300 JOSEPH BARRELL 



greater than for the United States as a whole, it would be concluded 

 that for the region east of the Mississippi the departure from the 

 levels giving complete compensation averages not more than no 

 ft. instead of the 275 ft. previously stated. 



Or, again, imagine a rise of ocean-level so that the average eleva- 

 tion of this part of the continent is reduced to 100 ft. without 

 changing the detail of the topography. The deflections would suffer 

 only small alterations due to the added mass of water. Although 

 the crust remained without change, the same reasoning would then 

 lead to the conclusion that the topography departed on the average 

 but II ft. from the levels which would give complete compensation. 



In computing the influence of the topographic irregularities and 

 their compensation upon the deflection of the vertical, all the 

 topography was taken into account up to a radial distance of 

 4,125 km. from each station. This radius is approximately the 

 length of 37° of latitude. It embraces the Pacific Ocean out to the 

 Hawaiian Islands and to ten degrees south of the equator, and the 

 Atlantic Ocean out to the Azores. The relief within this region 

 ranges from —8,340 m. north of Porto Rico to +6,220 m. in Mount 

 McKinley, +6,247 i^^ Chimborazo; a total differential relief of 

 about 14,590 m. About one-half of the topography surrounding 

 the coast stations consists of ocean bed. Even for the stations in 

 Minnesota, farthest removed from the sea, about one-third of the 

 surrounding topography within the limits is deep ocean, but lying 

 at a greater distance and carrying lesser influence. The average 

 depth of the oceans influencing the deflection of the station at mean 

 distance inland may be assumed for purposes of illustration to be 

 about 5,000 meters. This depth of water is equivalent in mass to 

 1,900 m. of rock of density 2 . 67, leaving an effective ocean depth of 

 3,100 m. Add the mean continental elevation of 760 m. to this, 

 and 3,800 to 3,900 m. represents about the effective mean relief 

 between continent and ocean. On coast stations this dift'erential 

 relief has greatest influence. For inland stations the several 

 portions of the continent have proportionately more effect. For 

 the United States as a whole it is this relief of between 3,500 and 

 4,000 m. between continent and ocean, more than the relief between 

 the major features of the continent, which is nine-tenths compen- 



