242 LIGHT SCIENCE FOR LEISURE HOURS. 



reservoir, though the slope from the reservoir to the 

 vent were very much greater than in the case of any 

 natural spring. Just as in the case of a spring it is 

 the difference of level, and not the rate of slope, which 

 is effective in causing the rate of outflow, so in the 

 case of the oceanic vertical circulation, it is the actual 

 difference of temperature, and not the rate of grada- 

 tion, which produces the activity of the circulation. 



Another objection has been urged against the 'heat 

 and cold theory' by a very skilful mathematician, Mr. 

 Croll. He reasons on this wise : Since the water which 

 is carried from the equator to the latitude of England 1 

 (say) must have partaken, when at the equator, of the 

 earth's rotation there, which has a velocity of more 

 than 1,000 miles per hour eastwards, whereas, when it 

 reaches our latitudes, it partakes of a rotation-move- 

 ment reduced to about 620 miles per hour, it follows 

 that, neglecting the drift motions as relatively quite 

 insignificant, friction has deprived the water which has 

 thus travelled from the equator to our latitudes of a 

 velocity amounting to no less than 380 miles per hour. 

 If friction is thus effective, how utterly inconceivable 

 is it, says Mr. Croll, that the descending currents of 

 Dr. Carpenter's theory (or the ascending currents of 

 the evaporation theory) should maintain their motion. 

 Hence, Mr. Croll is an earnest advocate of the trade- 

 wind theory. 



The worst of this reasoning is that it proves too 



1 I present the general nature of Mr. Croll's reasoning, without fol- 

 lowing him in details. 



