BRIDGE. 



sr nioot of tlie arch, independent of any arch on the other 

 fide of the pier. But the weight of the pier ought a httic 

 to preponderate, or exceed in effedt, the flioot of the arch ; 

 and, therefore, the thicknefs ought to be taken a httle 

 more than what the theorem will give : indeed, in moll 

 cafes occurring in praftice, the thicknefs mull be between 

 the fifll> and \\\q fevcnlh pait of the Ipan of the arch. 



The only remaining coufidcration in the theory, relates to 

 the form of tht ends of a pier, fo as to afford the leall refillance 

 to the force of the llream of water. Now, it may be found 

 by a fluxionnl procefs, that if the water ilrike every part of 

 the pier with equal velocity, the end of the pier (hould be a 

 right-lined triangle, when the force of the water upon it is 

 the lead poffible : when the variably increafed velocity, as 

 in the cafe of a flood, is ufed, the form of the ends comes 

 out a little curved. One third of the abfohitc force is taken 

 off, by making the ends of the pier fcmieirciilar ; J^ would 

 be taken off, if the cuds were parabolic; but when the 

 ends are right angled triangles, with tlie right angles pointed 

 into the flream, the abfolute force of the water upon the 

 pier is reduced to one half; and an acute angle pointed 

 to the flream will reduce its force llill more. But in rivers, 

 on which heavy craft navigate, and pafs the arch, it is ge- 

 nerally better to make the ends nearly femicircular : for, al- 

 though it does not divide the wat«r fo well as the triangle, 

 yet it will bear the fliock of the veffcls better, and, at the 

 fame time, be more likely to turn them off towards the mid- 

 dle of the arch. 



Bridge, in Gunnery, th« two pieces of timber which go 

 between the two tranfums of a gun-carriage, on which the 

 bed refls. 



Bflidg e, in the Military art. Flying iridge, font volant, or 

 pons chidarius, iignifies a bridge couilrufted of pontoons, 

 leather boats, beams, hollow calks, fheaves of rulhcs, blown 

 bladders, called afcogephyri, or the like, laid upon a river, 

 or marfhy and boggy ground, and covered over with planks, 

 for the paffage of a body of troops. 



Flying bridge, pont "volant, taken in a more particular 

 figniiication, denotes a bridge compofed of feveral boats, 

 connefted by a flooring of planks, and furrounded by a bal- 

 luftrade or railing. It is furnilhed with one or more malls, 

 to which is faltened a flrong cable, fupported at proper dif- 

 tances by boats, and extending to an anchor to which the 

 other end is made fall, in the middle of the water. By this 

 contrivance, the bridge becomes moveable, like a pendulum, 

 from one fide of the river to the other, without other help 

 than a rudder. Such bridges were formerly fomistimes con- 

 llrufted of two (lories, for the quicker paffage of a great 

 number of men, or that both infantry and cavalry might 

 pafs at the fame time. The ufe of this kind of flying 

 bridge is, however, attended with great difficulty and dan- 

 ger, and fubjeft to ths mofl: fatal accidents. An unfortu- 

 nate inltance of this occurred at the evacuation of Nime- 

 giien in the campaign of 1794, where, while the Dutch 

 g-arrifon were occupied in croifing the river, an unlucky 

 Ihot from the French batteries carried away the top of the 

 mall, and the bridge fwinging round to the enemy's fide of 

 the Waal, above 400 of the garrifon were immediately 

 made prifoners. Thofe who remained in the tower, to a 

 much greater number, bereft of the means of efeape, fur- 

 r-endered to the befiegers. 



Another kind of_y^7«_f, or Jloating bridge, is formed of 

 tvro fmall bridges laid over one another in luch a manner, as 

 that the jippermolt itretches and runs out by the affillance 

 of cords drawn through fmall pnllies, placed along the 

 fides of the undermoft biidge, which is thus puflied for- 



ward, till the farther extremity of it rolls againft the place- 

 it is iiitended to be fixed upon. 



