April 23, 1915.

THE COLLIERY GUARDIAN

857

used with a battery of ten wet Leclanche cells, the
current voltage being 15. The break-flash was still
more dangerous when a battery of ten dry cells was
employed, since the current available from them is
considerably greater than that obtainable from wet
Leclanche cells.

In general, the bells examined were “ overpowered.”
A single bobbin with a reduced number of layers of
wire was found in several cases, when fitted to the
original bell-frame, to actuate the bell as efficiently as
is required in practice. The use of only one bobbin
(z'.e.. a bar electro-magnet) with a small number of layers
of winding reduces the self-induction of the circuit and
thereby decreases the danger of the break-flash at the
signal wires.

(2)	Inasmuch as the current voltage is relatively of
little importance compared with that of the current
strength, so far as the safety of the break-flash on the
signal-wires is concerned, it is desirable that attention
should be directed towards not exceeding a certain
maximum number of cells in the battery, rather than
that care should be taken not to exceed a certain voltage.
Moreover, it is desirable that a cell of comparatively
high internal resistance, such as the wet Leclanche cell,
should be employed, so as to avoid the possibility of
obtaining large currents on short-circuiting the battery.

(3)	Taking as a standard battery ten wet Leclanche
cells, quart size, such as are commonly used for battery-
bell signalling systems, giving a voltage of 15 and a
maximum currentonshortcircuiringof about 1 5 amperes,
it is possible so to modify the usual pattern of bell as to
render the break-flash at the signal-wires safe in the
most sensitive methane-air mixture, without impairing

[tell

Figure 4.

>'///A

Fig. 5.

the ringing power of the bell. This can be done in
several ways, of which the simplest probaby are:—-

(i.) By the introduction of a non-inductively wound
resistance coil in series with the magnet-coils such
that it will reduce the current available at the break-
flash below the minimum igniting current.

(ii.) By increasing the resistance of the magnet
windings of the bell by the use of wire of fairly high
resistance, such as brass wire, for the same purpose as
in (i.).

(iii.) By the use of parallel winding ; and

(iv.) By the use of tinfoil strips between the layers
of winding, in the manner and for the purpose already
described.

Of these four methods the third may be open to the
objection that should the short-circuited winding be
accidentally broken, the bell might become unsafe.

No experiments were made as to the igniting power
of the maintained spark at the trembler of the bell, for
it was apparent from the bells examined that the pro-
vision of an adequate flametight casing, affording
complete security against ignition of a firedamp-air
mixture at the trembler of the bell, was not a difficult
matter. Nor were any experiments made with relays
such as are often introduced in battery-bell signalling
systems, for all the considerations advanced in this
report respecting bells apply, probably with equal force,
to relays. A relay, if introduced into the signalling
system, simply takes the place of the bell, and from the
point of view of danger at the break-flash on the signal
wires, can be regarded as a bell.

Method of Experiment.

The main feature of the method of experiment, which
required that there should be produced in an explosive
mixture the spark or flash that arises when an electric
circuit, containing a bell or inductance, is broken, was
the apparatus for producing such break-flashes mechani-
cally. The general arrangement of the circuit is shown
diagrammatically in fig. 4. A post-office resistance box,
the coils of which were non-inductively wound, was
included in the circuit to enable small changes in the
current to be made. The break-flash was produced within
a glass explosion vessel, the detail of which is shown in

fig- 5, in the following manner:—A brass rod fixed
through the side of the vessel carried at its end a metal
strip (A, fig. 5) to form one of the electrical contacts
at which the break-flash should be produced. The
otner contact was a metal rod B. This rod was
mounted, as shown in the diagram, on a glass support
which passed through the side of the explosion-vessel
through the ground-glass bearing 0, and could be
caused to revolve by means of the pulley D, driven by
an electric motor. The glass support was hollow so as
to enable elect1 ical connection to be established (by
means of a copper wire, E, passing through it) between
short pieces of stout platinum wire fused into either
end. The upper platinum wire carried the contact rod
B, and the lower wire dipped into a mercury-cup F,
whence the electric circuit could be completed. The
rod was revolved at such a speed as to make contact
with the strip every five seconds. The strip was bent
at an angle (in a manner which is not apparent from
fig- 5)> so that the rod as it revolved remained in
contact with it for about half a second and then released
it suddenly, forming a quick break of electric circuit.
The explosion-vessel, which was of 500 c.c. capacity,
was filled with the mixture of methane and air (pre-
viously prepared in a gas-holder and analysed) through
the tubulure G, by displacement of mercury. The
mouth H Hiving been closed by glueing a disc of oiled
paper over it, mercury from the reservoir J was allowed
to fill the vessel up to the level of the tubulure. The
small volume of air remaining above the mercury was
swept out with five or six times its volume of the
methane-air mixture through the tap K, which was then
closed. The mercury reservoir was then lowered and
the mixture drawn into it from the gas-holder. The
strip A and the rod B were of platinum. No difference
was observed in the current required for ignition of a
given mixture by the break-flash when the contacts
were of iron or steel but such contacts soon became
worn and pitted and the results obtained with them,
unless freshly polished, were irregular.

