ToBOMBARD,-BOMBARDING,BOMBARDMENT,
the act of assaulting a city or fortress, by throwing shells into it in order to set fire to and ruin the houses, churches, magazines, &c. and to do other mischief. As one of the effects of the shell results from its weight, it is never discharged as a ball from a cannon, that is, by pointing it at a certain object: but the mortars are fixed at an elevation of or about 45 degrees; that is, inclined so many degrees from the horizon, that the shell describes a curve, called the military projectile: hence a mortar, whose trunnions are placed at the breech, can have no point-blank range. Mortars should be so contrived, that they may be elevated to any degree required, as much preferable to those fixed at an angle of 45°; because shells should never be thrown at that angle but in one single case only, which seldom happens; that is, when the battery is so far off, that they cannot otherwise reach the works: for when shells are thrown from the trenches into the works of a fortification, or from the town into the trenches, they should have as little elevation as possible, in order to roll along, and not bury themselves; whereby the damage they do, and the terror they cause to the troops, is much greater than if they sink into the ground. On the contrary, when shells are thrown upon magazines, or any other buildings, with an intention to destroy them, the mortar should be elevated as high as possible, that the shells may acquire a greater force in their fall.
Shells should be loaded with no more powder than is requisite to burst them into the greatest number of pieces, and the length of the fuzes should be exactly calculated according to the required ranges; for, should the fuze set fire to the powder in the shell, before it falls on the place intended, the shell will burst in the air, and probably do more mischief to those who fired the mortar, than to those against whom it was discharged. To prevent this, the fuzes are divided into as many seconds as the greatest range requires, consequently may be cut to any distance, at an elevation of 45 degrees.
Mortars are not to be fired with two fires; for when the fuze is properly fixed, and both fuze and shell dredged with mealed powder, the blast of the powder in the chamber of the mortar, when inflamed by the tube, will likewise set fire to the fuze fixed in the shell.
BOMBARDIERS, artillery soldiers, who are employed in mortar and howitzer duty. They are to load them on all occasions; and in most services they load the shells and grenades, fix the fuzes, prepare the composition both for fuzes and tubes, and fire both mortars and howitzers on every occasion. In the English service, shells and grenades, composition for the same, fuzes, &c. are prepared in the laboratory by people well-skilled in that business.
In most other armies both officers and soldiers belonging to the companies of bombardiers, have an extraordinary pay, as it requires more mathematical learning to throw shells with some degree of exactness, than is requisite for the rest of the artillery. In the British service a specific number is attached to each company of artillery, and do not form a separate corps as in other countries.
BONAVOGLIE,Fr.a man that fora certain consideration voluntarily engages to row.
BONNET, in fortification, implies a small but useful work, that greatly annoys the enemy in their lodgments. This work consists of two faces, which make a salient angle in the nature of a ravelin, without any ditch, having only a parapet 3 feet high, and 10 or 12 feet broad. They are made at the salient angles of the glacis, outworks, and body of the place, beyond the counterscarp, and in the faussebray. SeeFortification.
Bonnetà Prêtre, orPriest’s Cap, in fortification, is an outwork, having three salient and two inward angles, and differs from the double tenaille only in having its sides incline inwards towards the gorge, and those of a double tenaille are parallel to each other. SeeFortification.
BORDER, in military drawings, implies single or double lines, or any other ornament, round a drawing, &c.
BOOKS. There are different books made use of in the army, for the specific purposes of general and regimental economy.
The general orderlyBookis kept by the brigade major, from which the leading orders of regiments, conveying the parole and countersign, are always taken.
The regimental orderlyBookcontains the peculiar instructions of corps which are given by a colonel or commanding officer to the adjutant—henceadjutant’s orderlyBook—and from him to the serjeant-major, who delivers the same to the different serjeants of companies assembled in the orderly room for that purpose—hence thecompany’s orderlyBook.
The regimentalBookis kept by the clerk of the regiment, and contains all the records, &c. belonging to the corps.
The CompanyBook, is kept by the commanding officer of every company; and contains returns of all incidents and payments.
The blackBookis a sort of memorandum which is kept in every regiment, to describe the character and conduct of non-commissioned officers and soldiers; when and how often they have been reduced or punished, &c.
Every quarter-master belonging to the cavalry and infantry, has likewise a book which may not improperly be called a book or inventory of regimental stores, &c.
PracticeBook. Every officer of the artillery ought to have a book in which he should note every useful fact that occurs in practice.
