THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.

FRIDAY, SEPTEMBER 20, 1918.

No. 3012.

Determining the Magnetic Meridian as a Basis for Mining Surveys.* * * § *

By T. LINDSAY GALLOWAY. M.A., F.R.S.E.

In surface surveying, and in the setting out of works
above ground, the most convenient line of reference is
undoubtedly the true north or geographical meridian.
This is the line to which all national standard maps and
plans are referred, and, when not already known, its
direction can always be ascertained with great accuracy
and comparative ease by astronomical methods. It
was a bold conception of Mr. Beanlands to extend the
use of astronomical methods to underground surveying.
Even in cases where there was no access by adit level or
inclined shaft, he proposed to determine the true north
line by observing the transit of stars from the bottom
of a vertical shaft.f But this idea was difficult to carry
out in practice, and Mr. Beanlands found it more prac-
ticable to place his transit instrument at the top instead
of at the bottom of the shaft, and, by looking down-
wards and fixing with great accuracy the position of
two marks at the bottom of the shaft, he was able to
determine a line which could be afterwards extended so
as to form the basis of a survey. Notwithstanding
some very successful results obtained by this method,
it is obvious that the risk of error is considerable, and a
small inaccuracy may give rise to serious consequences
by reason of the shortness of the base-line.

An improvement upon Mr. Beanlands’ method was
successfully employed by Mr. Richardson in setting out
the line of the Severn Tunnel—the principle of which
was that, instead of fixing marks at each side of the
shaft, a long wire was stretched horizontally across the
bottom, and its direction fixed by observation with a
transit instrument placed at the top. By this means
a longer base was obtained. J -

Compared with the transit instrument, it must be
admitted that the ordinary magnetic needle as fitted to
mining compasses, is a very inadequate appliance for
fixing and setting out lines; but its great convenience,
its moderate accuracy, and the circumstance that it can
be set up alm< st anywhere and its indications checked
as often as desired, are good reasons why it still remains
the most usual means of fixing directions underground,
[ts defects and limitations are obvious. As ordinarily
constructed, its indications can only be read to the
nearest degree—the smaller subdivisions being esti-
mated by the eye. Its mode of suspension, also, is far
from perfect—for a jewelled cap, resting upon a steel
pivot, however finely made, cannot be totally devoid of
friction.

It has been the aim of the writer for some time past
to devise a simple magnetometer, and a method of
determining the magnetic meridian from it, with the
aid of the theodolite, so as to secure the inestimable
advantages of the telescope and vernier. This is the
more desirable as accurate records of the variations of
the magnetic declination are now published, and a good
deal of attention has been drawn to the subject by a
recent presidential address§ and by Dr. Chree’s admir-
able and comprehensive paper. ||

In the method about to be described, the only instru-
ments required are a theodolite, three tripods, and a
simple appliance, which may be called a “magnetic
reflector.” The latter, which will be described. more in
detail afterwards, consists essentially of a small plate-
glass mirror, to which is cemented a flat magnetised
bar, the whole being delicately suspended by a single
fibre of unspun silk, in the manner frequently employed
in physical laboratories and observatories. The pro-
cedure for finding the magnetic meridian is as follows :
—Place the magnetic reflector on one of the tripods,
adjust it by the levelling screws, set free the mirror,
and allow it to swing until it comes to rest in the plane
of the meridian. Place the other two tripods, each
having a plumb line suspended from its centre, at con-
venient distances in front of the reflector—not directly
in front, however, but somewhat to each side, and far
enough apart to admit of distinct focussing when a
sight is taken from one to the other. Place the theodo-
lite on either of the tripods and cause the other to
be moved, if necessary, until the image of its plumb
line can be seen reflected in the mirror. The following
observations are now to be made, and at each observa-
tion the cross wires in the telescope must be brought
into exact coincidence with the image of the plumb
line, whether viewed direct or reflected, as the case
may be.

Referring to fig. 1, A, B, C represents the tripods, B
being the one which carries the reflector. Place the
theodolite at A, turn the zero towards B, clamp the
axis, and turn the instrument through the angle BAC ;

* From a paper read before the Institution of Mining
Engineers at Nottingham, Sept. 13, 1918

f “ On Mining Surveys,” by Arthur Beanlands, Trans.
N. E. Inst., 1855-1856, vol. iv., p. 267; and “ On Recent
Improvements in Underground Surveying,” by Arthur
Beanlands, ibid., 1870-1871, vol. xx., p. 85.

J Engineering, 1882, p. 48.

§ Presidential address, by George Spencer, Trans.
Institution Mining Engineers, 1918-1819, vol. Ivi., page.

|| Trans Institution Mining Engineers, 1917-1918, vol. Iv,
page 223.

clamp the vernier, unclamp the axis, and remove to 0;
turn the instrument towards A. 'the zero will now lie
in the direction CD, parallel but opposite to the original
direction AB. Clamp the axis, unclamp the vernier,
and turn the instrument towards B. The angle now
recorded on the horizontial circle is to be noted. It is
evidently the angle DCB. If the instrument be turned

Fig. 1.—Diagram Showing Positions of Tripods,
D C being parallel to A B, D C B is equal to A B E. There-
fore N C B, which is the half of D C B, is equal to n B E,
which is the half of A B E. Therefore N C is parallel to n B.

