1855.] Pendulum-experiments at Harton Colliery. 21 



care was taken for solidity of foundation and steadiness of tem- 

 perature. In each (the upper and the lower) was mounted an 

 invariable brass pendulum, vibrating by means of a steel knife edge 

 upon plates of agate, carried by a very firm iron stand. Close 

 behind it, upon an independent stand, was a clock, carrying upon 

 the bob of its pendulum an illuminated disk, of diameter nearly 

 equal to the breadth of the tail of the invariable pendulum ; and 

 between the two pendulums was a chink or opening of two plates 

 of metal, which admitted of adjustment, and was opened very 

 nearly to the same breadth as the disk. To view these, a telescope 

 was fixed in a wall, and the observer was seated in another room. 

 When the invariable pendulum and the clock pendulum pass the 

 central points of vibration at the same instant, the invariable pen- 

 dulum hides the illuminated disk as it passes the chink, and it 

 is not seen at all. At other times it is seen in passing the chink. 

 The observation, then, of this disappearance determines a coinci- 

 dence with great precision. Suppose the next coincidence occurs 

 after 400 seconds. Then the invariable pendulum (swinging more 

 slowly), has lost exactly two swings upon the clock pendulum, or 

 the proportion of its swings to those of the clock pendulum is 

 398 : 400. If an error of a second has been committed, the pro- 

 portion is only altered to 397 : 399, which differs by an almost 

 insignificant quantity. Thus the observation, in itself extremely 

 rude, gives results of very great accuracy. As the proportion 

 of invariable-pendulum-swings to clock-pendulum-swings is thus 

 found, and as the clock-pendulum-swings in any required time 

 are counted by the clock dial, the corresponding number of in- 

 variable-pendulum-swings is at once found. Corrections are then 

 required for the expansion of the metal (depending on the ther- 

 mometer-reading), for the arc of vibration, and for the buoyancy in 

 air (depending on the barometer-reading). 



But when the corrected proportion of upper-invariable-pendulum- 

 swings to upper-clock-pendulum-swings is found, and the proportion 

 of lower-invariable-pendulum-swings to lower clock-pendulum- 

 swings is found, there is yet another thing required : — namely, 

 the proportion of upper-clock-pendulum-swings to lower-clock- 

 pendulum-swings in the same time ; or, in other words, the pro- 

 portion of the clock rates. It was for this that the galvanic signals 

 were required. A galvanometer was attached to each clock, and 

 an apparatus was provided in a small auxiliary clock, which com- 

 pleted a circuit at every 15 seconds nearly. The wire of this 

 circuit, passing from a small battery through the auxiliary clock, 

 then went through the upper galvanometer, then passed down the 

 shaft of the mine to the lower galvanometer, and then returned to 

 the battery. At each galvanometer there was a small apparatus 

 for breaking circuit. At times previously arranged, the circuit 

 was completed by this apparatus at both stations, and then it was 

 the duty of the observers at both stations to note the clock times of 



