CHAPTER I. 
GENERAL DYNAMICAL THEORY OF SEISMOGRAPHS. 
THE most general movement of the ground in the vicinity 
of a point on the earth’s surface may be regarded as made up 
of the components of a linear displacement resolved along 
three mutually perpendicular axes and the components of a 
rotation resolved about these three axes, It is convenient to 
choose the geographical axes at the point, viz. North, East, 
and Vertical. 
In practical seismometry the horizontal components have 
been mainly the subject of measurement, and it is but recently 
that the vertical movement has been carefully studied. The 
rotations are not at present recorded, although experiments 
directed to that end are now in progress at Pulkowa. The 
principal seismic waves recorded are, however, many kilo- 
metres in wave length, while the amplitude at some distance 
from even a devastating earthquake is but a fraction of a 
millimetre, so that the twisting movement is practically small 
except in the vicinity of the earthquake, where actual measure- 
ments are for obvious reasons of a rough and hazardous nature. 
Thus the objective of a seismological station being primarily 
the recording of the earth movement experienced there, we have 
to consider the instruments by means of which records are ob- 
tained. 
The instruments are called seismographs, and each seismo- 
graph measures, or is supposed to measure, one component. 
Thus six instruments are theoretically required to determine 
the complete motion, but at present only a few stations are 
fitted with three seismographs for the three linear components, 
while most stations possess only two instruments for recording 
the two horizontal components. 
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