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FRANK, LESLIEF’S "NEW YORK JOURNAL:
281.
>, Pendulum Experiments i in a Coal-Mine
Winze a'l the world.is being agitated and shaken
to its centre by the fury of war, philosophy, it would
; appear, invariably, pursues the : even tenor of its
way—ever advancing, seldom retrograding. . And it
is singular to have to remark, that in modern times
the greatest practical discoveries or developments in
science were made in. periods, of, turbulence . and
» commotion.
,Electricity, as.an all-peryading element, was de-
fined and subjected to the manipulation of rules du-
ring a;period of intense’ political excitement. .. The
Marq: 11s of: Worcester devoted himself to the appli-
cation of. steam, as a motive power, during the hor-
rors of the great civil war ; ant it is well known that
steam navigation was first introduced conspicuously
to the notice of civilized. mankind just as England
was:commencing that grand struggle with France
which, after fifteen years’ almost incessant warfare,
resulted in the dethronement and exile of the Colos-
sus of Europe.’ ., It would: therefore seem that’ when
the human “passions. were at the, highest point of
, anger, philosophy, was in the ‘greatest state of acti-
vity: Perhaps, inspired by: the prevailing enthusiasm,
it became more prolific in suggestions, and Pursued
them with greater avidity.
Be this as it may, periods of war have, within the
past. three centuries, been distinguished for discove-
ries and improvements in art and science ; and after
having had lately to hear so. much ‘about the war in
the East, it is refreshing to have our attention direc-
ted to a series of experiments of the utmost impor-
tance to astronomy and science in general. « put
: Professor Airey, :the- Astronomer! Royal, undis-
turbed by the grand attempt to drive back the Great
Bear, of the North to his: native fastnesses, deter-
mined some weeks ago to pursue an inquiry on which
he had been engaged for many’ years. :That was no
less than to aceertain, by means of observation: and
calculations, the weight: of the earth—the weight of
the very gicbe on which we tive, and on which we are
annually carried round the sun. 91>! *
‘For this purpose he descended: Harton Pita coal
| mine in the neighborhood of South Shields—which
is'1200 feet deep, and .has underground passages of
more than 109 miles in’ extent. «.The apparatus he
employed consisted of two astionomical clocks, with
compensation pendulums, two invariable pendulums
—Kater’s—suspended on strong: tripod iron stands,
immediately in front of the clock pendulums. -' An
astronomical clock and an invariable pendulum were
placed at each station, with barometer and’ thermo-
meter ‘attached, which were regularly: observed—so
that the necessary corrections might be made for at- 1
mospheric resistance, variations of temperature, &c.
There was also: at each station a ‘galvanic! signal
needle—and these, connected‘ by means of insulated
wires, placed the observers in electric communication:
The upper and lower. stations at Harton Pit were ex-
actly in a vertical ‘line, and distant: nearly a quarter
of:a mile; and at each station the instruments were
carefully inclosed, to prevent the vibrations of the
pendulums being affected by currents of air.
:'The inclosure was effected’ by the construction of
what, for the moment, were called pendulum rooms.’
One was at the top of the pit—the other at the bot-
tom. We: give an illustration’ of the Pendulum
Room at the bottom of the pit.” The mode in which
the experiments were conducted may thus briefly. be
explained : '
~Thei instruments i in the upper and lower rooms were
fac-similes of each other; and at certain intervals,
to: correct: any irregularity, those at ‘the top were
placed below, and those below, above—changing pla-
ces. . The upper and lower astronomical clocks, ex-
actly regulated to each other, had each on the balls
of their pendulums an illuminated disc, about the
size of an eagle. ' Exactly in front, and within a few
inches, hung ‘ a free—Kater’ 's—pendulum; suspended
on very hard steel, shaped to an obtuse angle, mov-
ing on an agate plain. “Ths number of vibrations of
the ‘clocks within any given‘ time were of course
registered by the clocks themselves. The number of
the vibrations of the free’ pendulums, produced ex-
clusively by gravitation, were ascertained exactly by
their proportionate number to those of the clocks.
At intervals each free pendulum—which moves more
quickly | than the clock: pendulum—passed and re-
passed in front of the ‘latter, exposing, first on the
right and then on the left, the illuminated disc on the |. °
- clock pendulum: ‘ When a certain series had been
| gone through, the illaminated disc was covered for a
second or twoby the free pendulum in front; and this
obscuration or coincidence marked exactly the pro-
portionate numbers of vibrations due to each. The
exact time was then noted, the temperature, the bar-
ometric pressure, the relative time of the clocks above
and below, by the galvanic signal needle; and then :
the observation was complete, both in the mine and
at the surface... ).
