BORING, generally speaking, is the art of perforating a solid body. In the present article we propose to give some account of the boring of Cannon, of Cylinders, of Muskets, of Portland Stone, of Rocks, and of Wooden Pipes.

1. Boring of Cannon is performed by placing the cannon on an axis, which is turned by a very strong power, whilst a steel cutter, in form of a drill, is pressed against the metal, and excavates the cylindrical cavity which is required. Boring may be considered as a branch of the art of turning, which, in general, is the formation of cones, cylinders, and other figures that have an axis, by making a straight line or curve revolve round the axis on which the material is fixed, or by making the material revolve whilst the generating line remains at rest. In turning bodies of no great degree of hardness, and where it is required to take off only a small portion of the surface at once, a small power is sufficient to put the turning machine in motion; and the longer the edge of the cutter which is applied to the metal is, and the harder the metal, the greater force is required to turn the machine.

Cannon, at first, were frequently made of bars of malleable iron, placed longitudinally, and these bars covered with iron hoops, the whole welded or brazed together. Ordnance of this construction was not sufficiently strong to resist the explosion of the powder, and did not admit of the cylindrical cavity being formed with much accuracy. Its use was, therefore, gradually laid aside, and guns of cast-metal were employed. And before the casting of cannon became general, guns of cast-metal were reserved for the most important situations; thus the ships of the admiral and vice-admiral alone had cast-metal cannon, the other ships of war being armed with wrought-iron guns only.

Copper, without mixture, has been employed to cast guns, as appears from two large cannon made in the time of Henry VIII. and bearing his name, in the armoury of the Tower of London. But the only two materials now used for cannon are bronze, which is a mixture of copper and tin, and cast-iron. In modern times the use of cast-iron cannon has become more general, as that metal has the advantage of not being softened by the heat of the inflammation of the powder; whereas brass guns, when fired many times in rapid succession, become heated so nearly to the melting temperature of the metal, that the muzzle of the gun droops.

The first cannon made of cast-metal were cast hollow, with a cavity as nearly cylindrical as could be executed by casting. The surface of this cavity was then smoothed on a boring machine by steel cutters set in a copper head, and disposed so as to describe a cylinder terminated by a half spheroid. These cutters (in French alezoirs, and the operation alézer) are represented in the French Encyclopedic— planches, Ponte. This method of making guns has long been laid aside on account of the holes and inequalities in the cavity thus formed, and the difficulty of casting the cavity so as that its axis shall coincide with the axis of the piece. Cannon are now always cast solid, and the cylindrical cavity is formed by boring in this solid mass.

The power employed for boring cannon ought to be in proportion to the hardness of the metal of which they are composed, and to the size of the pieces. For the boring of guns of brass, as it is called, that is, a metal composed of ten parts of copper, one of tin, and two of brass, or of these metals in other proportions, a metal softer and more easily bored than cast-iron, horses are frequently employed as a moving power; but the strong moving powers of water or steam must be had recourse to for boring large guns of cast-iron, which is the material used for making the largest guns now in use, and is also the hardest substance used in their manufacture. Indeed some kinds of cast-iron are too hard to admit the action of the borer; and for the making of guns it is necessary to melt pig-iron of different qualities together, in order to have a metal that shall possess no more than the required degree of hardness.

The quality of pig-iron is known by the appearance of its surface, but more decisively by the appearance which its fracture presents. To obtain this fracture, a man takes one end of a pig in each hand, and lifting it as high above his head as he can, throws it with force, so that the middle of the pig shall fall across another pig placed on the ground. In this way the pig thrown down is broken. Soft or grey pig-iron, which is the most valuable, breaks with difficulty, and the surface of its fracture is of a grey colour, composed of pretty large crystalline grains. Hard or white pig-iron breaks easily; the surface of the fracture is white, and not sensibly granulated, the grains that compose it being small. The pig-iron here [7:5:2]spoken of is that smelted by the coak of pit-coal. Pig-iron smelted with charcoal of wood has a fracture of a different appearance, sometimes lamellar, like the fracture of metallic bismuth. Formerly guns used to be cast from the blast furnace; that is to say, immediately from the ironstone. This was attended with uncertainty in respect to the nature of the metal; for the nature of the metal given by the blast-furnace varies frequently and suddenly, from causes either unknown, or not under the command of the iron-master. For this reason guns are no longer cast from the blast-furnace, but pig-iron already formed is taken, of such qualities and in such proportions as to form a metal neither too soft nor too brittle and hard for guns. The different kinds of pig-iron thus selected are melted together in a furnace, called in iron manufactories an air-furnace, and by some writers a reverberatory furnace, by' the flame of pit-coal; the flame being impelled by a strong current of air produced by the rarefaction of the air in a chimney of thirty or forty feet in height. The column of the atmosphere of which the air in the chimney makes a part being lighter than the unrarefied columns of the atmosphere next it, its equilibrium with these columns is destroyed. The neighbouring columns, therefore, rush through the grate of the furnace, which is the only aperture by which they can attain the bottom of the rarefied column; and they carry the flame of the coal against the pig-iron, which is thereby brought into fusion. From the iron thus fused only one large gun is cast at a time, the furnace not being capable of melting more metal than is requisite for that purpose.

