On the Physical Geology of the United States, &§c. 7 
part of Africa and towards St. Helena and Ascension, meet nearly 
in opposition to each other under the tropics; and although the 
velocity of the currents is small, the opposite momenta have a 
tendency to elevate the waters of the ocean higher than the true 
water level, the same as when a tide is obstructed by a coast, it 
rises higher than it would if unobstructed. The tendency of the 
water to restore the equilibrium would be to the westward and 
to the eastward. 'The flow to the eastward is partially obstructed 
by the African coast, but the eastward current by Cape Palmas 
and Cape Threepoints, and around the Gulf of Guinea, may be 
caused in part by this tendency.* ‘The flow to the westward in 
the direction of the equatorial current, is the one by which the 
equilibrium is mostly restored, for two reasons; Ist, the flow in 
that direction is unobstructed; and 2d, the waters coming from 
higher latitudes, where the linear rotative velocity of the earth’s 
surface is less than under the tropics, the tendency of the water 
is to flow to the westward. 
Another cause tending to aid in the production of the polar 
and equatorial currents may be adduced, although its real influence 
may be very minute. The evaporation from the tropical regions 
of the earth exceeds the amount condensed as rain, fog, and dew, 
and this excess is carried to the temperate and frigid zones by 
the great currents of the atmosphere, where it is condensed as 
rain, snow, fog, and dew. The excess of water deposited from the 
atmosphere in the extra-tropical regions, more than is evaporated, 
falls and flows into the ocean. The ocean level in those regions, 
particularly along coasts where large rivers debouche, may be 
said to be slightly raised above the level of the spheroidal form 
of equilibrium of the earth, and the water in consequence of its 
mobility, will tend to flow from the polar and temperate zones 
towards the tropics. 
All the various causes that have been mentioned as railadinelie 
the great currents of the ocean and atmosphere, (and which are 
the legitimate results of the action of gravitation, variations of 
temperature, and inertia, while the earth revolves on its axis, ) 
concur in their effects, and are believed to be the true causes of 
the Gulf Stream and the great currents of the ocean. The cur- 
* Currents have been observed between Cape Palmas and Cape of Good Hope, 
indicating with much probability a gyrating mass of waters in a great eddy be- 
tween Cape Palmas, Cape Formosa, Cape Negro, and St. Helena. 
