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



place in this intermitting manner; according to him, the cir- 

 culation is constant. He asserts that there is a " continual 

 transference of water from the bottom of C to the bottom of W, 

 and from the top of W to the top of C, with a constant descend- 

 ing movement in C and a constant ascending movement in W '* 

 (§ 29). But such a condition of things is irreconcilable with 

 the idea of " the levels of the two columns, and consequently 

 their heights, being maintained at a constant equality" (§ 29). 



Although Dr. Carpenter does not admit the existence of a 

 permanent difference of level between the equator and the pole, 

 he nevertheless speaks of a depression of level in the polar basin 

 resulting from the contraction by cooling of the water flowing 

 into it. This reduction of level induces an inflow of water from 

 the surrounding area; "and since what is drawn away," to 

 quote his own words, " is supplied from a yet greater distance, 

 the continued cooling of the surface-stratum in the polar basin 

 will cause a ' set ' of waters towards it, to be propagated back- 

 wards through the whole intervening ocean in communication 

 with it until it reaches the tropical area." The slope produced 

 between the polar basin and the surrounding area, if sufficiently 

 great, will enable the water in the surrounding area to now 

 polewards ; but unless this slope extend to the equator, it will 

 not enable the tropical waters also to flow polewards. One of 

 two things necessarily follows : either the slope extends from the 

 equator to the pole, or water can flow from the equator to the 

 pole without a slope. If Dr. Carpenter maintains the former, 

 he contradicts himself; and if he adopts the latter, he contra- 

 dicts an obvious principle of mechanics. 



A confusion of ideas in reference to the supposed agency of Polar 

 Cold. — It seems to me that Dr. Carpenter has been somewhat 

 misled by a slight confusion of ideas in reference to the supposed 

 agency of polar cold. This is brought out forcibly in the fol- 

 lowing passage from his memoir in the ' Proceedings of the 

 Royal Geographical Society/ vol. xv. p. 54. 



"Mr. Croll, in arguing against the doctrine of a general 

 oceanic circulation sustained by difference of temperature, and 

 justly maintaining that such a circulation cannot be produced by 

 the application of heat at the surface, has entirely ignored the 

 agency of cold." 



It is here supposed that there are two agents at work in the 

 production of the general oceanic circulation. The one agent is 

 heat, acting at the equatorial regions ; and the other agent is 

 cold, acting at the polar regions. It is supposed that the 

 agency of cold is far more powerful than that of heat. In fact so 

 trifling is the agency of equatorial heat in comparison with that 

 of polar cold that it may be " practically disregarded " — left out 



