74 



fixpd olijcct, siipli as a star, in a basin of morcnrv. This plan was used by 

 JIalk't in oxpcrinients on the speed of transit of arlifieial earthquake waves 

 through sand and rock (Brit. Areoc. Report for 1851). The following descrip- 

 tion of a registering seismoscopc, capable of considerable sensibility, but some- 

 what lialile to give false indications, is given by Milne*: — "If a light, 

 small sensitive compass needle be placed on a table, it will be found that a small 

 piece of iron like a nail may be piislied .-o near to it that the needle assumes a 

 position of extremely unstable eijuilibrium. If the table now receives the slight- 

 est tap or shake this condition is overcome, and the needle flies to the iron and 

 there remains. By making the support of the needle and the iron the poles of 

 an electric circuit it wnuld be possible to register tin- time at wliich motion took 

 ]i]ace with cnnsideiable accuracy.'" 



*^ 73. OpfirnI methixl of muHipIying the duplaeemcnt of a pciididum. 



In the experiments referred to in the Introduction, Messrs. G. and H. Darwin 

 employed a very delicate means of measuring changes in the pcisition of a ]icndu- 

 lum bob, which may also be cm]iloycd in the measurement of very minute earth- 

 ([uakes. The method was to hang a light minor liy two fibres, one attached to the 

 bob and the other to an adjacent fixed supjiort. AVhen the bob moved the 

 jtlauc of the mirror was changed, and the amount of change was read either by 

 reflecting a beam of liglit from a lamp u])on a scale, or Iw observing the scale 

 reading reflected from tli»^ mirror info a telesco]»'. The experiments of Älessrs. 

 Darwin had for their object the defection of slow changes in the direction of the 

 vortical, an<l for this reason they fliminated sudden tremoi-s by hanging the 

 l)endulum in a licjuid. In the microseismic application of the same method, the 

 pendulum and minor .-hould b(> left as free as ]iossible to respond to sudden 

 tremors of the ground; but slow displacements are to be altogether discarded. 

 ^Ir. Milne t iia-i affempted by a similar method to register minute motions of the 

 soil. His pendulum bob was kept in contact with the ends of two wires laid in 

 fixed guiding pieces of gla,«s tube, and i)laccd at right angles to each other. The 

 other ends of the wires abutted against two mirrtirs, so as to turn them round if 

 the bob should Iiecome disj)laced. Some movement was found to have occurred 

 almost every time flic instrument was examined ; but it is impossible to tell how 

 much of this was caused by earth tremors, how much by changes in the vortical, 

 and how much by movements of the pendulum's supporting point, due to mois- 

 ture, unequal heating, and other causes. To arrange the apparatus in a man- 

 iici- which would eliminate all sources of error due to this last head would be 

 scarcely possible; and if, by continuous observation, actual tremors were distin- 

 guished, it would still be a matter of great difficulty to separate out those due to 

 .such obvious causes .as neighbouring vehicles and pedestrians from the natural 

 disturbances which if is the object of the a]iparatus to detect. 



* Loc. cit., p. 37. 

 t Ibiil., p. SO. 



