INSTALLATION OF SEISMOGRAPHS 31 
point of view, and becomes of ‘vital importance if the move- 
ments of the pendulum zero are to be examined for diurnal 
tilting of the ground due to earth tides. An underground 
chamber may be used to conduce to such uniformity of tem- 
perature, and it has been found at Pulkowa that the disturbing 
effects of local wind are considerably reduced in an under- 
ground room. 
The pendulums have to be carried on substantial piers 
which take up the earth movement. Here it is important to 
avoid the danger of making a pier which itself becomes a 
pendulum and so complicates the recorded motion. The pier 
ought therefore to be broad rather than high. Concrete 1 
metre square imbedded in clay to a depth of I metre gives 
satisfactory results. In the case of a complete Galitzin in- 
stallation the pier is enlarged so that all three pendulums may 
be carried on it. 
The co-ordinates of the station latitude, longitude, and 
height above sea-level must be known or determined in some 
suitable way, and it then becomes important tc determine the 
position of the geographical meridian. It is convenient to 
record the N.—S. and E.—W. components directly, and it is im- 
portant to make the adjustment correct from time to time or 
to determine how far the pendulums have deviated from the 
true positions. We also require to know the initial direction 
of motion of the recording point, when the earth moves ina 
given direction. Thus it is convenient to arrange that a move- 
ment up the sheet corresponds to an earth movement to north 
or east, and it is useful to remember here that if a sudden 
movement of the earth occurs, say to north, the initial motion 
of the centre of gravity of the pendulum will appear to be 
towards the south. As confusion on this point has occasion- 
ally arisen by the notion of forces applied to the pendulum, it 
is well to recall that we are concerned not with forces, but with 
a prescribed motion given to the point of support in which the 
recording part of the apparatus participates, so that it is only 
the relative movement of the pendulum that is observed. It 
may be that the prescribed initial motion is complicated and 
not instantaneous, so that the record is then mixed up with 
