ISOSTASY 577 
States and five specified areas of deficiency (over-compensation). 
The gravity observations confirm the existence of 10 of these 16 
anomalous areas, there being a confirmation in every case in which 
the gravity stations were so located as to give a thorough test. 
In no case was there a contradiction between the conclusions 
from the gravity observations and those from observations of 
deflections of the vertical. If the apparent residuals in each case 
were due to errors in assumptions, as contended by Mr. Lewis, this 
confirmation could not occur. In that case the geographic dis- 
tribution of the residuals would certainly be different in the two 
cases for the reasons indicated in the following paragraph. 
The intensity of gravity at a given station is a summation of 
the vertical components of the gravitational forces at that point 
minus the centrifugal force due to the earth’s rotation. The 
deflection of the vertical at a station dealt with in the publications 
criticized by Mr. Lewis depend, on the other hand, upon the sum- 
mation of the horizontal components of the gravitational forces at 
a given point. The pendulum responds mainly to masses which 
are above or below it. The plumb bob responds mainly to masses 
which are to the right or left, before or behind the station. Hence 
any errors of assumption as to the distribution of the isostatic 
compensation necessarily produce different effects in connection 
with gravity computations than those same errors of assumption 
produce in connection with computations of the deflections of the 
vertical. Hence a study of the two kinds of computations for the 
same region furnished a very severe test for errors of assumption. 
It is especially important to note that if a given error of assumption 
produces residuals in an investigation of deflections of the vertical 
having a certain geographic distribution that same error of assump- 
tion would produce a different distribution of residuals in an investi- 
gation of gravity. 
In the new gravity publication the computed corrections for 
the effects of topography and compensation are published in such 
detail, for every zone at every station, that Mr. Lewis or anyone 
else has abundant opportunity to test the effects of making the 
assumptions different from those upon which the computation 
