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.
The Gulf Stream cannot be viewed as a separate entity. Its very birth in the Gulf of Mexico depends on a great mass of water ever flowing from the south-east into the Caribbean Sea. Since so much water pours in, the same volume must pour out, and as it does so it gains the name of the “Gulf Stream.”
But after quitting the Gulf of Mexico the stream does not exist alone. It becomes a leading part of the North Atlantic circulation. The whole surface of that ocean is slowly turning round and round—“whirling as if stirred in the direction of the hands of a watch,”[1] and the Gulf Stream occupies one side or more of the vast maelström. In the centre of this revolving mass of water lies a district where the motion is slight, and at that centre floats an enormous collection of drift and sea-weed called the Sargasso Sea.
[1] Dr. H. R. Mill.
Suppose we pour some water in a large basin, drop into it a handful of small leaves and chips, and make the whole spin gently with one hand. We shall then see how the chips and leaves will collect at the centre, and will float there, almost stationary. That is what happens, on a large scale, in the North Atlantic Ocean.
Other Oceans also have this steady circular movement, not of the whole body of water, but of the surface-water, down to a greater or less depth—precisely how deep one cannot say; it lessens gradually with increasing depth. The same is found also in the South Atlantic, in the North and South Pacific, and doubtless in the Indian Ocean.