Part 5
We have seen that the winds and the currents carry the detritus of the coasts reduced to sand to great distances. The currents however do not arrest the motion of the waves and the ground swell; they bend to their forms, and as their direction necessarily tends to the shore, the sands clear the currents with the ground swell which contains them, and which thus conducts them to the shore. When the direction of the waves is oblique to the coast, the sands are borne to a distance, but when it is perpendicular to the coast, the waves raise the sands brought by the ground swell into dykes and banks, which protect the low shores. The most minute and lightest portions are accumulated at the more elevated points of the flats, where, being dried by the sun, they are soon carried away by the wind, which leaves them, in its turn, in the shape of downs. The ground swell, therefore, furnishes the materials of those downs, which usually border flat shores, and it is that which has drawn from the depths of the sea the sands of those immense deserts of Africa, and of so many other plains which are found in various parts of the globe.
“Often,” says M. Jomard, “have I remained for whole hours, contemplating the origin and progress of the phenomenon of the formation of sands. I saw the waves break and deposit a small line, scarcely discernible, of very fine sand. Another wave came, burdened like the first, and this new line of sand pushed the first slightly on. This, once beyond the reach of the water, and exposed to the rays of a burning sun, was quickly dried, and became the prey of the wind, which immediately seized and carried it off into the air; the less light particles of gravel did not reach so far, but, subjected to the alternate motion, diminished more and more, and were converted by degrees into sand.”
We may also say with Colonel Emy, that
“All river bars are deposits, brought or arrested by the ground swell, and without it these deposits would be repelled into the main as far as the rivers extend their course. The Delta of the Nile, those of the Mississippi, of the Ganges, of the Scheld, of the Meuse, of the Rhine, and the Camargue of the Rhone, were originally bars formed by this same ground swell.”
The tongues of sand which separate the lake of Thau from the gulf of the Lion, the tongue of earth upon which Alexandria is built, those which separate the lakes Bourlos and Menzaleh from the Mediterranean, are bars of sand formed by the ground swell. The sand bank which separates from the Red Sea the vast basin of the Bitter Lakes, was, without any doubt, a ford elevated by the ground swell, which, in tempestuous weather, ascends this sea with the current of the tides charged with sand. The ford, which answers at the present time at Suez, was certainly formed in this manner by the ground swell.
We may say also, that the whole Isthmus of Suez was formed by the maritime deposits of the Mediterranean and of the Red Sea. We believe that, previous to historic times, the two Seas were in communication with each other, that the detritus of the chains of mountains situated to the right and left, carried down by rain, filled up the space which separates them, and that when that space was elevated to such a height that the ground swell could reach it, its action was applied in such a way that by the meeting of the swell of the two Seas, a bank was formed, which is no other than the bar of El Guisr. After the formation of this bank, the combined action of the ground swell, both on one side and the other, and the accretions from the neighbouring mountains continued until the Isthmus was dry. Then the soil thus constituted was covered by the downs, which advanced upon it from the direction of Pelusium, driven by the north winds, and from the direction of Suez, driven by the winds and currents from the south.
In this state the Isthmus is at present, and the numerous soundings which we have asked for from His Highness the Viceroy, will prove whether our hypothesis is well founded or not.
The same theory may, as Colonel Emy observes in his remarkable work, throw a new light on important geological facts:—
“For instance, those ancient and elevated plains, composed of sand and pebbles, the formation of which, it has been attempted to explain, by the revolutions of the globe and violent convulsions of nature, or which have been regarded as deposits left by rivers, appear to be maritime accretions. If is, indeed, easy to conceive rivers capable of bringing down the fragments detached from mountains, by shocks, and by the decomposition of the rocks; but how could they extend those fragments uniformly, and over spaces so extensive as the plains in question? Moreover, was not the course from the summits of the mountains generally too short for the fragments of the excessively hard rock found in some of those plains, to have time to acquire their roundness? The rivers have prolonged their courses through these accumulations of pebbles; they may, in overflowing, have covered them with sand and earth, but it is more probable that they contributed in nowise to the formation of these accretions, unless it were by transporting the rough materials to the sea. Nothing but the ground swell could spread these fragments of mountains so uniformly as they are, convert them into shingle and sand by a long trituration on the shores, where it had driven them; gather them either into banks or plains, and thus fill up spaces over which the sea formerly extended.
“The ancient collections of shingle, pebbles and sand are owing, like those at present forming in a similar manner, to maritime accretions, and must henceforth be regarded as an incontestable proof that the ocean formerly reached and was long stationary at different heights far exceeding its present level.”
It is not surprising, then, to find on divers points of the Isthmus pieces of hard stone broken into small fragments, and half rounded, covering the sand-banks at variable heights above the level of the Mediterranean.
