518 



BRIDGE. 



Theory, mi^ht be given them with much advantage, and with- 



v out adding to the trouble of erection. _ 



It were perhaps to be wished, that in the horizon- 

 tal section of the curved part of the sterling, it should 

 have a contrary flexure, so as to make the point ot 

 it by being as sharp as possible, turn aside the waters 

 with the |reater ease; but this is attended with a 

 manifest disadvantage. The point will then be liable 

 to damage, and the most trivial variation in the thread 

 of the stream, would produce in an increased degree 

 all those gyrations we wanted to avoid ; such points 

 too, in navigable rivers, would be fatal to loaded craft. 

 But while we do not attempt to give this acutenesa 

 to the pier in the horizontal section, we may acquire 

 it with great facility in the longitudinal or vertical sec- 

 tion For that purpose, the lower courses should b 

 made gradually to project before the upper, and it the 

 formation of the pier admits of it, they may also be 

 made to project on the sides ; but care should be 

 taken in so doing to attend to the rules we have 

 formerly given for its stability, and net uselessly to 

 increase the mass of the pier. As we defend, how- 

 ever the courses of the sterling should become more 

 acute, and project further up the stream, and thus the 

 pier may be supposed ultimately to end in a mere 

 point. In short, the figure of a pier, or sterling o 

 this kind, will have a considerable resemblance to the 

 sock of a plough, an instrument which, in its appli- 

 cation, has a considerable analogy with the sterling. 

 It may be asked, whether these projecting courses 

 should be left by the mason in steps, or trimmed 

 away to a regular curve surface ? We conceive, that 

 there are good reasons for choosing to leave them in 

 steps. For, independent of the saving of labour ; 

 when the current has any obliquity to the direction 

 of the pier, it will flow over the acute point of the 

 .sterling and being reflected by these steps, will be in 

 a great measure prevented from injuring the bottom. 

 The safety, however, of craft would seem to make H 

 proper to round awav the fore corners of the courses. 

 As to the point of the pier down the stream, its 

 figure must, for the sake of uniformity, be assimi- 

 latod to the other. It were well if we could in this 

 case apply the contrary flexure above mentioned. 

 The same difficulties are not to be dreaded ; and al- 



j. ne aaiuc i*vu*wvw * - 



though it may not be easy or agreeable to iorm i 

 in the horizontal section, yet, by prolonging the tail 

 down the stream further than usual, we will virtual- 

 ly obtain the same thing. The tail of the pier is of 

 less consequence to the stability of a bridge, though 

 it may certainly be contrived so as to facilitate the 

 transmission of the water. Bridges seldom fail on 

 that side ; and as they are usually constructed, there 

 is an eddy below the pier, which, although it ob- 

 structs the current, and injures the bottom, yet c 

 up a shoal below each pier that serves to protect tl 

 foundations of the building. _ 



Bridges built in the way we have now mentioned, 

 will appear greatly stronger than those of the usual 

 construction ; and they will really be so. Indepen- 

 dent of their power of preventing the action on the 

 foundation at the shoulder of the pier, which we 

 have shewn in an early part of this section to be the 

 principal cause of the decay of bridge*, these ster- 



lings will appear stout, curved buttresses or as it 

 were knees, to support the lofty side- walls ; and by 

 expanding the lower courses of the arch, advantage 

 may be taken of the increased base, to cast the vault 

 into a kind of groin, and thus give a greater degree 

 of transverse strength to the whole structure. __ 



This idea of extending the lower courses or tl 

 pier, and breaking, as it were, by degrees, tl 

 of the current, is not new, and we do not give itassuc 

 It may be seen with various degrees of perfection in 

 many ancient and modern structures, particularly i 

 the celebrated Pont St Esprit over the Rhone, which 

 probably owes in a great measure its endurance 

 that expedient. But we wish merely to bring b 

 to the builder, a principle which, in modern times, 

 seems too often to have been lost sight of, and which 

 was, no doubt, originally the result of a succe 

 experience. 



When necessity obliges us to form a bridge 

 considerable obliquity to the current of a river, a 

 danger is thereby incurred of producing a gyratioi 

 on the lee bow of the piers, which will be attende 

 with all the evil consequences we have already e 

 plair.ed. The obstruction to the current may be pre 

 sumed to vary with the cosine of the angle of c 

 liquity, and consequently the additional head must 

 be as the versed sine of that angle. But if the sides of 

 the river be parallel straight lines, the water-way unde 

 the bridge will just increase as the secant of t 

 of obliquity, or inversely as the cosine. 1 his will 

 just counterbalance the obstruction caused by dt 

 fleeting the current, leaving only the reaction of 

 bank, and the weather-side of each pier against I 

 stream, to produce the deflection. 1 here will be an 

 accumulation on the one side of each arch therefore, 

 and a depression on the other ; while a strong gyra 

 ticn will take place on one of the shoulders of eac 

 pier, and which is likely to be attended with much 

 danger. The water-way of the arch too is likely to 

 be much contracted by this cause. All these appear- 

 ances frequently occur, although the bridge appea 

 to pass directly across the stream ; because in sue 

 a case the bridge has been designed for the low wa- 

 ters, whereas the stream, when in flood, may have a 

 very different direction. The course of the rive 

 therefore, above the bridge, should be carefully at- 

 tended to, especially when the waters are high, ar 

 either corrected, or the position of the bridge adi 



6 Tne effect of the gyration at the shoulder of the 

 piers is curious ; and, as it is one of the chief cause* 

 l( their destruction, it i. well deserving ot atte, 

 The beds of all rivers are porous, and will the 

 be glutted with water; which again everywhwe 

 pressed by the whole depth in the nver : but at 

 shoulder of the pier there is a void, so that the 1 

 torn at that spot will not have the same vertical pres- 

 sure that there is every where round it. Water v, 

 therefore lise out of the bottom at that place, k_ 

 spring or fountain, and so much the men- forcib 

 the void is deeper. Or, if prevented from issuing, 

 will exert a strong pressure upwards, tipc" whntc , 

 form, the bottom of the void. Now this void be- 

 ing sometimes even four or five feet deep, we ne, 



