210 REPORT — 1861. 



an achromatic telescope (a) with a single wire is similarly adjusted, so that 

 when the moveable blackened cover (e e) is placed over the trough, &c., no 

 light can reach the surface of the mercury except through the tube c. The 

 image of the cross wires in the latter is therefore seen through the tele- 

 scope a, clearly reflected and defined in the surface of the mercury, so long as 

 the fluid metal remains absolutely at rest ; but the moment the slightest 

 vibration or disturbance is by any means communicated to the instrument, 

 the surface of the fluid mirror is disturbed, and the image is distorted, 

 or generally disappears totally. The telescope magnifies 11*39 times 

 linearly, and the total magnifying power of the instrument to exalt the 

 manifestation to the eye of any slight disturbance of the mercurial mirror is 

 nearly twenty-three times. Its actual sensibility is extremely great. In 

 the present case, however, this was not needful, as the impulse transmitted 

 from these powerful explosions produced in all cases the most complete 

 obliteration of the image, and in those of the most powerful mines experi- 

 mented on caused a movement in the mercury of the trough that would 

 have been visible to the naked eye. Indeed, in that of the S-ith Novem- 

 ber, 1860, the amplitude of the wave that reached the seismoscope was so 

 great as to cause the mercury to sway forwards and backwards in the trough 

 to a depth that might have been measured. 



After the earth-wave has reached This instrument, a certain interval of 

 time is necessary for the production of the wave in the mercury, and for its 

 transit from the end of the trough next c, where it is produced, to the mid- 

 length where it is observed. This involves a correction in the gross transit- 

 time as observed with it. For the methods by which the constant for this 

 (seismoscope correction) was determined I must refer again to Report of 

 Brit. Assoc. 1851, pp. 280, 281. It amounts to 0"'065 in time ; and as the 

 eflect of this will in every observation appear to delay the arrival of the 

 earth-wave at the instrument, this constant in time, converted into distance, 

 must be added to the rate of wave-transit otherwise obtained. 



The chronograph (originally devised by Wheatstone) is shown in fig. 1*, 

 PI. IV. It consists, in fact, of a small and finely made clock, deprived of its 

 pendulum, but provided with a suitable detent (shown more at large in 

 fig. 4*), by which the action of the weight upon it is kept always arrested, 

 but can immediately be permitted to take place in giving it motion, upon 

 pressing the band quickly upon the lever g. 



The running down of the weight causes the anchor and pallets of the 

 escapement (k) rapidly to pass the teeth of the escapement-wheel («), so that 

 the clock " runs down " by a succession of minute descents ; and thus the 

 motion is practically a uniform one. It follows that as more weight is added 

 this velocity becomes greater, and by such addition the instrument may be 

 made to measure more and more minute fractions of time. 



It registers time upon two dials (fig. 2*), each with an index : one of these is 

 fixed on the axis of the escapement-wheel (a), and its dial is divided into thirty 

 smaller and six larger divisions ; the pinion on this axis is to the wheel 

 upon the weight-barrel (5) as 1 : 12. This carries the other index, and its dial 

 has twelve divisions, so that one of its divisions corresponds to an entire 

 revolution of the former one. The value in actual mean time due to the 

 movement of the instrument, as thus recorded, requires to be ascertained by 

 reference to a clock beating seconds, so that the number of revolutions of 

 the index b, and parts of revolutions of that of a, during an interval of, say, 

 30 seconds, may be determined by the mean of several experiments. For 

 the methods of performing this with the necessary correctness, I again refer to 



