UH 



70 



Ayrton in a jiaper " On a neglected Prinoiple that may be emjiloyed in Eartli- 

 «juake Measurement.'!."* The.se writers eonclnde, as the result of a mathematical 

 examination of the dynamics of a body attached to the earth by stiff sjirings 

 when the earth oscillates periodically, that such a body has, in certain ca.ses, 

 motion.s relative to the earth, which represent in miniature the motions of the 

 earth's surface itself. This result is obtained by making the .springs so stiff that 

 the free period of the body's vibration is much shorter than the period of the 

 eartlupiake waves. By the introduction of friction it is possible to get an 

 ap|)roximation to accuracy with less stiff springs ; and this is always desirable 

 when the earthquake vibrations are irregular. A proposed instrument is figured, 

 in which a mass is supported within a fixed case In- five strong spiral .sjirings, 

 and provided with three multiplying ]>ointers, adapted to show three rectangular 

 components of its motion relative to the ca.se. The pointers are to record their 

 displacements on a band of paper, drawn along by a clock which is set in motion 

 liy the earthquake. 



It a])pears that this suggestion has never been carried into effect. To sus- 

 pend a mass by s])rings wliicli woulil render its pcrind much less than that 

 of ordinary earthquake waves, and yet withmit practically ])reventing all 

 motion, would be a matter of great difficulty : and the sudden changes which 

 take place in actual earthquake motions would be a serious obstacle to the intel- 

 ligibility of the records. Compared with the .steady-jwint instruments of 

 Chapters II, III, and IV, this one ])resents many drawbacks, and few, if any, 

 advantages; and it is not remarkable that the ])rincii)le on whicii it is based 

 should, even after attention was directed to it, have remained neglected. 



§ 68. Kkctric Selsiiw copea. 



\ simple method of detecting the existence of small earth(|Uakes, whicli 

 lias l)cen very successfully used by Prof. I'almieri, consists in arranging o.seiliat- 

 ing pieces, sncii as short pendulum.s, in such a manni'r that a sligiit disturbance 

 will cause them to make an electric contact. The current so established may be 

 used to work a magnetic indicator, to stop or start or take time from a clock, to 

 start a record-receiving plate, or, in other ways, to register the occurrence of the 

 earthquake and serve useful purposes in connection with its measurement. For 

 the detection of horizontal motion, Palmieri uses a simple pendulum, a few 

 decimetres long, to the bottom of whose bob a platinum wire is fixed which 

 stands over and dips into a hole or depression in a cup of mercury j)laced 

 beneath it. To make a hole in the mercury, an iron jiin is fixed in the centre 

 of the cn]>, and the mercury is jKiured in .so tiiat its level is somewhat higher than 

 the top of the pin, but not so high as to make the liquid meet over the pin. 

 The pendulum hangs by a fine wire, forming part of the circuit; and the circuit 

 is closed when, by any shaking of the ground, the wire fixed to the under side of 

 the pendulum bob makes contact with the edge of the depression in the mercury. 

 * Pbil. Wag., July 1879, ji. 30: or Trans, of the Asiatic Soc'iety of Japan, Vol, V^ p. 181. 



