The effect of the gulf stream upon temperature has been nowhere more strikingly -observed than in high northern latitudes. Where the warns stream from the s.w. meets the arctic current in the latitude of Iceland, a difference of 17° has been observed.
In treating of the cause or causes of the gulf stream, we must take a general view of oceau-currents. Taken altogether, they form a connected system 1vhich has been aptly -compared to the circulation of the blood. The two prime movers are differences of tem perature and prevalent winds. Sea-water of average saltness does not freeze until it is cooled down to about and, unlike fresh water, it, continues to grow heavier down to that point. The effect of the intense cold of the polar regions is thus to cause a con stant sinking down of the surface-water, and to establish a current of ice-cold water along the bottom towards the equator; while to supply the place of what sinks down, an iudraught or northward flow takes place on the surface, which brings the warm water of the temperate and tropical regions toward the poles. This is the general theory of the vertical circulation of the ocean—a circulation which might almost be assumed from the well-known laws of the flow of liquids, and which recent observations have established as a fact. The general prevalence of cold currents along the lied of the ocean from the poles to the equator is now beyond dispute. The soundings taken recently by H.M.S. Challenger show the temperature of the bottom water between Som brero in the West Indies and Teneriffc to vary from 34°.4 to 35°.5; while at the equator it is in sonic places still lower, being only 32°.4. This is held to prove that the Antarctic bottom current extends to the equator and beyond it. Motion once thus begun, how • ever, is differently mpdified in each locality by the shape of the coasts, by prevalent winds, and other circumstances. But one cause which modifies all currents that tend' either is. or s., is the daily rotation of the earth. In the very same way that the rotation • Of the earth gives the trade-windg their peculiar directions (see WINDS), it causes the • cold currents coming from the poles to turn towards the s.w., and the surface-currents from the s. to take a n.e. direction. At the equator, any spot on the surface is moving
eastward at the rate of 1000 in. an hour; at GO° n. latitude, the velocity is only one half. Thus, the water of a current starting from the equator northward, is constantly coming to places where the bottom under it has less and less eastward velocity. But, by the law of inertia, the water tends to retain the same velocity eastward with which it • started, and thus it moves to the c. of n.—shooting ahead, as it wore, of the bottom over which it is flowing, as a rider does whose horse slackens his pace. The contrary happens to a stream flowing from n. to south. In this case, the eastward motion or • motal inertia of the water is too slow for the parts of the bottom to which it slimes ' sively conies; the bottom slips in a manner from under it. and it falls to w. of south. This, in combination with the action of opposing coasts, accounts for the circular sweep' which many of the currents make, returning partI7 into themselves.
Different in origin from this vertical circulation, though partly mixed tip with it, is the horizontal circulation caused by prevalent winds. The best example of this is the equatorial current, which sets from the w. coast of Africa to the e. coast of Brazil. and which is owing to the action of the trade-winds. Currents caused by winds are called "drift-currents," in opposition to the deeper-seated "stream-currents." In order to feed this westerly equatorial current, there spring up two in-draught currents, which also follow the prevailing•winds of their respective regions—the one from the is. along the w. coast of Portugal and Morocco, the other from the cape of Good Hope along the w. coast of Africa. as far as the gulf of Guinea. When the equatorial current reaches the coast of Brazil, it divides into two branches. One proceeds southwards. turning gradually eastwards across the Atlantic until it falls in with the northern in-draught from the cape of Good Hope. The other branch is deflected northwards into the Caribbean sea and the gulf of Mexico. The water thus driven into this pent-up sea now rushes with accumulated momentum through the strait or gulf between Florida and the Bahamas, and forms the famous gulf stream.