The Construction of Iron Bridges
As bridges and the method of constructing them are becoming objects of great importance throughout the United States, and as there are at this time proposals for a bridge over the Delaware, and also a bridge beginning to be erected over the Schuylkill at Philadelphia, I present the public with some account of the construction of iron bridges.
The following memoir on that subject written last winter at the Federal City, was intended to be presented to Congress. But as the session would necessarily be short, and as several of its members would be replaced by new elections at the ensuing session, it was judged better to let it lie over. In the meantime, on account of the bridges now in contemplation, or begun, I give the memoir the opportunity of appearing before the public, and the persons concerned in those works.
N.B.-The two models mentioned in this memoir will, I expect, arrive at Philadelphia by the next packet from the Federal City and will remain for some time in Mr. Peale’s museum.
To THE CONGRESS OF THE UNITED STATES
I have deposited in the office of the Secretary of State, and under the care of the Patent Office, two models of iron bridges; the one in paste- board, the other cast in metal. As they will show by inspection the manner of constructing iron bridges, I shall not take up the time of Congress with a description of them.
My intention in presenting this memoir to Congress is to put the country in possession of the means and of the right of making use of the construction freely; as I do not intend to take any patent right for it.
As America abounds in rivers that interrupt the land communication, and as by violence of floods and the breaking tap of the ice in the spring, the bridges depending for support from the bottom of the river are frequently carried away, I turned my attention, after the Revolutionary War was over, to find a method of constructing an arch that might, without rendering the height inconvenient or the ascent difficult, extend at once from shore to shore, over rivers of three, four or five hundred feet and probably more.
The principle I took to begin with and work upon was that the small segment of a large circle was preferable to the great segment of a small circle. The appearance of such arches, and the manner of forming and putting the parts together, admit of many varieties, but the principle will be the same in all. The bridge architects that I conversed with in England denied the principle, but it was generally supported by mathematicians, and experiment has now established the fact.
In 1786, I made three models, partly at Philadelphia, but mostly at Bordentown in the State of New Jersey. One model was in wood, one in cast iron, and one in wrought iron connected with blocks of wood, representing cast iron blocks, but all on the same principle, that of the small segment of a large circle.
I took the last mentioned one with me to France in 1787 and presented it to the Academy of Sciences at Paris for their opinion of it. The Academy appointed a committee of three of their own body-Mons. Le Roy, the Abbe Bossou, and Mons. Borda. The first was an acquaintance of Dr. Franklin, and of Mr. Jefferson, then minister at Paris. The two others were celebrated as mathematicians. I presented it as a model for a bridge of a single arch of four hundred feet span over the river Schuylkill at Philadelphia. The committee brought in a report which the Academy adopted-that an arch on the principle and construction of the model, in their opinion, might be extended four hundred feet, the extent proposed.
In September of the same year, I sent the model to Sir Joseph Banks, president of the Royal Society in England, and soon after went there myself.
In order to ascertain the truth of the principle on a larger scale than could be shown by a portable model five or six feet in length, I went to the iron-foundry of Messrs. Walker, at Rotherham, County of Yorkshire, in England, and had a complete rib of 90 feet span, and 5 feet of height from the chord line to the center of the arch, manufactured and erected. It was a segment of a circle of 410 feet diameter; and until this was done no experiment on a circle of such an extensive diameter had ever been made in architecture, or the practicability of it supposed.
The rib was erected between a wall of a furnace belonging to the ironworks and the gable end of a brick building, which served as butments. The weight of iron in the rib was three tons, and we loaded it with double its weight in pig-iron. I wrote to Mr. Jefferson who was then at Paris, an account of this experiment, and also to Sir Joseph Banks in London, who in his answer to me says-“I look for many other bold improvements from your countrymen, the Americans, who think with vigor, and are not fettered with the trammels of science before they are capable of exerting their mental faculties to advantage.”
On the success of this experiment, I entered into an agreement with the iron-founders at Rotherham to cast and manufacture a complete bridge, to be composed of five ribs of 210 feet span, and 5 feet of height from the chord line, being a segment of a circle 610 feet diameter, and sent it to London to be erected as a specimen for establishing a manufactory of iron bridges to be sent to any part of the world.
The bridge was erected at the village of Paddington, near London, but being in a plain field, where no advantage could be taken of butments without the expense of building them, as in the former case, it served only as a specimen of the practicability of a manufactory of iron bridges. It was brought by sea, packed in the hold of a vessel, from the place where it was made; and after standing a year was taken down without injury to any of its parts, and might be erected anywhere else.
At this time my bridge operations became suspended. Mr. Edmund Burke published his attack on the French Revolution and the system of representative government, and in defense of government by hereditary succession, a thing which is in its nature an absurdity, because it is impossible to make wisdom hereditary; and therefore, so far as wisdom is necessary in a government, it must be looked for where it can be found, sometimes in one family, sometimes in another. History informs us that the son of Solomon was a fool. He lost ten tribes out of twelve (2 Chron. ch. x). There are those in later times who lost thirteen.
The publication of this work by Mr. Burke, absurd in its principles and outrageous in its manner, drew me, as I have said, from my bridge operations, and my time became employed in defending a system then established and operating in America, and which I wished to see peaceably adopted in Europe. I therefore ceased my work on the bridge to employ myself on the more necessary work, “Rights of Man,” in answer to Mr. Burke.
In 1792, a convention was elected in France for the express purpose of forming a Constitution on the authority of the people, as had been done in America, of which convention I was elected a member. I was at this time in England and knew nothing of my being elected till the arrival of the person who was sent officially to inform me of it.
