420 



SCIENCE. 



[Vol. VII., No. 170 



of the earth's crust beneath. This is the principle 

 of his hypothesis, but it is not sufficient to an- 

 nounce it. It is also necessary to justify it in 

 proving that the cause is adequate to the effect. 

 This it is that M. Faye has neglected to do ; and 

 I would add, that, in my conviction, such a proof 

 is impossible. 



But, before attempting to show this, it would 

 be well, perhaps, to call attention to one singular 

 consequence, which is entailed if it is necessary 

 to admit the theory of the cooling of the earth's 

 crust by contact. No one is ignorant of the fact, 

 that, if the temperature of the bottom of the sea 

 is in the neighborhood of 0°, there are on the sur- 

 face of the continents wide stretches of country 

 which are still less favored. Without speaking of 

 mountainous regions covered with perpetual snow, 

 we will only mention the plains of Siberia, and 

 especially that of the district of Yakootsk, where 

 there reigns a mean temperature of — 10°C. This 

 temperature, as may be readily seen, was estab- 

 lished at the same time as the ice-caps around the 

 poles, and has tended to produce a change of tem- 

 perature of the crust for a time at] least as long 

 as that during which the cold waters have flowed 

 over the ocean-bottoms : consequently, as the 

 earth's surface affected by this cooling is far from 

 being negligible, it is there that the pendulum 

 ought to oscillate the most rapidly. 



But, aside from this argument from the facts in 

 the case, there are other strong reasons deduced 

 from what we know of the bad conductive power 

 of rocks. Experiments at Paris have shown that 

 a change in the mean monthly temperature propa- 

 gates itself in thirty-eight days to the depth of 

 one metre, and that at ten metres below the sur- 

 face all variation in the temperature of the air 

 becomes absolutely insensible. This being the 

 case, one would think that a cooling coming from 

 the surface could hardly exercise any effect on the 

 inside of the crust of the earth. 



To argue the possibility of such a cooling effect, 

 it would be necessary first to have some idea of 

 the probable thickness of the crust. Whatever 

 hypothesis we accept as to the interior constitution 

 of the earth, it is inadmissible that, at the time 

 when the glaciers took possession of the poles, the 

 thickness of the solid crust had not reached at 

 least twenty kilometres. Fossil botany teaches us 

 that in the middle of the tertiary period the re- 

 gions immediately around the poles possessed a 

 ricli vegetation of a character essentially tem- 

 perate, which certainly could not have existed in 

 the neighborhood of ice. The first appearance of 

 polar ice was therefore not in the carboniferous 

 period, when we know, moreover, that the arctic 

 seas were inhabited by corals like those which now 



live only in the tropics. This granted, if we take 

 account of the generally given thickness of the 

 gneiss and micaschists, by all estimated at many 

 thousands of metres ; if we add to this the Cam- 

 brian, Silurian, Devonian, and carboniferous de- 

 posits, even attributing to them only a small part 

 of the depth which they have in Europe, — we 

 find that a total thickness of twenty thousand 

 metres for the crust constitutes certainly a moder- 

 ate valuation. 



Let us suppose, then, a crust of twenty thou- 

 sand metres, of which the temperature, about 

 2000° C. on its lower side, decreases regularly up 

 to the surface, where it is about 20° C, — the 

 minimum of tropical regions, — or a diminution 

 of one degree for ten metres. Can we imagine a 

 difference of twenty degrees in the surface tem- 

 perature could have produced an appreciable dif- 

 ference in the interior even after millions of years ? 



Let us consider more closely in what way the 

 distribution of temperature exists in the interior 

 of the earth. We know that this temperature 

 increases constantly with the depth. But it has 

 long been granted that the flow of heat does not 

 contribute to the exterior temperature more than 

 the thirtieth of a degree. Let us reverse the cal- 

 culation, and ask how far a temperature of 0° C. 

 could contribute to the diminution of heat 

 which reigns at twenty kilometres depth. Cannot 

 the answer be made without discussion ? 



But we have the reply expressed in figures in 

 the results of some investigations in Siberia. In 

 1836 a merchant of Yakootsk, wishing to utilize 

 the internal heat, dug a well in the hope of 

 reaching water. In this well, dug to a depth of 

 115 metres, the temperature increased progres- 

 sively from — 10° C. to 0° C. 



The well was abandoned because such a great 

 depth rendered it useless for the purpose proposed ; 

 but a little later, in the steppes of Katchongin, 

 another well reached water at a depth of 126 

 metres. Therefore, below the constantly frozen 

 surface of Siberia, the temperature rises in 126 

 metres at least ten degrees to 0° C. The increase 

 is thus one degree for twelve metres and a half ; 

 that is to say, three times more rapidly than in 

 the temperate regions, where it is one degree for 

 from thirty-five to thirty-seven metres. 



What, therefore, is to be concluded? Even 

 that a great superficial cold only affects the layers 

 immediately in the neighborhood of the surface, 

 and that this influence at any considerable depth 

 must become absolutely insignificant. If, then, 

 the force of gravitation is not diminished above 

 the oceans ; if at the same time, on the authority 

 of all others with the exception of M. Faye, there 

 exists a sensible increase, — it is not to an increase 



