Mr. Toicn's neiv mode of Bridge-building. 159 



offspring of unimproved minds and whimsical fancies, which 

 are ever upon the rack to estabhsh new things, the creation 

 of their own imaginations; and which are therefore sure to 

 be wrong for this good reason, that their authors are so. 



Perhaps the following proposition comprises what is the 

 most important to be determined with regard to a general 

 system of Bridge-Building, viz. 



By what construction or arrangement w^ill the least quan- 

 tity of materials, and cost of labor, erect a bridge of any 

 practicable span or opening between piers or abutments, to 

 be the strongest and most permanent, and to admit of the 

 easiest repair f 



In giving the best answer to this proposition, which I am 

 capable of, after a number of years' attention to the theory 

 and practice of this subject, I shall refer to the plates ac- 

 companying this article. The mode of construction is so 

 simple and plain to inspection, as to require little explanation 

 of them. 



Figure 1, is an elevation of one of the trusses of a bridge; 

 one, two, or three of those trusses placed vertically upon 

 piers, are to be considered as the support of the bridge, and 

 are to be of a height, at least, sufficient to admit a waggon 

 to pass under the upper beams, which lie horizontally upon 

 the top string-piece of the side trusses; and on these same side-, 

 string-pieces rest the feet of rafters, which form a roof to 

 .shingle upon. In this case, a middle truss is used, whiclj 

 will always be necessary in bridges of considerable widih ; 

 the height of it will be as much greater than the side onet^ 

 as the height or pitch of the roof. The height of the trus- 

 ses must be equal to the whole height of the bridge requir- 

 ed, and is to be an exact continuation of the work represent- 

 ed in Fig. 1. 



The height of the trusses is to be proportioned to the 

 width of the openings between the piers or abutments, and 

 may be about one-tenth of the openings, when the piers are. 

 fifteen feet or more apart — a less span requiring about the 

 same height, for the reasons before stated. 



The diagonal bearing of these trusses, is composed of 

 sawed plank ten or eleven inches v/ide, and from three to 

 three and a half inches thick; it may be sawed from any 

 timber that will last well, when kept dry. White pine and 

 sprnco are probably the best kinds of timber for the pur- 



