422 



EARTHQUAKE PHENOMENA. 



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 will be 

 constant for them all; and if the true time he noted at the principal 

 station it can be got for the rest. 



Clocks with seconds pendulums only, should be chosen for this use. 

 They should be all set by one chronometer, and their errors after- 

 wards taken. • 



Where convenient, the pendulums should be all placed to swing 

 north and south, 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 tub of fluid to mark direction with each 

 clock. 



The positions chosen for the clocks must vary with circumstances, 

 but they should, as far as possible, surround the principal station; 

 their distances apart must be considerable, as the sp%ed of the wave 

 or shock is immense — probably five miles is the ordinary minimum, 

 and thirty to fifty miles a convenient maximum distance. Such 

 arrangements should be made as rapidly as possible after the first 

 shock has given the expectation of others to succeed. 



When practicable, the following method of fitting common clocks 

 may be advantageously adopted. Let a, fig. 8, be the pendulum- 

 bob ; fix a pin of stout wire into a hole in the centre of it, b, at right 

 angles to the plane of vibration; cut two small mortices through the 

 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 rack, of about 

 three-eighths of an inch in depth and breadth each. Place the lower 

 edge of the lath horizontally, and just above and clear of the pin b, 



secure the end of the lath d 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 d as a centre. Prepare a vertical log of wood/, of the 

 size and form already described, but cut its upper end to a square 

 pyramid, the flat surface at the top being reduced to about a quarter 



