23G 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



This balancing of the earth's crust was termed by C. E. Dutton. 

 isostasy, or equal pressure. The early writers on the subject of 

 isostasy developed the qualitative side of the theory, but it was only 

 during the past 30 years that any quantitative values were made 

 available. 2 John F. "Hayford, as chief of the Division of Geodesy 

 of the U. S. Coast and Geodetic Survey, conducted a very elaborate 

 investigation which showed that blocks of the earth's crust, of the 

 same cross section at their bases, were in equilibrium ; that is, that 

 each block had the same mass as any other block of the crust. The 

 base of each of these blocks was assumed to be at a definite depth 

 below sea level, which depth was termed the depth of compensation. 

 The compensation is defined as the deficiency of mass in the block 

 under any particular area and exactly balances the mass which ap- 

 pears at the top of the block above sea level. It is also the excess in 

 mass in a block under an ocean, and in this case it exactly offsets or 

 balances the deficiency of material in the space occupied by the water 

 of the ocean above the block. 



Fig. 1. — A simple case of isostatic equilibrium. 



If equal masses of different metals, each lighter than mercury, are moulded to the 

 same cross section they will sink to the same depth when placed in mercury. Their lower 

 surfaces will form a plane while their upper surfaces will be irregular. There will be 

 equal pressure at the base of the different blocks. This is isostatic equilibrium in its 

 simplest form. 



The results of the investigations of Hayford were reported in two 

 publications 3 of the U. S. Coast and Geodetic Survey. The investi- 

 gations started by Hayford have been continued by the U. S. Coast 

 and Geodetic Survey up to the present time. Later results 4 have 



2 The earliest attempt to prove the theory of isostasy was made by G. R. Putnam. 

 (See Report U. S. Coast and Geodetic Survey for 1894, pt. 2, app. 1, Relative determina- 

 tions of gravity with half-second pendulums, and other pendulum investigations.) 



3 The figure of the earth and isostasy from measurements in the United States, by 

 John P. Hayford. 



Supplemental investigations in 1909 of the figure of the earth and isostasy, by John P. 

 Hayford. 



* Effect of topography and isostatic compensation on the intensity of gravity, U. S. 

 Coast and Geodetic Survey Special Publication No. 10, by John F. Hayford and William 

 Bowie. 



Same, U. S. Coast and Geodetic Survey Special Publication No. 12, by William Bowie. 



Investigations of gravity and isostasy, U. S. Coast and Geodetic Survey Special Publi- 

 cation No. 40, by William Bowie. 



