August 9, 1918.

THE COLLIERY GUARDIAN.

285

this rock bends the booms, and if there is a weak
boom it breaks and is followed by the failure of the
other booms. If all the booms stand up under the
load, the entry may stand for a considerable time
with the booms bent, but eventually the rock higher
up breaks loose, as shown in fig. 1. This may happen
quite suddenly and without any warning, and when
it does happen a load greater than the booms can
support is placed on them and the entry caves. This
action has been noticed in a great number of cases.
A whole entry standing for some time with the timbers
bent has been seen to cave in one night when the rock
farther up parted.

Fig. 3.

4-

r ft

.-J—I

—!----J

Fig. 4.

In the pillars where props are used to support the
roof, the first layer of shale does not usually part
owing to the fact that there is less strain in props
than in booms with the same load. The props in
pillars usually stand for some time without appreciable
movement, and when the roof does break, it does high
enough up so that the weight is sufficient to cause
complete failure of the props. From 18 ft. to 30 ft.
of coal can usually be extracted without roof trouble,
and then, without very much warning, the roof comes
right in up to the coal face. Well packed cogs will
stop this break, but no method of setting props will
keep the face open.

SIZE OF CROC IN FIRECLAY BODIES.*

By F. A. Kirkpatrick.

In those industries which use crushed and ground
raw materials in forming bodies by means of a
cementing action of some sort, the size of the particles
plays a most important role. This may be, in many
instances, the main factor controlling the desired
properties in the body. This is nowhere more true
than in the ceramic industries, where in compounding
bodies the manufacturer is compelled to use materials
from the size of pebbles down to the dispersoid and
emulsoid (colloidal) states. All present evidence tends
to show that in disperse systems the size of particles
which we cannot now measure has as much effect upon
the properties of certain materials as have the larger
sizes which we are able to measure. In no other way
is it possible to account for the great differences in
cementing power exhibited by fine-grained bond clays
and other fine-grained materials. In practice the
cementing action takes place at temperatures from zero
to that of the electric arc, depending upon the nature
of the process, and is due to many different chemical
reactions. In the present experiments the bonding
power of mixtures composed of raw clay and calcined
clay was determined after these bodies had been heated
to 110 degs. Cent., and to 1,250 and 1,300 degs. Cent.

The control of the strength of raw fireclay bodies is
a difficult matter into which enter a number of factors.
Those which are directly connected with the size of
grog are size of the body, cracks formed in drying,
density or porosity, and size of grain of the clay.

If the grog is too large for the size of the body used,
the conditions of bond are different than for smaller
grog. Small cracks may or may not form in drying.
The size of grain of the bond clay also affects the
strength of the body. In regard to the grog, aside
from other considerations, the following condition is
necessary for highest strength: The mixture of sizes
must be such that the smaller particles fill the voids
between the larger, giving maximum density. The
proper proportions may be predicted qualitatively
from the ratio of sizes, but can be determined accu-
rately only by experiment.

One series of the strongest raw bodies investigated
had the following limits of grog composition : 25 to
66j per cent., 20 to 40 grog; 0 to 25 per cent., 40
to 80 grog,' and 33J to 66| per cent., 80 to dust grog.
In another series the strongest bodies were those con-
taining the greatest percentage of 80 to dust grog.

The control of strength in the burned state proved
to be not at all difficult. For all bodies used the
modulus of rupture was found to increase with increase
of surface factor. The relation was represented by
means of straight lines, except that the lower portions
of some of the curves representing large-sized grog
were of parabolic form. The rate of increase of
strength increased with the temperature of burning,
due to the more rapid rate of solution of the finer
particles at the higher temperatures. The porosity
at cone 12 varied much the same as in the raw bodies,
and had little relation to strength. The strongest
bodies were those with 20 to dust grog. No relation
was found between strength in the raw state and in
the burned state.

In the quenching tests from 600 degs. Cent, and
1,000 degs. Cent, mixtures of the larger sizes of grog
gave the more resistant bodies.

* Technologic paper No. 104, U.S. Bureau of Standards.

THE MAGNETIC MERIDIAN AND THE
ORIENTATION OF MINE SURVEYS.

By W. B. H. Lockerbie.

The publication of the weekly tables of mean mag-
netic variation at Kew, and Dr. Chree’s paper
recently read before the Institution of Mining Engi-
neers has aroused considerable interest among mine
surveyors.

A somewhat cursory study of these tables has led
some surveyors to the conclusion that the magnetic
meridian, owing to its so-called fickleness, is absolutely
unreliable for the orientation of mine surveys.

