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204 



EARTHQUAKE PHENOMENA. 



[Sect. VIL 



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eighteen iijubes long. 



We shall also require some common 

 barometer tubes of the same calibre : the open end beincr 

 turned up like an inverted syphon, and equal in bore to the 



(See Ftp, 4.) 



L 



the horiaontal, the others for the vertical elements. 



To fit the L tubes for use, fill each partly with mercury, 

 and so adjust it that a column of six inches in length shall 

 be in each limb of each tube, when held as in Fi^. 1 ; 



" r 



the limb a b horizontal, and the vertical 

 column being supported as in a baro- 

 meter. Tie four of these tubes so pre- 

 pared together, back to back, so that if 

 one horizontal limb face the north, the 



Sealed end 



Fiq. L 



5 



others shall face east. 



Sealed ends. 



, and west respectively, as in 



In this position 



ia. 2 



tube 



secure them all down 

 upon a broad stout board, 

 that can be itself fixed to 

 a surface of rock, or other 

 flat fixed surface of the 

 earth. 



An index or marker must now be prepared for each 



or ono of these cut a common piece of card two 



E 





inches long by rather more than five-eighths of an inch 

 wide, and double it down the long way, so that the two 

 segments shall stand at rather less than right angles to 



each other ; cut a cylindrical slice of cork one-eighth of 

 an inch thick, of five-sixteenths of an inch diameter, so 

 that it will go easily into the tubes (these being all three- 

 eighths of an inch in diameter) : attach the bit of cork 

 with glue or sealing-wax to the end of one wing or seg- 



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