Transcriber’s Notes
Texts printed in italics in the source document have been transcribed
_between underscores_; small capitals have been replaced by ALL
CAPITALS. ^{T} represents a superscript T, ~T~ a T-shape part rather
than the letter T.
More Transcriber’s Notes will be found at the end of this text.
ROCK BLASTING.
ROCK BLASTING.
A
PRACTICAL TREATISE
ON THE
MEANS EMPLOYED IN BLASTING ROCKS
FOR INDUSTRIAL PURPOSES.
BY
GEO. G. ANDRÉ, F.G.S., ASSOC. INST. C.E.,
MINING CIVIL ENGINEER; MEMBER OF THE SOCIETY OF ENGINEERS.
[Illustration]
LONDON:
E. & F. N. SPON, 46, CHARING CROSS.
NEW YORK:
446, BROOME STREET.
1878.
PREFACE.
During the past decade, numerous and great changes have taken place in
the system followed and the methods adopted for blasting rocks in
industrial operations. The introduction of the machine drill led
naturally to these important changes. The system which was suitable to
the operations carried on by hand was inefficient under the requirements
of machine labour, and the methods which had been adopted as the most
appropriate in the former case were found to be more or less unsuitable
in the latter. Moreover, the conditions involved in machine boring are
such as render necessary stronger explosive agents than the common
gunpowder hitherto in use, and a more expeditious and effective means of
firing them than that afforded by the ordinary fuse. These stronger
agents have been found in the nitro-cotton and the nitro-glycerine
compounds, and in the ordinary black powder improved in constitution
and fired by detonation; and this more expeditious and effective means
of firing has been discovered in the convenient application of
electricity. Hence it is that the changes mentioned have been brought
about, and hence, also, has arisen a need for a work like the present,
in which the subjects are treated of in detail under the new aspects due
to the altered conditions.
GEO. G. ANDRÉ.
LONDON, 17, KING WILLIAM STREET, STRAND,
_January 1st, 1878._
CONTENTS.
CHAPTER I.
THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN ROCK BLASTING.
PAGE
Section I. _Hand-boring Tools._--Drills. Hammers. Auxiliary
Tools. Sets of Blasting Gear 1
Section II. _Machine-boring Tools._--Machine Rock-drills. Borer-
bits. Drill Carriages 23
Section III. _Appliances for firing Blasting Charges._--Squibs.
Safety Fuse. Electric Fuses. Cables. Detonators. Electric Firing-
Machines 42
CHAPTER II.
EXPLOSIVE AGENTS USED IN ROCK BLASTING.
Section I. _Phenomena accompanying an Explosion._--Nature of an
Explosion. Heat liberated by an Explosion. Gases generated by an
Explosion. Force developed by an Explosion 64
Section II. _Nature of Explosive Agents._--Mechanical Mixtures.
Chemical Compounds 76
Section III. _Relative Strength of the common Explosive Agents._--
Force developed by Gunpowder. Relative Force developed by
Gunpowder, Gun-cotton, and Nitro-Glycerine 88
Section IV. _Means of firing the common Explosive Agents._--Action
of Heat. Detonation 92
Section V. _Some Properties of the common Explosive Agents._--
Gunpowder, Gun-cotton, Dynamite. Firing Temperatures 97
Section VI. _Some Varieties of the Nitro-Cellulose and the Nitro-
Glycerine Compounds._--Nitrated Gun-cotton. Tonite, or Cotton-
Powder. Schultze’s Powder. Lithofracteur. Brain’s Powder.
Cellulose-Dynamite 103
CHAPTER III.
THE PRINCIPLES OF ROCK BLASTING.
Line of least Resistance. Force required to cause Disruption.
Conditions of Disruption. Example of a Heading. Economical
Considerations. Tamping 106
CHAPTER IV.
THE OPERATIONS OF ROCK BLASTING.
_Hand Boring._--Boring the Shot-holes. Charging the Shot-holes.
Firing the Charges 128
_Machine Boring._--Boring the Shot-holes. Charging and Firing.
Removing the dislodged Rock. Division of Labour 142
_Examples of Drivings._--The St. Gothard Tunnel. The Hoosac
Tunnel. The Musconetcong Tunnel. Headings at Marihaye, Anzin, and
Ronchamp 157
CHAPTER V.
SUBAQUEOUS BLASTING.
Preparation of the Charge. Boring under Water. Submarine Rocks.
Obstructions in Water-courses 164
ROCK BLASTING.
CHAPTER I.
THE TOOLS, MACHINES, AND OTHER APPLIANCES USED IN BLASTING ROCKS.
SECTION I.--HAND BORING.
_Drills._--The operations of blasting consist in boring suitable holes
in the rock to be dislodged, in inserting a charge of some explosive
compound into the lower portion of these holes, in filling up,
sometimes, the remaining portion of the holes with suitable material,
and in exploding the charge. The subjects which naturally first present
themselves for consideration are: the nature, form, and construction of
the tools, machines, and other appliances used. Of these tools, the
“drill” or “borer” constitutes the chief. To understand clearly the
action of the rock drill, we must consider the nature of the substance
which has to be perforated. He who has examined the mineral constitution
of rocks will have recognised the impossibility of _cutting_ them, using
that term in its ordinary acceptation, inasmuch as the rock constituents
are frequently harder than the material of the tools employed to
penetrate them. As a rock cannot be cut, the only way of removing
portions of it is to fracture or to disintegrate it by a blow delivered
through the medium of a suitable instrument. Each blow so delivered may
be made to chip off a small fragment, and by this means the rock may be
gradually worn away. To effect this chipping, however, the instrument
used must present only a small surface to the rock, in order to
concentrate the force, and that surface must be bounded by inclined
planes or wedge surfaces, to cause a lateral pressure upon the particles
of rock in contact with them. In other words, the instrument must be
provided with an edge similar to that possessed by an ordinary _cutting_
tool.
The conditions under which the instrument is worked are obviously such
that this edge will be rapidly worn down by attrition from the hard rock
material, and by fracture. To withstand these destructive actions, two
qualities are requisite in the material of which the instrument is
composed, namely, hardness and toughness. Thus there are three important
conditions concurring to determine the nature and the form of a cutting
tool to be used in rock boring--1, a necessity for a cutting edge; 2, a
necessity for a frequent renewal of that edge; and 3, a necessity for
the qualities of hardness and toughness in the material of the tool.
In very hard rock, a few minutes of work suffice to destroy the cutting
edge, and then the tool has to be returned to the smithy to be
re-sharpened. Hence it is manifest that the form of the edge should not
be one that is difficult to produce, since, were it so, much time would
be consumed in the labour of re-sharpening. Experience has shown that
the foregoing conditions are most fully satisfied in the steel rod
terminating in a simple chisel edge, now universally adopted.
This form of drill is exhibited in Fig. 1, which represents a common
“jumper” borer. It consists of a rod terminating at each end in a chisel
edge, and having a swell, technically described as the “bead,” between
the extremities to give it weight. The bead divides the jumper into two
unequal portions, each of which constitutes a chisel bit, with its shank
or “stock.” The shorter stock is used while the hole is shallow, and the
longer one to continue it to a greater depth.
[Illustration: FIG. 1.]
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
With the jumper, the blow is obtained from the direct impact of the
falling tool. The mode of using the instrument is to lift it with both
hands to a height of about a foot, and then to let it drop. In lifting
the jumper, care is taken to turn it partially round, that the edge may
not fall twice in the same place. By this means, the edge is made to act
most favourably in chipping away the rock, and the hole is kept fairly
circular. So long as the holes are required to be bored vertically
downwards, the jumper is a convenient and very efficient tool, and hence
in open quarrying operations, it is very commonly employed. But in
mining, the shot-holes are more often required to be bored in some other
direction, or, as it is termed, “at an angle;” that is, at an angle with
the vertical. Or it may be that a shot-hole is required to be bored
vertically upward. It is obvious that in any one of these directions the
jumper is useless. To meet the requirements of such cases, recourse is
had to the hammer wherewith to deliver the blow, and the drill is
constructed to be used with the hammer. We have a suitable form of tool
for application in this wise when we cut out the bead of the jumper and
leave the ends flat for a striking face, as shown in Figs. 2 and 3. The
form of the two chisels thus obtained is that adopted for the ordinary
rock drill.
It will be understood from these descriptions that a rock drill consists
of the chisel edge or _bit_, the _stock_, and the _striking face_.
Formerly drills were made of wrought iron, and steeled at each end to
form the bit and the striking face. Now they are commonly made of cast
steel, which is supplied for that purpose in octagonal bars of the
requisite diameter. The advantages offered by steel stocks are numerous.
The superior solidity of texture of that material renders it capable of
transmitting the force of a blow more effectively than iron. Being
stronger than the latter material, a smaller diameter of stock, and,
consequently, a less weight, are sufficient. This circumstance also
tends to increase the effect of the blow by diminishing the mass through
which it is transmitted. On the other hand, a steel stock is more easily
broken than one of iron.
The cutting edge of a drill demands careful consideration. To enable the
tool to free itself readily in the bore-hole, and also to avoid
introducing unnecessary weight into the stock, the bit is made wider
than the latter; the difference in width may be as much as 1 inch. It is
evident that in hard rock, the liability of the edge to fracture
increases as the difference of width. The edge of the drill may be
straight or slightly curved. The straight edge cuts its way somewhat
more freely than the curved, but it is weaker at the corners than the
latter, a circumstance that renders it less suitable for very hard rock.
It is also slightly more difficult to forge. The width of the bit
varies, according to the size of the hole required, from 1 inch to 2½
inches. Figs. 4, 5, and 6 show the straight and the curved bits, and the
angles of the cutting edges for use in rock.
[Illustration: FIG. 4.]
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
The stock is octagonal in section; it is made in lengths varying from
20 inches to 42 inches. The shorter the stock the more effectively does
it transmit the force of the blow, and therefore it is made as short as
possible. For this reason, several lengths are employed in boring a
shot-hole, the shortest being used at the commencement of the hole, a
longer one to continue the depth, and a still longer one, sometimes, to
complete it. To ensure the longer drills working freely in the hole, the
width of the bit should be very slightly reduced in each length. It has
already been remarked that the diameter of the stock is less than the
width of the bit; this difference may be greater in coal drills than in
rock or “stone” drills; a common difference in the latter is ⅜ of an
inch for the longer. The following proportions may be taken as the
average adopted:--
+---------+---------+
| Width of| Diameter|
| the Bit.| of the |
| | Stock. |
+---------+---------+
|1 inch | ⅝ inch |
|1⅛ „ | ¾ „ |
|1¼ „ | ⅞ „ |
|1½ „ |1 „ |
|1¾ „ |1⅛ „ |
|2 inches|1⅜ „ |
|2¼ „ |1½ „ |
|2½ „ |1⅝ „ |
+---------+---------+
The striking face of the drill should be flat. The diameter of the face
is less than that of the stock in all but the smallest sizes, the
difference being made by drawing in the striking end. The amount of
reduction is greater for the largest diameters; that of the striking
face being rarely more than one-eighth of an inch.
