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CHAP. XXII.

PLATE XVII. OF BRIDGES OF WOOD



	The important objection to bridges of wood, is there rapid decay: and this objection is certainly well founded, when particular situations are alluded to, where timber is scarce, and consequently expensive; but in such countries such as America, where wood is abundant, I conceive is will be a fair criterion to judge of their application, by calculating on the expense of a bridge of stone and one of wood; then compare the interest of the principal saved in adopting the wood bridge, with the expense of its annual repairs.

	I have before exhibited the necessity of constructing bridges in America, of an extensive span or arch, in order to sunder the ice and collected waters to pass without interruption: and for this purpose, it must be observed, that a wood arch, may be formed of a much greater length, or span, than it is possible to erect on of stone, hence they are applicable to many situations, where accumulated waters, bearing down trees and fields of ice, would brush a bridge of stone from its foundations.

	It therefore becomes of importance, to render bridges of wood as permanent as the nature of the material will admit.

	Hitherto the immense quantity of mortising and tenants, which however well done, will admit and wet, consequently tend to expedite the decay of the weak plants, has been a material error in constructing bridges of wood, by a repetition of bracing, has also exposed almost every side of the whole of the timbers to the changes of the weather; consequently, the whole was reduced to the durability of one stick, similar to suspending a cage in open air, each stick is exposed to decay, without receiving shelter from each other.

	It has also been usual to place supporters in the water, subjecting them to the surge of floods, which shakes the whole fabric; which supporters decaying progressively let down the upper works.

But to render wood-bridges of much more importance than they have hitherto been considered; first, from their extensive span; second, by their durability; two things must be considered: first, that the wood-works should stand clear of the stream, in every part, by which it never would have any other weight to sustain them than that of the usual carriage; second, that it be so combined as to exclude as much as possible from the air and rain.

	For this purpose, in erecting a bridge of wood, I would proceed much on the same system as in constructing the one of iron staves. For instance, suppose a bridge three hundred feet span, thirty feet wide, the butments being rendered secure, and centers raised on piles; let timbers, if convenient, be procured thirty feet long, and of as great diameter as the country will produce; such timbers being squared and planed to the radius of the arch, with the holes to receive a bolt or trundle, about four feet distant from each other; the whole operation in springing the arch, will be to tar and paint the junctions with white lead, and insert the trundle, as at A, FIG II. and press them close, thus lay them side by side, by which means an arch might be laid in a few days, and as soon as the last timber is placed, the centers may be removed, and each piece pressing on its neighbor, will tend to render the whole compact; this being done, both down the belts C and D, distant from each other, in a lateral direction, about ten feet; then cover the whole with old sail-cloth, or tarpaulins, tar and sand; of the tar and sand, two or three coats may be laid on, which will fill every aperture, and form a permanent cover to the total exclusion of the air and rain. The arch being thus covered and kept dry, would present a segment of a cylinder, at least one foot thick, guarded from the change of seasons, and in all probability would last many ages. The first segment should be coated in like manner with tar, &c. previous to laying on the earth and gravel should be, perhaps, eighteen inches thick, in order that the wheels might never wear the timbers or coating. Having described the construction, and exhibited the formation, by the engraving, it is but necessary to observe, that the staves composing the arch not eat away by mortising, but preserve their whole strength, the whole of each timber is completely covered from the weather. I will now leave my reader to reflect on the strength and durability of this arrangement; and then proceed to consider to what extent of span such bridges bay be constructed.

	In this I must call to mind my former remarks, that the segment of a circle must be compressed into a straight line, or sink in one part and rise another, or the butments separate before it is possible for a bridge to fall; by longitudinal pressure, the lateral tendency shall hereafter be observed, admitting the above proportions; the questions is, whether the arrangement of parts is so calculated as to guard against accidents? which probably will appear by the following considerations:

	First the butments may be made to resist any weight, buy giving a greater weight of stone than there is weight of materials in the arch.

	Second, the timbers being laid side by side, like staves, and pressing on each other , leave not the least aperture into which the parts of mortised and framed timbers might be compressed; hence, several timbers, in a long arch, must absolutely be compressed into nothing, before the segment could become a straight line.

	Third, that it may not vary its position, by sinking in one part and rising in another, with the various weights, I will consider the staves and belts only: it must be observed, that by bolting down the belts, which belts may be from thirty to forty feet long, and break joint, as in FIG. II> the whole arch will become like one solid piece of timber, bent between the piers. I will now suppose such a bridge forty feet broad, the timbers combining eighteen inches thick; hence, admitting that it was constructed of wood as light as fir, each lineal foot would weight one ton and a half, hence every thirty feet forward would weigh forty-five tons; as a wagon, when loaded, seldom weighs more than five tons. And admitting four such wagons, weighing twenty tons, to stand abreast on the bridge, in which situation they would have the greatest possible pressure, by acting near the same spot; yet. to compress the part, thirty feet of the bridge, at least, on each side, must rise, and this in some measure raising the whole of the spring, which would be a weight of at least one hundred tons; hence, as twenty tons cannot move one hundred, without superiorr leverage, and there is no leverage obtained, consequently there is no weight which it is reasonable to suppose can come on a bridge at one time can injure it.

	The longitudinal pressure being considered, the tendency to yield sideways may be prevented, by constructing it wide at the ends and contracted at the center, the pressure will then resisted by an arch in every direction. Having exhibited the construction, and assigned my reasons for its permanency, I hope it will easily be admitted, that a bridge of three hundred feet would be perfectly safe; but if the reader should hesitate at this, he has my reason why it would stand, and that it will be weak to discover the reason why it would not: but supporting it admitted, that three hundred feet would be safe, I see no difference between that and on of five hundred, or even more feet span, the proportions being preserved by a spring one tenth of the span.


	Having made this assertion, I almost fear I have forfeited the confidence of my reader, who may now be inclined  to doubt the stability of my senses; but patience should accompany investigation, and I must beg of him to proceed of five hundred feet spa, fifty feet spring, and forty feet broad, take a board eleven feet long, ten inches wide, and half-inch thick, and bend it between two blocks till it rises twelve inches, and it will give a model of the spring of an arch composed of two rows of staves, each a foot thick, amounting to two feet in thickness; extend this idea, by measuring off five hundred feet in a field, and imagining a perpendicular in the center fifty feet high, then draw a segment by the eye, conceive the whole well wedged and bolted together, the proportions of the timbers preserved, and deliberate on the part where it could give way.