Sect. VII.] 



EARTHQUAKE niENOMENA 



211 



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sides of the clock-case, so that a lath of deal or other 

 light wood, of about an inch and a half wide by a quarter 

 of an inch thick, may be passed through from c to d, just 

 in front of the bob and clear of it. 



Mark the length of the arc of vibration on the lower 

 edge of the lath, and cut this length into nicks or teeth 

 like a racli, of about three-eighths of an inch in depth 

 and breadth each. Place the lower edge of the lath 

 boijzontally, and just above and clear of the pin b; 

 secure the end of the lath J by a wire pin or stud, as a 

 fixed point, so that the end c is free to move in an arc of 

 a few inches up and down round ^ as a centre. 



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descril)ed, but cut its upper end to a square pyramid, the 

 flat surface at the top being reduced to about a quarter 

 of an inch square ; adjust the length and position of the 

 log, so that it shall form a support for the end of the 

 lath c, as in the figure. 



It is obvious that the moment the log / is overthrown 

 by a shock the lath will drop at the end c (which should 

 be slightly weighted), and the teeth or rack nicks catch- 



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