Voi;. 6, 1920 
GEOPHYSICS: W. BOWIE 
553 
The theory of isostasy has some corroborative evidence in the form 
of a relation between the elevation of the ground and the density of the 
igneous rocks. Recently Dr. H. S. Washington delivered a paper at a 
meeting of the Geological Society of Washington in which he showed 
very conclusively that the higher the elevation at which igneous rocks 
are found, the smaller the density of such rocks. This tends to confirm 
the theory of isostasy which postulates the higher the land the less the 
density in the column under it, down to some indefinite depth called the 
depth of compensation, approximately 96 kilometers. 
It is hoped that these studies by Dr. Washington may be continued by 
him and by others, for their results will undoubtedly be of great value 
to geophysicists in general and especially to geologists working in the sub- 
ject of isostasy. 
Another matter that is assuming importance in geodesy is the deter- 
mination of earth tides. These were studied by the International 
Geodetic Association, a station having been established for this purpose 
in Europe some years ago. The apparatus used was of sufficient 
delicacy to show that there is an earth tide caused by the tide producing 
forces of the moon and sun, but it could not be considered that the re- 
sults obtained were entirely satisfactory. This condition has been changed, 
as a result of the construction of an earth tide apparatus by Prof. A. A. 
Michelson, of the University of Chicago. This apparatus consists es- 
sentially of two tubes placed horizontally in the ground, one in a north 
and south and the other in an east and west direction. These tubes are 
partially filled with liquid and the tide producing forces cause an inclina- 
tion of the liquid surfaces, producing the phenomena of tides. The 
actual amount of tilting of the surface of the liquid is determined by the 
interferometer method. 
The differences between the tides, as observed in the horizontal tubes, 
and the theoretical tides that would be produced on the earth, if it were 
rigid, give the measure of the actual distortion of the earth itself. This 
apparatus of Prof. Michelson is considered to be entirely satisfactory 
and it is hoped that it may be used nationally and internationally in 
determining the value of the earth tides at different parts of the earth and 
under different geological conditions. 
From the result of the earth tides, values can be deduced for some of 
the physical properties of the material of the earth, such as elasticity and 
viscosity, at different distances from the surface. A knowledge of these 
will be of great geophysical value. 
Seismology is a geodetic subject in the sense that it throws light on the 
constitution of the interior of the earth and thus assists in investigations 
of the shape of the earth. As was said in the first part of this paper, 
geodesy originally had as its function the determination of the figure of 
the earth, but in order to ascertain why the figure of the earth is such as 
