Part 4

| Taken from low water | Variation | in the Mediterranean | from the STATIONS. | at Tineh. | Levels of +------------+------------+ | 1853. | 1847. | 1847. +------------+------------+----------- Low water in the | | | Mediterranean at Tineh. | 0 _m._ 0000| 0 _m._ 0000| 0 _m._ 0000 | | | Stations of the German | | | Engineers at Tineh. | 1 _m._ 5586| 1 _m._ 7400| 0 _m._ 1814 | | | Station at the Staff 29 L. | | | 1853, point 26 of | | | Bourdaloue’s triangulation | | | of the most elevated Lagoons| | | of Lake Menzaleh at Ras el | | | Ballah. | 1 _m._ 9800| 1 _m._ 9800| 0 _m._ 0000 | | | Station 4 L. 1853, | | | Bourdaloue’s point | | | A, which was found and | | | verified. | 7 _m._ 8210| 7 _m._ 4300| 0 _m._ 3910 | | | Bourdaloue’s Station Staff | | | at the mouth of the Canal | | | (this staff is not certain).| 3 _m._ 8280| 3 _m._ 0800| 0 _m._ 7480 | | | Station 3 L. 1853, at the |16 _m._ 5950|16 _m._ 2300| 0 _m._ 3650 Serapeum, or Bourdaloue’s | 2 _m._ 4100| ---- | ---- No. 83. | | | | | | Upon the most elevated | 2 _m._ 0300| ---- | ---- deposits in the basin | 1 _m._ 8600| 1 _m._ 8000| 0 _m._ 0600 of the Isthmus. | | | | | | Station 2 L. 1853, and | | | Bourdaloue’s Station B. | | | 30, on a block of | | | petrified wood, covered | | | with sandy secretions, | | | placed upon the deposits in | | | the basin of the Isthmus. | 2 _m._ 4380| 2 _m._ 1100| 0 _m._ 3280 | | | Station 1 L. 1853, at the | | | Persepolitan monument, | | | upon a block of sandstone, | | | south of the Bourdaloue | | | excavations. |11 _m._ 6300|11 _m._ 3700| 0 _m._ 2600 | | | Station on the Caravan Road, | | | at the Staff Station, | | | 3 L. 1853. | 2 _m._ 3900| ---- | ---- | | | Station at the staff at | | | the starting point | | | No. 1, L. 1853. | 1 _m._ 5186| ---- | ---- | | | Station on the quay of | | | the Suez hotel, the same | | | as that of M. Bourdaloue. | 2 _m._ 4286| 2 _m._ 6100| 0 _m._ 1814

The most striking fact to be observed in the examination of this table is, the slight relief of the ground above high water of the Red Sea, in the whole extent of the Isthmus. There are only two points somewhat elevated. The first, proceeding from Suez, is met with before Lake Timsah, and is that which we shall call the Serapeum bar; its greatest elevation is 16 _met._, 5950, above low water in the Mediterranean. The second point is at the outlet of the lake, and its greatest elevation is fifteen _metres_, at the spot known as the bar of _El Guisr_; but the line of the Canal may be carried in a direction where but ten _metres_ are met with for some _kilometres_ of length. Supposing therefore the bed of the canal to be established at the depth of 6 _met._, 50, below low water in the Mediterranean, the greatest excavation would be at the bar of _El Guisr_, and would show a total depth of 16 _met._, 50, which is nothing extraordinary; supposing it even twenty _metres_, the requisite excavation would bear no comparison with what was executed in Mexico, during the Spanish occupation. For, in their then difficult position, and in the absence of tools and improved means, the Spaniards were able to effect, near the town of Mexico, which was threatened with invasion by the waters of the neighbouring lakes, the cutting of Huehuetoca, the total length of which is 20,585 _metres_, and its depth from forty-five to sixty _metres_, for a length of more than 800 _metres_, and from thirty to fifty _metres_ for a length of 3500 _metres_. And yet the expense of this work was only 31,000,000 francs.

The levelling also shows, that by adopting 6 _met._, 50, for the bed of the Canal, there will be a length of 18 _kil._ in the Bitter Lakes, where there will not be a shovelful to remove, and for another 18 _kil._ there will be very little to do; and as these lakes are dry at a depth of 8 _met._, 39, below low water, all the earth-works for the whole length of them could be performed in the dry, if found advantageous to do so.

The numerous transverse sections taken with the levelling of 1847, enable us to ascertain approximately the superfice of the Bitter Lakes at the water line. This superfice is about 330,000,000 _square metres_. If, then, the action of the tide, which brings two _metres_ of moving water, be admitted into these lakes, a disposable volume of 660,000,000 _cubic metres_ of water would be accumulated, and which might be raised to 800,000,000 by adding Lake Timsah and the retaining basins at Suez and Pelusium to these immense reservoirs.

