STANDARDIZATION OF SEISMOGRAPHS 25 
whenever y reverses. If now y,, #, v3 represent successive 
amplitudes while vw is the ratio of successive amplitudes of the 
periodic term, it is easy to see that 
SS CLG, 
Jott Jntr 
and hence 
pp SE +2 — 27 
I2-I3 Vat I3t 27 
while 
1 = (Yo" — I,Vs) (V1 - Is) 
which suffice to determine vy and e/z. In practice it is best 
to obtain ~ when the damping is cut out, so that v being 
nearly I we get approximately = }(y, — 75). 
The writing point may remain at rest anywhere within a 
range 27, and discontinuities of this magnitude may occur in 
the trace. 
It is also clear that the motion can never start unless the 
impressed acceleration exceeds a certain amount, and this ex- 
plains the fact that so many more earthquakes are recorded 
on instruments that use photographic registration even with 
smaller magnification. 
But a more serious matter arises. Experiments of my own 
on an 80 kg. Wiechert showed clearly that 7 was not a con- 
stant, but depended on the state of the smoked surface and the 
amplitude of the movement. This has been more recently 
established by Galitzin (Vorles. ii. Seis.) whose elaborate ex- 
periments show that a more complex and non-linear equation 
corresponds better with the facts. This, however, rather sug- 
gests that if cases arise where the solid friction is so great as to 
seriously vitiate the records, we should do well frankly to 
abandon mechanical registration. With reasonable care the 
magnitude of 7 does not exceed a few tenth millimetres in the 
Wiechert instruments. 
The motion of the Galitzin horizontal pendulum is given 
by the usual equation 
6+ 2604+ 20 = -x/0. 
In practice T =nearly 24° or = 0'2618, and when in use we 
make e as nearly=x as possible. The length / is a definite 
