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SCIENCE. 



[N. S. Vol. I. No. 7. 



inci'ease from the siirface toward the center. 

 Various laws of its increase have been pro- 

 posed, of which that of Laplace seems to be 

 on the whole the most plausible. 



It is im2Dortant to appreciate that the 

 strata rest vipon one another substantially 

 as if fluid, because the arch of the crust is 

 so flat. The compressive stress on any 

 portion considered as a keystone is 30 times 

 the crushing strength of steel, and 500- 

 1000 times that of granite and limestone, 

 whence it follows that the earth is prac- 

 tically in hydrostatic equilibrium. It also 

 follows that the pressures in the interior 

 are excessive, and that at the center the 

 pressure is about 3,000,000 atmospheres. 

 The earth is ' solid,' as the word is used 

 by Lord Kelvin, that is, it has no cavities 

 below a comparatively shallow depth. The 

 explanations of the changes of latitude lately 

 advanced and based on internal hollows in 

 which loose matter rolls around are absurd. 

 There is perfect continuity of matter, and 

 there is only fluidity when for some local 

 cause the pressure is somewhat relieved. 

 As Major Button has shown, the trans- 

 mission of vibrations from the centrum of 

 the Charleston earthquake indicated a 

 medium nearly as homogeneoiis as steel. 



Geologists have had to account for move- 

 ments of the crust, such as subsidence, ele- 

 vation, crumpling, folding, etc. Two ele- 

 m^entarjr forces are necessarily appealed to. 

 The first is Ch-avity ; the second that due to 

 the Em-til's Internal Heat. The idea of the 

 earlier geologists that the earth cooled and 

 contracted and hence caused the disturb- 

 ances has been mostly relied on as an ex- 

 planation, but for the last ten or fifteen 

 years it has been felt to be insuflicient. The 

 idea of Babbage and Herschel that loaded 

 areas, or areas of sedimentation, sink and 

 crumple up the adjacent areas as moun- 

 tains, tending thus to renew and perpetuate 

 regionsof upheaval, has also had believers. 

 This has had its best formulation in the re- 



cent doctrine called isostafsy, which regards 

 the earth as a body in essentially hydrosta- 

 tic equilibrium, and as balancing inequali- 

 ties of pressure by subterranean flow. Tlie 

 speaker regarded this doctrine, however, as 

 insuflicient in that it furnishes no start and 

 tends to run rapidly down. We need secu- 

 lar contraction to keep isostasy at work. 

 The earth's internal heat is the great store 

 of energy available for this purpose. How 

 to explain the earth's internal heat is a 

 hard and dark problem. The nebular hy- 

 pothesis, first outlined in Leibnitz's Proto- 

 gea has been most widely believed. The 

 critical stage in this method of development 

 came when convection ceased and the sphere 

 was all at the same temperature, the stage 

 usually called consistentior status. Then 

 came the formation of a crust and the be- 

 ginning of geological phenomena as usually 

 discussed. The speaker had reason to ques- 

 tion the reliability of the nebular hypothe- 

 sis and whether the earth had ever been 

 gaseous, etc. An origin for the globe and an 

 explanation of its heat are perhaps as well 

 to be found in the collision of meteoric 

 bodies. 



The time that has elapsed since the con- 

 sistentior status has been an interesting sub- 

 ject for computations, and widelj' varying 

 estimates have been made. Lord Kelvin in 

 1862, on very questionable data, placed the 

 limits of geological phenomena at 20,000,- 

 000-400,000,000 years in the past. On the 

 same line, Tait estimated 10,000,000, but it 

 was doubtless true that in England the 

 weight of Kelvin's authority had fettered 

 geological thought in the last thirty years 

 to too narrow limits of time, for no geolo- 

 gist of eminence had questioned his results. 

 Yet within a month Lord Kelvin has raised 

 his upper limit to a possible 4,000,000,000. 

 All must appreciate that if the data are un- 

 reliable, the finest processes of mathematics 

 will lead to no certain result. 



The speaker concluded that to secular 



