INSTALLATION OF SEISMOGRAPHS 35 
earlier phases of a seismogram, instrumental terms have a pro- 
nounced influence in any case, but the interpretation of the 
record is greatly facilitated if we can depend on the rapid 
decay of the “free terms ”. 
Under the influence of periodic waves the magnification is 
given by 
L pose pare 2 42h - 
F Pat py tga py? 
and in the case of an aperiodic pendulum this becomes 
7; LBV a bo 
We on Dy Vanes 
Thus the magnification is dependent on the period of the im- 
pressed vibrations, and we can extend the range over which 
approximate uniformity is obtained only by an increase of the 
primary period T,. But this means that if we use heavy 
damping we must be prepared to sacrifice true aperiodicity. 
For rapid vibrations the magnification is L/? and for slow 
vibrations the magnification is 
Py Lp? 
We can thus increase the magnification for rapid vibrations 
by reducing 7 and that for long waves by reducing z, and this 
might be done without any great change in # or 27/T, from 
the values at present attainable, say T,=twenty seconds. 
Now / may be reduced by reducing the dimensions of the 
pendulum, and if z is correspondingly reduced T, would not be 
altered. The reduction of dimensions would not greatly alter 
e, but the reduction of the mass would increase e considerably. 
The point I wish to put is this, that we have much to gain 
and little to lose by a substantial reduction in mass and 
length of the pendulums as at present used. To be definite it 
appears to be practically possible by the use of a fine quartz. 
Zollner suspension to make a pendulum in which 7 is of the 
order 1 cm., M of the order 1 gram, which is highly damped (e of 
order say 1), and which could be placed inside a vessel the 
size of an ordinary tumbler. With optical registration at a 
distance of 2 metres the magnification for rapid vibrations 
3 * 
