Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers / Comprising the history of his professional life, together with reminiscences dating from the commencement of the century to the present time.

by Sir John Rennie

The extraordinary and at the same time most beautiful and novel appearance of the city, with numerous towers and spires, about which I had read and heard so much, and had so long wished to see, now stood before me, and its loveliness more than realized my most sanguine expectations. When I considered its origin, a few fishermen’s huts built upon the mud banks of the lagoon by men flying from the invasion of Attila, then the rise of the great republic whose wealth, conquests, and influence were destined hereafter to play such an important part in the world, and lastly the fallen and degraded state in which it then presented itself before me, I was lost in astonishment; I was for a while transported as it were in a dream, and could scarcely believe where I was.

The Grand Canal first attracted our notice,—perhaps there is no thoroughfare in the world lined with so many magnificent palaces,—and along which we passed until we came to the Rialto, a drawing of which I made and sent to my father. But if I was delighted, and I may almost say astonished, at the Grand Canal, I was still more so with the Place of St. Marc and its surrounding buildings, so varied in their architectural styles, yet each so picturesque and elegant in itself, and combined together forming at once the most interesting and beautiful scene of the kind in the world. It is one of those sights, at least speaking for myself, that never satiates—the more I looked the more I admired it. As to the details of these different buildings which we saw, they are so much better described in the numerous guide books that it is unnecessary to repeat them here.

I visited every part of the lagoons, including the various islands, all of which are more or less deserving of notice, particularly the island of Murano, the seat of the celebrated glass manufactories; and also the Moravian establishment. But what really most interested me were the lagoons, and the means which must have been resorted to for keeping them open, notwithstanding the numerous causes which were and are constantly in operation to fill them up with the alluvial matter brought down from the mountains and plains by the various rivers and streams which discharge their waters into this portion of the Adriatic, also from the alluvium brought in from the adjacent shores, by the tide, which rises from 2 to 4 feet, and at times, during heavy gales from the southward, as much as 6 feet, overflowing the quays of St. Marc’s Place.

There was a long-continued discussion amongst the numerous distinguished mathematicians, engineers, and others who have written upon this subject, as to the best way of preventing the filling up of the harbour. Some contended that the only method of effecting this was to admit all the rivers into the lagoons freely; for although they might deposit a certain amount of alluvial matter, nevertheless the great quantity of water discharged would alone be sufficient to carry away this deposit. But they forgot that when the rivers met the sea the current would necessarily be checked and rendered powerless to carry forward any matter which might be held in suspension, and that consequently the detritus would be deposited and form banks and shoals which the waters could not remove; thus in time the lagoons would be filled up, grass marshes would be formed, the city of Venice would be united to the mainland, and the harbour would be destroyed. On the other hand, it was argued, for the reasons above mentioned, that the only way to preserve the lagoons and the port of Venice was to exclude the rivers when densely charged with alluvial matter, and only to admit their waters at certain times, when they were comparatively clear; thus all the advantage would be obtained from the scour of these rivers, without the disadvantage arising from their deposits. Ultimately the arguments of the latter prevailed, the rivers were excluded from the lagoons by making a capacious canal all around them with sluices at their mouths, by means of which the waters were discharged into the lagoons when they were tolerably clear of alluvial matter; the surplus waters were discharged into the adjacent sea clear of the lagoons, and any alluvial matter which was brought in from the sea was removed by dredging from the main channels of the lagoons, so that they were in a fit state to admit the tidal waters and thus to keep the lagoons open.

But there was another important agent to be provided against, namely, the alluvial matter brought in by the winds, waves, and currents from the scouring of the adjacent shores of the Adriatic; this is done to a certain extent by dredging. Originally these banks contributed materially to the formation of the outer banks, which protect the lagoon on the sea side. If these banks were broken through or completely swept away, which the storms of the Adriatic frequently threatened to do, the lagoon, and with it the port, would be seriously injured or totally destroyed. This was remedied by defending this outer barrier bank of the lagoon by facing it with stone, and where the sea was most violent by constructing a solid breakwater of stone, and protecting it further by stone filters carried out a sufficient distance into the sea in order to divert the current, and to enable the alluvial matter to be deposited between them so as to form an additional protection to the main breakwater; this was accordingly done, and thus an extraordinary work at great expense has been constructed between Lido and Malamocco, the principal entrances from the sea to the lagoon, for a length of four miles, where the effects of the sea are greatest.

