172 Cx. F. BECKER ISOSTASY AND RADIOACTIVITY 



deptlis as are reported by the geodesists is compatible with the lapse of 

 something- like a thousand million years since the ocean was gathered 

 together. 



Geolog}' as a science is conditioned by the state of the earth's interior 

 and our knowledge of its constitution is now advancing. So late as the 

 foundation of this Society, in 1889, the Cartesian doctrine of a fluid 

 earth, inclosed in a very rigid shell, a score or two of miles in thickness, 

 was held by most geologists. We now know that the globe is solid and, 

 on the whole, of great rigidity, and probably divisible into at least four 

 distinct shells, each more rigid than that overlying it;^ that the irregu- 

 larities in density and structure, which are so marked at the surface, 

 extend only to a depth of something like a fiftieth of the earth's radius; 

 that open cavities or cracks may exist at depths of 20 miles and very 

 possibly down to the level of isostatic compensation. We know, too, that 

 the earth is radioactive, but tliat the radioactivity is superficial, reaching 

 only to a moderate, though uncertain, level; we also know, however, that 

 the earth's heat is not wholly of radioactive origin. ]\]"ore information 

 is certainly in store for us, for Mr. Michelson is now measuring the 

 terrestrial tides in terms of the wave-length of light,* while methods 

 have been developed by which the distribution of density above the level 

 of isostatic compensation can be studied. 



Thus the future is full of hope. The rational method of attaining it 

 is to make trial hypotheses and to devise methods of testing them. 



Premonitions of Isostasy 



Observations bearing on the problem of isostasy are as old as geodesy 

 itself. During the measurement of the Peruvian arc, 1T35 to 1745, 

 Pierre Bouguer^ observed and computed the attraction of Chimborazo. 



^ L. Geiger and B. Gutenberg, continuing Investigations by Wiechert, Zoeppritz, and 

 themselves on the intensity of longitudinal and transverse earthquake waves, find in the 

 earth three surfaces of discontinuity at depths of 1,1^^3, 1,712, and 2.454 kilometers. 

 The values found for Poisson's ratio only slightly exceed one-fourth. Gottingen, Nach- 

 richten, 1912, p. 675. 



* The first attempts to measure bodily tides in the earth were made by George H. and 

 Horace Darwin with the horizontal pendulum, but without satisfactory results. Better 

 success attended the experiments by B. von Rebeur-Pashwitz with a similar instrument. 

 O. Hecker at Potsdam, in 1907, and A. Orloff at Dorpat, in 1911, demonstrated small 

 terrestrial tides and the high rigidity of the earth, also with the horizontal pendulum. 

 Their results and others were discussed by W. Schweydar, Roy. Preus. Geod. Inst, 1912. 

 To obviate certain difficulties and uncertainties attending the use of the horizontal pendu- 

 lum. A. A. Michelson is experimenting with an apparatus consisting of two water-levels 

 (acting on the principle of a spirit-level) at right angles to one another, each about 500 

 feet long. Astrophys. Jour., vol. 39, 1914, p. 97. 



■^ La figure de la terre, 1749, p. 379. On page .391 he remarks that mountains attract 

 much less than the greatness of their volume would promise. 