The making and re-sharpening of rock drills constitute an extremely
important part of the labour of the mine smith. The frequent use of the
drill, and its rapid wear, necessitate a daily amount of work of no
trifling proportions, and the judgment and skill required in proper
tempering render some degree of intelligence in the workman
indispensable; indeed, so much depends upon the smith whose duty it is
to repair the miners’ tools, that no pains should be spared to obtain a
man capable of fulfilling that duty in the most efficient manner
possible.
When the borer-steel bars are supplied to the smith, he cuts them up, as
required, into the desired lengths. To form the bit, the end of the bar
is heated and flattened out by hammering to a width a little greater
than the diameter of the hole to be bored. The cutting edge is then
hammered up with a light hammer to the requisite angle, and the corners
beaten in to give the exact diameter of the bore-hole intended. As the
drills are made in sets, the longer stocks will have a bit slightly
narrower than the shorter ones, for reasons already given. The edge is
subsequently touched up with a file. In performing these operations,
heavy hammering should be avoided, as well as high heats, and care
should be taken in making the heat that the steel should be well covered
with coal, and far enough removed from the tuyere to be protected from
the “raw” air. Overheated or “burned” steel is liable to fly, and drills
so injured are useless until the burned portion has been cut away.
[Illustration: FIG. 7.]
[Illustration: FIG. 8.]
[Illustration: FIG. 9.]
Both in making and in re-sharpening drills, great care is required to
form the cutting edge evenly, and of the full form and dimensions. If
the corners get hammered in, as shown in Fig. 7, they are said to be
“nipped,” and the tool will not free itself in cutting. When a
depression of the straight, or the curved, line forming the edge occurs,
as shown in Fig. 8, the bit is said to be “backward,” and when one of
the corners is too far back, as in Fig. 9, it is spoken of as
“odd-cornered.” When either of these defects exist--and they are
unfortunately common--not only does the bit work less effectively on the
rock, but the force of the blow is thrown upon a portion only of the
edge, which, being thereby overstrained, is liable to fracture.
The hardening and tempering of steel is a matter requiring careful study
and observation. It is a well-known fact that a sudden and great
reduction of temperature causes a notable increase of hardness in the
metal. The reason of this phenomenon is not understood, but it is
certain that it is in some way dependent upon the presence of carbon.
The degree of hardness imparted to steel by this means depends upon the
amount of the reduction of the temperature, and the proportion of carbon
present in the metal, highly carburetted steel being capable of
hardening to a higher degree, under the same conditions, than steel
containing less carbon. Thus, for steel of the same quality, the wider
the range of temperature the higher is the degree of hardness. But here
we encounter another condition, which limits the degree of hardness
practically attainable.
The change which takes place among the molecules of the metal in
consequence of the change of temperature causes internal strains, and
thereby puts portions in a state of unequal tension. This state renders
the strained parts liable to yield when an additional strain is thrown
upon them while the tool is in use; in other words, the brittleness of
the steel increases with its hardness. Here again the proportion of
carbon present comes into play, and it must be borne in mind that for
equal degrees of hardness the steel which contains the least carbon
will be the most brittle. In hardening borer-steel, which has to combine
as far as possible the qualities of hardness and toughness, this matter
is one deserving careful attention. It is a remarkable fact, and one of
considerable practical value, that when oil is employed as the cooling
medium instead of water, the toughness of steel is enormously increased.
The tempering of steel, which is a phenomenon of a similar character to
that of hardening, also claims careful consideration. When a bright
surface of steel is subjected to heat, a series of colours is produced,
which follow each other in a regular order as the temperature increases.
This order is as follows: pale yellow, straw yellow, golden yellow,
brown, brown and purple mingled, purple, light blue, full clear blue,
and dark blue. Experience has shown that some one of these colours is
more suitable than the rest for certain kinds of tools and certain
conditions of working.
The selection of the proper colour constitutes a subject for the
exercise of judgment and skill on the part of the smith. For rock
drills, straw colour is generally the most suitable when the work is in
very hard rock, and light blue when the rock is only of moderate
hardness.
The processes of hardening and tempering drills are as follows: When the
edge of the bit has been formed in the manner already described, from 3
to 4 inches of the end is heated to cherry redness, and dipped in cold
water to a depth of about an inch to harden it. While in the water, the
bit should be moved slightly up and down, for, were this neglected, the
hardness would terminate abruptly, and the bit would be very liable to
fracture along the line corresponding with the surface of the water. In
cold weather, the water should be slightly warmed, by immersing a piece
of hot iron in it, before dipping the steel. When a sufficient degree of
hardness has been attained, the remainder of the hot portion is immersed
until the heat is reduced sufficiently for tempering. At this stage it
is withdrawn, and the colours carefully watched for. The heat which is
left in the stock will pass down to the edge of the bit, and as the
temperature increases in that part the colours will appear in regular
succession upon the filed surface of the edge. When the proper hue
appears, the whole drill is plunged into the water and left there till
cold, when the tempering is complete. When the edge is curved or
“bowed,” the colours will reach the corners sooner than the middle of
the bit. This tendency must be checked by dipping the corners in the
water, for otherwise the edge will not be of equal hardness throughout.
As the colour can be best observed in the dark, it is a good plan to
darken that portion of the smithy in which tempering is being carried
on.
The degree of temper required depends upon the quality of the steel and
the nature of the work to be performed. The larger the proportion of
carbon present in the metal, the lower must be the temper. Also the
state of the blunted edges, whether battered or fractured, will show
what degree of hardness it is desirable to produce. From inattention to
these matters, good steel is not unfrequently condemned as unsuitable.
To form the striking face, the end of the stock is heated to a dull red,
and drawn out by a hammer to form a conical head. The extremity is then
flattened to form a face from ½ inch to 1 inch in diameter. This head is
then annealed to a degree that will combine considerable toughness with
hardness. The constant blows to which the head is subjected tend to wear
it down very rapidly. There is great difference in the lasting qualities
of steel in this respect; some drills will wear away more quickly at the
striking than at the bit end.
A smith will, with the assistance of a striker, sharpen and temper about
thirty single-hand drills of medium size in an hour, or twenty
double-hand drills of medium size in the same time. Of course, much will
depend on the degree of bluntness in the cutting edge; but assuming the
drills to be sent up only moderately blunted, this may be taken as a
fair average of the work of two men.
It will be evident from the foregoing remarks, that to enable a drill to
stand properly it must be made of good material, be skilfully tempered
in the smithy, and provided with a cutting edge having an angle and a
shape suited to the character of the rock in which it is used. To these
conditions, may be added another, namely, proper handling; for if the
drill be carelessly turned in the hole so as to bring all the work upon
a portion only of the cutting edge, or unskilfully struck by the sledge,
fracture or blunting will speedily result. Improper handling often
destroys the edge in the first five minutes of using.
Drills, as before remarked, are used in sets of different lengths. The
sets may be intended for use by one man or by two. In the former case,
the sets are described as “single-hand” sets, and they contain a hammer
for striking the drills; in the latter case, the sets are spoken of as
“double-handed,” and they contain a sledge instead of a hammer for
striking. It may appear at first sight that there is a waste of power in
employing two men, or, as it is termed, the double set, for that two men
cannot bore twice as fast as one. This rate of speed can, however, be
obtained, and is due less to the greater effectiveness of the stroke
than to the fact that two men can, by repeatedly changing places with
each other, keep up almost without intermission a succession of blows
for an indefinite length of time; whereas, with the single set, the man
is continually obliged to cease for rest.
_Hammers._--To deliver the blow upon a rock drill, hammers and sledges
are used. The distinction between a hammer and a sledge is founded on
dimensions only: the hammer being intended for use in _one_ hand, is
made comparatively light and is furnished with a short handle, while the
sledge, being intended for use in _both_ hands, is furnished with a much
longer handle and is made heavier. The striking face of the blasting
sledge should be flat, to enable the striker to deliver a direct blow
with certainty upon the head of the drill; and to facilitate the
directing of the blow, as well as to increase its effect, the mass of
metal composing the head should be concentrated within a short length.
To cause the sledge to fly off from the head of the drill in the case of
a false blow being struck, and thereby to prevent it from striking the
hand of the man who holds the drill, the edges of the striking face
should be chamfered or bevelled down till the diameter is reduced by
nearly one-half. This requirement is, however, but seldom provided for.
[Illustration: FIG. 10.]
[Illustration: FIG. 11.]
[Illustration: FIG. 12.]
[Illustration: FIG. 13.]
The head of a sledge is of iron; it consists of a pierced central
portion called the “eye,” and two shanks or “stumps,” the steeled ends
of which form the striking faces or “panes.” The form of the head varies
in different localities, but whatever the variations may be, the form
may be classed under one of four types or “patterns.” A very common
form is that shown in Fig. 10 and known as the “bully” pattern. By
varying the width, as shown in Fig. 11, we obtain the “broad bully,” the
former being called for the sake of distinction the “narrow” bully.
Another common form is the “pointing” pattern, represented in Fig. 12.
The form shown in Fig. 13 is designated as the “bloat” pattern; and that
given in Fig. 14 the “plug” pattern. Each of these forms possesses
peculiar merits which renders it more suitable for certain uses than
the others. The same forms are used for hammers. The eye is generally
made oval in shape, but sometimes, especially with the bloat pattern, it
is made circular, as shown in Fig. 13. The weight of a sledge head may
vary from 5 lb. to 10 lb., but a common and convenient weight is 7 lb.
The length of the helve varies from 20 inches to 30 inches; a common
length for blasting sledges is 24 inches. The average weight of hammer
heads is about 3 lb., and the average length of the helve 10 inches.
[Illustration: FIG. 14.]
[Illustration: FIG. 15.]
Fig. 15 represents a blasting sledge used in South Wales. The stumps are
octagonal in section, and spring from a square block in the centre. The
panes or striking faces, however, are circular and flat. The length of
the head is 8¾ inches, and that of the helve 27 inches, and the weight
of the tool complete 7 lb.
[Illustration: FIG. 16.]
Fig. 16 represents a blasting sledge used in North Wales. The central
block is an irregular octagon in section, formed by slightly chamfering
the angles of a square section, and the stumps are chamfered down to
form a regular octagon at the panes, which are flat. The length of the
head is 7¾ inches, and that of the helve 22 inches, and the weight of
the tool complete 6 lb. 7 oz.
[Illustration: FIG. 17.]
The sledges used in the north of England have shorter heads, and are
lighter than the foregoing. Fig. 17 represents one of these blasting
sledges. The head is nearly square in section at the centre, and the
panes are flat. The length of the head is 5 inches, and that of the
helve 24½ inches, and the weight of the sledge complete 4 lb. 14 oz.