Malamocco is the principal entrance for large vessels, and the channel from thence to Venice has been deepened, chiefly by dredging, to the extent of 24 feet at low water. Lido, which is the next chief entrance and the nearest to Venice, being about 1½ mile distant from it, serves for the general class of merchant vessels. The other entrances of Foggia, Tre Porte, and the Piave, are seldom used except for fishing vessels, and it is not necessary to do more to these than to keep them in their present state, that is, to prevent deterioration, as it is an object of importance to allow the great mass of water by which the lagoon is chiefly preserved to pass in and out of the main entrances, Malamocco and Lido. Upon the whole it appears to me that this latter plan is the wisest that could be adopted, and the result has proved that it has been so far successful, although attended with considerable expense. It is in fact a choice of two evils, and the least has been chosen. It is, I believe, admitted that the port of Venice is now capable of receiving as large, if not larger, vessels than she ever received before; for it should be recollected that in the most flourishing times of Venice there were no ships drawing 23 or 24 feet, and vessels of this size can now enter and depart at all times. Hence Venice has been converted into a port fit for modern requirements; but it must always be borne in mind that so strong are the natural obstacles against its maintenance that nothing can preserve it in its present condition but the most constant vigilance and care. Fortunately the method of dredging by steam has been introduced, and this may be done to any extent, at a comparatively moderate cost, but it can only be compensated for when there is a sufficient amount of trade to pay for it. Still, in whatever way we may consider the question, it must be admitted that the port of Venice has been preserved in a most extraordinary manner during so many centuries, notwithstanding the natural obstacles against it; and now that it has been connected with the kingdom of Italy, there is no further drawback to its full development, and it only requires self-reliance and energy to render it what it once was, one, if not the most important, of the commercial cities of Italy.

At Bologna we had the pleasure of making the acquaintance of the celebrated linguist Mezzofanti, whose modest and simple manner, accompanied by his extraordinary acquirements, quite enchanted us. The singular fact is that at the time of our meeting him he had never been out of the province of Bologna, and yet he had acquired the knowledge of twenty-four languages, and, as far as our limited acquaintance went, he spoke the English, German, French, Greek and Latin tongues perfectly; and those conversant with the Oriental languages informed us that he spoke them equally well. He appeared to be completely absorbed in languages, and was scarcely acquainted with any other branches of knowledge; still his wonderful mastery of this branch of study was a great acquirement, and must have required vast powers of memory as well as indefatigable study.

From thence we passed through Ravenna to Ancona, the position of which is good, occupying as it does a salient point of the coast. The water is deep and there is a commanding height for a citadel. The Roman emperors resolved to take advantage of the situation, and built a town here, the place being well adapted for a seaport. They accordingly made a mole on the south side to protect the harbour against the most dangerous winds, namely, those blowing from the south; they commenced the work from the shore by throwing down large blocks of rough stone, which were obtained from the rocks in the vicinity, in order to form a base for the superstructure; these blocks of stone were deposited promiscuously in the sea and left to the action of the waves, which in a comparatively short time consolidated them until they formed a mass that at length became immovable, always adding more stone as required until the whole became solid; this foundation was carried up to the level of low-water mark. They then commenced the superstructure of masonry, of squared stone and brickwork cemented together by pozzolana or hydraulic mortar, which was best adapted to make the work permanent. The inside or quay wall was formed by first placing a close framework of timber in front and at the angle or slope at which the quay wall was to be formed. They then threw down a mass of pozzolana, lime, and small rubble stone, mixed together, between the wooden frames and the rubble stone which had been previously deposited to form the base of the mole. This in a comparatively short time become solid, as the mixture of pozzolana, lime, and stone possesses the quality of setting under water. When it had become sufficiently solid, the timber frame was removed and transferred to another section to form a continuation of the quay wall, and thus the whole line of inner quay wall which formed the roadway was made by backing the quay wall to its full height with rubble, and a parapet of masonry was erected on the outside; this superstructure, although rough, stood very well with occasional repairs. In this manner the mole was carried out to a great extent. A fine triumphal arch was erected at the end to commemorate the completion of the work.

It is singular that the same emperor should have constructed a like work in a similar situation and in the same manner at Civita Vecchia, on the opposite coast of Italy, after every attempt had been made to establish a port by his predecessors at Ostia, which was built at extraordinary expense, and has been filled up by the alluvial matter brought down by the Tiber and from the sea by the littoral currents, so that it is now three miles from the sea.

The principle of construction used in these works resembles a good deal the mode adopted by the Phœnicians at Tyre, and subsequently by the Carthaginians at Carthage, and by the moderns. The Romans also employed the hollow mole, that is, a mole constructed with arches, by means of which the current charged with alluvial matter was enabled to pass through the mole, and thus any deposit within or without the mole was to a great extent prevented. The harbour of Civita Vecchia remains serviceable at the present day for vessels drawing 20 feet of water, although, as may be naturally expected, a certain deposit has taken place during the lapse of so many centuries which requires to be dredged out occasionally. At Ancona only one pier was built on the south side, and consequently an eddy and stagnation took place on the north side, as the littoral current runs from south to north, and therefore there is a tendency to deposit on the north side. The French when masters of Italy commenced another mole on the north side, thus enclosing a considerable space of sea so as to form a close harbour, which no doubt is of great service; still, from the nature of the local circumstances, a certain deposit may always be expected. This, however, can be removed by dredging, and the harbour may always be preserved in a state of efficiency if the extent of trade will warrant the expenditure necessary for the purpose.