Page:Physical Geography of the Sea and its Meteorology.djvu/208

182 waste from canals by evaporation, in the summer-time, is an element which the engineer, when taking the capacity of his feeders into calculation, has to consider. With him it is an important element: how much more so must the waste by evaporation from this sea be when we consider the physical conditions under which it is placed! Its feeder, the Arabian Sea, is a thousand miles from its head; its shores are burning sands; the evaporation is ceaseless; it is a natural evaporating dish (§ 525) on a grand scale; none of the vapours which the scorching winds that blow over it carry away are returned to it again in the shape of rains. The Red Sea vapours are carried off and precipitated elsewhere. The depression in the level of its head waters in the summer-time, therefore, it appears, is owing to the effect of evaporation, as well as to that of the wind blowing the waters back. The evaporation in certain parts of the Indian Ocean is supposed to be (§ 103) from three fourths of an inch to an inch daily. Whatever it be, it is doubtless greater in the Red Sea. Let us assume it, then, in the summer-time to average only half an inch a day. Now, if we suppose the velocity of the current which runs into that sea to average, from mouth to head, twenty miles a day, it would take the water fifty days to reach the head of it. If it lose half an inch from its surface by evaporation daily, it would, by the time it reaches the Isthmus of Suez, have lost twenty-five inches from its surface. Thus the waters of the Red Sea ought to be lower at the Isthmus of Suez than they are at the Straits of Babelmandeb. Independently of the forcing out by the wind, the waters there ought to be lower from two other causes, viz., evaporation and temperature; for the temperature of that sea is necessarily lower at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 13°. To make it quite clear that the surface of the Red Sea is not a sea level, but is an inclined plane, suppose the channel of the Red Sea to have a perfectly smooth and level floor, with no water in it, and a wave ten feet high to enter the Straits of Babelmandeb, and to flow up the channel, like the present surface current, at the rate of twenty miles a day for fifty days, losing daily, by evaporation, half an inch; it is easy to perceive that, at the end of the fiftieth day, this wave would not be so high by two feet (twenty-five inches) as it was the first day it commenced to flow. The top of that sea, therefore, may be regarded as an inclined plane, made so by evaporation.