_Auxiliary Tools._--Besides the drill and the hammer, other tools are
needed in preparing the hole for the blasting charge. If the bore-hole
is inclined downwards, the débris or “bore-meal” made by the drill
remains on the bottom of the hole, where it is converted into mud or
“sludge” by the water there present. This sludge has to be removed as
the work progresses, to keep the rock exposed to the action of the
drill. The removal of the sludge is effected by a simple tool called a
“scraper.” It consists of a rod of iron from ¼ inch to ½ inch in
diameter, and of sufficient length to reach the bottom of the bore-hole.
One end of the rod is flattened out on the anvil and made circular in
form, and then turned up at right angles to the stem. The disc thus
formed must be less in diameter than the bore-hole, to allow it to pass
readily down. When inserted in the hole, the scraper is turned round
while it is being pressed to the bottom; on withdrawing the instrument,
the sludge is brought up upon the disc. The operation, two or three
times repeated, is sufficient to clear the bore-hole. The other end of
the scraper is sometimes made to terminate in a ring for convenience in
handling, as shown in Fig. 18. Instead of the ring, however, at one end,
a disc may be made at each end, as shown in Fig. 19, the discs in this
case being of different diameter, to render the scraper suitable for
different size bore-holes. Sometimes the scraper is made to terminate
in a spiral hook or “drag-twist,” as represented in Fig. 20. The use of
the drag is to thoroughly cleanse the hole before inserting the charge.
A wisp of hay is pushed down the hole, and the drag end of the scraper
introduced after it, and turned round till it has become firmly
entangled. The withdrawal of the hay by the drag wipes the bore-hole
clean. Instead of the twist drag, the “loop” drag is frequently
employed. This consists of a loop or eye, through which a piece of rag
or tow is passed. The rag or tow is used for the same purpose as the
hay, namely, to thoroughly cleanse and dry the bore-hole previous to the
introduction of the charge. Very frequently the “swab-stick” is used
instead of the scraper to clear out the bore-hole. This is simply a deal
rod bruised at one end by blows with a hammer until the fibres separate
to form a kind of stumpy brush or “swab.” When this is pushed down the
hole, the sludge passes up around and between the fibres, which are then
spread out by being pressed against the bottom of the hole. On
withdrawing the swab, the sludge is brought out with it.
[Illustration: FIG. 18.]
[Illustration: FIG. 19.]
[Illustration: FIG. 20.]
When the charge has been placed in the bore-hole, and the fuse laid to
it, the hole needs to be tamped, that is, the portion above the charge
has to be filled up with some suitable substance. For this purpose, a
“rammer,” “stemmer,” or “tamping iron,” as the instrument is variously
called, is required. This instrument is illustrated in Fig. 21. It
consists of a metal bar, the tamping end of which is grooved to receive
the fuse lying against the side of the bore-hole. The other end is flat,
to afford a pressing surface for the hand, or a striking face for the
hammer when the latter is needed. To prevent the danger of accidental
ignition from sparks caused by the friction of the metal against
silicious substances, the employment of iron stemmers has been
prohibited by law. They are usually made of copper or phosphor-bronze,
the latter substance being more resisting than the former.
[Illustration: FIG. 21.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
Sometimes in wet ground it becomes necessary to shut back the water from
the bore-hole before introducing the charge of gunpowder. This happens
very frequently in shaft sinking. The method employed in such cases is
to force clay into the interstices through which the water enters. The
instrument used for this purpose is the “claying-iron” or “bull,”
represented in Fig. 22. It consists of a round bar of iron, called the
stock or shaft, a little smaller in diameter than the bore-hole, and a
thicker portion, called the head or poll, terminating in a striking
face. The lower end of the shaft is pointed, to enable it to penetrate
the clay, and the head is pierced by a hole about an inch in diameter to
receive a lever. Clay in a plastic state having been put into the
bore-hole, the bull is inserted and driven down by blows with the
sledge. As the shaft forces its way down, the clay is driven into the
joints and crevices of the rock on all sides. To withdraw the bull, a
bar of iron is placed in the eye and used as a lever to turn it round to
loosen it; the rod is then taken by both hands and the bull lifted out.
To allow the bull to be withdrawn more readily, the shaft should be made
with a slight taper and kept perfectly smooth. As the bull is subjected
to a good deal of heavy hammering on the head, the latter part should be
made stout. This tool, which should be considered as an extra instrument
rather than as an essential part of a blasting set, is a very
serviceable one, and should always be at hand in wet ground when loose
gunpowder is employed.
Another instrument of this auxiliary character is the beche, Fig. 23,
used for extracting a broken drill. It consists of an iron rod of nearly
the diameter of the bore-hole, and hollow at the lower end. The form of
the aperture is slightly conical, so that the lower end may easily pass
over the broken stock of the drill, and, on being pressed down with
some force, may grasp the stock in the higher portion of the aperture
with sufficient firmness to allow of the two being raised together. When
only a portion of the bit remains in the hole, it may often be extracted
by means of the drag-twist end of the scraper, or the swab-stick may be
driven down upon the broken portion, and latter withdrawn with the swab.
_Sets of Blasting Gear._--On Plates I., II., and III., will be found
three sets of blasting gear; a set of coal-blasting gear; a set of
single-hand stone-blasting gear; and a set of double-hand stone-blasting
gear. In the first set, the drill, shown in Fig. 1, is 22 inches in
length; the cutting edge is straight and 1½ inch wide, and the weight is
2½ lb. The other drill, Fig. 2, is 42 inches in length; it has a
straight cutting edge 1⁷/₁₆ inch wide, and weighs 4 lb. 10 oz. The
hammer used in this set and shown in Fig. 3 weighs 2 lb. 14 oz.; the
length of the head is 4½ inches, and that of the handle 7¾ inches. In
the second or single-hand stone set, the shorter drill, Fig. 6, Plate
II., is 22 inches in length; the cutting edge is strongly curved, and is
1½ inch in width, and the weight is 3 lb. 10 oz. The longer drill, Fig.
7, is 36 inches in length; the width of the cutting edge, which is
curved as in the shorter drill, is 1⁷/₁₆ inch, and the weight is 6 lb. 5
oz. The hammer used with this set, and represented in Fig. 8, weighs 3
lb. 6 oz.; the length of the head is 5 inches, and that of the handle
10 inches. In the third or double-hand stone set, Plate III., the first
or shortest drill, Fig. 12, is 18 inches in length, 1¾ inch wide on the
cutting edge, and weighs 4¼ lb. The second drill, Fig. 13, is 27 inches
in length, 1¹¹/₁₆ wide on the cutting edge, and weighs 6 lb. The third
or longest drill, Fig. 14, is 40 inches in length, 1⅝ inch wide on the
cutting edge, and weighs 9¼ lb. The cutting edges of all these drills
are strongly curved as in the preceding set. The sledge used with this
set, and represented in Fig. 15, weighs about 5 lb.
SECTION II.--MACHINE BORING.
_Machine Rock-Drills._--The most remarkable advance, which in recent, or
perhaps in any, times has been made in the practice of mining consists
in the substitution of machine for hand labour in rock boring. The
importance of this change is obvious, and very great. Not only is the
miner relieved thereby of the labour of boring, but the speed with which
the shot-holes may be bored is increased a hundredfold. This gain of
speed offers many practical advantages. The ability to sink a shaft or
to drive a heading rapidly may ensure the success of an undertaking, and
save indirectly the expenditure of large sums of money; and, in all
cases, it allows the time spent in preparatory work to be materially
shortened. Indeed, it would be difficult to over-estimate the magnitude
of the advantage accruing from the increased rate of progress due to
the substitution of machine power for hand labour, and in the future we
may expect to see its application greatly extended. In making this
substitution, numerous difficulties have had to be overcome, and in
encountering these many failures have had to be recorded. But it must
now be conceded by the most prejudiced that rock-boring machines have
successfully passed through what may be described as the tentative stage
of their existence, and have taken a foremost place among the mechanical
appliances which experience has shown to be capable of effectually
performing the work required of them. In the author’s work on ‘Mining
Engineering,’ the requirements of a rock drill will be found fully
discussed, and the principles and the construction of the most important
machines now in use carefully explained and described. In the present
work, only one example can be given.
Machine drills penetrate rock in the same way as the ordinary hand
drills already described, namely, by means of a percussive action. The
cutting tool is in most cases attached directly to the piston rod, with
which it consequently reciprocates. Thus the piston with its rod is made
to constitute a portion of the cutting tool, and the blow is then given
by the direct action of the steam, or the compressed air, upon the tool.
As no work is done upon the rock by the back stroke of the piston, the
area of the forward side is reduced to the dimensions necessary only to
lift the piston, and to overcome the resistance due to the friction of
the tool in the bore-hole. The piston is made to admit steam or air into
the cylinder, and to cut off the supply, and to open the exhaust, as
required, by means of tappet valves, or other suitable devices; and
provision is made to allow, within certain limits, a variation in the
length of the stroke. During a portion of the stroke, means are brought
into action to cause the piston to rotate to some extent, for the
purposes that have been already explained. To keep the cutting edge of
the tool up to its work, the whole machine is moved forward as the rock
is cut away. This forward or “feed” motion is usually given by hand, but
in some cases it is communicated automatically. The machine is supported
upon a stand or framing which varies in form according to the situation
in which it is to be used. This support is in all cases constructed to
allow of the feed motion taking place, and also of the cutting tool
being directed at any angle. The support for a rock drill constitutes an
indispensable and a very important adjunct to the machine, for upon the
suitability of its form, material, and construction, the efficiency of
the machine will largely depend.
The foregoing is a general description of the construction and mode of
action of percussive rock-drills. The numerous varieties now in use
differ from each other rather in the details of their construction than
in the principles of their action, and the importance of the difference
is, of course, dependent upon that of the details. It is but just to
remark here that the first really practical solution of the
rock-drilling problem is due to M. Sommeiller, whose machine was
employed in excavating the Mont Cenis tunnel.
_The Darlington Drill._--The machine which, in England, has stood the
test of experience most satisfactorily, and which, consequently, is
surely working itself into general favour in this country, and also in
some of the important mining districts of the Continent, is the
invention of John Darlington, and is known as the “Darlington drill.”
This drill is remarkable as the attainment of the highest degree of
simplicity of parts possible in a machine. The valve gear of a machine
drill is especially liable to derangement. It must necessarily consist
of several parts, and these parts must as necessarily be of a somewhat
fragile character. Besides this, when actuated by the piston through the
intervention of tappets, the violence of the blow delivered at each
stroke is such as to rapidly destroy the parts. In some machines, the
force of these blows and their destructive tendency have been reduced to
a minimum; but when every means of remedying the evil has been employed,
there remains a large amount of inevitable wear and tear, and a
liability to failure from fracture or displacement exists in a greater
or less degree. Moreover, as these effects are greatly intensified by
increasing the velocity of the piston, it becomes at least undesirable
to use a high piston speed. To remedy these defects, which are inherent
in the system, Darlington proposed to remove altogether the necessity
for a valve gear by radically changing the mode of admitting the motor
fluid to the cylinder. This proposal he has realized in the machine
which is illustrated on Plate IV.
