HYPOGEIC WORK. 377 



sustained the same conclusion, stating that " if it were true tliat the earth 

 is a fluid ball coated witli a crust, that crust must be of fabulous rigidity to 

 resist the tidal surgings of the subjacent fluid" (1888). At the same time, 

 according to the same authority, the weight of the water of a high flood-tide 

 probably occasions, owing to the elasticity of the crust, " a local elastic yield- 

 ing along the coast-line of continents"; and " there is reason to believe 

 that such flexure has actually been observed by a delicate form of level on 

 the coast of the Bay of Biscay." Kewcomb favors the same conclusion in a 

 paper discussing the cause of the periodic variations of latitude (1893). 



0. Fisher, of Cambridge, England, questions the above conclusion from 

 the tides (1892). Basing his mathematical calculations on an investigation 

 by Darwin of the tides upon a yielding earth according to the canal theory, 

 he obtains the result, that the height of the tide for a liquid earth would be 

 only a fifth less than that for a rigid earth, and suggests, as the difference 

 is so small, that the existing tides may have just the height appropriate to 

 a liquid interior. He observes, further, that the heat generated within the 

 earth by the tides in the earth's mass from their commencement — calcu- 

 lated by Darwin to be sufficient ''to give a supply of heat, at the present 

 rate of loss, for 3560 millions of years " — would have been only to a 

 small extent expended or wasted, and that, through convection currents, 

 it keeps the liquid layer in fusion, and prevents the crust from growing 

 thicker. Other considerations have led Fisher to make the thickness of the 

 crust about 18 miles. The conclusion of Fisher is objected to by G. F. Becker, 

 on the basis of calculations which lead him to the conclusion that " for a 

 fluid earth the canal theory and the equilibrium theory give the same result, 

 viz.: no relative tide." He adds, that "on any theory of the tides, the ex- 

 istence of semi-diurnal tides indicates an earth presenting great resistance 

 to deformation" (1893). 



2. Earth-shaping. — AVhether solid to the surface or not, the earth is 

 believed to be so far fluid-like in its mass as to admit of adjustments to 

 gravitational pressure through molecular flow, if not through a liquid layer, 

 and to owe its shape primarily to the principle of gravitational equilibrium, 

 as if liquid. This view of adaptation to gravitational pressure was rec- 

 ognized geologically by Herschel in his Appendix to Babbage's Ninth Bridge- 

 water Treatise (1837), where he attributed changes of level to "changes in 

 the incidence of pressure on the general substratum of liquefied matter which 

 supports the whole," and argued therefrom that the rise in level going on in 

 Scandinavia might be caused by the accumulation of sedimentary deposits 

 over the adjacent ocean bed. The earth's interior liquidity was then gen- 

 erally admitted. In 1888, C. E. Dutton proposed the term isostasy for " the 

 condition of equilibrium to which gravitation tends to reduce a planetary 

 body irrespective of whether it be homogeneous or not," that is, whether 

 solid to the surface or partly liquid beneath it, and whatever its constitution. 



The rate of adjustment to changing load would necessarily be very slow 

 in a solid globe, in which it could take place only through molecular flow in 



