Chap. VIII

BRITAIN.

477

Fig. 480.

I think that they almost all act like the rafters,da, b e, of a mansard or kerb roof from whichthe middle tie-beam has been taken away. Thecrown breaks in by the opening below and crush-ing above, and it springs at some intermediatepoints, by the joints opening there on the upperside. Sir Christopher Wren considered an archentirely in this way, and makes no use what-ever of the mathematical theory of cymlibration, although then carefully studied by thegreat mechanicians. The drawings do not clearly show how the different arch-stonesor frames are united, yet plainly point out something like joggles in one frame fittingconcavities in the one adjoining, by which they are prevented sliding on each other. 1highly approve of this plan, because it connects at least half of this rib of frames : as astraight line can be drawn through, causing it to act as one rafter, I am confident that thisring alone, if con-sidered as consistingof six pieces, the jointsof which a b c arcconnected with theroadway by uprights,a a', bb', and trussrafters, a y, fy, ah, bhabutting on the arch,would make it firmenough, stiff enough,and a great deal lighter

titan by the plan proposed. But I do not mean to prefer this method, because, although sixpoints of bearing may be perfectly sufficient, it is far preferable, with so brittle a material ascast-iron, to make the bearings as numerous as possible, that no one point may be muchmore strained or compressed than another; yet I think that this is overdone in the pro-posed plan, and that it might be much lighter in the haunches.

I should certainly construct the main or lowest ring as an arch of equilibration, makingeach of its frames bear on its neighbour, as a stone in an arch of masonry. I think that anycement would be inefficient, and would be hazardous in the extreme; I know no practicablecement whose cohesion will be of any significancy, none that will not be a little compressedby the enormous horizontal thrust of near 10,000 tons. The consequence of the compres-sion, by the arch taking its set, will be almost certain destruction, as I shall show by-and-by. The frames should all abut on each other with perfect accuracy. If they are tohave this form, the radial joints must all be ground on each other lengthways, so thatthey may touch all over; should cement be put on such a joint, the smallest hard bitabout the middle would cause the piece to snap without remedy. Grinding will procurea full contact and bearing, therefore the joggles should be loop pieces, like pound balls,that the grinding may be practicable. Perhaps this form may be still more secure againstsnapping, and not unsuitable to the style of ornament, which is very pure Gothic. Iown that I prefer Mr. Burden’s construction of the ring, as the most susceptible of ac-curacy in the execution, the firmest union, by means of the wrought-iron straps, andperfectly free from snapping. I foresee immense difficulty in casting everything true, soas to have the radiated joints all straight, and all bearing, and for this reason am disposedto prefer a construction with loose pieces, butting endways, like rafters. The differentexpansion of different fonts of iron by heat will have some effect here; whenever a jointseems open, or not in an exact line with the rest of that radial joint, it should be filled upwith a plate of good copper. A forged iron plate would perhaps exfoliate with the weather.

1 look upon this circumstance of accurate joining as the most important of all in thisparticular construction, and I apprehend that it was this difficulty that induced Mr. Burdonto fill up the haunches of his arch at Wearmouth with a series of circles, which servemerely to prevent the main arch from rising in the haunches, and to support the roadway.But if it can be attained, the bridge will be both stiffer and stronger.

That the hazard from the compression of cement or the closing of bad joints may clearlyappear, we must now consider the weight of the whole, and the different thrusts excited inits different parts. I consider it as two masses resting on the abutments, and leaning oneach other in the middle ; I suppose the carriage-way to have 2 feet of gravel, each cubicfoot weighing 109 pounds. The footpaths may be paved hollow with free-stone, so as notto require more than 18 inches in thickness. One half of this, including the increasingwidth at the ends, will weigh about 1800 tons ; this added to 3250 tons of iron workmakes 5050 tons. I think that its centre of gravity is situated at nearly two-fifths ofthe halt-breadth of the arch from the abutments, that is, at about 120 feet. We have,therefore,