Page:Physical Geography of the Sea and its Meteorology.djvu/212

186 Suez to the Straits of Babelmandeb, which were afterwards examined by Dr. Giraud, who reported the following results:

These observations agree with the theoretical deductions just announced, and show that the surface waters at the head are heavier and Salter than the surface waters at the mouth of the Red Sea.

382. Evaporation from.—In the same paper, the temperature of the air between Suez and Aden often rises, it is said, to 90°, "and probably averages little less than 75° day and night all the year round. The surface of this sea varies in heat from 65° to 85°, and the difference between the wet and dry bulb thermometers often amounts to 25°—in the kamsin, or desert winds to from 30° to 40°; the average evaporation at Aden is about eight feet for the year." Now assuming," says Dr. Buist, "the evaporation of the Red Sea to be no greater than that of Aden, a sheet of water eight feet thick, equal in area to the whole expanse of that sea, will be carried off annually in vapour; or, assuming the Red Sea to be eight hundred feet in depth at an average—and this, most assuredly, is more than double the fact—the whole of it would be dried up, were no water to enter from the ocean, in one hundred years. The waters of the Red Sea, throughout, contain some four per cent, of salt by weight—or, as salt is a half heavier than water, some 2.7 per cent, in bulk—or, in round numbers, say three per cent. In the course of three thousand years, on the assumptions just made, the Red Sea ought to have been one mass of solid salt, if there were no current running out." Now we know the Red Sea is more than three thousand years old, and that it is not filled with salt; and the reason is, that as fast as the upper currents bring the salt in at the top, the under currents carry it out at the bottom.

383. The Mediterranean Currents.—With regard to an