Autobiography of Sir John Rennie, F.R.S., Past President of the Institute of Civil Engineers. Sir John Rennie
a powerful sun, now produce most injurious exhalations, might have been deprived of their baneful influence.
Having passed Sion, we left Brieg early on the 10th of August, and as soon as we began the ascent I descended from the carriage, and with line and rule I measured every bridge until we reached Boveno, on the Lake Maggiore. I sent off from Milan, as I had promised, a detailed account of this celebrated road to my father, giving a drawing and account of every work, which I afterwards had the gratification of knowing afforded him great pleasure. The whole Pass must have at first sight been appalling to the engineers who traced the line of road; and although many other works of the kind of greater magnitude have been since executed, nevertheless, all things considered, it is worthy the approbation of mankind, and does great credit both to those who designed and those who executed it.
We reached Milan[1] on the 12th of August, and I was much struck with the fine canal which unites it with Pavia, and which can only be compared with the canals of Belgium. It has been said that the first pound lock was invented and executed by the famous Leonardi da Vinci, but subsequent inquiries have induced me to believe that our own country is entitled to the honour in the Exeter ship canal. I was also greatly pleased with the system of irrigation employed generally throughout Lombardy. This system was originally introduced by the Italians themselves, and during the Austrian rule was carried to the greatest perfection, Lombardy being by nature peculiarly well adapted to it. The vast and fertile valley of the Po is for the most part destitute of rain during the summer, when it is most wanted; but it fortunately happens that at this season water is most abundant from the melting of the snows on the Alps, which descend into the adjacent lakes and rivers, and would be otherwise wasted and thrown away if not employed for fertilizing the land. The water, therefore, at this season is conducted by an elaborate system of artificial canals, and distributed over the adjacent lands according to their respective levels, at a certain price, varying with the quantity distributed. Thus the constant supply of water, the high temperature, and the fertility of the soil combined, produce the most abundant crops, and the plains of Lombardy are rendered the most productive and valuable in Europe; whereas in winter, when the temperature is lowest, the snow is congealed on the Alps, and comparatively little water comes down when it is least wanted.
From Milan we reached Verona, where the bridge, consisting principally of brick, with binding courses of marble, can boast of one of the largest brick arches in the world, an excellent example of what may be done with this material when properly handled. After passing Vicenza and Padua, we reached Venice early in September, 1819.
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,