and carried into effect some time ago, and more recently has been
applied to lines of tubes in connection with the General Post Office.
These tubes pass from Euston Station down Drummond Street, Hampstead
Road, Tottenham Court Road, to Broad Street, St. Giles’s, whence, with a
sharp bend, they proceed to the Engine Station at Holborn, and then to
the Post Office. The tube is formed chiefly of cast iron pipes of a
⌓-shaped section, 4 ft. 6 in. wide and 4 ft. high, in 9 ft. lengths.
There are curves with radii of 70 ft. and upwards, and at these parts
the tube is made of brickwork and not of iron. The carriages run on four
wheels, and are so constructed that the ends fit the tubes nearly, and
the interval left is partly closed by a projecting sheet of india-rubber
all round. The carriages are usually sent through the tube in trains of
two or three, and the trains are drawn forward by an exhausting
apparatus formed by a fan, 22 ft. in diameter, worked by two horizontal
steam engines having cylinders 24 in. in diameter and a stroke of 20 in.
The air rushes by centrifugal force from the circumference of the fan,
and is drawn in at the centre, where the exhaust effect is produced. The
tubes which convey the air from the main tube open into the latter at
some distance from its extremities, which are closed by doors, so that
after the carriage passes the entrance of the suction tube, its momentum
is checked by the air included between it and the doors, which air is,
of course, compressed by the forward movement of the carriage. At the
proper moment the doors are opened by a self-acting arrangement, and the
carriage emerges from the tube. There are two lines of tube—an “up” and
a “down” line—and means are provided for rapidly transferring the
carriages from one to the other at the termini. The time occupied in the
transit is about 12 minutes. Some of the inclines have as much slope as
1 in 14, yet loads of 10 or 12 tons weight are drawn up these gradients
without difficulty. The mails are sent between Euston Station and the
Post Office by means of these tubes. Passengers have also made the
journey as an experiment by lying down in the carriages. Fig. 174 shows
one of the carriages and the entrance to the tubes.
Great expectations have been formed by some persons of the applications
of pneumatic force. Some have suggested its use for moving the trains in
the proposed tunnel between England and France. But calculations show
that for long distances and large areas such modes of imparting motion
are enormously wasteful of power. Thus, in the tunnel alluded to it must
be remembered that not only the train, but the whole mass of air in the
tunnel would have to be drawn or pushed forward. The drawing of a train
through by exhausting the air would be very similar to drawing it
through by a rope; in fact, the mass of air may be regarded as a very
elastic rope, but by no means a very light one, or one that could be
drawn through without some opposing force which has a certain
resemblance to friction coming into operation. Indeed, it has been
calculated that in the case named, only five per cent. of the total
power exerted by the engines in exhausting the air could possibly
produce a useful effect in moving the train.
Air has also been made the medium for conveying intelligence in another
manner than by shooting written messages through tubes, for its property
of transmitting pressure has been applied to produce at a distance
signals like those made use of in the electric telegraph system. A few
years ago, an apparatus for this object was contrived by Signor
Guattari, whose invention is known as the “Guattari Atmospheric
Telegraph.” In this there is a vessel charged with compressed air by a
compression-pump, and the pressure is maintained by the same means,
while the reservoir is being drawn upon. A valve is so arranged that the
manipulator can readily admit the compressed air to a tube extending to
the station where the signals are received, at which the pressure is
made to move a piston as often as the sender opens the valve. This
movement is made to convey intelligence when a duly regulated succession
of impulses is sent into the tube—the receiving apparatus being arranged
either to give visible or audible signals, or to print them on slips of
paper, according to any of the methods in use with the electric
telegraph. Certain advantages over the electric system are claimed for
this pneumatic telegraph—as, for example, greater simplicity and less
liability to derangement. The tubes, which are merely leaden piping of
small bore, are also exempt from the inconvenient interruptions which
electric communication sometimes suffers from electrical disturbances in
the atmosphere. The pneumatic system is easily arranged, and from its
great simplicity any person can in a few hours learn to use the whole
apparatus, while it is calculated that the expense of construction and
working would not be above half of that incurred for the electric
system. For telegraphs in houses, ships, warehouses, and short lines,
this invention will doubtless prove very serviceable; but for long lines
a much greater force of compression would be required, and the time
needed for the production of an impulse at the distant ends of the tubes
would be considerably increased. [1875].
