CHAPTER VII. 
INTERPRETATION OF SEISMOGRAMS AND THE INTERIOR 
OF THE EARTH. 
Ir may be remarked of most seismograms that, on first 
acquaintance, it is difficult to see the wood for the trees. 
Only by experience and study is it possible to disentangle 
those effects that are characteristic and essential from those 
that are accidental. Not only so, but we must keep in view 
that any seismogram is influenced by the particular instrument 
from which it is obtained. A record from an undamped 
instrument is for instance dominated throughout by instru- 
mental periodicity. Heavily damped instruments, on the other 
hand, agree wonderfully well in presenting the same general 
features, and it is chiefly as regards relative magnitude of effect 
in different parts of the seismogram that they differ. The 
speed of registration also plays an important part, as move- 
ments that are resolved with high speeds get crushed together 
at lower speeds. 
It is unfortunate that general statements with regard to 
the character of seismic waves have obtained credence, which 
are really dependent on one particular instrument. I am thus 
diffident about giving a general description of a seismogram 
that may convey a false impression, but as some description 
must be given it may be well to state that I have in view 
heavily damped seismographs in general, and in particular 
Galitzin’s aperiodic pendulums with galvanometric registration 
at a high speed. I would add that anyone who desires to 
work at any of the theoretical problems awaiting solution will 
do well to study actual seismograms for himself and not accept 
descriptions made by other people. 
We shall suppose that records of the horizontal motion 
(X,Y) and of the vertical motion (Z) are available, and in 
48 
