274 Dr. J. Croll on the Cause of 



the surface of the Atlantic is lowest at the equator, and rises 

 with a gentle slope to well nigh the latitude of England. 

 This curious condition of things is owing to the fact that, in 

 consequence of the enormous quantity of warm water from 

 intertropical regions which is being continually carried by the 

 Gulf-stream into temperate regions, the mean temperature of 

 the Atlantic water, considered from its surface to the bottom, 

 is greater, and the specific gravity less, in temperate regions 

 than at the equator. In consequence of this difference of 

 specific gravity, the surface of the Atlantic at latitude 23° N. 

 must stand 2 feet 3 inches above the level of the equator, and 

 at latitude 38° N. 3 feet 3 inches above the equator. In this 

 case it is absolutely impossible that there can be a flow in the 

 Atlantic from the equatorial to the temperate regions result- 

 ing from difference of specific gravity. If there is any motion 

 of the water from that cause, it must, in so far as the Atlantic 

 is concerned, be in the opposite direction, viz. from the tem- 

 perate to the equatorial regions. 



All, or almost all, the heat which the Arctic seas receive 

 from intertropical regions in the form of warm water comes 

 from the Atlantic, and not from the Pacific ; for the amount 

 of warm water entering by Behring Strait must be compara- 

 tively small. It therefore follows from the foregoing con- 

 siderations that none of that equatorial heat can be conveyed 

 by a circulation resulting from difference of specific gravity 

 produced by difference of temperature. 



It is assumed as a condition in this theory that a submer- 

 gence of the Arctic land of several hundred feet must have 

 taken place in order to convert that land into a series of 

 islands allowing of the free passage of water round them. But 

 the evidence of Geology, as was shown on a former occasion*, 

 is not altogether favourable to the idea that those warm cli- 

 mates were in any way the result of a submergence of the 

 polar land. Take the Miocenee poch as an example : all the 

 way from Ireland and the Western Isles, by the Faroes, Ice- 

 land, Franz- Joseph Land, to North Greenland, the Miocene 

 vegetation and the denuded fragmentary state of the strata 

 point to a much wider distribution of Polar land than that 

 which now obtains in those regions. 



Mr. Alfred R. Wallace on Mild Arctic Climates. — The theory 

 that the mild climates of Arctic regions were due to an inflow 

 of warm water from intertropical and temperate regions has 

 also been fully adopted by Mr. Alfred R. Wallace. But, 

 unlike Sir William Thomson, he does not attribute this 



* Geol. Mag., September 1878. 



