The Express.
London, England, 1855-02-05 00:00:00

ROYAL

Professor Airy, the Astronomer Royal, delivered
a lecture on Friday evening, "On the:Pendulum Semi-
meats lately made in the Herten Colliery for ascertaining
the mean Density of the Barth." He commenced by or
plenum the importance in utronomical investigations of
acciairiag a merest knowledge of the men of the earth, as
the seam of determining the force of gravitation and the
density of other planetary bodies. He then lotion' the
differest plans which bad been adopted to ascertain the
mean density of the earth, including the attraction of •
plummet to the side of the mountain Scheehallian, and
kaown as the Bcheehelliae experiment, Mr. Cavendish's
cabinet experiment, in which he bed attempted to arrive at
the same result by the attractions of large spheres
of lead on light bodies; and the pendulum experiments
which, with the solstice* of Dr. Whewell, Profess
gee Airy bad made in 1826, and again in 1828,
lathe Dalarith Mine, in Cornwall. Both those experiments
were Worn, for in each ease, after many days of persever-
ing toil and 011etrilti:)11. unforeseen accidents injured the
instruments, and prevented the attainment of any satiates.
tory results. Professor Airy minutely explained the prin-
ciple on which the •ibration of a pendulum depends, and
ou which the pendulum experiments were contrived to de-
termine the density of the earth. As the rapidity of the
vibration of a pendulum increases with the increase of the
6n that attract its downward., it was expected that by
*sting accurately the number of vibrations is a given time
On the surface and at the bottom of • deep IBIS*, the differ-
ing. would afford a measure or the relative attractions of
Pseitenee at the two points. la those early experimeste
Ihe great difficulty experienced oasis aseertair isg precisely
the correspondence or difference in the vibration. of the pos.
datum placed at the tap and at the bottom of the saise,which
*maw meted to be done by means of a chronometer. The re.
met application el voltaic Ilkaltitity in the Observatory at
Greenwich bad supgested a ready mode of noting the
variations of the peadalums, and the Astronomer Royal,
therefore, with this new power at command, determined to
repeat the experiment* With the 'askew, of a sonorous
staff of observers, drawn from the different elrervatorles in
the kingdom, he commenced operation. in the Herten Col-
bert. near Newcastle, which is 1,600 feet deep. The Eke-
Ms Telegraph Company gave their aid in laying down in-
Wand war, and, by means of a clock, to make and Wank
_lMMast with the voltaic battery, galvanometers at the
Mega *ad at the top of the pit made ennulteeecus
illestiem every fifteen Attends. Professor Airy explained
lie erreageinests adopted to herniate the obeervetioos and
ffile precoitions taken to avoid error. Several observations
orre mads:in TRH d forma, oath one be ing coot inued for 104
hams, and the ctrropoodese. between them was so exact
so to leave no doubt of the accuracy of the irxlverimerde•
The mutt skewed a difference of two semods and a qu irter
"inn, four hours between the pendulum at the lower
psis% and that at the mouth of the pit, the lower one haying
finised that mach in consequence of the greater attraction
avevitatioo. Professor Airy said it could be 'asthenia-
flakof the e demonstrated that the counteracting attraction ot the
arth oboe* the pendulum would be exactly
Mrstralised by the attraction of the Malt thichneee of matter
athsr parts of the sphere, so that the actual fore. of
tenon acting on the pendulum would be represented mass henesth, not includingrepresented
the shell, of 1,000 Get.
he requisite computations have not yet bees made, bat
essikient has been done to ascertain that the mean density
d the earth is greater than the estimated &tory by the
Illiesehalliee experiment. By that experiment it appeared
am the mean density is between five and six time* the
speeific gravity of water, whilst the pendulum experiments
make it between six and seven times more dense. Pro-
fessor Airy avoided catering into any mathematical calcu-
labial, and explained the phenomena in the simplest man-
ner, offering at the same Limo to enter more (ally into the
sietheeritice of the subject after the lecture, should any ef
bis iodises, &sirs it.