52 On (he Currents of the Atlantic 



at the isthmus of Suez than at the straits of Babelmandel. 

 They ought to be lower from two causes — viz., evaporation 

 and temperature ; for the temperature of that sea is neces- 

 sarily lower at Suez, in latitude 30°, than at Babelmandel, 

 in latitude 13°. To make this quite clear, suppose the 

 channel of the Red Sea to have no water in it, and a wave 

 ten feet high to enter the straits, and flow up this channel 

 at the rate of twenty miles a day for fifty days, losing daily 

 by evaporation two-tenths of an inch, it is easy to perceive 

 that at the end of the fiftieth day it would not be so high, by 

 ten inches, as it was the first day it commenced to flow. 

 The top of this sea, therefore, is probably an inclined plane. 

 But the salt water which has lost so much of its freshness 

 by evaporation, becomes Salter, and therefore heavier. The 

 lighter water at the straits cannot balance the heavier, colder, 

 and Salter water at the isthmus, and therefore the heavier 

 water must either run out as an under- current, or it must 

 deposit its surplus salt, and thus gradually make the bottom 

 of the Red Sea a salt-bed. As we know that this latter 

 process is not going on, we infer that there is from the Red 

 Sea an under or outer current, as from the Mediterranean 

 through the Straits of Gibraltar. The rivers which dis- 

 charge into the Mediterranean are not sufficient to supply 

 the waste of evaporation, and it is by this under-current that 

 the salt carried in from the ocean is returned to it again ; 

 were it not so, the bed of that sea would be a mass of solid 

 salt. Thus it is that by a system of compensation the equili- 

 brium of the seas is maintained* 



Lieutenant Maury said that he had noticed this fact, that, 

 inasmuch as the Gulf Stream was a bed of warm water, 

 lying between banks of cold water, the warm water was 

 lighter, and therefore the surface of the Gulf Stream was in 

 the shape of a double inclined plane, like the roof of a house, 

 down which there was a shallow surface or roof current, 

 from the middle towards either edge of the stream. This 

 fact had been confirmed in a singular way : a person who 

 had been engaged on the coast survey, with observations on 

 the Gulf Stream, had noticed that when he tried the cur- 

 rent in a boat, he found it sometimes one way, sometimes 