The Darlington rock-drill consists essentially of only two parts: the
cylinder A, Figs. 20 and 21, with its cover; and the piston B, with its
rod. The cover, when bolted on, forms a part of the cylinder; the piston
rod is cast solid with the piston, and is made sufficiently large at its
outer end to receive the tool. These two parts constitute an engine, and
with less than one fixed and one moving part it is obviously impossible
to develop power in a machine by the action of an elastic fluid. The
piston itself is made to do the work of a valve in the following manner:
The annular space affording the area for pressure on the fore part of
the piston gives a much smaller extent of surface than that afforded by
the diameter of the cylinder, as shown in the drawing; and it is obvious
that by increasing or diminishing the diameter of the piston rod, the
area for pressure on the one side of the piston may be made to bear any
desired proportion to that on the other side. The inlet aperture, or
port C, being in constant communication with the interior of the
cylinder, the pressure of the fluid is always acting upon the front of
the piston, consequently when there is no pressure upon the other side,
the piston will be forced backward in the cylinder. During this backward
motion, the piston first covers the exhaust port D, and then uncovers
the equilibrium port E, by means of which communication is established
between the front and back ends of the cylinder, and, consequently, the
fluid is made to act upon both sides of the piston. The area of the back
face of the piston being greater than that of the front face by the
extent occupied by the piston rod, the pressure upon the former first
acts to arrest the backward motion of the piston, which, by its
considerable weight and high velocity, has acquired a large momentum,
and then to produce a forward motion, the propelling force being
dependent for its amount upon the difference of area on the two sides of
the piston. As the piston passes down, it cuts off the steam from the
back part of the cylinder and opens the exhaust. The length or thickness
of the piston is such that the exhaust port D is never open to its front
side, but, in the forward stroke, it is opened almost immediately after
the equilibrium port is closed, and nearly at the time of striking the
blow. It will be observed that the quantity of fluid expended is only
that which passes over to the back face of the piston, since that which
is used to effect the return stroke is not discharged.
The means employed to give a rotary motion to the tool are deserving of
special attention, as being simple in design, effective in action, and
well situate within the cylinder. These means consist of a spiral or
rifled bar H, having three grooves, and being fitted at its head with a
ratchet wheel G, recessed into the cover of the cylinder. Two detents J,
J, Fig. 22, also recessed into the cover, are made to fall into the
teeth of the ratchet wheel by spiral springs. These springs may, in case
of breakage, be immediately renewed without removing the cover. It will
be observed that this arrangement of the wheel and the detents allow the
spiral bar H to turn freely in one direction, while it prevents it from
turning in the contrary direction. The spiral bar drops into a long
recess in the piston, which is fitted with a steel nut made to
accurately fit the grooves of the spiral. Hence the piston, during its
instroke, is forced to turn upon the bar; but, during its outstroke, it
turns the bar, the latter being free to move in the direction in which
the straight outstroke of the piston tends to rotate it. Thus the
piston, and with it the tool, assumes a new position after each stroke.
The mode of fixing the cutting tool to the piston rod is a matter
deserving some attention. As the tool has to be changed more than once
during the progress of a bore-hole, it is important that the change
should be accomplished in as short a time as possible; and as the
vibration of the machine and the strain upon the tool are necessarily
great, it is equally important that the tool be firmly held. It is also
desirable that the mode of fixing the tool shall not require a shoulder
upon the latter, a slot in it, or any peculiarity of form difficult to
be made in the smithy. The Darlington machine fulfils the requirements
of expedition in fixing, firmness of retention, and simplicity of form
most satisfactorily. The means and the method are the following: The
outer end of the rod or holder is first flattened to afford a seat for
the nut, as shown in Figs. 21 and 25. The slot is then cut and fitted
tightly with a piece of steel K forged of the required shape for the
clamp, and the holder is afterwards bored to receive the tool while the
clamp is in place. This clamp K is then taken out, its fittings eased a
little, and its end screwed and fitted with a nut. When returned to its
place in the holder, the clamp, in consequence of the easing, can be
easily drawn tight against the tool, by which means it is firmly held in
position. The shank of the tool is turned to fit the hole easily, and
the end of it is made hemispherical to fit the bottom of the hole, upon
which the force of the reaction of the blow is received.
It would seem impossible to attain a higher degree of simplicity of
form, or to construct a machine with fewer parts. The absence of a valve
or striking gear of any kind ensures the utmost attainable degree of
durability, and allows a high piston speed to be adopted without risk or
injury. As the piston controls its own motion, there is no liability to
strike against the cylinder cover. The stroke may be varied in length
from half an inch to four inches, and as the machine will work
effectively with a pressure of 10 lb. to the inch, holes may be started
with the greatest ease. With a pressure of 40 lb., the machine makes
1000 blows a minute, a speed that may be attained without causing undue
strains or vibration. This alone constitutes a very great advantage. It
must indeed be conceded that an unprejudiced consideration of the merits
of this drill shows it to be admirably adapted to the work required of
it.
_Borer-Bits._--The form and the dimensions of the cutting tools,
variously described as “drills,” “borers,” and “bits,” used with machine
rock-perforators are matters of great practical importance. The
dimensions are determined mainly by two conditions, namely, the
necessity for sufficient strength in the shank of the tool, and the
necessity for sufficient space between the shank and the sides of the
hole to allow the débris to escape. Experience has shown that the latter
condition is best fulfilled when the distance between the sides of the
hole and the shank of the tool is from ³/₁₆ inch to ¼ inch, regard being
had to the former condition.
The form of the cutting edge is determined by several conditions, some
of which have been already discussed in relation to hand drills. The
form first adopted was naturally that possessed by the hand drill,
namely, the chisel edge. To increase the useful effect of the blow, the
cutting edge was subsequently doubled, the bit being formed of two
chisel edges crossing each other at right angles. This bit, which from
its form was called the “cross” bit, was found to penetrate the rock
more rapidly than the straight or chisel bit. The gain in speed was very
marked at the commencement of the hole; but it diminished gradually as
the hole progressed in depth, owing to the difficulty with which the
débris escaped. To remedy this defect, the cutting edges were next made
to cross each other obliquely, so as to form the letter X. In this way,
the two chisel edges were retained, while the breadth of the bit was
considerably reduced. This form, described as the X bit, cleared the
hole much more effectively than the cross, but not in a manner that was
altogether satisfactory. Another modification of the form was,
therefore, made, and this time that of the Z was adopted, the upper and
the lower portions of which were arcs of circles struck from the centre
of the bit in the direction contrary to that of the rotation.
This form of tool, which is known as the Z bit, readily cleared itself
of the débris. But besides this advantage, it was found to possess
others of an important character. With the chisel-edge forms, the
corners of the bit were rapidly worn off by friction against the sides
of the hole. With the Z form, this wearing no longer occurred, by
reason of the large surface exposed to friction. Another advantage of
the Z form of bit lies in its tendency to bore the hole truly circular.
Generally then, it may be stated that this form satisfies most fully the
determining conditions. The form of bit, however, that is most suitable
in a given case will, in some degree, be determined by particular
circumstances. Of these, the nature and the character of the rock will
operate most strongly to influence the choice. Thus the cross bit will
generally be found the most suitable in fissured rock, while the single
chisel edge may be used with advantage in rock of a very solid and hard
character. Indeed, on the judicious selection of the most suitable form
of cutting edge, the success of machine boring largely depends. The
chisel bit, the cross bit, the X bit, and the Z bit, are shown in Figs.
24 to 27.
[Illustration: FIG. 24.]
[Illustration: FIG. 25.]
[Illustration: FIG. 26.]
[Illustration: FIG. 27.]
The sharpening of bits of a form other than that of the chisel is done
by means of “swages.” The tempering is effected in the way already
described in reference to hand drills. As in the latter case, the
degree of temper must be suited to the hardness of the rocks to be
penetrated. Generally the straw colour will be found to be the best
degree. It is a remarkable fact that the wear of the cutting edge of a
machine drill is, for a given length of boring, five or six times less
than that of a hand drill. Steel of the best quality should always be
used.
As in the case of hand boring, each successive length of drill must
diminish slightly in the width of its cutting edge; a diminution of
about ¹/₃₂ inch may be considered sufficient. Care should, however, be
taken to ensure the proper dimensions being given to the edge, and it
will be found advantageous to have at hand an accurate gauge through
which the tool may be passed previously to its being fixed to the
machine. It is important that the tool be truly “centred,” that is, the
centres of the edge of the bit, of the shank, and of the piston rod,
should be perfectly coincident.
_Rock-Drill Supports._--A machine rock-drill may satisfy every
requirement, and yet, by reason of the defective character of the
support to which it is attached, it may be unsuitable to the work
required of it. Hence it becomes desirable to carefully study the design
and construction of a drill support, and to consider the requirements
which it is needful to fulfil. Assuming the necessity for a high degree
of strength and rigidity in the support, a primary condition is that it
shall allow the machine to be readily adjusted to any angle, so that the
holes may be bored in the direction and with the inclination required.
When this requirement is not fulfilled, the machine is placed, in this
respect, at a great disadvantage with hand labour. If a machine drill
were not capable of boring in any position and in any direction, hand
labour would have to be employed in conjunction with it, and such
incompleteness in the work of a machine would constitute a serious
objection to its adoption.
Besides allowing of the desired adjustment of the machine, the support
must be itself adjustable to uneven ground. The bottom of a shaft which
is being sunk, or the sides, roof, and floor of a heading which is being
driven, present great irregularities of surface, and, as the support
must of necessity in most cases be fixed to these, it is obvious that
its design and construction must be such as will allow of its ready
adjustment to these irregularities. The means by which the adjustment is
effected should be few and simple, for simplicity of parts is important
in the support as well as in the machine, and for the same reasons. A
large proportion of the time during which a machine drill is in use is
occupied in shifting it from one position or one situation to another;
this time reduces, in a proportionate degree, the superiority of machine
over hand labour, in respect of rapidity of execution, and it is
evidently desirable that it should be shortened as far as possible.
Hence the necessity for the employment of means of adjustment which
shall be few in number, rapid in action, and of easy management.
For reasons similar to the foregoing, the drill support must be of small
dimensions, and sufficiently light to allow of its being easily
portable. The limited space in which rock drills are used renders this
condition, as in the case of the machine itself, a very important one.
It must be borne in mind that, after every blast, the dislodged rock has
to be removed, and rapidity of execution requires that the operations of
removal should be carried on without hindrance. A drill support that
occupies a large proportion of the free space in a shaft or a heading is
thus a cause of inconvenience and a source of serious delay. Moreover,
as it has to be continually removed from one situation to another, it
should be of sufficiently light weight to allow of its being lifted or
run along without difficulty. In underground workings, manual power is
generally the only power available, and therefore it is desirable that
both the machine and its support should be of such weight that each may
be lifted by one man. Of course, when any endeavour is made to reduce
the weight of the support, the necessity for great strength and rigidity
must be kept in view.