Before pointing out the various directions of the adopted track, it appears necessary to arrive at a fixed opinion as to the formation of the Isthmus and of the downs by which it is partly covered, and also as to the accumulations of sand which exist both on the coast of Pelusium and at the bottom of the Gulf of Suez; for it is from the explanation of these phenomena that we shall start in our justification of the arrangements of the direct track in general and in detail.

By attentively observing what is passing before our eyes at the present time, in respect of the destruction and recomposition of continuity, we may come to an exact conclusion regarding the laws which operated towards the first ages of the world in the formation of alluvial lands.

Let us first examine what is going on in the English Channel; for this narrow sea having a large number of ports both on the French and English coasts, has on that account been the object of numerous observations by engineers.

The first well recognised fact is the destruction of the coast from the point of Barfleur as far as the Somme, a distance of 338 _kilometres_; and on the other side of the channel, from the Isle of Wight to Dover, a distance of 250 _kilometres_. This action is produced by the alternation of frost and thaw, by dry and moist winds, and by the saline evaporation of the sea. The abrasion observed on the coast of Calvados is an average of 0 _met._, 25, _per ann._ and on the coasts of Normandy and England 0 _met._, 30. The mean height of the cliffs on either side being sixty _metres_, it follows that the channel swallows up an amount of 10,000,000 _cubic metres_ of earth and stones every year, which must find a place somewhere.

The second fact, equally well established, and which, though opposed to the opinion of the ancients, can no longer leave any doubt on the mind, is, that rivers, with a few rare exceptions,—such as the Loire for instance,—only carry to the sea an extremely thin mud, destined to be lost in the mass of matter held in suspension by the latter; that the sands of rivers do not in general reach the sea, and that the muddy or sandy deposits observed in tidal rivers, are entirely owing to the matters brought by the tide. This discovery has been arrived at as follows.

In making the analysis of the alluvial lands forming the Bay of St. Michael, it was found that the principal substances of their formation are silex and the carbonate of lime; that the nearer the sea is approached, the more the proportion of silex increases; the more it is receded from, the more considerable the proportion of carbonate of lime becomes. Now if the basins of the three rivers which discharge themselves into this bay, the Sée, the Selime and the Couësnon, be examined, they will be found entirely destitute of calcareous substances. It is the same with the coasts of the channel and of Brittany. It cannot, therefore, be either from these rivers or from the coasts that the enormous proportion of silex proceeds which has just been described. If samples are examined with a magnifying glass, commencing with those nearest the sea, and afterwards proceeding farther into the bay, in the first, fragments of shells are perceived quite distinguishable, then these fragments are reduced and become so impalpable, that the best glass will no longer enable us to distinguish the form in the most calcareous portions.

It is, therefore, certain that the calcareous part comes exclusively from the sea, and even from the bottom of the roadstead of Cancale. As for the silex and clay, a part in their deposit may be attributed to the rivers; but it should first be understood how unimportant these three small rivers are, each discharging not more than an average of eight to ten _cubic metres_ of water per second. Farther, if the contributions of the rivers reckoned for anything in the deposits which are made in this locality, clayey or gravelly stratifications would be seen on their banks at the parts where the tide is least felt. Nothing of the kind occurs. The mixture of the calcareous matter, the grains of silex, and the argillaceous atoms is so intimate, that it is evident it could only be made at the very centre of the production of the calcareous matter; that is to say, at the bottom of the sea. If the fluviatile deposit was appreciable, it would counterbalance entirely, or in part, the calcareous overplus in the drift taken from the top of the roadstead, as compared with that taken at the bottom. Far from this being the case, the progression of the calcareous element, which can only come from the sea, is seen in proportion to the elevation of the shores. Finally, if the fluviatile deposit ought to be reckoned for anything, a larger proportion of clay would be seen upon the brink of the Sée, which traverses fissile lands, than in the neighbouring channel of the Couësnon, which traverses lands of a much harder character, furnishing less clay than the fissile ground of the Sée and the Selime. Now, the contrary is the case; the drifts of the neighbouring channel of the Couësnon are more clayey than the others, solely because this channel being more sheltered than the beds of the other two rivers, the muddy matter which the sea always holds so abundantly in suspension, and which it deposits in the basins of ports, can be carried there concurrently with the drifts.

