300 
MR. AIRY’S ACCOUNT OF THE HARTON EXPERIxMENT 
9. Many years passed on before sufficient leisure or sufficient motive for again 
trying the experiment presented itself to me. The only great improvement in the 
application of the pendulum to the measure of gravity was Bessel’s discovery, 
reduced to a practical form (as regarded the English construction of the pendulum) 
by Colonel Sabine, of the necessity of an increased correction for the pendulum’s 
buoyancy in the atmosphere. I had, however, opportunities of observing the dif- 
ficulties inherent in the Cavendish experiment, from witnessing the repetition of that 
experiment by my late friend Mr. F. Baily. At length, in the year 1854, a new 
power was placed in my hands. The galvanic system was established at the Royal 
Observatory, and in the familiarity which we now possessed with telegraphic appli- 
cations I perceived that the difficulty of comparing the upper and lower clocks would 
be almost entirely removed. The coal-mines of the Durham coal-field had been 
worked much deeper, and the facility of access to these mines would materially 
diminish the labour of the experiment; while the intimate acquaintance with the 
geological character of the country possessed by the coal-owners, and the general 
regularity of the beds, would give great confidence in the ultimate calculations of the 
attraction of the mass of matter principally affecting the experiment. After a lapse 
of twenty-six years from the last attempt in Cornwall, 1 therefore seriously took up 
the subject again, and proceeded personally to examine the fitness of the coal-mines 
of Durham for the experiment. 
10. Assisted by the introductions of David Lietch, Esq., M.D., and by the local 
knowledge of James Mather, Esq., I had little difficulty in fixing on a mine. The 
deepest mines in the Durham coal-field are near the coast. The deepest of all is the 
Monk Wearmouth Colliery ; but it is so close to the sea (its workings in fact extending 
far under the sea), that it seemed probable that more of disadvantage would be intro- 
duced by the complication of the elements of final computations than of advantage 
by its extreme depth. The next in depth, I believe, is the Harton Pit, at the distance 
of somewhat more than two miles from South Shields, and about the same distance 
from the coast. The general circumstances of form of surface, &c. are as favourable 
as can^usiially be found. Its depth is reputed to be 1260 feet. On making known 
to William Anderson, Esq., the principal owner of the coal-mine, my wish to try 
the experiment in that place, I was at once assured that every assistance should be 
given to me. In company with C. W. Anderson, Esq. and with G. W. Arkley, Esq. 
(local viewer and superintendent of the mine-works) in 1854, August 5, I examined 
the buildings on the surface and the workings underground in the “Bensham seam,” 
and stations for the pendulums were speedily chosen. The two stations were exactly 
in the same vertical. The upper station was a stable near the Mine Office. The 
lower station was a wide part of a gallery (now disused), less than 200 yards from 
the bottom of the shaft: in going from the bottom of the shaft to the station, one- 
half (roughly speaking) of the way passed along one of the tram-ways of the mine, 
and the other half was along the disused gallery; and at this distance the sound of 
