October 28, 1904.] 



SCIENCE. 



553 



Darwin have shown; but there are many 

 problems too broad and too laborious to be 

 solved by individual effort, and these are 

 as essential to the rounding out of the sci- 

 ence of physics as they are to the develop- 

 ment of geology and astrophysics. 



In the brief review which precedes, I 

 have endeavored to show that the history 

 of the earth bristles with problems, few of 

 them completely solved, though in many 

 eases we have some inkling of the solution. 

 This sketch has been drawn for the pur- 

 pose of considering the strategy of a cam- 

 paign against the series of well intrenched 

 positions occupied by our great enemy, the 

 unknown. 



Generalizing the results of the sketch 

 presented, it is easy to see that nearly all 

 the problems suggested involve investiga- 

 tion of the properties of solids, or of 

 liquids, or of the transition from one phase 

 to the other. It is the business of the ex- 

 perimental physicist to establish linear 

 relations; it is the occupation of the mathe- 

 matical physicist to draw logical inferences 

 from these relations. Each will have 

 plenty to do in a methodical study of 

 geophysics. 



There can be no doubt that the character 

 of the earth's interior and the physical 

 laws which there prevail constitute the 

 most fundamental object of geological and 

 geophysical research, while the results of 

 successful investigation would be immedi- 

 ately applicable at least to the moon and 

 Mars. No one questions that enormous 

 pressures and very high temperatures exist 

 near the earth's center, while the quality 

 of matter which constitutes the interior 

 can not be satisfactorily determined until 

 we know how substances woiild behave 

 under extreme pressures and at tempera- 

 tures approaching 2000° C. There is 

 every reason to suppose that under purely 

 cubical compression, dense, undeformed 

 solids are perfectly elastic. Hence the 



basal problem of geophysics is to find the 

 law of elastic compressibility. This can 

 not be accomplished by direct means, but 

 the task is, nevertheless, as pointed out 

 above, not a hopeless one, and has been 

 taken in hand. Should success be achieved, 

 researches will follow on the variation of 

 elasticity with temperature. This feature 

 of the investigation Avill present very great 

 experimental and theoretical difficulties, 

 but there is no good reason to despair of 

 success. 



When the law of resistance of solid 

 bodies becomes known as a function of both 

 temperature and pressure, even for iso- 

 tropic substances with only two moduluses 

 of elasticity, the way will be opened to 

 various important investigations, largely 

 mathematical in character. It is true that 

 thoroughly isotropic bodies are seldom met 

 with, yet geological masses must, neverthe- 

 less, often approach closely to this ideal. 

 Many of the most important rocks are 

 chiefly composed of triclinic feldspars, 

 which, indeed, occur about as abundantly 

 as all other minerals found at the surface 

 of the earth put together. A triclinic 

 feldspar crystal rejoices in the full possible 

 number of elastic moduluses, 21. Yet a 

 large spherical mass of small, fortuitously 

 oriented feldspars will behave to external 

 forces of given intensity and direction in 

 the same way, no matter how the sphere 

 may be turned about its center, and will, 

 therefore, act as an isotropic body. This 

 fact is enough to show that an infinite 

 variety of intimate molecular structures 

 are compatible with molar isotropy. 



Thus a knowledge of isotropic elasticity 

 will suffice as a basis for testing reasonable 

 hypotheses of the constitution of the 

 earth's interior, taking into account its 

 known rigidity and density. Still greater 

 light can be thrown on this subject by in- 

 cluding in the investigation the moon and 

 Mars: for their masses and dimensions are 



