University of Virginia Library

6316. PAINE (Thomas), Iron bridge.—[continued].

I feel myself interested in your bridge, and it is with great pleasure
that I learn that the execution of the arch of
experiment exceeds your expectation. In your
former letter, you mention that instead of arranging
your tubes and bolts as ordinates to
the chord of the arch, you had reverted to your
first idea, of arranging them in the direction of
the radii. I am sure it will gain both in beauty
and strength. It is true that the divergence of
those radii recurs as a difficulty, in getting the
rails upon the bolts; but I thought this removed
by the answer you first gave me, when I suggested
that difficulty, to wit, that you should

place the rails first, and drive the bolts through
them, and not as I had imagined, place the
bolts first, and put the rails on them. I must
doubt whether what you now suggest, will be as
good as your first idea; to wit, to have every
rail split into two pieces longitudinally, so that
there shall be but the halves of the holes in
each, and then to clamp the two halves together.
The solidity of this method cannot be
equal to that of the solid rail, and it increases
the suspicious part of the whole machine,
which, in a first experiment, ought to be rendered
as few as possible. But of all this, the
practical iron men are much better judges than
we theorists. You hesitate between the
catenary and portion of a circle. I have lately
received from Italy, a treatise on the equilibrium
of arches by the Abbé Mascheroni. * * * I find that the conclusions of his demonstrations
are that every part of the catenary is in
perfect equilibrium. It is a great point, then,
in a new experiment, to adopt the sole arch,
where the pressure will be equally borne by
every point of it. If any one point is pushed
with accumulated pressure, it will introduce a
danger foreign to the essential part of the plan.
The difficulty you suggest is, that the rails being
all in catenaries, the tubes must be of different
lengths, as these approach nearer, or recede
farther from each other, and therefore, you
recur to the portions of concentric circles,
which are equi-distant in all their parts. But
I would rather propose that you make your
middle rail an exact catenary, and the interior
and exterior rails parallels to that. It is true
they will not be exact catenaries, but they will
depart very little from it; much less than portions
of circles will.—
To Thomas Paine. Washington ed. ii, 546.
(P. 1788)