BRIDGE.

and is confined to the more general and obvious conditions which such bridges must fulfil, avoiding the mechanical theory of their stability as too abstruse for popular exposition.

An arched B. rests between masses of masonry on opposite sides of a river, called its abutments (q. v.). The intermediate points of support of the arches are the piers (q. v.), which are rarely built so strong as to be able of themselves to resist the lateral thrust of the arches resting on them, if the thrust of one arch did not counteract that of another. The arch itself is the curved construction between adjacent piers. The chief terms used in speak-way in which the stream affects the form. If it is ing of the arch itself are explained under ARCH. In addition, may be noticed the spandril, the name given to the filling in above the extrados to the roadway. The chord or span is the distance between the piers; while the rise of the arch is the perpendicular distance between the level of the springing and the horizontal through the key.

When a B. has to be erected, the question of what form it should be falls to be settled by a variety of considerations. Regard to appearance affects the question, but the material points are its sufficiency for the purposes for which it is intended, and its security and durability. The nature of the embankments and of the soil in the water-bed, together with the nature of the water-shed, or country drained by the stream, may make it necessary that the B. should not be an arched bridge at all, but a sus pension or tubular bridge. But if it is to be an arched B., then the most important questions respect the number of its piers and the form of its arches. If vessels must be free to pass under it, the arches must be lofty, and the abutments high; so also must they be if the river is exposed to sudden elevations of its level by floods. Formerly, a prejudice existed against laying a B. across a stream at any other angle than at right angles to its course. The reason was, that, the theory of the skewed arch (q. v.) being unknown, the obliquity of the B. to the water-course involved a corresponding obliquity of its piers to the water, which greatly increased the risk of the B. suffering from floods. That the pressure of the current on piers increases B with their obliquity to its course, may be seen at once from the annexed figures, which represent the same section of a pier set first, as in A, dead up and down the stream, and next, as in B, obliquely to it. The mass of water which strikes B is equal in breadth to a'b', the distance between lines through the extremities of B, parallel to the stream; while the mass which strikes A is in breadth only equal to ab, the thickness of the pier. But the skewed arch allows a B. to be thrown at any angle across a river, with its piers all parallel to the stream; and many an awkward turn in our public roads would have been spared us, had the skewed arch only been earlier known.

From this consideration, the effect of too many
piers will be obvious; but indeed this is not matter
of speculation, for many bridges-among others, a
B. of Smeaton's at Hexham-have been destroyed
from this cause, thus falling from the very overabun-
dance of support! To know how many piers may
with safety be used, the volume of water that flows
through the channel, both ordinarily and in winter-
floods, must be ascertained, which can be done very
nearly by calculating the mean of many soundings
taken at different states of the river, and at a suc-
cession of points across its bed. There is another
liable to floods, care must be taken to make the
piers so high as to elevate the spring of the arches
above the highest level attainable by the water.
The annexed figure sufficiently shews how greatly
the pressure of the water on the B. increases when
ever it reaches above the
pier-head-the breadth
above the springing, as
at abcd, greatly exceed-
ing the breadth of the
pier itself. In con-
nection with this part
of the subject, it must
be remembered, too,
that floods are apt to
carry down trees and
other floating masses, which, if the arches do not
afford them passage, become powerful levers for the
destruction of the bridge.

The form of the B. being determined on, the remaining questions relate to its stability. This depends on the strength of the abutments and piers, and the balanced equilibrium of the arches. The importance of securing proper foundations for the abutments and piers cannot be overestimated, and very frequently their foundations, owing to the nature of the soil, have to be artificially constructed. See PILES, COFFER-DAM, and CONCRETE. In considering the stability of the B., the first thing is to ascertain the forces which will act to destroy it. This is ascertained by calculating the extreme passing load, and also the weight of the structure above the arches, and of the arches themselves. A scientific and skilled engineer is then able to judge what amount of strain or destructive pressure will be exercised by these weights on the several parts of the structure, and thus to adapt the strength at every point to the strain. As to the passing load, it is usual to calculate on 240 lbs. per foot, superficial, of the whole area in ordinary bridges, and on 960 lbs. in railway-bridges. The weight of the superstructure and arches is a question for practical measurement. As to the remaining pressure-viz., that of the stream-it must be ascertained for the highest floods. It is calculated from knowing the mean velocity of the stream, and the amount of surface exposed to it. The surface velocity is readily observed by means of floats; and when this is under 10 feet per second, the mean velocity is found to be about one-fifth less. The stress of After making allowance for the requirements of the stream on the bridge is dimiposition and traffic, the form next must be con- nished by the expedient known as a sidered, more particularly in relation to the stream. cut-water, which is an angular proThe stream principally affects the form, through jection from the pier, as shewn in prescribing the number of piers. Each pier takes the annexed figure. The best form up so much of the water-course, and thus narrows for a cut-water has practically been the effective passage of the water. The immediate ascertained to be an equilateral prism, consequence of narrowing the channel is to increase presenting an angle of 60° to the the velocity of the stream. As the velocity of the water-course. In all bridges, these stream increases, it tends more and more to carry are to be found on the sides of the piers presented off the soil in the neighbourhood of the piers, and to the stream; and in tidal rivers, they are built on finally, by deepening its course, to undermine them. I the lower side as well.

