THE SOUTH SHIELDS PENDULUM EXPERIMENTS
BY THE ASTRONOMER ROYAL
WE have already briefly explained the arrangements made for this important
series of experiments with the pendulum, in the Harton Coal Pit, at South Shields. At
the request of several scientific gentlemen, the Astronomer Royal, Professor Airy, gave an explanation of these
experiments to the members of the Mechanics' Institution and Working
Mens Institution of South Shields, on Tuesday week, the 24th ult. ; but it will be some time before the astronomical observations
obtained will have been sufficiently reduced to arrive at the anticipated results.
The experiments at the colliery, which extended over from three to four weeks, having been brought to a close the
instruments had been removed to the lecture-room, and were placed on the opposite ends of the platform in exactly the
same relative positions as they had occupied at the pit-one set below, and the other above ground. The apparatus
consists of two astronomical 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 thermometer attached which were regularly observed, so that
the necessary corrections might be made far atmospheric resistance, variations of temperature, &c. There was also at each station a
galvanic signal-needle and these were connected by means of insulated wires,
placing the observers in electric communication. The upper and lower stations at Harton Pit were
exactly in a vertical line, and distant nearly a quarter of a mile, and at each station the instruments were carefully enclosed to
prevent the vibrations of the pendulums being affected by currents of air.
James Mather Esq., occupied the chair at the meeting at Shields and, with a few
well-timed observations, introduced the distinguished lecturer.
The Astronomer Royal commenced by observing that the present lecture was due to the people of South Shields and the
county of Durham generally, in return for the very kind assistance which he had received.
He also acknowledged his obligation to the Durham Observatory. Mr. Airy next proceeded to state that his grand object was to arrive at the
weight of our globe-the earth-it being necessary to know this before we can proceed to determine the weight or mass of
the sun, moon and planets, which is of the utmost importance to practical astronomy. The size and figure of the earth
being now pretty accurately known, it only remains to determine its mean density, i.e., the average weight of say a cable foot, and the total
weight of the globe is easily calculated.The Astronomer Royal then proceeded to detail the experiments. Harton Pit is 1260 feet deep; its mode of ascent and
descent is by "cage and tub;" which Is made in either way, with the smoothness of a railway carriage, in less than two
minutes. The extensive nature of this pit may be judged of by the amount of passages which it contains
in its workings, in connection with its other shaft, St. Hilda. They are upwards of 100 miles; and there are
underground passages more than two & half miles in a direct line.
A model Zenith-Sector was explained in connection with the late Dr. Maskelyne's experiments and observations at the
mountain Schehallien, in Scotland, in 1774. Professor Airy next detailed the various experiments with the Torsion
Balance by Cavendish, Retch, and the late Francis Baily.
The results of the above experiments show the mean density of the earth to be five to six times the weight of water.
This was the state of the inquiry in 1826, when Professor Airy and Dr. Whewell saw hat the problem might be solved in
an entirely different manner. A little reflection Is sufficient to show that, if the mean density of the earth be
from five to six times that of water, as indicated by previous experiments, the interior of the earth must be
considerably heavier than its super stratum, which, so far as we have been able to penetrate, we know to
be little less than one-half that weight. Now the question arose,
how was this to he determined ? How were we to ascertain whether the centre of the globe
was a mass of matter, probably as dense as gold or platinum? Messrs. Airy and Whewell 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 correct measure of
the power of gravitation; and, as we penetrate the comparatively light super-strata 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 pendulum, i e. to make it go quicker. This
principle forms the basis of the Harton Pit experiments. Mr. Airy here enumerated the many difficulties to be
encountered, and the many precautions to be used in the prosecution to such delicate inquiries. He then proceeded to
state that he and Dr. Whewell attempted the experiment at the Dolcoath mines in Cornwall in the year
1828, but were completely defeated. One principal difficulty was the comparison of the clocks at the top and bottom in the mine,
which at that time could only be managed by conveying a chronometer, in the most inconvenient manner,
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, an they were "drowned
out" Mr. Airy here remarked that as it was necessary in these cases to combine philosophy with
pertinacity he had lately resolved 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 to be noted at both stations, which was done
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 take 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
observations commenced : in this way any difference in the lengths of the pendulums was got rid
of: Mr. Airy thought that, with care, the final error of the pendulum 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 globe which could be pretty easily arrived at. The Astronomer Royal
demonstrated by the assistance of a diagram, the effect which the outer crust would ; produce on the lower pendulum,
which, fortunately, maybe said to be none at all. Professor Airy, in
conclusion stated that, before he could say whether the results would show any difference at all, many long calculations and deductions would require to be made.
