29-2 



NA TURE 



[July 26, 1694 



A paper on Geological Climate continues what we 

 are treating as the geological portion of the volume 

 under review. One of the chief subjects therein discussed 

 is the probable cause of the warm Arctic climate once ex- 

 perienced, so that not only are remains of forest trees 

 found within fifteen degrees of the North Pole, with 

 every evidence of their having grown there, and that not 

 so very long ago, but the return of the Arctic expedition 

 in 1876 brought "evidences of a very warm climate, 

 probably as warm as we have it now in the tropics, 

 within nine degrees of the North Pole" ! 



For explanation, at any rate of the more moderate pine- 

 forest temperature, we are told to look in the direction 

 indicated by Lyell's twelfth chapter, viz. to a redistribu- 

 tion of land and water. 



The Arctic Ocean is a land-locked sea, and the effect 

 of the surrounding coasts is to hem the ice in and prevent 

 free oceanic circulation, while the land itself serves to 

 receive and accumulate snow wherewith to load the 

 neighbouring sea with a thin layer of surface ice. 



But now lower the land 2000 feet: the sea would 

 be open, but for a few islands, and the water would be 

 deep enough for plenty of warm currents to flow in, 

 sufficient to clear the ice away and keep it clear. Even 

 now the ice seems to be only 5 feet thick, evidently melt- 

 ing away underneath. It is asserted that the climate of 

 a small island in an iceless circumpolar sea would be 

 probably " temperate and free from frost except in 

 hollows." Considering all the defences, the heat- 

 capacity of moist air, the formation of dew, and so on, 

 it appears that the same defences as protect a large 

 continent in temperate zones from destructive cold 

 during a summer night, would "prevent even so much as 

 hoar-frost on a small island at the very pole during its 

 whole winter six months' night, if it were surrounded by 

 a deep ocean with no land to obstruct .free circulation 

 between it and tropical seas." 



And in the other direction Lord Kelvin agrees that the 

 simplest cause of the glaciation of India is some 15,000 

 feet extra elevation. But at the same time " the astro- 

 nomical cause invoked by Herschel must have had, and 

 must now have, its effect," the well-known fact, namely, 

 of the varying distance of the sun, and the periodic coin- 

 cidences of its least distance with the northern summer. 

 The sometimes postulated shift of the earth's axis to 

 account for changes in climate does not satisfy Lord 

 Kelvin ; he says that there is no evidence, either 

 geological or astronomical, for any considerable shifting 

 of the position of the poles. As to the warmer climate 

 evidenced all over the earth at one time ; underground 

 heat is often appealed to, but it is hopelessly inadequate, 

 it can never have sensibly influenced the climate during 

 the period of the stratified rocks. " The earth might be 

 a globe of white hot iron covered with a crust of rock 

 2000 feet, or there might be an ice-cold temperature within 

 50 feet of the surface, yet the climate could not on that 

 account be sensibly different from what it is, or the soil 

 be sensibly more or less genial than it is for the roots 

 of trees or smaller plants." 



The simple and in every way "almost infinitely prob- 

 able" hypothesis to account for past high temperature is 

 (/ warmer sun. 



Persons who are inclined to imagine a future limit to 

 NO. 1291, VOL. 50] 



the duration of life on the earth as in any way dependent 

 on a failure in the supply of heat from below, will do welt 

 to note the above strong pronouncement. 



The next essay, on "The Internal Condition of the 

 Earth," asserts that on the meteoric theory the earth 

 would once have been just about molten throughout, by 

 reason of the heat of its own formation ; but subsequent 

 occurrences must depend on whether solid rock sinks or 

 swims in molten rock. Definite experimental information 

 on this fundamental point appears to be still wanting, 

 but so many facts show that the earth is rigid, that 

 practically the author seems to have little doubt but that 

 it would sink. He is careful to point out, however, that 

 the crude notion sometimes met with of a rise of tem- 

 perature in arithmetic progression at different depths in 

 the crust is certainly false. The law of increase is an 

 asymptotic one, and the temperature at the centre need 

 not be higher than two or three thousand degrees. He 

 denies altogether the intensely high temperature often 

 imagined, and thus has no difficulty in accepting the 

 solidity and rigidity otherwise indicated. 



Some ingenious arithmetical calculations attempted ir> 

 Dr. Croll's book on Climate and Time, form the subject of 

 the next paper, on "Polar ice-caps and their influence in 

 changing sea-levels." It is interesting to know that if 

 the ocean were of quicksilver instead of water, it would 

 not spread over the earth as it does, but would accumu- 

 late itself entirely at one side. The stability of the 

 ocean depends on the low specific gravity of sea-water 

 as compared with earth. Now, any great piling of 

 material over a large area, such for instance as over the 

 gigantic Antarctic continent, would have the effect of 

 drawing by gravitation some of the ocean toward itself, 

 and thereby lowering the sea-level all over the rest of the 

 earth. And if the material so piled up were itself ice, 

 having been withdrawn by distillation from the ocean 

 itself, the fall of level would be greater still. By the 

 formation of an ice-cap on the Antarctic continent, twelve 

 miles thick, an immense change of sea-level can be pro- 

 duced ; the amount of lowering thereby caused in 

 distant parts of the globe being differently estimated 

 as 3S0 feet, 650 feet, 1 14.0 feet, and 2000 feet. 



But Lord Kelvin denies the possibility of a coat of ice 

 twelve miles thick, such as Dr. Croll postulates ; the. 

 pseudo-viscosity of ice forbids it ; but suppose it only 

 a quarter-mile, suppose the thickness varied by 1200 feet 

 from some cause, the area of the Antarctic continent is 

 something like ,'„th of the whole earth, and so, if such a 

 coat of ice melted ofl^ it, the sea-level all over the world 

 would rise twenty-five feet, — sufficient to make many 

 important changes. 



The greatest permissible thickness of ice at the South 

 Pole seems to be about iS,ooo feet, such a height as that 

 with a gradual slope could stand ; and a comparatively 

 small fluctuation in its thickness would have very im- 

 portant geological results. What the actual condition of 

 affairs now is at the .South Pole can only be settled by an 

 Antarctic expedition, of which the feasibility and import- 

 ance are now in some quarters being seriously considered. 

 The thickness of the ice-cap would depend on the 

 annual snowfall and on the rate of viscous sliding down. 

 The internal heat of the earth has nothing to do wiih the 

 problem — the underground heat could not melt a niilli- 



