More reasons than one may help to explain why these two currents slant off to the eastward instead of pouring due north. The shape of the various coast-lines has something to do with it; also the presence of ridges and hollows in the ocean-beds, and the resistance of other contending currents. A river, either on land or in the sea, will always travel where it finds least opposition. One main cause, however, is the whirl of our Earth upon its axis. This, which greatly affects the directions of prevailing winds, alters also the lines followed by ocean rivers. A current starting from near the equator for the north shares in the rapid rush of the Earth’s surface, which at the equator spins eastward at a rate of about one thousand miles each hour. As the volume of water gets farther north, it reaches parts of the Earth which are whirling more slowly, while it has not lost much of its own eastward whirl. A sideways flow is the result, changing the northward into a north-eastward direction. But a stream starting from the far north for the south is affected in the opposite way. Near the north pole the Earth’s surface hardly moves at all; and the southward-flowing current, being weighted with northern inertia, takes a contrary course to the current flowing north. It lags more and more _behind_ the faster-revolving surface, and so wanders westward instead of eastward. Or, if prevented by the land from so doing, it hugs the coast which hinders it. So the _pull_ of the two great streams in the Atlantic is exactly opposed, each to the other. That of the Gulf Stream is towards the east; that of the Labrador Stream is towards the west; and the resolute manner in which the two refuse to mingle may be partly due to this fact. If our Earth could be made to change the present whirl from west to east, and to revolve instead from east to west, those two great currents would alter their directions. The Gulf Stream would hug the American coasts, and the Labrador would find its way over to Europe. Then the British Isles in winter would know a temperature of 30° or 40° below zero, and the Canadians would experience soft damp winters and moderate summers. Perhaps they would no more welcome the exchange than we should. Then, too, the Black Stream would cling to the Asiatic side, transforming the climate of western Siberia, and the cold Arctic river would put a speedy end to humming-birds in Alaska. But abundance of ice floes would soon be awaiting the walruses which would have to emigrate from the other side of the continent. A good deal of discussion has been held as to whether we do truly owe our mildness to the Gulf Stream alone, or whether that Gulf Stream is merely part of a general northward movement of Atlantic waters from the tropics towards the pole. The question has been warmly contested; and no doubt on both sides, as generally is the case, truth has been mixed with error.