178 G. F. BECKER ISOSTASY AND RADIOACTIVITY 



at the surface, are confined to a superficial shell; in other words, they 

 lend probability to the hypothesis that a level of approximate isostatic; 

 compensation underlies an heterogeneous external shell. 



Direct Investigations of Isostasy 



Turning now to researches addressed more directly to elucidating the 

 conditions existing in the earth's superficial shell. Sir John Herschel and 

 Charles Babbage seem to have been the first to indicate a tendency to 

 isostasy as the controlling factor in at least some recent upheavals and 

 subsidences.^^ Babbage confined himself to eft'ects of temperature change. 

 Herschel relied on "the variation of the pressure, and the infinity of sup- 

 ports broken by weight, or softened by heat, to jn-oduce tilts." He finds 

 in erosion and deposition the primuvi mobile of geology through the sub- 

 version uf equilibrium of pressure. Neither of these authorities appears 

 to have pursued the matter, nor was the subject resumed for many years. 



Archdeacon John H. Pratt, in 1855, called attention to the fact that 

 the attraction of the Himalayan range produces in the plains of India a 

 deflection of the plumb-line far smaller than was to have been expected, 

 but he then ofliered no satisfactory explanation.^*' Airy in the same year 

 attempted to explain the facts by the hypothesis that the solid crust of 

 the liquid earth, supposed only a score or two of miles in thickness, extends 

 downward under mountain chains and is of relatively small density, 

 while beneath the oceans it is thin, the whole crust being supported by 

 fiotation.^^ In answer Pratt pointed out that no possible law of cooling 

 could produce such a crust as Airy described, and furthermore that W. 

 Hopkins^^ had shown the crust to be at least 900 miles thick and prob- 

 ably more than 1,000 miles. ^^ But in 1858 Airy's hypothesis gave Pratt 

 an idea, namely, that although the earth is solid to a very great depth, if 

 not throughout, there is a relative deficiency of matter under mountain 

 ranges and relative excess of matter beneath oceanic depressions; in short, 

 an approach to isostasy.'" This idea, not unknown to Laplace, he elab- 

 orated in a number of papers, most authoritatively, of course, in his well 

 known work on Laplace's Functions and the Figure of the Earth.^^ 



1^ Babbage's paper on the Temple of Serapis was read before the Geol. Soc. Lond. iu 

 March, 1834, but published in full in the Proceedings, vol. 3, 1847, p. 186. Herschel's 

 letters to Leyell and Murchisou were printed in I'roc. Geol. Soc. Lend., vol. 2, 1833-1838, 

 pp. 548 and 596. 



i" Phil. Trans., vol. 145, 1855, p. 53. 



" Phil. Trans., vol. 155, 1855, p. 101. 



i»Phll. Trans., 1830 to 1842. 



1" Airy does not seem to have made any reply at the time, but in a popular lecture in 

 1878, Nature, vol. 18, 1878, p. 43, again expressed his belief in a lumpy crust. 



-^ Phil. Trans., vol. 149, 1859, p. 747. 



21 The copy before me is the 4th edition, 1871. 



