THE OCEA^. 299 



civil life, that by his complete theory of tides, Laplace has 

 enabled us to predict in our astronomical ephemerides the 

 height of spring tides at the periods of new and full moon. 



Oceanic currents, which exercise an important influence 

 on the climate of neighbouring coasts, and on the intercourse 

 of nations, depend concurrently on a variety of causes of 

 unequal magnitude and operation. Amongst these we may 

 reckon the propagation of the tide-wave in its progress round 

 the globe, the duration and strength of prevailing winds, 

 the variations of density which sea-water undergoes in 

 different latitudes and depths by changes of temperature and 

 of the relative quantity of its saline contents, ( 368 ) and, finally, 

 the horary variations of the atmospheric pressure, so regular 

 in the tropics, and propagated successively from east to west. 

 The currents of the ocean present a remarkable spectacle ; 

 maintaining a nearly constant breadth, they cross the sea in 

 different directions, like rivers of which the adjacent undis- 

 turbed masses of water form the banks. The line of de- 

 marcation between the parts in motion, and those in repose, 

 is most strikingly shewn in places where long bands of sea- 

 weed, borne onward by the current, enable us to estimate its 

 velocity. Analogous phenomena are sometimes presented 

 to our notice in the lower strata of the atmosphere, when, 

 after a violent storm, the path of a limited aerial current 

 may be traced through the forest by long lanes of over- 

 thrown trees, whilst those on either side remain unscathed. 



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

 tween the tropics, known by the name of the equatorial 

 or rotation current, is regarded as the joint effect of the 

 trade winds, and of the progressive propagation of the tide- 

 wave. Its direction is modified by the resistance which it 



