252 LIGHT SCIENCE FOR LEISURE HOURS. 



the slow removal, the creeping aivay, as it were, of that 

 which it replaces. That this cause, per se, can ever 

 become one of sufficient activity 1 to generate a complete 

 system of vertical oceanic circulation seems at the least 

 open to grave question. It appears to me also that 

 when applied to the North Pacific this theory fails. 

 Very little water can pass through Behring s Straits, and 

 beyond Behring s Straits there is an island-locked and 

 shallow sea of enormous area, altogether unlike the deep 

 North Atlantic. 



I would further point out that the interesting fact 

 above mentioned, namely that the equatorial heat exerts 

 no perceptible effect at a depth exceeding 200 fathoms, 

 is in reality almost a necessity for my theory. For if 

 the whole of the equatorial ocean were heated, and, 

 therefore, of reduced specific gravity, the water arriving 

 from higher latitudes would flow to the bottom, and so 

 have to force up the intervening strata, in order to pro 

 duce the observed effects ; and this may be regarded as 

 impossible. As it is, such colder and heavier water 

 would be in dynamical equilibrium within a very short 

 distance of the surface. 



Next, as to the question of rainfall. Dr. Carpenter 

 considers that I have overlooked the considerations (1) 

 that the rainfall of Europe and North America may be 

 accounted for by the evaporation in the Mid-Atlantic, 



1 In passing I may notice that I did not suppose Sir J. Herschel to 

 be humorous in reference to the intensity of the polar action, but in his 

 use of the word emphasis. I should not have touched on the point, 

 did I not thoroughly sympathise with the emphatic utterance of specula- 

 tire or theoretical opinions. 



