184 THE OCEAN. [Book VIII, 



and by dilution with rain, the reverse qualities,' 

 he admits that ' in this fact we have a vera causa^ 

 though a very feeble one, for the production of 

 an indraught on both sides towards the lines of 

 maximum evaporation and minimum precipitation : ' ^ 

 but argues that its action is insufficient to cause the 

 currents which exist, and, that it would not tend to 

 give them the direction which they actually have. 

 And, as regards the direct action of the sun's rays, 

 Sir John Herschel says : ' The surface of the ocean 

 becomes most heated, and the heated water will, 

 therefore, neither directly tend to ascend (which it 

 could not do without leaving the sea) nor to descend, 

 which it cannot do, being rendered buoyant, nor to 

 move laterally, no lateral impulse being given, and 

 which it could only do by reason of a general decHvity 

 of surface — the diluted portion occupying a higher 

 level : ' and argues to show that this may be dismissed 

 ' as a cause capable of creating only a very triflmg 

 surface drift, and not worth considermg, even were 

 it in the proper direction to form, by concentration, a 

 current from east to west ; which it would not be, hut 

 the very reverse.^ ^ 



1 Sec. 59. 



^ Sec. 57. I do not profess to endorse all the details of 

 Herschel's argument in the above quotations, but only the cor- 

 rectness of the opinion based upon them, which, since the text 

 above was published in 1868, has been corroborated in a very 

 decided manner by the extract from Sir Wyville Thompson given 

 on page 5 of this volume. 