In spacious headings, such as are driven in railway tunnel work,
supports of a special kind may be used. In these situations, the
conditions of work are different from those which exist in mines. The
space is less limited, the heading is commenced at surface, and the
floor is laid with a tramway and sidings. In such a case, the support
may consist of a more massive structure mounted upon wheels to run upon
the rails. This support will carry several machines, and to remove it
out of the way when occasion requires, it will be run back on to a
siding; but for ordinary mining purposes, such a support is suitable.
_The Stretcher Bar._--The simplest kind of support is the “stretcher
bar.” This consists essentially of a bar so constructed that it may be
lengthened or shortened at pleasure, by means of a screw. It is fixed in
position by screwing the ends into firm contact with the sides, or with
the roof and the floor, of a heading. The machine is fixed to this bar
by means of a clamp, which, when loosened, slides along the bar, and
allows the drill to be placed in the required position, and to be
directed at the required angle. The bar illustrated in Fig. 26, Plate
V., is that which is used with the Darlington drill; in it, lightness
and rigidity are combined in the highest possible degree by the adoption
of the hollow section. The mode of setting the bar in a heading is shown
in the drawing; the end claws are set against pieces of wood on the
floor and the roof, and are tightened by turning the screw with a common
bar.
The simple stretcher bar is frequently used in narrow drivings and in
shafts of small diameter. But a more satisfactory support in drivings is
afforded by a bar suitably mounted upon a carriage designed to run upon
rails. The carriage consists simply of a trolly, to the fore part of
which the bar is fixed usually by some kind of hinge-joint. It is
obvious that the details of the construction of this support may be
varied greatly, and numerous designs have been introduced and adopted.
In Figs. 27 and 28, Plate VI., is shown a support of this character
designed by J. Darlington. A single vertical bar is carried on the fore
part of the trolly, and fixed, by the usual means, against the centre of
the roof. This vertical bar carries an arm, which is capable of turning
upon it, as upon a centre, and of sliding up and down it. This arm
carries the drill. The central bar having been fixed in position, the
arm is slid up to the highest position required, and fixed against the
side of the heading. A row of holes are then bored from this arm. When
these are completed, the arm is lowered the requisite distance, and
another row of holes are bored. This is continued until all the holes
are bored over one-half the face. The arm is then swung round, and fixed
against the other side of the heading, and the holes are bored over that
half the face in like manner. In this way, one-half the heading is kept
clear to allow the operations of removing the dislodged rock to be
carried on at the same time. If desired, two arms may be used. This
arrangement gives undoubtedly great facilities for working the drill,
and leaves the heading comparatively unencumbered.
In shaft sinking, the same support, slightly modified, is used without
the trolly. The arrangement adopted in this case is shown in Fig. 29,
Plate VII. The central bar is held firmly in its position by a cross
stretcher bar set against the sides of the shaft. The arms are made to
revolve upon this bar to allow the holes to be bored in the positions
required. When all the holes have been bored, the support, with the
machines, is hauled up, by means of a chain attached to the central bar,
out of the way of the blast. With this support, the time of fixing,
raising, and lowering is reduced to a minimum; while the facility with
which the machines may be slid along and fixed to the arm, and the
positions of the latter changed, allows the boring to be carried on
rapidly.
For open work, as in quarrying, where the stretcher bar cannot be used,
the tripod stand is adopted.
_The Dubois-François Carriage._--The support commonly used in France and
in Belgium consists of a kind of carriage carrying bars upon which the
drills are set. This carriage is used in drivings of all kinds; but it
is particularly suitable for tunnelling. It has been adopted, with but
slight modification, in the St. Gothard tunnel, and in several other
important works of the like character.
A modification of the carriage is shown in Figs. 30 and 31. Being
designed for ordinary mining operations, it carries but two machines;
but it will be readily perceived that, by increasing the number of
vertical screws, the same support may be made to carry a larger number.
It consists essentially of a vertical frame of flat bar iron _a b c d_,
8 feet in length, and 4 feet 9 inches in height above the rails, the
hinder portion of which rests upon a cast-iron plate _e f g h_, carried
upon two wheels; on this are fixed the two uprights _l_, _l′_, which,
being bound to the upper part by a transverse bar _m m′_, form a framing
to serve as a support to the two vertical screws _p′_, _q′_. The front
framing is formed of two longitudinals _b c_ and _b′ c′_ and the
uprights _a_, _a′_, and the vertical screws _p_, _q_, which are
connected to the upper part by the single piece _a d_. This framing is
supported below upon a small trolly with four wheels, connected to the
two longitudinals of the framing by a pivot bolt _n_ of ~T~ form, the
bar of the ~T~ being inserted into the elongated openings _o_ cut
through the middle of the curved portion of the longitudinals. The
cast-iron plate behind, the use of which is only to give stability to
the carriage, carries above it, by means of the two curved pieces _h_,
_h′_, a wrought-iron plate V, upon which the small tools needed for
repairs are kept. Two screws, _s_, _s′_, carried by lugs cast upon the
back of the plate, serve, by turning them down upon the rails, to fix
the carriage, the latter being slightly lifted by the screws.
Each machine is supported at two points. Behind, the point of support is
given by a cast-iron bracket _t_, having a projecting eye which enters
between the two cheeks formed at the back end of the machine by the
continuations of the two longitudinals of the framing. A pin bolt,
carried by the machine, allows the latter to be fixed to the bracket,
while leaving sufficient freedom of motion to allow of its being
directed at the required angle. This bracket, shown in plan in Fig. 33,
is supported by a kind of nut, Fig. 32, having two handles whereby it
may be easily turned. By raising or lowering this, the hinder support of
the drill may be brought to the requisite height. To prevent it turning
upon the screw, a pin is passed through the hole _o_, which pin forms a
stop for the handles aforementioned. The rotation of the bracket itself
is rendered impossible by the form of the vertical screw upon which it
is set, as shown in Fig. 33. In front, the support is a fork, the shank
of which slides along in the piece U, Figs. 30 and 31. This support,
which is not screwed on the inside, rests upon a nut of the same form as
that already described, and the same means are employed to prevent
rotation as in the case of the hinder supports.
SECTION III.--APPLIANCES FOR FIRING BLASTING CHARGES.
In the foregoing sections, the machines and tools used in rock boring
have been treated of. It now remains to describe those which are
employed in firing the charges after they have been placed in the
bore-holes. In this direction, too, great progress has been made in
recent times. With the introduction of new explosive agents, arose the
necessity for improved means of firing them. Attention being thus
directed to the subject, its requirements were investigated and its
conditions observed, the outcome being some important modifications of
the old appliances and the introduction of others altogether new. Some
of the improvements effected are scarcely less remarkable than the
substitution of machine for hand boring.
[Illustration: FIG. 28.]
[Illustration: FIG. 29.]
The means by which the charge of explosive matter placed in the
bore-hole is fired constitute a very important part of the set of
appliances used in blasting. The conditions which any such means must
fulfil are: (1) that it shall fire the charge with certainty; (2) that
it shall allow the person whose duty it is to explode the charge to be
at a safe distance away when the explosion takes place; (3) that it
shall be practically suitable, and applicable to all situations; and (4)
that it shall be obtainable at a low cost. To fulfil the second and most
essential of these conditions, the means must be either slow in
operation, or capable of being acted upon at a distance. The only known
means possessing the latter quality is electricity. The application of
electricity to this purpose is of recent date, and in consequence of the
great advantages which it offers, its use is rapidly extending. The
other means in common use are those which are slow in operation, and
which allow thereby sufficient time to elapse between their ignition and
the explosion of the charge for a person to retire to a safe distance.
These means consist generally of a train of gunpowder so placed that the
ignition of the particles must necessarily be gradual and slow. The old,
and in some parts still employed, mode of constructing this train was as
follows: An iron rod of small diameter and terminating in a point,
called a “pricker,” was inserted into the charge and left in the
bore-hole while the tamping was being rammed down. When this operation
was completed, the pricker was withdrawn, leaving a hole through the
tamping down to the charge. Into this hole, a straw, rush, quill, or
some other like hollow substance filled with gunpowder, was inserted. A
piece of slow-match was then attached to the upper end of this train,
and lighted.
The combustion of the powder confined in the straw fired the charge, the
time allowed by the slow burning of the match being sufficient to
enable the man who ignited it to retire to a place of safety. This
method of forming the train does not, however, satisfy all the
conditions mentioned above. It is not readily applicable to all
situations. Moreover, the use of the iron pricker may be a source of
danger; the friction of this instrument against silicious substances in
the sides of the bore-hole or in the tamping has in some instances
occasioned accidental explosions. This danger is, however, very greatly
lessened by the employment of copper or phosphor-bronze instead of iron
for the prickers. But the method is defective in some other respects.
With many kinds of tamping, there is a difficulty in keeping the hole
open after the pricker is withdrawn till the straw can be inserted. When
the holes are inclined upwards, besides this difficulty, another is
occasioned by the liability of the powder constituting the train to run
out on being ignited. And in wet situations, special provision has to be
made to protect the trains. Moreover, the manufacture of these trains by
the workmen is always a source of danger. Many of these defects in the
system may, however, be removed by the employment of properly
constructed trains. One of these trains or “squibs” is shown full size
in Fig. 28.
_Safety Fuse._--Many of the defects pertaining to the system were
removed by the introduction of the fuse invented by W. Bickford, and
known as “safety fuse.” The merits of this fuse, which is shown full
size in Fig. 29, are such as to render it one of the most perfect of the
slow-action means that have yet been devised. The train of gunpowder is
retained in this fuse, but the details of its arrangement are changed so
as to fairly satisfy the conditions previously laid down as necessary.
It consists of a flexible cord composed of a central core of fine
gunpowder, surrounded by hempen yarns twisted up into a tube, and called
the countering. An outer casing is made of different materials,
according to the circumstances under which it is intended to be used. A
central touch thread, or in some cases two threads, passes through the
core of gunpowder. This fuse, which in external appearance resembles a
piece of plain cord, is tolerably certain in its action; it may be used
with equal facility in holes bored in any direction; it is capable of
resisting considerable pressure without injury; it may be used without
special means of protection in wet ground; and it may be transported
from place to place without risk of damage.
In the safety fuse, the conditions of slow burning are fully satisfied,
and certainty is in some measure provided for by the touch thread
through the centre of the core. As the combustion of the core leaves, in
the small space occupied by it, a carbonaceous residue, there is little
or no passage left through the tamping by which the gases of the
exploding charge may escape, as in the case of the squibs. Hence results
an economy of force. Another advantage offered by the safety fuse is,
that it may be made to carry the fire into the centre of the bursting
charge if it be desired to produce rapid ignition. This fuse can be also
very conveniently used for firing charges of compounds other than
gunpowder, by fixing a detonating charge at the end of it, and dropping
the latter into the charge of the compound. This means is usually
adopted in firing the nitro-glycerine compounds, the detonating charge
in such cases being generally contained within a metallic cap. In using
this fuse, a sufficient length is cut off to reach from the charge to a
distance of about an inch, or farther if necessary, beyond the mouth of
the hole. One end is then untwisted to a height of about a quarter of an
inch, and placed to that depth in the charge. The fuse being placed
against the side of the bore-hole with the other end projecting beyond
it, the tamping is put in, and the projecting end of the fuse slightly
untwisted. The match may then be applied directly to this part. The
rate of burning is about two and a half feet a minute.