BRIDGE-BUILDING BROTHERHOODS-BRIDGE-HEAD.

or other annoyances.

Private bridges are those
erected and maintained under contracts authorised
by private acts of parliament. See ROAD.
BRIDGE-BUILDING

BROTHERHOODS

Whether or not the

After the conditions already mentioned are satis fied, taste has more to do with the form of the arches than anything else. The forms in use are the old semicircular, the elliptical-usually got at by putting together several circular arches of (Fr. Frères pontifes; Lat. Fratres pontifices) were different radii-and the segmental arch. The semi-religious societies that originated in the south of circular arch was almost exclusively used in the France in the latter half of the 12th century. Their more ancient bridges. This arch is the most solid purpose was to establish hospices at the most fre and most easily constructed, as all the voussoirs may quented fords of large rivers, to keep up ferries, and be worked from the same mould. It requires, how-to build bridges. The church during the middle ever, high banking, as its height is equal half its ages regarded the making of streets and bridges as breadth; and where the water-level greatly changes, meritorious religious service. it is particularly unsuitable, from the great height herdsman Benezet, subsequently canonised, was the necessary to be given to the piers, to carry the founder or only a member of this fraternity, is as The elliptical arch uncertain as the tradition which attributes to him intrados out of water-reach. and the segmental of 60° are, besides, far more the completion of the bridge over the Rhone at Avignon in 1180. The fraternity was sanctioned pleasing in appearance. by Pope Clemens III. in 1189; its internal organisation was similar to that of the knightly orders, and the members wore as their badge or insignia In France, they a pick-hammer on the breast. 400 FEET laboured very actively, but were gradually absorbed into the order of St John. Similar associations sprang up in other lands, but under different

50

100

200

300

Elliptical Arches.-London Bridge.

In possible extent of span, the masonic bridge is far exceeded by suspension and girder bridges. At Chester there is a stone arch with a span of 200 feet, perhaps the greatest in Britain; in the Britannia Tubular Bridge the span is 460 feet; in the suspension-bridge over the Menai Strait, 600 feet; and in the suspension-bridge at Freiburg, Switzerland, 870 feet.

It has been already mentioned that bridges are built of various materials-wood, iron, stone, and brick. The principal objection to the wooden B. is its liability to decay, besides which it is liable to warping, through the swelling and contracting of its beams. The latter objection applies also to iron bridges, but in their case, the contractions and expansions from heat and cold may be compensated for, as in the compensation-balance of a watch, or the compensation-pendulum.

Public bridges are maintainable at the expense of the counties in which they are situated; but in many cities and boroughs, the inhabitants have acquired by prescription a liability for this expense, and by the 13 and 14 Vict. c. 64, the management and control of such bridges is given to the council of the city or borough. If part of a public bridge be within one county or other place on which the liability rests, and the other part of the bridge be within another, each party or body shall repair that part of the bridge which is within its own boundaries.

Besides the

bridge itself, the county liable is bound by the 22
Henry VIII. c. 5, to repair 300 feet of the road either
way from the bridge. And such is still the state
of the law as to all bridges built prior to the pass-
ing of the Highway Act, 5 and 6 Will. IV. c. 50.
But by that act it is provided that, in the case of all
bridges thereafter to be built, the repair of the road
itself passing over or adjoining to a bridge, shall
be done by the parish, or other parties bound to
the general repair of the highway of which it forms
a portion-the county being still subject, how
ever, to its former obligation as regards the walls,
banks, or fences of the raised causeways, and raised
approaches to any bridge, or the land arches thereof.'
See Stephen's Com., vol. iii. p. 234. The neglect to
make such repairs is treated in law-books as a kind
of negative offence; but there are positive offences
against bridges, which in the statutes are called
nuisances, as to which, see the 43 Geo. III. c. 59,
s. 1, by which it is enacted that the surveyors of
county bridges shall have the power and authority of
removing all nuisances in the form of obstructions

names.

