ON THE FACTS OF EARTHQUAKE PHiENOMENA. 285 



each an accelerated one, and each equal to the preceding, and hence the 

 run down is practically an uniform motion of the clock. It follows, that in 

 proportion as the weight is a<ided to, the velocity with which the clock runs 

 down is increased, and hence by such additions the instrument may be made 

 a measurer of more and more minute fractions of time. 



Two dials and two hands register the revolutions, and parts of revolutions, 

 made by the instrument. One hand is fixed upon the axis of the escapement- 

 wheel (a, figs. 4, 5, 6, PI. XIII.), and its dial is divided into 30 smaller and 

 6 larger divisions. The pinion on this axis has one-twelfth the number of 

 teeth in the wheel {b) upon the weight barrel shaft, which carries the other 

 hand, and hence one division of its dial (which has twelve divisions) corre- 

 ponds to a whole revolution of the former dial. 



I devoted Prof. Wheatstone's instrument to determining the time of " hang 

 fire," and distinguished it as the Firing Chronograph ; Sharp's chronograph was 

 used for determining the time of wave transit, and became known as the Ob- 

 server's Chronograph. 



I now proceed to describe the peculiar additions made to these instruments 

 for the purpose of starting and stopping their motions at the required instants ; 

 and first as to the Firing Chronograph. In PI. XIII. fig. 2. is shown a front 

 elevation of the whole instrument, a. The front of the clock and dials. 

 b. The weight composed of shot poured into a small brass bucket attached by 

 a silk string to the barrel and descending in the inside of the small clock 

 case, the front door of which is removed in the figure, c c. The base of ma- 

 hogany upon which the whole is secured. At the back of the instrument is 

 a small lever, d, shown at full size in fig. 6, which is fixed on the arbor e, 

 which carries also the lever ef. 



The large brass lever g (figs. 2 and 6) is maintained always, except while 

 under pressure from the hand, by means of a stout spring h, in close contact 

 at the end next the clock with the lever e d, and while so in contact, the 

 lever e/is held down upon the anchor k of the escapement, so as to prevent 

 any motion in the clock although wound up ; but as soon as the lever g is 

 pressed down by the application of the hand on the end m, the small spring 

 n pushes back the lever ef from the anchor k, and the clock begins to run 

 down. The moment again that the hand is removed from m, the lever m g 

 flies up, pushes back e d, and with it restores ef to its former position on 

 the anchor k, and so instantly stops the clock. 



But besides this, it was necessary that the same motion of the hand that 

 started the clock into motion, should " make contact" with two separate gal- 

 vanic batteries, one of which should act upon the other chronograph, and the 

 second fire the mine. 



The lever m g is of brass; it carries an adjustable copper wire o, dipping 

 at its lower extremity into a cup of mercury p, into which is also inserted the 

 long wire/)*, which leads to one pole of the "firing battery"; r^ is the wire 

 from the other pole dipping into the mercury cup r, which is connected by a 

 fixed wire with another flat and shallow mercury cup s placed on the top of 

 the wood pillar r s, and at such a level, that the hooked or turned down ex- 

 tremity t of the lever g m, hangs just clear by a quarter of an inch above the 

 surface of the mercury when the chronograph is at rest. 



The extremity of the lever at t is amalgamated, as are also all other con- 

 nexions. It is now obvious that the instant the lever g in is depressed a 

 quarter of an inch, by placing the hand upon it at m, contact is completed 

 through r^, r, s, t, m, o, p and p*, and the mine or other charge fired. 



But for reasons hereafter to be described, contact required to be made at 

 precisely the same instant with a second galvanic battery also; for this 



