Improvement in the Construction of Bridges, §¢c. 291 
resent the horizontal length of the model, from interior to exte- 
rior of the two piers, w its weight, w the weight it will just sus- 
tain at its middle point B before it breaks. Let x J, the length of 
a bridge actually constructed of the same material as the model, 
and all its dimensions similar: then, its weight will be * w, and 
its resisting power to that of the model, as n?.to 1, being=n? 
w+3w.) Hence n? (w+4$ w)—3$ n2%0=n?w—$n? (n—1)w, 
the load which the bridge itself would bear at the middle point. 
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 acct the braces in size between the joints, and of cast- 
ing flanges to them where they intersect, thereby making it un- 
necessary to have more than one bolt and nut to each joint or in- 
tersection. 
When it is considered that bridges, —" from the weather, 
will last eight or ten times as long as those not covered, and that 
the cheapness of this mode will admit.of its being generally 
adopted, with openings or spans between piers which are compo- 
sed of piles, and at a distance of 150 to 200 feet apart, then the 
construction of long bridges over mud bottomed rivers, like those 
at Washington, Boston, Norfolk, Charleston, &c., on this princi- 
ple, will be perceived to be of great importance ; especially as the 
common mode of piling is so exposed to freshets, uncommon 
tides, drift-wood, and ice, as not to insure safety or economy in 
covering them, i consequently continual repairs, and often re~ 
building them, become necessary. There is very little doubt, 
that one half of the expense, computing stock and interest, that 
would be required to keep up, for 100 years, one of the common 
pile bridges, like those at Boston, would be sufficient to maintain 
one built in this new mode, keep it covered, and have all or | 
hearly all the piers built with stone at the end of 100 years. If 
this be the case, it would be great economy to commence rebuild- 
ing by degrees, in this manner. The saving in theone 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 the time. 