On making the same investigations for the Seine, it was found that the sands transported by this river do not pass Rouen, and that all the accretions that are seen lower down, as far as the flats which are met with at its mouth, are deposits by the sea.

The same results were arrived at for the Scheld.

As to the Meuse and the Rhine, the following deductions have been made.

The abrasion of the coasts of the channel supplies the sea with fragments of chalk and siliceous rocks, which being rolled about by the sea become shingle. This shingle forms banks along the English and French coasts, and forced by the double action of flood and wind towards the straits it approaches them; but the shingle on the coast of France continually decreasing in size, reaches the mouth of the Somme, where it finds the point of Cayeux formed by its accumulation. Stopped at this point by the waters of the Somme, and by the change in the direction of the current of the sea which turns towards the Pas de Calais, this shingle increases the point of Cayeux, so long as its continual collision has not sufficiently reduced the size of the stones for them to be carried away by the sea; but when they are small enough, the flood bears them away and distributes them on the numerous banks which are found between the Somme and the Pas de Calais. From the inspection of Marine Charts, it is seen that the fineness of the deposit increases in proportion as these banks are nearer to the straits, and if the banks disappear in the Straits, it is because the force of the current does not allow the sands, which from being sifted for a long time have become finer and finer, to stop in that passage. They pass it therefore and some go to form the downs between Dunkirk and the Scheld, others in like manner to form downs on the English coast, others remaining in the strongest currents are carried as far as the mouths of the Humber in England, and of the Meuse and Rhine on the Continent.

If the shape of the English and French coasts to the north and south of the Straits is observed attentively, it will strike every one that those to the south are cut out into concave indentations, while those to the north all affect the convex form. It is because the coasts to the south of the Straits are abraded by the tide, and those to the north, on the contrary, are fed by the accretions. As for the muddy matters in this long course, they can only be deposited in a few perfectly tranquil creeks, or in the basins of open ports on either coast. Wherever the tide penetrates they are carried with it, and, when finally it has entered the northern sea, and made a course sufficiently long to abate its swiftness, it finds itself in an excellent condition for depositing these muddy matters, which it holds in suspension. This is what it does at the mouth of the Humber, where it completely chokes up the port of Hull.

In like manner, the muddy matters form at the mouth of the Rhine, of the Meuse, and of the Scheld, those immense polders, which constitute such an essential part of the territory of Holland, and the numerous banks at the mouth of these rivers are only composed of sand and carbonate of lime. Now the rapidity of the current, long before reaching the mouth, is not sufficiently great to carry down the sands; in fact, no trace of them is perceived; these banks are therefore the production of the sea.

Finally, in order to appreciate at the _maximum_ the power of the fluviatile deposit in the formation of the coasts, observations have been made upon the Yssel, that branch of the Rhine which discharges itself into the Zuyderzee. This sea has but very feeble tides, 0 _met._, 40, at ordinary high water, and very much resembles the Mediterranean, the Black Sea, and the Adriatic Gulf in this respect. A muddy Delta has also been formed at the mouth of the Yssel, of the same shape as those of the Rhone, the Po, the Nile, &c. &c. This Delta cannot be exclusively owing to the Yssel, because, although it is true that the tides of the Zuyderzee are very feeble, on the other hand the shores which surround it are of an exceedingly friable nature; now, however feeble the tides may be, they yet attack the banks, and what proves it, is, that the Hollanders are obliged to defend them. By considering the Delta of the Yssel as a fluviatile deposit solely, we shall therefore have an extreme case. Now, this Delta has a superfice of only 1500 _hectares_, while the superfice of the land in Holland, which is evidently of modern deposit, is at least 1,000,000 _hectares_. If it is observed that the Yssel only emits a fifteenth of the whole volume of the Rhine and the Meuse united, it will give 22,500 _hectares_ for the deposit of the river, against 1,000,000 deposited by the sea; which is scarcely two per cent. of what the sea has furnished in the formation of the polders of Holland.

From the examination of all these facts, it evidently results, as we have said, that in seas with tides, the rivers not only do not form banks, alluvium, or deltas at their mouths, but farther, that the alluvium found in the regions of these rivers submitted to the action of the tide, is deposited by the sea.

We shall now prove that these conclusions are equally true for the rivers of the Mediterranean, notwithstanding the opinion of the Italian engineers, who have considered the fluviatile origin of their deltas as demonstrated.

To give an idea of the propagation of the waves or billows of the sea agitated by the wind, they have been compared to a field of corn under the action of the air. It seems as if the ears of corn had an impulsive swiftness, which however does not exist, since they do not quit their places. Farther, if the wind is feeble, it is only the ears which waver without the stalks being shaken; but, in proportion as the wind rises, the stalks take part in the movement to a greater and greater depth down to the root.

