■p' 



244 



REPORT- -1H81. 



.; I 



As tlio (liHturbanro dies nut tho period of ilicsc irregular movcmoiits 

 increascH, iiiul waves with ii [icriod of two (»:• tlireo Hoconds havo Itccii 

 recorded. 



For information respoetinfif the veloeity of propat^ation, I will vfer 

 to my report ol" IHHl, wliere .some general results wen; iriven. i sliortiv 

 expect to be able to give more delinito information on this subject. 



E.qycrhnc)itii oa /he Direction of Motion of a Tutnt. 



Hitherto the only means that we havo had at our disposal for de- 

 termiiiing tho direction of motion of a point, has l)eon cither to eombiiie 

 tho records of two rectangular components, or to trace a few of the nioro 

 conspicuous curves in a record given by a seismograph writing on a 

 stationary plate. JJoth of these methods can only be applied to jn'oniinciit 

 vibrations in a record, and each of them, unless under ppeeiul circiiin- 

 stances, is liable to error. Tho records giv(>n by seismographs with 

 single indices writing on moving plates, aro ibr several reasons also open 

 to error, especially perhaps on account of the friction of tho moving plivto 

 exerting a drag on the recording index. To partially overcome these 

 difficulties, I have constructed a record receiver which works as follows: 

 — Shortly after tho commeuceraent of the disturbance, tho smoked plate 

 on which the index of a .seismograph is writing, is suddenly dropped 

 vertically out of range of tho index. It is next pushed along horizontally, 

 and then raised vertically back to its original level, so that it is again in 

 contact with tho recording point. 



This operation is quickly repeated twelve limes, at intervals of every 

 two seconds, so that twelve ditlei'cnt diagi'amii are obtaitied on a strip of 

 smoked glass, each one being written on a different part of the plate. In 

 this way all etfects of drag produced by ho moving plate upon the pointer 

 are eliminated. As T have thus fur only obtained one set of diagram.^, I 

 must reserve a description of the results until a future occasion. 



The simullancoua ohservatlon of Earth iiual;c!i at three stations in 

 Teleijraphic connection. 



Tho advantages to bo gained by tho observation of earthquakes at three 

 or more stations ir telegraphic connection were first definitely pointed 

 out by Professor J. A. Ewiug, in a communication to the Seismologieal 

 Society. A very similar method had, however, been previously followed 

 by ]\Ir. T. Gray and myself, in our observations on artiOcially produced 

 disturbances. 



Tho method which I am now following is briefly as follows : — Near i 

 to my house I havo established, at tho corners of a triangle, the sides oti 

 which are each approximately 800 feet, similar instruments. These are 

 fixed on the heads of stakes level with the surface of tho ground. The | 

 I'ocords are written on smoked glass plates which at the time of an earth- 

 quake are drawn by means of a falling weight beneath the writing indices. 

 By means of electrical connections, these plates are simultaneously set in I 

 motion by the withdrawal of a catch. As they move along, time intervals 

 are marked by levers deflected by electro-magnets every time a small 

 pendulum passes a cup of mercury. The pendulum, which is usually beWl 



diff 

 .Sta 

 .'I 111 

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Stat: 



