22 MODERN SEISMOLOGY 
The paper speed is 4 mm. per minute, and for a period of 
185 this implies 24 mm. on the paper for twenty vibrations. 
Fic. 8. 
There is thus no difficulty in measuring the apparent period 
T’ to o°1. We may also determine with considerable accuracy 
the ratio of successive amplitudes by measuring the ratio for 
say ten vibrations. We may then compute z and e as 
follows :— 
Since 
T’ = 27/(x? — e*) 
and the ratio of successive amplitudes 
eieiebila 
we find that 
ZANT ; 
log, 2 p= F005N/T 
where 
r= log,, v 
and 
A Se NCU S 2 aa : 23 
N= aR = ar {Lt / (ar log e)*} Tu +0°5372A7H. 
From the curve (fig. 8) I found fifteen vibrations in 19 mm. 
and the amplitude fell from 5 mm. to I mm, in ten vibrations, 
so that we get IT =19'0 and v=1:084 and so T=19°o) 
m= 0'331, and e=0'0085. 
We have pointed out that Milne has always provided a 
screw by which a known static tilt can be given to the instru- 
