EARTHQUAKE PHENOMENA. 



421 



observe this, but such observations can seldom be very numerous or 

 extend over a large tract of country, and without automatic instru- 

 ments shocks are almost certain to be missed at one or more stations; 

 yet it is most desirable that a network of such observing points should 

 be stretched over the shaken country. For this purpose common 

 house-clocks, situated at several distant points, may bo easily ar- 

 ranged, so that the pendulum shall be brought to rest and the clock 

 stopped at the moment that the shock passes. 



Fig. 7 shows part of the case and pendulum of a common clock. To 

 fit it for this purpose, bore two holes <5f a quarter of an inch diameter, 

 one through either side of the clock case, at a b, at the level of the 

 lowest point of the pendulum-bob, and in the plane of its vibration; 

 round off the edges of these holes, and grease them. 



In the centre of a piece of fishing-line or stretched whip-cord 

 make a loop and pass it round the screw or other lower projection of 

 the pendulum-bob; pass the two free ends of the cord out, one through 

 each of the holes in the sides of the clock-case; provide a squared log 

 of heavy wood of about five or six inches thick each way, and from 

 four to five feet in height; cut both ends off square, and stand the log 

 upright on one end directly opposite the dial of the clock. 



Measure off equal lengths of the cord at each side of the pendulum, 

 and make fast their extremities to the two opposite sides of the up- 

 right log, c d, close to the top; bring the log backwards from the 

 clock now, until the pendulum being at rest, both cords are drawn 

 tight; and then advance it two or three inches towards the clock, so 

 that the cords may be slacked down into a festoon or bend at each 

 side of the pendulum, and within the clock-case, so that the pendulum 

 may have room to swing freely; and very slightly wedge the cord to 

 keep it so, through the holes in the clock-case, and from the outside; 

 see that the log rests firmly and upright upon a firm floor; and now 

 set the clock going. The length of the cords, or the distance of _ the 

 log from the clock in relation to its height, must be such that if it 

 fall towards the clock it shall bring the cords up tight before the 

 upper part of the log touches the ground. It is now obviousthat, 

 -vin whatever direction the log may fall, it will arrest the motion of 