Safety fuse is sold in coils of 24 feet in length. The price varies
according to the quality, and the degree of protection afforded to the
train.
_Electric Fuses._--The employment of electricity to fire the charge in
blasting rock offers numerous and great advantages. The most important,
perhaps, is the greatly increased effect of the explosions when the
charges are fired simultaneously. But another advantage, of no small
moment, lies in the security from accident which this means of firing
gives. When electricity is used, not only may the charge be fired at the
moment desired, after the workmen have retired to a place of safety, but
the danger due to a misfire is altogether avoided. Further, the facility
afforded by electricity for firing charges under water is a feature in
this agent of very great practical importance. It would therefore seem,
when all these advantages are taken into account, that electricity is
destined to become of general application to blasting purposes in this
country, as it is already in Germany and in America.
An electric fuse consists of a charge of an explosive compound suitably
placed in the circuit of an electric current, which compound is of a
character to be acted upon by the current in a manner and in a degree
sufficient to produce explosion. The mode in which the current is made
to act depends upon the nature of the source of the electricity. That
which is generated by a machine is of high tension, but small in
quantity; while that which is generated by a battery is, on the
contrary, of low tension, but is large in quantity. Electricity of high
tension is capable of leaping across a narrow break in the circuit, and
advantage is taken of this property to place in the break an explosive
compound sufficiently sensitive to be decomposed by the passage of the
current. The electricity generated in a battery, though incapable of
leaping across a break in the circuit, is in sufficient quantity to
develop a high degree of heat. Advantage is taken of this property to
fire an explosive compound by reducing the sectional area of the wire
composing a portion of the circuit at a certain point, and surrounding
this wire with the compound. It is obvious that any explosive compound
may be fired in this way; but for the purpose of increasing the
efficiency of the battery, preference is given to those compounds which
ignite at a low temperature. Hence it will be observed that there are
two kinds of electric fuses, namely, those which may be fired by means
of a machine, and which are called “tension” fuses, and those which
require a battery, and which are known as “quantity” fuses.
In the tension, or machine fuses, the circuit is interrupted within the
fuse case, and the priming, as before remarked, is interposed in the
break; the current, in leaping across the interval, passes through the
priming. In the quantity or battery fuses, the reduction of the
sectional area is effected by severing the conducting wire within the
fuse case, and again joining the severed ends of the wire by soldering
to them a short piece of very fine wire. Platinum wire, on account of
its high resistance and low specific heat, is usually employed for this
purpose. The priming composition is placed around this fine wire, which
is heated to redness by the current as soon as the circuit is closed.
The advantages of high tension lie chiefly in the convenient form and
ready action of the machines employed to excite the electricity. Being
of small dimensions and weight, simple in construction, and not liable
to get quickly out of order, these sources of electricity are
particularly suitable for use in mining operations, especially when
these operations are entrusted, as they usually are, to men of no
scientific knowledge.
Another advantage of high tension is the small effect of line resistance
upon the current, a consequence of which is that mines may be fired at
long distances from the machine, and through iron wire of very small
section. A disadvantage of high tension is the necessity for a perfect
insulation of the wires.
When electricity of low tension is employed, the insulation of the
wires needs not to be perfect, so that leakages arising from injury to
the coating of the wire are not of great importance. In many cases, bare
wires may be used. Other advantages of low tension are the ability to
test the fuse at any moment by means of a weak current, and an almost
absolute certainty of action. For this reason, it is usually preferred
for torpedoes and important submarine work. On the other hand, the
copper wires used must be of comparatively large section, and the
influence of line resistance is so considerable that only a small number
of shots can be fired simultaneously when the distance is great.
[Illustration: FIG. 30.]
[Illustration: FIG. 31.]
[Illustration: FIG. 32.]
In Fig. 30 is shown an external view of an electric tension fuse. It
consists of a metal cap containing a detonating composition, upon the
top of which is placed the priming to be ignited by the electric spark.
The ends of two insulated wires project into this priming, which is
fired by the passage of the spark from one of these wires to the other.
The insulated wires are sufficiently long to reach a few inches beyond
the bore-hole.
Sometimes the fuse is attached to the end of a stick, and the wires are
affixed to the latter in the manner shown in Fig. 31. The rigidity of
the stick allows the fuse to be readily pushed into the bore-hole. When
the ground is not very wet, bare wires are, for cheapness, used, and the
stick is in that case covered with oiled paper, or some other substance
capable of resisting moisture. These “blasting sticks,” as they are
called, are extensively used in Germany. When heavy tamping is employed,
the stick is not suitable, by reason of the space which it occupies in
the bore-hole.
A mode of insulating the wires, less expensive than the guttapercha
shown in Fig. 30, is illustrated in Fig. 32. In this case, the wires are
cemented between strips of paper, and the whole is dipped into some
resinous substance to protect it from water. These “ribbon” wires may be
used in ground that is not very wet. They occupy little or no space in
the bore-hole, and therefore are suitable for use with tamping.
To connect the fuses with the machine or the battery, two sets of wires
are required when a single shot is fired, and three sets may be needed
when two or more shots are fired simultaneously. Of these several sets
of wires, the first consists of those which are attached to the fuses,
and which, by reason of their being placed in the shot-hole, are called
the “shot-hole wires.” Two shot-hole wires must be attached to each
fuse, and they must be of such a length that, when the fuse has been
placed in its proper position in the charge, the ends may project a few
inches from the hole. These wires must also be “insulated,” that is,
covered with a substance capable of preventing the escape of
electricity.
The second set of wires consists of those which are employed to connect
the charges one with another, and which, for this reason, are called
“connecting wires.” In connecting the charges in single circuit, the end
of one of the shot-hole wires of the first charge is left free, and the
other wire is connected, by means of a piece of this connecting wire, to
one of the shot-hole wires in the second hole; the other wire in this
second hole is then connected, in the same manner, to one of the wires
in the third hole; and so on till the last hole is reached, one
shot-hole wire of which is left free, as in the first. Whenever the
connecting wires can be kept from touching the rock, and also from
coming into contact one with another--and in most cases this may be
done--bare wire may be used, the cost of which is very little. But when
this condition cannot be complied with, and, of course, when blasting in
water, the connecting wires, like the shot-hole wires, must be
insulated. When guttapercha shot-hole wires are used, it is best to have
them sufficiently long to allow the ends projecting from one hole to
reach those projecting from the next hole. This renders connecting wire
unnecessary, and moreover saves one joint for each shot.
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
_Cables._--The third set of wires required consists of those used to
connect the charges with the machine or the battery. These wires, which
are called the “cables,” consist each of three or more strands of copper
wire well insulated with guttapercha, or better, indiarubber, the
coating of these materials being protected from injury by a sheathing of
tape or of galvanized iron wire underlaid with hemp. Two cables are
needed to complete the circuit; the one which is attached to the
positive pole of the machine, that is, the pole through which the
electric current passes out, is distinguished as the “leading cable,”
and the other, which is attached to the negative pole, that is, the pole
through which the current returns to the machine, is described as the
return cable. Sometimes both the leading and the return cables are
contained within one covering. When a cable having a metallic sheathing
is used, the sheathing may be made to serve as a return cable, care
being taken to make good metallic contact with the wires that connect
the sheathing to the fuses and to the terminal of the machine. The best
kind of unprotected cable consists of a three-strand tinned copper wire,
each 0·035 inch in diameter, insulated with three layers of indiarubber
to 0·22 inch diameter, and taped with indiarubber-saturated cotton to
0·24 inch diameter, as shown in Fig. 33. The best protected cable
consists of a similar strand of copper wire, covered with guttapercha
and tarred jute, and sheathed with fifteen galvanized iron wires of 0·08
inch diameter each, to a total diameter of 0·48 inch, as shown in Fig.
34.
_Detonators._--The new explosives of the nitro-cotton and
nitro-glycerine class cannot be effectively fired by means of safety or
other fuse alone. To bring about their instantaneous decomposition, it
is necessary to produce in their midst the explosion of some other
substance. The force of this initial explosion causes the charge of
gun-cotton, or dynamite, as the case may be, to detonate. It has been
found that the explosion of the fulminate of mercury brings about this
result most effectively and with the greatest certainty; and this
substance is therefore generally used for the purpose. The charge of
fulminate is contained in a copper capsule about a quarter of an inch in
diameter, and from 1 inch to 1¼ inch in length. These caps, with their
charge of fulminate, which are now well known to users of the
nitro-compounds, are called “detonators.” It is of the highest
importance that these detonators should contain a sufficiently strong
charge to produce detonation, for if too weak, not only is the whole
force of the explosive not developed, but a large quantity of noxious
gas is generated. Gun-cotton requires a much stronger charge of
fulminate than dynamite.
[Illustration: FIG. 35.]
In the electric fuses illustrated, the metal case shown is the
detonator, the fuse being placed inside above the fulminate. When safety
fuse is used, the end is cut off clean and inserted into the cap, which
is then pressed tightly upon the fuse by means of a pair of nippers, as
shown in Fig. 35. When water tamping is used, and when, with ordinary
tamping, the hole is very wet, a little white-lead or grease must be put
round the edge of the cap as a protection. The electric fuses are always
made waterproof; consequently, they are ready for use under all
circumstances. When the safety fuse burns down into the cap, or when, in
the other case, the priming of the electric fuse is fired, the fulminate
explodes and causes the detonation of the charge in which it is placed.
_Firing Machines and Batteries._--The electrical machines used for
firing tension fuses are of two kinds. In one kind, the electricity is
excited by friction, and stored in a condenser to be afterwards
discharged by suitable means provided for the purpose. In the other
kind, the electricity is excited by the motion of an armature before the
poles of a magnet. The former kind are called “frictional electric”
exploders; the latter kind are known as “magneto-electric” exploders.
When a magneto-electric machine contains an electro-magnet instead of a
permanent magnet, it is described as a “dynamo-electric exploder.”
Frictional machines act very well as exploders so long as they are kept
in a proper state. But as they are injuriously affected by a moist
atmosphere, and weaken rapidly with use by reason of the wearing away of
the rubbers, it is necessary to take care that they be in good
electrical condition before using them for firing. Unless this care be
taken, the quantity of electricity excited by a given number of
revolutions of the plate will be very variable, and vexatious failures
will ensue. If, however, the proper precautions be observed, very
certain and satisfactory results may be obtained. In Germany and in
America, frictional exploders are generally used.