But, be it as it may, it is certain that throughout the length of the line, from the roadstead of Suez to that of Pelusium, the excavations will only be in light earth, which can be easily removed by hand as far as the water line, and with dredges down to the bed of the Canal.
The track which we have followed for the Canal was prescribed by the very nature of the locality, and by the condition that the two Seas were to be brought into direct communication in the most economical manner.
The line begins at the roadstead of Suez, turns to the east of the town, making a curve to reach the ancient track, which it leaves to the west, and follows the channel of the valley until it joins the Bitter Lakes, which anciently formed the extremity of the gulf of the Red Sea. It traverses those lakes throughout their length, following their sinuosities, so as to avoid the inequalities of the ground. On leaving the lakes, the line crosses the bar of the Serapeum, at its lowest point, and enters Lake Timsah, leaving the heights of Cheik Ennedek to the east.
The last-mentioned lake is to serve in the formation of an inland port, in which ships may be revictualled and repaired, while it will be the point of junction between the Maritime Canal and the Canal communicating with the Nile.
In traversing this lake, the line forms several curves, in order to avoid the extensive downs which have encroached upon a part of that region.
On leaving the lake, the line proceeds to the bar of El Guisr, at its lowest point, and then goes towards Lake Menzaleh, which it follows directly along its eastern shore as far as Pelusium, and is prolonged into the sea until it reaches a depth of 7 _m._, 50.
The dimensions of the Canal have been determined by the idea of creating a grand passage for maritime navigation, open to steam and sailing vessels of considerable burthen. The Caledonian Canal is the only known analogous work. This Canal, however, is but 37 _m._ broad at the water line, and but 6 _m._, 10 deep. The locks, to the number of 23, have been enlarged so as to admit forty-four gun frigates; they are 52 _m._, 40 in length between the gates, 13 _met._ in breadth, and have a depth of water of 6 _m._, 10.
For cutting through the Isthmus of Panama by a maritime canal, as projected by Mr. Garella, it was proposed that the width of the canal, at the water line, should be 44 _met._, and the depth of water 7 _met._
Prince Louis Napoleon, who, in 1846, published a remarkable work, inserted in the _Revue Britannique_, under the title of _Canal de Nicaragua_, adopted the same dimensions as Mr. Garella, in the project which he proposed to execute for establishing the communication between the Atlantic and Pacific Oceans.
We have assumed on considerations hereinafter to be explained, that paddle and screw frigates as well as vessels of 1000 to 1500 tons, ought to be able to traverse the Canal in order to satisfy to the fullest extent the demands of navigation. We have therefore fixed the width of the Canal at the water line at 100 _met._; its _minimum_ draught of water at 6 _m._, 50, below low water in the Mediterranean. The locks, two in number, are to be 100 _met._ long, 21 _met._ wide, with a _minimum_ depth of water of 6 _m._, 50. These works will be established at the two extremities of the Canal, immediately before the dykes forming the channel which on each side unites the Canal with the two Seas. These two locks will form part of a sluiced barrage, and thus convert the whole Canal into one immense dam, receiving the waters of the Red Sea during the highest tides, and storing them up successively in order to raise the level and create a rush of water in each channel when necessary. The highest tides of the Red Sea being from 2 _met._ to 2 _m._, 50 above low water in the Mediterranean, a depth of 9 _met._ of water will be obtained in the canal at certain times, but a mean super elevation of 1 _m._ may be depended on, which will usually give a minimum depth of 7 _m._, 50 to 8 _met._ Under these conditions, screw steamers will be enabled to pass easily along the Canal without the presence of its bed re-acting in an inconvenient manner on the motion of their screws. We have, however, calculated the earth-works for three different depths of water, viz. 6 _met._; 6 _m._, 50; and 7 _met._ below low water in the Mediterranean. If the Company should require a depth of 8 _met._, it would be easy to obtain it by means of dredges, without stopping the navigation on the Canal.
The length of each barrage, including the lock, is 100 _met._; and in order farther to facilitate the entrance of the rising tide into the Canal, a third barrage has been added at Suez on the site of the existing channel. This last work will be separated from the first by a platform raised above the level of high water, so that the two together will unite the road from Cairo passing by Suez to Mecca. Its length has also been fixed at 100 _met._
For reasons of economy the width of the Canal has been reduced to 65 _met._ wherever the height of the ground reaches 6 _met._
To prevent the degradation of the banks of the Canal, the slope has been fixed at two on the base to one in height, and it is proposed to have a causeway 2 _met._ broad to receive, 1st. A covering of the broken stones found along the Canal, 2nd. Any earth falling from the higher grounds, which would otherwise encumber the bed of the Canal. This is only an imitation of what has proved so successful on the Caledonian Canal.
