4 THE OCEAN. [Book I. 



aerial and oceanic circulation, led the first investigators 

 of the subject to regard them as cause and ejBPect ; but 

 difficulties met with by subsequent inquirers en- 

 deavouring more accurately to reconcile the observed 

 efi'ects with this suggested cause, led to the con- 

 sideration that as the burning rays of the sun are 

 constantly pouring down on the equatorial waters, 

 leaving those of the polar regions icy cold, the 

 differences of temperature resulting from this must 

 also to some extent tend to cause currents, as the 

 water endeavours to maintain its equilibrium. 



Those investigators who came to regard the force 

 or the direction of the winds as not forming a suffi- 

 cient explanation of the observed circulation of the 

 ocean, suggested, as above stated, that differences of 

 temperature must tend, by disturbing the equilibrium 

 of the water, to cause a constant circulation between 

 the equator and the poles ; and have by various 

 theories endeavoured to explain the action of this 

 effect in such a manner as to reconcile it with what is 

 known of the actual circulation.^ According to the 

 most popular form in which the temperature theory has 

 been propounded, polar cold causes the water to sink in 

 those regions, and thence to travel along the bottom 



^ Maury, as a practical investigator, was compelled to abandon 

 the idea of the circulation of the ocean being caused by the winds, 

 and therefore turned his attention to the current-creating action 

 of differences of temperature, but he has not attempted systemati- 

 cally to trace the connection between the observed effects and the 

 suggested cause. 



