13 



Thus in tlio tigiirt', wliin tlif slu'll «S' is roni pressed liy tlie I'assanc nf a 

 normal wave whose source is at 0, a particle P at tlie surface is (lis|ilaci(l aloni; 

 Pm l>ei-.iu.so of the latei-al ex]iansioii »if tlie soil, as well as along Pn ; ami again 

 when the shell AH is dilateil, tlie particle is displaced downwards as well 

 as along PO, and hence its direction of oscillation lies between Pn and Ptn. 



§ 13. Prt'lim'niary Stcdetiienf of the liesiilfs of Observatlmis. 



Automatic records given by seismographs confirm what has been said, on 

 theoretical grounds, as to the complexity which earthquake motions may be 

 expected to i)resent. Tiiey show that, as observed at a station on the surface of 

 the earth, an eartli(juake consists of a very large nunil)er of successive viiirations 

 — in some casi's as many as tiiree hundred have been distinctly registered. These 

 are irregular both in period and amplitude, and the amplitude does not exceed a 

 few millimetres even when tiie earthquake is of sufficient severity to throw down 

 chimneys and crack walls, while in many instances the greatest motion is no more 

 than a fraction of a millimetre. The ]ieriods of the princiital motions are usually 

 from half a .second to a .second, but, as has been already said, the early part of the 

 disturbance often contains vibrations of much greater frequency. The earthquake 

 genei-ally begins and always ends very gradually, and it is a noteworthy fact that 

 there is in general no one motion standing out from the rest as greatly larger 

 than those which precede and inllow it. The dii'ection of motion varies irregu- 

 larly (luring the distm-banee— so much .<o that in a protracted shock the horizontal 

 movements at a singli- .station oc<-ur in all pos.silde azimuths. The duration, that 

 is to say the time during which the shaking lasts at any one point, is rarely less 

 than one minute, often two or three, and in one cast' in the writer's ex])eriencc 

 w;ls .is much as twelve minutes. 



A more particular account of tiie .actual residts of eartlujuake nieasin-emeut 

 will be given in a later chapter. The facts just stated are deduced from the 

 writer's olwrvatioas, made at the University of Tokio, and they describr, in 

 brief, the rliaraeteristies of the moderate eaithquakfs which occur with great 

 frequ<'U<-y in the I'lain of Yedo, occ;L<ionally wilii destiiiiti\-e ellect, but much 

 more comrnunly without doing any damage or attracting iiioi-c tiiaii a momentary 

 attention from the inliai)ilants. This oulline stalcin( iil of actual residts has been 

 pnsented here as affiirdiiig da(a by which we iiia\' more readi!\' discii.ss the practi- 

 cal value of various s<'ismom<-ters. Mu<h time and labour has been wasted in 

 s<'ismometrv through fal>e preconceptions xs to ilie diaiaiter <if c;ullic|iiakc 

 motion. Many instruments havi- bi'cn constructed under th<! idea that an earth- 

 quake con.-isl.-^ es.>ientially of one relatively large impul.se, easily distinguishalilc 

 iViim any minor shakings which may accouipauy it. Ilow far this mav be true 

 of gre:it earth(|uakes, or of earthquakes in other localities, it is as yet im|)ossible 

 to Miy ; nothing could be Ics (rue of the earthquakes we expei'ieuee in .lapaii, 

 whoäe frii|ueucy and nmnageuble hize make them goinl subjeet.s ior measurement. 



