jetties with the little mole on which the Lazaretto stands form the boundary of the basin. It contains forty thousand seven hundred and twenty-two superficial yards, and can float sixty vessels, of which about thirty, may be vessels of three hundred tons, and some few, eight hundred tons. Vessels of a larger class anchor outside the basin. The greatest depth of water is sixteen and a half feet; but this may be increased by dredging. The Cheredin jetty and the mole were found in a state of complete dilapidation when Algiers fell into the hands of the French. These two works constructed of loose stone, (sometimes called rip-rap in the United States,) were levelled to their base. The Deys were in the habit every year of having the stones replaced which were carried away in the winter by the sea.

Laugier de Tassy, one of the most faithful historians of the Algerine regency, who resided there in 1727, says:

"The great mole (the Cheredin jetty,) being entirely exposed to the north, to prevent it from being carried away by the furious swells of the sea, which roll up the sand bank, stretching along this mole and out into the sea, they were obliged to keep the slaves of the beylick employed the whole year carrying hard stones from a place near point Pescade, to put them along the mole. The sea soon carried away the stones thus deposited, but care was always taken that they should be replaced."

Large magazines of military supplies are placed on the Cheredin jetty and it naturally claimed the first attention of government.

The preservation of these magazines required that the loose stone upon which they rested, at the base of the jetty, should be secured.

This undertaking was confided to M. Noel, the engineer, in charge of the hydraulic works at Toulon, from which he was temporarily relieved.

He rebuilt the entire body of the jetty to a height of sixteen and a half feet above the water, with a thickness of six and a half feet. The new masonry is of the very best kind and possesses great solidity; unfortunately the insufficiency of funds placed at the disposal of the engineer and his limited time did not permit him to turn his attention to the foundation of the jetty which will soon require considerable repair.

The extremity of the mole, called the chop, in which the sea made a large breach, was repaired in 1831, but the new masonry being built upon the fragments which the action of the sea had brought down, was entirely destroyed by the first storm in the winter of 1832. All the repairs made to the top of the work were necessarily liable to the same catastrophe, as the base upon which they rested was insecure. It became necessary, therefore, before proceeding farther, to reconstruct the base permanently and durably.

The locality did not permit the engineer's resorting to the ordinary means of establishing a foundation by throwing over loose stones, (riprap.) The shore to the west, where the quarries are, has not a single creek or harbour where a vessel could load; it is open to the ocean and skirted by a reef of rocks which make the landing dangerous even in a calm. The transportation of blocks of stone could only be effected by carriages, a tedious and difficult operation with masses, which, to resist the action of the waves, should measure at least four cubic yards. Besides which it would have been necessary to carry these blocks through the most frequented and populous part of the city, very much to the inconvenience of the inhabitants passing to and fro. Another difficulty presented inself, even if the obstacles to an easy transportation had been overcome. To give sufficient stability to the work at the end of the mole, a long slope of at least one in ten was necessary, which would have entirely obstructed the navigation, as the entrance to the basin was already very narrow, being

only one hundred and thirty-four yards wide, measuring from the end of the mole on which the Lazaretto stands, to that of the work in question.

Under these circumstances, the engineer was obliged to resort to other expedients, and he was thus led to form and execute a new plan for establishing foundations at sea, which five years experience of the works at Algiers bas proved to be, according to all accounts, superior to all those which have heretofore been put in practice and particularly to those made of rip-rap work; a method much approved of since the construction of the Cherbourg and Plymouth breakwaters, the two most important maritime works executed in modern times.

The principle feature of this plan is the use of blocks made of béton. These blocks are of two kinds; one being constructed in the water at the place it is intended to occupy, the other made on shore and launched.

The first is made by pouring the béton into cases without bottoms, sunk on the place where the block is to rest. The frames of these cases as shown in the annexed cut, are made by putting together pieces of scantling, in a rectangular form, and the sides are made by nailing to this frame two courses of plank placed at right angles to each other. The lower edges of the cases are cut out to fit the profile of the surface on which they are to rest. They are lined with tarred cloth. throughout the whole extent of the inside up to the level of the water. The cloth at the bottom is allowed sufficient fullness to accomodate itself to the inequalities of the ground. The cases are thus, in fact, converted into cloth sacks, the sides of which are strengthened by the timber work on which they are stretched and fastened. The cloth sacks enable the mass of béton to accommodate itself perfectly to the surface which receives it, the inequalities of which serve to bind together the rock forming the bottom, and the béton. This is a great advantage in the use of these cases, for with the flat bottomed ones generally used, it is necessary to level the surface to be built upon, which is a difficult and uncertain operation.

