CURRENTS OF THE SEA. 
127 
241. The evaporation in certain parts of the Indian Ocean 
33) is from three fourths of an inch to an inch daily. Suppose 
it for the Red Sea 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, lose twenty-five inches 
from its surface. 
- 242. Thus the waters of the Red Sea ought to be lower at 
the Isthmus of Suez than they are at the Straits of Babelman- 
deb. Independently of the waters forced out by the wind, they 
ought to be lower from two other causes, viz., evaporation and 
temperature, for the temperature of that sea is necessarily low- 
er at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 
13°. 
243. 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 Babel- 
mandeb, and to flow up the channel 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. 
244. The top of that sea, therefore, may be regarded as an in- 
clined plane, made so by evaporation. 
245. But the salt water, which has lost so much of its freshness 
by evaporation, becomes Salter, and therefore heavier. The light- 
er water at the Straits can not balance the heavier water at the 
Isthmus, and the colder and Salter, and therefore heavier water, 
must either run out as an under current, or it must deposit its sur- 
plus salt in the shape of crystals, and thus gradually make the 
bottom of the Red Sea a salt-bed, or it must abstract all the salt 
from the ocean to make the Red Sea brine — and we know that 
neither the one process nor the other is going on. Hence we in- 
fer that there is from the Red Sea an under or outer current, as 
there is from the Mediterranean through the Straits of Gibraltar, 
