7.3 



rcncc, rcjiistonil in tliis way, an exceed infjly indefinit«' «jiiantity. A valuable 

 addition to any time-taker is a contriNance by whicli tlie instant when time 

 is taken shall lie marked im the eontinu^>nsly revolving plate of a reeordini^ seis- 

 niiiiiietiT. AVe ean then see, by examiiiinji the reeord, how lonij the earthi^nake 

 has been <ji>intc on before the timc-takin«^ scismoscope acts, an<l olttain a inneh 

 more definite notion of the connoetion between the instant of time registered 1)V 

 the elork and the principal motions in the distnrVianee. 



Ü 71. lif).ms tHeLfinoiCope and Micro-ieimnic Apparatus* 



In Rossi's reijisteriniij seisnioscope, a secon<ls pondnliini witii a heavy bol) is 

 liunj; IVom a rijjid frame, with freedom to swinu; in any azimnth. The bob is 

 tied by fonr silk threads to four npriijhts which siii>p!>rt tlie i)endulnm. The 

 threads are made somewhat loose, and eanv at the niiiMlc point of eaeli a 

 metallic ncfdli', whose weiu^ht causes the branches of tlie thread to make a verv 

 oiitusc angle with each other. Kach needle stands just over a cup of mercury, 

 with which it makes contact when the pendulum swings, or raflur when the 

 frame moves relatively to the bob. The vertical moli<in of the needle is ^tan J- 

 tiincs the horizontal motion of the ground (in the plane of the thread), being 

 the angle formed liy the two halves of the thread at the needle. Fig. •}•'>, Plate 

 XXI, gives a sketch of this ingenious method of multiplying the motion of a 

 ])endulum. There; n is the bob: the jwint of suspension is carried by four post.-*, 

 a part of one of which a])pears at b. cr is one of the four .<ilk threads; (/ the 

 corresponding ne;'dle, which is partially supported liv the delicate spiral spring 

 e; andy is the oup of mercury. The four cups arc arranged to close four sepa- 

 rate circuits, each being jimvided with an electromagnet and pencil, which 

 H'cords the closure of the circuit on a strip of" paper drawn by clockwork. Be- 

 si<Ies acting as a sei.smoscope, the apparatus therefore shows the quadrant towards 

 which the pendulum moves each time contact occurs. 



In till' microscismos<^'ope thei-c are five |)cn(lulums of different lengths — one 

 in the centre and four round it. Kach of the latter is tied to the central pendu- 

 lum by a lunt silk thread, which (a.s the former apparatus) causes a needle to 

 make contact with a cup of mercury whenever a relative di.«placemcnt of the 

 conm-ctcd bolis taki's place. The pendulums have dilffrcnt periods of oscillation, 

 and are therefore likely to acijuire ditlerent amount- of swinging motion dui'ing 

 any «listurbanr-c The ap|>aralus Una l)cen successfully used in Italy to detect 

 very minute eartluiuakc« since 1876. 



§ 72. Other ilicroneUmoscopai. 



UocNsi * in Italy and Milnef in Japan have us<'d microphones to detect 

 minute disturbances of (lie ground. Another wav of detecling the feeblest earth 

 tremor!^, if wc know when to look for liiem, is to observe the image of a 



• Tnlrjfmphic Jonrnal, Vol IX, p. 4flO. 



t TruiiH. of tlio iSuinmological Koviuly of Japan, V'ul. Ill, p. ü7. 