- The gun is cast with two appendages, which are to come off before it is finished and ready for use. The one is a square piece beyond the cascabel, for fixing the gun so as to revolve with the axis of the boring-mill; and the other is the head.

The head in cast-iron cannon is a mass of cast-iron two or three feet long, and somewhat bell-shaped. It is a prolongation of the mass of metal beyond the muzzle ring, and, in the position in which the gun is cast, the bead is the top of the whole mass, the square beyond the cascabel being the lowest part. 'After the metal has cooled, the upper surface of the head is cavernous, as is the case with the surface that is uppermost during the casting and cooling of any large body of cast-iron. The sides of the cavities in the head are frequently formed of cast-iron crystallized in a fern-leaf form. The intention of the head is to prevent these cavities, which are formed most abundantly at the upper surface of the cooling cast-iron, from forming in the gun itself. But, notwithstanding the precaution of casting the gun with a large head, and of mixing proper kinds of cast-iron in the air-furnace, it frequently happens that small cavities occur in the guns.

The gun with its head being cast and allowed to cool, it is conveyed to the boring-mill, where the head is to be taken off, the cylindrical cavity or bore is to be formed, and the outside of the gun is to be turned. Formerly the boring of guns was done in an upright position; the gun being placed above the boring-bar, was fixed in a frame sliding vertically in grooves. This frame was suspended on each side by a block and tackle, and the end of each of the two ropes was wound round a windlass. By turning these windlasses the gun might be raised or lowered, and by this means might be allowed either to press with its whole weight on the boring-bit, or with any part of its whole weight. A figure of this apparatus may be seen in the French Encyclopédie— planches, Ponte. Another vertical apparatus for boring cannon is represented in Rinman, Bergwerks Lexico, Stockholm, 1789, tab. iv.

I The practice which has long been followed in this country is to place the gun horizontally in the boring-mill; and it is fixed on the axis of the mill by means of the square piece at the cascabel.

In a boring-mill constructed by Smeaton, one gun is placed on the horizontal axis of the water-wheel itself, and, consequently, revolves with the same velocity. On this same axis is a toothed wheel with seventy-eight teeth, which works two wheels, one placed on each side of it, and each having twenty-nine teeth. On the axis of each of these a gun is placed; their power is ^ŷths of the power of the centre wheel. (See Smeaton’s Reports, vol. i.) On the axis where the power is least, smaller sized guns are bored; on the axis of the greatest power the large guns are bored. A crane, movable on a vertical axis, with a sweep that extends over all the carriages, with a tackle hanging from its beam, and wrought by a windlass, serves to place the gun on the carriage where it is to be bored, or to remove it from one carriage to another if required; and afterwards, when the gun is bored and turned, the crane serves to remove the gun from the boring-mill.

The gun, when placed on the. machine, has the square at the cascabel fixed in a square iron box (G, Plate CXI. fig. 5) on the axis. This box has a screw passing through each of its sides, and by the operation of these screws the square of the gun is adjusted, centred, and fixed; the chace of the gun is also fixed in a collar N, in which it is to revolve. (The collar in the figure is represented too near the muzzle ring.)

