ON Tin; K.vnriK^UAKK 1'iiknomi;na or .lAr.vN. 



243 



tlie 



•rtiuu 

 c(l as 



;on in 

 stuv\)- 

 A few ' 

 ,8t 150 

 land 

 iqnako 

 -oV)sev- 

 ,. Tho 



)ng tlio 



5 of the 



all the 



ia, ov on 



arc sin- 



)lcr as it 

 em. The 

 G,000 to 



m holds a 

 . centre of 

 tainorphic 

 tlic vents 



bes. 



ocp ocean, 



les chietly 

 1 Atnerica 



|Llar origin. 



[er months 

 )nsider the 

 [aken, then 

 icr luoutlis 



shock per 

 Ian seiswo- 

 lan are ex- 



Lhe records 

 if means of 

 Lee closely 

 uses of tho 

 I advance ot 

 L In con- 

 |act that tlio 

 • the earth- 

 alov/ly and 



n. There has honn no inavki'd connection between the occuvrcnco of 

 oarthiiniikes and the position of tlio moon. 



(i. Kiirtliqnakca liavo been II'- ])er cent, moro nnmcrons nt low 

 water than at higli water. Jt is lVe(|nently assiinieil tliiit earthquakes aro 

 more frequent at one time rather than at anotliei-. I havo spent much 

 time in the tabulation of tliu oaith(|iiakes of Japan and other countries, 

 comparing tofrctlu'i- the IV('(iui'ncv of eartb(|tiiikt's nt certain jdiases of tho 

 moon, at particnlar seasons, during tlie day as compared with tlio night, 

 relatively to the state (if the Iiaronioter ami other meteorological changes, 

 &c,., with tile g(!neral result, fliat there are no strongly marked ])eriods 

 wlieu eartlKpiakes niiiy bt; expected, tho c.\-ception.s to rules which may 

 he formulated being .'dmost as numerous as the cnses whieh were tho 

 foundation for the rules. Tlie most marked rule about earthquakes i.** 

 tliat tliey chietly occur during the eold months. 



7. With regard to tlie natuiv of (>arthquako motion ris deduced from 

 the numerous diagriims which have been obtaini-d, 1 cannot say that; 

 they do more than ciuitirm the riisults which 1 have already comninni- 

 •cated to the llritish Association. The greater nundier of shocks had a 

 duration of from twenty to sixty seconds, but some lasted mon; than four 

 minutes. Tho duration recorded depended on tho situation of tho 

 •oh.scrver, and on t' <j nature of tho instruments. Two observers, with 

 similar instruments, two or three hundred yards apart, might con- 

 f^iderably diil'er as to the length of time assigned for tho duration of a 

 disturbance. If one observer was situated on a marsh whilst the other 

 was on iiai'd ground, the formei* woidd record the longer time I'or tho 

 duration of sensible moticm. 



An instrument with a large niultijdying index, and sensitive to small 

 ^)ut ([uiek movements, will ol'teu eommence to write a record before an 

 ■justiniment which has only a small multiplying ])ower. Again, an instru- 

 ment with very little friction, and susceptible to very slow movements, 

 will contin\ie to write a record, after an instrument with considei-ablo 

 friction has ceased to move. Strictly speaking it would appear that 

 the whole of an earthquake has never yet been recorded ; many of tho 

 preliminary tremors at the coniniencoment of a disturbance and the slow 

 pulsations whieh bring a distui-banco to a close being lost. 



The preliminary tremors have an amplitude which is a small fraction 



of a millimeter, and a period of twenty-five secouds to sixteen seconds. 



Tlie tremors may be followed by a shock which consists of three or fonr 



very rapidly performed back and forth motions, having an amplitude of 



from one to ten millimeters. The maximum acceleration during such a 



movement, calculated on tho assumption of harmonic motion, sometimes 



reaches live hundred millhueters [vr second. Such a shock is on tho 



verge of being dangerous, hi ordiniiry disturbances it is from ten to 



j one hundred millimeters per second. After tho shock wo get a series of 



I irregular motions, perhaps accompanied by other shocks. These irregular 



motions are tho chief features in ordinary disturbances, and the tremor.s 



and shock may he absent. The maximum amplitude recorded is from a 



jtraction of a millimeter to one or two nulllmeters. The period is from 



five seconds to three seconds. TIk^ direction of motion of these irrej^ular 



jvihrations constantly chauires. They do not appear to havo any direct; 



Icomiection with that, in which the earthquake is being propagated. Tho 



jdirection of a shock, however, seems to coincide with the direction of 



]propagat'on. 



k2 



