SKCT. xiii.] CUKRENTS IN THE OCEAN. Ill 



in Tonquin, where the tides arrive by two channels of 

 lengths corresponding to half an interval, there is neither 

 high nor low water on account of the interference of the 

 waves. 



The initial state of the ocean has no influence on the tides ; 

 for, whatever its primitive conditions may have been, they 

 must soon have vanished by the friction and mobility of the 

 fluid. One .of the most remarkable circumstances in the 

 theory of the tides is the assurance that, in consequence of 

 the density of the sea being only one-fifth of the mean den- 

 sity of the earth, and the earth itself increasing in density 

 towards the centre, the stability of the equilibrium of the 

 ocean never can be subverted by any physical cause. A 

 general inundation arising from the mere instability of the 

 ocean is therefore impossible. A variety of circumstances, 

 however, 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. 157). The tangential force, though small, is sufficient 

 to make the fluid particles within the polar circles tend to- 

 wards 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, partly from their low temperature and partly 

 from their gravitation being less diminished by the action 

 of the sun and moon than that of the seas of lower lati- 

 tudes. In consequence of the combination of all these cir- 

 cumstances, two great currents perpetually set from each 

 pole towards the equator. But, as they come from latitudes 



