210 



EARTHQUAKE PHENOMENA. 



[Sect. VII. 



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vious that in whatever direction the h)g may fall, it Mill 

 arrest the motion of the pendulum and stop the clock 

 within less than a second of the true time of transit of the 

 wave at the spot. 



If the adjustments are similar for all the clocks this 

 error v/ill be constant for them all ; and if the true time 

 be noted at the principal station it can be got for the 



rest. 



Clocks with seconds pendulums cmly should be chosen 

 for this use. They should be all set by one chronometer, 

 and their errors afterwards taken. 



Where conyenient, the pendulupas should be all placed 

 to swing north and soutli, or east and west ; and in this 

 case the sides of the logs will face the cardinal points, 

 and the directions of their fall (where not entangled) be a 

 rude index of that of the wave. It will be also desirable 

 to place a bovvd of fluid to mark direction with each 

 clock. 



The positions chosen for the clocks must vary with cir- 

 cumstances, but they should, as far as possible, surround 

 the principal station ; their distances apart must be con- 

 siderable, as the speed of the wave or shock is immense 



probably five miles is the ordinary minimum, and thirty 

 to fifty miles a convenient maximum. Such arrangements 

 should be made as rapidly as possible after the first shock 

 has given the expectation of others in succession. 



When practicable, the following method of fitting 



conmion clocks may be adopted advantageously. — Let 

 a^ Fig. 6. be the pendulum-bob ; fix a pin of stout wire 

 into a hole in the centre of it, J, at right angles to the 

 plane of vibration ; cut two small mortices through the 



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