Vol. 6, 1920 
GEOPHYSICS: H. F. REID 
557 
For our present purposes we can classify seismological problems into: 
A. World-wide. 
B. Regional. 
C. Theoretical and Auxiliary. 
A. World-wide Problems 
The noted seismologist, the late John Milne, has estimated that fifty 
or sixty earthquakes, strong enough to be recorded on seismographs in 
all parts of the world, occur every year. The study and comparison of 
the seismograms made at these scattered observatories requires inter- 
national cooperation, and this will be provided for by the new International 
Geodetic and Geophysical Union. 
The problems facing us are numerous, but perhaps the most important 
is the accurate determination of the time of the earthquake waves to 
pass from their origin to points on the earth's surface at various dis- 
tances from it. We already have a fair knowledge of these times, but 
need more accurate determinations. This information serves two main 
purposes: it enables us to determine the origins of earthquakes when 
they occur in inaccessible regions or under the sea, and thus gives us in- 
formation of the regions of seismic and therefore of geologic activity; 
and it enables us to trace out the paths and the velocity of the earthquake 
waves through the earth, thus throwing light on the physical condition 
of the earth's interior. 
The records of earthquakes at distances of more than about 110° from 
the origin have proved very unsatisfactory. It is still uncertain whether 
this is due to the imperfections of our observations or, as Oldham thinks, 
to the existence of a central core in the earth having different physical 
properties from the material surrounding it, which materially changes 
the direction of propagation of the seismic vibrations passing through it, 
and may even damp some of them out entirely. This very interesting 
suggestion deserves to be tested. 
Professor Wiechert and his collaborators have announced the existence 
of several shells in the earth where the velocity of propagation of the 
waves suddenly changes. These results are based on the form of the 
transmission curve; but, unfortunately, this form is not known with 
sufficient accuracy to justify the conclusions. 
Do earthquake waves travel under the continents and the oceans at a 
different rate? We can say that the difference is certainly small, but 
we cannot say that there is no difference. These are some of the 
problems dependent for their solution on increased accuracy of the trans- 
mission curve. 
The comprehensive work of the Count de Montessus de Ballore on 
the distribution of earthquake centers on the earth, has shown that 95% 
of all earthquakes on land occur in two broad zones, one surrounding the 