The axis on which each gun is fixed maybe set in gear, or put in connection with the revolving axis of the machine, so as to move round with it; or taken out of gear so as to remain at rest, although the other parts of the machine continue in movement. There are various methods of doing this. One is given by Smeaton in the work above cited. After the gun is fixed on the axis, and before beginning the operation of boring, the head, which has been described above, is cut off’near the muzzle ring. For this purpose the gun is set in gear so as to revolve on its axis with the moving power; and a bar of steel, in shape and size like the coulter of a plough, is applied at right angles to the axis of the gun. The narrow side of this bar is sharpened to a cutting edge, so that it has the form of one tooth of a very large saw; and this cutting edge is opposed to the direction of the revolving motion of tfie gun, and held strongly on to the gun by a screw pressing on the bar; the cutter takes off an angular portion at right angles to the axis, till the cylindrical part connecting the head with the gun is so much diminished, that the head is made to fall off by the blow of a hammer applied on it. In brass guns, cast with a core, the head was sawed off by hand with a blade of steel, whose edge was toothed as a saw, while the sides were toothed as files. See the French Encyclopédie— planches, Ponte.

A great degree of heat is generated by the violent friction of the steel-cutter on the cast-iron during the operation of cutting off the heads of guns. The quantity of this heat has been estimated by Rumford in one of his Essays on Heat.

After the head is taken off, the workmen proceed to bore the gun. This is done by exposing the revolving gun to the action of a steel-cutter, fixed on the end of a bar, which bar is placed on a carriage, and impelled continually towards the gun. The operation of boring is done on the same axis on which the head was cut off, if the power be sufficient; if not, the gun is removed, by means of the crane, to an axis, where it is made to revolve by a stronger power.

The boring-bar is fixed on a carriage sliding in iron grooves, which are truest when made triangular. The carriage, which, in the apparatus represented at fig. 5, consists merely of the bar on which the rack is, is pressed [7:5:3]forward by a pinion P, whose gudgeons are on a fixed frame BB; and this pinion works into a rack R. The axis of the pinion has mortised holes in it, through which one end of a lever L is passed; and the other end of this lever is loaded with a weight W, which causes the pinion to propel the carriage and boring-bar towards the gun. In many boring-machines there are two pinions on the same axis, acting on two racks; in others, the carriage is propelled by two upright levers, on the end of one of which acts a weight, hanging from a rope, that passes over a pulley; the lower end of the upper lever acts on the upper end of the lower, whilst the lower extremity of the lower lever presses forward the carriage. This method, which is free from any inequalities that may arise from the teeth of the rack, is figured by Smeaton in his Reports, vol. i. p. 396. Another method of propelling the carriage of the boringbar, is by a screw acting on the end of the carriage. See Meyer in the Transactions of the Academy of Stockholm, 1782, tab. ix.

The boring-bar is a very strong piece of wrought iron, of less diameter than the intended calibre of the piece, in order that the boring dust or shavings detached by the cutter may be got out. The boring-bar is increased in diameter near the end, for some inches, see fig. 6, B; in this part there is a superficial groove for receiving the sides of the steel-cutter or bit, which is to be firmly fixed in the bar. The bit T, fig. 6, is made from a rectangular piece of a steel bar, in which the two upper angles are cut off obliquely, so as to form two cutting edges like an obtuse-angled drill; the side of the rectangle, opposite to the point of the drill, is then hollowed out in the form of a pigeon hole; and this hollow fits into and embraces the solid part of the boring-bar, whilst the sides of the pigeon hole fit into the grooves of the bar. The point of this obtuse-angled bit is pressed against the revolving metal of the gun, by the force which propels the boring-bar; and the edges coming in contact with the revolving metal, a conical cavity is produced; so that, by taking off successively a multitude of similar shells or shavings, the cylindrical bore, with a conical termination, is formed. The diameter of the pointed bit first used must be less than the intended calibre of the piece, as the boring is to be repeated again at least once, in order to make the internal cylindrical surface as smooth as possible, by taking off any inequalities that have been left by the first cutter. In finishing the bore, a cross bit may be employed. It is a rectangular piece of steel, ground to a cutting edge at each end, and put through a hole in the boring-bar, in which it is fixed. The edges of this cutter, in revolving, describe a cylindrical surface. After the cylindrical surface of the bore is made sufficiently true, and of the required calibre, a bit without a point, and rounded off to the desired curve, is used to form the bottom of the chamber.

