Pendulum

A pendulum suspended in the same manner, with a small con cave mirror attached to the rod at the centre of rotation, has recently (1927) been erected by Mr. M. Ishimoto at Tokyo and other places in order to measure very small tilts of the ground.

Vertical Motion Seismograph.

All the instruments de scribed above are designed to record the horizontal component or components of the earthquake motion. Instruments intended for recording the vertical component depend as a rule on a principle first recognised by Prof. T. Gray in 1881. Gray's vertical motion seismograph, with improvements suggested by Sir J. A. Ewing, is represented in fig. 7. In this in strument, AC is a horizontal metal beam, carrying near one end the cylindrical heavy mass M. The beam turns about a fulcrum at the end A, which consists of two steel points fixed to a projec tion from the frame and pressing into a hole and knife-edge at the end of the beam. It is supported by two long springs S, adjustable at the upper end D, while their lower ends are attached to a hang ing cross-bar B, in the middle of which a point presses against the lower side of the beam. The length of the springs S is adjusted so that the moments about the fulcrum of the weight of the pendulum and the upward pull of the springs are approximately equal. When the fulcrum A is dis placed in the vertical direction, the line that remains steady lies within the mass M on the side of its centre away from the fulcrum, and the movements of the ground are magnified by a lever pro jecting downwards from the free end of the arm. In Gaiitzin's vertical motion seismograph, which is a compact form of the above, the weight of the beam and mass is about 54 pounds and the length of the beam 3o inches. At the end of the beam are

attached a copper plate and coils of wire placed between magnets, the one for damping the instrument and the other for recording the velocity of the beam, as in the horizontal motion seismograph.

BIBLIoGR PHY.--The literature of seismometry is very extensive, and it is only possible here to refer to a few of the works in which the theory and construction of different instruments are described—R. Ehlert (Brit. zur Geoph., vol. 3, 1896-98, PP. ; J. A. Ewing, Earthquake Measurement (Tokyo Imp. Univ., Mem. Sci. Dept., no. 9, 1883, pp. 1-92) ; Prince B. Galitzin, Vorlesungen caber Seismometrie (1914, pp. 1-538, German trans. edited by 0. Hecker) ; C. G. Knott, The Physics of Earthquake Phenomena (1908, pp. 48-89) ; H. F. Reid, Theory of the Seismograph (Californian Earthquake of 1906, vol. 2, 1910, pp. 143-190) and On the Choice of a Seismograph (Amer. Seis.

Soc. Bull., vol. 2, 1912, pp. 8-3o) ; G. W. Walker, Modern Seismology (1913, pp. 1-36) ; and E. Wiechert, Theorie der automatischen Seis mographen (Abhand. der kon. Gesell. Wissen. zu G8ttingen, Math. Phys. Kl., vol. 2, 1903, pp. 1-128). Brief descriptions of several in struments specially adapted for recording near earthquakes, such as Ewing's three component seismograph, the Gray-Milne seismograph, and Ewing's duplex pendulum seismometer, will be found in C. Davison's Manual of Seismology (1921, pp. 17-22).