CHI ETAT VPES OR SELSMOGRAPHS IN ACTUAL USE 17 
on the surface of the bromide paper wound on the surface of 
the recording cylinder. The cylinder driven by a spring clock- 
work rotates in about four hours, the paper speed being about 
250 mm. per hour or nearly 4 mm. per minute. By means of 
a deep helix cut in the spindle the cylinder is made to move 
sideways as it revolves by about 6 mm. per revolution, so that 
the paper is available to run for one day or two days as the 
case may be. Every hour the light is cut off by an electric 
shutter operated by a good pendulum clock, so that accurate 
time marks are thus put directly on the record and eliminate 
any irregularity in the driving spring clock, which cannot be 
depended on to give sufficiently accurate time. 
In the twin boom instrument the two pendulums are carried 
at right angles by the vertical pillar, but the booms are brought 
out parallel to each other so that the two horizontal com- 
ponents are recorded on one sheet. 
The instrument is subject only to such natural damping as 
may occur, and this is very small. 
The Omori Seismograph resembles the Milne instrument, 
but is much larger, the stationary mass being about 100 kg. 
A multiplying lever and registration on smoked paper is also 
used. 
The Bosch Seismograph is also similar to the Milne, but of 
a somewhat heavier build. Itis fitted with artificial air damp- 
ing and registers photographically by means of a mirror at- 
tached to the pendulum. 
The Wiechert 1000 kg. astatic pendulum is made by 
Herr Bartels, Gottingen. To him and to Prof. Wiechert I am 
indebted for permission to use the photograph, Plate 2. We 
must refer also to the diagrammatic sketch in Chapter II. 
The stationary mass is built up of iron plates and supported 
by a strong iron pillar from the pier. The support consists 
of a double set of Cardan springs so as to avoid friction and 
allow the pillar and mass to rotate about two horizontal axes 
at right angles to each other. A rigid framework carries the 
registering apparatus, supports for the damping boxes, etc. 
Stops are also provided to prevent any large motion of the 
mass, which would be fatal to the instrument. 
2 
