12 MODERN SEISMOLOGY 
We have to note that rotation about an axis perpendicular 
to the plane of the paper would also be recorded, and that it is 
most important that the line joining the C.G. and P should 
be accurately horizontal, otherwise horizontal motion of the 
ground in the plane of the paper would also contribute to the 
observed motion. 
The best known types are those of Wiechert and Galitzin. 
Those familiar with the practical difficulties of mechanical 
construction will understand that long periods combined with 
stability are far more difficult to attain for vertical than for 
horizontal motion. The best result obtained by Galitzin was 
a period of thirteen seconds. While this is a remarkable 
practical achievement, it is only about half what can be 
obtained with horizontal seismographs. 
We have found it desirable to retain a frictional term in 
the equation of motion and we have now to consider this 
matter more fully. 
The Milne Seismograph is the best known throughout the 
world, and in that instrument no artificial damping is intro- 
duced. It is, however, subject to such friction as may exist 
at the pivot and to the natural damping action of the air. As 
the pendulum is comparatively light (only about 1 kg.) we 
need hardly expect that with reasonable care the effect of the 
pivot should be serious; and my own experience confirms 
this. The natural air damping is comparatively small, but 
conforms as nearly as one can measure on experimental decay 
curves to the law of proportionality to the velocity. It may 
be expected to vary somewhat with the temperature and 
humidity of the air. The Omori Seismograph is also without 
artificial damping, but as the mass is very great, trouble does 
arise with the pivot in this case, and the trouble can only be 
avoided by the use of Cardan springs. 
We have observed that when the damping is very slight, 
the record of an earthquake is largely influenced by instru- 
mental vibration, making it difficult to determine the period 
and absolute magnitude of the seismic waves, especially when 
these happen to possess, as they often do, a period nearly that 
of the pendulum, viz. about eighteen seconds. Thus for 