Some recommend that the boring-bit for cast-iron should have its cutting edges brought to an acute angle, by being filed hollow; but in this case the two edges cannot be brought into one point; for the obtuse-angled edge formed by the thickness of the metal of the bit joins the two cutting edges crossways, and forces itself forwards by being near the centre, requiring, however, a considerable pressure. These hollow-edged bits are not so well adapted to continuance of grinding as the plain ones, but they make amends by their much less frequently wanting sharpening. It does not appear, however, that these hollow-edged bits have been found advantageous in gun-boring.

The howitzer appears to have had its origin in Germany. This piece of ordnance, the mortar, and the carronade, in all of which the diameter of the chamber for the powder is smaller than the diameter of the rest of the bore, are first bored all through, nearly to the intended calibre of the chamber, and then that part of the bore that requires it is enlarged.

The cutters in gun-boring become magnetic, in consequence of being continually rubbed in the same direction, so that the boring dust is seen adhering and hanging from their edges when they are withdrawn from the gun. *

It is required that the bore shall be a Cylindrical cavity whose axis coincides with the axis of the gun: for this purpose, care must be taken to place the axis of the boring-bar, and that of the gun, both in one horizontal line, and it is requisite that these two lines continue in this position during the whole operation of boring. The centring of the boring-bar for this purpose requires to be done by an experienced workman, and an accurately-constructed boring-machine is necessary for the continuance of the right position.

Whilst on the axis of the mill, the gun has a smooth outer surface given it by turning tools, which are applied in the way usual in turning metals; a wooden gauge, or cut-out profile, of the gun, with its intended mouldings, being applied to know when the turning has been continued to a proper depth. When this is done the gun is taken out of the boring-mill; the square at the cascabel is cut off by the chisel; and the trunions, and other parts which are not susceptible of being turned, are dressed by the chisel. The cyphers and arms which had been cast on the gun are finished by the chisel.

A cannon is said to constitute the ultima ratio regurn, the last argument that governments have recourse to; and even this severe kind of argument has sometimes been embellished. Amongst ornamented cannon, the brass three-pounder in the Tower, brought from Malta, is a masterpiece; it is covered with carving in a good taste by a sculptor of Rome.

The touch-hole is drilled by stock and bit, or by drill and bow; the drill being propelled by a lever placed on a carriage, movable on wheels. A figure of this apparatus is given in the Encyclopédie— planches, Ponte. Another apparatus for this purpose is figured in Rinman, Bergwerks Lexico, table xiv. fig. 9, 10. See also Monge, Description de l’Art de Pabriguer les Canons, in 4to, Paris, 1794. This work was published by order of the revolutionary government, and distributed to the iron-masters and founders in different parts of France, for their instruction. It contains, amongst others, figures and descriptions of two kinds of vertical boring machines, of three kinds of horizontal boring machines, of a machine for turning the trunions, of two different machines for boring the touch-hole, of a machine for putting copper boshes in brass guns, and of various instruments for examining and proving guns.'

Before the gun is sent off, it is examined and proved in various ways. And first, to ascertain whether the bore is free from holes, an instrument is employed, consisting of several elastic steel prongs disposed in a circle, and with their sharp points turned outwards. This being fixed on a pole, is introduced into the bore of the gun, and drawn to and fro; the points of the prongs press against the sides of the bore, and the presence of a hole is known by one of the prongs getting into the hole, and preventing the instrument from being drawn out directly, unless by the use of a ring that is pushed over the prongs to unbend them.

There is another instrument, composed of a board twice as long as the bore of the piece. Along the middle of the board is a groove proceeding in a straight line.' In this groove a button is movable, and on the button, as a centre, are fixed two radii or arms; the two ends of these arms within the gun describe a line on the inside of thebore when the button is pushed inwards, whilst the extremities of the [7:5:4]arms on the outside describe two similar lines on the part of the board that is situate without the bore. In this way the outline of a longitudinal section of the bore is described, and its sinuosities or deviation from the axis are rendered sensible. This instrument is seldom used; it requires to be made by a workman skilled in the construction of mathematical instruments, or in watchmaking.

A lighted wax-candle is introduced into the gun for the purpose of seeing any defects there may be in the bore, or the light of the sun is reflected into the bore by a mirror. The strength of the gun is proved by firing it with a large charge of powder; and by forcing water into the bore by a powerful forcing pump, the touch-hole being stopped, and also the mouth of the piece, so that water forced in by the mouth cannot return that way.

