14 MODERN SEISMOLOGY 
several months that they have to be moved a little closer, so 
as to increase the field and maintain strict 
aperiodicity. 
We should expect that light pendulums would 
be used in attempting to get aperiodicity, and it 
is somewhat curious that the Bosch, Wiechert, 
and Galitzin pendulums are actually heavier than 
even the Milne pendulum. We shall consider 
this in a later chapter. 
Ne The values of the angular quantity @ are small 
Fic. 7. and we have now to consider the manner in which 
a permanent record of the changes of 9 are obtained on a 
linear scale of sufficient magnitude. The indicator length L 
thus determines the final scale of the record, ie. the sensitive- 
ness, but I have preferred to keep this separate from the prob- 
lem of the relation of 6 to the earth movement. 
We may imagine a very light but rigid rod of length L at- 
tached to the pendulum, so that the end of the rod gives the 
linear quantity which is to be registered. This is the method 
actually used by Milne, an aluminium rod of about I metre 
length being attached to the pendulum mass. 
We cannot, however, practically proceed to great length of 
a straight rod, so that in some instruments that aim at higher 
magnification a multiplying arrangement of light levers is in- 
troduced. These involve the use of heavier pendulums, and 
where pivots are used give serious trouble by introducing solid 
friction and often lead to dislocation of the record owing to 
loose joints. Galitzin’s arrangement of replacing pivots by 
fine wire and spring control gets rid of this objection. 
The indicating end may be made to write by means of a 
style on smoked paper or by a pen with ink on smooth white 
paper. Of the sharpness of the lines so obtained there is no 
question and its cheapness is a great recommendation. It is 
used in the Wiechert and Omori Seismographs. Unfortun- 
ately solid friction is introduced by mechanical registration, the 
fundamental equation of motion has to be modified, and recent 
investigation has made it doubtful whether the matter can be 
dealt with in a satisfactory way. 
