Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers. Sir John Rennie
main ribs solid cast-iron wedges the whole depth of the main rib were fitted, then drawn home against the ends of the rib; by this means the ribs were kept firmly within their places, and as an additional precaution strong diagonal braces, having a strong feathering rib on each side, were inserted diagonally between the ribs from one end of the arch to the other, and secured to the ribs by projecting dovetailed cheeks on them, and wedges and bolts, so that these cast-iron arches were constructed in the same manner as a stone arch, being almost as it were a solid mass depending upon the equilibrium of the different pieces for its stability. The depth of the main rib of the centre arch at the crown is 6 feet and 8 feet at the piers, whilst the depth of the ribs of the side arches at the crown is 6 feet and 8 feet at the abutments. As these ribs with their attendant transverse frames and diagonal braces formed the main part of the arches upon which the whole of the superstructure depended, it was necessary that they should be extremely well put together and properly united to the piers and abutments. Contrary to the usual mode of constructing stone arches, they were commenced at the centre instead of the sides, a strong wooden centring supported upon tiers of piles having been previously constructed between the piers and abutments to support them whilst being put in their places. Each piece of each rib was carefully placed upon the centre, resting upon nicely-adjusted strong wooden double wedges, and connected together as they proceeded by the transverse frames and diagonal braces before mentioned. By this means the whole of the arches were constructed at the same time from the centre to the skewbacks or bearing parts of the piers and abutments; but in order to connect them properly with them it was necessary to devise a particular arrangement. For this purpose a transverse frame, similar to those already described for connecting the rib pieces together, was accurately imbedded and fitted to the skewbacks or bearing places on piers and abutments, resting on a bed of sheet lead, and the joints were filled in with melted lead also; this formed a solid and to a certain extent elastic bearing, upon which the main ribs were ultimately to rest.
At the ends of the arched rib-plates next to the piers and abutments there was another transverse frame plate of the same kind as those previously described, and fixed there in a similar manner; this brought the ends of the arches within 6 inches of the abutting or bearing plates fixed in the skewbacks or springing places of the piers and abutments.
Between the frame plates fitted on the skewbacks or masonry of the piers and abutments, and those fitted on the ends of the rib plates of each arch, solid cast-iron wedges, 9 feet long and 6 inches thick at the back, and 2 inches thick at the bottom, 9 inches wide, three being behind each rib, were accurately fitted by chipping and filing, so that it would slide down to within 12 inches of the bottom; when these wedges were all accurately adjusted at the same temperature to the same depth, they were simultaneously driven home by wooden rams to their full depth, so as to reach about an inch below the bottom of each rib; by this means the whole of the three arches were gradually brought to their bearing without being raised wholly from their centres. Matters were then allowed to remain in this state for a few days in order to give time for every part to come to its bearing and to ascertain whether there was any defect in any part.
After the minutest search in every part no defect could be discovered; the wedges between the centres and the under sides of the ribs were then gradually slackened until the whole of the arches came to their full bearing, and were removed entirely, leaving the arches perfectly free of support. During the whole of these operations, from first to last, which occupied about a week, not the slightest accident or fracture occurred; the total subsidence of the main arch barely exceeded 2½ inches, whilst the subsidence of the two side arches barely exceeded 2 inches, which had been allowed for in the construction.
In order to ascertain the effects of expansion and contraction of the arches by the variation of the temperature of the atmosphere, I constructed steel, brass, and wooden gauges, accurately divided into decimal parts of an inch, and erected them upon different parts of the centres, where the effects were most likely to be apparent, and I kept the register for several weeks, during the height of summer, autumn, winter, and spring. I found that the variation in the rise and fall of the crown of the arches, the abutments being fixed, was 1/10th of an inch for every 10° of temperature, so that, taking the extremes of temperature at London to be 10° below freezing point of Fahrenheit in winter, and 80° in summer; the utmost rise and fall of the arches may be taken at 7/10ths, or at most one inch; but as any variation in the temperature, unless continued for some time, has no sensible effect upon such a large mass of iron, so, in our variable climate, the rise and fall of the crowns of the arches may be taken upon the average somewhat below the amount above given.
After the arches had been brought to their bearing and had been relieved from the centres, the superstructural framework was carried up and firmly connected and bracketed together by diagonal ties and wedges; in doing this the ends of the superstructural frames were too tightly wedged to the masonry of the piers, without my knowledge, so that they would not allow the main ribs of the arches to play freely, and some of the masonry courses above the main ribs were slightly splintered and deranged; the wedges were then slackened, and some of them removed entirely, and thus the evil was immediately remedied; the whole structure has ever since remained in a perfect state.
The bridges and approaches were finally completed and opened to the public traffic in March, 1819, the ceremony being performed by Sir John Jackson, the chairman, the directors, and a few friends.
In the month of August, 1818, having worked very hard, I may say almost night and day, for some time, I was nearly worn out, and was permitted to have a short holiday. I therefore determined to go to Belgium and visit the celebrated field of Waterloo, which closed the long and eventful revolutionary war, and attracted the admiration and interest of the whole civilized world. I accordingly started for Dover, in company with my old friend, the late Mr. Joseph Gwilt, architect, and crossed over to Calais.
On landing we repaired to Dessin’s hotel, at that time one of the best in Europe, and rendered famous by Sterne, whose rooms are still shown as one of the most interesting curiosities (to Englishmen at least) of the place. Here we passed the remainder of the day very agreeably. Everything was new to us—the people, their language, their manners, their mode of living. We had been so long considering the French as our deadly foes that we could hardly believe ourselves to be at peace with them, and to be actually in France and so civilly treated by them. Then the living was so good and cheap, compared to that of England; champagne, which with us was considered the greatest luxury, and only within reach of the highest and most wealthy, was here obtainable for four francs a bottle, whilst in England it was twenty-four, and almost everything in the same proportion. In the town of Calais there is nothing to see. The harbour is but indifferent, and almost dry at low water, and is chiefly maintained open by sluicing and dredging. On such a flat, sandy shore, with the prevailing winds and currents always driving on and accumulating the sands, it is very difficult to make and maintain a good harbour; still I think that a great deal more might be effected by pursuing a different course, and having a proper system of open piers and breakwaters on the outside. After breakfast on the following day we started in a travelling calèche, with two horses, for St. Omer, about 26 miles distant. On our way we stopped to examine the celebrated quadrangular bridge, called the Pont Sans Pareille, across the junction of the two canals of Picardy. This is certainly a meritorious, well-executed work, but the idea is by no means new, as the Gothic triangular bridge of Crowland, in Lincolnshire, across the junction of the Welland, was executed many centuries previous to the Pont Sans Pareille, and is still extant in a perfect state. Near this bridge is the celebrated maiden fortress of Ardres, which is said to have never been taken. Here also, according to Froissart, was the Champs d’Or, or Field of the Cloth of Gold. Ardres is a poor, miserable little fortress, surrounded with earthworks and ditch, on a flat plain. In ancient times it might have been formidable, with their means of attack, but now it would be utterly defenceless. From Ardres we passed through St. Omer and Lille, and after visiting Tournay and Ghent we proceeded to the capital of Belgium, where we took up our quarters at the Hôtel de Flandre, in the highest and best part of the city. We were particularly struck with the magnificent Hôtel de Ville and its lofty spire; also the remarkable place or square in front of it, which, looking to its picturesque mediæval buildings and the remarkable historical events which have taken place there, renders it one of the most interesting in Europe. The fine old Gothic cathedral of St. Gudule, the museum, fine canals,