362 G. B. Airy on Pendulum experiments 
errors to which observations are liable, that the resulting error . 
in the density, in this form of experiment, would be less than in 
the others. 
Accordingly, in 1826, the speaker, with the assistance of his 
friend Mr. Whewell (now Dr. Whewell), undertook a series of 
experiments at the depth of nearly 12U0 feet, in the Doleoath 
mine, near Camborne, in Cornwall. The comparison of the 
upper and lower clocks (to which further allusion will be made) 
was found to be the most serious difficulty. The personal la- 
bor was also very great. They had, however, made a certain 
progress when, on raising a part of the instruments, the straw 
packing took fire—(the origin of the fire is still unknown)—and 
partly by burning, and partly by falling, the instruments were 
nearly destroyed. 
In 1828 the same party, with the assistance of Mr. Sheep- 
shanks and other friends, repeated the experiment in the same 
place. After mastering several difficulties, they were stopped by 
a slip of the solid rock of the mine, which deranged the pumps 
and finally flooded the lower station. 
The matter rested for nearly twenty-six years, the principal 
progress in the subjects related to it being the correction to the 
computation of “buoyancy” of the pendulum, determined by 
Colonel Sabine’s experiments. But in the spring of 1854, the 
manipulation of galvanic signals had become familiar to the As- 
tronomer Royal, and the assistants of the Greenwich Observa- 
tory, and it soon occurred to him that one of the most annoying 
difficulties in the former experiment might be considered as being 
practically overcome, inasmuch as the upper and lower clocks 
could be compared by simultaneons galvauic signals. Inquiries, 
made in the summer, induced him to fix on the Harton colliery 
near South Shields, where a reputed depth of 1260 feet could 
tendence of the observations. : 
The two stations selected were exactly in the same vertical, 
excellently walled, floored, and ceiled; the lower station im par 
