livyal Institution, 313 



selection was known, every possible facility and assistance were 

 given by the owners of the mine. Arrangements were made for 

 preparing an expedition on a scale sufficient to overcome all antici- 

 pated difficulties. A considerable part of the expense was met by a 

 grant from the Board of Admiralty. The Electric Telegraph Com- 

 pany, with great liberality, contributed (unsolicited) the skill and 

 labour required in the galvanic mountings. The principal instru- 

 ments were lent by the Royal Society. Two observers were fur- 

 nished by the Royal Observatory, one by the Durham Observatory, 

 one by the Oxford Observatory, one by the Cambridge Observatory, 

 and one by the private observatory of Red Hill (Mr. Carrington's). 

 Mr. Dunkin, of the Royal Observatory, had the immediate superin- 

 tendence of the observations. 



The two stations selected were exactly in the same vertical, ex- 

 cellently walled, floored, and ceiled; the lower station, in particular, 

 was a most comfortable room, or rather suite of rooms. Every care 

 was taken for solidity of foundation and steadiness of temperature. 

 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 pen- 

 dulum an illuminated disc, of diameter nearly equal to the breadth 

 of the tail of the invariable pendulum ; and between the two pen- 

 dulums was a chink or opening of two plates of metal, which ad- 

 mitted of adjustment, and was opened very nearly to the same 

 breadth as the disc. 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 pendulum hides the illuminated 

 disc 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 dis- 

 appearance determines a coincidence 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 com- 

 mitted, the proportion is only altered to 397 : 399, which differs by 

 an almost insignificant quantity. Thus the observation, in itself ex- 

 tremely rude, gives results of very great accuracy. As the propor- 

 tion 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 invariable- 

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

 for the expansion of the metal (depending on the thermometer- 

 reading), for the arc of vibration, and for the buoyancy in air (de- 

 pending on the barometer-reading). 



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

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

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



