58 REPORTS ON THE STATE OF SCIENCE.—1914. 
method of optical registration could well be devised. No expensive 
lenses are needed, and, with the exception of a few parts of the 
mechanism, no specially high-class work is required in manufacture. 
The whole of the apparatus is self-contained and does not take up 
much floor-space. It does not require a continuously darkened room 
in which to work. Two pendulums to record both N.S. and E.W. 
movements can be installed in the same case and record on the same 
drum. 
The Galitzin instrument, on the other hand, is a very much more 
complicated affair. It is designed to follow a somewhat elaborate 
mathematical theory, and high-class workmanship and accuracy are 
needed in its construction. Its pendulum is shorter than the Milne 
and much heavier—say, seven kilograms. It is hung by two steel 
wires (Zollner system), and has no pivot at all in some cases, Pro- 
vision, however, is made on the pendulum and frame for a steel point 
and cup to be inserted if required. The supporting wires might, with 
advantage, be made of tungsten if corrosion were feared. At the outer 
end of the boom are fixed to the frame four powerful horseshoe 
magnets. Between the poles of one pair of these moves a set of wire 
coils fixed to the boom and coupled in series with a delicate galvano- 
meter placed in any convenient position elsewhere. Between. the 
other pair is a large copper plate, also fixed to the boom, and this last 
acts as a magnetic damper. The magnets can be adjusted as desired 
to vary the.magnetic field between the poles. 
The galvanometer is of the moving coil type, and has a long period 
of oscillation when undamped. ‘This galvanometer is an excellent, 
piece of work and is electrically damped so that it can be rendered 
just aperiodic. With the whole instrument in normal working it is 
necessary that the undamped periods of both pendulum and galvano- 
meter should be the same, and that they both should be damped just 
to the limit of aperiodicity. 
The optical registration consists of a collimator with a fine slit 
powerfully illuminated. The beam is reflected from a mirror on the 
galvanometer and thence to the recording drum, where a cylindrical 
lens condenses the line of light into a point on the paper. 
The two pendulums for recording N.S. and E.W. movements are 
under entirely separate covers, and in a more refined installation two 
separate drums are also used; but it is possible to use one drum only 
and arrange the spots of light from the two galvanometers side by 
side. 
A good deal of floor space is required, and the room in which the 
recording parts are placed must be kept dark. 
The galvanometers and recording drum may be placed in a separate 
room altogether; and, in fact, are better so placed. The presence of 
the attendant is likely to disturb the pendulum if he brings his weight 
near the pillar on which it stands. The recording part of the 
apparatus is quite unaffected by disturbances in the room in which it 
is placed. 
For a further description of the Galitzin instrument see 
(1) ‘ Modern Seismology,’ by G. W. Walker, F.R.S., chapters 2 and 3. 
