72

THE COLLIERY GUARDIAN.

January 12, 1917:

An excess of oxygen or a diminution of oxygen such
as often occurs in many mines, if present in buildings’
above ground where people congregate, would be indi-
cative of extremely bad ventilation. However, in mines
having such an excess or diminution the ventilation
may be excellent. In most buildings above ground, the
problem of good ventilation is not to correct a diminished ,
content of oxygen or an increased content of carbon
dioxide, but to maintain proper temperature and rela-
tive humidity, and to keep the air moving, although the
presence of an excessive proportion of carbon dioxide,
more than 0-1 or 0-2 per cent., may be a reliable indi-
cation of air that will produce injurious effects on men.
In these buildings the carbon dioxide comes chiefly from
the air exhaled by the persons present, and, if fresh air
is not admitted, invariably accompanies bad- conditions
such as stagnant and oppressive air, high temperature,
and frequently high humidity. In coal mines, the
carbon dioxide is principally from the action of the
oxygen of the air on the coal, and 0*2. per cent, is
frequently found in the cool, swiftly moving air of
returns where 50,000 or more cu. ft. of air is passing
per minute. Hence, a proportion that accompanies
good conditions of ventilation in a coal mine may indi-
cate extremely poor conditions of ventilation in the
room of a house.

A similar statement applies to oxygen. The oxygen
content is scarcely ever normal in a coal mine, owing to
the gas being absorbed by the coal; in fact, a diminution
of 1 per cent, is not uncommon. Such a diminution in
a building filled with people would be accompanied by
intolerable conditions of ventilation. However, even in
coal mines the oxygen content of the air should not be
allowed to become too low, and the authors believe that
it should not fall below 19 per cent. That this limit
can easily be maintained in coal mines is indicated by
many mine air analyses made by the authors. The
maximum percentage of carbon dioxide allowed in
English coal mines is 1*25 per cent.

.The principal cause of the depletion of oxygen in coal
mine air, and the increase of carbon dioxide, is the
reaction between the oxygen of the air and coal. Some
of the oxygen is actually held dissolved in the coal sub-
stance. Part of the oxygen is converted into water,
part into carbon dioxide, and part (by far the larger
part) is retained as combined oxygen to give compounds
richer in oxygen than the coal itself. Part of the carbon
dioxide is retained by the coal.

Explosive proportions of methane in air 'become non-
explosive when the proportion of oxygen in the atmo-
sphere falls below about 14 per cent. Carbon dioxide
has only a slightly greater effect in reducing the explo-
sibility of methane-air mixtures than nitrogen has; for
instance, when the oxygen is kept. constant at 20 per
cent., part of the nitrogen must be replaced by 10 per
cent, of carbon dioxide to raise the low limit for
methane from 5*8 to 6*2 per cent.

The specific gravity of blackdamp varies consider-
ably in certain mixtures. When methane is present,
the combined gases may be lighter than air. Great
caution should be observed when one detects an accu-
mulation of blackdamp of lighter density than air,
especially in coal mines in which naked lights are used,
as this lesser density is probably due to the presence of
methane.

Except directly over a fire area, or close to a mine
fire, a large proportion of carbon dioxide (more than 3
to 5 per cent.) is unusual in the air of a coal mine. An
oil-fed flame becomes extinguished when the oxygen in
air falls to about 17 per cent.; an acetylene flame is
extinguished when the oxygen falls to about 12 or 13
per cent.

Lack of oxygen is the important factor in extinguish-
ing lights. In some experiments conducted by the
authors, the oxygen content fell to 16-3 per cent, before
the flame became extinguished, but the presence of 10
per cent, of carbon dioxide ' raised the extinguishing
percentage of oxygen to 17*3.

Atmospheres that do not contain enough oxygen to
support an oil-fed flame (about 17 per cent,) may be
explosive when the oxygen content is as low as 14 per
cent., if enough methane is present. When a burning
part of a mine has been successfully sealed, the compo-
sition of the atmosphere within changes. The oxygen
decreases to a proportion (probably about 17 per cent.)
that will not support flame; ultimately the oxygen con-
tent becomes so small that the rate of combustion is
extremely low, so low that combustion entirely ceases,
the embers cool, and the admission of air when the mine
is re-opened does not re-kindle them. '

In 111 samples of gas from 20 mines represented, the
average percentage of carbon dioxide in the blackdamp
was 11’5 per cent., and the average percentage of
nitrogen 89’5 per cent. In six mines of 22 examined
the temperature was higher than it should be (75 degs.
Fahr:, wet bulb) under the best ventilating conditions.

Analyses of a large number of samples .show how
mine air changes as it traverses the workings. The
average composition of the blackdamp was 9*2 per cent,
carbon dioxide and 90-8 per cent, nitrogen. Except for
two or three samples, in which carbon dioxide was high
and the oxygen low, the quality of the air was good.

