346 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



thin portion on the right hand and through the same point a similar surface 

 separating a like portion on the left. The sphere consists, then, of three 

 parts, the middle portion being of a symmetrical form and attracting the point 

 in the direction of the radius, and the two slender slices attracting it equally 

 to the right and left of that radius. If one of these slices became fluid and of 

 less density than the other, its attraction would be overcome by that of the 

 other, and the fluid would be drawn away to the other parts of the sphere. 



" It does not follow that the whole of the fluid would be drawn over. The 

 above process would go in till the surface of the fluid at the circumference of 

 the slice had become so inclined as to be at right angles to the direction of 

 the resultant attraction of the whole mass, solid and fluid. If, however, a 

 narrow channel were cut through this circumference (which would otherwise 

 act as an embankment) the whole of the water would be drawn off. 



" Now in the case of the earth there is a channel opening a passage from 

 the New Zealand hemisphere into the opposite one, viz., the North and South 

 Atlantic, and yet the ocean remains in that hemisphere. There must, there- 

 fore, be some excess of matter in the solid parts of the earth between the 

 Pacific Ocean and the earth's centre which retains the water in its place. 

 This effect may be produced in an infinite variety of ways; and therefore, 

 without data, it is useless to speculate regarding the arrangement of matter 

 which actually exists in the solid parts below." 



Pratt then discusses the original fluidity of the earth, and says 



finally : 



"The conclusion at which we have arrived in Art. 132, that the parts 

 of the crust below the more elevated regions are of less density, and the 

 parts beneath the depressed regions in the oceans are of greater density than 

 the average portions of the surface, seems to bear additional testimony to the 

 fluid theory. For it shows, that notwithstanding the varied surface, seen 

 at present in mountains and oceans, the amount of matter in a vertical 

 prism drawn down at various places to any given spheroidal stratum is the 

 same although its length varies from place to place as the earth's contour 

 varies. No better explanation of this phenomenon can be conceived than 

 that which the fluid hypothesis furnishes; viz., that these prisms though now 

 of different lengths, were, when the crust was fluid, of the same length; and 

 as their lengths are now various simply from the fact that the surface in 

 solidifying has contracted unequally, of course the amount of matter which 

 they contain is the same in all of them." ^ 



The views here expressed are very generally recognized by 

 geodesists, and have been treated in different ways by Clarke, 

 Tisserand, Helmert, and other leading authorities. 



§ 29. Results obtained by the United States Coast and Geodetic 

 Survey from recent researches on the average depth of isostatic 

 compensation. 



^ " Figure of the Earth," 3d edition, p. 162. 