Professor. Airey has not yet published any of. “hia
calculations and deductions, but he has given to the
public a brief. summary of the results of his experi-
ments.’ , From that we gather that the great difficulty
to be overcome was to ascertain the character of the
mass of matter in the centre of the globe.
A little reflection is sufficient to show that, if the
mean density of the earth be from five to siz times
that of water, as indicated by previous experimenis,
; the interior of the earth must be considerably heavier
than its ‘superstratum, which,.so, far as’ we, have
been able to penetrate, we know to belittle less than
one half that weight. .. Now the question arose—how
was this to be determined ?,. How were we to ascer-
tain whether the centre of the globe was a mass of
matter, probably as dense as gold or platinum ? Mr.
Airey directly saw that if this was the case, it would
be most. easily settled by. observing the number of
vibrations made by a pendulum swung at the surface
of the earth and at the bottom of the deepest mine.
The number of vibrations of the pendulum, is a cor-
rect measure of the power of gravitation ; and as we
penetrate the comparatively, light ; superstrata + or
crust of the globe, and approach the much denser
and heavier mass forming the interior, the attraction
of gravitation will be considerably increased, and the
effect will be to accelerate the vibration of the pen-
dulum—i.'¢ , to make it go quicker. .,This principle
forms the basis of the Harton. Pit experiments... -
.Professer Airey states that'he and Dr. Whewell
attempted the experiment. at the ‘Dolcoath mines in
Cornwall, in the year 1828, but were completely de-
feated. One principal difficulty was the comparison
of.the clocks at the, top, and. bottom ofthe mine,
which at that time could only be managed. by con-
veying a chronometer, in the most inconvenient man-
ner, down a series of ladders, altogether | different
from being lowered direct to the bottom, in the cage
at Harton Colliery. °, In spite of this difficulty. they
~| persevered until, from some serious accident, the
mine :was | inundated , with .water, and; they, were
“drowned out. ”.; Mr. Airey remarks, that as it was
necessary in these cases to combine philosophy with
pertinacity; he had resolyed.to renew, the- attempt,
but with’ the new element, before: mentioned—viz.,;
the Electric Signal. ..This enabled the two observers
to know the exact. instant at which the clock time
was tobe noted at both stations—which was’ done
a
Hi
Hk
. He
ih
!
every four hours, night and day, for four or five days;
when the invariable pendulums were changed—the
pendulum which had been swinging at the upper
station was taken down the pit and hung up in front
of the clock at the lower station, and that which had
been down the previous five days was brought to the
station at the surface, and a new series of observa-
tions commenced : in this way any difference in the
length of the pendulums was got rid of. Mr. Airey
thinks that, with care, the final error of the pendu-
lums would not exceed one-tenth of a second per day.
The necessary elements or data for the calculation
of the mean density of the earth by this method are
—first, the difference in the number of the vibrations
at the top and bottom of the mine ; .and, secondly,
the thickness and mean density, or specific gravity, of
the outer shell of the slobe—which | could -be pretty
easily arrived at.
The experiments were interesting but ‘intricate,
and their results will require many tests before any
true data on the subject can be accepted by the philo-~
sophical world. :.This is self evident, when it is re-
quisite that every description of stratum lying be-
tween the upper and lower pendulums, for the 1260
feet of the Harton : Pit; should have its weight ex-
actly found, as well as its thickness measured. ‘In
some places, near the surface,' there are ‘clay and
earth ; in others, as you descend, sandstone, lime-
stone, shale, and then seams of coal. Some of the
strata are saturated! with * water—others are : quite
dry. All these. varieties of. deposits in their respec-
tive conditions must be distinctly examined and
weighed.’ : Difficult, however, as it may. appear, all
this may be done, and the genius that has hitherto
arratiged and developed this important experiment,
will doubtless find means to complete the operation,
-.A Common Exnor.—The vulgar. are apt to con-
clude, that where a great deal. is said,, something
must be true; and adopting that: lazy contrivance
for. saving the. trouble - of thinking, ‘splitting the
difference, imagine that they show a laudable cau-
tion.in believing only part of what is said: This is
to: be as simple asthe clown who thinks-he has
bought: a/ great. bargain of a Jew, because he has
beat down the price from :five dollars to: one for
some article that is not really worth a shilling.
» JustiFIaABLy ANGER.—Wise anger is like fire from
a flint; there is great ado to get it out; and. when
it'does come, it is out again immediately. :.. 9.