BOOM, in marine fortification, is a long piece of timber, with which rivers or harbors are stopped, to prevent the enemy’s coming in: it is sometimes done by a cable or chain, and floated with yards, topmasts, or spars of wood lashed to it.
BORE, in gunnery, implies the cavity of the barrel of a gun, mortar, howitzer, or any other piece of ordnance.
BOSSE,Fr.a term used in the French artillery, to express a glass bottle which is very thin, contains four or five pounds of powder, and round the neck of which four or five matches are hung under, after it has been well-corked. A cord, two or three feet in length, is tied to the bottle, which serves to throw it. The instant the bottle breaks, the powder catches fire, and every thing within the immediate effects of the explosion is destroyed.
BOTTES,Fr.boots.
GrossesBottes,Fr.jack-boots.
BOTTINE,Fr.half-boots worn by the hussars and dragoons in foreign armies.
BOUCHE,Fr.means the aperture or mouth of a piece of ordnance, that of a mortar, of the barrel of a musket, and of every species of fire-arms from which a ball or bullet is discharged.
BOUCHESà feu,Fr.is generally used to signify pieces of ordnance.
BOULERla Matiere,Fr.to stir up the different metals which are used in casting cannon.
BOULETSà deux têtes, chain-shot.
BOULEVART,Fr.formerly meant abastion. It is no longer used as a military phrase, although it sometimes occurs in the description of works or lines which cover a whole country, and protect it from the incursions of an enemy. Thus Strasburgh and Landau may be called two principal boulevarts or bulwarks, by which France is protected on this side of the Rhine.
The elevated line or rampart which reaches from the Champs Elysées in Paris beyond the spot where the bastille was destroyed in 1789, is stiled the Boulevart.
In ancient times, when the Romans attacked any place, they raised boulevarts near the circumference of the walls. These boulevarts were 80 feet high, 300 feet broad, upon which wooden towers commanding the ramparts were erected covered on all sides with iron-work, and from which the besiegers threw upon the besieged stones, darts, fire-works, &c. to facilitate the approaches of the archers and battering rams.
BOULINER,Fr.a French military phrase.Bouliner dans un camp, means to steal or pilfer in a camp.Un soldat boulineur, signifies a thief.
BOURGUIGNOTE,Fr.Is a helmet or morion which is usually worn with a breast-plate. It is proof against pikes and swords.
BOURRELET,Fr.the extremity of a piece of ordnance towards its mouth. It is usually cast in the shape of a tulip on account of its aptitude to fit the construction of embrasures. Bourrelet means likewise a pad or collar.
BOURRER,Fr.to ram the wad or any other materials into the barrel of a fire-arm.
BOURRIQUET,Fr.a basket made use of in mining, to draw up the earth, and to let down whatever may be necessary for the miner.
BOUSSOLE,Fr.a compass which every miner must be in possession of to direct him in his work.
BOUTE-SELLE,Fr.the signal or word which is given to the cavalry to saddle their horses.
BOUTON,Fr.the sight of a musquet.
BOW, an ancient weapon of offence, made of steel, wood, or other elastic matter; which, after being bent by means of a string fastened to its two ends, in returning to its natural state, throws out an arrow with prodigious force.
The use of the bow is, without all doubt, of the earliest antiquity. It has likewise been the most universal of all weapons, having obtained amongst the most barbarous and remote people, who had the least communication with the rest of mankind.
The bow is a weapon of offence amongst the inhabitants of Asia, Africa, and America, at this day; and in Europe, before the invention of fire-arms, a part of the infantry was armed with bows. Lewis XII. first abolished the use of them in France, introducing, in their stead, the halbert, pike, and broadsword. The long-bow was formerly in great use in England, and many laws were made to encourage the use of it. The parliament under Henry VII. complained of the disuse of long bows, theretofore the safeguard and defence of that kingdom, and the dread and terror of its enemies.
Cross-Bow, is likewise an ancient weapon of offence, of the eleventh century. Philip II. surnamed the Conqueror, introduced cross-bows into France. In this reign Richard I. of England, was killed by a cross-bow at the siege of Chalus.
BOWMAN. SeeArcher.
BOWYER. The man who made or repaired the military bows was so called.
BOXES, in military affairs, are of several sorts, and for various purposes.
Battery-Boxes. SeeBattery.
Cartouch-Boxes. SeeCartouch.
NaveBoxes, are made of iron or brass, and fastened one at each end of the nave, to prevent the arms of the axle-tree, about which the boxes turn, from causing too much friction.
