INTRODUCTION xi 
request of the German Government, he made a tour to the 
Italian observatories, and as a result of his studies designed 
and set up the inverted astatic pendulum now known by his 
name. By the introduction of fairly large damping, of in- 
creased magnification, and of increased speed of registration, 
the accuracy was greatly increased, and the division of a seis- 
mogram into three principal phases corresponding to the longi- 
tudinal, transversal, and long waves became a comparatively 
simple and definite process. 
In 1903 Wiechert published a most important memoir on 
the “Theory of Automatic Seismographs” (“Abhand. der 
K@6nig. Gesell. der Wiss. Gottingen”) showing, among other 
things, the quantitative relation of the recorded movement of 
the instrument to that impressed on the pier. This memoir 
was followed in 1907 by a paper on “ Earthquake Waves” by 
Wiechert and Zoppritz (‘‘ Gott. Nachrichten,’ 1907). On the 
experimental side greatly improved time curves giving the 
time of arrival of the longitudinal and transversal waves as 
functions of the epicentral distance are obtained. ‘The results 
have been used by Zeissig in the preparation of his interpo- 
lated tables giving the epicentral distance for the time interval 
between the arrival of the two sets of waves now known as 
PandS. These tables (published by the Imper. Academy of 
Sciences, St. Petersburg) are now in general use and are the 
most accurate we have. On the theoretical side Wiechert adds 
greatly to the interpretation of seismograms, and shows how 
the time curves lead to a knowledge of the physical properties 
of the interior of the earth. Wiechert and his pupils are still 
actively engaged in the extension of our knowledge in this 
direction. 
Galitzin’s investigations began about the same time as 
those of Wiechert and have proceeded on somewhat different 
lines. It may be said that the problem he set himself was to 
make instrumental seismometry a truly quantitative art as 