2. Boring of Cylinders for steam-engines, and for blowing machines, and the boring of the working barrels of large pumps, and other hollow cylinders in which pistons are to work, is performed by making the steel-cutters describe a cylindrical surface on the inside of the cylinder, whilst the cylinder remains fixed. The first steamengine cylinders in this country were of brass, or of a mixture of copper and tin. This was the case with the cylinder of the steam-engine erected in the early part of the eighteenth century for lifting water from the colliery of Elphinston in Stirlingshire. But since that period the construction of steam-engines, and the manufacture of cast-iron, have been greatly improved; the uses of both have been much extended; and cast-iron has now for a long time been the only material employed in making cylinders for steam-engines, and other large cylinders in which pistons are to move.

In the boring of cylinders the steel-cutters are fixed in a cutter-head, which revolves with the boring-bar at the same time that it is impelled along the interior surface of the cylinder by a rack, with a pinion moved by a lever and weight as already described. The axis or boring-bar employed for cylinders is a hollow tube of cast-iron, and has a groove passing through it; the length of this groove being proportioned to the length of the cylinder to be bored. The cutter-head consists of two cast-iron rings, the first of which is accurately fitted on the boring-bar, which is turned truly cylindrical, so that this ring may slide along the boring-bar; the second ring is fixed round the first by wedges, its diameter being proportioned to the diameter of the cylinder to be bored; and on its circumference are eight notches to receive the steel-cutters, which are fixed in by wedges. The firstring is fixed on the boring-bar so as to make the whole cutter-head move round with the boring-bar, by means of two small iron bars, which go through notches in the first ring, and pass through the groove of the boring-bar. These small bars have each a round hole in the part which passes through the geometrical axis of the boring-bar; through these round holes there passes a bolt, which forms the end of the rack; a key is put through the end of the bolt, which prevents the rack from being drawn back by the lever and weight; and by this means the rack, impelled by the lever and weight, pushes forward the cutter-head, which is at the same time revolving with the boring-bar; while the connection of the rack and cutter-head being round, and in the axis of motion, the rack is thereby free from the circular motion of the cutter-head. This mode of constructing the boring-bar was invented in the works of Mr Wilkinson, at the time when accurately-bored cylinders came to be required in consequence of Mr Watt’s improvements in the steamengine. In the machines about to be mentioned the cutters are made to advance by a train of wheels deriving their motion from the power that turns the boring-bar.

An apparatus of great merit was contrived and describe ed in 1802 by Mr Billingsley, engineer of the BowIing iron-works, near Bradford. (See Repertory of Arts, second series, vol. ii. p. 322.) According to his method the cylinder is placed with its axis perpendicular to the horizon. The object of this is, first, that the boring-dust may fall out, and not remain on one side of the cylinder, wearing the cutters; so that in this way the cylinder may be bored through without changing the cutters, by which means a more regular bore is obtained. Secondly, That the cylinder may not deviate from its cylindrical form by its own weight, a deviation which is found to take place in large and slender cylinders when laid on their side; the vertical diameter being then less than the horizontal diameter. A similar loss of shape may happen to cylinders that are improperly wedged and strapped down for the purpose of being bored. In this method the cylinder is fixed with screws by the flanges, where it is most capable of resistance, and the screws are disposed so as to press the cylinder equally all round. Thirdly, That the operation may be sooner completed, which is effected in consequence of less time being employed to fix the cylinder in this method. In the usual mode of propelling the cutters described above, the attendance of a man is necessary to change the position of the bar on the axis of the pinion, and to raise the weight. This attendance is dispensed with in the machine under consideration, the mechanism for propelling the cutters being as follows: A leather strap passing over the boring-bar communicates the revolving motion of the boring-bar to a wheel, which communicates a slow motion by a train of wheels and pinions to an axis, bearing two pinions which work into two racks; and these racks push the boring-head and cutters slowly forward on the boring-bar, at the same time that the boring-head is revolving with the boring-bar. The velocity with which it is required that the cutters shall advance varies as the diameter of the cylinder varies, the moving power remaining the same. And by altering the train of wheelwork, the cutters may be made to advance with any velocity required.