During my residence in France, which was from 1792 to 1802, an iron bridge of 236 feet span, and 34 of height from the chord line, was erected over the river Wear near the town of Sunderland, in the County of Durham, England. It was done chiefly at the expense of the two members of Parliament for that county, Milbanke and Burdon.
It happened that a very intimate friend of mine, Sir Robert Smyth (who was also an acquaintance of Mr. Monroe, the American Minister, and since of Mr. Livingston), was then at Paris. He had been a colleague in Parliament with Milbanke, and supposing that the persons who constructed the iron bridge at Sunderland had made free with my model, which was at the iron-works where the Sunderland bridge was cast, he wrote to Milbanke on the subject, and the following is that gentleman’s answer.
“With respect to the iron bridge over the river Wear at Sunderland, it certainly is a work well deserving admiration, both for its structure and utility, and I have good grounds for saying that the first idea was suggested by Mr. Paine’s bridge exhibited at Paddington. What difference there may be in some part of the structure, or in the proportion of wrought and cast iron, I cannot pretend to say, Burdon having undertaken to build the bridge, in consequence of his having taken upon himself whatever the expense might be beyond between three and four thousand pounds sterling, subscribed by myself and some other gentlemen.
“But whatever the mechanism might be, it did not supersede the necessity of a center.” (It is the technical term, meaning the boards and numbers which form the arch upon which the permanent materials are laid; when a bridge is finished the workmen say they are ready to strike center, that is to take down the scaffolding.-Author) (The writer has here confounded a center with a scaffolding.) “Which center (continues the writer) was esteemed a very ingenious piece of workmanship, and taken from a plan sketched out by Mr. Nash, an architect of great merit, who had been consulted in the outset of the business when a bridge of stone was in contemplation.”With respect therefore to any gratuity to Mr. Paine, though ever so desirous of rewarding the labors of an ingenious man, I do not feel how, under the circumstances already described, I have it in my power, having had nothing to do with the bridge after the payment of my subscription, Mr. Burdon then becoming accountable for the whole. But if you can point out any mode according to which it would be in my power to be instrumental in procuring him any compensation for the advantages the public may have derived from his ingenious model, from which certainly the outline of the bridge at Sunderland was taken, be assured it will afford me very great satisfaction. (The original is in my possession.-Author.)
The year before I left France, the Government of that country had it in contemplation to erect an iron bridge over the river Seine, at Paris. As all edifices of public construction came under the cognizance of the Minister of the Interior (and as their plan was to erect a bridge of five iron arches of 100 feet span each, instead of passing the river with a single arch, and which was going backward in practise, instead of forward, as there was already an iron arch of 230 feet in existence) I wrote the Minister of the Interior, the citizen Chaptal, a memoir on the construction of iron bridges. The following is his answer:
“The Minister of the Interior to the citizen Thomas Paine,-I have received, citizen, the observations that you have been so good as to address to me upon the construction of iron bridges. They will be of the greatest utility to us when the new kind of construction goes to be executed for the first time. With pleasure, I assure you, citizen, that you have rights of more than one kind to the thankfulness of nations, and I give you, cordially, the particular expression of my esteem.
“-CHAPTAL.” (The original, in French, is in my possession.-Author.)
A short time before I left France, a person came to me from London with plans and drawings for an iron bridge of one arch over the river Thames at London, of 600 feet span, and 60 feet of height from the chord line. The subject was then before a committee of the House of Commons, but I know not the proceedings thereon.
As this new construction of an arch for bridges, and the principles on which it is founded, originated in America, as the documents I have produced sufficiently prove, and is becoming an object of importance to the world, and to no part of it more than to our own country, on account of its numerous rivers, and as no experiment has been made in America to bring it into practise further than on the model I have executed myself and at my own expense, I beg leave to submit a proposal to Congress on the subject, which is:
To erect an experiment rib of about 400 feet span, to be the segment of a circle of at least 1,000 feet diameter, and to let it remain exposed to public view, so that the method of constructing such arches may be generally known.
It is an advantage peculiar to the construction of iron bridges that the success of an arch of a given extent and height can be ascertained without being at the expense of building the bridge; which is, by the method I propose, that of erecting an experiment rib on the ground where advantage can be taken of two hills for butments.
I began in this manner with the rib of 90 feet span and 5 feet of height, being a segment of a circle of 410 feet diameter. The undertakers of the Sunderland bridge began in the same manner. They contracted with the iron-founder for a single rib, and, finding it to answer, had five more manufactured like it and erected into a bridge consisting of six ribs, the experiment rib being one.
But the Sunderland bridge does not carry the principle much further into practise than had been done by the rib of 90 feet span and 5 feet in height, being, as before said, a segment of a circle of 410 feet diameter; the Sunderland bridge being 206 feet span and 34 feet of height, gives the diameter of the circle of which it is a segment to be 444 feet, within a few inches, which is but a larger segment of a circle 30 feet more diameter.
The construction of those bridges does not come within the line of any established practise of business. The stone architect can derive but little from the theory of practise of his art that enters into his construction of an iron bridge; and the iron-founder, though he may be expert in moulding and casting the parts, when the models are given him, would be at a loss to proportion them, unless he was acquainted with all the lines and properties belonging to a circle.
If it should appear to Congress that the construction of iron bridges will be of utility to the country, and they should direct that an experiment rib be made for that purpose, I will furnish the proportions for the several parts of the work and give my attendance to superintend the erection of it.
But, in any case, I have to request that this memoir may be put on the journals of Congress, as an evidence hereafter that this new method of constructing bridges originated in America.
FEDERAL CITY, January 3, 1803.