Magneto-electric machines possess the very valuable quality of
constancy. They are unaffected, in any appreciable degree, by
atmospheric changes, and they are not subject to wear. These qualities
are of inestimable worth in an exploder used for ordinary blasting
operations. Moreover, as they give electricity of a lower tension than
the frictional machines, defects of insulation are less important. Of
these machines, only the dynamo variety are suitable for industrial
blasting. It is of primary importance that an exploder should possess
great power. The mistake of using weak machines has done more than
anything else to hinder the adoption of electrical firing in this
country.
[Illustration: FIG. 36.]
The machine most used in Germany is Bornhardt’s frictional exploder,
shown in Fig. 36. This machine is contained in a wooden case 20 inches
in length, 7 inches in breadth, and 14 inches in depth, outside
measurement. The weight is about 20 lb.
To fire the charges by means of this exploder, the leading wire is
attached to the upper terminal B, and the return to the lower terminal
C, the other ends of these wires being connected to the fuses. The
handle is then turned briskly from fifteen to thirty times, according to
the number of the fuses and the state of the machine, to excite the
electricity. The knob A is then pressed suddenly in, and the discharge
takes place. To ascertain the condition of the machine, a scale of
fifteen brass-headed nails is provided on the outside, which scale may
be put in communication with the poles B and C by means of brass chains,
as shown in the drawing. If after twelve or fourteen turns, the spark
leaps the scale when the knob is pushed in, the machine is in a
sufficiently good working condition. To give security to the men
engaged, the handle is designed to be taken off when the machine is not
in actual use; and the end of the machine into which the cable wires are
led is made to close with a lid and lock, the key of which should be
always in the possession of the man in charge of the firing operations.
[Illustration: FIG. 37.]
[Illustration: FIG. 38.]
In America, there are two frictional exploders in common use. One, shown
in Fig. 37, is the invention of H. Julian Smith. The apparatus is
enclosed in a wooden case about 1 foot square and 6 inches in depth.
The handle is on the top of the case, and is turned horizontally. This
handle is removable, as in Bornhardt’s machine. The cable wires having
been attached to the terminals, the handle is turned forward a certain
number of times to excite the electricity, and then turned a quarter of
a revolution backward to discharge the condenser and to fire the blast.
By this device, the necessity for a second aperture of communication
with the inside is avoided, an important point in frictional machines,
which are so readily affected by moisture. The aperture through which
the axis of the plate passes, upon which axis the handle is fixed, is
tightly closed by a stuffing-box. A leathern strap on one end of the
case allows the machine to be easily carried. The weight of this
exploder is under 10 lb.
The other exploder used is that designed by G. Mowbray. This machine,
which is shown in Fig. 38, is contained in a wooden barrel-shaped case,
and is known as the “powder-keg” exploder, the form and dimensions of
the case being those of a powder-keg. The action is similar to that of
the machine last described. The cable wires having been attached to the
terminals at one end of the keg, the handle at the other end is turned
forward to excite the electricity, and the condenser is discharged by
making a quarter turn backward, as in Smith’s machine. The handle is in
this case also removable. The weight of the powder-keg exploder is about
26 lb.
Both of these machines are very extensively used, and good results are
obtained from them. They stand well in a damp atmosphere, and do not
quickly get out of order from the wearing of the rubbers. They are also,
especially the former, very easily portable.
[Illustration: FIG. 39.]
The machine commonly used in England is the dynamo-electric exploder of
the Messrs. Siemens. This machine, which is the best of its kind yet
introduced for blasting purposes, is not more than half the size of
Bornhardt’s frictional exploder; but it greatly exceeds the latter in
weight, that of Siemens’ being about 55 lb. The apparatus, which is
contained within the casing shown in Fig. 39, consists of an ordinary
Siemens’ armature, which is made, by turning the handle, to revolve
between the poles of an electro-magnet. The coils of the electro-magnet
are in circuit with the wire of the armature; the residual magnetism of
the electro-magnet cores excites, at first, weak currents; these pass
into the coils, thereby increasing the magnetism of the cores, and
inducing still stronger currents in the armature wire, to the limit of
magnetic saturation of the iron cores of the electro-magnets. By the
automatic action of the machine, this powerful current is, at every
second turn of the handle, sent into the cables leading to the fuses.
To fire this machine, the handle is turned gently till a click is heard
from the inside, indicating that the handle is in the right position to
start from. The cable wires are then attached to the terminals, and the
handle is turned quickly, but steadily. At the completion of the second
revolution, the current is sent off into line, as it is termed, that is,
the current passes out through the cables and the fuses. As in the case
of the frictional machines, the handle is, for safety, made removable.
This exploder is practically unaffected by moisture, and it is not
liable to get out of order from wear.
Induction coils have been used to fire tension fuses; but it is
surprising that they have not been more extensively applied to that
purpose. A coil designed for the work required of it is a very effective
instrument. If constructed to give a spark not exceeding three inches in
length, with comparatively thick wire for quantity, it makes a very
powerful exploder. An objection to its use is the necessity for a
battery. But a few bichromate of potash cells, provided with spiral
springs to hold the zincs out of the liquid, and designed to be set in
action by simply pressing down the zincs, give but little trouble, so
that the objection is not a serious one. The writer has used an
induction exploder in ordinary mining operations without experiencing
any difficulty or inconvenience. It is cheap, easily portable, and
constant in its action.
Batteries are used to fire what are known as “quantity” or “low tension”
fuses. Any cells may be applied to this purpose; but they are not all
equally suitable. A firing battery should require but little attention,
and should remain in working order for a long time. These conditions are
satisfactorily fulfilled by only two cells, namely, the Léclanché and
the Bichromate of Potash. The latter is the more powerful, and generally
the more suitable. The Léclanché is much used in this country for firing
purposes, under the form known as the “Silvertown Firing Battery.” This
battery consists of a rectangular teak box, containing ten cells. Two,
or more, of these may be joined up together when great power is
required. In France, the battery used generally for firing is the
Bichromate. This battery is much more powerful than the Léclanché, and
as no action goes on when the zincs are lifted out of the liquid, it is
equally durable. It is moreover much cheaper. At the suggestion of the
writer, Mr. Apps, of the Strand, London, has constructed a bichromate
firing battery of very great power. It is contained in a box of smaller
dimension than the 10-cell Silvertown. The firing is effected by simply
lowering the zincs, which rise again automatically out of the liquid, so
that there is no danger of the battery exhausting itself by continuous
action in case of neglect. Externally, this battery, like the
Silvertown, appears a simple rectangular box, so that no illustration is
needed. With either of these, the usual objections urged against the
employment of batteries, on the ground of the trouble involved in
keeping them in order, and their liability to be injured by ignorant or
careless handling, do not apply, or at least apply in only a very
unimportant degree.
To guard against misfires, the machine or the battery used should be
constructed to give a very powerful current. If this precaution be
observed, and the number of fuses in circuit be limited to one-half that
which the machine is capable of firing with a fair degree of certainty,
perfectly satisfactory results may be obtained. The employment of weak
machines and batteries leads inevitably to failure. In the minds of
those who have hitherto tried electrical blasting in this country, there
seems to be no notion of any relation existing between the work to be
done and the force employed to do it. The electrical exploder is
regarded as a sort of magic box that needs only to be set in action to
produce any required result. Whenever failure ensues, the cause is
unhesitatingly attributed to the fuses.
CHAPTER II.
EXPLOSIVE AGENTS USED IN BLASTING ROCKS.
SECTION I.--PHENOMENA ACCOMPANYING AN EXPLOSION.
_Nature of an Explosion._--The combination of oxygen with other
substances for which it has affinity is called generally “oxidation.”
The result of this combination is a new substance, and the process of
change is accompanied by the liberation of heat. The quantity of heat
set free when two substances combine chemically is constant, that is, it
is the same under all conditions. If the change takes place within a
short space of time, the heat becomes sensible; but if the change
proceeds very slowly, the heat cannot be felt. The same _quantity_,
however, is liberated in both cases. Thus, though the quantity of heat
set free by a chemical combination is under all conditions the same, the
degree or _intensity_ of the heat is determined by the rapidity with
which the change is effected.
When oxidation is sufficiently rapid to cause a sensible degree of heat,
the process is described as “combustion.” The oxidation of a lump of
coke in the furnace, for example, is effected within a short space of
time, and, as the quantity of heat liberated by the oxidation of that
weight of carbon is great, a high degree results. And it is well known
and obvious that as combustion is quickened, or, in other words, as the
time of change is shortened, the intensity of the heat is proportionally
increased. So in the case of common illuminating gas, the oxidation of
the hydrogen is rapidly effected, and, consequently, a high degree of
heat ensues.
When oxidation takes place within a space of time so short as to be
inappreciable to the senses, the process is described as “explosion.”
The combustion of a charge of gunpowder, for example, proceeds with such
rapidity that no interval can be perceived to intervene between the
commencement and the termination of the process. Oxidation is in this
case, therefore, correctly described as an explosion; but the combustion
of a train of gunpowder, or of a piece of quick-match, though
exceedingly rapid, yet, as it extends over an appreciable space of time,
is not to be so described. By analogy, the sudden change of state which
takes place when water is “flashed” into steam, is called an explosion.
It may be remarked here that the application of this expression to the
bursting of a steam boiler is an abuse of language; as well may we speak
of an “explosion” of rock.
From a consideration of the facts stated in the foregoing paragraphs,
it will be observed that oxidation by explosion gives the maximum
intensity of heat.
_Measure of Heat, and specific Heat._--It is known that if a certain
quantity of heat will raise the temperature of a body one degree, twice
that quantity will raise its temperature two degrees, three times the
quantity, three degrees, and so on. Thus we may obtain a measure of heat
by which to determine, either the temperature to which a given quantity
of heat is capable of raising a given body, or the quantity of heat
which is contained in a given body at a given temperature. The quantity
of heat requisite to produce a change of one degree in temperature is
different for different bodies, but is practically constant for the same
body, and this quantity is called the “specific heat” of the body. The
standard which has been adopted whereby to measure the specific heat of
bodies is that of water, the unit being the quantity of heat required to
raise the temperature of 1 lb. of water through 1° Fahr., say from 32°
to 33°. The quantity of heat required to produce this change of
temperature in 1 lb. of water is called the “unit of heat,” or the
“thermal unit.” Having determined the specific heat of water, that of
air may in like manner be ascertained, and expressed in terms of the
former. It has been proved by experiments that if air be heated at
constant pressure through 1° Fahr., the quantity of heat absorbed is
0·2375 thermal units, whatever the pressure or the temperature of the
air may be. Similarly it has been shown that the specific heat of air at
constant volume is, in thermal units, 0·1687; that is, if the air be
confined so that no expansion can take place, 0·1687 of a thermal unit
will be required to increase its temperature one degree.