Figs. 1, 2, 3, and 4, Plate CXI., are different views of the machine for boring cylinders, invented by Mr Murray of Leeds. Fig. 1 is an elevation, and fig. 2 a plan, of the machine. W, figs. 1 and 2, is the spur wheel, deriving its motion from water or steam, and communicating a revolving motion to the boring-bar. The toothed wheel A, fig. 1, moves round with the boring-bar B on which it is fixed; it gives motion through the wheels D and E, and to the screw S, whose threads act on the two racks, which racks are fixed to the cutter-head H, and revolve with it. The velocity with which the cutter-head is impelled along the cylinder depends upon the number of threads of the screw in a given length, and on the proportions of the wheels A, C, I), and E, to each other. By varying the velocity of the screw, the cutter-head maybe made to move in either direction up or down the cylinder. F is a pinion whose axis ends in a square, which may be wrought by a key, so as to bring the cutter-head out of the cylinder, or push it home by the hand when that is required. The cylinder is fixed in its bed by screws passing through two iron rings, as represented at fig. 4; and in this way the cylinder is equally pressed in the different parts of its circumference. Fig. 3 is a transverse elevation of the collar in which the end of the bar at A, fig. 1, turns; X is the gudgeon in which the spindle X, fig. 1, turns. In fig. 3 are also seen the two apertures through which the two racks pass. By this machine also the flanges are turned truly plane, so that the lid of the cylinder may fit on exactly.

The patent granted in 1799 to Mr Murdoch, engineer, Redruth, for new methods of constructing steam-engines [7:5:5](See Repertory of Arts, vol. xiii.), contains some articles relative to boring. He employs an endless screw, which is turned by the moving power, and works into a toothed wheel, whose axis carries the cutter-head; and this method, he says, produces a more smooth and steady motion than the usual mode of fixing the boring-bar immediately on the axis turned by the moving power.

Another article in Mr Murdoch’s patent that relates to boring, is his method of forming the cylinder and steamcase. He casts them of one solid piece, and then bores a cylindrical interstice, by means of a boring-tool, made of a hollow cylinder of iron, with steel-cutters fixed to its edge, and acting like a trepan.

The chambers of brass pumps, whose diameter does not exceed a few inches, are fixed within iron rings, by means of screws, in the manner described above when speaking of Mr Murray’s apparatus. The rings are made accurately cylindrical by turning, as is also the boring-bar. The boring-bar has four cross arms on its outer extremity, to one of which a handle is fixed, whereby a workman makes the boring-bar revolve. The cutter-head is made to advance along the boring-bar by a screw.

3. Boring the Barrels of Muskets and other Small Arms. Rectangular pieces of iron are forged of a proper length and breadth; these are heated in the fire, and the two long edges, which had been previously thinned off, are welded together on a mandril. The barrel thus formed is fixed by a screw on a carriage that moves in iron grooves; this carriage is propelled towards the boringbar by a rope which passes over pullies, and has a weight hanging from its end. The boring-bar is turned by the power of the same mill that turns the grinding-stones for polishing the outside of the barrels. (See Encyclopedic— planches, Arquebusier; and Rozier, Introduction aux Observations sur la Physique, tom. i. p. 157.) Water is thrown on the barrels whilst boring from a trough placed underneath. After the barrel is bored, the interior surface of the bore is polished by the action of the boringbar. The barrel is tried during the operation, by an iron gauge of an inch and a half in length, and of a diameter equal to the intended diameter of the musket. When the barrel is bored, it is held to the light and looked through, and if it contains any flaw, the place of that flaw is marked on the outside with chalk, and the barrel is put on the mandril again, and the defective place heated and hammered; the workman also examines with a gauge whether the barrel is crooked. When the bore has no flaws, the barrel then undergoes the operation of the grinding-mill, to the effect of polishing its exterior surface.

Rifled barrels are put on a bench twelve feet long. The boring-bar is guided by a matrix or female-screw, whose spiral curve is similar to the spiral of the rifles intended to be made; the boring-bar being fixed to a male-screw, which passes through the female-screw, and fits it exactly. The female-screw is fixed to the bench, and has four threads and as many furrows; and these threads, in general, return to the point of the circumference from which they set out, or make a revolution in the length of two feet. The male-screw, which fits into the female-screw, has at one end an iron bar attached to it, by which it is put in motion; at the other extremity is fixed the boring-bar, which passes through the barrel to be rifled; and the boring-bar has a cutter fixed in it, which forms a spiral furrow in the barrel when the screw is turned by the handle. The number of spiral threads in rifle-barrels is from three to twelve. Sometimes the threads and furrows of the riflebarrel are required to be in straight lines; in which case a straight-lined matrix is used. In order that the threads may be placed at an equal number of degrees of the circumference from each other, the bench is furnished with a brass plate, divided in the same way as the plate of the machine for cutting the teeth of clock-wheels.

