156 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 off 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 affect the health of 

 its inhabitants. 



300. The principal agents that are supposed to be concerned 

 in giving circulation to the atmosphere, and m 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 which 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 pow^erful and active agen- 

 cy in the system of oceanic circulation is derived from the salts of 

 the sea w^ater, 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 w^ould 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, w^ould 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 w^hence power is given 

 to the system of oceanic circulation ; but, though a feeble one^ it 