MANCHESTER GEOLOGICAL AND
MINING SOCIETY.

A meeting of the Manchester Geological and Mining
Society was held on the 13th inst. in the Geological
Lecture Theatre, Beyer Building, Manchester Univer-
sity, Mr. Leonard R. Fletcher presiding.

The Late Col. Hollingworth.

The Chairman announced that since their last meeting
they had lost their hon. treasurer, Col. Hollingworth,
who died about a month ago. He was elected a member
of the society in 1878, and was appointed vice-president
in 1899. In October of the same year he was elected
hon. treasurer, an office he continued to hold up to the
day of his death. He had also contributed several
papers to the Records, and frequently took part in the
discussions. They had lost a most useful and valuable
member. He moved that their condolences be sent to
the widow and family in the great loss they had
sustained.—Mr. G. Harrison, H.M. inspector of mines,
in seconding, said he had known Col. Hollingworth
since the first Geological meeting he attended, which
was about 21 years ago.—The resolution was carried by
members rising from their seats.

The following new members were elected :—Mr.
Christopher Lambourne, 8, Neville-pIace, Cardiff; and
Mr. Harry Barker Winstanley, 42, Deansgate, Man-
chester.

Drift Deposits of Manchester and Neighbourhood.

Mr. J. E. Wynfield Rhodes, B.Sc., read a paper on
“ The Drift Deposits of Manchester and Neighbour-
hood,” which was illustrated by a series of lantern
slides. The objects of the paper were twofold—first,
to give an account of the composition and mode of
occurrence of the drift deposits of Prestwich; and,
secondly, to give the results of the author’s micro-
scopical research on the composition of some of them.
He said the country between Manchester and Bury was
almost entirely covered with drift. This attracted the
attention of the Manchester geologists in the early days
of the society, particularly Mr. E. W. Binney, who
wrote several papers dealing with the geology of Man-
chester. The area to which this paper refers lies
between the rivers Irwell and Irk, and is most of it
included in Sheet 96 S.W. (1910) of the Six-inch
Ordnance Survey Maps of Lancashire. With one excep-
tion, all the specimens examined came from the area
under consideration, and 25 slides were, prepared from
them.

The lower boulder clay, middle sands, -and upper boulder
clay are separable formations in this area. The upper clay
over a large part of the area is consistently present: in fact,
over a wider area than is indicated in the One-inch Geological
Survey Map, but at its southern edge it is not always map-
able. owing to the way it merges into the sands there. The
sandy material in the middle sand and gravels and in the
upper clay is derived from the Bunter pebble beds rather
than from the carboniiiferous, though traces of characteristic
minerals from the carboniferous are occasionally found,
indicating that a small proportion of the sand may have
been derived from that source. As is stated to be the case,
in other districts the lower boulder clay was produced by
the first advance of the ice in this region, and was com-
pleted before its final retreat. On the other hand, the
moraine-like hills bordering the river vallevs of the Irk and
Irwell were caused by the final melting of the last glacier,
and mark a stage in the final retreat of the ice. The shape
of the hills bordering the valleys is chiefly original, and not
due to post-glacial denudation. The present valleys of the
Irwell and of the Irk and the cloughs are the result of post-

glacial denudation, but the Irwell valley is on the site of a
much older valley, probably determined by the Irwell fault,
which here throws down the soft Bunter pebble beds against
the coal measures.

