8 MODERN SEISMOLOGY 
pendulum is used because the length of the pendulum lies 
nearly along a horizontal axis, not because the pendulum 
measures the horizontal component of motion. 
It is not possible to make the axis of rotation strictly 
vertical as a practical limit is reached at which the instrument 
becomes unstable. This depends on the limit of accuracy of 
mechanical workmanship. 
The arrangement is represented in the diagram (fig. 2) where 
BV is the vertical, AB the axis of rotation making an angle 
2 with BV, while CM is the rod and mass. 
Fic. 2. Fig. 3. 
The line AB may be actually a rigid rod pivoted at fixed 
points A and B about which the rotation takes place. The 
instruments of Hecker and Rebeur-Paschwitz are of this type. 
But we can also make C a fixed pivot and support M bya 
string or wire toa fixed point A, and AB is then an ideal line 
about which the system turns. Types of these are the Milne, 
Bosch, and Omori Seismographs. Pivots are liable to become 
defective, and in any case introduce frictional effect that cannot 
be adequately considered theoretically, so that flat steel springs 
replace the pivot C in the pendulums of Mainka and Galitzin 
pendulums for second order stations. 
Another form of suspension that possesses great advantages 