BRIDGE, MILITARY, is a temporary construc

tion, to facilitate the passage of rivers by troops,
are described under PONTOON; but there are many
cannon, and military wagons. The most efficient
other kinds. A bridge of boats is formed by small-
craft, especially cargo-boats, collected from various
places up and down the river; trestles are placed in
them to bring their tops to one common level; the
boats are anchored across the river, and baulks of
timber, resting on the trestles, form a continuous
road from boat to boat across the whole breadth of
the river; the boats ought to be of such size that,
when fully laden, the gunwales or upper edges shall
not be less than one foot above the water. Rope-
bridges are sometimes but not frequently used by
A boat-and-rope bridge con
military engineers.
sists of cables resting on boats, and supporting a
A cask-bridge
consists of a series of timber-rafts resting on casks;
platform or road of stout timber.
the casks are grouped together in quadrangular
masses; at certain intervals, timbers are laid upon
them to form rafts, and several such rafts form
a bridge; it is an inferior kind of pontoon-bridge.
A trestle-bridge is sometimes made for crossing a
small stream in a hilly country; it consists of
trestles hastily made up in any rough materials that
may be at hand, with planking or fascines to form a
flooring, cables to keep the trestles in a straight
line, and heavy stones to prevent them from floating.
Raft-bridges, consisting of planks lashed together,
are easily made of any rough materials that may be
found on the spot; but they have little buoyancy,
A swing-flying
and are not very manageable.
bridge consists of a bridge of boats, of which one
end is moored in the centre of the river, and the
other end left loose; this loose end is brought to
the proper side of the river, the boats are laden,
and they make a semicircular sweep across the
river by means of rudders and oars, until the
loose end of the bridge reaches the other bank.
A trail-flying bridge is a boat or raft, or a string of
boats or rafts, which is drawn across a river by
ropes, in a line marked out and limited by other
ropes.

BRIDGE-HEAD, or TÊTE-DU-PONT, in Mili. tary Engineering, is a fortified post intended to defend the passage of a river over a bridge. It is a fieldwork, open at the gorge or in the rear, and having its two flanks on the banks of the river. The most favourable position is at a re-entering sinuosity of

843

BRIDGE OF ALLAN-BRIDGET.

the river, where the guns can work better with the supporting batteries opposite. Bridge-heads are

Bridge-head Defence Work.

always happy. Her brain seems to have been unduly excited for a blind person; she not only held imaginary dialogues with herself, but dreamed incessantly by night; and during these dreams, while asleep, talked much on her fingers. She learned to write a fair, legible, square hand, and to read with great dexterity, and at last, even to think deeply, and to reason with good sense and discrimination. Keen, sensitive, and lively, in various occupation, her days now pass rapidly and pleasantly, mainly owing to the unremitting skill and kindness of Dr Howe. She was saved by him from a life of hopeless, helpless darkness; educated and trained to take her part in the world; and now, as a teacher of the blind and deaf and dumb, is conferring on them the blessings she has herself received. She is usually temporary works, hastily constructed. Their probably among the most skilful of Blind teachers. BRIDGENORTH, a town of Salop or Shropmost frequent use is to aid a retiring army to cross the river in good order, and to check an enemy shire, on both sides of the Severn, 20 miles southpressing upon it. Openings are left to allow the retir-east of Shrewsbury. It consists of an upper and ing army, with guns and carriages, to file through lower town, connected by a bridge over the Severn. without confusion; and parapets are so disposed as The larger part of the town is on the right bank, to flank and defend these openings. and is built on a sandstone rock rising 60 feet above the river. Pop. 7160. It returns two members to parliament. The navigation of the Severn formerly employed many of the inhabitants, but the traffic has been greatly injured by the introduction of railways. The town, which was at one time called Bruges or Brug, is said to be of Saxon origin. In the beginning of the 12th c., the Earl of Shrewsbury defended the town unsuccessfully against Henry I. It was besieged in the same century by Henry II.; and during the civil wars it resisted the Parliamentary forces for three weeks. A great portion of the town was on this occasion destroyed by fire. It has carpet and worsted manufactories. Bishop Percy was born here.

BRIDGE OF ALLAN. See ALLAN.

