182 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



waste from canals by evaporation, in the siunmcr-time, is an 

 clement 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 bo when we connider 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 Eed 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 

 Ked 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 evapo- 

 ration daily, it w^ould, by the time it reaches the Isthmus of 

 Suez, have lost twenty-five inches from its surface. Thus the 

 waters of the Eed 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 

 SO'-", than it is at Babelmandeb, in latitude 13°. To make it 

 quite clear that the surface of the Eed Sea is not a sea level, but 

 is an inclined plane, suppose the channel of the Eed 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. 



