The bottom of the beach, descending by a very gentle inclination, will, moreover, have the effect of abating the waves, and diminishing their action against the jetties. This is a well ascertained fact, and one which may, indeed, be easily conceived; for, supposing that the bottom of the sea, from a depth below the limit of the motion of the waves, rises by an extremely gentle slope, until it meets that limit; this meeting taking place at a very small angle, the bottom will be almost insensibly substituted for the limit of motion.
At the point of this meeting the undulating motion is nil, it is very feeble at the adjacent points, and easily abated by the resistance and friction which the molecules experience against the bottom. The abatement will thus extend vertically up to the surface, and the waves will then gradually diminish in volume as they approach the shore.
We have, therefore, adopted the system of loose stones for the jetties and for the mole, with but slight modifications suggested by our own experience, modifications which consist in making the jetty-heads in hydraulic masonry to a certain height, as well as the interior surface of the windward jetty, which is to serve for the towing of vessels.
What we have said of the gulf of Pelusium we may repeat, still more forcibly, with regard to the roadstead of Suez. The sands have long ceased to accumulate in any perceptible manner. And if maritime alluvium is still brought up by the ground swell and the current, it is driven by the west and south-west winds on to the eastern shore, without reaching the extremity of the gulf. In fact, the plan of the roadstead was taken in 1799, and the soundings of the channel are marked, as well as the shape of the sandbank, which forms a kind of bar at its extremity towards the roadstead. In 1847, the plan was taken again with the same soundings, and it is impossible to find the least difference between the two results, which also agree with those given by Commander Moresby, in his excellent chart of the Red Sea.
There is, then, nothing to fear on that side, either from the sand or the violence of the sea. The jetties will be of the simplest construction, and as the materials are, as it were, at hand, their erection will present no difficulty.
With regard to the excavation of the Canal to a depth of 6 m., 50 below low water in the Mediterranean, in a very porous soil, the task, at first sight, presents what appear to be considerable difficulties. We cannot, indeed, hope to accomplish the whole of the excavation in the dry, or by pumping, on account of the nature of the ground. It will be necessary, then, for all that portion below water, to have recourse to dredging; and, as the quantity of earth to be removed by this means is 57,205,342 cubic metres, at first sight it is difficult to conceive how the work is to be accomplished. But, upon examining the matter more closely, nothing is found to frighten the most timorous. In fact, a single steam dredging machine, of twenty horse power, such as those which have been employed on the Nile, working night and day, can, in twenty-four hours, raise 1000 cubic metres of sand, from a depth of seven metres. According to this calculation, and supposing the year to consist of only 270 working days, it would take forty dredging machines five years to complete the labour; but if, instead of such small machines, dredges of thirty to thirty-five horse power were adopted, it would be easy to raise 1500 cubic metres per diem, and the dredging would be more economical.
The quantity of earth to be raised by manual labour amounts to 17,473,790 cubic metres, and the deepest cutting does not exceed 10 metres above the water. This is a small matter when compared with the earth-works performed in many canal and railway undertakings, and even with those accomplished before the present century; such, for example, as the one mentioned by Michel Chevalier, in his investigations concerning the maritime canal of the Isthmus of Panama (Recherches sur la Canalisation maritime de l’Isthme de Panama).
“It required,” says he, “the treasure which the Viceroys of Mexico had at their disposal to undertake the cutting at Huehuetoca, the total length of which is 29,585 met., with a depth of from 45 to 60 met., for a length of 800 met., and from 30 to 50 met. for 3500 met. The expense was 31,000,000.”
Farther on, he adds:—
“Nowadays, however, in a case of necessity, by displaying the improved appliances at the command of engineering art, it would be possible to effect cuttings of great depth, and to remove large quantities of earth at no extraordinary expense. On the Arles canal, at Bouc, for instance, the plateau of the Lecque was cut through by a trench 2100 met. in length, by 40 to 50 met. in depth, at the culminating point. The expense was under 4,000,000, and yet the cutting was performed by the old method. In cuttings of magnitude, the soil is now broken by instruments of enormous power, and the earth is removed by means of railways and locomotives. All that has to be done by manual labour is to collect the loose earth and load the waggons. For so important an object as the uniting of two seas, even the impossible might be attempted.”