Tin-Boxes, such as are filled with small shot for grape, according to the size of the gun they are to be fired out of.
Wood-Boxes, with lids, for holding grape-shot, &c. each calibre has its own, distinguished by marks of the calibre on the lid.
Boxesfor Ammunition. The dimensions of the common ammunition boxes vary according to the ammunition they are made to contain, in order that it may pack tight: this variation, however, is confined to a few inches, and does not exceed the following numbers.
Table of general dimensions of Ammunition Boxes.
Weight when filled, and number contained in each.
[7]Shells called four and an half, are really four and two-fifths.
[7]Shells called four and an half, are really four and two-fifths.
The common ammunition waggon will hold from 9 to 13 of these boxes in one tier.
The tonnage of ammunition in boxes is equal to its weight: about 12 boxes make one ton.
BOYAU, in fortification, is a particular trench separated from the others, which, in winding about, incloses different spaces of ground, and runs parallel with the works of the place, that it may not be enfiladed. When two attacks are made at once, one near to the other, the boyau makes a communication between the trenches, and serves as a line of contravallation, not only to hinder the sallies of the besieged, but likewise to secure the miners.
BRACES, in a military sense, are a kind of armor for the arm: they were formerly a part of a coat of mail.
BRACKETS, in gunnery, are the cheeks of the travelling carriage of a mortar; they are made of strong wooden planks. This name is also given to that part of a large mortar-bed, where thetrunnions are placed, for the elevation of the mortar: they are sometimes made of wood, and more frequently of iron, of almost a semicircular figure, well fastened with nails and strong plates.
BRANCH. SeeMineandGallery.
BRAND, an ancient term for a sword; so called by the Saxons.
BRAQUER,Fr.is improperly used to express the movement of a cannon to any particular side. The correct expression is, to point the cannon,pointer le canon.
BRASSARTS,Fr.thin plates of beaten iron which were anciently used to cover the arms above the coat of mail.
Brassarts and cuirasses were worn in the days of St. Louis.
BRASSERla Matiere,Fr.to mix the different ingredients which are required for the making of gunpowder or other combustible matter.
BREACH, in fortification, a gap, or opening, in any part of the works of a fortified place, made by the artillery or mines of the besiegers, preparatory to the making an assault.
The batteries to make a breach, should commence by marking out as near as possible, the extent of the breach intended to be made; first, by a horizontal line within a fathom of the bottom of the revetement in a dry ditch, and close to the water’s edge in a wet one; and then by lines perpendicular to this line, at short distances from each other, as high as the cordon; then, by continuing to deepen all these cuts, the wall will give way in a body. The guns to produce the greatest effect should be fired as near as possible insalvosor vollies. The breach should be one third the length of the face, from the centre towards the flanked angle. When the wall has given way, the firing must be continued to make the slope of the breach practicable.
Four 24 pounders from the lodgement in the covert way will effect a breach in 4 or 5 days, which may be made practicable in 3 days more.
Another way of making a breach is by piercing the wall sufficiently to admit two or three miners, who cross the ditch, and make their entry during the night into the wall, where they establish two or three small mines, sufficient to make a breach. SeeArtilleryat Siege; see alsoBattery.
To repair aBreach, is to stop or fill up the gap with gabions, fascines, &c. and prevent the assault.
To fortify aBreach, is to render it inaccessible by means of chevaux-de-frize, crow’s-feet, &c.
To make a lodgment in theBreach. After the besieged are driven away, the besiegers secure themselves against any future attack in the breach.
To clear theBreach, that is, to remove the ruins, that it may be the better defended.
BREAKoff, a term used when cavalry or infantry are ordered to diminish its front. It is also used to signify wheeling from line; asBREAKING-off to the left, forwheelingto the left.
BREAK-Ground, the first opening of the earth to form entrenchments, as at the commencement of a siege. It applies also to the striking of tents and quitting the ground on which any troops had been encamped.
ToBreakground, to begin, to open and work at the trenches in a siege, &c.
BREAST PLATE, in military antiquity, a piece of defensive armor worn on the breast of both men and horses. They are but seldom used now.
Breast-workSeeParapet.
BREECHof a gun, the end near the vent. SeeCannon.
BREVETrank, is a rank in the army higher than that for which you receive pay; and gives a precedence (when corps are brigaded) to the date of the brevet commission.
Brevet,Fr.commission, appointment. Under the old government of France it consisted in letters or appointments signed by the king, by virtue of which every officer was authorised to discharge his particular duty. All officers in the old French service, from a cornet or sub-lieutenant up to a marshal of France were stiledOfficiers à Brévet.
