- they are used. 
160 = Mr. Town’s new mode of Bridge-building. 
pose, on account of their lightness, and their not being so 
. 
- subject to spring or warp as white oak. 
he nearer those braces are placed to each other, the 
more strength will the truss have, and in no case are they 
to be halved or gained, where they intersect each other; 
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; @ 
chain or clamp 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 au 
edge at the top end; they are then to be seasoned before 
=e 
— The string-pieces are ‘composed of two. thicknesses of 
ih 
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 als 
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 sufii- 
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 oF 
intersections of the braces, and in wide spans the. fleor- 
beams may be placed on the second string-piece as show? 
at Fig.-1. : ; : 
Fig. 3, shows on a larger scale how each joint is secured, 
by which it is seen that the trunnels take hold of the whole 
thickness of each piece. - ni Cae 
Fig. 4, is a section of a bridge of this construction, and 
- shows the manner in which the braces and string-pieces 
the bridge, and of putting beams.and braces over head, 
: ; <ddte:anaae for! 
come together, and also the manner of making the floor 0 
nh are to eonnected with the mi e 
- purpose of bracing the bridge against. lateral rack or mo- 
