206 



EARTHQUAKE PHENOMENA, 



[Sect. VII 



the four horizontal tube-markers and this vertical one, 

 the direction of emergence of the wave is determinable. 



These iiistriiraents are of the nature of fluid pendulums. 

 They are much superior to common solid pendulums, for 

 these uses, where the dimensions of the shocks are mo- 

 derate ; but where these are great and very violent, 

 heavy solid suspended pendulums, with a quick time of 

 vibration, will be found alone applicable : the seconds 

 pendulum for lat. Greenwich will always be desirable* 

 Where fluid pendulums are not attainable, a solid pen- 

 dulum to answer some of the purposes may be thus 

 prepared : — Fix a heavy ball, such as a four-pound 

 shot, at one end of an elastic stick, whose direction passes 

 througli its centre of gravity : a stout rattan will do. Fix 

 the stick vertically in a socket in a heavy block of wood 

 or stone, and adjust the length above the block as near 

 as may be to that of the seconds pendulum frjr Green- 



wich. 



Prepare a hoop 



of wood, or other convenient 

 material, of about eight inches diameter ; bore four 

 smooth holes through the hoop in the plane of its circle, 

 and at points ninety degrees distant from each other ; 

 adjust through each of these a smooth round rod of wood 

 (an uncut pencil will do well), and make them, 

 greasing, &c., slide freely, but with slight friction through 

 the holes. 



Secure the hoop horizontally at the level of the centre 

 of the ball by struts from the block, and the ball being in 

 the middle of the huop, slide in the four rods through 

 the hoop until just in contact with the ball. 



It is no., ^.bvious that a shock, causing the ball to os- 

 cillate in any direction, will move one or more of the rods 



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