298 
MR. AIRY’S ACCOUNT OF THE HARTON EXPERIMENT 
vibrations of a seconds’ pendulum). Then ^ — 2 • ^"3^^ • 439 QQQ oi’ the mean 
density would be determined with an error not exceeding 4-oth part of the whole. 
This error, I apprehend, is far less than those to which Cavendish’s experiments 
were liable (Reich’s and Baily’s experiments had not been made at the date of this 
investigation, but I do not except thetn from the same remark). It is also quite as 
small as that to which the mere astronomical determination in the Schehallien 
experiment was subject. When other elements of calculation are examined, as the 
general simplicity in the form of the ground for the mine-experiment and the com- 
plexity for the mountain-experiment, the accurate knowledge of the geology for the 
mine-experiment and the obscurity for the mountain-experiment, the difference 
becomes still more striking in favour of the mine-experiment as compared with that 
on Schehallien. 
4. In a subsequent section, means will be given for enabling the reader to judge 
whether an accuracy like OT vibration per day in the difference of the rates of pen- 
dulums above and below, in tlie mine-experiment, has really been obtained. Methods 
will also be indicated for computing the corrections depending on the irregularities 
of the earth’s surface. I advert to these at present only to have the opportunity of 
explaining that the form of computation exhibited above is not final, but is merely 
intended for a preliminary calculation, showing the antecedent plausibility of the 
experiment. 
5. Upon communicating my views to Mr. (now Dr.) Whewell (then, like myself, 
a resident Fellow of Trinity College, Cambridge), I found him entirely disposed to 
join me in undertaking the experiment. My first idea had been, to ascertain the 
rate of a clock at the top and the bottom of a mine; and the locality which first 
occurred to me was the Ecton Mine in Staffordshire. But it was soon settled, on 
discussion with Mr. Whewell, that detached pendulums would be preferable to 
clock pendulums; and, the Ecton Mine having proved on examination to be ill 
suited to our purpose, the Dolcoath Mine near Camborne in Cornwall was selected 
(partly at the suggestion of John Taylor, Esq.). The Royal Society and the Board 
of Longitude most liberally lent to os invariable pendulums, clocks, and other 
apparatus, sufficient for the simultaneous observation of a pendulum above and one 
below ; and the Board of Admiralty lent us box and pocket chronometers, to be used 
principally for the comparison of the upper and lower clocks. 
6. About the end of May 1826 we proceeded to Dolcoath, assisted by the friendly 
introductions of the late Davies Gilbert, Esq. and of Dr. Paris ; and we received 
from the resident authorities of the mine every possible assistance to our experiment ; 
and from the late Lord De Dunstanville, E. W. W. Pendarves, Esq., and other 
gentlemen of the country the most hospitable attention to our personal comforts. 
And, in spite of the labours and misfortunes of the Cornish enterprises, I do not doubt 
that they are regarded by my companion (as well as by myself) as among the most 
