Mr. Towii's new mode ofBridge-huiiding. ]61 



tion. Very flat pitched roofs will be pfe?ferable, as it will, 

 in that case, be a greater support to the upper.part of the 

 bridge. 



Fig. 5, is the floor or plan of the bridge, showing the 

 mode of bracing and the floor-joist. 



Fig. 6, is a view of the bottom or top edge of the 

 string-pieces, and shows how the joints are broke in using 

 the plank, and also how the trunnels are distributed. 



This mode of construction will have the same advantages 

 in iron as in wood, and some in cast-iron which wood has 

 not, viz. that of reducing the braces in size between the 

 joints and of casting flaunches to them where they intersect, 

 thereby making it unnecessary to have more than one bolt 

 and nut to each joint or intersection. 



When it is considered that bridges, covered from the 

 weather, will last seven or eight times as long as those not 

 covered, and that the cheapness of this mode v/ill admit of 

 its being generally adopted, with openings or spans be- 

 tween piers composed of piles, and at a distance of one 

 hundred and twenty to one hundred and sixty feet apart, 

 then the construction of long bridges over mud bottomed 

 rivers, like those at Washington, Boston, Norfolk, Charles- 

 ton, &;c. will be perceived to be of great importance, esper 

 cially as the common mode of piling is so exposed to fresh- 

 ets, uncommon tides, drift-wood, and ice, as not to insure 

 safety or economy in covering them, and consequently con- 

 tinual repairs, and often rebuilding them, become neces- 

 sary. There is very little, if any, doubt, that one half of 

 the expense, computing stock, and interest, that would 

 be required to keep up, for one hundred years, one of the 

 common pile bridges, like those at Boston, would be suffi- 

 cient to maintain one built in this new mode, keep it cover- 

 ed, and have all or nearly all the piers built with stone at 

 the end of the one hundred years. If this be the case, it would 

 be great economy to commence rebuilding, by degrees, in this 

 manner. The saving, in the one article of floor planks, ii 

 kept dry, would be very great, as by being so mucii wet 

 they rot and wear out in about half the time. 



For aqueduct bridges of wood or iron no other mode 

 can be as cheap or answer as well ; this mode has equal ad- 

 vantages also in supporting wide roofs of buildings, centres 

 of wide arches in masonry, trussed flooring?, partitions, 



Vol. III.....N0. L * 21 



