FOR DETERMINING THE MEAN DENSITY OF THE EARTH. 
299 
interesting in our lives. Stations were provided for us at the surface and near the 
lowest part of the mine ; the latter being at a short distance from the South Shaft or 
Harriet Shaft, near the junction of the granite and the killas, in the 180-fathom 
level (the depth being measured from the adit for the discharge of the water pumped 
from the mine), or nearly 1200 feet below the surface. This place, as is usual in the 
Cornish mines, could only be reached by ladders. 
7 . Our intention was, to compare the vibrations of a detached pendulum in each 
station with the vibrations of a clock pendulum, in the manner which has acquired 
so much celebrity from the labours of Kater and Sabine, and to compare the two 
clocks by means of chronometers carried on the person of an attendant. One set of 
observations, extending over several hours (including if necessary more than one 
series of coincidences), was to be observed each day. When this should be carried 
far enough, the pendulums were to be reversed and the observations were to be 
repeated, or the two pendulums were to be compared, as might seem best. After 
overcoming some difficulties, and with very great personal labour, we found on com- 
puting approximately the results that the chronometer-comparisons were not trust- 
worthy. We resumed the work, with a modification of the method of using the chro- 
nometers which promised to render the results more accurate. We were raising the 
lower pendulum up the South Shaft for the purpose of interchanging the two pendu- 
lums, when (from causes of which we are yet ignorant) the straw in which the pendu- 
lum-box was packed took fire, the lashings were burnt away, and the pendulum with 
some other apparatus fell to the bottom. This terminated our operations of 1826. 
8. In the summer of 1828 we again attempted the experiment, in the same locali- 
ties, and with the same general instrumental means. Our personal powers however 
were far greater than in 1826. We had now the assistance of the Rev. R. Sheep- 
shanks and of two junior observers (my brother Mr., now the Rev. W. Airy, and 
the late Rev. S. Jackson). Our plan, principally at the instance of Mr. Sheepshanks, 
was so modified as to admit of incessant observations being made, day and night, for 
several days consecutively; and this arrangement greatly diminished the injurious 
effects of chronometer-errors. A new difficulty now presented itself in the irregular 
and varying foi’in of the pendulums’ knife-edges. After tedious experiments on these, 
which seemed at last likely to be successful, our labours were suddenly stopped by 
the occurrence of a “ fall” in the mine. The lodes or metalliferous veins in the 
Cornish mines are usually bounded by nearly parallel planes inclined perhaps 30° or 
40° to the vertical, and the removal of the vein-stuff (even when, as in this case, the 
vacuity has been filled up as much as circumstances permit) endangers the falling of 
the rock which is on the upper side of the lode. On this occasion, the general fall 
(in consequence of the precautions above described) did not exceed a few inches ; 
but large masses of rock were detached from their places, and interrupted the work- 
ing of the pumps; and our lower station was soon flooded by the rise of the water. 
And thus for the second time our attempts were defeated. 
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