_Heat liberated by an Explosion._--In the oxidation of carbon, one atom
of oxygen may enter into combination with one atom of that substance;
the resulting body is a gas known as “carbonic oxide.” As the weight of
carbon is to that of oxygen as 12 is to 16, 1 lb. of the former
substance will require for its oxidation 1⅓ lb. of the latter; and since
the two enter into combination, the product, carbonic oxide, will weigh
1 + 1⅓ = 2⅓ lb. The combining of one atom of oxygen with one of carbon
throughout this quantity, that is, 1⅓ lb. of oxygen, with 1 lb. of
carbon, generates 10,100 units of heat. Of this quantity, 5700 units are
absorbed in changing the carbon from the solid into the gaseous state,
and 4400 are set free. The quantity of heat liberated, namely, the 4400
units, will be expended in raising the temperature of the gas from 32°
Fahr., which we will assume to be that of the carbon and the oxygen
previous to combustion, to a much higher degree, the value of which may
be easily determined. The 4400 units would raise 1 lb. of water from 32°
to 32 + 4400 = 4432°; and as the specific heat of carbonic oxide is
0·17 when there is no increase of volume, the same quantity of heat will
raise 1 lb. of that gas from 32° to
4400
32 + ---- = 25,914°.
0·17
But in the case under consideration, we have 2⅓ lb. of the gas, the
resulting temperature of which will be
25,914
------ = 9718°.
2⅓
In the oxidation of carbonic oxide, one atom of oxygen combines with one
atom of the gaseous carbon; the resulting body is a gas known as
“carbonic acid.” Since 2⅓ lb. of carbonic oxide contains 1 lb. of
carbon, that quantity of the oxide will require 1⅓ lb. of oxygen to
convert it into the acid, that is, to completely oxidize the original
pound of solid carbon. By this combination, 10,100 units of heat are
generated, as already stated, and since the carbon is now in the gaseous
state, the whole of that quantity will be set free. Hence the
temperature of the resulting 3⅔ lb. of carbonic acid will be
4400 + 10,100
32 + ------------- = 23,516°.
0·17 × 3·667
It will be seen from the foregoing considerations that if 1 lb. of pure
carbon be burned in 2⅔ lb. of pure oxygen, 3⅔ lb. of carbonic acid is
produced, and 14,500 units of heat are liberated; and further, that if
the gas be confined within the space occupied by the carbon and the
oxygen previously to their combination, the temperature of the product
may reach 23,516° Fahr.
In the oxidation of hydrogen, one atom of oxygen combines with two atoms
of the former substance; the resulting body is water. As the weight of
hydrogen is to that of oxygen as 1 is to 16, 1 lb. of the former gas
will require for its oxidation 8 lb. of the latter; and since the two
substances enter into combination, the product, water, will weigh 1 + 8
= 9 lb. By this union, 62,032 units of heat are generated. Of this
quantity, 8694 are absorbed in converting the water into steam, and
53,338 are set free. The specific heat of steam at constant volume being
0·37, the temperature of the product of combustion, estimated as before,
will be
53,338
32 + -------- = 16,049°.
0·37 × 9
Hence it will be observed that if 1 lb. of hydrogen be burned in 8 lb.
of oxygen, 9 lb. of steam will be produced, and 53,338 units of heat
will be liberated; and further, that the temperature of the product may
reach 16,049°.
_Gases generated by an Explosion._--It was shown in the preceding
paragraph that in the combustion of carbon, one atom of oxygen may unite
with one atom of carbon to form carbonic oxide, or two atoms of oxygen
may unite with one atom of carbon to form carbonic acid. When the
combination takes place according to the former proportions, the
reaction is described as “imperfect combustion,” because the carbon is
not fully oxidized; but when the combination is effected in the latter
proportions, the combustion is said to be “perfect,” because no more
oxygen can be taken up. The products of combustion are in both cases
gaseous. Carbonic oxide, the product of imperfect combustion, is an
extremely poisonous gas; it is this gas which is so noisome in close
headings, and in all ill-ventilated places, after a blast has been
fired. A cubic foot of carbonic oxide, the specific gravity of which is
0·975, weighs, at the mean atmospheric pressure, 0·075 lb., so that 1
lb. will occupy a space of 13·5 cubic feet. Thus 1 lb. of carbon
imperfectly oxidized will give 2⅓ lb. of carbonic oxide, which, at the
mean atmospheric pressure of 30 inches and the mean temperature of 62°
Fahr., will occupy a space of 13·5 × 2⅓ = 31·5 cubic feet. The product
of perfect combustion, carbonic acid, is a far less noxious gas than the
oxide, and it is much more easily expelled from confined places, because
water possesses the property of absorbing large quantities of it. In an
ill-ventilated but wet heading, the gas from a blast is soon taken up.
Carbonic acid is a comparatively heavy gas, its specific gravity
relatively to that of common air being 1·524. Hence a cubic foot at the
ordinary pressure and temperature will weigh 0·116 lb., and 1 lb. of the
gas under the same conditions will occupy a space of 8·6 cubic feet.
Thus if 1 lb. of carbon be completely oxidized, there will result 3⅔ lb.
of carbonic acid, which will fill a space of 8·6 × 3⅔ = 31·5 cubic feet.
It will be observed that, though an additional pound of oxygen has been
taken up during this reaction, the product occupies the same volume as
the oxide. In complete combustion, therefore, a contraction takes place.
In the oxidation of hydrogen, as already pointed out, one atom of oxygen
combines with two atoms of the former substance to form water. In this
case, the product is liquid. But the heat generated by the combustion
converts the water into steam, so that we have to deal with this product
also in the gaseous state, in all considerations relating to the effects
of an explosion. A cubic foot of steam, at atmospheric pressure and a
temperature of 212° Fahr., weighs 0·047 lb.; 1 lb. of steam under these
conditions will, therefore, occupy a space of 21·14 cubic feet. Thus the
combustion of 1 lb. of hydrogen will produce 9 lb. of steam, which,
under the conditions mentioned, will fill a space of 21·14 × 9 = 190·26
cubic feet.
Usually in an explosion a large quantity of nitrogen gas is liberated.
This gas, which is not in itself noxious, has a specific gravity of
0·971, so that practically a cubic foot will weigh 0·075 lb., and 1 lb.
will occupy a space of 13·5 cubic feet, which are the weight and the
volume of carbonic oxide. Other gases are often formed as products of
combustion; but the foregoing are the chief, viewed as the results of an
explosion, since upon these the force developed almost wholly depends.
_Force developed by an Explosion._--A consideration of the facts
enunciated in the foregoing paragraphs will show to what the tremendous
energy developed by an explosion is due. It was pointed out that the
combustion of 1 lb. of carbon gives rise to 31·5 cubic feet of gas. If
this volume of gas be compressed within the space of 1 cubic foot it
will obviously have a tension of 31·5 atmospheres; that is, it will
exert upon the walls of the containing vessel a pressure of 472 lb. to
the square inch. If the same volume be compressed into a space
one-eighth of a cubic foot in extent, say a vessel of cubical form and 6
inches side, the tension will be 31·5 × 8 = 252 atmospheres, and the
pressure 472 × 8 = 3776 lb. to the square inch. Assuming now the oxygen
to exist in the solid state, and the two bodies carbon and oxygen to
occupy together a space of one-eighth of a cubic foot, the combustion of
the carbon will develop upon the walls of an unyielding containing
vessel of that capacity a pressure of 252 atmospheres. Also the
combustion of 1 lb. of hydrogen gives rise, as already remarked, to
190·26 cubic feet of steam; and if combustion take place under similar
conditions with respect to space, the pressure exerted upon the
containing vessel will be 22,830 lb., or nearly 10·5 tons, to the
square inch, the tension being 190·26 × 8 = 1522 atmospheres.
The force thus developed is due wholly to the volume of the gas
generated, and by no means represents the total amount developed by the
explosion. The volume of the gases evolved by an explosion is estimated
for a temperature of 62°; but it was shown in a former paragraph that
the temperature of the products of combustion at the moment of their
generation is far above this. Now it is a well-known law of
thermo-dynamics that, the volume remaining the same, the pressure of a
gas will vary directly as the temperature; that is, when the temperature
is doubled, the pressure is also doubled. By temperature is understood
the number of degrees measured by Fahrenheit’s scale on a perfect gas
thermometer, from a zero 461°·2 below the zero of Fahrenheit’s scale,
that is, 493°·2 below the freezing point of water. Thus the temperature
of 62° for which the volume has been estimated is equal to 461·2 + 62 =
523°·2 absolute.
It was shown that the temperature of the product of combustion when
carbon is burned to carbonic oxide is 9718° Fahr., which is equivalent
to 10179°·2 absolute. Hence it will be observed that the temperature has
been increased
10179°·2
-------- = 19·45 times.
523°·2
According to the law above enunciated, therefore, the pressure will be
increased in a like ratio, that is, it will be, for the volume and the
space already given, 3776 × 19·45 = 73,443 lb. = 32·8 tons to the square
inch.
When carbon is burned to carbonic acid, the temperature of the product
was shown to be 23,516° Fahr., which is equivalent to 23977·2 absolute.
In this case, it will be observed that the temperature has been
increased
23977·2
------- = 45·83 times.
523·2
Hence the resulting pressure will be 3776 × 45·83 = 173,154 lb. = 77·3
tons to the square inch. It will be seen from these pressures that when
combustion is complete, the force developed is 2·36 greater than when
combustion is incomplete; and also that the increase of force is due to
the larger quantity of heat liberated, since the volume of the gases is
the same in both cases. If we suppose the carbon burned to carbonic
oxide in the presence of a sufficient quantity of oxygen to make
carbonic acid, we shall have 31·5 cubic feet of the oxide + 15·7 cubic
feet of free oxygen, or a total volume of 42·7 cubic feet of gases. If
this volume be compressed within the space of one-eighth of a cubic
foot, it will have a tension of 42·7 × 8 = 341·6 atmospheres, and will
exert upon the walls of the containing vessel a pressure of 5124 lb. to
the square inch. The temperature of the gases will be
4400
32 + ------------- = 6347° Fahr. = 6808°·2 absolute,
0·190 × 3·667
the mean specific heat of the gases being 0·190; whence it will be seen
that the temperature has been increased
6808°·2
------- = 13·01 times.
523·2
According to the law of thermo-dynamics, therefore, the pressure under
the foregoing conditions will be 5124 × 13·01 = 66,663 lb. = 29·8 tons
to square inch. So that, under the conditions assumed in this case, the
pressures developed by incomplete and by complete combustion are as 29·8
to 77·3, or as 1 to 2·59.
Similarly, when hydrogen is burned to water, the temperature of the
product will be, as shown in a former paragraph, 16,049 Fahr. = 16510·2
absolute; and the pressure will be
16510·2
22,830 × ------- = 720,286 lb. = 321·1 tons to the square inch.
523·2
It will be observed, from a consideration of the foregoing facts, that a
very large proportion of the force developed by an explosion is due to
the heat liberated by the chemical reactions which take place. And hence
it will plainly appear that, in the practical application of explosive
agents to rock blasting, care should be taken to avoid a loss of the
heat upon which the effects of the explosion manifestly so largely
depend.