PHYSICAL GEOGRAPHY. 309 



of the sun would, if they were of long duration, be reflected 

 with more certainty in the mean temperature of the sea, than 

 in that of the solid land. 



The zones, at which occur the maxima of the oceanic 

 temperature and of the density (the saline contents) of its 

 waters, do not correspond with the equator. The two 

 maxima are separated from one another, and the waters of 

 the higliest temperature appear to form two nearly parallel 

 lines north and south of the geographical equator. Lenz, in 

 his voyage of circumnavigation, found in the Pacific the 

 maxima of density in 22 north and 17 south latitude; 

 whilst its minimum was situated a few degrees to the south of 

 the equator. In the region of calms the solar heat can exer- 

 cise but little influence on evaporation, because the stratum 

 of air impregnated with saline aqueous vapour, which rests on 

 the surface of the sea, remains still and unchanged. 



The surface of all connected seas must be considered as 

 having a general perfectly equal level with respect to their 

 mean elevation. Local causes (probably prevailing winds and 

 currents) may however produce permanent although trifling 

 changes in the level of some deeply indented bays, as for 

 instance the Red Sea. The highest level of the water at the 

 Isthmus of Suez is at different hours of the day from 24 to 

 30 feet above that of the Mediterranean. The form of the 

 Straits of Bab-el-Maudeb, through which the waters appear to 

 find an easier ingress than egress, seems to contribute to this 

 remarkable phenomenon, which was known to the ancients * 

 The admirable geodetic operations of Coraboeuf and Delcrois, 

 show that no perceptible difference of level exists between the 

 upper surfaces of the Atlantic and the Mediterranean, along 

 the chain of the Pyrenees, or between the coasts of northern 

 Holland and Marseilles.! 



mountains, and the drying up of lakes and marshes. Each age might 

 easily transmit to the succeeding one some few data, which would 

 perhaps furnish tne most simple, exact, and direct means of deciding 

 whether the sun, which is almost the sole and exclusive source of the 

 he it of our planet, changes its physical constitution and splendour, like 

 the greater number of the stars, or whether, on the contrary, that lumi- 

 tutry has attained to a permanent condition." Arago, in the Comptee 

 Hindus des Seances de I'Acad. des Sciences, t. xi. pt. 2, p. 309. 



* Humboldt, Asie centrale, t. ii. pp. 321, 327. 



t See the numerical results, in pp. 328-333 of the volume just 



