Mr. J. Croll on the Physical Cause of Ocean-currents. 101 



do not fully counterbalance the contraction of the superficial 



film by arctic cold Speaking in the total absence of all 



reliable data, it is my general impression that if we were to set 

 aside all other agencies, and to trust for an oceanic circulation 

 to these conditions only which are relied upon by Dr. Carpenter, 

 if there were any general circulation at all, which seems very 

 problematical, the odds are rather in favour of a warm under- 

 current travelling northwards by virtue of its excess of salt, ba- 

 lanced by a surface return-current of fresher though colder arctic 

 water.-" (< The Depths of the Sea/ pp. 376 & 377.) 



This is what actually takes place on the west and north-west 

 of Spitzbergen. There the warm water of the Gulf-stream 

 ilows underneath the cold polar current. And it is the opinion 

 of Dr. Scoresby, Clements Markham, and Lieut. Maury that 

 this warm water, in virtue of its greater saltness, is denser than 

 the polar water. Mr. Leigh Smith found on the north-west of 

 Spitzbergen the temperature at 500 fathoms to be 52°, and once 

 even 64°, while the water on the surface was only a degree or two 

 above freezing*. Mr. Aitken, of Darroch, in a paper lately read 

 before the Royal Scottish Society of Arts, showed experimen- 

 tally that the polar water in regions where the ice is melting is 

 actually less dense than the warm and more salt tropical waters. 

 Nor will it help the matter in the least to maintain that differ- 

 ence of specific gravity is not the reason why the warm water of 

 the Gulf- stream passes under the polar stream — because if dif- 

 ferences of specific gravity be not the cause of the warm w r ater 

 underlying the cold water in polar regions, then difference of 

 specific gravity may likewise not be the cause of the cold water 

 underlying the warm at the equator; and if so, then there is 

 no necessity for the gravitation hypothesis of oceanic circulation. 



There is little doubt that the superheated stratum at the 

 surface of the intertropical seas, which stratum, according to 

 Dr. Carpenter, is of no great thickness, is less dense than the 

 polar water; but if we take a column extending from the sur- 

 face down to the bottom of the ocean, this column at the equator 

 will be found to be as heavy as one of equal length in the polar 

 area. And if this be the case, then there can be no difference 

 of level between the equator and the poles, and no disturbance 

 of static equilibrium nor any thing else to produce circulation. 



Under currents account for all tlie Facts better than Dr. Car- 

 penter's Hypothesis. — Assuming, for the present, the system of 

 prevailing winds to be the true cause of oceanic currents, it ne- 

 cessarily follows (as will be shown hereafter) that a large quan- 

 tity of iUlantic water must be propelled into the Arctic Ocean ; 

 and such, as we know, is actually the case. But the Arctic 

 * The Threshold of the Unknown Region, p. 95. 



