THE DOCTRINE OF ISOSTATIC COMPENSATION 705 



Let us then see from examination of his figures whether they betray 

 evidence of large local as against broad systematic distribution 

 only of the concealed matter beneath and about the station. For 

 reasons already given, this test of his figures is possible only in 

 case of the deflections of the vertical. 



Distribution of residuals in excess of 50" of arc. — Hayford has 

 computed with considerable care the topographic deflections for 

 each station, and this resultant horizontal pull of the determinable 

 masses at the station, when deducted from the measured deflec- 

 tion, must leave a residual fixed by the hidden masses whose dis- 

 tribution of density is unknown. By assembling his topographic 

 corrections and applying them we obtain residual deflections which 

 range from zero to 89'' 46 of arc. 1 We have arbitrarily selected in a 

 first survey of the results an arc of 50", or something more than 

 half the maximum residual, as a minimum of excessive anomaly 

 in order to determine the plan of distribution. ' The results are 

 shown in Table I. 



Thus it is seen that 100 stations out of a total of 774 (numbering 

 stations separately for latitude and for longitude or azimuth) gave 

 residuals, after correction for known distribution of matter, which 

 are in excess of 50" of arc. These constitute 12.9 per cent, or 

 about one-eighth of the entire number, and it is a fact of much 

 significance that, with the exception of two stations which are 



1 We may here ignore the sign of the residual deflection, which indicates the 

 direction of displacement of the zenith. The topographic correction is for the coastal 

 regions particularly large and often several times the measured deflection. It is 

 therefore interesting to find that the uncorrected deflections are also in the Pacific 

 coastal region the largest for the United States. Deflections of the vertical for the 

 entire series of Hayford which are in excess of 15" are the following, with station 

 numbers given: 



In meridian: 238, Santa Barbara (—18.38); 245, Los Angeles ( — 17.99); 3°4, 

 Mt. Wilson (-28.50). 



Prime vertical: 1, Point Arenas (+16.98); 224, Tepusquet (+20.15); 22 S> 

 Arguello (+15.90); 227, New San Miguel (+20.62); 228, Santa Cruz W. (+19. 71); 

 229, Santa Barbara (+15.04); 230, Los Angeles N.W. (+19.56); 231, Los Angeles 

 S.E. (+21.55); 236, Sulphur Pk. (+18.82); 237, Ross Mt. (+18.18); 241, Avila 

 (+29.93); 2 4 2 > San Buenaventura (+19.38); 244, Soledad (+28.06); 245, San 

 Diego (+32.43); 23, Genoa (-18.65); 4 2 , Ogden (+16.25); 43, Waddoup (+24.84); 

 44, Salt Lake City, Long. Sta. (+15.37); 45, Salt Lake City, Az. Sta. (+18.15); 46, 

 Mt. Nebo (+18.69); 61, Colorado Springs (—18.74). 