[Illustration]
[Illustration:
FIG. 178.—_The Sommeiller Boring Machines._
]
ROCK BORING.
Allusion has already been made to one great characteristic of our age,
namely, the replacement, in every department of industry, of manual
labour by machines. A brief notice of even the main features of the
various contrivances which have been made to take the place of men’s
hands would more than occupy this volume. Accordingly, we must omit all
reference to many branches of manufacture, although the products may be
of very great utility, and the processes of very high interest; and in
taking one example here and another there, we must be guided mainly by
the extent and depth of the influence which the new invention appears
destined to exert. This consideration has, with scarcely an exception,
decided the selection of the topics already discussed, and it has also
determined the introduction of the present article, which relates to
machines of no less general importance than the rest, although at first
sight it might seem to enter upon the details of merely a special branch
of industry. But so general are the interests connected with the subject
we are about to lay before our readers, that we are not sure it would
not have been more logical to have placed the present article before all
the rest. For whence comes the iron of which our steam engines, tools,
rails, ships, cannon, bridges, and printing presses are made?—whence
comes the fuel which supplies force to the engines?—whence come, in
fine, the substances which form the _matériel_ of every art? Plainly
from the earth—the nurse and the mother of all, and in most cases from
the bowels of the earth, for her treasures are hidden far below the
surface—the coal, and the ores of iron and other metals, are not ready
to our hand, exposed to the light of day. The railways also, and the
canals, can be made only on condition that we cut roads through the
solid rocks, and pierce with tunnels the towering mountains. Hence the
tools which enable us to penetrate into the substance of the earth
present the highest general interest from a practical point of view, and
this interest is enhanced by the knowledge of the structure and past
history of our planet acquired in such operations.
The operations by which solid rocks are penetrated in the sinking of
shafts for mines, or in the driving of tunnels, drifts, headings,
galleries, or cuttings for railways, mines, or other works, are easily
understood. In the first place a number of holes—perhaps 3 ft. or 4 ft.
deep and 2 in. or 3 in. in diameter—are formed in the rock. The holes
are then charged with gunpowder or other explosive materials, a
slow-burning match is adjusted, the miners retire to a safe distance,
the explosion takes place—detaching, shattering, and loosening masses of
the rock more or less considerable; and then gangs of workmen clear away
the stones and _débris_ which have been detached by the explosion, and
the same series of operations is renewed. The holes for the blasting
charges are formed by giving repeated blows on the rock with a kind of
chisel called a _jumper_—the end of which is formed of very hard steel,
so that the rock is in reality chipped away. The _débris_ resulting from
this operation is cleared away from time to time by a kind of auger or
some similar contrivance. But for many purposes it is necessary to drill
holes in rocks to great depths, hundreds of feet perhaps, as for
example, in order to ascertain the nature of underlying strata, or to
verify the presence of coal or other minerals before the expense of
sinking a shaft is incurred. These bore-holes were commonly formed in
exactly the same manner as the blast-holes already mentioned, by
repeated blows of a chisel or jumper, which was attached to the end of a
rod; and as the hole deepened, additional lengths of rod were joined on,
and the rods were withdrawn from time to time to admit of the removal of
the _débris_ by augers, or by cylinders having a valve at the bottom.
