Practically the Mediterranean, despite its depth, may be looked upon as an inland sea. The one opening which connects it with the open ocean is not only comparatively narrow, but also is shallow, being less than two hundred fathoms deep. The water on this dividing ridge remains at about 55°, and the Mediterranean throughout, to its greatest depths, keeps to about that same degree of warmth. No entrance is afforded to the heavy cold currents from polar regions. The Red Sea is separated from the Indian Ocean by a similar ridge, and the same result is seen there. Over the floor of the Indian Ocean lies a carpet of chill water. But the whole body of the Red Sea never sinks, in summer or winter, below 70° F. Here, too, the cold streams are not able to surmount the barrier. In many deep mid-ocean hollows, cut off by surrounding sub-ocean walls of rock, the same is found again--warm water within the hollow, cold water on the ocean-bed outside. CHAPTER VI. RIVERS IN THE SEA “Sinuous or straight, now rapid and now slow.”--COWPER. “As winds that coerce the sea.”--C. G. ROSSETTI. Actual Rivers in the Ocean; distinct streams of water, flowing over a bed of water, with banks of water. Not merely one or two such rivers, but scores of them, hundreds of them, great and small, in all parts of the world. Chief perhaps in importance is the Gulf Stream, that vast flood which pours out of the Gulf of Mexico, and acts as a winter heating apparatus for the west of Europe. Though by no means the largest of ocean streams, it is one of the most useful to man. After quitting the Gulf, it hurries at speed through the Straits of Florida; then spreads out into a river, about fifty miles wide and over two thousand feet deep, journeying at a rate of some sixty miles in twelve hours. For a while it hugs the American coast; but, happily for Europe, it forsakes this friend of its youth, and wanders to the north-eastward across the Atlantic. To call it a “river” is no mere fiction of speech. Near Halifax the separation between warm and cold water is so sharp, that those on board a ship may know what latitude they have reached, on entering or leaving the stream, by simply dipping a bucket in the water and taking the temperature. Literally the Gulf Stream is a warm river, flowing over a bed of cold water, with cold-water banks. So far as Cape Hatteras the stream clings to its early friend; and after that the American coast knows it no more, being left to the mercies of a very different acquaintance. An icy stream flows southward from the far north, clinging to the coast of North America, while we in western Europe benefit by the presence of the warm current which travels over to us. This stream has been described as forming a “cold wall” to the mild Gulf Stream.