PENDULUM which turns about an axle F nected with the ground. The short arm FE of the lever ends a horizontal fork, the branches of which pass round a vertical steel axle connected with the mass M. This axle turns with great free dom, so that there is very little friction between it and the lever. The other end D of the lever is a light aluminium pointer which rests lightly on a sheet of thinly smoked paper wrapped round the rotating drum P. The drum is about 36 inches in circumference, rotates once an hour, and advances parallel to its axle one-sixth of an inch with every revolution. Once every minute, time-marks are made on the paper by a small pointer (not shown in fig. 4) connected with a clock. The chief advantage of the Omori pendu lum is the open diagram that it provides, but, being undamped, the oscillations of the pendulum lessen the value of the record. The Omori pendulum is also made in other forms. In the portable form, the mass weighs about 64 pounds and the length of the rod is nearly 3o inches. For recording local shocks, two other forms are used. In one, the mass weighs iio pounds, the length of the rod is nearly 8 inches, and the movements of the ground are magnified 20 times; in the other, the corresponding figures are 33 pounds, 21 inches and 7o times. A somewhat similar instru ment is the Bosch-Omori horizontal pendulum, one form of which records mechanically and the other photo graphically.
The Darwin bifilar pendulum (fig. 5) is a modification of the instrument used by G. H. and H. Darwin in their experi ments on the lunar disturbance of gravity. In this instrument, the mirror M itself is the bob of the pendulum. It is carried by a fine silver wire, passing through two hooks C, D, in the rim of the mirror and attached to two points of support at a very small distance apart horizontally. The in strument is damped by being entirely im mersed in oil, and its movements are re corded by light reflected from the mir ror to a sheet of bromide paper wrapped round a revolving drum. Though many
earthquakes have been recorded by the bifilar pendulum, it does not, owing to its immersion in oil, respond to the prelimi nary tremors of short period. The instrument is, however, ex tremely sensitive to small tilts of the ground, and it has been found possible to measure a tilt of of a second, which is equal to the angle formed by lifting through an inch the end of a line one thousand miles in length.
The Galitzin seismograph, like the Darwin pendulum, belongs to the third type of horizontal pendulum. The arrangement of the mass M and the supporting wires AB, CD are as shown in fig. 6. The weight of the mass is about 110 kg. At one end of the beam, a copper plate is attached horizontally and placed between a pair of powerful horseshoe magnets. When the pendulum is displaced, currents are induced in the copper plate, and the attraction between them and the magnets damps the swing. By adjusting the distance between the magnets, the pendulum can be rendered absolutely aperiodic. Close to the copper plate, coils of fine wire are also attached to the beam and placed between another pair of horseshoe mag nets. When the beam moves, an electric current is generated in the coils, and is transmitted to a sensitive d'Arsonval galvanome ter, the movements of which are registered by a ray of light reflected by the galvanometer mirror on to photographic paper wrapped round a revolving drum. This current is proportional, not to the displacement of the beam, but to its velocity. Thus, the record does not correspond to the actual movements of the ground, but it gives with great accuracy the epochs of the various phases of an earthquake. Though not without disadvantages, the in strument possesses many compensations. It can be accurately damped, it can be arranged so as to magnify the movements of the ground 1,000 times or more, owing to its suspension by wires the beam moves without friction, and the recording apparatus can be placed at some distance from the pendulum, if desired.