Brevetd’Assurance ou de Retenue d’Argent,Fr.certain military and civil appointments granted by the old kings of France, which were distinguished from other places of trust, in as much as every successor was obliged to pay a certain sum of money to the heirs of the deceased, or for the discharge of his debts. Hence the termbrevet d’Assurance ou de retenue.
BRICKS, in military architecture, supply the place of stone in common buildings, and are composed of an earthy matter, hardened by art, to a resemblance of that kind: they may be very well considered as artificial stone. The Greeks and Romans, &c. generally used bricks in their buildings, witness the Pantheon, &c. In the east they baked their bricks in the sun. The Romans used them unburnt, having first left them to dry in the air for 3, 4, or 5 years.
The bestbricksmust not be made of any earth that abounds with sand or gravel, not of such as is gritty or stony; but of a greyish marle, or yellow clay, or at least of reddish earth. But if there is a necessity to use that which is sandy, choice should be made of that which is tough and strong.
The best season for makingbricksis the spring; because they are subject to crack, when made in the summer: the loam should be well steeped or soaked, and wrought with water. They are shaped in a mould, and, after some drying inthe sun or air, are burnt to a hardness. This is our manner of making bricks; but whether they were always made in this manner admits a doubt. We are not clear what was the use of straw in the bricks for building in Egypt, or why in some part of Germany they mix saw-dust in their clay for bricks.
We are in general tied down by custom to one form, and one size; which is truly ridiculous: 8 or 9 inches in length, and 4 in breadth, is the general measure: but beyond doubt there might be other forms, and other sizes, introduced very advantageously.
CompassBricks, are of a circular form; their use is for steening of walls; we have also concave, and semi-cylindrical, used for different purposes.
Grey-Stocks, are made of the purest earth, and better wrought: they are used in front in building, being the strongest and handsomest of this kind.
Place-Bricks, are made of the same earth, or worse, and being carelessly put out of hand, are therefore weaker and more brittle, and are only used out of sight, and where little stress is laid on them.
Red Stocks, are made of a particular earth, well wrought, and little injured by mixtures: they are used in fine work, and ornaments.
HedgerlyBricks, are made of a yellowish colored loam, very hard to the touch, containing a great quantity of sand: their particular excellence is, that they will bear the greatest violence of fire without hurt.
BRICOLE, an improved kind of traces used by the French in drawing and manœuvring artillery; analogous to the old drag rope, but having the addition of a leather strap or girdle with a buckle, to which the drag is affixed; and an iron ring and hook at the end to drag by.
BRIDGES. Manner of laying a pontoon bridge across a river.
The bank on each side, where the ends of the bridge are to be, must be made solid and firm, by means of fascines, or otherwise. One end of the cable must be carried across the river; and being fixed to a picket, or any thing firm, must be drawn tight by means of a capstan, across where the heads of the boats are to be ranged. The boats are then launched, having on board each two men, and the necessary ropes, &c. and are floateddownthe stream, under the cable, to which they are lashed endwise, by the rings and small ropes, at equal distances, and about their own breadth asunder; more or less, according to the strength required. If the river be very rapid, a second cable must be stretched across it, parallel to the first, and at the distance of the length of the boats; and to which the other ends of the boats must be lashed. The spring lines are then lashed diagonally from one boat to the other, to brace them tight; and the anchors, if necessary, carried out, up the stream, and fixed to the cable or sheer line across the river. One of the chesses is then laid on the edge of the bank, at each end of the bridge, bottom up; these serve to lay the ends of the baulks upon, and as a direction for placing them at the proper distances, to fit the chesses that cover the bridge. The baulks should then be laid across the boats, and keyed together: their numbers proportioned to the strength required in the bridge. If the gangboards are laid across the heads and sterns of the boats from one side of the river to the other, they will give the men a footing for doing the rest of the work. Across the baulks are laid the chesses, one after another, the edges to meet; and the baulks running between the cross pieces on the under side of the chesses. The gangboards are then laid across the ends of the chesses on each edge of the bridge.
Precautions for passing a bridge of boats.
Whatever size the bridge may be, infantry should never be allowed to pass at the same time with carriages or cavalry. The carriages should always move at a certain distance behind each other, that the bridge may not be shook, by being overloaded. The horses should not be allowed to trot over the bridge; and the cavalry should dismount and lead their horses over. Large flocks of cattle must not be allowed to cross at once.
For the dimensions, weight, and equipage of a pontoon, see the wordPontoon.
