100 CURRENTS IN THE OCEAX. SECT. XIII. 



the assurance that, in consequence of the density of the sea being 

 only one-fifth of the mean density of the earth, and the earth itself 

 increasing in density towards the centre, the stability of the equi- 

 librium of the ocean never can be subverted lay any physical cause. 

 A general inundation arising from the mere instability of the 

 ocean is therefore impossible. A variety of circumstances, how- 

 ever, tend to produce partial variations in the equilibrium of the 

 seas, which is restored by means of currents. Winds and the 

 periodical melting of the ice at the poles occasion temporary water- 

 courses ; but by far the most important causes are the centrifugal 

 force induced by the velocity of the earth's rotation, and variations 

 in the density of the sea. 



The centrifugal force may be resolved into two forces one 

 perpendicular, and another tangent to the earth's surface (N. 161). 

 The tangential force, though small, is sufficient to make the fluid 

 particles within the polar circles tend towards the equator, and 

 the tendency is much increased by the immense evaporation in 

 the equatorial regions from the heat of the sun, which disturbs 

 the equilibrium of the ocean. To this may also be added the 

 superior density of the waters near the poles, from their low tem- 

 perature. In consequence of the combination of all these circum- 

 stances, two great currents perpetually set from each pole towards 

 the equator. But, as they come from latitudes where the rotatory 

 motion of the surface of the earth is very much less than it is 

 between the tropics, on account of their inertia, they do not imme- 

 diately acquire the velocity with which the solid part of the 

 earth's surface is revolving at the equatorial regions ; from whence 

 it follows that, within twenty-five or thirty degrees on each side 

 of the line, the ocean has a general motion from east to west, 

 which is much increased by the action of the trade winds. Both 

 in the Pacific and Atlantic currents of enormous magnitude are 

 deflected by the continents and islands to the north and south 

 from this mighty mass of rushing waters, which convey the 

 warmth of the equator to temper the severity of the polar regions, 

 while to maintain the equilibrium of the seas counter currents of 

 cold water are poured from the polar oceans to mingle with the 

 warm waters at the line, so that a perpetual circulation is main- 

 tained. 



Icebergs are sometimes drifted as far as the Azores from the 

 Polar seas, and from the south pole they have come even to the 



