. Mr. Town's new mode of Bridge-building. 161 
tion. Very flat pitched roofs will be preferable, as it will, 
a a case, be a greater support to the upper part of the 
ridge 
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 
he pip and also how the trunnels are distributed. 
is mode of construction - have the same advantages 
in iron as in wood, and some in cast-iron which wood has 
not, viz. that of reducing the iiscos 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 interse 
When it is conadeted that bridges, covered from: the 
weather, will last seven or eight times as long as 
covered, and that the cheapness of this mode will 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 bondred and sixty se i a 
then the construction of long bridges over mud med 
rivers, like those at Washington, Boctrn. Norfolk, thes. 
ton, &c. will be perceived to be of great importance, espe- 
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- 
sa 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 reste one built in this new mode, keep it cover- 
stead b ave all or nearly all the piers built with stone at 
psp ine) of the one hundred years, if this be the CASE, itwould 
manner. The saving, in the one article of floor planks, if 
kept dry, would. be very great, as by being so much wet 
they rot and wear out in about half thetime. 
For aqueduct bridges of wood or iron no other 
can be as cheap or answer as well; this: mode has equal ad- 
- vantages also in supporting wide roofs: of buildings, centres 
of one ae tae in ists Arosees floorings, “partitions, 