The cloth bottomed cases are built upon stocks, launched and floated to the place they are to occupy. They are then sunk by means of smal wooden boxes, one foot square, filled with cannon balls or pig-iron strung entirely around on the outside of the case, about one foot and a half from the top, by means of a cable passing through iron rings fixed in the uprights. The arrangements for sinking the cases is shown in the annexed figure.

A similar use of béton was made by the Italians to prevent the disinte

gration of masonry immersed in water. They filled, with béton, bags similar to those used in fortification for making earth defences, and placed them compactly, one upon the other, and in such a manner as to fill up the inequalities of the surface on which they rested. The cement which oozed out through the interstices of the cloth, bound the little rolls of béton together and soon formed a very compact and durable mass. The cloth between the joints rotted and disappeared in a few years. On one occasion, they filled a much larger sack with béton than those above described, and threw it into the sea in stormy weather; some days after the storm had subsided they found this block very hard and strong. From the result of this experiment it was natural that the adoption of very large blocks of béton should be thought of, but the difficulty consisted in making bags of those dimensions which would not burst, and fixing them in position, while being filled with béton.

When the case is moored, the béton is lowered and deposited in it by means of a trough, which has a vertical and semi-rotary motion communicated to it by a cylinder worked at each end by a crank. This trough which contains a little more than a cubic yard, gives the advantage of putting in the case a large quantity at a time. The operation is thus made more rapid and there are fewer seams.

The béton blocks made on shore are moulded in cases consisting of four sides made of thick planks and lined on the inside with another course of plank jointed together at the bottom and removable at pleasure. The bottom rests upon two large sills connected transversely, forming an inclined plane which terminates at the point where the block is to be launched. These cases like the others, are entirely empty and without shores. When they are filled with béton, and it becomes sufficiently hard, the sides are taken off and the block thus stripped is launched into the sea.

The mortar used in the large cases with cloth bottoms, is formed of one part fat lime and two parts of Italian puzzolana; that used for blocks on shore is composed of puzzolana and sand in equal proportions.

The lime should be made from the grey transition limestone, fine grained and very hard; slaked in the ordinary way, and reduced to the consistency of thick paste, it absorbs two and a half times its weight of water. Its bulk is increased in the proportion of 1 to 1.8.

The puzzolana is the same as that used along the Mediterranean coast in the formation of hydraulic mortars. It is to be found in the neighbourhood of Rome. The best comes from Saint Paul's cave, near the church of that * name. This puzzolana is brought by wagons to the Tiber, and thence by batteaux to Civita Vecchia, whence it is exported. It is sent abroad in the natural condition in which it is found, the pieces varying in size from that of an egg to the smallest grain of sand. M. Jullien, the engineer, found by experiment that the very finest grains were the only ones that could be used with effect in hydraulic mortars, and that when it was used in grains as large, for example, as the largest grains of sea sand, it was as ineffectual as the sea sand itself. From this it appears to be necessary that the finest grained puzzolana alone should be used in hydraulic works; and as its efficacy, and quickness in hardening are in proportion to the fineness, too much pains cannot be taken to pulverize it.

Acting on this principle, the puzzolana brought from Italy to Africa for the work on the mole was sifted at Algiers before being used. One half, forming the residue, was ground in a mortar mill and sifted again, leaving a residue of one tenth.

That ground and sifted was of a quality inferior to that furnished by the

first sifting. The price of the puzzolana delivered at the work was thirtysix francs per cubic yard, and the cost of sifting, grinding, &c., twelve francs, making the total cost forty-eight francs.

The cost of labour at Algiers, independently of the inferior quality of the puzzolana obtained by trituration, and the consequent increase of expense, made it desirable that it should be sifted at Rome and the refuse left there. The soil on which this city and its environs stand, is composed of this material, and is of course very cheap. The only difference in the price would therefore arise from the cost of sifting, which could be more than balanced by the freight saved in leaving the refuse.

Influenced by these considerations, the author, on the requisition of the Governor General was authorized by the Secretary of War to repair to Rome and superintend in person, the details of the operation. He there fixed up a number of strong bolting cloths pierced with small rectangular holes. The price of sifting one cubic yard of puzzolana with labour hired of the pontifical government, was about twenty cents.

