause however can have but little influence in the production of the trade 
winds, for, if we suppose uprising columns of air to exist and to ascend twenty 
miles in height, which is far more than we have any evidence of in the clouds, 
the increased velocity due to a point at this distance would be only 62-83 miles 
more in twenty four hours, than that of a point on the surface at the equator, or 
less than three miles per hour, which would produce a retrograde or westwardly 
wind scarcely perceptible there, and would influence the currents near the surface 
of the earth still less, in an almost infinitesimal degree. 
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