160 Mr. Towti's new node of Bridge-building.^ 



pose, on account of their lightness, and their not being so 

 subject to spring or warp as white oak. 



The nearer those braces are placed to each other, the 

 more strength will the truss have, and in no case are Xhey 

 to be halved or gained, where they intersect each other j 

 but they are to stand in close contact, depending entirely 

 on three or four trunnels, which go through each joint or in- 

 tersection, and where the string-pieces pass over these joints 

 the trunnels go through them also, and are each of them 

 wedged at each end to keep the timber in close contact ; a 

 chain or cl-amp is necessary to bring the work tight to- 

 gether. 



The trunnels may be made of white oak, one and a half 

 inches in diameter. They are made very cheaply and ex- 

 cellently, by being rived out square, and driven, while green 

 or wet, through a tube fitted to a block and ground to an 

 edge at the top end ; they are then to be seasoned befora 

 they are used. 



The string-pieces are composed of two thicknesses of 

 plank, of about the same dimensions as the braces, and 

 they are so put together as to break joints as shewn at Fig. 

 6. This renders long hewn timber unnecessary, as also 

 any labor in making splices, and putting on iron work. 



For any span or opening not exceeding one hundred and 

 thirty feet, one string-piece at top and one at bottom of each 

 truss, if of a good proportion and well secured, will be suffi- 

 cient, (see Fig. 2. ;) but as the span is extended beyond 

 one hundred and thirty feet, two or more at top and bottom 

 would be required as shown in Fig. 1 where two string- 

 pieces run over the two upper and lower series of joints or 

 intersections of the braces, and in wide spans the floor- 

 beams may be placed on the second string-piece as shown 

 at Fig. 1. 



Fig. 3, shows on a larger scale how each joint is secured, 

 by which it is seen that the trunnels take hpld of the whole 

 thickness of each piece. 



Fig. 4, is a section of a bridge of this construction, and 

 shows the manner in which the braces and string-pieces 

 come together, and also the manner of making the floor of 

 the bridge, and of putting beams and braces over head, 

 which are to be connected with the middle truss for the 

 purpose of bracing the bridge against lateral rack or mo- 