The waves have been again compared to the movements of a cord, which is made to undulate by shaking one of its extremities in the hand. It seems as if the cord was going at a rapid rate, while in reality it does not quit the hand that shakes it, only each point of it rises and falls alternately, and this movement is the greater according as the impulse is stronger; if the extremity of the cord opposite to that which receives this impulse encounters an obstacle, as the surface of a wall for instance, it will strike it at each movement of the hand.

It is exactly the same with the waves of the sea; every fluid molecule placed at the surface of the billow experiences an oscillatory movement nearly vertical, so that if a body floating on the surface of a wave is watched it will be seen to remain in the same place, sometimes in the hollow of the wave, sometimes on its summit, and if at length it changes its place, that depends upon other circumstances, such as the force of the wind or the direction of the currents.

This oscillatory movement which is perceived on the surface of the sea, is necessarily developed to a certain depth, which will be greater in proportion as the undulations are stronger at the surface. This fact has been confirmed by experiment; it has been ascertained in effect that the agitation of the sea caused by the wind, is communicated to a certain depth, variable according to the wind, according to the sea, and the places where the observations were made, and that beyond that depth the sea is perfectly calm. Thus it may be admitted as an observed and well proved fact, that the waves require a certain depth for their free developement; if an obstacle is presented to this developement, there will be a forcible re-action of the wave against this obstacle which will be carried off, if it is moveable, and will enter into the system of the wave. This action of the waves against the deeps is what is called the _ground swell_.

This established, it has already been seen that the coast of the sea, as well as the projecting capes, resign to the sea every year a certain amount of earthy and rocky matters. These matters are removed by the waves which break upon the shore, the soft portions are quickly disintegrated by this powerful action, and form muddy sand and mud, and the hard portions are rounded into pebbles the size of which is diminished more and more by the prolonged action of the force which set them in motion and which reduces them to sand; but in proportion as these matters arrive at a sufficient degree of tenuity, they become susceptible of submitting to the transporting force of the waves and currents, and quit the place where they were formed.

This transporting force depends both upon the height of the tides and the direction of the winds, as well as their intensity, combined with that of the currents which are observed in all seas. So that while considerable masses of matter are set in motion along the shores, the rivers, especially those which traverse a great extent of country, transport as far as their mouths only muddy matters, so light that they are carried to a distance, and afterwards deposited in the depths of the sea. This is remarkably the case with the Nile, whose waters at the time of the inundation are distinguished by their colour for more than ten leagues into the sea. All the deposits and accretions of the river up to 20 _kilom._ above its mouth are muddy, while all the banks which are at its mouth are composed of sand alone.

Thus, all the impediments of the mouth of the Nile evidently emanate from the sea. To demonstrate it by still farther evidence, we will repeat the reasoning of the engineer M. Bonniceau relative to the alluvium of the river Mersey, in his excellent work upon the navigation of tidal rivers: “If the deposits emanated from the elevated lands in a sensible degree, the quantities deposited from time to time ought to be proportional to the quantity of rain that falls at the same epochs, because the same amount of matters descending from the elevated lands and transported by the river, ought to be partly regulated by the quantity of water that carries them; but it is a fact well ascertained, that the accumulations of sand which exist in the vicinity of the mouth are greater in proportion as the waters of the river are less abundant, while on the contrary at the time of the increase, when the Nile contains nearly 0 _met._ 008 of matter in suspension, the sand banks are removed and thrown back far off into the sea.”

It is said that Alexander the Great was determined in his choice of the situation for the port of Alexandria by the consideration of the winds and littoral currents which carry eastward the matters held in suspension by the Nile, and thus cover the coast with sand. If this theory were true, no alluvium ought to be perceived westward of the mouth of the river. Now all the coast from Tripoli as far as El Aritch is covered with sand banks, which frequently form downs, and these downs are found at the present time transported several leagues into the interior of the lands westward of the Rosetta mouth.

The port of Alexandria itself has not escaped the action of the ground swell, for a sand bank has been formed which occupies a good third of the total superfice of the roadstead. Happily for the port the accumulation of sand appears to have been arrested long ago, or rather its increase has become imperceptible.

The roadstead of Alexandria owes its depth to the disposition of its sides in respect to the winds and currents. It is like the roadstead of Algiers, which is everywhere very deep, while the neighbouring ports have sand banks. It cannot be said that these sand banks are owing to the presence of rivers, which do not exist in the whole extent of the coast of Barbary, for the few land streams that are scattered along the shore cannot be called by that name.