Page:Physical Geography of the Sea and its Meteorology.djvu/207

Rh though the water were drawn into a chasm below. The celebrated Maelstrom is caused by such a conflict of tidal or other streams. The late Admiral Beechey, R.N., gave diagrams illustrative of many " rotatory streams in the English Channel, a number of which occur between the outer extremities of the channel tide and the stream of the oceanic or parent wave." "They are clearly to be accounted for," says he, " by the streams acting obliquely upon each other."

375. Marine currents do not, like those on land, run of necessity from higher to lower levels.—It is not necessary to associate with oceanic currents the idea that they must, of necessity, as on land, run from a higher to a lower level. So far from this being the case, some currents of the sea actually run up hill, while others run on a level. The Gulf Stream is of the first class (§ 83).

376. The Red Sea current.—The currents which run from the Atlantic into the Mediterranean, and from the Indian Ocean into the Red Sea, are the reverse of this. Here the bottom of the current is probably a water-level, and the top an inclined plane, running down hill. Take the Red Sea current as an illustration. That sea lies, for the most part, within a rainless and riverless district. It may be compared to a long and narrow trough. Being in a rainless district, the evaporation of it is immense; none of the water thus taken up is returned to it either by rivers or rains. It is about one thousand miles long; it lies nearly north and south, and extends from latitude 13° to the parallel of 30° north. From May to October, the water in the upper part of this sea is said to be two feet lower than it is near the mouth. This change or difference of level is ascribed to the effect of the wind, which, prevailing from the north at that season, is supposed to blow the water out. But from May to October is also the hot season; it is the season when evaporation is going on most rapidly: and when we consider how dry and how hot the winds are which blow upon this sea at this season of the year; that it is a narrow sea; that they blow across it and are not saturated, we may suppose the daily evaporation to be immense. The evaporation from this sea and the Persian Gulf is probably greater than it is from any other arms of the ocean. We know that the