THE GEODETIC EVIDENCE OF ISOSTASY 33 



count, it appears that, if the compensation is uniformly distrib- 

 uted with respect to depth, the most probable value of the limit- 

 ing depth is 71 miles and that it is practically certain that the 

 limiting depth is not less than 50 miles nor more than 100 miles. 



No conclusive evidence has yet developed in the investigation 

 that the depth of compensation is different for different parts of 

 the United States. 



In all that has been said thus far, and in the corresponding 

 parts of the investigation, it has been assumed that the compen- 

 sation is uniformly distributed with respect to the depth. This 

 assumption is not necessarily true and it must, therefore, be 

 examined. It was adopted as a working hypothesis because it 

 happened to be the one reasonable assumption which lends itself 

 most readily to computation, and because it also seemed to the 

 speaker to be the most probable simple assumption. 



It is probably impossible to determine the distribution of the 

 compensation with respect to depth from investigations based 

 simply upon deflections of the vertical. Possibly pendulum 

 observations combined with deflection observations may detect 

 the manner of distribution. 



All that can be done with deflections of the vertical is to 

 determine the depth of compensation on various assumptions in 

 regard to distribution with respect to depth. 



Just as the limiting depth of the compensation, if it is uni- 

 formly distributed with respect to depth, has been determined 

 by this investigation to be about 71 miles, so it has also been 

 determined by a later portion of the investigation that if the 

 compensation is greatest at the surface and diminishes uniformly 

 with respect to depth until it fades out to zero, the limiting depth 

 is about 109 miles. 



Again, it has been determined by the investigation that if the 

 compensation all occurs within a stratum ten miles thick the 

 bottom of the stratum is at a depth of about 37 miles. 



My belief is that the depth 71 miles and the corresponding 

 assumed manner of distribution are nearer the truth than either 

 the depth 37 or 109 miles with its corresponding assumption. 

 This belief rests on insecure foundation. If anyone will tell me 

 the manner of distribution of the compensation with respect to 



