The Iron Bridge

Letter to Sir George Staunton, Bart.

As I know you interest yourself in the success of the useful arts, and are a member of the society for the promotion thereof, I do myself the pleasure to send you an account of a small experiment I have been making at Messrs. Walker's iron works at this place. You have already seen the model I constructed for a bridge of a single arch, to be made of iron, and erected over the river Schuylkill, at Philadelphia; but as the dimensions may have escaped your recollection, I will begin with stating those particulars.

The vast quantity of ice and melted snow at the breaking up of the frost in that part of America render it impracticable to erect abridge on piers.  The river can conveniently be contracted to 400 feet, the model, therefore, is for an arch of 400 feet span; the height of the arch in the center, from the chord thereof, is to be about 20 feet, and to be brought off on the top so as to make the ascent about I foot in 18 or 20.

The judgment of the Academy of Sciences at Paris has been given on the principles and [begin page 223] practicability of the construction.  The original, signed by the Academy, is in my possession; and in which the fully approve and support the design.  They introduce their opinion by saying:

It is certain that when such a project as that of making an iron arch of 4O~ feet span is thought of, and when we consider the effects resulting from an arch of such vast magnitude, it would be strange if doubts were not raised as to the success of such an enterprise, from the difficulties which at first present themselves.  But if such be the disposition of the various parts, and the method of uniting them, that the collective body should present a whole both firm and solid, we should then no longer have the same doubts of the success of the plan, 

The Academy then proceed to state the; reasons on which their judgment is founded; and conclude with saying:

We conclude from what we have just remarked that Mr. Paine's plan of an iron bridge is ingeniously, imagined, that the construction of it is simple, solid and proper to give it the necessary strength for resisting the effects resulting from its burden, and that it is deserving of a trial.  In short, it may furnish a new example of the application of a metal, which has not hitherto been used in any works on an extensive scale; although on many occasions it is employed with the greatest success.

As it was my design to pass some time in England before I returned to America, I em- [begin page 224] ployed part of it in making the small essay I am now to inform you of.  My intention, when I came to the iron works, was to raise an arch of at least 200 feet span; but as it was late in the fall of last year the season was too far advanced to work out of doors, and an arch of that extent too great to be worked within doors, and as I was unwilling to lose time, I moderated my ambition with a little "common sense," and began with such an arch as could be compassed within some of the buildings belonging to the works.  As the construction of the American arch admits, in practise, any species of curve with equal facility, I set off in preference to all others a catenarian arch of 90 feet span and 5 feet high.  Were this arch converted into an arch of a circle, the diameter of its circle would be 410 feet.  From the ordinates of the arch taken from the wall where the arch was struck, I produced a similar arch on the floor whereon the work was to be fitted and framed, and there was something so apparently just when the work was set out that the looking at it promised success. 

You will recollect that the model is composed of four parallel arched ribs, and as the number of ribs may be increased at pleasure to any [begin page 225] breadth an arch sufficient for a roadway may require, and the arches to any number the breadth of a river may require, the construction of one rib would determine for the whole; because if one rib succeeded all the rest of the work, to any extent, is a repetition.

In less time than I expected, and before the winter set in, I had fitted and framed the arch, or properly the rib, completely together on the floor; it was then taken in pieces and stowed away during the winter, in a corner of a workshop; used in the meantime by the carpenters, where it occupied so small a compass as to be hid among the shavings; and though the extent of it is 90 feet, the depth of the arch at the center 2  feet 9 inches, and the depth at the branches 6 feet, the whole of it might, when in pieces, be put in an ordinary stage wagon, and sent to any part of England.

I returned to the works in April, and began to prepare for erecting; we chose a situation between a steel-furnace and a workshop, which served for butments.  The distance between those buildings was about 4 feet more than the span of the arch, which we filled up with chumps of wood at each end.  I mention this as I shall have occasion to refer, to it hereafter.
[begin page 226]

We soon ran up a center to turn the arch upon, and began our erections.  Every part fitted to a mathematical exactness.  The raising an arch of this construction is different to the method of raising a stone arch.  In a stone arch they begin at the bottom, on the extremities of the arch, and work upward, meeting at the crown.  In this we began at the crown by a line perpendicular thereto and worked downward each way.  It differs likewise in another respect.

