To His Excellency Benjamin Franklin, Esquire June 6th, 1786
From the original letter at the American Philosophical Society.
Borden Town June 6th 1786
The gentleman, Mr. Hall, who presents you with this letter, has the Care of two models for a Bridge, one of wood, the other of cast Iron, which I have the pleasure of submitting to you, as well for the purposes of shewing my respect to you, as my patron in this Country as for the sake of having your opinion and Judgment thereon.
The European method of Bridge Architecture, by piers and Arches, is not adapted to the condition of many of the Rivers in America on account of the Ice in the winter. — The Construction of those I have the honor of presenting to you is designed to obviate that difficulty by leaving the whole passage of the River clear of the incumbrance of Piers.
The Wooden Model was the first executed. The Timbers are an Inch square, and twelve inches and a half long, the extent of the model is thirteen feet, and the height of the Arch about a twenty eighth part of the length of the chord. — It is made of cherry tree which is not a very strong wood. What weight it will bear, as it cannot be ascertained without breaking it, I am unwilling to put to an experiment. Four men have been on it at one time, without the least injury to it, or signs of any.
The objections against the wooden model are the compressibility and perishableness of the Material. The ends of the Timber by continually pressing against each other will in time diminish something in their length either by splitting up or wearing away.
The Iron Model is intended to obviate those objections. Tho’ the principle is the same as in the other, the preparations and dimensions are not exactly the same. The least angle in the wooden Model is a right angle, and the greatest angle in the Iron one is about 88 degrees, and the height of the Arch is a sixteenth part of the length of its chord which is twelve feet.
It does not appear to me, that increasing the height of the Arch is an advantage, on the Contrary I think it a disadvantage. Whatever the chord, may be, I should prefer making the height of the arch about a 1/22 part of it.
My first design in the Wooden Model was for a Bridge over the Harlem River for my good friend General Morris of Morrisania. It is made on a Scale of one to twenty four, that is, the Timbers being increased twenty four the length of those in the Model, the same Number of Angles (nine) substituted in the room of Arches, would extend across that river, which is three hundred feet.
It would be tedious in a letter to give you all the observations that have occurred to me in the progress of putting the parts together in the two Models. — As to what may be the best angle, or the best proportions of the several parts I do not undertake to decide. The longer the legs are the fewer (of consequence) will be the number of the Angles, and the shorter the most; but neither case, if the same Angle be used, the quantity of Timber or Iron will be nearly the same, for the eighteen legs which compose each side of either of the Bridges are only equal to one angle whose base would be the extent of the length of the Bridge. Therefore as the quantity of Timber or Iron will be nearly the same in a greater or less number of Angles for any given length the only consideration is the relative proportion.
To what extent, on a great Work, such a construction of a Bridge can be carried, is what I am unwilling to venture too much opinion on; but I cannot help thinking that it might be carried across the Schuylkill. On the proportions I have mentioned it would have an elevation of about twenty feet in the Center, but it might be made either more or less.
As I do not think it possible for any Bridge constructed with Piers to withstand the Ice which comes down that River in some winters, there appears to me no hope of a permanent Bridge unless it can be carried clear across. — A floating Bridge at all times obstruct the Navigation, and in winter, when a Bridge is most wanted, is often of no use; and frequently carried away by driving Ice. But could a Bridge be erected on the plan of the Iron Model, it would exceedingly benefit the City and Country, and besides its usefulness would, I believe, be the most extensive Arch in the world, and the longest Bridge without Piers: I should therefore wish to see it undertaken and performed during your Presidency, as any share I might have therein would be greatly heightened by that Circumstance, but of this, and other matters relating thereto I reserve myself till I have the honor of seeing you which I hope will be on Thursday, in the mean time, Mr. Hall, who has been with me at Borden Town, and has done the chief share of the working part, for we have done the whole ourselves, will inform you of any Circumstance relating to it which does not depend on the mathematical Construction.
Mr. Hall will undertake to see the Models brought safe from the Stage Boat to you, they are too large to be admitted into the house but will stand very well in the Garden.
Should there be a vessel going round to N. York within about a week after my arrival at Philadelphia, I shall take that convenience for sending them there, at which place I hope to be in about a fort’night. —
I am Dear Sir your Affectionate and Obedient Humble Servant