BRIDGEMAN, LAURA. This famous blind mute was born in Hanover, New Hampshire, United States, on the 21st December 1829. She was a bright, intelligent child, but at two years of age was seized with a violent fever, which utterly destroyed both sight and hearing. For a time this so shattered her system, that there seemed no hope of recovery; but she rallied, and soon learned to find her way about the house and neighbourhood,

and even learned to sew and to knit a little.

A

strong passion for imitation began to develop itself, and by assiduously cultivating this power, she was at last enabled to emerge out of her life of unbroken darkness and silence, and take her place among the educated people of the day. In 1839, Dr Howe of Boston undertook her care and education at the Deaf and Dumb School. The first attempt was to give her a knowledge of arbitrary signs, by which she could interchange thoughts with others. Then she learned to read embossed letters by the touch; next, embossed words were attached to different articles, and she learned to associate each word with its corresponding object. A pat on the head told her when she was right in her spelling-lesson. Thus far, however, the work was only an exercise of imitation and memory, roused into exertion by the motive of love of approbation, but seemingly without intellectual perception of the relation between words and things. It was like teaching a clever dog a variety of tricks. But at last the truth flashed upon her, that by this means she could communicate to others a sign of what was passing in her own mind. Her whole being seemed changed. The next step was to procure a set of metal types, with the letters cast at the ends, and a board with square holes for their insertion, so as to be read by the finger. In six months, she could write down the name of most common objects, and in two years had made great bodily and mental improvement. She grew happier, and enjoyed play like other children, amusing herself with imaginary dialogues, spelling old and new words, and with her left hand slapping the fingers of her right, if they spelled a word wrong; or giving herself a pat of approval, as the teacher did, when correct. Her touch grew in accuracy as its power increased; she learned to know people almost instantly by the touch alone. In a year or two more, she was able to receive lessons in geography, algebra, and history. She received and answered letters from all parts of the world, and was always employed, and therefore

BRIDGEPORT, a seaport of Connecticut, U. S., at the mouth of the Pequomock, which empties itself into an inlet of Long Island Sound. It is in lat. 41° 11' N., and long. 73° 12′ W., being 178 miles to the south-west of Boston, and 58 to the north-east of New York.

In 1850, the population was 7560, having gained 2990 in 10 years. B. is connected by railways both with the interior and with the other places generally on the seaboard. Though the harbour does not admit large ships, having only 13 feet on the bar at high-water, yet B. has a considerable coasting-trade, and a number of vessels engaged in the whale-fishery. Its manufactures are extensive, particularly of carriages and harness.

BRIDGET, ST (or, more properly, Birgit or Brigitte), a famous Roman Catholic saint, was born in Sweden about the year 1302. Her father was a prince of the blood-royal of Sweden. When only sixteen, she married Ulf Gudmarson, Prince of Nericia, a stripling of eighteen, by whom she had eight children, the youngest of whom, named Catherine, born in 1336, died in 1381, became par excellence the female saint of Sweden. Her husband and she now solemnly vowed to spend the remainder of their lives in a state of continence, and, to obtain strength to carry out their severe resolution, made a pilgrimage to the shrine of St Jago de Compostella in Spain. On their return, Ulf died in 1344, and B. founded about the same time the monastery of Wadstena, in East Gothland. Sixty nuns and twenty-five monks were its first inmates. They received the rule of St Augustine, to which St B. herself added a few particulars. They constituted a new order, sometimes called the order of St B., sometimes the order of St Salvator, or the Holy Saviour, which flourished in Sweden until the Reformation, when it was suppressed, but it still possesses some establishments in Italy,

BRIDGETON-BRIDGEWATER TREATISES.

BRIDGEWATER, a town and port of Somerset-president of the Royal Society of London, Davies shire, on both sides of the Parret (which is here Gilbert, to whom the selection of the author was spanned by an iron bridge), 6 miles in a direct left, with the advice of the Archbishop of Canterline, and 12 by the river, from the Bristol Channel, bury, the Bishop of London, and a noble friend of and 30 miles south-west of Bristol. It stands on the border of a marshy plain which lies between the Mendip and Quantock Hills, but the country around is well wooded. It is chiefly built of brick. St Mary's Church has a remarkably slender and lofty spire. The Parret admits vessels of 200 tons up to the town; it rises 36 feet at spring-tides, and is subject to a bore or perpendicular advancing wave, 6 feet high, often causing much annoyance to BRIDGEWATER TREATISES, eight celeshipping. Pop. 10,317. B. returns two members to | brated works' On the Power, Wisdom, and Goodness

BRIDLINGTON-BRIEL.