870 



SCIENCE 



[N. S. Vol. XXXVI. No. 938 



extent would be supported by the partial 

 rigidity of the earth's outer materials. 



In a recent report' Hayford and Bowie 

 study the question of local versus general 

 compensation by a comparison of the residuals, 

 observed minus theoretical gravity, for 41 sta- 

 tions in the United States, and 4 outside. The 

 observations are reduced according to the new 

 method in four different ways : first, with com- 

 plete local compensation; second, regional 

 compensation for a zone 19 kilometers (12 

 miles) in radius; third, the same for radius 

 of 59 kilometers (37 miles), and fourth, the 

 same for radius of 167 kilometers (104 miles), 

 this study having been made along these lines 

 at the suggestion of the writer. In the last 

 three reductions the surface layers are taken 

 as rigid to the respective distances from the 

 station, and it is assumed that there is a uni- 

 form compensation of the area as a whole. 

 In the last, for instance, an area of the earth's 

 surface of 167 kilometers radius is taken as 

 being in general equilibrium with a uniform 

 compensation beneath this area, but local 

 irregularities as mountains or valleys within 

 this area are assumed to be not locally com- 

 pensated, but supported rigidly. 



The authors state that the resulting evidence 

 " is necessarily slight and possibly inconclu- 

 sive." In a number of comments, however, it 

 is indicated that the authors favor the idea 

 that the results point to fairly close but not 

 complete local compensation, and finally that 

 " the evidence, slight as it necessarily is, indi- 

 cates that the assumption of local compensa- 

 tion is nearer the truth than the assumption 

 of regional compensation uniformly distrib- 

 uted to zone 18.8 kilometers." 



It is believed that a close scrutiny of the 

 figures does not support this conclusion so far 

 as evidence from these results is concerned. 

 The differences in the mean of the residuals 

 with the four different reductions are insig- 

 nificant. The suggested advantage for local 

 compensation is based (page 101) on small 

 differences in the relative number of larger 



^ ' ' The Effect of Topography and Isostatic Com- 

 pensation upon the Intensity of Gravity, ' ' Coast 

 and Geodetic Survey, 1912. 



and smaller residuals with the respective 

 methods. Along this line, however, it may be 

 noted that of the 41 stations there are only 7 

 where the local compensation anomaly is lower 

 than any of the three regional, while there are 

 24 where one of the three regional compensa- 

 tion anomalies is lower than the local. Of the 

 four outside stations three show anomalies in 

 favor of regional compensation. In a later 

 paper by Mr. Bowie under the same title 

 (second paper) a general result is given of an 

 extension of this study to 124 stations in the 

 United States, and it is stated (page 22) that 

 for all these stations " these mean anomalies 

 give only negative evidence," though from a 

 portion of the data he draws a conclusion un- 

 favorable to regional compensation to zone 167 

 kilometers. 



It would seem that the best evidence as to 

 local completeness of compensation from pres- 

 ent available observations will be afforded by 

 comparing the residuals at pairs of stations in 

 the same general locality, but differing con- 

 siderably in elevation, as in such a comparison 

 distant effects and various uncertainties will 

 be largely eliminated, and furthermore it is in 

 such regions that lack of local compensation 

 might be most likely to occur. It is on some- 

 what the same principle that the most accu- 

 rate latitude determinations are obtained from 

 observations of pairs of stars. In the 45 sta- 

 tions reduced by the four methods as outlined 

 above there are only five such pairs of stations, 

 that is, stations horizontally not remote, and 

 yet having considerable differences of eleva- 

 tion. The following table gives the facts for 

 these ten stations, and also the differences be- 

 tween the anomalies, subtracting that of the 

 lower from that of the higher station in each 

 instance. 



In the second paper there is one more such 

 pair, Cloudland, Tennessee, 1,890 meters, and 

 Hughes, Tennessee, 994 meters, with difference 

 in anomaly for local compensation -j- .033, sub- 

 tracting the lower from the higher station; for 

 the three regional compensation reductions 

 (not yet published) the differences are, 19 km. 

 + .031, 59 km. + .031, 167 km. + .031. 