As regards the unfavourable effect of blackdamp on
men, on lights, and on the explosibility of methane-air
mixtures, the diminution of oxygen in the atmosphere,
resulting in the formation of more nitrogen, is mainly
responsible. The presence of carbon dioxide is far less
important; hence the objection to making the terms
“ blackdamp ” and “ carbon dioxide ” synonymous.

. Many of the mines of the Cripple Creek region are
menaced with gas that is loosely held in the rock strata,
and that issues at times into the mines, so that work-
men cannot enter certain drifts, and occasionally a
whole mine. This gas, according to Lingren and Ran-
some, is of deep-seated origin, and probably represents
the last exhalations of the extinct Cripple Creek volcano.

It contains about 14 per cent, carbon dioxide and 86
per cent, nitrogen. The gas is confined in the rock
strata under very low pressure, so that changes in out-
side . atmospheric pressures affect its outflow. A limited
number of observations. made by the authors indicate
that the direction of the wind influences the outflow of
the gas.	’ :

The average percentage of blackdamp found in the
Cripple Creek mines was 2'67 per cent. The average
composition of the blackdamp was 9’3 per cent, carbon
dioxide and 90*7 per cent, nitrogen, or about the same as
that found in coal mines. In general, the composition
of the air in these metal mines, as shown by the carbon
dioxide and oxygen content, was good. Difficulty is
experienced, however, in keeping down the wet and dry
bulb temperature. In many cases the temperatures
were so high as to be detrimental to health.

STONE DUSTING IN COLLIERIES.

The use -of stone dust in underground workings and
roadways has recently excited a considerable amount of
attention amongst colliery owners, particularly in the
case of mines where the workings are of a dry character,
causing considerable deposits of fine coal dust to accu-
mulate. Any method of eliminating the ever-present
risk of explosion from coal dust, with its attendant.loss
of life and property, deserves the serious consideration
of all colliery companies which are operating dry and
dusty workings; and the very exhaustive tests which
have been carried out under the direction of the Home
Office, both at Altofts and Eskmeals, demonstrate
unquestionably that stone dust, when pulverised to a
certain degree of fineness, and applied in suitable pro-
portions to the underground roads and galleries, acts very
satisfactorily as a preventive of explosions from coal
dust. ‘Whilst not making any claim that stone dust-
ing will absolutely prevent explosions, there is not the
slightest doubt that the application of properly prepared
dust considerably minimises the risk; and if, unfortu-
nately, explosions should occur, the consequences are

T A. STEEL BALLS.

B.	PERMANENT PLATES with PERFORATIONS.

C.	INTERNAL CHECK SCREENS.1

D.	OUTER CONTROLLING SCREEN.	___

I E. APERTURES tor RETURN or REJECTIONS from

-1	INTERNAL OUTER SCREENS, to INTERIOR

or MILL ror FURTHER REDUCTION.

f F. DISCHARGE ro. FINISHED DUST.
G RENEWABLE WEARINC PLATES fitted to

SIDE PLATES or DRUM.	_____

The Scholefield “Ideal” Stone Dust Mill.

less serious, and the danger of recurring explosions is
avoided, where stone dusting has -been efficiently carried
out. In view of the importance of 'the subject, there-
fore, we may draw attention to a special machine which
has been put on the market by Messrs. Scholefield, of
Burley Vale Foundry, Leeds, for grinding shale, bind—
or .similar materials where the content of free silica is
not too high—to a suitable powder for use in spreading
on under ground roads and workings. The advantages
claimed are that the machine is self-contained, and no
auxiliary plant beyond the mill is required for carrying
out the necessary operations of grinding and screening.
The grinding is effected by steel balls, and after the
material passes through the permanent grinding plates,
it is dealt , with by a very ingenious arrangement of
screens combined with the mill, and this absolutely
ensures all material being reduced to the required fine-
ness before passing out of the machine. All coarse
material is automatically returned to the interior of the
mill for further reduction. The machine' is provided
with a steel plate dust casing, and there is therefore no
nuisance from a dust-laden atmosphere in the vicinity
of the mill when working. The parts subject to wear
and tear are . few -and the power necessary for driving
very moderate compared with the output. The mill
delivers a finished product, of which 30 per cent, will
pass la 200-mesh test screen, and 70 per cent, go through
a 100-mesh screen, whilst every particle will pass
through a 30-mesh (900 holes per square inch) screen.
A special type of feed hopper enables the machine to be
fed with comparatively large pieces. The makers supply
various sizes of machines, having outputs varying from
10 cwt. and upwards per day. In order to obtain the
maximum life from the grinding plates, these are made
of a special chilled iron, which it is claimed is superior
to steel for this particular purpose. The machine illus-
trated is the No. 1 size, which has a capacity of about
10 cwt. of finished dust per hour, the exact output
depending upon the hardness of the shale 'and size of

pieces fed into the machine. During the past two years
a considerable number of these machines have been
installed at some of the largest collieries in the country
in Yorkshire, Lancashire, Notts, Staffordshire, etc.

MANCHESTER GEOLOGICAL AMD
MINING SOCIETY.

