« AnteriorContinuar »
The Academy then proceed to state the reasons on which their judgment is founded, and conclude with saying,
"Nous concluons de tout ce que nous venons d'exposer que la pont de fer de M. Paine est ingenieusement imaginé, que la con struction en est simple, solide, et propre à lui donner la force necessaire pour résister aux effets resultans de sa charge, et qu'il merite qu'on en tente l'execution. Enfin, qu'il pourra fournira un nouvel exemple de l'application d'un métal dont on n'a pas jusqu'ici fait assez d'usage en grand, quoique dans nombre d'occasions il est peu être employé avec plus grand succès."
As it was my design to pass some time in England before I returned to America, I employed part of it in making the small essay I am now to inform
My intention, when I came to the iron works, was to raise an arch of at least two hundred 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 practice, any species of curve with equal facility, I set off in preference to all others, a catenarian arch of ninety feet span, and five feet high. Were this arch converted into an arch of a circle, the diameter of its circle would be four hundred and ten 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 paralle. arched ribs, and as the number of ribs may be increased at pleasure to any breadth an arch sufficient for a road way may require, and the arches to any number the breadth of a river may require, the
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.
*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.
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 work shop, used in the mean time by the carpenters, where it occupied so small a compass as to be hid among the shavings, and though the extent of it is ninety feet, the depth of the arch at the centre two feet nine inches, and the depth at the branches six 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 four 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.
We soon ran up a centre 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 upwards, 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 centre is different, for as stone arches sometimes break down the centre by their weight, this, on the contrary, grew lighter on the centre as the arch increased in thickness, so much so, that before the arch was completely finished, it rose itself off the centre 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 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 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 keystone 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, 66 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 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 reenjoyment 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, to be distributed into thirteen ribs, in conmemoration 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