SHAPING THE EARTH BOWIE 331 



values of gravity to test this flotational hypothesis. It is the only 

 method, so far as I am aware, by which the idea can be quantita- 

 tively tested. We have a direct measure of the extent to which the 

 plumb line deviates from the line that is at right angles to the 

 spheroid surface, and a measure of the difference between the theo- 

 retical and observed values of gravity. The idea of isostasy can be 

 tested by means of these data. 



If the earth were a true spheroid and there were no irregularities 

 on its surface and if the densities along each radius were normal, 

 gravity would increase slightly as one proceeded from the Equator 

 to one of the poles. The attraction of the earth at sea level would 

 be about one two-hundredths part greater at a pole than at the 

 Equator. Enough work has been done to prove conclusively that 

 gravity does follow very definite laws. For instance, it changes on 

 the average about one part in a million for a mile change in latitude. 

 It changes one part in a million for about 10 feet change of elevation. 

 These changes are perfectly normal, for the centrifugal force is a 

 maximum at the Equator and zero at the poles, and, besides, the at- 

 traction at either pole is greater than it is at a point on the Equator. 

 Necessarily, too, a particle is attracted less by the mass of the earth 

 when elevated than when it is exactly at sea level. 



It is not necessary to go into details regarding the geodetic tests 

 of isostasy, for the methods used and the results obtained have all 

 been set forth in a number of publications of geodetic organizations. 

 It is sufficient to state that when isostasy is taken into account in 

 computing geodetic data, harmonious or practically harmonious re- 

 sults are obtained. By means of geodetic data it has been possible 

 to determine the approximate depth below sea level to which these 

 abnormal densities extend. The most probable depth obtained from 

 mountain and plateau stations of the United States is about 96 kilo- 

 meters, approximately 60 miles, below sea level. This depth is con- 

 firmed by determinations of A. H. Miller, of the Dominion Observa- 

 tory at Ottawa, Canada, who found, from analysis of gravity data 

 at mountain stations in the western part of that country, a depth also 

 of approximately 60 miles. 



COMPARISON OF PRATT AND AIRY HYPOTHESES 



There has been much discussion in literature on isostasy of the 

 question as to whether the Pratt or the Airy hypothesis is the true 

 one. Pratt postulated that the densities vary under the different 

 classes of topography. Under the oceans the density would be ab- 

 normally great and under the continents it would be abnormally 

 small. Airy, on the other hand, suggested that the depth of com- 



