rammer, and be made slightly wet, so as to be somewhat softened and brought into proper state to unite with the succeeding layer. As the work rises, care must be taken to surround it with straw, or some similar substance, which must be kept always moist during the first summer. If the work have but little altitude, earth may be banked against it. In all constructions of this kind, it is proper to make the work with a little excess of dimensions, so that the following year some tenth of an inch of the outside, which, having dried too quickly, will be less hard than the interior, may be cut away; pressure should always be applied, in any way most convenient, where the nature of the structure will permit it.

If it be thought proper, these factitious stones may be coloured at the time they are made, by mixing in the mortar composing the surface, some suitable metallic oxide.

If the factitious stones are to be of large dimensions, gravel or small stones may then, for the sake of economy, be mixed with the mortar: in such cases the resulting mass will be concrete. This substance was used by the Romans: several of their works are found in which the faces of the walls are of stones, and all the interior of a mixture of mortar and pebbles. Mr. Fleuret having enumerated, as stated in the beginning of this chapter, the great monuments supposed to have been made of factitious stones, says "There are even great walls constituting the enclosure of towns, large aqueducts, and piers of bridges, still remaining, nearly entire, from the time of the Romans, of which the faces are made of very small stones, and the heart of the wall of pebbles mixed with stones larger or smaller, thrown at hazard between these light facings. This masonry made of fragments, and rammed in an encasement, becomes but a single mass, and is rendered so compact by continuity, that in a short time the walls becomes indestructible.”

I do not think that this masonry was made in encasements; the facing walls were substitutes therefor; and had there been no facings, the concrete, as I have before observed, might have been built up without moulds. But we see by the quotation, that the Romans made much use, in constructions in the air, of small materials mixed with their mortar, and it is this kind of construction which, as I have said, we now call concrete. I have stated, in the first section, that in repairing, in 1816, one of the dams which sustained the navigable canal of Strasburg in its passage across the ditches of the fortification, I found that the facings alone were of free stone, and that all the interior was of concrete, of great hardness, which led me to presume it had been made of hydraulic lime, and induced me to make researches, whereby I became possessed of the qualities of Obernai lime, and afterward of many other hydraulic limes in the neighbourhood.

In constructing, with concrete, the interior of baterdeaux that have sufficient thickness, we may be certain of their great durability. In rubble or regular masonry, the mortar does not always unite itself perfectly to the stone-so that if there be a head of water, it will at least find a passage between the joints. This cannot happen in masonry made of small materials, as concrete, because all these small stones are separated from each other by a portion of the mortar which will oppose filtration. In the two baterdeaux of Strasburg which I have cited, the facings were displaced; but the central mass of concrete prevented a single drop of water from passing, notwithstanding the bad condition of the facings.

Mr. Rondelet says at page 116 of his work "There is found near Metz in France, a very hard stone with which they make lime of very superior

quality; this lime, newly slaked and mixed with gravel, affords a concrete or species of mortar, of which the consistence is so great that arches may be constructed of it, without bricks or stones; these arches form a single piece as hard as stone.

Lieut. Col. Finot has constructed at Strasburg, within two years, an arch of concrete of about thirteen feet four inches diameter, which has succeeded well: the piers are of masonry. A similar arch has been coustructed at Schelestadt. This kind of arch offers great advantage in many circum

stances.

When the arches are to be underground; this will be the best means of preventing filtration; but that the underground room may be perfectly dry it will be necessary to make the piers and end walls also of concrete.

It often happens that there are casemates and cellars which in time of floods, fill with water. This may be prevented by putting a structure of concrete upon the bottom: and if the water filters through the walls, by reinforcing them with a plaster of concrete. By these means we turned to profit, at Strasburg, several casemates, of which the floor was below the level of the water in the river, and in the ditches, in time of floods.

We are frequently obliged to carry a canal over a stream of water, and vice versa. In such cases aqueducts are built. It is here particularly, as in aqueducts in general, that concrete is indispensable. If it be thought proper to build the arches of stones or bricks, it is indispensable, in order to resist filtration, to make all the masonry above the arches, of concrete. In countries where proper materials for making the arches cannot be obtained at a reasonable rate, it will be advantageous to make the arches of concrete, in which case the aqueduct will be formed of a single piece. Arches of large dimensions may be constructed in this way, either for aqueducts or for ordinary bridges.