The writer would like to show in these notes that, in
the hands of a capable surveyor, very precise orientation
can be made by meins of the magnetic needle; and in
some instances the accuracy of orientation exceeds
that of other methods which depend on an arbitrary
meridian for their orientation.

It is conceivable that, when bearings are carried
forward by traversing, considerable errors may creep
in, due to short drafts, inaccurate centring, collimation
error, etc., and, unless refined methods of measuring
angles are adopted, the combined error in minutes of
arc, from these causes, may be of the order of the
square root of the number of stations occupied; and,
as the error is accumulative, a serious state of affairs
may accrue. Furthermore, without repetition traversing,
one cannot have absolute confidence in the bearing
carried forward. The frequent use of the needle would
tend to stop these errors, and would, without much
labour, afford a valuable check on the work performed.
Whilst the writer must not be understood to say that
orientation by the needle is superior to orientation from
an arbitrary meridian, he is of opinion that it affords a
valuable check on the alternative method.

The errors of orientation which occur when the mag-
netic meridian is used are chiefly due to the following
factors:—

(a)	Neglect of the diurnal variation.

(b)	Ignorance of the errors attaching to the ordi-
nary compasses which the mine surveyor is
called upon to use.

(c)	The crudity of the magnetic instruments ordi-

narily used by the mine surveyor.

(d)	Want of precautions to overcome attraction,
whether due to the presence of iron or electric
currents.

Diurnal Variation.

We find on examination of the tables of mean mag-
netic variation that the needle, for days together, has
the same variation at the same hour of the day, this
being especially so daring the hoars of darkness. The
next important feature is the magnetic character of the
day, which is of the highest importance to mine sur-
veyors, and its continued publication will, indeed, be a
boon to surveyors.

Taking the period April 28 to June 15,1918, inclusive,
the average magnetic variation on all days between the
hours of 8 and 10 a.m. is 14 degs.48’2 min.; the average
of all quiet days at the same time is 14 degs. 47 7 min.,
and the average of the three quiet days taken at random
is 14 degs. 48'5 min. Bearing these facts in mind,
we can now generalise on the methods to be adopted to
reduce errors due to the diurnal variation to a negligible
quantity.

First, then, when it is required to orient an important
theodolite traverse, the bearing of the underground line
should be taken, say, two or three days in succession, at
a given time of the day, and at about the same time the
bearings of the surface orientation lines should be taken.
As an alternative the bearing could be taken on the
surface on one day and the bearing of the underground
line taken at the same time on the next day. This
procedure should be repeated two or three times—the
substantial agreement of these readings would show the
accuracy of the determination and the presence or
absence of disturbing conditions.

If a second instrument and observer are available,
readings can be taken over a period of hours, and data
obtained for correcting the underground bearing if
taken at a different time to the surface bearing.

Another method, but wh;ch is not so accurate as the
previous methods, is to take the bearing of the surface
line or lines, then proceed underground and take the
bearing of the underground line, return at once to the
surface and again take the bearings of the surface lines.
The mean of the two surface bearings should give a
bearing about the same as would have been observed
had it been taken at the same time as the underground
line.

It has been suggested that the tables of mean mag-
netic variation should be used to correct bearings taken
at any time to the mean position for that day. Such a
procedure, however, would lead to error, as it has been
proved that the amplitude of the diurnal variation is
not the same for every part of the country. If correc-
tions are to be made from tables or curves, the values
must be observed in the immediate locality of the
colliery concerned.

If the true north meridian is shown on the plan, and is
also marked on the ground—or the true north bearing
of a surface line or lines is known—the variation of the
needle would be obtained instead of the bearings of
orientation lines. Of course, the variation would be
taken at a predetermined time, and the bearing of the
underground line taken at the same time—exactly the
same procedure being gone through as indicated in
the case of orientation lines.

The true north azimuth of the underground line
can now be calculated.

If the underground survey in question has been made
from a line of which the true north bearing was known,
the value of the bearing, as carried forward by the
traverse, can be checked against the true north azimuth,
as calculated from the observed magnetic azimuth.
This method the writer has used for a number of years,
and so close has been the agreement of the azimuths
that had the survey been orientated by the needle alone,
he would have had perfect confidence in the work.

A large discrepancy would prove to be due to faulty
traversing or to attraction of the needle, and would
sound a note of alarm, so that steps could be taken to
prevent an error creeping into the work.

If, however, instead of the true north meridian we
have on the plan the magnetic meridian, the procedure,
so far as observation goes, is precisely the same, with
the exception that the bearings of the orientation lines
are taken instead of the variation of the needle.

