200 



SCIENCE 



[N. S. Vol. XXXIII. No. 841 



defect of density and, therefore, of mass in 

 the material in the same vertical line below 

 sea level and above the depth of compensa- 

 tion. 



Note that isostasy is defined in terms of 

 masses and densities without regard to the 

 manner in which this arrangement of 

 masses and densities has been produced. 



Isostasy is a condition of approximate 

 equilibrium, not perfect equilibrium. The 

 total weight of column A being the same 

 as that of column B, the pressure at the 

 depth of compensation due to weight is the 

 same under the two columns, and at this 

 level there is equilibrium. Above any 

 selected higher level in the two columns 

 such as that marked depth X in the figure, 

 the mass is greater in column A than in 

 column B.^ Therefore, at depth X the 

 pressure due to weight is greater in A than 

 in B, equilibrium does not exist, and the 

 material in A at this level tends to move 

 downward and to the right into B. 



The geodetic observations which have 

 furnished a positive proof that a close ap- 

 proximation to the condition called isos- 

 tasy exists in the earth are, first, 765 series 

 of astronomic observations scattered over 

 the United States from the Atlantic to the 

 Pacific and from Canada to Mexico, and all 

 connected by continuous triangulation,* 



^The density in column A, in which a defect of 

 density exists to compensate for the excess of 

 mass at the surface, being less than in column B, 

 in which the reverse condition exists, the mass in 

 column A below depth X is necessarily less than 

 in column B below that level. Hence the total 

 masses in the two columns being equal, the mass 

 in column A above depth X must be greater than 

 in column B, as stated. 



^ The evidence from these observations is given 

 in full in " The Figure of the Earth and Isostasy 

 from Measurements in the United States " and 

 " Supplementary Investigation in 1909 of the 

 Figure of the Earth and Isostasy," both by John 

 F. Hayford and both published by the Coast and 

 Geodetic Survey. 



and, second, determinations of the inten- 

 sity of gravity at 89 stations scattered over 

 the whole of the United States.* 



The geodetic observations show that the 

 most probable depth of compensation is 76 

 miles and that it is practically certain that 

 it is not less than 62 nor more than 87 

 miles.'' 



Let the isostatic compensation be con- 

 sidered complete if in every column, such 

 as those shown in Fig. 1, the mass above 

 the depth of compensation is the same as 

 in every other column. If the mass is 

 greater or less than this in any one column, 

 let us characterize the isostatic compensa- 

 tion as incomplete and measure the degree 

 of incompleteness in terms of the excess or 

 defect of mass. 



The geodetic observations show that the 

 isostatic compensation under the United 

 States is nearly complete. It is not merely 

 a compensation of the continent as a whole, 

 it is a compensation of the separate, large, 

 topographic features of the continent. 



' These have furnished evidence which corrobo- 

 rates that from the astronomical observations 

 and triaugulation. This evidence has not been 

 published except in brief and incomplete form 

 (report of the sixteenth general conference of the 

 International Geodetic Association, Vol. 1, pp. 

 365-389, " The Effect of Topography and Isostatic 

 Compensation upon the Intensity of Gravity," by 

 John F. Hayford), but it will probably be pub- 

 lished in full within a year in a paper which is 

 being prepared by Mr. William Bowie, inspector 

 of geodesy, Coast and Geodetic Survey, and the 

 speaker. It is expected that this will be pub- 

 lished by the Coast and Geodetic Survey under 

 the same title as the report presented at the 

 International Geodetic Association to which ref- 

 erence has just been made, 



" This is the depth of the compensation if uni- 

 formly distributed with respect to depth, which 

 seems to be the most probable assumption. If 

 the compensation is distributed in some other 

 manner with respect to depth, the limiting depth 

 of compensation is different, see pp. 77-78 of the 

 " Supplementary Investigation in 1909 of the 

 Figure of the Earth and Isostasy.'' 



