14 



would give means of determining the earth's mean density with 

 accuracy, perhaps superior to that obtained in the Schehallien or the 

 Cavendish experiment ; and which induced him first in the summer 

 of 1826 (in concert with Dr. Whewell), and again in 1828 (with 

 Dr. Whewell, Mr. Sheepshanks and others), to try the experiment hi 

 the Dolcoath mine near Camhorne in Cornwall. These attempts were 

 both frustrated by accidents having no connexion with the essential 

 parts of the experiment. After a lapse of many years, he found that 

 several circumstances (of which one was the general familiarity with 

 the manipulation of the galvanic telegraph and the facility of applying 

 it to the comparison of widely separated clocks) were very favourable 

 to a repetition of the experiment ; and having selected the Harton 

 Colliery in the neighbourhood of South Shields as a fit place, in 

 which two stations could be found in exactly the same vertical but 

 at 1256 feet difference of height, and being assured of every assist- 

 ance from the owners of the mine, he proceeded with the experi- 

 ments in the months of September and October 1854. 



The principal instruments employed were two detached pen- 

 dulums on iron stands, the property of the Royal Society, which 

 were most carefully repaired by Mr. Simms ; graduated arcs, baro- 

 meters, thermometers, &c.; two clocks, one the property of the 

 Royal Society, which were fitted for this purpose with inclined 

 gilded reflectors upon the pendulum bobs, intended to be illuminated 

 by the light of lamps passing through holes in the side of the clock- 

 cases ; galvanometer-needles attached to the clock-cases, with circuit, 

 breakers ; a galvanic battery at the upper station ; a journeyman- 

 clock at the upper station, fitted with an apparatus by which it 

 completed the galvanic circuit at every 15 s of its own time ; and two 

 galvanic wires passing down the mine-shaft and forming a closed 

 circuit through the battery, the journeyman-clock, and the two 

 galvanometers. 



The working party consisted of Mr. Dunkin (superintendent) and 

 Mr. Ellis from the Royal Observatory, Mr. Pogson from the Obser- 

 vatory of Oxford, Mr. Creswick from the Observatory of Cambridge, 

 Mr. G. Riimker from the Observatory of Durham, and Mr. Sim- 

 monds from Mr. Carrington's Red Hill Observatory. 



The plan of operations was this. Simultaneous observations of 

 the two pendulums (one in the upper and the other in the lower 



