With respect to the subject of tunneling thus incidentally introduced,
in subaqueous work of this kind, I have already alluded to that which
is done by "cut and cover," but where the influx of water is a source
of great difficulty, as it was in the old Thames tunnel (though in
this case for water one should read silt or mud), I do not know that
anything has been devised so ingenious as the Thames tunnel shield;
improvement has, however, been made by the application of compressed
air.
In the instance of the Hudson River tunnel, the work was done in the
manner proposed so long ago as the year 1830 by Lord Cochrane (Earl
Dundonald) in that specification of his, No. 6,018, wherein he
discloses, not merely the crude idea, but the very details needed for
compressed air cylinder-sinking and tunneling, included air-locks
and hydraulically-sealed modes for the introduction and extraction of
materials. I may, perhaps, be permitted to mention that some few years
ago I devised for a tunnel through the water-bearing chalk a mode of
excavation by the use of compressed air to hold back the water, and
combined with the employment of a tunneling machine. This work, I
regret to say, was not carried out. But there are, happily, cases of
subaqueous tunneling where the water can be dealt with by ordinary
pumping power, more or less extensive, and where the material is
capable of being cut by a tunneling machine. This was so in the Mersey
tunnel, and would be in the Channel tunnel. In the Mersey tunnel, and
in the experimental work of the Channel tunnel, Colonel Beaumont and
Major English's tunneling machine has done most admirable work. In the
7 foot 4 inch diameter heading, in the new red sandstone of the Mersey
tunnel, a speed of as much as 10 yards forward in twenty-four hours
has been averaged, while a maximum of 11-2/3 yards has been attained;
while in the 7 foot heading for the Channel tunnel, in the gray chalk,
a maximum speed of as much as 24 yards forward in the twenty-four
hours has been attained on the English side; and with the later
machine put to work at the French end, a maximum speed of as much as
27-1/3 yards forward in the twenty-four hours has been effected. In
ordinary land tunneling since 1862 there has been great progress, by
the substitution of dynamite and preparations of a similar nature
for gunpowder, and by the improvements in the rock-drills worked by
compressed air, which are used in making the holes into which the
explosive is charged. For boring for water, and for many other
purposes, the diamond drill has proved of great service, and most
certainly its advent should be welcomed by the geologist, as it has
enabled specimens of the stratum passed through to be taken in the
natural, unbroken condition, exhibiting not only the material and the
very structure of the rock, but the direction and the angle of the dip
of the beds.
Closely connected with tunneling machines are the machines for
"getting" coal. This "getting," when practiced by manual labor,
involves, as we know, the conversion into fragments and dust of a very
considerable portion of the underside of the seam of coal, the workman
laboring in a confined position, and in peril of the block of coal
breaking away and crushing him beneath it. Coal-getting machines, such
as those of the late Mr. Firth, worked by compressed air, reduce to
a minimum the waste of coal, relieve the workman of a most fatiguing
labor in a constrained position, and save him from the danger to which
he is exposed in the hand operation. It is a matter of deep regret on
many grounds, but especially as showing how little the true principles
of political economy are realized by working men, who are usually well
informed on many other points, that the commercial failure of these
machines is due to their opposition. In connection with colliery
work, and indeed in connection with explosives, in the sense of a
substitution for them of sources of expansion acting more slowly,
mention should be made of the hydraulic wedges. The employment of
these in lieu of gunpowder, to force down the block of coal that had
been undercut, is one of the means to be looked to for diminishing the
explosions in collieries. Another substitute for gunpowder is found in
the utilization of the expansion of lime when wetted. This has given
birth to the lime cartridge, the merits of which are now universally
recognized, but it is feared that trade prejudices may also prevent
its introduction. While on this subject of "accidents in mines," it
will be well to call attention to the investigations that have been
made into the causes of these disasters, and into the probable part
played by the minute dust which prevails to so great an extent in dry
collieries.
The experiments of our honorary member, Sir Frederick Abel, on this
point have been of the most striking and conclusive character, and
corroborate investigations of the late Macquorn Rankine into the
origin of explosions in flour mills and rice mills, which had
previously been so obscure. The name of Mr. Galloway should also be
mentioned as one of the earliest workers in this direction. At
first sight, pile driving appears to have but little connection with
explosives, but it will be well to notice an invention which has been
brought into practical use, although not largely (in this country at
all events), for driving piles, by allowing the monkey to fall on
a cartridge placed in the cavity in the cap on top of the pile; the
cartridge is exploded by the fall, and in the act of explosion drives
down the pile and raises the monkey; during its ascent, and before the
completion of its descent, time is found for the removal of the empty
cartridge and the insertion of a new one.