94 



APPENDIX. 



taper so that their width shall equal the diameter of the cylinders standing on them 

 at various points, the two slips being placed one due north and south, the other east 

 and west ; soft, dry sand is then filled in up to the level of the two slips. Under 

 the influence of an earthquake shock a greater or less number of the cylinders are 

 overthrown, and the horizontal component of the shock is then intermediate between 

 that required for the overthrow of the largest cylinder overturned and the smallest 

 still standing. 



The cylinders may be made of stone, pottery, metal, or any other homogeneous 

 material whose density is immaterial, as the overthrow is due to inertia, which is 

 proportional to the mass : probably the most convenient material will be found to be 

 good, straight-grained, seasoned wood, free from knots, which can be turned to the 

 requisite diameter with great ease. The dimensions for a series' of cylinders, 12 inches 

 in height, are given below, with the horizontal velocity of shock required for their 

 respective overthrow : — 





Diameter of cylinder 

 in inches. 



Horizontal velocity of shock for 

 overthrow in feet per second. 



1 



11| inches. 



5-54 



2 



10S 



•■■"4, » 



5-08 



3 



QS. 



04. i) 



4-48 



4 



9 



4-05 . 



5 



8 



3-50 



6 



7 



2-98 



7 



6 



2-50 



8 



5 



2-03 



9 



3| „ 



1-49 



10 



%i >) 



•98 



11 



■•■2 J) 



•57 







It will be seen that this form of seismometer is only adapted to the measurement 

 of moderate shocks, but it is just these which are most important from a scientific 

 point of view. 



As regards the fixing of the instrument, the following modification of Mr. 

 Mallet's plan will, we think, be found advantageous, as it allows of equally accurate 

 results being obtained with a single series of cylinders ; or if the same total number is 

 used, the instrument may be made to give more closely accurate results. If, then, 

 it be determined to set up an apparatus consisting of 11 cylinders, as given in the 

 table, a patch of ground, 12 feet by 10 feet, should first be carefully levelled and 

 smoothed, with the longer axis running preferably north and south. Then let a 

 series of stands be made for the cylinders, each exactly the same diameter as the 

 ( 94 ) 