When bridges are made to facilitate the communication between different parts of the approaches at a siege, they should, if possible, be placed above the town; or the besieged will take advantage of the current to float down large trees, or other bodies, in order to destroy the bridge. Two of such bridges should always be placed close to each other, in order to prevent the confusion of crossing and recrossing on the same bridge; the one being intended to pass over one way, and the other to return. Pontoon bridges will generally not support a greater weight than 4 or 5,000 pounds. Pontoons, when united as a bridge, will no doubt bear more in proportion, than when acted upon separately: but the weight which a pontoon will bear may be easily ascertained, by loading it with water till it sinks to any required depth, and then by calculating the number of cubic feet of water it contains, ascertain the number of pounds required to sink it to that particular depth.
Bridges, in military affairs, are of several sorts and denominations, viz.
Rush-Bridges, are made of large bundles of rushes, bound fast together, over which planks are laid, and fastened: these are put in marshy places, for an army to pass over on any emergency.
PendantorhangingBridges, are thosenot supported by posts, pillars, or butments, but hung at large in the air, sustained only at the two ends; as the new bridge at the Falls of Schuylkill, five miles from Philadelphia, 1809.
Draw-Bridge, that which is fastened with hinges at one end only, so that the other may be drawn up (in which case the bridge is almost perpendicular) to hinder the passage of a ditch, &c. There are others made to draw back and hinder the passage; and some that open in the middle; one half of which turns away to one side, and the other half to the other, and both again join at pleasure.
Flying-Bridge, is generally made of two small bridges, laid one over the other, in such a manner that the uppermost stretches, and runs out by the help of certain cords running through pullies placed along the sides of the upper bridge, which push it forwards, till the end of it joins the place it is intended to be fixed on. They are frequently used to surprise works, or out-posts that have but narrow ditches. There is a curious bridge of this kind on the Ohio, worthy of attention.
Bridgeof boats, is a number of common boats joined parallel to each other, at the distance of 6 feet, till they reach across the river; which being covered with strong planks, and fastened with anchors and ropes, the troops march over.
Bridgeof communication, is that made over a river, by which two armies, or forts, which are separated by that river, have a free communication with one another.
Floating-Bridge, a bridge made use of in form of a work in fortification called a redoubt; consisting of two boats, covered with planks, which are solidly framed, so as to bear either horse or artillery. Bridges of this kind are frequently used.
Floating bridges made of large logs of light timber bound together with a floor along them are common in the United States.
Ponton-Bridge, a number of tin or copper boats placed at the distance of 7 or 8 feet asunder, each fastened with an anchor, or a strong rope that goes across the river, running through the rings of the pontons. They are covered with baulks, and then with chests or planks, for the army to march over. SeePonton.
Cask, orBarrelBridge, a number of empty casks that support baulks and planks, made as above into a bridge, where pontons, &c. are wanting. Experience has taught us that 5 ton of empty casks will support above water 9000 pounds: hence any calculation may be made.
Bridgesare made of carpentry or masonry. The number of arches of a bridge is generally made odd; either that the middle of the stream or chief current may flow freely without interruption of a pier; or that the two halves of the bridge, by gradually rising from the ends to the middle, may there meet in the highest and largest arch; or else, for the sake of grace, that by being open in the middle, the eye in viewing it may look directly through there as we always expect to do in looking at it, and without which opening we generally feel a disappointment in viewing it.
If the bridge be equally high throughout, the arches, being all of a height, are made all of a size, which causes a great saving of centering. If the bridge be higher in the middle than at the ends, let the arches decrease from the middle towards each end, but so that each half have the arches exactly alike, and that they decrease in span proportionally to their height, so as to be always the same kind of figure. Bridges should rather be of few and large arches, than of many and small ones, if the height and situation will allow of it.
Names of all the terms, peculiar toBridges, &c.
Abutment.SeeButments.
Arch, an opening of a bridge, through or under which the water, &c. passes, and which is supported by piers or butments. Arches are denominated circular, elliptical, cycloidal, caternarian, equilibrial, gothic, &c. according to their figure or curve.
Archivolt, the curve or line formed by the upper sides of the voussoirs or arch-stones. It is parallel to the intrados or under side of the arch when the voussoirs are all of the same length; otherwise not.
By the archivolt is also sometimes understood the whole set of voussoirs.
Banquet, the raised foot-path at the sides of the bridge next the parapet: it is generally raised about a foot above the middle or horse-passage, and 3, 4, 5, 6, or 7, &c. feet broad, according to the size of the bridge, and paved with large stones, whose length is equal to the breadth of the walk.
