2. The preservation of the clue position of the frames is made to depend too much upon wrought-iron bars, and bolts, which should be. as much as possible, excluded from structures of this kind.
3. The circles in the spandrels, placed as supports for the roadway, we have already stated, are improper in a situation where they are not equally pressed around.
We shall observe nothing respecting the timber in the superstructure, because this is mere economy ; if pro perly managed, any bad effects from the difference of expansion and contraction in the two materials, may be easily avoided ; and the timber, not interfering with the essential parts, it may, when necessary, be removed, with little interruption to the intercourse over, and none to that under this noble arch. We cannot here resist drawing the attention of our reader to the perfection of this double accommodation, in crossing this deep ravine with facility, while vessels of 200 tons are passing full rigged below.
A cast-iron bridge has lately been built over the river Witham, at Boston, in Lincolnshire, from a design by Mr Rennie. See Plate XCIV. The span is about 85 feet, the rise is about five feet six inches, the breadth is 36 feet, and there are eight ribs; each rib is composed of eleven frames, three feet depth in the direction of the radius. At each joining there is a cast-iron grating across the arch, which connects the frames, on the same principles as practised at Pontcysylte aqueduct. Instead of three pieces in the direction of the curve, as at Sunderland, here there arc only two, but they arc se ven inches by 41. These arc, in each frame, connected in the direction of the radius, by pieces four by three in ches. Upon the back of the ribs, pillars, four by three inches, are placed perpendicularly to support the road way. The superstructure resembles that of the first iron bridge at Coalbrookdalc. The arch has been kept very flat, to suit the tide below, and the streets above. The rise being only about of the span, is another proof of the facilities which may be acquired by using cast-iron. The frames being made about four times the length of those at Sunderland, and being connected with cast-iron gratings instead of wrought-iron, are es sential improvements ; but from the pieces in the frames, which are in the direction of the radius, being only four by three inches, while the main pieces, in the direction of the curve, are 7 by 41, a great proportion of the for mer are broken. This is a defect ; and the pillars which
support the roadway, being perpendicular, do not cor respond with the radiated pieces of the frames. The ribs, in springing from the perpendicular face of the masonry of the abutment, have also a crippled appear ance.
In improving the port of Bristol, Mr Jessop found it necessary to change the course of the river Avon, and to make two cast-iron bridges over the new channel. See Plate XClV. The span of the iron-work of each arch is 100 feet ; the rise 12 feet 6 inches, or 4. of the span ; the breadth is 30 feet ; and there are six ribs ; each rib is composed of two pieces meeting in the mid dle, and they are connected crosswise by nine cast-iron tics, which are dovetailed, and wedged into the ribs ; the cross sections of these ties are in this form T. The ribs stand upon abutment-plates, which are laid in the direction of the radius. These plates arc 32 feet in length, 2 feet 4 inches in breadth, and 4 inches in thick ness ; in each plate are five apertures, each 5 feet long and 20 inches in width. The ribs are 2 feet 4 incites in depth in the direction of the radius, and two inches in thickness, and have each 80 apertures, one foot square, separated by bars three inches broad, excepting oppo site the cross tics, where the solid is 12 inches broad. 'Where the ribs meet in the middle, they have flaunches eight inches broad and two thick, and they are connect ed by cast-iron screw-bolts three inches diameter. Be tween the ribs and the bearers of the roadway, perpen dicular pillars, with cross sections of this form T, are placed. The bearers are of the same form. The whole is covered with cast-iron plates, and there are railings of cast-iron.