82 



revolving phites. Tliis apparatus is to be started by the same con- 

 tact which starts the plates of [B] and [E] and actuates [I] ; and it 

 must be arranged to cease acting before one revolution of the plates 

 is com])leted, else the marks will become nninteliigible. 

 The use of [J] is to allow tlie conteiiiporarv parts of all tlie records (excej)t 

 those of [D] ) ti) be readily identified, and to sliow at what part of the disturbance 

 the time-taking apjiliance has acted. It also provides, on the plates, a scale of 

 time which is convenient in determining the period, etc. of the waves. 



For [A] it appears difficult to iind any instrument superior to the horizonttd 

 pendulum with light pivotted frame and pivotted bob (§ 24). The construction 

 shown in Plate II will !)e found very convenient; but the stand, there repre- 

 sented as made of wood, might with advantage be entirely of metal. This would 

 probably prevent, in part, a tendency to "creep" which the pointers are found to 

 iw&se.ss. The instrument has the advantage of great compactness, facility of 

 transport, and an almost entire freedom from liability to get out of adjustment, 

 or out of order in any way. 



[1]] is desirable as a supplement to [A] in oidcr that wc may have a record 

 which is not obscured by the somewhat broad datum-line forimd when | AJ runs 

 for some time, c-ither before or after the eartlw|uak('.* The driving-clock for (]5| 

 and [K| should of course be arranged to run for no more tiian a few minutes, so 

 that tiie record may not be obliterated by "creejiing" after the disturbance 

 is over. [Bj may suitably consist nf a jiair of horizontal jiendidums with 

 flexil>le jiiints at one or both of the twn points wliich determine the axis of 

 supjiort of eac-h (§ 27 or § 2X). Their eijuilibriuni may be more nearly neutral 

 than that of [A], and their frictional resistance shoulil be reduced to (he lowest 

 jjossible value. 



[C] may conveniently consist of a long pendulum like that of Plate VIII. 

 By j)lacing a weight on the nudti])]ying lever, near the top, we may approximate 

 to the duplex pendulum of § 41, and in tliat case tlie length may be reduced. 

 The record given when a double-freedom horizontal seismograjiii writes with one 

 pointer on a revolving plate is not so completely intelligible as a pair of comj-.o- 

 nent records, each showing motion in one azinmth, for it merges the tangential 

 jiartof the earthquake motion in the continuous motion of the plate itself; but 

 the single-pointer jilan has the merit of showing, at a glance, the kind o{ motion 

 and the changes of direction which occur during the di>turbance. These are, 

 of course, deducible from two-component records, but only by a somewhat 



* An iiist.incfi of tlio usefulness of .in a|.]iarnUis of this liind was furnished hy a recent very 

 sliglit earlliquake, the niaxinuiin amplitude of whose motions was so small that the wide datum- 

 lines on two continuously running plates obscured, almost completely, the records traced on 

 them. The disturbance, however, actuated an electric seismoscope and started a third plate, on 

 which a very clear record was given by a pair of horizontal pendulums and by the vertical-motion 

 seismograph of S 48. The greatest horizontal motion was about 0.3 mm., and the greatest 

 vertical motion about 0.1 mm. 



