Seismological Investigations. 105 



simple arrangement for keeping this float in position is em- 

 ployed and the real contact is between two platinum surfaces, 

 being thus much more desirable and lasting. 



Several other forms of seismoscopes have been suggested and 

 tried with more or less success. One of these is a modification 

 of the well known Zollner's horizontal pendulum, the extreme 

 sensitiveness to change in level of which has been so well shown 

 by Professor Rood.* It is easy to arrange this so that an elec- 

 tric circuit is closed when a disturbance occurs ; and, although 

 its greatest sensitiveness is for disturbance in one plane it is 

 generally sufficiently delicate to respond to very feeble motions 

 in all azimuths. 



I have recently modified . the instrument first described, re- 

 ducing its dimensions and cost, and greatly increasing the ease 

 with which it is adjusted and "set." The alteration consists 

 principally in extending the multiplying needle upward instead 

 of downward. This brings the circuit-closing part of the ap- 

 paratus above, where it is open to inspection and convenient for 

 adjustment. The supporting frame rests upon a square or tri- 

 angular base, and can be placed upon any convenient pier or 

 table, instead of necessarily being screwed to a vertical support 

 as in the earlier form. The latter plan was adopted with a 

 view to fastening the instrument to a post driven in the earth, 

 but it has been found inconvenient in practice, and it will gen 

 erally be better to rest it upon the top of the post, if one is 

 used, or upon a stone imbedded in the earth, or upon a bracket 

 shelf attached to a foundation wall. 



The new form of the instrument is conveniently covered by 

 an ordinary glass shade to protect it from dust and disturbance 

 by air currents. An improvement is made in the arrangement 

 for adjusting the position of the circuit closer, which is held to 

 the table upon which it rests by means of a spiral spring, so that 

 while it moves freely upon the application of a slight pressure 

 it is sufficiently firm to resist accidental disturbances. The new 

 form will be readily understood by examination of fig. 2 (p. 103.) 



Any mechanical device by means of which a temporarily 

 "steady point" is provid_ed may be utilized as a seismoscope. 

 The horizontal pendulum, first suggested by Chaplin and utilized 

 by Ewing in his seismograph, satisfies the requirements, except 

 that its sensitiveness exists in only one plane. The use of some 

 form of link motion for an astatic suspension was suggested by 

 Professor West a few years ago, at a meeting of the Seismolog- 

 ical Society of Japan.-}* It is clear that an ideal arrangement 

 would be the Peaucellier linkage for straight line motion in a 

 horizontal plane. But it is difficult if not impossible to avoid 



* This Journal, III, June, 1875. f Trans. Seis. Soc. Japan, vol. vi. 