The width of the towing path has been fixed at 4 _met._, which is quite sufficient for a maritime canal where steam towing will be so much in use.
Lake Timsah, situated nearly midway between the two Seas, at the entrance of the _Wady Tomilat_, will form, as we have said, an inland port, to which both the outward and inward navigation will tend. On its shores will be established magazines, stables, workshops for repairs, as well as 1500 _metres_ of quay walls for mooring vessels and embarking merchandize. For, as the illustrious author of the work on the canal of Nicaragua well expresses it, the proposed Canal must not be a mere cutting destined solely to form a passage from one sea to the other for the produce of Europe, but it must make Egypt a prosperous state by enabling her to dispose of her interior produce, and a powerful one by the extent of her commerce.
As for the two entrances, whether from the Red Sea or the Mediterranean, all that is necessary is, that ships shall be able to approach at all seasons, and find certain and effectual shelter in bad weather. Now the roadstead of Suez is sheltered from every wind except the south-east. It will therefore be sufficient to prolong the eastern jetty to a certain distance beyond the western to render the shelter complete.
All the vessels which now take their stations in the roadstead ride out the bad weather very well, and the magazine corvette belonging to the English Company which has been anchored there for the last two years and a half has suffered no damage.
Thus, at the Suez extremity, it will be sufficient to establish two jetties, forming the entrance channel from the Red Sea, and to prolong them sufficiently far into the roadstead to reach the required depth of water, in order that vessels entering may have a draught of 7 _m._, 50 to 8 _met._ at low water. The eastern jetty must be 150 _met._ longer than the western for the reasons we have just given.
At Pelusium, the two jetties, in order to reach the depth of 7 _m._, 50 to 8 _met._ must be at least 6000 _met._ in length; but if it should be feared that the channel thus formed would not be sufficiently safe for the approach of vessels, and in order to meet objections, the real value of which have yet to be tested, we have projected a sheltered roadstead in front of these jetties by means of a grand mole from 450 to 500 _met._ in length, placed in such a manner as to afford shelter to vessels in bad weather, and to enable them to enter the channel at their convenience.
At all events no one can doubt that the Canal would be really and practically navigable for all vessels willing to avail themselves of the passage. But it will be asked whether jetties extending 6000 _met._ into the sea do not present great difficulties; whether a trench of 65 _met._ in width, dug 16 _m._, 50 deep, a part of which is under water, is not an impossibility; and whether, supposing the engineering difficulties to be surmounted, the results obtained would be in proportion to the expenses incurred. Doubts have also been started on the navigation of the Red Sea; finally, several authors have put the question, without however solving it, whether, even if the Canal were once established, commerce would not prefer the old way by the Cape as the safest and most advantageous.
These questions we are about to examine: these doubts we shall endeavour to clear up.
The Gulf of Pelusium is said to be constantly filled with sand or mud brought down by the Damietta branch of the Nile, and it is objected that the advanced works to be established on that part of the shore would only have the effect of increasing the accumulations. We admit that this portion of the Egyptian shore has been formed by maritime alluvium brought by the ground swell, as we have already proved at the commencement of our memorial. We also admit, that the object of the dykes forming the entrance channel to the Canal, would be to stop the sand thus brought by the waves, and to accumulate it against the dyke opposed to the prevailing wind, namely, against the western dyke.
But most of the ports already in existence are open to the same objections; and if they were sufficient to prevent the construction of a port, we may safely say that very few of those we are at present acquainted with would ever have been formed.
According to our idea the essential question is, to know whether, when once the port is established, it can be maintained without too great an expense.
Now it appears, that for many ages the sands have ceased to extend the Pelusiac shore, as is manifest from the well ascertained position of Pelusium, the ruins of which still remain. Strabo, in his Itinerary, says that Pelusium is situated at the distance of twenty stadia from the sea. The French engineers of the expedition have verified this distance, by measuring 1600 toises, or 3000 _met._ from its remains to the shore.
In 1847, the distance between these two points had not varied, as it is marked on the plan with the figure 3000 _met._, and at the present day it is still the same.
In fact, by reading all the accounts of ancient authors, and comparing them with what actually exists, we arrive at the conclusion that the shores of the Delta have varied very little in historic times.
The sea sands then have long ceased to accumulate, and the fact may be explained by assuming that the destruction of the coasts of Morocco, Algeria, Candia, and other parts,—which destruction, we repeat, alone furnishes the materials of maritime alluvium, —has abated from some cause or other. It may also be assumed, that the sands which were formerly driven by winds and currents into the Gulf of Pelusium, are now cast on the African coast between Tripoli and Alexandria, and driven inland in the shape of downs. The fact is, that no new downs are now seen forming in the Isthmus; those on the seashore being of ancient formation, and nearly all naturally fixed by vegetation. In conclusion, the extension of the Pelusiac shore, if such extension there be, is too insignificant to be taken into consideration.
