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SCIENCE. 



[N.S. Vol. XX. No. 513. 



known and there seems every probability 

 that they are composed of the same ma- 

 terials as the earth, though in different 

 proportions. If a given hypothesis as to 

 the chief constituents satisfies the known 

 conditions of all three planets, it will 

 doubtless find acceptance. Such a result 

 would open the way to fresh advances in 

 geodesy and terrestrial magnetism, and 

 cast backward through the vista of time 

 a ray of light on the nebular hypothesis. 



Again, when the law of elasticity and 

 the approximate constitution of the globe 

 are known, it will be possible to work out 

 a satisfactory theory of the simpler modes 

 of vibration in a terrestrial sphere, and 

 then seismological observations can be ap- 

 plied to determining more precisely the 

 intrinsic elastic moduluses of the earth 

 along the paths of earthquake waves. 



It will also be practicable to examine 

 critically the possible rupture of the globe 

 as a consequence of change of figure and 

 to study intelligently the simpler cases of 

 the crumpling of strata, fissuring and other 

 problems in the mechanics of orogeny. 



The science of elasticity has had a very 

 disappointing history. Simple as is the 

 assumption ut tens-io, sic vis, the attempt 

 to solve even such seemingly elementary 

 problems as the flexure of a uniformly 

 loaded rectangular bar leads to insoluble 

 equations ; so that the science has been 

 relatively unfriiitful. It remains to be 

 seen whether a truer relation between load 

 and strain will not simplify formulas and 

 increase the applicability of algebra to 

 concrete cases. 



From an astrophysieal point of view 

 the dialytic action of mineral septa is un- 

 important, but it is very interesting in its 

 bearing on metamorphism and ore deposi-. 

 tion, and may readily contribute to eco- 

 nomic technology. 



The relations of viscosity to the diffusion 

 of matter have not vet been elucidated even 



for ordinary temperatures. This subject 

 is one of much importance in connection 

 with the genesis of rock species, and of 

 course it should be studied at 10° before 

 undertaking researches at 1000°. 



High temperature work is essential even 

 to the investigation of the elastic problem 

 and it is almost a virgin field. Even 

 thermometry is very imperfect above the 

 melting point of gold, though it is destined 

 soon to become exact at least as high as 

 2000°, a range which will probably suffice 

 for geophysics. But we are also in almost 

 total ignorance of the extent to which the 

 laws of physics, studied at ordinary tem- 

 peratures, prevail at one or two thousand 

 degrees. One of the less difficult problems 

 of this group is that of thermal conduc- 

 tivity and specific heat of solid bodies at 

 high temperatures. For the principal 

 metals this is already known as far as 100°, 

 but not for rocks or minerals. It would 

 be especially desirahle to have such deter- 

 minations for granite, basalt and andesite, 

 the last representing the average composi- 

 tion of the accessible part of the litho- 

 sphere. 



It seems to me that when the thermal 

 diffusivities are known for these rocks, over 

 a range of a thousand degrees, the question 

 of upheaval and subsidence can be attacked 

 M'ith a good prospect of success. A cooling 

 sphere is conceivable in which the distri- 

 bution of thermal diffusivity is such that 

 the flow of heat Avould be 'steady,' in 

 Fourier's sense, and thus accompanied by 

 no superficial deformation. With any 

 other distribution of diff'usivities, deforma- 

 tion would occur, and the globe would act 

 as an imperfect heat engine, the work done 

 being that of upheaval or subsidence. Now 

 ■when the assuredly variable value of dif- 

 fusivity for the materials of the globe is 

 known, the mathematical conditions for 

 steady flow can be worked out, and if these 

 are not consistent with the facts of the 



