BRIDGE. 



521 



near to considerable towns, from 26 to 30 ; and in 

 or near large cities, they are made from SO to J0 

 feet. Tlir roadway should have a declivity from the 

 middle ot the length each way towards and beyond 

 the abutments, of about 1 in 2i ; and the roadway, as 

 well as the footpaths and parapets, should, near their 

 extremities, diverge, to suit .the approaches, whether 

 one or more, at each end of the bridge. 



3. The decorations should be varied, according tp 

 the situation and accompany ments. In the country, 

 the utmost simplicity, consistent with distinguishing 

 the essential parts, should be preserved ; and even in 

 the most splendid cities, or adjacent to palaces, all 

 decora ions should be kept perfectly subservient to, 

 and in unison with, the essential parts : the neglect 

 of this i, a frequent error in designing bridges. C- 

 lunctns and entablatures, though proper in a Grecian 

 temple, are ill suited to an edifice, where forms un- 

 known to the Greeks are the leading features. As 

 columns can only be placed over the piers and abut- 

 ments, the entablature, intended to represent beams of 

 timber, cannot be supposed to be wholly upheld by 

 supports placed at such great distances from each 

 other. And the introduction of columns, in place of 

 carrying up the piers, deprives the superstructure of 

 powerful buttresses in situations where they would 

 prove very beneficial. The affectation of preserving 

 the entablature upon a perfect level, has led to making 

 .the roadway along the bridge also level, which is 

 nothing less than constructing, at a vast expenie, a 

 piece of road more imperfect than what is formed by 

 the common labourer in the open country ; and be- 

 sides, this mode of construction gives an appearance of 

 feebleness to the outlines of the bridge. This false 

 taste was introduced by some of the French engi- 

 neers, and has of late been, in some instances, copied 

 in Britain. It cannot be too early reprobated, because 

 bridges, when substantially constructed, remain for 

 many ages, and are not easily altered. 



In making out the design, the engineer should fur- 

 nish a plan, shewing the form and dimensions of the 

 foundations of the piers, abutments, wing wails, and 

 wharf walls connected with the bridge ; an elevation 

 shewing the general facade; also vertical, longitudinal, 

 and cross sections, shewingthe construction of the inte- 

 rior parts. In large bridges, there should be a plan or 

 horizontal section taken at about one, third way up 

 the rise of the arches, in order to shew the mode of 

 filling up the spandrels. There should likewise be- a 

 plan of the roadway, footpaths, and parapets ; besides 

 separate drawings of all those parts which cannot be 

 made sufficiently distinct in the general drawings. 

 A specification should likewise be added, describing 

 in words, the quality and dimensions ui every part 

 of the work, and the form and manner of its construc- 

 tion. 



3. MATERIALS. 



The materials consist of timber and iron for piles, 

 cc ( li rd, in... caissons, scaffolding, and centres ; of 

 tone, lime, and sand for the masonry ; also gravel 

 for embanking at the ends, and forming the roadway 

 oer the bridge. 



Of timber, oak is the most generally useful, and 



TOt. IV. PART 11. . 



where exposed to be alternately wet and dry, ii most P/riif. 

 durable ; and next to it art- pitch, pine, and fir grown ' v 

 from the seed. Under low water, elm it very suitable ; 

 it does not easily split, and is very durable : indeed, in 

 this latter situation any timber is sufficiently durable. 

 For gangways, scaffolding, and center*, sound lir tim- 

 ber of natural growth is the fittest ; it should be 

 free of knots and sap. In ties, bolts, and nails, mal- 

 leable iron, made from wood charcoal, should be used. 

 For receiving thrusts, forming supports, or dowelling 

 stones, cast iron is the most proper. 



With regard to stones for the piers, abutments, 

 arch-stones, and parapets, no pains should be spared 

 to procure the best which can be obtained at a rea- 

 sonable expense. They should be of a quality not to be 

 decomposed by the operations of the atmosphere, and 

 they should not be intersected by cross seams. Where 

 the quarry produces them with flat beds, much labour 

 is saved ; but when they are laminated, great can- 

 must be taken to have them laid on their natural beds. 

 Their dimensions must, of course, be regulated by the 

 magnitude of the work in which they are to be em- 

 ployed. If new quarries are to be opened, experienced 

 workmen should carefully examine their quality ; and 

 specimens of the stone should be exposed for at least one 

 winter and summer, in situations similar to that where 

 they are to be used : experiments should also be made 

 to prove their consistence. For the spandrels, wing 

 walls, and backing, inferior stones may answer ; but 

 they should be such as can be laid with proper bond, 

 and in regular courses. Those used on the external 

 faces must be fit to withstand the effects of the wea- 

 ther : they should correspond with those of the piers 

 and arches as to colour ; and if regularly squared, 

 should be brought from the same quarry, though 

 they may be of smaller dimensions. 



In procuring lime for mortar, it is of great impor- 

 tance that it will set or indurate under water ; and 

 where immediately exposed to agitated water, the 



outer edge of the joints should be laid with the Bri- 

 tish cement, discovered by Mr Parker, which in a 

 few minutes becomes sufficiently hard. Where these? 

 cannot be procured, recourse must be had to substi- 

 tutes : Burnt and pounded iron stone, scales from an 

 iron forge, hard burnt tiles ground and mixed with 

 quick lime, all become hard under water, and in damp 

 situations. . In all cases, clean, sharp, or angular sand, 

 is a necessary ingredient 4 and in" rubble work it is 

 better for being very coarse, or approximating to 

 clean small gravel. In making mortar, we nwe 

 known great advantage derived from using water 

 which* contained a solution of iron, and was of a dark 

 reddish colour, approaching to black, which is fre- 

 quently found adjacent to peat mosses. The lime 

 should be used when fresh slacked, and be well beaten, 

 or made with a machine called a mortar mill. 



In using sand, Vitruvius very judiciously makes a 

 distinction between sea or river, and pit sand ; of the 

 former he allows two, and three of the latter, to one 

 cf lime in powder. Dr Higgin, who made many 

 experiments, and published a treatise upon calcareous 

 cements, recommends (by weight) one of lime to 

 eight of sand. Loriot, in describing what he con- 

 ceives the secret of the ancients, mixes a portion of 

 URilackcd lime ground to a powder, with the mortar 

 S r 



