The Sun.
London, England, 1839-04-06 00:00:00

INTERESTING EXPERIMENTS
ON BLASTING AT CRAIGLEITII QLTARRY.

(From the Scotsman.)
On Tuesday, the 26th ult., a large party of gentle-
men assembled in Craigleith Quarry, at two o'clock, to
witness some experiments on blasting by means of gal-
vanism, which were made, at the request of the Direc-
tors of the Highland and Agricultural Society of Scot-
land, by Martin Roberts, Esq. Among the company
we observed Sir Robert Abercromby, Bart., Sir James
Foulis, Bart., Sir George M'Pherson Grant, Bart., Sir
Charles Gordon, Secretary of the Society, Mr. L'Amy,
Sheriff of Forfarshire, Mr. Cunningham, of Thornton,
Captain Blackwood, R.N., Mr. Campbell, of Jura, Mr.
Grieve, Count Moligny, Mr. Graham, of Redgorton,
Captain Grant, of Carnoustie, Mr. Gregory, Advocate,
Captain Chrystie, H.E.I.C.S. Mr. Inverarity, of the
Madras Engineers, Mr. Learmonth of Leuchars, Major
Allan, Mr. Jameson Torrie, Dr. David Boswell Reid,
Mr. Adie, optician, and Mr. James Jardine and Mr.
Alan Stevenson, civil engineers. Mr. George Johnson,
the lessee of the Quarry, was also present.
It has long been known that the ignition of gunpowder
can be very effectually produced by the application of
the electric fluid ; but Mr. Roberts has succeeded in
producing an apparatus for this purpose, whici. is simple
it its structure, very portable, and which, above all, is
easily managed. He has also, in the application of this
apparatus to blasting rocks, introduced various modifi-
cations of its arrangements, and effected great improve-
ments in the mode of charging.
The apparatus consists of a small trough about a
foot in length, and four inches square on the end, and
a battery containing ten pairs of plates. Along the
battery runs a bar upon which a tin disc slides freely.
This disc, when drawn to the end of the bar, touches
another disc, and thus completes the connection be-
tween the opposite poles of the battery. To prevent
accidents, the sliding disc is kept in the middle of the
bar by means of a spring of coiled wire ; and it is impos
sible to put the battery in action although slink in the
trough, without shifting the plate along the bar to the
opposite end of the trough. The copper wires which con-
vey the electric fluid to the gunpowder, are kept separate
during their whole course by a sheath of cotton thread,
which is wrapped closely round them, in the same man-
ner as in the strings of the guitar, or as in the wire
which stiffens the rim of a lady's bonnet. At their
termination these wires are bent outwards, and their
extremities are connected by means of a fine steel wire
half an inch long, so as to form a small triangle like
the Greek capital delta. This triangular end is
inserted into a small tin cartridge, and ignition of the
powder contained in the cartridge is produced by
the deflagration of the steel wire which connects
the ends of the two copper wires. So rapid is the pro-
gress of the electric fluid, that it is impossible to mea-
sure the interval of time which elapses between the
action at the trough, and the explosion of the call-
ridge. The cost of this apparatus is only about fifteen,
shillings ; and the price of the materials required for
the solution is such, that a shilling will cover the ex-
pense of keeping the trough in a working state for
months. The copper wire, which, if properly shielded
may last for years, costs about one farthing for each
yard. In applying this apparatus to blasting, Mr.
Roberts makes the following arrangements :—ln regard
to the mode of charging, which is perhaps the most
important peculiarity of his method, he leaves a
space of about one foot containing atmospheric air
above and below the gunpowder, and thus obtains,
over and above the effect of the gunpowder, all the
power which the sudden increase of its volume pro-
duces ; and thus the same effect is obtained from a
smaller charge. He also inserts the tin cartridge into
the heart of the charge of powder, and as the cart-
ridge explodes at both ends, the gunpowder is
much more instantaneously ignited. Lastly, in tamp-
ing no vent-hole is left as in the common system,
by the withdrawing of the needle ; but the tamping is
pressed closely round the wire which conveys the
electric fluid from the trough to the cartridge. When
the tamping is completed, the battery is plunged into
the trough, which is at the distance of 40 feet from
the bore-hole, and may of course be removed as far
as may seem desirable, by giving a small increase to
the power of the battery if required, which is easily
effect by adding a pair of plates; The spring of coiled
wire still keeping the tin disc in the middle of the bar,
there is no risk of an unexpected explosion, a danger
which occasionally happens by the too rapid ignition of
a train or fuse in the common method of blasting,
Every one having retired, a person stationed at any
safe distance pulls a string, which makes the tin disc
pass along the bar, and the instant the connection of
the opposite poles of the battery is established, the
explosion takes place. We shall briefly detail the chief
advantage of this new system of blasting, which we
conceive to be as follows :-
1. Freedom from the dangers which always attend
blasting is obtained from various causes. In the com-
mon system, the fuse or train must be fixed at, or very
near, the bore-hole, long trains being expensive and un-
certain in their action ; and accidents, from the too
rapid burning of the fuse, are unfortunately very com-
mon. But in Mr. Roberts' system, the person who
pulls the string which puts the battery in action, may
be stationed at any convenient distance. In the present
system, perhaps the most common source of accident is
the withdrawing of the needle ; and this is completely
avoided in Mr. Roberts' plan. Lastly, there is less
chance of failure, and when failure does occur, the bore-
hole may be at once approached without risk of accident,
as the moment the string is slackened, the action of the
battery ceases.

2. The next advantage is, the great facility which
this mode gives for blasting under water. This is one
of the most inconvenient, expensive, and uncertain of all
engineering operations.
3. The great advantage of a much more rapid igni-
tion of the gunpowder, which incloses the cartridge on
all sides, and receives the action of the flame over the
greater part of its surface at the same instant, gives the
new system a great superiority.
4. There is absolutely no vent-hole in the mode of
tamping pursued by Mr. Roberts, which mode cannot
be applied to the present system of blasting.
5. The advantage of inclosing a column of atmos-
pheric air, as practised by Mr. Roberts, is obvious, for
the force exerted during its expansion is added to that
of the gunpowder itself. What that expansion may be
it is difficult to tell, as we have no good means of ascer-
taining the increase of temperature which accompanies
the explosion of gunpowder ; but as the volume of at-
mospheric air is doubled for every increase of temper-
ature of 4.50 deg. of Fahrenheit, the force produced by
the expansion of the inclosed column of atmospheric
air, must form an important addition to the effect of
the gunpowder. .
6. It follows necessarily from what has been said
.above, that the combined effects of the instantaneous ig-
nition of the gunpowder, the absence of all vent-hcles,
and the expansion of the enclosed column of atmos-
pheric air, must cause a much greater effect than
the explosion of the powder alone in the common
system can produce, and consequently that a great
economy in the article of gunpowder must result. This
is a far more important item in the expense of quar-
rying and rock excavation, than is generally imagined
by those who are unacquainted with such works.
In the excavation for the Philadelphia Water-works,
for example, nearly 3,000/. were expended in
gunpowder, and at the rock-cutting for the new
approach to Edinburgh, by the Calton Hill, 1,0001. was
spent in this item alone. In granite quarries the powder
for a single shot often costs 3/. If the method of Mr.
Roberts produced a saving of about two-thirds of the
quantity of gunpowder required for blasting, as would
appear from the experiments which were made on
Tuesday, some idea may be formed of the great eco-
nomy which would follow on the adoption of the new
system.
7. The system of Mr. Roberts makes the simulta-
neous firing of several blasts easily practised ; and in
many situations, where the removal of the men to a
place of safety is difficult, this is an important advan-
tage.