PHYSICAL GEOGRAPHY. 311 



has been shown by the analysis of the great geometrician 

 Laplace, that supposing the depth to be wholly inconsiderable 

 when compared with the radius of the earth, the stability of 

 the equilibrium of the sea requires that the density of its 

 fluid should be less than that of the earth; and as we have 

 already seen, the earth's density is in fact five times greater 

 than that of water. The elevated parts of the land cannot 

 therefore be overflowed, nor can the remains of marine 

 animals found on the summits of mountains, have been con- 

 veyed to those localities by any previous high tides."* It is 

 no slight evidence of the importance of analysis, which is too 

 often regarded with contempt amongst the unscientific, that 

 Laplace s perfect theory of tides has enabled us in our astro- 

 nomical ephemerides, to predict the height of spring-tides at 

 the periocls of new and full moon, and thus put the inhabitants 

 of the sea shore on their guard against the increased danger 

 attending these lunar revolutions. 



Oceanic currents, which exercise so important an influence 

 on the intercourse of nations and on the climatic relations of 

 adjacent coasts, depend conjointly upon various causes, differ- 

 ing alike in nature and importance. Amongst these we may 

 reckon the periods at which tides occur in their progress round 

 the earth ; the duration and intensity of prevailing winds ; the 

 modifications of density and specific gravity which the particles 

 of water undergo in consequence of differences in the tem- 

 perature and in the relative quantity of saline contents at dif- 

 ferent latitudes and depths ; f and lastly, the horary variations 



* Bessel, Ueber Fluth und Ebbe, in Schumacher's Jahrbuch, 1838, 

 s. 225. 



t The relative density of the particles of water depends simulta- 

 neously on the temperature and on the amount of the saline contents, a 

 circumstance that is not sufficiently borne in mind in considering the 

 cause of currents. The submarine current, which brings the cold polar 

 water to the equatorial regions, would follow an exactly opposite course, 

 that is to say, from the equator towards the poles, if the difference in 

 saline contents were alone concerned. In this view, the geographical 

 distribution of temperature and of density in the water of the ocean, 

 under the different zones of latitude and longitude, is of great import- 

 ance. The numerous observations of Lenz (Poggendorff's Annalen, 

 bd. xx. 1830, B. 129), and those of Captain Beechey, collected in hia 

 Voyage to the Pacific, vol. ii. p. 727, deserve particular attention. See 

 Humboldt, Relat. hist., t. i. p. 74, and Asie centrale, t. iii. p. 356. 



