Part 16

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.

_To_ BREAK _ground_, 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-_work_ See PARAPET.

BREECH _of a gun_, the end near the vent. See CANNON.

BREVET _rank_, 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 stiled _Officiers à Brévet_.

BREVET _d’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 term _brevet 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 best _bricks_ must 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 making _bricks_ is 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 in the 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.

_Compass_ BRICKS, 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.

_Hedgerly_ BRICKS, 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 floated _down_ the 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 word _Pontoon_.

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.

_Pendant_ or _hanging_ BRIDGES, are those not 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.

BRIDGE _of 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.

BRIDGE _of 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. See PONTON.

_Cask_, or _Barrel_ BRIDGE, 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.

BRIDGES are 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 to BRIDGES, &c.

_Abutment._ See _Butments_.

_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 notched or 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._ See CAISSON.

_Coffer-dam._ See BATTARDEAU.

_Drift_, _Shoot_, or _Thrust_, of an arch, is the push or force which it exerts in the direction of the length of the bridge. This force arises from the perpendicular gravitation of the stones of the arch, which being kept from descending by the form of the arch, and the resistance of the pier, exert their force in a lateral or horizontal direction. This force is computed in _Prop. 10_, of Mr. Hutton’s _Principles of Bridges_, where the thickness of the pier is determined that is necessary to resist it, and is greater the lower the arch is, _cæteris paribus_.

_Elevation_, the orthographic projection of the front of a bridge, on the vertical plane, parallel to its length. This is necessary to shew the form and dimensions of the arches and other parts, as to height and breadth, and therefore has a plain scale annexed to it, to measure the parts by. It also shews the manner of working up and decorating the fronts of the bridge.

_Extrados_, the exterior curvature or line of an arch. In the propositions of the second section in Professor Hutton’s _Principles of Bridges_, it is the outer or upper line of the wall above the arch; but it often means only the upper or exterior curve of the voussoirs.

_Foundations_, the bottoms of the piers, &c. or the bases on which they are built. These bottoms are always to be made with projections, greater or less, according to the spaces on which they are built: and according to the nature of the ground, depth and velocity of water, &c. the foundations are laid and the piers built after different manners, either in caissons, in battardeaux, on stilts with sterlings, &c. for the particular method of doing which, see each under its respective term.

The most obvious and simple method of laying the foundations and raising the piers up to the water-mark, is to turn the river out of its course above the place or the bridge, into a new channel cut for it near the place where it makes an elbow or turn; then the piers are built on dry ground, and the water turned into its old course again; the new one being securely banked up. This is certainly the best method, when the new channel can be easily and conveniently made. This, however, is seldom or never the case.

Another method is, to lay only the space of each pier dry till it be built, by surrounding it with piles and planks driven down into the bed of the river, so close together as to exclude the water from coming in; then the water is pumped out of the inclosed space, the pier built in it, and lastly the piles and planks drawn up. This is coffer-dam work, but evidently cannot be practised if the bottom be of a loose consistence, admitting the water to ooze and spring up through it.

When neither the whole nor part of the river can be easily laid dry as above, other methods are to be used; such as to build either in caissons or on stilts, both which methods are described under their proper words; or yet by another method, which hath, though seldom, been sometimes used, without laying the bottom dry, and which is thus: the pier is built upon strong rafts or gratings of timber, well bound together, and buoyed up on the surface of the water by strong cables, fixed to the other floats or machines, till the pier is built; the whole is then gently let down to the bottom, which must be made level for the purpose; but of these methods, that of building in caissons is best.

But before the pier can be built in any manner, the ground at the bottom must be well secured, and made quite good and safe, if it be not so naturally. The space must be bored into, to try the consistence of the ground; and if a good bottom of stone, or firm gravel, clay, &c. be met with, within a moderate depth below the bed of the river, the loose sand, &c. must be removed and digged out to it, and the foundation laid on the firm bottom on a strong grating or base of timber made much broader every way than the pier, that there may be the greater base to press on, to prevent its being sunk; but if a solid bottom cannot be found at a convenient depth to dig to, the space must then be driven full of strong piles, whose tops must be sawed off level some feet below the bed of the water, the sand having been previously dug out for that purpose; and then the foundation on a grating of timber laid on their tops as before: or, when the bottom is not good, if it be made level, and a strong grating of timber, 2, 3, or 4 times as large as the base of the pier be made, it will form a good base to build on, its great size preventing it from sinking. In driving the piles, begin at the middle, and proceed outwards all the way to the borders or margin; the reason of which is, that if the outer ones were driven first, the earth of the inner space would be thereby so jammed together, as not to allow the inner piles to be driven; and besides the piles immediately under the piers, it is also very prudent to drive in a single, double, or triple row of them round, and close to the frame of the foundation, cutting them off a little above it, to secure it from slipping aside out of its place, and to bind the ground under the pier firmer: for, as the safety of the whole bridge depends on the foundation, too much care cannot be used to have the bottom made quite secure.

_Jettée_, the border made round the stilts under a pier. See STERLING.

_Impost_, is the part of the pier on which the feet of the arches stand, or from which they spring.

_Key-stone_, the middle voussoir, or the arch-stone in the top or immediately over the centre of the arch. The length of the key-stone, or thickness of the archivolt at top, is allowed to be about ¹⁄₁₅th or ¹⁄₁₆th of the span, by the best architects.

_Orthography_, the elevation of a bridge, or front view, as seen at an infinite distance.

_Parapet_, the breast-wall made on the top of a bridge to prevent passengers from falling over. In good bridges, to build the parapet but a little part of its height close or solid, and upon that a balustrade to above a man’s height, has an elegant effect.