Page:Physical Geography of the Sea and its Meteorology.djvu/58

32 thousand (§ 75) such streams as the Mississippi River—a power at least sufficient to overcome the resistance required to reduce from two miles and a half to a few feet per minute the velocity of a stream that keeps in perpetual motion one-fourth of all the waters in the Atlantic Ocean. Not only so, we must admit the existence of an engine in the Gulf of Mexico, which, being played upon by the gentle forces of the trade-winds, is capable of sending a stream of water from the shores of the New World to the shores of the Old.

93. Nor by the trade-wind theory.—The advocates of the trade-wind theory, whether, with Franklin (§ 77), they make the propelling power to be derived from a head of water in the Gulf, or, with Herschel (§ 79), from the rebound, à la billiard-balls, against its shores, require that the impulse then and there communicated to the waters of the Gulf Stream should be sufficient to send them entirely across the Ocean; for in neither case does their theory provide for any renewal of the propelling power by the wayside. Can this be? Can water flow on any more than cannon-balls can continue their flight after the propelling force has been expended?

94. Illustration.—When we inject water into a pool, be the force never so great, the jet is soon overcome, broken up, and made to disappear. In this illustration the Gulf Stream may be likened to the jet, and the Atlantic to the pool. We remember to have observed as children how soon the mill-tail loses its current in the pool below; or we may now see at any time, and on a larger scale, how soon the Niagara, current and all, is swallowed up in the lake below.

95. Gulf Stream the effect of some constantly operating power.—Nothing but a continually-acting power can keep currents in the sea, any more than cannon-balls in the air or rivers on the land, in motion. But for the forces of gravitation the waters of the Mississippi would remain at its fountain, and but for difference of specific gravity the waters of the Gulf Stream would remain in the caldron, as the inter-tropical parts of the Atlantic Ocean may be called.

96. The production of currents without wind.—For the sake of further illustration, let us suppose a globe of the earth's size, and with a solid nucleus, to be covered all over with water two hundred fathoms deep, and that every source of heat and cause of radiation be removed, so that its fluid temperature becomes