The contractor who has leased from the Roman government, the monopoly in the puzzolana trade, regarding the project as impracticable, asked an exorbitant price for taking charge of it, but as soon as he discovered it was both easy and cheap, he came forward and offered for the future to send none but the sifted puzzolana to Algiers. It was delivered there in 1837, in this state, for forty-two francs the cubic yard, and could, without doubt, be delivered for forty francs. By adding one half sand, quite as good a commodity as the rough puzzolana is produced, and you get for twenty francs what formerly cost thirty-nine. At this price this material is likely to supersede all the bydraulic lime and artificial cements made at the different localities. It is easier worked, and the quality is superior or at least equal.

Algiers is not the only place where this measure can be adopted advantageously; it can be practised with advantage on the whole Mediterranean coast and wherever the puzzolana of Italy is used. The engineers of Toulon and Marseilles have already made arrangements for the importation of the sifted puzzolana, and there is little doubt but that it will become an exfensive article of traffic.

The mortar is made with one part lime in paste, and two parts puzzolana. If the puzzolana is in the rough state the mortar becomes hard in four days and resists the Vicat rod; if it is sifted through the bolting cloths it will be come hard in two days, and if the puzzolana is sifted through a fine hair sieve, it will become hard in twenty-four hours.

It takes six days for mortar to become hard, which is made of one part lime, one of bolted puzzolana and one of sand.

Béton is composed of one part mortar and two of stones broken to the size of from one to two inches, making two parts of béton.

*

A cubic metre (35.317 cubic feet) of béton weighs 5886 pounds. It acquires in twenty-four hours, sufficient cohesion to withstand the shock of a heavy sea without disintegration. In November 1836, a block containing two hundred and fifty cubic yards, which had been immersed only thirty-six hours was stripped of its enclosure, and resisted the action of one of the most violent storms. M. Fénéon, a mining engineer, then at Algiers, was an eye witness of this remarkable fact.

The blocks made in the cases with cloth bottoms, measure generally from

* Whether cubic or superficial measure, is not stated. Tr.

VOL XXV.-No. S.-MARCH, 1840.

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one hundred and eighty to two hundred and fifty cubic yards; those made on shore, from fifteen to sixty cubic yards. When constructing the mole at Algiers, they placed first a set of the large blocks, and then, in advance of them, to protect their bases, they placed a number of the second size. The large cases serve as a platform from which to launch the small blocks. The two lines of blocks are bound together at intervals by large blocks of béton, and these intervals are filled by stone measuring from five to eight cubic yards.

The following is an estimate of quantities and labour for a béton block, of thirty-six cubic yards, using mortar made of lime, sand and puzzolana : 36 cubic yards of Broken stone.

The cases used cost about one hundred dollars, and one will answer for twenty blocks.

The whole cost of making and laying this béton at Algiers is about five dollars and seventy cents the cubic yard.

Estimate of quantities and labour for a béton block of one hundred and eighty-two cubic yards, sunk in a case with a tarred cloth bottom, caulked, using mortar made of lime and puzzolana:

The construction and moving of the case cost about four hundred dollars; it can also be used twenty times.

The caulking at the angles, the cloth bottom and the removal of the case, cost about one hundred and seventy dollars.

The whole cost of making and laying this béton at Algiers, is estimated to be about eight dollars and seventy-five cents the cubic yard.

Colonel Emry published a work in 1831, containing many purely theoretical views, and at the same time many useful suggestions, in which he set forth, strongly, the inconveniences of the present system of building stone work in the sea, and proposed as a substitute, blocks made of béton.

The blocks that he proposed were also of two kinds, one kind made in the water and the other on shore; the first was to be built in a flat bottomed case, and the other he proposed to transport to the place for immersion; a plan of doubtful success; he proposed, too, that these blocks, which were to be hexagonal prisms, should be laid regularly one upon the other, which we regard as impossible.

During the execution of the new system at Algiers, some engineers thought the success of it very doubtful; but the manner in which the end of the mole stands, puts all doubt to rest. This work projects into the sea towards the quarter whence the winds blow with most violence, and it stands without having sustained the slightest injury from the most furious tempests. Besides other unquestioned advantages presented by the use of béton blocks instead of loose stone, the difficulty is avoided of transporting the stone of the requisite size when the quarries are remote. This consideration amounts