The reciprocating movement is given to the chisels and rods either by
hand or by steam or water power. When the length of the rods becomes
considerable, of course the difficulty of giving the requisite blows in
rapid succession is greatly increased, for the whole length of rods has
to be lifted each time, and if allowed to fall with too much violence,
the breaking of the chisel or the rods is the inevitable result. The
time requisite for drawing out the rods, removing the fragments chipped
out, and again attaching the rods and lowering, also increases very much
as the bore gets deeper. Messrs. Mather and Platt, the Manchester
engineers, have, in order to obviate these difficulties, constructed
machines in which the chipping or cutting is done by the fall of a tool
suspended from a rope, the great advantage resulting from the
arrangement being the facility and rapidity with which the tools used
for the cutting and for the removal of the _débris_ are lowered to their
work and drawn up. It is necessary in using the jumper, whether in
cutting blast-holes or bore-holes, to give the tool a slight turn after
each blow, in order that the rock may be chipped off all round, and the
action of the tool equalized. Many attempts have been made to drill
rocks after the fashion in which iron is drilled—that is, by drilling
properly so called, in which the tool has a rapid rotary motion. But
even in comparatively soft rock, it is found that no steel can
sufficiently withstand the abrading action of the rock, for the tool
becomes quickly worn, and makes extremely slow progress. We shall have
presently to return to the subject of bore-holes; but now let us turn
our attention to an example which will illustrate the nature and
advantages of the machinery which has in recent times been applied to
work the jumpers by which the holes for blasting are formed.
_THE MONT CENIS TUNNEL._
The successful construction, by the direction of Napoleon, of a broad
and easy highway from Switzerland into Italy, crossing the lofty Alps
amid the snows and glaciers of the Simplon, has justly been considered a
feat of skill redounding to the glory of its designers. But we have
recently witnessed a greater feat of engineering skill, for we have seen
the Alps conquered by the stupendous work known as the Mont Cenis
Tunnel. This tunnel is 7½ English miles in length; but it is not the
mere length which has made the undertaking remarkable. The mountain
which is pierced by the tunnel is formed entirely of hard rock, and what
added still more to the apparently impracticable character of the
proposal when first announced was the circumstance that it was quite
impossible to sink vertical shafts, so that the work could not, as in
the usual process, be carried on at several points simultaneously, but
must necessarily be continued from the two extremities only, a
restriction which would occasion a vast loss of time and much expense,
to say nothing of the difficulties of ventilating galleries of more than
three miles in length. The reader must bear in mind that the importance
of this question of ventilation depends not simply on the renewing of
the air contaminated by the respiration of the workmen, but on the quick
removal of the noxious gases produced in the explosions of the blasting
charges. A work surrounded by such difficulties would probably have
never been attempted had not Messrs. Sommeiller and Co. invited the
attention of engineers to an engine of their invention, worked by
compressed air, and capable of automatically working “jumpers” which
could penetrate the hardest rock. These rock-boring machines, having
been examined by competent authorities in the year 1857, were pronounced
so efficient that the execution of the long-spoken-of Alpine tunnel was
at once resolved upon, and before the close of that year the work had
actually been commenced, after a skilful and accurate survey of the
proposed locality had been made, and the direction of the tunnel set
out. The tunnel does not pass through Mont Cenis, although the post road
from St. Michel to Susa passes over part of Mont Cenis, which gives its
name to the pass. The mountain really pierced by the tunnel is known as
the Grand Vallon, and the tunnel passes almost exactly below its summit,
but at a depth the perpendicular distance of which is as nearly as
possible one mile. The northern end of the tunnel is near a village
named Fourneaux.
Pending the construction of the Sommeiller machines, and other machinery
which was to supply the motive force, the work of excavation was
commenced at both ends, in 1857, in the ordinary manner, that is, by
hand labour, and in 1858 surveys of the greatest possible accuracy were
meanwhile made, in order that the two tunnels might be directed so that
they would meet each other in the heart of the mountain. The reader will
at once perceive that the smallest error in fixing on the direction of
the two straight lines which ought to meet each other would entail very
serious consequences. The difficulties of doing this may be conceived
when we remember that the stations were nearly 8 miles apart, separated
by rugged mountains, in a region of snows, mists, clouds, and winds,
over which the levels had to be taken, and a very precise triangulation
effected. So successfully were these difficulties overcome, and so
accurately were the measurements and calculations made, that the
junction of the centre lines of the completed tunnel failed by only a
_few inches_, a length utterly insignificant under the conditions.
