H. S. Woodward — Mathematical Theories of the Earth. 345 



density and precession ; so that to one who can divest his mind 

 of the notion that pressure and continuity are important fac- 

 tors in the mechanics of such masses, the picture which Roche 

 draws of the constitution of our planet will present nothing 

 incongruous. 



In a field so little explored and so inaccessible, though 

 hedged about as we have seen by certain sharply limiting con- 

 ditions, there is room for a wide range of opinion and for great 

 freedom in the play of hypothesis ; and although the prepon- 

 derance of evidence appears to be in favor of a terrestrial 

 mass in which the reign of pressure is well nigh absolute, we 

 should not be surprised a few decades or centuries hence to 

 find many of our notions on this subject radically defective. 



If the problem of the constitution and distribution of the 

 earth's mass is yet an obscure and difficult one after two centu- 

 ries of observation and investigation can we report any greater 

 degree of success in the treatment of that still older problem 

 of the earth's internal heat, of its origin and effects? Con- 

 cerning phenomena always so impressive and often so terribly 

 destructive as those intimately connected with the terrestrial 

 store of heat, it is natural that there should be a considerable 

 variety of opinion. The consensus of such opinion, how- 

 ever, has long been in favor of the hypothesis that heat is the 

 active cause of many, and a potent factor in most of the 

 grander phenomena which geologists assign to the earth's 

 crust ; and the prevailing interpretation of these phenomena is 

 based on the assumption that our planet is a cooling sphere 

 whose outer shell or crust is constantly cracked and crumpled 

 in adjusting itself to the shrinking nucleus. 



The conception that the earth was originally an intensely 

 heated and molten mass appears to have first taken something 

 like definite form in the minds of Leibnitz and Descartes.* 

 But neither of these philosophers was armed with the necessary 

 mathematical equipment to subject this conception to the test 

 of numerical calculation. Indeed, it was not fashionable in 

 their day, any more than it is with some philosophers in ours, 

 to undertake the drudgery of applying the machinery of anal- 

 ysis to the details of an hypothesis. Nearly a century elapsed 

 before an order of intellects capable of dealing with this class 

 of questions appeared. It was reserved for Joseph Fourier to 

 lay the foundation and build a great part of the superstructure 

 of our modern theory of heat diffusion, his avowed desire 

 being to solve the great problem of terrestrial heat. " The 

 question of terrestrial temperatures," he says, "has always 



* Protogee, ou De la Formation et des Revolutions du Globe, par Leibnitz, 

 Ouvrage traduite .... avec une introduction et des notes par le Dr. Bertrand 

 de Saint Germain, Paris, 1859. 



