30G COSMOS. 



evidence of tiie importance of analysis, which is loo often re 

 parded 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 at tend - 

 ng 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. 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 differences in the 

 temperature and in the relative quantity of saline contents at 

 different 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 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 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 differ- 

 ence 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 numerous observations of Lenz (Poggendorf'a 

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

 ed in hw Voyage to the acific, vol. ii., p. 727, deserve particular at- 

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

 t. iii., p. 356. 



