200 O. C. JONES, GEORGE D. HUBBARD Vol. XXII, No. 7 



of the focus in the Ischian disturbance was about 500 meters., 

 and of the Andalusian of 1884, 12^^^ kilometers; the Charleston 

 showed a depth of seven and one-third kilometers, and the 

 Rivera 18 kilometers. The San Francisco earthquake showed 

 a depth of disturbance of about 24 kilometers. 



Many attempts have been made to measure the velocity of 

 the waves at the surface, but with such varying results as to 

 indicate inaccuracy of method or data, or perhaps to some 

 degree to indicate varying density. 



THE DATA OF ISOSTASY." 



Isostasy is the theory that any given column of rock taken 

 from the center of the earth to the surface will have the same 

 mass as any other column of the same cross-sectional 

 dimensions. But as density of rocks varies the height of the 

 column will vary. The inequality of density, however, is 

 found only in the part of the column near the surface, and the 

 difference in the length of the column is due to difference of 

 density in the upper 122 kilometers (77 miles) of the column. 

 The adjustment of material toward this balance under the 

 force of gravity is called "isostatic adjustment." Defective 

 density resulting in a taller column and excessive density 

 causing oceanic basins, that is, a shorter column, constitute 

 "isostatic compensation." 



The density of any given column is calculated from the 

 vertical component of the forces operative on a swinging 

 pendulum or the forces deflecting a plumb-bob. -Corrections 

 must be made for the effect of topography. The pendulum 

 method has been found most satisfactory and is generally used. 



The scheme followed by the U. S. Coast and Geodetic 

 Survey is as follows: eighty-nine stations were selected in 

 the United States, and sixteen over the rest of the world. At 

 these stations pendulum apparatus is set up, and the vertical 

 component of gravity worked out. In order to correct for 

 topography the entire earth is divided into 317 compartments, 

 and the effect of each compartment computed. These cor- 

 rections are applied to obtain the actual value of gravity due 

 to density below the station. Next the value of gravity at 

 the station is computed, a mean surface density of 2.67 being 



22Special Rpt. No. 10, U. S. C. Sz G. S., pp. 6, 7, 108. 