Now, the direction of the jetties being nearly perpendicular to the shore, in order to be at right angles with the prevailing wind from W. N. W., the sand, when the wind is perpendicular to the shore, will be driven on to the coast and increase its height, as hitherto, no change being occasioned by the jetties. During the parallel winds, which mostly prevail, the littoral current, finding an obstacle in the jetties, will form an eddy to windward, which will increase the force of the current between the points of the jetties and the mole, so that the sand will be carried far away; and the probability is, that the bottom will become deeper.
It is only the oblique winds then, that will carry the sand into the angles formed by the shore and the windward jetty.
In calm weather, the sea-current which flows along the coast from west to east has not sufficient force to affect the equilibrium of the beach. Thus, to sum the matter up, the most that can be feared is the accumulation of a small quantity of the loose sand in the Gulf at the angle of the windward jetty. Supposing that even 10,000 _cubic metres per ann._ should be so deposited, which, according to what we have said, is an exaggeration, it would take 100 years to advance the beach 400 _metres_, and such an advance would produce no perceptible effect at the extremities of the jetties.
It may be objected, that by all these movements of the sands, some portion will necessarily find its way into the channel, and thus, by degrees, end in obstructing it. To obviate this inconvenience we have at our disposal dredging machines, and the most powerful means of clearance derived from a mass of 700,000,000 _cubic metres_ of water, which can be stored up, above the level of low water in the two Seas, throughout the whole extent of the Canal, and in the immense reservoir of the Bitter Lakes.
But are jetties extending 6000 _metres_ into the sea possible? and if possible, would they not require so much time and such an expenditure of money as, practically, to cause the undertaking to be given up?
With regard to the possibility, there can be no doubt, for more than a century ago the Dutch Government constructed a jetty 8000 _met._ in length in the Bay of the Lion, near the Cape, in water more than sixteen _met._ deep, in spite of the continued tempestuous weather which succeeds the settled calms in those latitudes. Such a work, considering the depth of water, must have required a quantity of materials at least four times as great as that required for the two jetties and the mole at Pelusium. It was undertaken by a nation not over rich, at a time when steam was unknown, and before the invention of machinery, which saves so much time, expense and labour. There can be no doubt then, that if the cutting of the Isthmus is admitted to be advantageous, it will be easy to overcome all difficulties.
With regard to the method of constructing these works, opinions are no less divided. Some engineers, grounding their opinions on ancient constructions, recommend that the moles should be formed of immense blocks of stone of thirty to forty _cubic metres_. Others are of opinion that the only means of preserving the roads from the accumulation of sand, is to construct the moles and dykes of open masonry. There are also some in favour of walls in hydraulic masonry with vertical facings. But, our own opinion is, that in so important an enterprise, every theoretical hypothesis should be discarded, and that we ought to be guided solely by the experience we have acquired in works of an analogous character already executed. And this is what we have done in adopting the system of loose stones, as it has been carried out with success: 1. For the dyke at Cherbourg which is 3768 _met._ long in a depth of water of 14 _m._, 80; 2. For the jetty at Plymouth which is 1364 _met._ long in a depth of 11 _met._ and more; 3. For the dyke in the Bay of Delaware 1200 _met._ long, with a depth of 14 _met._; 4. For that of the Bay of the Lion 8000 _met._ long, in depths of more than 16 _met._
Objections to this system may, indeed, be raised on account of the damage sustained at Cherbourg and Plymouth as well as at Algiers, before the introduction of factitious blocks, but it is necessary to observe, that both at Cherbourg and Plymouth, the tidal current is exceedingly strong, its velocity being as much as 4 _met._ per second; that the sea at these points is very rough, and that there is reason to suppose that the damage would not have occurred had the blocks been rather larger, and the interstices well filled up. With regard to the roadstead at Algiers, it is, as is well known, constantly beaten over by heavy seas, no other point in the Mediterranean presenting such difficult conditions. We have in favour of our system most of the moles erected in the various ports of the Mediterranean, Genoa, Cannes, Barcelona, Valencia, Cadiz, &c. &c., all of which are constructed of natural blocks, the largest not exceeding 2 _m._, 50 cube, and which are nevertheless established at considerable depths of water. Finally, we have on our side the opinions of the most distinguished English engineers; opinions which have prevailed in Parliament, and in accordance with which, all the moles in the harbours of refuge in course of construction are being made, according to the system of natural blocks sunk into the sea, at certain slopes.
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.