When thcfc two bridges are extended to their uttnofr 

 length, fo that the two middle ends meet, they (hould not 

 be above four or five fathoms long ; for if longer, they wilt 

 break. Their chief ufe is for furprifing oiit-works, or for- 

 tified pods that have but narrow moats. In the memoirs of 

 the R^oyal Academy of Sciences, we find a newcontrivancs 

 of a floating bridge, which lays itfelf on the other fide of 

 the river. Vide Hilt. Acad. R. Scienc. an. 1713, p. 104. 



Draw-bridge, or pons fubdii^arius, is a bridge fallened at 

 one end with hinges, fo that the other end may be lifted 

 up or let down by fome eafy contrivance. The mod com- 

 mon method is by a kind of balance called plyers (which 

 fee) ; in which cafe the bridge ftands upright, to hindtr the 

 paffage of a moat, or the hke ; the breadth of this bridge 

 K ufually about nine or ten feet, and its length about liftecu 

 feet. There are others fo conllrufted as to" be drawn back, 

 for hindering a padage, and to be thrud over again for 

 aflfording a palfage. Others open in the middle, half turn- 

 ing to one fide, and the other half to the other, being joined 

 again at pleafure ; but thefe are fubjed to an obvious in- 

 convenience, as one half of them remains on the enemy's 

 fide. The marquis de L'Hopital has given the condruc- 

 tion of a curve, in which a weight will always be a counter- 

 balance to a draw-bridge ; which the younger Bernouilli has 

 (hewn to be no other than the cycloid. Ael. Erud. Lipf 

 an. 1695.. 



Drawbridges are likewife frequently ufsd on canals, navi- 

 gable rivers, and wet-docks ; for fmall canals they confid of 

 one leaf or frame only, moveable on hinge'; ; but for large 

 canals, fuch as the Forth and Clyde canal, in Scotland, and 

 for wet-docks, &c. they are made in two pieces which meet 

 in the middle, forming an arch, and are railed or lowered 

 by means of balance frames, moveable on the tops of up- 

 rights, fuited in height to the magnitude of the bridge. 

 Such bridges, however, have been found inconvenient in 

 ufe, owing to the obdruaion they give to the yards and 

 rigging of diips in pafiing through them. This gave rife to 

 the invention of a different fort of bridge, which, for fmall 

 canals, confills of one frame or leaf only, turning on a centre 

 or feries of balls or rollers ; and for large canals, or navigabl» 

 rivers, they are formed of two parts, which meet iiv th- 

 middle. The fird that have come to our knowledtre are 

 thofe at Cherbourg and Toulon. Neither of them," how. 

 ever, are fo complete as thofe that have lately been con- 

 flrufted art the Wed India and London docks ; the latter 

 fpans 40 feet, and 15 feet wide in the roadway, and is made 

 of thin ribs of cad iron, about an inch and a half thick, turn- 

 ing on a number of concentric rollers, moving between two 

 circular rings of cad iron, which are very nicely turned, and 

 there is a flap for each leaf, which lets down by a fcrew,' and 

 abuts againd the done work on each fide, forming the 

 whole, when diut, into an arch, capable of carryino any 

 weight which can ever pafs over it. " 



The whole, though weighing 8,^ tons, moves wiih 

 great eafe, and can be opened and (hut in lefs than three 

 minutes, thereby occafioning very little obdruaion to tra- 

 vellers, while veffels pafs through the locks. 



Biidge of communication, is a bridge made over a river 

 to preferve a free interconrfe between two armies, or for! 

 tified places, feparated by the dream. 



The bridge now mod generally employed, and which, by 

 reafon of Us fupenor efhcacy, has gradually almod fupcr- 

 ieded the ufe of all thofe above-mentioned, is that con- 

 druaed of copper or wooden boats, fadcned with flakes or 

 anchors to the bed of the river, and covered over «-ith 



planks* 



