MEDRHODS OF ATTAINING SENSITIVENESS, LLC, 13 
instance the first phase of an earthquake on a Milne seismo- 
gram indicates distinct periodicity, whereas on a Galitzin 
seismogram the first phase appears extremely irregular. A 
most interesting confirmation of this came under my notice at 
Eskdalemuir where, owing to the action of some spiders’ threads, 
the east component of the Milne Seismograph was rendered 
nearly aperiodic while the north component remained periodic. 
An earthquake of considerable magnitude occurred, and the 
profound difference in the appearance of the component records 
was exceedingly instructive. 
For these reasons it has appeared desirable to most investi- 
gators to introduce large artificial damping in the seismograph 
so that the absolute measurement and analysis of seismograms 
should be rendered easier. Inasmuch as increased damping 
on a given pendulum would reduce its effective magnification, 
we must obviously increase the sensitiveness to start with. 
The Bosch and Wiechert pendulums are arranged with 
artificial air damping. This is done by attaching to the 
pendulum a multiplying arrangement with a piston at the 
end, so that the piston moves inside a fixed cylinder. The 
ends of the cylinder are connected by air passages with each 
other and with the external air, so that the amount of resist- 
ance offered to the piston can be varied within certain limits. 
In this way a frictional term is introduced in the equation of 
motion and it is possible to attain aperiodicity if so desired. 
The results obtained by the use of artificial air damping 
appear to indicate that it is only as an approximation that the 
frictional term introduced is proportional to the velocity. 
The most important advance in recent years has been made 
by Galitzin, who successfully introduced electromagnetic damp- 
ing. A horizontal copper plate attached to the Zollner pendu- 
lum moves in the field produced bya pair of very strong 
permanent horse-shoe magnets fixed above and below it. The 
eddy currents induced in the plate when it moves retard the 
motion, and here there can be no doubt that the retarding 
force is proportional to the velocity. 
Aperiodicity can readily be obtained. The magnets have 
proved remarkably constant and it is only at intervals of 