The author described the samples examined in detail.
He said the middle sands were characterised by the
occurrence of iron stained quartz grains, and by the
small proportion of heavy minerals they contain. These
characteristics are also shared by the Bunter pebble beds,
whilst, on the other hand, the carboniferous sandstones
contain a larger proportion and a wider range of mineral
species than the middle sands. Apatite, rare in the
drift, is characteristic of both the coal measures and the
millstone grit sandstone, whilst the last-named also is
characterised by its abundant garnets.

Mr. Atherton, in moving a vote of thanks to the
author, expressed the opinion that it would be better to
reserve the discussion until they had an opportunity
of seeing the paper in print, when they would be better
able to appreciate the points which had been dealt with.

Mr. Sydney A. Smith seconded, and the resolution was
carried.

Mr. Harrison having given expression to his sense of
the value of the paper, the discussion was adjourned.

Supporting the Roof in Mines.

A discussion followed on Mr. Frank N. Siddall’s
(H.M. inspector of mines) paper on “ Some Notes on
Supporting the Roof in Coal Mines,” which was read at
the last meeting of the society, and reported in our
issues of March 12 and 19.

Mr. Siddall said Mr. Lamb appeared to think that he
(Mr. Siddall) was advocating the Durham system of
deputies setting the timber. He did not mean to do so,
but, as Mr. Gerrard remarked, to recommend the
appointment of special men, under the firemen, to super-
vise the setting of the timber and the building of packs.
With regard to his statement that “ every longwall roof
must and will sink,” what he meant was, that, the solid
coal having been removed, subsidence of the roof in
general would take place to a greater or less extent, and
no timbering or packing could prevent it, in that nothing
so solid as the original coal could be put in its place.
He agreed with Mr. Lamb that the roofs of coal mines
varied enormously, but, taken generally, in the majority
of cases solid packing well done would prove safer and
more economical in the long run than most other
methods of supporting the roof. In answer to Mr. P. L.
Wood, he might say that he made the suggestion that
props ought to be set at right angles to the seam, from
his own experience, in steep mines, steeper even than
Mr. Wood’s, the gradient at times being as much as
1 in 1|. In every case he found that props so set stood
much better than those set nearer to the vertical, and
he also found that where the “ riding weight ” spoken of
by Mr. Ollerenshaw came on, the props pushed uphill
at the head or downhill at the foot. In fact, so great
was this movement that props left far back in the goaf
would eventually lie at such an angle that any set
nearer to the vertical invariably fell out. He had never
seen props in a steep mine do anything but ride over at
the head. He had never seen them ride back or go
uphill at the foot. For that reason he did not agree
with Mr. Ollerenshaw that props set at less than right
angles would stand the riding weight better than those
at right angles. If the weight was riding over two
props, one at right angles to the seam, and the other at
a less angle, he took it that the latter would fall out
before the former, as it had a less distance to travel. He
agreed with the observations of Mr. Orchard. Props
which were set for the particular purpose of keeping up
some loose piece of stone should not be tapered, but
should be as strong as possible, because if that stone
gave way at all it might liberate others.

Mr. V. Bramall said he did not agree with the setting
of props at right angles to the inclination, but probably
half-and-half. In their case, the greatest danger was
not that the prop would fly out, but the fact that in
most instances it had a loose end, and the dirt fell back
into the waste. The dirt, as it were, had a free end,
and would slide over, and in falling would pull the prop
out. That was the main point. At the same time, he
did not think that a prop in a perfectly fast unbroken
roof would have a tendency to fall otit if it was set at
right angles, and, theoretically, that would perhaps be
the best position.

Mr. Harrison (H.M. inspector of mines) said that
some time ago, in the course of an address, he adopted
the same line of argument as Mr. Siddall, that props
should be set practically at right angles to the inclina-
tion of the seam, but he qualified it by the statement
that different seams might necessitate different treat-
ment. A man would have to learn by experience how
his props rode after the setting, and he could quite
conceive cases where, as Mr. Siddall remarked, there
would be a pressure tending to press them towards the
face, even if the face was advancing uphill. With a
roof that would pull neither one way nor the other, there
would still be the force of gravity and the settlement,
and in that case the prop ought to be set at dead right
angles if there was a solid roof. Differences might
occur in the same seam with the same dip where they
were following a longwall face.

The Chairman said the question of timber setting was
a very controversial on’e, and there was a great difference
between saying it should be set “ absolutely at right
angles ” and “ practically at right angles,” because the
latter allowed a little variation, which was important.
As one speaker had remarked, circumstances altered