The figures recorded " looked well," and reflected the highest credit on his six assistants, who were the most careful
observers he could desire. Mr. Ingham, M.P., in moving a vote of thanks to the Astronomer Royal, thanked him for
having, at his request issued a ticket to each of the members of the South Shields Mechanics' Institution.
To this report of the lecture, abridged from the Newcastle Chronicle, we add a few
details. The position of the two pendulum rooms, was one at the top of the pit, the other at the bottom, within 100 yards of it, exactly vertical. The instruments in the upper and lover rooms were facsimiles of each other; and at certain intervals, to correct any irregularity, those at the top were placed below, and those below,
above, changing places. The upper and lower astronomical clock, exactly regulated to each other, had each on the balls of their pendulums
an Illuminated disk, about the size of a crown-piece. Exactly in front, and within a few inches, hung a free (Kater's) pendulum, suspended on very hard steel, shaped to an obtuse angle, moving on an agate plane. The number of vibrations of the clocks within any given time, are of course registered by the clocks themselves. The number of the vibrations of the free pendulums, produced exclusively 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, passes and
re-passes in front of the latter, exposing, first on the right and then on the left, the illuminated disk on the clock pendulum. When
a certain series has been gone through, the illuminated disk is covered for a second or two by the free pendulum in front; and this ; obscuration or coincidence, marks exactly the proportionate number of vibrations due to each. The exact time is then noted, the temperature, the barometric pressure, the relative time of the clocks above and below, by the galvanic signal needle; and then the observation is complete, both in the mine and at the surface.
This coincidence requires so exact an observation, and from a direct line in front, that a small telescope is fixed in the most precise manner within a few feet, to make the
observations in which so much precision and care were used that the tenth of a second was frequently noted, which develops in the calculations in some instances nearly the
four hundredth part of that brief period of time.
To ascertain the weight of the earth's crust will be more open to error. It requires that every description of stratum, lying between the upper and lower pendulum for these 7260 feet should have its weight exactly found, as well as
its thickness measured. In some places, near the surface, there are clay and earth ; in others, as you descend, sandstone, limestone, shale, and then seams of coal ; some of the strata are saturated with water, others quite dry. All these varieties of deposits, in their respective conditions, must be distinctly examined and weighed. Difficult, as it may appear, all this may be done; and the genius that has hitherto arranged and developed this important experiment will doubtless find means to complete the operation.
Mr. Dunkin, of the Royal Observatory, Greenwich, conducted the experiments in Professor Airy's absence, assisted by Mr. Ellis, of the same Observatory; Mr. Pagson, of the Radcliffe Observatory, Oxford; Mr. Rumkin, of the Durham Observatory; Mr. Creswick, of the Cambridge Observatory; and Mr. Simmonds, of the Red-hill Observatory, Surrey.
The accompanying Illustrations show a view of Harton Pit, which is merely interesting as being the scene of the observations.
The lower Engraving shows Professor Airys apparatus in situ in the room built for its reception at the bottom of the
pit shaft 1260 feet below the surface. The lantern placed on the block
is for the purpose of illuminating the pendulum disk, there being a glazed aperture in the side of
side of the clock to admit the light. The spiral wires are the telegraphic conductors, which were carried up to the ceiling through the wall, and so up the shaft to the upper station. The instruments suspended
from the iron support on the left, are two thermometers. No figures are introduced,
inasmuch, as during the observations, no one was admitted into the room; the observer being seated in a separate apartment, and making his observations by means of a telescope through an aperture in the wall.
Professor Airy has kindly agreed to forward his lecture, explanatory of the noble subject, written in extense to South Shields for publication.