306 COSMOS. 



evidence of tiie importance of analj'sis, which is loo often re- 

 garded with contempt among the unscientific, that Laplace's 

 perfect theory of tides has enabled us, in our astronomical 

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

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

 sea-shore on their guard against the increased danger attend - 

 ng these lunar revolutions. 



Oceanic currents, v/hich 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. Among 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 par- 

 ticles of water undergo in consequence of difTerences in the 

 temperature and in the relative quantity of saline contGnts at 

 dilTerent latitudes and depths ;* and, lastly, the horary varia- 

 tions of the atmospheric pressure, successively propagated from 

 east to west, and occurring with such regularity in the trop- 

 ics. These currents present a remarkable spectacle ; like riv- 

 ers of uniform breadth, they cross the sea in different direc- 

 tions, while the adjacent strata of water, which remain un- 

 disturbed, form, as it were, the banks of these moving streams. 

 This difference between the moving waters and those at rest 

 is most strikingly manifested where long lines of sea-weed, 

 borne onward by the current, enable us to estimate its veloc- 

 ity. In the lower strata of the atmosphere, we may some- 

 times, during a storm, observe similar phenomena in the lim- 

 ited aerial current, which is indicated by a narrow line of 

 trees, which are often found to be overthrown in the midst of 

 a dense wood. 



The general movement of the sea from east to west be- 



* The relative density of the particles of water depends simultane- 

 ously on the tempei'ature and ou the amount of the sahne contents — a 

 circumstance that is not sufficiently borne in mind in considering the 

 cause of curi'ents. The submarine current, which brings the cold po- 

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

 course, that is to say, from the equator toward the poles, if the ditler- 

 euce in saline contents were alone concerned. In this view, the geo- 

 graphical distribution of temperature and of density in the water of 

 the ocean, under the different zones of latitude and longitude, is of 

 great importance. The n^^merous observations of Lenz (Poggendorf 'a 

 Annalen, bd. xx., 1830, s. 129), and those of Captain Beechey, collect- 

 ed in his Voyage to the acijic, vol. ii., p. 727, deserve particular at- 

 tention. See Humboldt, Relat. Hist., t. i., p. 74, and Asie Central, 

 t. iii., p. 356. 



