SEISMOMETER. The term seismometer (from aftcru6s, an earthquake, and Aerpov, a measure) was invented by David Milne (afterwards Milne Home) in 1841 to denote an instru ment for recording and measuring the movements of the ground during an earthquake. It is our earliest seismological term. A few years later, the name seismograph (ypay5n, a writing) was given to an instrument erected in 1855 by L. Palmieri in the (then) new observatory on Vesuvius. A much simpler type of instrument is included under the heading of seismoscope and aims at little more than the announcement of the fact that a shaking has occurred, or the record of its time or direction.
The first seismometer deserving of the name was one invented in 1841 by J. D. Forbes for "measuring earthquake shocks and other concussions." The scientific prin ciples underlying the construction of seismographs were realized in 188o by some of the British teachers in Japan—J. A. Ewing, T. Gray and J. Milne—and the instruments erected by them in Tokyo and elsewhere, threw much light on the nature of the earthquake-motion. In 1881-82, the attempt of G. H. and H. Darwin at Cambridge to measure the lunar disturbance of gravity led to the construction, in 1893, of H. Darwin's bifilar pendulum and of Rebeur-Paschwitz's horizontal pendulum (1887).
The earliest instrument of the kind known to us may be classed as a seismoscope. The invention of a Chinese scholar Chang Heng, it dates as far back as the year A.D. 132. It consisted of a column so suspended that it could move in one of eight directions. A ball was held lightly along each of these lines and, when thrown down by the rod, was caught in a cup below and so revealed the direction of the motion.
Later seismoscopes were frequently designed to give the time of occurrence of a shock. They consisted as a rule of a hori zontal rod lightly pivoted at one end and provided with teeth below so that, when the rod fell, the teeth caught a pin projecting from the pendulum of a clock.
The essential feature of a seismometer is that some point or line within it shall remain at rest, or very nearly so, during the complicated movements of the ground in an earth quake. Various methods of obtaining such steady points have been proposed, but the instruments in general use are various forms of pendulums, either common pendulums, in which a heavy mass is suspended by a wire or rod from a fixed point above, or inverted pendulums, which consist of a vertical rod with its lower end pointed and working in a conical cavity and carrying a heavy mass at its upper end, or horizontal pendulums in which a horizontal rod carrying a heavy mass is suspended from two points in various ways. A few instruments are a combination of
two forms of pendulum—the Ewing duplex pendulum seismometer of common and inverted pendulums, and the instrument used by the Darwins of common and horizontal pendulums. Two methods of registration are now used—the one mechanical, a fine point tracing the movement on smooth paper covered with a thin layer of smoke, the other photographic, the beam of light either passing through a hole at the end of the pendulum or, more generally, reflected from a mirror connected with it.
An important point to remember about all pendulums is that they tend to oscillate with their own proper period. If the period of vibration of an earthquake should approach that of the pendu lum, such vibrations will then be unduly magnified in the record. The tendency is met, and the record corrected, by some form of damping, such as the resistance of a liquid (as in the Darwin bifilar pendulum), or a confined air-space (as in the Wiechert pendulum), or electro-magnetic reactions (as in the Galitzin and Milne-Shaw seismographs).
Pendulums.—Seismometers based on the common pendulum were in early use. One was erected near Comrie in Perthshire in 184o. It was 39 inches long and the bob carried at its lower end a pointer that traced the movement of the pendulum in a thin layer of sand below it. Instruments of this class have always been favoured in Italy, and many records of distant earth quakes have been obtained by them. They agree closely in their methods of recording the movements on smoked paper wrapped round a revolving drum. A thin rod, projecting downwards from the bob of the pendulum, works in slits in the short arms of two horizontal levers. The short arms are at right angles to one an other, so that the pointed ends of the long arms record two per pendicular components of the motion. One of the levers is straight and the other is bent at right angles, or both are bent at an angle of 45°, and the pointed ends rest near one another on the smoked paper in motion below. The instruments differ chiefly in the length of the pendulum and the weight of the bob.