304 COSMOS. 



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 tem- 

 perature 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 highest tem- 

 perature 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, while its minimum was 

 situated a few degrees to the south of the equator. In the 

 region of calms the solar heat can exercise but little influence 

 on evaporation, because the stratum of air impregnated with 

 saUne aqueous vapor, 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 theii 

 mean elevation. Local causes (probably prevaiUng winds and 

 currents) may, however, produce permanent, although trifling 

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

 stance, 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-Mandeb, through which the waters appear 

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

 this remarkable phenomenon, which was known to the an- 

 cients.* The admirable geodetic operations of Coraboeuf and 

 Delcrois show that no perceptible difference of level exists be- 

 tween the upper surfaces of the Atlantic and the Mediterra- 

 nean, along the chain of the Pyrenees, or between the coasts 

 of northern Holland and Marseilles.! 



our planet, changes its physical constitution and splendor, like the great 

 er number of the stars, or whether, on the contrary, that luminary has 

 attained to a permanent condition." — Arago, in the Comptes Rendu* 

 des Sianceg de V Acad, des Sciences, t. xi., Part ii., p. 309. 



* Humboldt, Asie Centrale, t. ii., p. 321, 327. 



t See the numerical results in p. 328-333 of the volume just named. 

 From the geodesical levelings which, at my request, my friend General 

 Bolivar caused to be taken by Lloyd and Falmarc, in the years 1828 

 and 1829, it was ascertained that the level of the Pacific is at the ut- 

 most 3i feet higher than that of the Caribbean Sea; and even that at 

 different hours of the day each of the seas is in turn the higher, accord- 

 ing to their respective hours of flood and ebb. If we reflect that in a 

 distance of 64 miles, comprising 933 stations of observation, an error of 

 three feet would be very apt to occur we may say that in these new 



