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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 so close as their 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 way of the butments, so generally fatal to stone arches, would have upon this.

We loaded the rib with six tons of pig iron, beginning at the centre, and proceeding both ways, which is twice the weight of the iron in the rib, as 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 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 key-stone in a stone arch of these dimensions, such an alteration at the butment would have endangered the safety of a 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 cord to 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 cord 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 cord should exactly correspond to the lowering of the arch, which it did through all their corresponding ordinates. The cord had risen something more than two inches at the centre, 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 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 matter 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 between two and three-tenths of an inch at the centre, diminishing in exact mathematical proportion each way; and no sooner does an ascent or descent of half a hair's breadth appear at the centre, 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 an 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 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 supposed, 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, &c. 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 could well be used in the experiment. It has already a weight of 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 a bridge 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 amongst 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 a bridge of a single arch for this river. Our beloved General had engaged in rendering another river, the Patowmac, navigable. The quantity of iron I had allowed in my plan for this arch was five hundred and twenty tons,

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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 expence 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 neighbourhood, 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 any thing I ever saw." I shall likewise mention, that it is much visited and exceedingly 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, náturally 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 any body 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 for 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 the same time that it greatly increases the magnificence, elegance, and beauty of bridges, it considerably lessens their expence, and their appearance by re-painting 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.

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