586 BRIDGES. 



yet easily admitting through its arches the vessels that navigate 

 upon it, and all the water, even at high tides and floods : the ne- 

 glect of this precept has been the ruin of many bridges. Bridges 

 are commonly continued in a straight direction perpendicular to 

 the stream ; though some think they should be made convex to- 

 wards the stream, the better to resist floods, &c. And bridges of 

 this sort have been executed in some places, as Pont St. Esprit, 

 near Lyons. Again, a bridge should not be made in too narrow a 

 part of a navigable river, or one subject to tides or floods ; be. 

 cause, the breadth being still more contracted by the piers, this 

 will increase the depth, velocity, and fall of the waU.-r under the 

 arches, and endanger the whole bridge and navigation. There 

 ought to be an uneven number of arches, or an even number of 

 piers ; both that the middle of the stream or chief current may 

 flow freely without the interruption of a pier ; and that the h\<> 

 halves of the bridge, by gradually rising from the ends to the mid* 

 die, may there meet in the highest and largest arch ; and alt>o, that 

 by being open in the middle, the eye in viewing it, may look di- 

 rectly through there. When the middle and ends are of different 

 heights, their difference however ought not to be great in propor. 

 tion to the length, that the ascent and descent may be easy ; and 

 in that case also it is more beautiful to make the top in one con- 

 tinued curve, than two straight lines forming an angle in the mid- 

 dle. Bridges should rather be of few and large arches than of 

 many smaller ones, if the height and situation will possibly allow 

 of it ; for this will leave more free passage for the water and navi. 

 gation, and be a great saving in materials and labour, as there will 

 be fewer piers and centres, and the arches, &c. will require less 

 materials ; a remarkable instance of which appears in the difference 

 between the bridges of Westminster and Blackfriars, the expence 

 of the former being more than double the latter. 



For the proper execution of a bridge, and making an estimate of 

 the expence, &c., it is necessary to have three plans, three sec- 

 tions, and an elevation. The three plans are so many horizontal 

 sections, viz. first a plan of the foundation under the piers, with 

 the particular circumstances attending it, whether of gratings, 

 planks, piles, &c. ; the second is the plan of the piers and arches ; 

 and the third is the plan of the superstructure, with the paved road 

 and banquet. The three sections ore vertical ones, the first of 

 them a longitudinal section from end to end of the bridge, and 



