Seismometer

Inverted Pendulums.

The oldest form of inverted pendu lum seismometer, indeed the oldest seismometer of any kind, was that designed by Prof. J. D. Forbes in 1841 and erected in the same year at several places near Comrie. Its essential features are shown in fig. 1. The heavy ball C is carried on a vertical metal rod AB, on which it can be adjusted at any height desired. The rod ends below in a cylindrical steel wire D fixed to the base of the instrument, which can be made more or less stiff by altering its free length. As this wire is cylindrical, the direction of the displacement is indicated by the plane of vibration of the pendu lum. The self-registering part of the instrument consists of a spherical segment E of copper lined with paper, against which a pencil F, in the top of the pendulum rod, is pressed by a spring.

The most widely used form of inverted pendulum is that de signed by Prof. E. Wiechert. In one, the heavy mass consists of iron plates, weighing a little more than a ton. This is carried on a stout iron rod, supported at the lower end on springs which enable it to turn freely in any direction and yet, after disturbance, restore it to its vertical position of equilibrium. The dis placements of the pendulum are magnified by two levers ending in fine glass rods which inscribe the records of their move ments on a sheet of moving smoked paper. By means of air-damping cylinders, the pendulum can be made as nearly aperiodic as is desired. The movements of the ground are magnified about 500 times. In a still more sensitive form of the instru ment, the iron plates forming the heavy mass are replaced by a cylinder that can be filled with the heavy mineral barytes. The weight is thus increased to nearly 17 tons, while the movements are magnified 2,200 times.

Horizontal Pendulums.

The hori zontal pendulum in its simplest form con sists of a metal rod or frame rotating about a nearly vertical axis, and carrying a heavy mass near one end or the middle. It may be (a) supported on two conical steel points, or (b) one end of the rod may be pointed and rest in a conical hole in the frame while the other is supported by a wire, or (c) the rod may be supported by two wires, the weight being either between or outside the two points of attachment of the wires. Whatever the mode of sus pension, however, it is essential that the horizontal distance be tween the points of support shall be very small compared with the vertical distance between them, that is, that the line joining S inches. It consists (fig. 2) of three thin brass tubes, C, D, E, in the form of an isosceles triangle, carrying a small mass M. The two equal tubes are prolonged at A and B and end in small spherical agate cups. These rest against steel points fixed to the frame of the instrument. The rod G projects at right angles to the tube E, and to it is attached the mirror F, which passes through the frame of the support. The photographic method of registration is used.

The Milne seismograph (fig. 3) was used by Prof. Milne in Japan for the registration of distant earthquakes and, with im provements in detail, became for many years the standard instru ment at the observatories under the care of the British Association Seismological Committee. The boom of the pendulum BD is an

aluminium rod about 39 inches long. The bob M consists of two brass balls, each about half a pound in weight and attached to the ends of a short bar crossing the boom at right angles. At the end B of the boom there is a small agate cup that rests on a steel point in the cast-iron AB, which is about 20 inches in height. The boom is further supported by a silk thread CA, which passes over a small pulley at the top of the column. It ends at D in a small aluminium plate, in which there is a narrow slit. In the top of the case containing the recording part of the instrument a similar plate is fixed in the direction at right angles to the other. A ray of light from an adjacent lamp is reflected by a mirror so as to pass through both slits and form a small spot of light on a sheet of bro mide paper wrapped round the brass cylindrical drum P. The drum is 39 inches in circumfer ence, rotates once in four hours, and, with every revolution, ad vances one-quarter of an inch in the direction of its axis. Close to the end D, a small shutter at tached to the long hand of a watch cuts off the light once every hour, thus providing the scale of time.

It would be difficult to over estimate the value of the records made all over the world with the Milne seismograph. The magni fying power of the instrument is, however, low (about 7 or 8 times), and it is unprovided with any form of damping. Both of these defects have been removed by Mr. J. J. Shaw, and, since 1915, the Milne-Shaw seismograph has been gradually replacing the earlier form. In this, a boom, 16 inches long, carries a heavy mass weighing one pound, and also a damping vane which moves in a magnetic field so strong that the pendulum is brought to rest after each oscillation. The beam of light is reflected by a mirror which rotates in an agate setting and is coupled to the free end of the boom. This gives a magnifying power of 300. After pass ing through two cylindrical lenses at right angles to one another, the beam of light converges to so fine a point on the paper that waves with a period of two or three seconds are dearly shown on paper moving at the rate of about 20 inches an hour.

The Omori horizontal pendulum (fig. 4) belongs to the same type as the Milne seismograph, the chief difference being that the record is made on smoked paper, A thus allowing a much more open diagram. The boom BC is an iron rod about 4o inches long and the bob M is a cylinder filled with lead and weighing about 3o pounds. The pointed end B rests D F in a conical socket fixed to the 13 iron supporting column, the other support being a wire CA that passes over a pulley at the top of FROM DAVISON, "A MANUAL OF SEISMOLOGY" the column. The recording lever FIG. 4.-0MORI'S HORIZONTAL DE is a light aluminium rod