The next procedure is to calculate the azimuth of the
underground line to the ancient meridian, which is
simply performed by adding or subtracting the differ-
ence in azimuth of the orientation lines at the present
time and when they were originally observed at the
date when the meridian was first taken for the plan.

An illustration will make this clear. Suppose the
bearing of an orientation line, observed at the time the
meridian of the plan was first taken, to be 84 degs.
10 min., and that careful observation shows that the
bearing of this line is now 84 degs., then any bearing
taken at the present time would need 10 min. adding
to its azimuth to obtain the azimuth to the meridian of
the plan.

If, however, the dial in use read the azimuth as
84 degs. 20 min., then the present underground azimuth
would need to be decreased by 10 min., to bring the
azimuth to the meridian of the plan. It is obvious that
this method is superior to any attempt which may be
made to alter the meridian graphically for so small an
amount of arc; indeed, any attempt to alter it in such
a manner can only lead to error. Finally, corroboration
of the accuracy of orientation of the survey can be
obtained by noting the azimuth as carried forward from
the permanent marks, and the azimuth observed and
corrected to the old meridian.

Compass Errors.

It is a proved fact that any two compasses very rarely
exhibit the same variation. This is due to divers
imperfections in the manufacture of the ordinary shop
instruments. It may be that the needles themselves do
not exactly point to the same spot in the horizon—
apart from instrumental differences.

The most common error is that due to the poles of
the needle not being in the same line as the centre of
support. In refined compasses this error is eliminated
by inverting the needle, readings being taken before
and after the inversion. The mean of the two readings
then gives the correct reading on the dial plate.
Another cause of error is due to the graduations not
being of the same size throughout the 360 degs. Again,
errors may be caused by the sights not being in line
with the centre of the instrument, or (in the case of
instruments where a vernier is used to determine the
bearing), the two plates may not be concentric.

In the case of trough compasses, as usually supplied
' to theodolites, in addition to the error due to the poles
of the needle not being in the line of support, there is
frequently an error due to the fact that the optical axis
of the telescope is not parallel to the north and south
ends of the compass box.

The various factors enumerated are a prolific cause of
error in the orientation of mine surveys. Take, for
instance, the reprehensible practice of altering the mag-
netic meridian from values observed in a distant
locality, or, as is frequently done, plotting a magnetic
meridian from a true north meridian—the variation
being taken from the smooth isogonals published on
maps, without regard to the fact that the needle in use
may differ from the standard instrument by as much as
half a degree, whilst, in addition, there may be consider-
able local attraction.

To eliminate errors due to the aforementioned instru-
mental differences, each instrument for orienting
surveys should, at the time of orientation, be used in
the manner indicated previously, i.e., the bearings of the
surface orientation lines and the selected underground
lines should be taken at or about the same times—
the bearings being then corrected to the true or
ancient magnetic meridian, as the case may be.
For ordinary loose needle surveys of secondary import-
ance, it will be sufficient if the variation of the
magnetic meridian is allowed for, say, once in two years,
by moving the protractor eastward to the extent of the
variation of the particular instrument in use, or, in the
case of the true north meridian, by twisting the north
end of the protractor to the west to the amount of the
variation of the particular instrument.

With regard to finding the annual variation of any
particular needle, the observations for this purpose
should be made in the same month of the year and at
the same time of the day. In order to obviate error
from differences in the size of the graduations of the
instrument, prominent objects all round the horizon
should be taken, and the central angles measured with
a theodolite. The bearings of these lines should then
be taken—the difference between any two being equal
to the angle at the centre. The mean difference in
bearing of these lines from when last taken may be
regarded as the annual or bi-annual variation of the parti-
cular instrument used. Greater accuracy can be obtained
if the compass is taken to each of the objects in turn
and sighted to the central mark, this procedure
eliminating any attraction which may be present, since
any bearing which is faulty can be rejected from the
results.

If the central mark and one of the distant objects be
marked on the plan, the current magnetic meridian can
be put on to the plan by simply setting off the bearing
of the line as read, at the date in question.

Crudity of Magnetic Instruments.

It is well-nigh impossible to read a bearing, unless
special precautions are taken, with the ordinary com-
passes in use with a certainty of 5 or 6 minutes.
This is largely due to the difficulty of placing the
needle accurately on the zero line of the instrument.

In many instruments an appreciable space exists
between the north end of the needle and the zero
mark. Another fault is want of general stability,
movement of the lower plate taking place while the
upper or outer plate is being rotated.

Dropping the needle by means of a spring instead of
lowering it by a screw is detrimental to the pivot upon