554 
GEOPHYSICS: W. BOWIE 
Proc. N. a. S. 
we find it, a knowledge of the physical characteristics of the material com- 
posing the earth is essential. Seismology in connection with the investi- 
gation of the earth tides should furnish much information regarding the 
physical properties of the material of the earth at various depths. 
A subject that is of interest to many geophysicists as well as to astron- 
omers is the variation of latitude which has been studied for a number of 
years by the International Geodetic Association. While certain parts 
of the variation have been found to follow certain laws of change there 
are still apparent irregularities not yet explained. 
From investigations dating back to Euler's time it was at first supposed 
that the latitude variation, if found, would have a period of about ten 
months, this being on the supposition of a rigid earth. It is found that 
the principal part of the polar motion has a period of 14 months. From 
this fact it is inferred that the earth is not rigid, and from the numerical 
amount of the lengthening of the period we may draw inferences as to 
the elastic properties of the earth as a whole; these inferences agree in a 
general way with what has been found by investigations along other lines. 
We naturally expect the shifting of the pole to be accompanied by a shifting 
of the ocean waters responding to the change in the centrifugal force of 
rotation. This latitude-variation tide has already been studied to some 
extent without in any way exhausting the subject. 
This problem of determining the variation of latitude at observatories 
is really an astronomic one, although it has a geodetic bearing. The figure 
of the earth, determined from triangulation and astronomic data, is af- 
fected though very slightly by this variation. When it was found de- 
sirable to carry on systematic observations on an international scale, 
there was no astronomic organization with sufiicient funds to do the work. 
The problem was presented to the International Geodetic Association, 
which had a source of funds not available to the International Astronomic 
Society. The result was that the International Geodetic Association un- 
dertook the work and has carried it on continuously, even during the war, 
for twenty years or more. The work must not be discontinued nor 
should there be any break in the observations at the international stations 
in Italy, Japan and the United States. 
Geodesists are interested in the science of volcanology or at least in 
certain phases of the work in that science which may throw light on the 
constitution of the material, with densities and temperatures, in the outer 
portion of the earth, and the cause of volcanic activity. It is believed 
that these data will supplement the information obtained from the several 
other lines of investigation such as gravity determinations, earth tides, 
seismology and variation of latitude. 
There is another subject which is of particular importance to geophysi- 
cists and, I might say, of very great interest to geodesists. This is what 
may be called geo-mechanics. It would embrace an investigation to 
