24 MODERN SEISMOLOGY 
number of vibrations on the paper when an artificial disturb- 
ance is given, to determine the apparent period T’ exactly. 
Hence the artificial damping is cut out so that ¢ is reduced 
considerably to ¢’, and ¢ and z are then determined exactly as 
we should do in the case of the Milne instrument. As the 
paper speed is one minute= 10 mm, it is generally possible to 
determine T’ to o”'1. 
The damping is again introduced, and after getting v nearly 
5, by trial, its exact value is obtained and z being now known 
e is determined. The period T=27/z aimed at is from 10° 
to 12° in the 1000 kg. instruments. 
Since the instrument also measures tilt we might theoreti- 
cally now get /, by giving a known static tilt. But it is not 
easy practically to give a known tilt to the 80 kg. pendulum, 
and in the case of 17,000 kg. pendulum it is out of the 
question. Hence resort is had to another way (cf. ‘“‘ Phys. Zeit.,” 
1903, l.c.). Ifa small known mass wz say 10 gm. is placed on 
the large mass M at a distance ~ of say 10 cm. horizontally 
from the centre of gravity, a small couple is produced which 
gives an angular deflexion that may be conveniently measured 
on the paper. 
We have 
(uh! — Mgh)@, = mgp 
L6, =; 
where y, is the deflexion on the paper and L is known from 
the linear dimensions of the multiplying levers. Hence we 
obtain (us? —- Mgh) and hence knowing z we get Z. As Z is 
proportional to L we really do not require to know L in getting 
the magnification. The value of L// is readily made several 
hundred units. 
Wiechert, however, recognized that the mechanical regis- 
tration introduced frictional forces that are not properly 
allowed for by a term proportional to 6. He assumes 
(Theorie Auto. Seis.) that the solid friction was such that the 
equation took the form 
| J+ 2ey+n(y tr)=0 
where the sign of 7 is such as to oppose the motion, reversing 
and 
