110 Bowie — Recent Gravity Work in the United States. 



Correction for Topo/jruphi/ nm! Isostatic Compensation — Separate Zones. 



(Continued.) 





Kansas 



Pik 



e's 



Colorado 



Grand 



Beau- 



San 





City. 



Peak. 



Springs. 



Junction. 



fort. 



Francisco, 



Zone 



No. 39 



No. 



43 



No. 42 



No. 



46 



No. 18 



No. 54 



7 



+ 8 



+ 



(V 



i 



+ 7 



+ 



rv 

 i 



+ 5 



+ 10 



6 



+ 9 



+ 



9 



+ 9 



+ 



9 



+ G 



+ 9 



5 



+ 11 



+ 



9 



+ 9 



+ 



9 



+ 8 



+ 9 



4 



+ 7 



+ 



8 



+ 3 



+ 



8 



+ 7 



+ 8 



3 



+ 5 



+ 



5 



+ 5 



+ 



5 



+ o 



+ 5 



2 



+ 3 



+ 



o 

 



+ 3 



+ 



3 



+ 3 



+ 4 



1 



+ 1 



+ 



1 



+ 1 



+ 



1 



+ 1 



+,1 



Total, -12 +1891 - 68 -511 +361 +446 



The six stations in the above table were selected as repre- 

 senting typical classes. Kansas City is an inland station, far 

 from the coast of the continent and not in a mountainous 

 region. Pike's Peak is a station on a high mountain summit 

 in the interior of the country. Colorado Springs is a station 

 near the foot of the same mountain and at the edge of a great 

 plain. Grand Junction is a station in the interior of the con- 

 tinent in a deep valley among high mountains. Beaufort and 

 San Francisco are coast stations near deep water. 



It is a most interesting fact and one brought out for the first 

 time by Hayford, that there is a change of sign in the combined 

 effect of topography and compensation near zone J, outer 

 radius about 12 kilometers. This change is due to the fact that 

 the effect of the compensation for more distant zones is greater 

 than that of the topography. It will be seen that the effect of 

 the distant topography and its compensation, beyond zone J 

 for example, is in some cases entirely too large to be neglected. 

 At Pike's Peak, it is — *136 dyne and at San Francisco it is 

 + -034 dyne. 



The results of the computations for certain stations are shown 

 in the following two tables. In the first <£ is the latitude, X 

 is the longitude, and <y is the theoretical value for the latitude 

 computed by Helmert's formula. II is the elevation of the 

 station above sea level in meters. The other headings are self- 

 explanatory. It will be noticed that the effect of topography 

 and compensation at some stations is very small, zero in two 

 cases, and large at others. For Pike's Peak it is nearly "200 

 dyne. 