The work was carried on by manual labour only, until the beginning of
1861, for it was found, on practically testing the machinery, that many
important modifications had to be made before it could be successfully
employed in the great work for which it was designed. After the
machinery had been set to work, at the Bardonnêche end, breakages and
imperfections of various parts of the apparatus, or the contrivances for
driving it, caused delay and trouble, so that during the whole of 1861
the machines were in actual operation for only 209 days, and the
progress made averaged only 18 in. per day, an advance much less than
could have been effected by manual labour. The engineers, not
disheartened or deterred by these difficulties and disappointments,
encountered them by making improvement after improvement in the
machinery as experience accumulated, so that a wonderful difference in
the rate of progress showed itself in 1862, when the working days
numbered 325, and the average rate of advance was _three feet nine
inches per day_.
At the Fourneaux extremity more time was required for the preparation of
the air-compressing machinery, and the machines had been at work in the
other extremity, with more or less interruption, for nearly two years
before the preparations at Fourneaux were completed.
The illustration at the head of this article, Fig. 178, represents the
Sommeiller machines at work, the motive power being compressed air,
conveyed by tubes from receivers, into which it is forced until the
pressure becomes equal to that of six atmospheres, or 90 lbs. per square
inch. The compression was effected by taking advantage of the natural
heads of water, which were made to act directly in compressing the air;
the pressure due to a column of water 160 ft. high being made to act
upwards, to compress air, and force it through valves into the
receivers; then the supply of water was cut off, and that which had
risen up into the vessel previously containing air was allowed to flow
out, drawing in after it through another valve a fresh supply of air;
and then the operations were repeated by the water being again permitted
to compress the air, and so on, the whole of the movements being
performed by the machinery itself. The compressed air, after doing its
work in the cylinders of the boring tools, escaped into the atmosphere,
and in its outrush became greatly cooled, a circumstance of the greatest
possible advantage to the workmen, for otherwise, from the internal
warmth of the earth, and that produced by the burning of lights,
explosions of gunpowder, and respiration, the heat would have been
intolerable. At the same time, the escaping air afforded a perfect
ventilation of the workings while the machines were in action. At other
times, as after the explosion of the charges, it was found desirable to
allow a jet of air to stream out, in order that the smoke and carbonic
acid gas should be quickly cleared away. Even had the work been done by
manual labour alone, a plentiful supply of compressed air would have
been required merely for ventilation, so that there was manifest
advantage in utilizing it as the motive power of the machines.
[Illustration:
FIG. 179.—_Transit by Diligence over Mont Cenis._
]
The experience gained in the progress of the work suggested from time to
time many improvements in the machinery and appliances, which finally
proved so effectual that the progress was accelerated beyond
expectation. At the end of 1864, when the machines had been in work
about four years, it was calculated that the opening of the tunnel might
be looked for in the course of the year 1875. But in point of fact it
happened that on the 25th December, 1870, perforator No. 45 bored a hole
from Italy into France, by piercing the wall of rock, about 4 yards
thick, which then separated the workings from each other. The centre
lines of the two workings, as set out from the different sides of the
mountain, failed to coincide by only a foot, that set out on the
Fourneaux side being this much higher than the other, but their
horizontal directions exactly agreeing. The actual length of the tunnel
was found to be some 15 yards longer than the calculated length, the
calculation having given 7·5932 miles for the length, whereas by actual
measurement it was found to be 7·6017 miles. The heights above the
sea-level of the principal points are these:
Feet.
