CURRENTS MITIGATE THE CLIMATE. 37 



tor and the poles, and touches the surface in lat. 

 56 26'. 



In this inequality of temperature of the fluid par- 

 ticles there is a clear cause of an internal movement? 

 which will continue until an equilibrium is restored. 

 But the sun is a disturbance sufficiently powerful to 

 maintain the inequality, by producing a vast evapora- 

 tion from the equatorial ocean, and also by warming 

 the surface waters of the sea in proportion to the 

 angle at which the rays of heat fall on them. The 

 constant transfer of water from the sea to the air by 

 evaporation in the torrid zone, causes a waste which 

 can only be supplied by water flowing in from neigh- 

 bouring regions. Currents will therefore set towards 

 the points of greatest evaporation. But besides this, 

 the lower strata in the tropical ocean being colder than 

 the upper, are continually attracting heat, and being thus 

 rendered specifically lighter than before, are constantly 

 rising to the surface, and pushing aside (so to say) the 

 still warmer superficial strata. This generates a strong 

 current which flows north and south from the equator. 

 And thus it comes to pass that an interchange or cir- 

 culation of the particles is maintained throughout the 

 ocean, the colder streams becoming more and more 

 undercurrents as they near the equator, and the warmer 

 completing the circuit by returning along the surface 

 towards the north and south, until gradually dissipated 

 in higher latitudes, where they encounter the chilling 

 waters of the great polar streams. 



The climate of the earth is powerfully mitigated by 

 this natural tendency of the warmer and colder cur- 



