32 MODERN SETSMOLOGY 
instrumental terms. The initial kinematical result remains true, 
but whether we succeed in detecting the true apparent initial 
movement of the pendulum on the record is another matter. 
Accurate time marks must be put on the records auto- 
matically, and the station thus requires a good clock and a 
knowledge of standard Greenwich Mean Time, so that the 
occurrence of events at different stations may be compared 
with an accuracy of one second. Absolute time is less im- 
portant than the consideration that all stations should have 
the same time. The use of the wereless time signal promises 
the best solution of this problem. 
A word with regard to photographic registration may not 
be out of place. If sharp traces are to be obtained only the 
highest quality of optical work is permissible. Mirrors must 
be optically plane and palladianized on the /ron¢ surface, lenses 
must be properly corrected for spherical and chromatic aberra- 
tion, and the use of thick plates of glass through which the 
light has to pass at a high angle must be avoided. 
The number of seismographs that have at various times 
been devised is very large, but only a few of these have sur- 
vived to practical use at the present time. We cannot attempt 
to discuss these obsolete forms, although a study of them will 
well repay anyone interested in the improvement of practical 
seismometry (for references see Milne, “‘ Earthquakes ”). 
We have already remarked that the rotations are not yet 
recorded although instruments for measuring tilting have 
been proposed. The bifilar pendulum of Darwin and Davison 
(“B. A. Reports,” 1881) and the klinograph of Schliiter (“ Gott. 
Dissert.,” 1900) have not come into use, as they record other 
things besides tilting. Galitzin (Vorlesungen) has recently pro- 
posed to record tilting by the combination of two similar 
horizontal aperiodic pendulums at different heights working 
in opposition on a single galvanometer. 
Thus if 6, represents the motion of the lower pendulum we 
have 
6, + 260, +220, = —(4-gh)/l 
while if 6, represents the motion of the precisely similar pen- 
dulum at a height s 
6, + 266, +220, = -(% -—gh+sp)/d 
