Mr. Town’s new mode of Bridge-building. 159 
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, which 
will always be necessary in bridges of considerable width ; 
the height of it will be as much greater than the side ones 
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. ae ee + TE 
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 
fifieen 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 wide, 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 
spruce are probably the hest kinds of timber for the pur- 
