34 MODERN SEISMOLOGY 
The statement sometimes made that the Omori Seismo- 
graph has a natural period of sixty seconds, I take to refer to 
the apparent period when friction of the recording system’ is 
introduced. Personally I found the Omori became unstable 
with a natural period above 16° and I am convinced that there 
is no piece of physical apparatus of reasonable dimensions that 
could have a natural period of sixty seconds. Instability due 
to mechanical imperfection sets in long before this. 
The differences in M are very great and Wiechert even used 
17,000 kg. in his celebrated instrument at Gottingen. If 
we go back to the physical equation of motion of a pendulum, 
we see that except for any controlling spring action or solid 
friction there is no point in using a large mass. On the con- 
trary, if we admit the desirability of high damping we have 
everything to gain by using a small mass, as we then make e 
larger without altering z and 7 There will be a practical 
limit to M depending on very small spring action that might 
ordinarily be neglected, but I do not think the practical limit 
of small mass has yet been reached. 
We have already commented on the advantage of high 
damping in removing the effect of instrumental vibration, and 
we must now consider more fully the question of aperiodicity. 
The mathematical advantage of ideal aperiodicity is to some 
extent discounted by the fact that it cannot be precisely main- 
tained, and its practical advantage of freeing the record from 
instrumental periodicity might be also secured in an over- 
damped pendulum. But aperiodicity alone does not solve the 
problem of deducing the earth movement. If an aperiodic 
pendulum is given a sudden displacement it would still show 
I per cent of its initial displacement when z¢= 6°64 or if 
277 
= a 24 Sec. = )2.5See 
Now ¢z can only be reduced by increasing e and we could 
then only retain aperiodicity by shortening the primary 
period. Further, if galvanometric registration is used, the 
record corresponding to a sudden displacement of the ground 
presents the appearance of a single wave. 
Thus for certain types of movement such as occur in the 