Fourneaux, or northern entrance 3,801
Bardonnêche, or southern entrance 4,236
Summit of tunnel 4,246
Highest point of mountain vertically over the tunnel 9,527
The tunnel is lined with excellent brick and stone arching, and it is
connected with the railways on either side by inclined lines, which are
in part tunnelled out of the mountain, so that the extremities of the
tunnel referred to above are not really entered by the trains at all;
but these lateral tunnels join the other and increase the total distance
traversed underground to very nearly 8 miles, or more accurately, 7·9806
miles. The time required by a train to pass from one side to the other
is about 25 minutes. What a contrast is this to the old transit over the
Mont Cenis pass by “diligence”! We have the scene depicted in Fig. 179,
where we perceive, sliding down or toiling up the steep zigzag ascents,
a series of curious vehicles drawn by horses with perpetually jingling
bells.
The cost of the Mont Cenis Tunnel was about £3,000,000 sterling, or
upwards of £200 per yard; but as a result of the experience gained in
this gigantic work, engineers consider that a similar undertaking could
now be carried out for half this cost. It is supposed that the profit to
the contractors for the Mont Cenis Tunnel was not much less than £100
per yard. The greatest number of men directly employed on the tunnel at
one time was 4,000, and the total horse-power of the machinery amounted
to 860. From 1857 to 1860, by hand labour alone, 1,646 metres were
excavated; from 1861 to 1870 the remaining 10,587 metres were completed
by the machines. The most rapid progress made was in May, 1865, in which
month the tunnel was driven forward at one end the length of 400 feet.
When the workings were being carried through quartz, a very hard rock,
the speed was greatly reduced—as, for example, during the month of
April, 1866, when the machines could not accomplish more than 35 ft.
The perforators used in the Mont Cenis Tunnel were worked by compressed
air, conveyed to a small cylinder, in which it works a piston, to the
rod of which the jumper is directly attached. The air, being admitted
behind the piston, impels the jumper against the rock, and the tool is
then immediately brought back by the opening of a valve, which admits
compressed air in front of the piston, at the same time that the air
which has driven it forward is allowed to escape, communication with the
reservoir of compressed air having previously been closed behind it. The
whole of these movements are automatic, and they are effected in the
most rapid manner, four or five blows being struck in every second, or
between two and three hundred in one minute. Water was constantly forced
into the holes, so as to remove the _débris_ as quickly as it was
formed. A number of these machines were mounted on one frame, supported
on wheels, running on the tramway which was laid along the gallery. The
perforators had no connection with each other, for each one had its own
tube for the conveyance of compressed air, and its own tube to carry the
water used for clearing out the hole, and the cylinders were so fixed on
the frames that the jumpers could be directed in any desired manner
against any selected portion of the rock. They were driven to an average
depth of about 2½ ft., and the process occupied from forty to fifty
minutes. When a set of holes had thus been formed, the cylinders were
shifted and another series commenced, until about eighty holes had been
bored, the formation of the whole number occupying about six or seven
hours, and the holes being so arranged that the next operation would
detach the rock to the required extent. The flexible tubes, which
conveyed the air and water to the machines from the entrances, were then
removed from the machines and stowed away, the frame bearing the
perforators was drawn back along the tramway, workmen advanced whose
duty it was to wipe out the holes, charge them with powder, and fix the
fuses ready for the explosion. When the slow-burning match was ignited,
all retired behind strong wooden barricades, at a safe distance, until
the explosion had taken place; and after the compressed air had been
allowed to stream into the working, so as to clear away all the smoke
and gas generated by the explosion, the workmen ran up on a special
tramway the waggons which were to carry away all the detached stones;
and when this had been done, the floor was levelled, the tramways were
lengthened, and the frame bearing the drilling machines was brought up
to begin a fresh series of operations, which were usually repeated about
twice in the course of every twenty-four hours. A great part of the rock
consists of very hard calcareous schist, interspersed with veins of
quartz, one of the hardest of all rocks, which severely tries the temper
of the steel tools, for a few blows on quartz will not unfrequently
cause the point of a jumper to snap off.
_ROCK-DRILLING MACHINES._