A stone arch is raised by sections of the curve, each stone being so, and this by concentric curves.  The effect likewise of the arch upon the center is different, for as stone arches sometimes break down the center by their weight, this, on the contrary, grew lighter on the center as the arch increased in thickness, so much so that before the arch was completely finished it rose itself off the center the full thickness of the blade of a knife from one butment to the other, and is, I suppose, the first arch of ninety feet span that ever struck itself.

I have already mentioned that the spaces between the ends of the arches and the butments were filled up with chumps of wood, and those rather in a damp state; and though we rammed them as close as we could we could not ram them [begin page 227] so close as the drying and the weight of the arch, or rib, especially when loaded, would be capable of doing; and we had now to observe the effects which the yielding and pressing up of the wood, and which; corresponds to the giving away of the butments, so generally fatal to stone arches, would have upon this.

We loaded the rib with six tons of pig-iron, beginning the center, and proceeding both ways, which is twice the weight of the iron in the rib, and I shall hereafter more particularly mention.

This had not the least visible effect on the strength of the arch, but it pressed the wood, home, so as to gain in three or four days, together with the drying and the shrinking of the wood, above a quarter of an inch at each end, and consequently the chord or span of the arch was lengthened above half an inch.  As this lengthening was more than double the feather of the keystone in a stone arch of these dimensions, such an alteration at the butment would have endangered the safety of the stone arch, while it produced on this no other than the proper mathematical effect.  To evidence this I had recourse to the cord still swinging on the wall from which the curve of the arch was taken.  I set the chord [begin page 228] at ninety feet span, and five feet for the height of the arch, and marked the curve on the wall.

I then removed the ends of the cords horizontally something more than a quarter of an inch at each end.  The chord should then describe the exact catenarian curve which, the rib, had assumed by the same lengthening at the butments; that is, the rising of the chord should exactly correspond to the lowering of the arch, which it did through all their corresponding ordinates.  The chord, had risen something more than two inches at the center, diminishing to nothing each way, and the arch had descended the same quantity and in the same proportion.

I much, doubt whether a stone arch, could it be constructed as flat as this, could sustain such an alteration; and on the contrary, I see no reason to doubt but an arch on this construction and dimensions, or corresponding thereto, might be let down to half its height, or as far as it would descend, with safety.  I say “as far as it would descend," because the construction renders it exceedingly probable that there is a point beyond which it would not descend, but retain itself independent of butments; but this cannot be explained but by a sight of the arch itself.

In four or five days the arch having gained [begin page 229] nearly all it could gain on the wood, except what the wood would lose by a summer's drying, the lowering of the arch began to be scarcely visible.  The weight still continues on it, to which I intend to add more, and there is not the least visible effect on the perfect curvature or strength of the arch.  The arch having thus gained nearly a solid bearing on the wood and the butments, and the days beginning to be warm, and the nights continuing to be cool, I had now to observe the effects of the contraction and expansion of the iron.

The Academy of Sciences at Paris, in their report on the principles and construction of this arch, state these effects as a matter of perfect indifference to the arch, or to the butments, and the experience establishes the truth of their opinion.  It is probable the Academy may have taken, in part, the observations of M. Peronnet, architect to the King of France, and a member of the Academy, as some ground for that opinion.

From the observations of M. Peronnet, all arches, whether of stone or brick, are constantly ascending or descending by the changes of the weather, so as to render the difference perceptible by taking a level, and that all stone and brick buildings do the same.  In short, that mat- [begin page 230] ter is never stationary, with respect to its dimensions, but when the atmosphere is so; but that as arches, like the tops of houses, are open to the air, and at freedom to rise, and all their weight in all changes of heat and cold is the same, their pressure is very little or nothing affected by it. 

I hung a thermometer to the arch, where it has continued several days, and by what I can observe it equals, if not exceeds, the thermometer in exactness.

In twenty-four hours it ascends and descends two and three-tenths of an inch at the center, diminishing in exact mathematical proportion each way; and no sooner does an ascent or descent of half a hair's breadth appear at the center, but it may be proportionally discovered through the whole span of ninety feet.  I have affixed an index which multiplies ten times, and it can as easily be multiplied a hundred times: could I make a line of fire on each side the arch, so as to heat it in the same equal manner through all its parts, as the natural air does, I would try it up to blood heat. 

I will not attempt a description of the construction; first, because you have already seen the model; and secondly, that I have often observed that a thing may be so very simple as to [begin page 231 baffle description.  On this head I shall only say that I took the idea of constructing it from a spider's web, of which it resembles a section, and I naturally suppose that when nature enabled that insect to make a web she taught it the best method of putting It together.