4. Boring of Portland Stone, so as to form pipes. That kind of calcareous stone called by geologists oolite, which is quarried for building at Portland, Bath, in the neighbourhood of the city of Paris, and other places, admits of being cut by means of an iron blade, acting as a saw, with sand and water. The more compact limestones and marbles are also cut in this way, but not so easily. The other kinds of stone that can be squared for building, namely, sandstone and granite, scarcely yield to the saw, but are formed into the desired shape by the chisel and hammer. A modification of this mode of working Portland stone consists in forming it into pipes. The method of Sir George Wright, proposed in 1805, is as follows: A hole is drilled through the block of stone, in which a long iron bolt is inserted for the saw to work round as a centre; this bolt forms the axis of the cylinder which is to be taken out, and projects considerably beyond the block at both ends. Another hole is drilled in the intended circumference; and into this the blade of the saw is introduced. The frame of the saw is so disposed, that when it is wrought to and fro, the blade is guided, by means of the centre bolt, so as to describe the intended cylindrical circumference. In this way a solid cylindrical core of stone is detached, and a cylindrical cavity or pipe left in the block. Or the saw may be made to describe a circle without drilling a hole in the centre, by drilling a hole in the circumference, and fixing on the surface of the stone two metallic concentric rings, so that the hole shall be included in the interstice between the rings. The saw is then introduced into the hole, and being worked, it cuts in the circular path formed by the interstice of the rings. See Repertory of Arts, second series, vol. viii.

Mr Murdoch’s method, for which he obtained a patent in 1810, is preferable in practice to the above-mentioned method. He employs a cylindrical saw to form the pipe. A plug of wood is inserted in the centre of the intended pipe; this plug receives the lower end of a vertical spindle, longer than the intended pipe; and this spindle is square, with sockets sliding on it. On the upper part of the spindle is a pulley or toothed-wheel, by which the spindle is made to revolve. Near the lower end of the spindle is a wheel, having a circumference like a hoop, three inches broad. The diameter of this wheel is somewhat less than that of the pipe to be bored. It regulates the motion, and fits in the inside of a tube of metal attached to the spindle. The diameter of the tube is nearly equal to that of the intended pipe; but its length is greater by two feet. On the lower edge of the tube is a rim of metal, so much thicker than the tube that the groove cut in the stone by the rim may admit the tube to move freely in it. This rim has an edge like that of a stone-cutter’s saw, and in fact performs the office of a saw. The tube is caused to make a reciprocating circular motion round the spindle. There is a cistern placed above the tube, for the purpose of conveying a mixture of sand and water into the cylindrical groove formed in the stone, whilst the machine is working.

Stone pipes, made in the above described way, have been tried for conveying water in London. They were joined by means of Parker’s cement, which consists of clay ironstone, burnt, and ground to a fine powder. This was the best material that could be got for forming the joints; but these joints cracked and allowed the water to escape, in consequence of the motion of the carriages on the streets under which the pipes were laid; and the adventurers found that they “had hewed out unto themselves broken cisterns, that could hold no water.”

5. Boring of Rocks, for the purpose of splitting them by means of gunpowder. We have already treated this subject[7:5:6] under Blasting, and shall only add here the mode of boring for this purpose practised in the mines of Germany.

