INSTALLATION OF SEITSMOGRAPHS 33 
hence 6, — 0, + 2€(0, — 6,) + 2(6, - 0,)= spl 
so that the differential motion is quite independent of x and 
depends only on y. Experiments at Pulkowa on an experi- 
mental table give very satisfactory results, but it remains to be 
seen how this arrangement does for continuous recording. 
I am not aware that any experiments have been made with 
a view to recording rotation about the vertical, but the use of 
electromagnetic registration appears to offer a way of record- 
ing this on a large enough scale. Suppose that we have a 
heavy rod suspended by a vertical wire which passes through 
its centre of gravity so that the rod rests horizontally. If now 
the rod carries similar flat coils at its ends moving in strong 
magnetic fields, the coils being coupled through a galvano- 
meter so as to assist each other when the rod rotates, the equa- 
tion will then take the form 
6+20+”0= -— x 
and the motion will be independent of x and w. 
We must remember that, however well we may be able to 
adjust apparatus to measure artificial rotations in the labora- 
tory, for practical continuous recording of earth tilting we 
have to make sure that with slight secular changes of the con- 
stants the apparatus does not develop a tendency to record a 
part of the comparatively large values of %. 
With regard to the recording of the linear displacements 
there still seems to be ample scope for the improvement of 
existing forms of apparatus. The ideal to aim at is the pre- 
cise reproduction of the earth movement on a suitable scale of 
magnification. No instrument does this although some come 
nearer it than others. Thus consideration of the dimensions 
of a seismograph appears to me to merit more attention than 
has already been given. The following table shows the diver- 
sity of magnitudes in existing forms :— 
Stationary Primary Equivalent Damping 
Seismograph. Mass. Period. length. Modulus. 
M Ti l efit 
Milne. c c I kg. 18 sec. 16 cm. 0°026 
Wiechert . : 1000 kg. 12 sec. about 100 cm. 0°45 
Galitzin : ; 7 kg. 24 Sec. I2 cm. I‘00 