A meeting of the Manchester Geological and Mining
Society was held on Tuesday last at the rooms of the
society, John Dal ton-street, Manchester.

The Late Mr. L. R. Fletcher.

The President (Mr. Wm. Pickup) wished the mem-
bers the compliments of the season, and expressed the
hope that the new year would bring peace again to the
world. He went on to say that he deeply regretted to
have to announce the death of Mr. Leonard R. Fletcher,
who had been a member of the society for 20 years, had
served on the council for six years, and had been its
esteemed president for two years up to July of last year.
He was a most valuable member of the society, having
a very intimate knowledge of its requirements, and
taking the greatest possible interest in all matters
appertaining to the question of mining. He devoted a
large amount of his time and thought to the, promotion
and well-being of the organisation, and contributed in
no small measure to its usefulness and success. He
moved that a letter be forwarded to Mrs. Fletcher and
family, conveying the condolence land very sincere
sympathy of the meeting with them in their bereave-
ment, and expressing how greatly Mr. Fletcher’s ser-
vices had. been valued and appreciated by the mining
community generally.

Mr. John Gerrard, in seconding, said the passing
away of Mr. Fletcher, with whom he had been in close
touch for 20 years, was a great blow to the society land
his friends. The deceased had attained that period in
life when his knowledge and experience were bearing
fruit more and more each year. He was in all respects
a most worthy man, and a most excellent mining engi-
neer, who had a kindly feeling for his officers and men,
and he would be greatly missed at the Atherton
Collieries.

Prof. Boyd Dawkins associated himself with the
remarks made by ’the previous speakers. The loss
caused by the death of Mr. Fletcher would fall, not
merely on the Manchester Geological and Mining
Society, but upon all those organisations with which
he was connected.

The resolution was adopted.

Mir. W. T. Anderson read his paper on “ Notes on an
Old Colliery Pumping Engine (1791).”

Notes on an Old Colliery Pumping Engine (1791).

The author stated that he had recently been connected
with the installation of certain plant at a colliery (the
Pentrich Colliery), which threw into disuse—or partial disuse
—an engine of the “ atmospheric” type, which, there was
every reason' to believe, had been in commission for 125
years.

A close examination of the engine proved that it was
undoubtedly the adaptation of Newcomen’s model experi-
mentally determined upon by John Smeaton, in 1769, the
design of which was first put into practical use at the Long
Benton Colliery, near Newcastle-on-Tyne, in 1772 or 1774,
in a machine of 40 horse-power. His improvements appear
to have consisted in constructing Newcomen’s model to
correct proportions, and—by increasing the area of the
cylinder and the length of the stroke—in greatly increasing
the speed of the piston.

The Pentrich engine, according t<5 lettering moulded round
the collar of the cylinder, appears to have been built by
F. Thompson in 1791. Ab Smeaton’s success was only
achieved 18 years previous to that date, if may reasonably
. be assumed that the apparatus was among the earliest of the
type in use. Of Thompson himself, no mention appears to
be made in the technical works of the period, except that
the engine erected at Yatestoop mine, Derbyshire (about
1’780) is said to have been “planned and executed by Mr.
Francis Thompson,-of Ashover, in Derbyshire, an operative
engineer, who had an extensive practice in that district, at
the period when its mines were in great activity.” In 1792
he took out a patent (No. 1884) for an atmospheric engine
with cylinders in tandem—in other words, a double-acting
engine.

The writer has had access to documents of considerable
historic interest, which throw some light on the life of
Thompson and his surroundings. He appears to have
worked intermittently, from the year 1768, at the Gregory
lead mine, Ashover (which was one of the most important
metalliferous mines in the country between 1550 and 1787).
His exact position there is obscure, although it is probable
that he was the son of an engine or pump tender called
Stephen Thompson, whose name figures regularly through-
out a period of many years in the pay records now in the
possession of Mr. J. H. Twigg, of Kelstedge.

In 1774 the Gregory mine is said to have been saved, and
to have become wonderfully profitable, through the installa-
tion of an engine, an original drawing and detailed descrip-
tion of which (bearing the name of Francis Thompson) is
still extant, inscribed “ Multum en Parvo by Fr*. Thompson,
1774—A Draught of the fire engine at Gregory’s Mine, near
Ashover.” The description ends with an assurance that no
danger need be apprehended from it.,

Whether Thompson was at this time a representa/tive of
the manufacturers or was the actual designer is not clear;
he may have been both, especially as it was a custom of the
period to pay for machinery by result, either in the form of
a percentage on the quantity of minerals raised, or on the
number of strokes recorded.

It has been suggested that Thompson’s 1774 engine was
manufactured by Boulton and Watt, but this is more than
doubtful, as Watt did not go to the Soho Works until that
year, the partnership between the two eminent engineers
being arranged in 1775. The machine also was of earlier
design than their standard type — without a separate
condenser.

As to the soundness of his work, the following details of
the Pentrich engine, recorded after a century and a quarter’s
service, will bear witness :—There does not seem to be any