UltlDOE. 



BRIDGE. 



3tt 



r 'activity may be observed to have taken place u among our- 

 and the gigantic works exscoted In France, Belgium, Germany, 

 BallsaiUnd, Italy, and even in Spain, within the Urt few yean, prove 

 that thorn nation* are u deeply impressed with the necessity fur pro- 

 riding at almost any cost the mean* of easy and uninterrupted loco- 

 motion, which M indeed the end bridgei are deaigned to subserve. 

 The inhabitants of the United States have not been behind in this 



race of improvement, and we owe to them several of the boldest 

 innovation! in the art of bridge-building, to which they have been 

 driven by the peculiar condition! of their country. The Bridge of 

 Trenton over the Delaware, the Upper and Lower Schuylkill bridges 

 near Philadelphia, the great Portage Viaduct, and the Niagara 

 Suspension Bridge may challenge comparison with any similar work* 

 erected in Europe. 



London Bridge- 



Southwark Iron Bridge. 



1'iiunm.i. 



200 feet. 



The SchQjrlkiU Bridge : one half showing the construction ; the other half the external elevations. 



Niagara 



_ t practical bridge builder* the following terms are generally 

 applied to the various descriptions of bridges, and to their various 

 parts ; and the several meaning! thus assigned will be attributed to 

 the respective words in the sequel of this article : 



A bridge is said to be either a culvert, when it conveys a small stream 

 of water under an embankment, or a raised causeway ; OTA foot bridge, 

 when it is only designed for the passage of foot travellers ; or a bridle 

 bridge, when it is Urge enough for the passage of led horses ; or an 

 ompatim bridge, when large enough for the passage of farm waggons, 

 and exclusively reserved for the use of the neighbouring landowners. 

 Public road bridges may be either jixtd, moteaUt, fying, or Jtoating ; 

 sad according to the resource of the country wherein they are erected 

 they may be constructed of stone, brick, wood, or iron ; or again they 

 mar bs constructed upon the principles of nupauim, or of the appli- 

 cation of the built, or girder beams. The extreme mii.ni of masonry 

 constructed to rssist the thrust of a series of arches, are called the 

 almtmntt ; whilst the intermediate points of support of such a series 

 are called pirn. The lower surface of ad arch is called the intradt*, 

 and the upper surface its titradm ; the stones themselves which con- 

 stitute the ring of an arch are called the rmatoin, and the central one 

 is distinguished by the name of the feyXoM. The Aowufei of an arch 

 are those portions respectively midway between the keystone and the 



-i, or ihtrfra/-* ; and the filling in above the arch, to the under- 



side of the roadway, is called the nandnl, or the tpandnl filing, as it 

 may bs upon the face, or in the interior of the work. The construc- 

 tion between two piers, or between any two points of support, is called 

 an ore* in stone, brick, or cast-iron bridges; but a bay, in timber or 

 wrought-iron ones. The distance between the points of support of a 



bay, or arch, IB called its chord, or tpan ; and the vertical height from 

 the underside of the centre of the intrados to the horizontal line 

 pi mring from the respective springing^ of the arch, is called the 

 verted nut, or rite ; in the case of beams, the rise is, however, com- 

 monly designated the camber. 



The first important point to be settled in designing a bridge is, the 

 water way absolutely required under the most unfavourable circum- 

 stances of the particular case. This dimension will depend, 1st, upon 

 the area of the district contributing to the stream ; 2nd, upon the 

 quantity and conditions of its rainfall ; 3rd, upon the configuration, 

 and the geological character of the watershed whose drainage is in- 

 tended to be passed under the bridge. Every geological formation, in 

 fact, parts with the excess of the rain which may fall upon it (beyond 

 the quantity absorbed by the ground, or by the vegetation), under 

 circumstances varying with the greater or less inclination of its surface, 

 and with the accidental interferences that surface may offer to the 

 escape of the surplus waters, whether arising from the state of the 

 upper strata, the outlines of the valleys, or the vegetation covering 

 their aides. Porous limestone, sandstone, or sandy strata, for instance, 

 yield streams which are, generally speaking, constant in their volume, 

 and are rarely visited by floods ; whilst, on the contrary, the dense, 

 non-absorbent granites, slate rocks, marbles, clays, and loams, give rise 

 to streams which are notoriously capricious in their flow, and liable to 

 serious and unexpected floods. It is, moreover, to be observed that 

 the denser and non-absorbent formations usually occur in mountain 

 chains, at considerable' elevations above the sea level, and that their 

 outlines are sharp and precipitate ; whilst those of the absorbent 

 strata are for the most part gently undulated. It thence happens that 



