THE OCEAN. 297 



maximum of density resulting from their saline contents, do not 

 coincide with each other, or with the geographical equator. 

 The waters of highest temperature appear to form two bands 

 not quite parallel, one on either side of the equator. Lenz, 

 in his voyage of circumnavigation, found in the Pacific the 

 maxima of saltness in 22 N. and in 17 S. lat., and be- 

 tween them a minimum zone was interposed a few degrees 

 south of the equator. Within the region of calms the 

 solar heat has little effect in augmenting the relative propor- 

 tion of the saline particles by the process of evaporation, 

 because the stratum of air saturated with aqueous vapour 

 which rests on the surface of the water is rarely disturbed 

 by the action of the wind. 



The surfaces of all connected seas must be regarded as 

 possessing a general equality of mean level, although local 

 causes (such as prevailing winds or currents) may produce 

 small permanent differences in particular seas which form 

 deep gulfs or inlets. An instance of this occurs in the Bed 

 Sea, whose level, near its northern extremity at the Isthmus of 

 Suez, is, at different hours of the day, from twenty-four to thirty 

 Trench feet above that of the neighbouring part of the Medi- 

 terranean : the form of the Straits of Bab-el-Mandeb, which 

 is more favourable to the ingress of the waters of the Indian 

 ocean than to their egress, being probably the cause of this 

 remarkable fact, which was not unknown to the ancients. ( 365 ) 

 The excellent geodesic operations of Corabceuf and Delcros 

 have shewn that at the two extremities of the Pyrenean 

 Chain, as well as at Marseilles and the northern coast of 

 Holland, there is no sensible difference between the level of 

 the Atlantic and of the Mediterranean Seas. ( 366 ) 