Another idea I have taken from nature is that of increasing the strength of matter by causing it to act over a larger space than it would occupy in a solid state, as is evidenced in the bones of animals, quills of birds, reeds, canes, etc., which, were they solid with the same quantity of matter, would have the same weight with a much less degree of strength. 

I have already mentioned that the quantity of iron in this rib is three tons; that an arch of sufficient width for a bridge is to be composed of as many ribs as that width requires; and that the number of arches, if the breadth of a river requires more than one, may be multiplied at discretion.

As the intention of this experiment was to ascertain, first, the practicability of the construction, and secondly, what degree of strength any given quantity of iron would have when thus formed into an arch, I employed in it no more than three tons, which is as small a quantity as [begin page 232] could well be used in the experiment.  It has already a weight or six tons constantly lying on it without any effect on the strength or perfect curvature of the arch.  What greater weight it will bear cannot be judged of; but taking even these as data, an arch of any strength, or capable of bearing a greater weight than can ever possibly come upon .any bridge, may be easily calculated.

The river Schuylkill, at Philadelphia, as I have already mentioned, requires a single arch of four hundred feet span.  The vast quantities of ice render it impossible to erect abridge on piers, and is the reason why no bridge has been attempted.  But great scenes inspire great ideas.  The natural mightiness of America expands the mind, and it partakes of the greatness it contemplates.  Even the war, with all its evils, had some advantages.  It energized invention and lessened the catalogue of impossibilities.  At the conclusion of it every man returned to his home to repair the ravages it had occasioned, and to think of war no more.

As one among thousands who had borne a share in that memorable Revolution, I returned with them to the re-enjoyment of quiet life, and, that I might not be idle, undertook to construct [begin page 233] a bridge of a single arch for this river.  Our beloved General had engaged in rendering another river, the Potomac, navigable.  The quantity of iron I had allowed in my plan for this arch was five hundred and twenty tons to be distributed into thirteen ribs, in commemoration of the thirteen United States, each rib to contain forty tons; but although strength is the first object in works of this kind, I shall; from the success of this experiment, very considerably lessen the quantity of iron I had proposed. 

The Academy of Sciences, in their report upon this construction, say, "there is one advantage in the construction of M. Paine's bridge that is singular and important, which is, that the success of an arch to any span can be determined before the work be undertaken on the river, and with a small part of the expense of the whole by erecting part on the ground."

As to its appearance, I shall give you an extract of a letter from a gentleman in the neighborhood, member in the former Parliament for this county, who, in speaking of the arch, says, "In point of elegance and beauty it far exceeds my expectations, and it is certainly beyond anything I ever saw."  I shall likewise mention that it is much visited and exceedingly [begin page 234] admired by the ladies; who, though they may not be much acquainted with mathematical principles, are certainly judges of taste.  I shall close my letter with a few other observations naturally and necessarily connected with the subject.

That, contrary to the general opinion, the most preservative situation in which iron can be placed is within the atmosphere of water, whether it be that the, air is less saline and nitrous than that which arises from the filth of streets, and the fermentation of the earth, I am not undertaking to prove; I speak only of fact, which anybody may observe by the rings and bolts in wharfs and other watery situations.  I never yet saw the iron chain affixed to a well-bucket consumed or injured by rust; and I believe it is impossible to find iron exposed to the open air in the same preserved condition as that which is exposed over water.

A method of extending the span and lessening the height of arches has always been the desideratum of bridge architecture.  But it has other advantages.  It renders bridges capable of becoming a portable manufacture, as they may, on this construction, be made and sent to any part of the world ready to be erected; and at [begin page 235] the same time it greatly increases the magnificence, elegance and beauty of bridges, it considerably lessens their expense, and their appearance by repainting will be ever new; and as they may be erected in all situations where stone bridges can be erected, they may, moreover, be erected in certain situations where, on account of ice, infirm foundations in the beds of rivers, low shores and various other causes, stone bridges cannot be erected.

The last convenience, and which is not inconsiderable, that I shall mention is that after they are erected they may very easily be taken down without any injury to the materials of the construction, and be re-erected elsewhere.

I am, sir, 
Your much obliged and obedient humble servant,

Thomas Paine,

(Rotherham, spring of 1789.)