42 



consists always of sluwly (lyiiiy; nut undulations whosf ])eiiod is considei'aV)It' and 

 whose am|(litude is small. The same objections wciuKl ai)]>ly, with soarcelv 

 diminished force, to an instrument in which the frictional resistance was even 

 two or three times less than th'- value jriven above. 



ii 40. Mtthiiil< of making a xhort pendKhim ti-ftatie. 



A'arious jilans have been proposed for reducint^ tlie stahilitv of a short 

 pendulum sufficiently to tit it for seisnionietric work. Mr. (iray* has siicgested 

 to fi.x on tiie boh a circular trough containing some liquid, wliich, when the 

 ])endulum is deflected, will accumulate on the inner side so as to bring the centre 

 of gravity of the sy.«tem back to (or nearly to) the vertical line through the point 

 of suspension. Another plan is to use a hall rolling in a hollow curved surface 

 fixed to tln' iiob, such as a sphere whose radius is longer tiian the [)endulura. 

 " Another method would be to attach a vertical spiral S])ring to a point in the 

 axis of the jiendulum a littl«' below the point of suspension and to a fixed point 

 above it, so that when the pendulum is deflected it would introduce a couple." 



The present writerf ha.« suggested the following arrangement: — Let the 

 bob of the pendulum be jointed to the to]) of a short vertical lever Avhich is 

 I ivolted below the bob hy a gimbal, or ball and .socket joint, to a fixed support, 

 and let the connection of the lever with the bob be a ball and tube joint, .so that 

 the top of the lever may l)e free to accompany th.' pendulum in all its (small) 

 displacements. J.vl a point on this lever anywhere between its pivot and the 

 boll of the iienduhnn be connected by a stretched spiral spring to a point (fixed 

 to the groimdl vertically below and at some distance from the pivot of the lever. 

 Then when the pendulum is deflected to any side this spring will exert, through 

 the lever, a force tending to increa.se the deflection ; and by a proper ar- 

 rangement of jaits this force may be made ver\- nearly suflicient to hold the 

 pendulum deflected against gravity, and so to give a condition of nearly neutral 

 (feebly stable) equilibrium throughout a range of motion eijual to that of the 

 largest eartlKjuakes the apparatus is intended to record. 



§ 41. Diiple.v I'enditlum Seismometer. 



An instrument resembling the.^e in its action, though materially different in 

 its design, has been invented and successfully applied to the measurement of 

 earthquake motion, by the ])resent writer, under the title of the Duplex Pendu- 

 lum. It consi.sts of a combination of a common with an inverted pendulum. 

 The common |)cndulum is stable : the inverted pendulum with a rigid pivottcd 

 .sujiporting rod is unstable : by placing an inverted ])enduluni below a common 

 one and connecting the bobs so that any horizontal displacement must be common 

 to both, we may make the equilibrium of the jointed system neutral or as feebly 

 stable as may be desired. The following description is taken, with little alteration, 



* TransactionB of the Seismological Society of Japan, Vol. Ill, p. 145. 



t Ibid., p. 147. 



