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CHAP. XX.

PLATE XIII. REPRESENTS AN AQUEDUCT OF CAST IRON

	In constructing an aqueduct by this means, the butments and piers being raised, it will only be necessary to extend two pieces of timber across the span; each to be braced back to the piers, and covered with plank to form a stage or scaffolding; which will answer every purpose of centers necessary to works of stone.

	The iron-work, as in the section, may all be cast in open sand, and of the following dimensions; supposing the span one hundred feet, and the spring of sixth of the span.

	First, Three segments of a circle, each in three pieces, about thirty-six feet long, eight inches by four diameter, to be united as at A.  Second, three strait bars, to extend from one pier to the other, to be of the above diameters, may also be cast in three pieces; which bars are to extend along the top of the segments to the piers, and form a line parallel to the horizon; the bars and segments to be united by perpendicular stirrups, like B, ten or fifteen feet distant from each other.

	 The mortise in the lower end of the stirrup being thirteen inches long, will be sufficient to receive the segment, and leave room for  a hole two inches square; through which a cross-brace, C, is to pals, and fasten the segments at proper distances; the brace to have a mortise cast on each side of the stirrup, in order to tighten the work by wedges.

	On the top of the stirrup, the square hole to receive the crossbrace may be beneath the mortise, as in the Figure; by which means, the whole may be combined and form an iron stage to support the troughs.

	The trough plates should be at least one inch thick, the side plates six feet broad, and as great a length as can conveniently be cast; which may be performed twelve feet, and perhaps more, in length: the flange to be outside on these plates.

	The bottom plates may be fix feet wide, thirteen feet long, seven feet plate, and four arms projecting, each three feet long, in order to support the horse-path and braces; as exhibited by a bottom and side-plate at D.

	Two of these plates laid across the stage, and screwed together, with the flange under, will compose a length equal to one of the side-plates; which may either meet or break joint as is thought proper.  The whole may, in this manner, be screwed together, on packing of wool and tar; and have the seams pitched like those of a ship.

	On the plates composing one side of the trough, small brackets, about three feet from the top, must be cast, as E, in order to support the horse-path; perpendicular rails, eight feet long, being raised from the arms of the bottom plates, will support the outside of the horse-path, also the iron railing, as in the section.

	By this mode, two patterns will answer for the whole of the trough-plates, and but few will be required for the springs, rails, and spurs; while the saving in time and expense will be considerable; particularly where it is necessary to bring the stone by long land-carriage;  for the arches being dispensed with, and the piers not more than one-third the dimensions necessary to an aqueduct of stone, will most materially reduce the quantity of masonry.

	But, according to the various circumstances of situation, carriage of stone, iron, &c. the disparity between the two modes will be easily determined, added to which, the durability may be of some importance.

	In aqueducts of stone, one of the great difficulties is to line and puddle to tough, as to prevent the water penetrating into, and injuring the masonry; but in one of iron, should a leak take place, it will instantly appear; and on shutting the stop-gates at each end, and discharging the water, it may be stopped in a few hours, if not minutes: the circumstance, in aqueducts, is, perhaps, one of the greatest preservatives; they are consequently less liable to injury, and only subject to the corroding tooth of time.