Battardeau, or-Coffer-dam,
a case of piling, &c. without a bottom, fixed in the river, water-tight or nearly so, by which to lay the bottom dry for a space large enough to build the pier on. When it is fixed, its sides reaching above the level of the water, the water is pumped out of it, or drawn off by engines, &c. till the space be dry; and it is kept so by the same means, until the pier is built up in it, and then the materials of it are drawn up again. Battardeaux are made in various manners, either by a single inclosure, or by a double one, with clay or chalk rammed in between the two, to prevent the water from coming through the sides: and these inclosures are also made either with piles only, driven close by one another, and sometimes notchedor dove-tailed into each other, or with piles grooved in the sides, driven in at a distance from one another, and boards let down between them in the grooves.
Butments, are the extremities of a bridge, by which it joins to, or abuts upon, the land, or sides of the river, &c.
These must be made very secure, quite immoveable, and more than barely sufficient to resist the drift of its adjacent arch, so that, if there are not rocks or very solid banks to raise them against, they must be well re-inforced with proper walls or returns, &c.
Caisson, a kind of chest, or flat-bottomed boat, in which a pier is built, then sunk to the bed of the river, and the sides loosened and taken off from the bottom, by a contrivance for that purpose; the bottom of it being left under the pier as a foundation. It is evident, therefore, that the bottoms of the caissons must be made very strong and fit for the foundations of the piers. The caisson is kept afloat till the pier be built to the height of low water mark; and for that purpose, its sides must either be made of more than that height at first, or else gradually raised to it, as it sinks by the weight of the work, so as always to keep its top above water: and therefore the sides must be made very strong, and kept asunder by cross-timbers within, lest the great pressure of the ambient water crush the sides in, and so not only endanger the work, but also drown the workmen within it. The caisson is made of the shape of the pier, but some feet wider on every side to make room for the men to work; the whole of the sides are of two pieces, both joined to the bottom quite round, and to each other at the salient angle, so as to be disengaged from the bottom, and from each other, when the pier is raised to the desired height, and sunk. It is also convenient to have a little sluice made in the bottom, occasionally to open and shut, to sink the caisson and pier sometimes by, before it be finished, to try if it bottom level and rightly; for by opening the sluice, the water will rush in and fill it to the height of the exterior water, and the weight of the work already built will sink it: then by shutting the sluice again, and pumping out the water, it will be made to float again, and the rest of the work may be completed. It must not however be sunk except when the sides are high enough to reach above the surface of the water, otherwise it cannot be raised and laid dry again. Mr. Labelye states, that the caissons in which he built Westminster bridge, London, contained above 150 load of fir timber, of 40 cubic feet each, and were of more tonnage or capacity than a 40 gun ship of war.
Centres, are the timber frames erected in the spaces of the arches to turn them on, by building on them the voussoirs of the arch. As the centre serves as a foundation for the arch to be built on, when the arch is completed, that foundation is struck from under it, to make way for the water and navigation, and then the arch will stand of itself from its curved figure. The centre must be constructed of the exact figure of the intended arch, convex, as the arch is concave, to receive it on as a mould. If the form be circular, the curve is struck from a central point by a radius; if it be elliptical, it should be struck with a double cord, passing over two pins fixed in the focusses, as the mathematicians describe their ellipses; and not by striking different pieces or arcs of circles from several centres; for these will form no ellipsis at all, but an irregular misshapen curve made up of broken pieces of different circular arches; but if the arch be of any other form, the several abscissas and ordinates should be calculated; then their corresponding lengths, transferred to the centering, will give so many points of the curve; by bending a bow of pliable matter, according to those points, the curve may be drawn.
The centres are constructed of beams of timber, firmly pinned and bound together, into one entire compact frame, covered smooth at top with planks or boards to place the voussoirs on; the whole supported by off-sets in the sides of the piers, and by piles driven into the bed of the river, and capable of being raised and depressed by wedges contrived for that purpose, and for taking them down when the arch is completed. They should also be constructed of a strength more than sufficient to bear the weight of the arch.
In taking the centre down, first let it down a little, all in a piece, by easing some of the wedges; then let it rest a few days to try if the arch makes any efforts to fall, or any joints open, or any stones crush or crack, &c. that the damage may be repaired before the centre is entirely removed, which is not to be done till the arch ceases to make any visible efforts.
Chest.SeeCaisson.
Coffer-dam.SeeBattardeau.