4S6 
THE PHYSICAL GEOGRAPHY OF THE SEA. 
by which a general interchange and comminghng of the waters 
take place? In like manner, the constituents of the atmosphere, 
whether it be analyzed at the equator or the poles, are the same. 
By cutting olF and shutting up from the general channels of cir- 
culation any portion of sea water, as in the Dead Sea, or of at- 
mospheric air, as in mines or wells, we can easily fill either with 
gases or other matter that shall very much affect its character, 
or alter the proportion of its ingredients, and alFect the health of 
its inhabitants. 
300. The principal agents that are supposed to be concerned 
in giving circulation to the atmosphere, and in preserving the ratio 
among its components, are light, heat, electricity, and magnetism 
231). But with regard to the sea, it is not known what office 
is performed by electricity and magnetism, in giving dynamical 
force to its waters in their system of circulation. The chief mo- 
tive power from w^hich marine currents derive their velocity has 
been ascribed to heat ; but a close study of the agents concerned 
has suggested that an important — nay, a powerful and active agen- 
cy in the system of oceanic circulation is derived from the salts of 
the sea water, through the instrumentality of the winds, of marine 
plants, and animals. These give the ocean great dynamical force. 
• 301. Let us, for the sake of illustrating and explaining this 
force, suppose the sea in all its parts — in its depths and at the 
surface, at the equator and about the poles — to be of one uniform 
temperature, and to be all of fresh water ; and, moreover, that 
there be neither wind to disturb its surface, nor tides nor rains to 
raise the level in this part, or to depress it in that. In this case, 
there would be nothing of heat to disturb its equilibrium, and there 
would be no motive power 288) to beget currents, or to set the 
water in motion by reason of the difference of level or of specific 
gravity due to water at different densities and temperatures, 
302. Now let us suppose the winds, for the first time since the 
creation, to commence to blow upon this quiescent sea, and to 
ruffle its surface ; they, by their force, would create partial sur- 
face currents, and thus agitating the waters to a certain depth, 
would give rise to a feeble and partial aqueous circulation in the 
supposed sea of fresh water. 
303. This, then, is one of the sources whence power is given 
to the system of oceanic circulation ; but, though a feeble one, it 
