«9 



Four balls ap^ nsrd, facing tlie four fardinal points. Wlu'u any oik- of the lialls 

 is project«! hy au cartlupiake out of its L-snpport, it runs up an inclined plane 

 and then falls liack a>rain, and the interval hclwecn its Icavintj and rcturninir to 

 the suppvrt is r(■!Ji^tl.'red hy means of a <;alvanie chronoj^raph. The expoot:ition 

 was, that the tirst arrival of a .shock would displace tlie i'all facinii- the (>ri<j:in of 

 disturhance, and hence, by breakinjj an electric contact, would register tlie instant of 

 arrival. Then the ball facinj; in the opposite direction would be carried forward 

 iiy its L-support, and s<i aopiire a vek>city ei|ual to the niaxinunn velocity of a 

 .»surface particle. When this maximum was passed, it would leave its sujiport 

 and run up and back asrain alonj; the inclined ])lane facint;; it. The interval of 

 its abs<'nc(i from its supjiort woidd be a function of the velocity of displacement, 

 in calculatinjr which th<' author ignores those movements of the suj)port which 

 take place during the interval in (juestion. The supports for the balls are fixed to 

 a ]>latforni held up by a spii-al spring, whose compression was intended to give 

 some information as to the vertical component. 



A simpler form consi.sts of two paii-s of Ijalls .set, in L-.shaped .snp]>orts, on 

 the tojjs of jiillars, round the base of which damp .<and is placed. The velocity 

 of ]>r"jw-tiiin of any ball is givfu iiy the horizontal distance from the foot of the 

 pillar to the |ioint where it falls. 



Aj)art from the erroi-s which these metliods of «led rmination involve, a 

 knowle<lge of the maxinuim horizontal velocity of a jiarticlc on the earth's sur- 

 face is not of great interest, and furnishes, of itself, a very incomplete account of 

 the motion. 



§ 66. I'euththimy inlvnrhd to fuhirj. 



Attempts wc ri' made (befoi-c llie introduclion of tlii' absuliite measnriu'^' iii- 

 strnments des<TilMd in (■ha|)ters II and III) by Mr. Milne,* by Mr. Gray.f and 

 also by the present writi-r, to determine roughly the period of an e:irth(juake's 

 vibnitions by seeing what length a ])cndu1um (or other bodv ca|)abli' of os<'il- 

 lation) should have in order to l»; .set into mo>t violent motion liv ihr ilisturlianec. 

 In th«' author's ex|)eriments a s>-t of inverted pendulums were set up, consisting 

 of had balls stuck on the ends of upright wires oi" various Icnijfhsand diameter!-- 

 Ivuh was provided with a simple index, consisting of a thrca<l which was at- 

 tacheil to the top of the ball, and passed through a hi«Ie in a fixed plate above, 

 on the surfu'c of which it re>(cd. The pendulum whose free jwiiod agreed most 

 «•loM'ly with that of the rarfhi|Uake wav<s would sh()W that it had been most 

 violently ilisturbcd by pulling its thnad farthest through the hole. I5ut the 

 pcri-nl (if till- wa\i- is too irregular to make tl bsc^ivatinti of much value. 



^' liT. I'lrrij mill Aijitoug propn.ied fjilnmoi/niph. 

 .\ univii->al sci-mograph, who-«' ai-l ion docs not depend on the pnwlni'tion of 

 a -teady or a|>|iroxin)ntely .«tcinly jKiint, Uns In-en propoM«! bv Mes.>^rs. I'errv and 



Tmim. iif thi; Scimiioloirical StK-iuly of Jiibaii, Vdl. Ill, ii. -JK. 

 t fhi.l. ).. W. 