A boring bar of steel is applied to the stone by its lower end, whilst its upper extremity is struck with a hammer of two pounds in weight. The form of the lower end of the boring-bar is various; some were fashioned like a swallow’s tail, ending in two points; but this form is no longer in use. Another kind has the and formed by the intersection of two wedge-shaped edges, with a point· at each corner, and one in the middle. A third kind has the end composed of four pyramidal points, with cavities between them. A fourth kind, which is that most frequently used, has the end in form of a wedge. (See Rinman, Eergwerhs Lexico. Stockholm, 1789, tab. ii.) Three sizes of boring bars are employed to make one hole, the first is the shortest and thickest, the second is longer and less in diameter, the third is the longest and the least in diameter. When a hole is to be made, a small opening is first formed with a pick in the place where the boring-iron is to be applied, and all pieces of the rock are removed that might impede the action of the powder. Then the workman uses the first boring-iron, which he drives with blows of the hammer till the boring-iron can reach no farther; he then employs the second and third boring-bars in like manner; and after each stroke of the hammer, the boring-bar is turned round a portion of the circumference. The stone, pulverized by the action of the boring-bar, as it hinders the progress of the operation, must be removed from time to time by means of an iron-rod, terminated at right angles by a small round plate. From the different diameter of the boring-bars, it follows that the end of the hole is of a smaller diameter than that of the beginning. The depth to which the hole is bored is proportioned to the nature of the rock. It varies from 10 to 15 and 20 inches. When the rock is solid a great way round, a deep hole is not used, because the resistance at a considerable depth, in such a situation, is too great; so that the explosion does not split the rock round the powder chamber, but acts upwards against the ramming, where it meets with less resistance. But if the rock be laid bare on one side, a deep hole is advantageous. Water is poured into the hole during the operation, to facilitate the action of the boring-iron. When the hole is perpendicularly downwards, it is kept full of water; when the hole is driven from below upwards, no water can be used. The water must be taken out, and the hole dried, before the cartridge be introduced, t The most frequent case is, that one man performs the work, holding the boring-iron in his left hand,’ and striking on it with the two-pound hammer in his right. Sometimes two men are set to do the work, one holding the boring-iron, whilst the second strikes it with a hammer of four or five pounds: this is done where it is required to make the hole thirty or thirty-six inches deep. When a still deeper hole is wanted, two men strike alternately with heavier hammers.

6. Boring of Wooden Pipes is done by means of a long auger, beginning with one of small diameter, and proceeding to employ successively spoon-formed augers of larger diameter. Notwithstanding the frequent employment of cast-iron pipes, some wooden pipes are still used for conveying water in London: they are of elm, which is the kind of tree most frequent in the neighbouring country. A pipe is bored out of one trunk of elm, and the bark is left on. When a tree is to be bored, it is fixed on a carriage, with a rack on the under part. This rack fits into a pinion, the axis of which passes through gudgeons on a fixed frame. On the axis of the pinion is a ratchet wheel, moved by two catches, which derive their motion from the wind or water power that turns the auger; and the pinion is moved in a direction that brings the tree towards the auger. See a figure in Belidor, Architecture Hydraulique, i. 1, 341. This apparatus is the same as the one employed in saw-mills. In the boring of pipes for the water-works in London, the tree is made to advance by ropes, which pass over a windlass wrought by men, whilst the auger is turned by a horse-mill. Wooden pipes are frequently bored by an auger having at its outer end a wooden drift or handle, which is put in motion by the workman. The trees are placed on tressels, and there are also tressels of a convenient height that support the auger; there is also a lathe to turn one end of the tree conical, so as to fit into a conical cavity in the end of the adjoining tree, and thus form a water-tight joint. The end of the tree which receives the adjoining pipe within it has a surface at right angles to the axis of the pipe. Into this surface is driven an iron hoop, the diameter of which is some inches greater than the diameter of the aperture of the pipe. This precaution prevents the tree from splitting when the conical end of the next tree is driven home. When the tree is crooked, a bore is driven in from each end, and the two bores meet, forming an angle. An auger whose stalk is formed spirally for some way up is figured in Bailey’s Machines of the Society of Arts. The objects of this is that the chips may be delivered without taking the auger out of the hole.

There is a patent granted in 1796 to Mr Howell, coalmaster, of Oswestry, for boring wooden pipes by a hollow cylinder made of thin plates of iron, about an inch less in diameter than the hole to be bored. To one end of this cylinder is fixed a flange about a quarter of an inch in breadth, and one part of this flange is divided, so that, being of steel, a cutter is formed thereby. The object of this method is to bore out a solid cylinder of wood, capable of being converted into a smaller pipe, or of being applied to some other use in carpentry. (Repertory of Arts, vol. ix.) This kind of borer is like the trepan, which is a hollow cylinder of steel, saw-toothed on the edge, and, when made to revolve rapidly on its axis in the hand of the surgeon, it saws or bores out circular pieces of the flat bones of the head. (b. b.)