APPENDIX. 



03 



vised, and, for moderate shocks, the accuracy with which it gives the horizontal direc- 

 tion of motion of the wave particles, make it particularly useful. In the case of a 

 great earthquake, one should, if no other instruments are set up, be immediately impro- 

 vised, and the intervals between, and direction of, subsequent shocks be noted. 



(2) This is one of the most untrustworthy forms of seismometer that has been 

 suggested. It is impossible to arrange it so that the centre of gravity shall lie in the 

 axis of oscillation, and consequently the pendulum, first set in motion in a vertical 

 plane, will very soon acquire a circular motion ; added to which, the dragging of the 

 bob by violent shocks, and the subsequent throwing of it forwards, are more noticeable 

 than in the first described form. 



(3) This is undoubtedly that form of pendulum seismometer which is least open 

 to objection ; but as it is more troublesome to construct than the instrument which will 

 be described below, while at the same time its observations are both less exact and not 

 comparable with those of other instruments not very carefully constructed to be du- 

 plicates of itself, we will not here describe it further. 



These pendulum seismometers, though under certain circumstances of great value, 

 should be regarded rather as makeshifts than as instruments to be permanently es- 

 tablished ; but the two which will be now described differ from them in that they give 

 absolute and not merely roughly comparative determinations of the velocity of motion 

 of the wave particles. 



The first of these depends on the circumstance that a solid right cylinder will be 

 overturned towards the point from which an earthquake shock comes, and that it re- 

 quires a definite minimum force to overthrow it, which can be readily calculated. In 



fig. 2 we have given 

 a rough perspective 

 view of an instru- 

 ment of this sort, 

 which is copied from 

 Mr. E. Mallet's arti- 

 cle in the Admiralty 

 Manual of scientific 

 enquiry. The ins- 

 trument consists of 

 two series of right 

 cylinders, all of equal 

 altitude but of gra- 

 dually diminishing 

 diameters, the two 

 series being exactly 

 similar. These 

 stand on a pair of 

 slips of wood, which 

 ( 93 } 



Fig 2. 