Fig. 2.—Front View of
Magnetic Reflector.

p

f

Fig. 3.—Side View of
Back Showing the
Suspension.

P is the counterpoise, C C,
C C, supporting clamps,//
the fibre, M the mirror,
and G the’guard.

back through half this angle, the centre line of the
telescope will be parallel to the face of the mirror, and
therefore in the plane of the magnetic meridian. The
graduation of the theodolite has been assumed to be in
clockwise direction. If otherwise, the order of procedure
is to be reversed. The process, it will be seen, is
extremely simple and geometrically exact. Having
determined the magnetic meridian, the surveyor may
proceed with the remainder of the survey in the usual
manner, and can check the meridian line at any other
suitable point by repeating the same process.

It is evident that the magnetic reflector (figs. 2 and
(3) should be as simple in construction as is consistent
with accuracy. It would not be practicable to use
elaborate or delicate instruments under the conditions
ordinarily to be met in mines. Keeping this circum-
stance in view, the writer has endeavoured to design an
appliance which is quite simple and portable. Externally
it consists of an oblong box having plate-glass windows

in front. The back is removable and carries the
suspension and its supports. The mirror, with its little
flat-bar magnet cemented to the face, is attached, as
already stated, to one end of a single fibre of unspun
silk. To the other end of the fibreJis attached a small
counterpoise of exactly the same weight as the mirror
and magnet. Both mirror and counterpoise, when not
in use, are held fast by clamps which can be operated
from behind, so that when the box is closed, either the
mirror or the counterpoise can be set free as desired
without opening the box. The object of the counter-
poise is to enable all twist to be taken out of the
fibre by the very ingenious method employed by
Prof. Tanakadate of Tokio. For this purpose the box
is first set up with the mirror uppermost and the
counterpoise below. The counterpoise is unclamped
and allowed to turn about freely until it comes to rest.
One may then be certain that there is no twist remaining
in the fibre. The counterpoise is clamped, and the box
inverted, with the mirror downwards. It is now ready
for use. But one must make sure that the fibre does
not again become twisted. A slight accident, when the
mirror is set free, might cause it to spin round through
one or more revolutions. Not only must such an
accident be prevented, but one must be certain that
when the mirror comes to rest its position relatively to
the counterpoise will remain practically the same as it
was when the latter was hanging free. Around the
lower edge of the mirror, therefore, is placed a guai d,
which prevents it from turning through more than a
fraction of a revolution. Accordingly, when the box
has been set up and the mirror unclamped, it will
generally be found necessary, after levelling, to turn
the box about its vertical axis until the mirror is seen
to be swinging free within the limits allowed by the
guard. It will then oscillate under the influence of the
earth’s magnetism until it comes to rest with the magnet
exactly in the plane of the meridian. If the axis of the
magnet is not perfectly parallel to the face of the mirror,
the latter will deviate slightly from the actual direction
of the meridian by a small constant difference. The
amount of this can be ascertained eithei’ by com-
parison with a standard instrument, or by direct
experiment. But generally such comparison or experi-
ment would not be necessary. For what the surveyor
requires is not the absolute direction of the magnetic
meridian, but the relative directions of two lines, one
above ground and one underground, or one in one part
of a mine and another in another part. So long,
therefore, as he uses the same reflector on each occasion,
it is immaterial whether it agrees exactly with a
standard instrument, or differs from it by a small
constant amount.

Certain difficulties and objections to the use of the
magnetic meridian as a basis for mining Purveys remain
to be considered, which the writer has no desire to
ignore or minimise. First of all, there is the influence
of local attraction due to the presence of iron. This
is a difficulty which can only be overcome by removing
all iron from the immediate vicinity of the reflector, or
by setting up the latter only where no iron exists. As
regards the chance of error arising from the unseen
presence of iron, this can be eliminated to a large
extent by making two or more determinations of the
meridian in different places, and comparing them
together.

A further difficulty arises from the fact that the mag- •
netic meridian is itself never, strictly speaking, a fixed
line. Its direction is continually changing at all hours
of the day and night. These fluctuations are not
usually large enough to obtrude themselves on one’s
notice in using the ordinary compass needle, and do not
seriously interfere with compass surveying. But they
become quite apparent in using the method herein
described, and as the diurnal variation may amount to
5' or 10' or more, its amount is too large to be over-
looked in very accurate work. There are circumstances,
however, which make this difficulty less formidable than
would appear at first sight. Observations made in the
evening or early morning do not deviate so much from
the daily average as those made within a few hours of
noon. The diurnal variation also follows a fairly
regular law, so that, in this respect, one day is not
unlike another. Thus two observations made on different
days at the same hour would generally be in close
agreement, particularly if the mid-day hours are
avoided.

With regard to the irregular disturbances caused by
magnetic storms, these cannot be foreseen or provided
against; but, as all movements take place simul-
taneously, affecting all instruments alike, wherever
situated, it will always be possible, by referring to the
published records, to find out the variation on any
particular day at any given hour. Allowance can thus
be made, if necessary, for any unusual disturbance that
may have occurred at the time of an observation. A
point worth noting in this connection is that, as the
instruments can be set up almost anywhere, it is easy
for the surveyor to continue his observations as long as
he wishes, or to repeat them as often as he thinks
necessary, in order to secure reliable results, without
in any way interfering with the other operations at the
mine.

* A Magnetic Survey of Japan, by Messrs, Knott and
Tanakadate.