362 Professor Ewing [June 1, 



with such rapidity that a point on the earth's surface wriggled 

 through a path like the form a loose coil of string might take if it 

 were ravelled into a state of the utmost confusion. The mechanical 

 problem in seismometrj was to find a steady point, to suspend a body 

 so that some point in it, at least, should not move while this com- 

 plicated wriggling was going on ; the steady point would then serve 

 as a datum with respect to which the movement of the ground might 

 be recorded and measured. The simple pendulum had often been 

 suggested as a steady-j^oint seismometer, but in the protracted series 

 of oscillations which made up an earthquake the bob of a pendulum 

 might, and often did, acquire so much oscillation that, far from 

 remaining steady, it moved more than the ground itself. 



The lecturer illustrated this by showing the cumulative effect of 

 a succession of small impulses on a pendulum when these happened 

 to agree in period with the pendulum's swing. The fault of the 

 pendulum, from the seismometric point of view, was its too great 

 stability, and its consequently short period of free oscillation. To 

 prevent the body whose inertia was to furnish a steady point from 

 acquiring independent oscillation, the body must be suspended or 

 supported astatically ; in other words, its equilibrium must be very 

 nearly neutral. Methods of astatic suspension which had been used 

 in seismomctry were described and illustrated by diagrams and 

 models, in particular the ball and block seismometer of Dr. Verbeck, 

 the horizontal j^endulum, and a method of suspension by cross cords 

 based on the Tchebicheff straight-line link-work.* 



The comj^lete analysis of the ground's motion was effected by a 

 seismograph which resolved it into three components ; two horizontal 

 and one vertical, and recorded each of these separately, with respect 

 to an appropriate steady-point, by means of a multiplying lever, on a 

 sheet of smoked glass which was caused to revolve at a uniform rate 

 by clock-work. The clock was started into motion by the action of 

 the earliest tremors of the earthquake on a very delicate electric 

 seismoscope, the construction of which was shown by a diagram. In 

 this way a record was deposited upon the revolving plate which gave 

 every possible particular regarding the character of the earth's motion 

 at the observing-station. A complete set of the instruments as now 

 manufactured by the Cambridge Scientific Instrument Comj^any was 

 show^n in action.f Professor Ewing also described his duplex 

 pendulum seismograph, which draws on a fixed plate of smoked glass 

 a magnified picture of the horizontal motion of the ground during an 

 earthquake.! Apparatus was shown for testing the accuracy of the 

 seismographs by means of imitation earthquakes, which shook the 



* See a memoir on 'Earthquake Measurement,' by Professor Ewing, pub- 

 lished by the University of Tokio, 1883. Also 'Proc. Koy. Soc.' No. 210, 1881 ; 

 and ' Transactions of the Seismological Society of Japan/ from 1880. 



t See ' Nature,' vol. xxxiv. p. 343. 



ij Trans. Seis. Soc. Jap. vol. v. p. 89, and vol. viii. p. 83; ' Encyclopsndia 

 Britannica,' Art. 'Seismometer'; ' Prcc. Koy. Soc.,' June 21, 1888. 



