CHAPTER IL. 
METHODS OF ATTAINING SENSITIVENESS, DAMPING, 
REGISTRATION. 
IN general the amplitude of the earth movement to be 
measured at a seismological station is small, the convenient 
unit being ;,4,,th mm. or micron. Thus a seismograph to be 
of practical value must give a large magnification of the earth 
movement. 
The expression for the magnification 
ys 
M = TU 
where U = {(#? — 1)? + 4e?u?/n?2 and u=n/p 
shows that it depends on the indicating length L. It will be 
convenient to consider L in connexion with registration and 
for the present to treat it as prescribed. 
For very rapid vibrations U equals 1 and thus we should 
gain by making Z small. I do not think the question of the 
best dimensions for a pendulum has received much attention, 
but it deserves consideration. 
For very slow waves we find that we gain by making 77/ 
small. Now z?= //and so “7=g and thus the dimensions 
are without influence. Thus attention has been directed to 
making what we may call the static sensitiveness great, by 
reducing the effective control. 
It is possible to reduce the influence of gravity in con- 
trolling a compound pendulum. If the axis of rotation is 
gradually altered from the horizontal to the vertical direction 
the effective part of gravity in controlling rotation of the body 
about that axis becomes less and less, until when the axis 
is vertical the influence is zz/. This is the principle on which 
horizontal pendulums are constructed. The name horizontal 
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