83 



tedious process, which it is practically difficult to apply when the motion of tlio 

 plate is slow and the period of the waves is short.* 



The same steady ]ioint — namely, that of [CJ — may be used to write a pair 

 of two-coni])oncnt records : Hi is is easily done when the elVt.ctive inertia is great, 

 as in the instrument of Date VIII, where tlie arrangement in (jiiestion is 

 adojitcd (figs. 27 and 28). 



For a static record, [D], the duplex ])endulnm will he found convenient. 

 There is, however, no reason why we should not take a ."^tatic record, as well as 

 a record on a moving plate, from the same long pendulum as gives [C'J, i)rovided 

 its efleetive inertia is so great as to make the friction of an additional marking 

 pointer scarcely noticeable. To do this a second transversely hinged pointer 

 miglit project from the vertical multiplying lever in fig. 25, Plate VIII, and 

 write on a fixed plate carried bv a bracket above or alongside of the revolving 

 plate. 



A double-freedom instrument might be made for [C] or [D] by combining 

 two single-freedom seismometei-s so that, instead of tracing separate records, their 

 motions should be compounded by a separate marking lever, pivotted with freedom 

 to move in any azimuth. It is scarcely likely, however, that this j)]an would 

 have any advantage as compared with the use of a double-freedom instrument of 

 the kind already described ; and it would of course involve a greater number of 

 part.s. For [E] either of the horizontal bar instruments described in §§ 4G-48 

 is suitable. The circuit-closers of Palmieri and Rossi apjiear perfectly satis- 

 fa<-tory for [M]; and Mr. Milne's time-taker should servo well for [I]. 



The various .seismographs may usefully be constructed ^\■i(ll dillerent ratios 

 of nudtiplication, appropriate to earthquakes of various amplitudes. One, pre- 

 ferably [C], should be cajialde of a considerable range of movement, and should 

 Im! provided with a snppii'mentary pointer, writing witiiout nuiltiplication, to 

 .M-rve for tlic registration of umisually large shock.«, who.sc aniplitude is so great 

 as to throw the ordinary indices out of gear. 



§ 83. Determination nf the Direction and Velocity of Tranxit. 



The foregoing account of methods of earthi|uake measurement ha.s referred 

 entirely to the det(>rmination of tlie character of the motions at a single observing 

 .st.atiryn. Anotlicr |>robh'm, nf much interest, is the determination of tlie direction 

 anci vcIiK-ily (if propagation of tlie earliiijuakc waves. The dii-ection, especially, 

 is ini|Mirtant on account of tlie information it gives a.s to the position of the origin. 

 If we know, rigorously, the time of arrival of an earth-wave at three stations on 

 the surface, and assume its velocity aii<l direction of propagation to be coiL-^tant 

 lietween them, an obvious geometrical solution givc's the direction in azimuth 



• To fncililalo llio proeoH« of compounding the principnl molionB recorded l>y n two-compo- 

 nent inntrumi'Ml, tlio plato, Itoforn lM.>inf( witlnlrnwn u(\vr an earlhi|imki', .stiniild Im tiinind round 

 liy hniid, and held nuccoHHivcly in a niimlNT of posilioiiR, wliilu liiicR, liUu tliosi.' in I'latc XIX, 

 arc iiininltancously drawn by dixplacing both pointers. 



