The tension rod is 1-A- inch round, the king-rod inch, and the queen-rods y inch round.
The timber between the bars in the rafters serves to fix battens for the covering, The third example is for a roof 35 feet wide, constructed wholly of malleable iron. It consists of rafters and tic-rod, suspended at three points by king-rod and queen-rods, with struts from the bottom of the king-rod to the top of the queen-rods, and from the root of the latter to the rafters.
The tie rods a e 1 inch round, the king-rod inch, and the queen-rod 1 inch.
The fourth example is fur a span of 60 feet, and consists of rafters and tie-rod, suspended at five points by a king and four queen-rods, two on either side. The four struts spring from the feet of the inner suspension-rod to the head of the outer.
The rafters are formed in the same manner as in the second example, of two parallel plates of bar-iron, with plank between ; the scantlings are, however, somewhat heavier, the depth of the bar-iron being 4 inches, and its thickness The struts are of T iron, the top table being 3 inches wide and 1 inch deep, and the rib 31 inches deep and b thick.
The tension-rod is of round iron, and is, from the extremity to the first suspension-rod, I inch thick, thence to the second rod inch, and thence to the king-rod 11 inch. The king-rod is 11- inch round, the intermediate I inch, and the outermost inch. The fifth example is for the same span, and consists of rafters and tension-rod, suspended at live points by king rod and four queen-rods, placed closer together than in the last example. There are six struts placed as above, but the last on either side buts against the rafter where there is no queen-rod.
The entire roof is of malleable iron, the rafters of the T form measuring 3 inches across the top table, which is b inch thick, and 31 inches in the depth of the rib, which is 1 inch in thickness. The struts are also of '1' iron, the inner ones being 21 inches wide over the top table, which is inch thick, and 31- inches in the depth of the rib, which is -!,? inch thick. The outer struts are inches wide over the top table,
and 21 inches in the depth of the rib, the former being inch thick, and the latter 1 inch.
The tension-rods are 1-g- inch round near the springing, 11 inch in the middle, and 11 near the king-rod.
In all such roofs, it is necessary to strengthen them with iron tie-rods, passing obliquely from the springing to the ridge across one bay, and secured on the under side of the purlins. The distance between the principals varies from 5 to 7 feet.
The roof over the Museum of Practical Geology-, Picca dilly, of which Mr. Pennethorne was architect, is entirely of cast-iron, which is somewhat differently applied to any instance we have alluded to. The total width of the building is 67 feet, but there is an intermediate bearing on either side 6 feet from the external wall, thus making the clear width of the root' 55 feet. Each rib is composed of five separate pieces or girders, securely bolted together at the ends ; the first piece, one end of which rests upon the wall, and the other on the intermediate bearing, is of a triangular form, with a per forated spandrel ; the lower side is horizontal, resting upon the supports, the upper taking the slope of the roof, and the inner one springing from the inner support, with a curve of 5 feet radius, to range with the curvature of the other part of the roof, the radius of which is 48 feet. This girder is 1 foot 6 inches in depth, with a rib on either side in the centre. The next girder is of the same depth throughout, viz. 1 foot o inches, and in the cross section has a rib in the centre, and one of less projection at the lower edge ; this portion is covered with slate slabs, inch thick. The central girder differs from the rest, being 2 feet in depth, with a flambe at top and bottom, the girder between being perforated for the sake of diminishing the weight ; this portion is covered with glass. Each of the girders have abutment plates at the end, by means of which they are bolted together. The total rise or versed sine of the roof is about 12 feet.