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ed over a stream liable to freshets, to secure each span as soon as possible, so as to stand in case the scaffold should happen to be carried away; the introduction of the secondary timbers is generally deferred until those essential to the support of the bridge, in such an event, are put together. Therefore, the diagonal braces are not inserted until after the girders are put into their places.

As these braces are tenoned into the girders at both ends, they could not be inserted into the mortises in the girders, unless some play were allowed at one end; this play is afterwards filled up by a pair of double wedges, as shown in fig. 11.

Where the counterbraces of the trusses intersect the main braces, the former are merely tenoned into the latter, as shown in fig. 3, at Y. Where the chords and queens intersect, they are notched equally into each other, so as to bring the two pieces composing the chords within. about an inch of each other, figs. 2 and 4.

The planking of the canal trunk is single, and well caulked. The courses of plank are from six to fifteen inches wide.

REMARKS. This aqueduct evinces, more strikingly than any other structure I know of, the capability of timber for the purpose of bridge building. The weight of water in the canal trunk on a single span, when four feet three inches deep, amounts to 275 tons, of 2240 lbs.; and we may safely say that that weight is frequently increased to at least 300 tons, during the passage of boats; for although a boat, of course, displaces a bulk of water of equal weight with herself, yet it may readily be conceived that the water so displaced does not instantaneously leave the span, on her entrance; and I think we may assume that at least twenty-five tons of it are frequently on a span at the same moment with the boat. Yet on a most critical examination, made with that view, I could not detect in any part of the timbers. the slightest symptom of what might with propriety be called crushing. Slight compressions, (if I may be allowed to draw such a distinction,) were visible at the heads of the queen-posts, but not to a greater extent, apparently, than invariably attends all trusses of this kind in common bridges of large spans, after having been some time in use. In all bridges there is a tendency to settle, or sag, in the centre; and this tendency, of course, brings a heavy compressing strain upon the pole plates; but beside this compression, incident to the truss considered as a whole, there is another, acting at the several points at which the heads of the posts tenon into the poles. This compound compression explains a fact for which I was for some time at a loss to assign a satisfactory reason. I have already stated that in the inner trusses of this aqueduct, a straining-piece, like that shown at T, fig. 3, was inserted between the heads of the posts, in

preference to the short butting piece, P, figs. 3, 5, employed in the outer trusses. This was evidently done under the impression that it opposed a more perfect resistance to the compressions alluded to, than the shorter pieces; and, at first sight, it will probably strike most of my readers in the same manner. But it is of great importance to know, that although the long piece is almost invariably introduced, both by engineers and bridge builders, whenever extraordinary compression of the pole is anticipated, it is in fact entirely ineffective: whereas the short butting pieces perform the duty assigned them perfectly.

I shall endeavor to point out the cause of this.

The compression of the poles evidently increases from the piers towards the centre of the span, in the same manner as in a single long piece of timber, supported at two ends, when it sags in the middle: consequently, when the bridge settles, as it always will, more or less, the head of any one post is moved a greater distance towards the centre of the span than the post behind it, that is, between it and a pier. Therefore, the opening, p', behind the post, Q', must be a little wider than the opening p, behind the post Q; and, consequently, the inner end of the straining-piece, T, cannot be forced up into contact with the head of the post Q, but must remain distant from it an amount equal to the difference of the compression which takes place in that part of the pole between Q' and the centre of the span, and that part which extends from p' to p. This difference in the amount of compression between any two consecutive posts, is very perceptible in all large bridges, being generally about one-eighth of an inch, that is if there be seven spaces in the truss, between a pier and a king post, the opening at the inner one will generally be about seven-eighths of an inch, at the next one six-eighths, at the next five-eighths, and so on to the queen post near the pier, where it will diminish to nothing. In some bridges, and those excellent ones, I have seen the openings behind the queen posts much greater than this, at least double; but, I believe, only in such bridges as have no chords to confine the feet of the ribs. Of course some portion of these openings, in every case, is due to the compression which takes place in the heads of the posts themselves. This is frequently very perceptible. I could just detect it in a few of the queen posts of the aqueduct.

But it may be objected that if the explanation I have given be correct, then even the short butting piece, P, should also be ineffective; because, if the compression of the pole increases so perceptibly towards the centre, then supposing the length of the butting-piece to be one-fourth of the distance between two queens, the inner end of the butting-piece should not come into contact with the pole, by an

amount equal to one-fourth of the opening which occurs between those two queens. Plausible as this deduction would seem, it is, nevertheless, incorrect, for as I have before remarked, the short butting-pieces act admirably, and, as I conceive, for this reason, that although the entire length of that portion of a pole, or cap, between two adjacent posts, is in a state of compression, which, considering the whole truss as one great beam, gradually increases towards the centre. Still the action of the main braces against the back part of the head of each post, tends to bring an additional strain upon the portion of the pole next adjoining the inside of the post head. This additional strain produces a compression of its own, which, unlike that operating on the truss considered as a whole, decreases towards the centre. Therefore, that part of the pole into which the head of any post tenons, is more compressed than the part at the end of the butting-piece, and, consequently, the latter is brought into full action.

This matter is a very important one, and my remarks on it were suggested by seeing that in this aqueduct the long straining-piece had superseded the short butting-piece, evidently in expectation of its greater efficiency. In the Market street bridge, at Philadelphia, the finest in existence, the same defect exists; also in the viaduct of the Columbia and Philadelphia Railroad, near Philadelphia, and many others, built by the most talented and experienced bridge builders in the country.

But in all these bridges, as well as in this aqueduct, the inner ends of none of these long straining-pieces are in contact with the heads of the adjoining posts, against which they were intended to exert a powerful compression. Consequently, they are not only useless, but positively injurious, as they add unnecessarily to the weight of the truss, and thus absolutely increase that tendency to settle, which they are intended to prevent.

I noticed a splintering, or spalling off, of the stones supporting the feet of some of the arches. The stones in this part of a bridge, as well as those forming the facing of the starlings, should always be of good quality; and, in the former case especially, attention should be paid to their toughness; soft sandstone should never be admitted.

In an extreme case like this, of such an immense weight and so soft a stone, or indeed with stone of the best quality, I should certainly prefer to have the recesses in the top of the pier for receiving the feet of the curved ribs and pier posts, somewhat deeper than in this instance, where they are but eight inches deep, figs. 1, 2, and 6. The pressure on the piers and abutments, so long as the bridge maintains itself, is almost altogether vertical, and as it sometimes happens that the bearing is not very fair, every precaution should be used to pre

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