Uplift and Subsidence. 5 



tigation in the subject of gravity and isostasy. Shortly 

 after he began this he severed his connection with the 

 Coast and Geodetic Survey and for a short time the inves- 

 tigation was conducted by Dr. Hayford and the writer, 

 and later by the writer alone. 



The investigations in gravity and isostasy confirmed 

 the results obtained by Hayford from the investigations 

 of the figure of the earth and isostasy. Isostasy was 

 found to be in about as complete a state from the gravity 

 investigations as from the figure of the earth investiga- 

 tions. The depth of compensation computed from the 

 gravity investigations was found to be approximately 96 

 kilometers. This agrees with Hayford 's determination 

 of the depth of compensation from the figure of the earth 

 investigations when he used only data in mountain 

 regions. The depth of 96 kilometers was obtained by the 

 writer from gravity stations in areas of high relief. It 

 is not necessary to go into details in regard to the compu- 

 tation of the depth of compensation, but when we consider 

 the fact that a disc of material, of uniform density and 

 thickness and of infinite horizontal extent, has the same 

 attraction on a particle, regardless of the distance of this 

 particle from it, we can see that it is difficult to obtain a 

 value for the depth of compensation in a flat area, such as 

 the coastal plain or a plateau. The determination of the 

 depth from data in mountain regions is very much more 

 sensitive. 



Beliahility of geodetic data. 



Some question has been raised at various times as to 

 the reliability of the geodetic data. It is safe to assert 

 that the accidental and systematic errors in the astro- 

 nomic observations, in the triangulation, and in the 

 pendulum observations are so small that they need not 

 worry us at all. For instance, if we should say that the 

 observed deflection of the vertical is 5". 5 in the meridian 

 or in the prime vertical, it is reasonably certain that its 

 error is not greater than 0".5. Similarly, if the gravitv 

 observations give an intensity of 980.025 dynes, it is 

 practically certain that this value is correct within three 

 or four in the last place of decimals. No one will doubt 

 but^ that these degrees of accuracy are well within the 

 limit' desirable in geodetic investigations. Published 

 reports on the investigations of the figure of the earth and 