It is necessary sometimes to enlarge a wharf, in cases where the river cannot be restricted; and it is done by making all the courses of masonry, projecting, or corbel, courses-a very expensive construction, on account of the large dimensions which the stones must have. The same result may be obtained with concrete, which will form but a single piece of great solidity, at small comparative cost. By similar means machicoulis, which are sometimes necessary to protect the entrance to forts, may be made.

Researches are now being made as to the means of preserving grain in Silos. It is requisite to success, that the grain be preserved from the contact of the air, and from humidity. Both these conditions will be secured at the same time, by making the Silos of concrete. After the grain is deposited therein, it may be hermetically sealed by closing the mouth with the same substance. Silos may be of great advantage in provisioning fortified places: and it is important that attention be directed towards them.

In some departments in the north of France, no good building stones are to be found, and revetment walls are there constructed of fragments of chalk, there being a facing of bricks to preserve the chalk from contact of the air. But in several of these places the bricks are of bad quality, and this facing scales off, requiring considerable, and perpetual, outlays for repairs. In such circumstances it would be, I think, advantageous and economical, to build the revetment entirely of concrete. It should be composed of hydraulic mortar in which would be mixed gravel, fragments of chalk, bricks or any similar matters, in the proportions indicated in the

first section. Taking care to apply on the outside a thick plaster of the same kind of mortar. The plaster should be applied at the time the concrete is laid, so that it may unite with it perfectly. By this means the chalk will be preserved from contact of air. As the wall rises, humid earth should be placed against the back of it, and the front should be covered with straw-mats or a thick layer of straw, which should be kept wet during all the first summer. In countries where good arenes are to be had, revetments in concrete would not be dear; and, besides the advantage of avoiding considerable expense in repairs, it would be more difficult to make breaches in revetments of concrete than in those constructed of stone.

I will observe that in the construction of revetments, which have, commonly, about thirty-three feet in height, and sustain a great mass of earth, the bad quality of the mortar often makes a much greater thickness of masonry necessary than if the mortar were good. By making, as I have proposed, all the masonry of hydraulic mortar, the expense will be augmented a little; but on the other hand it will be possible to diminish the thickness of the walls, which will be a compensation. By making the revetments of concrete they will be of a single piece, and the thickness may be sensibly reduced; which will amount to a great saving wherever good arenes are at hand. Nothing would prevent these revetments being made with counterforts or with relieving arches.

In the south of France particularly, are to be seen revetments of great exterior slope. This mode of construction, permits the thickness of masonry to be considerably reduced: but it cannot be adopted in the north, because of the humidity which favours the development of vegetation in the joints, causing the ruin of the masonry. By constructing these revetments with concrete, taking care to apply a strong exterior coat of plaster immediately, and to unite it perfectly with the beton, there will be nothing to fear from vegetation, and the talus of the wall will allow its thickness to be considerably lessened. This will be worthy of trial at those places in the north where the materials are not generally good, and where arenes are to be obtained.

When repairing revetments from which portions had scaled off, from the effect of vegetation, I had occasion to remark that sometimes the lime seemed to have disappeared from the mortar almost entirely; to such a degree that it was thought the lime had been fraudulently reduced in quantity in making the mortar. Examining these separated parts of the wall, I remarked that all the joints of the stones were filled with roots which had penetrated to a great depth. There remained hardly any thing but sand in the joints of the stones. I was led to believe from this that fat lime is absorbed by vegetation. I saw no such effect in masonry made of good hydraulic mortar. It is possible that the hardness of these mortars resists the development of vegetation; or that this kind of lime is less favourable than fat lime. In making revetments of concrete, giving them a good coat of plaster made of hydraulic mortar, of which the surface should be made very smooth, it seems to me that there would be nothing to fear from vegetation.

In the northern department of which I have spoken, the buildings are not more solid than the revetments. They are obliged to make them of bricks which are at the same time of bad quality and very dear. In many southern districts houses are made of Pisé. 'This kind of construction is

not in my opinion solid enough for military establishments; and I doubt if it would succeed in the humid climate of the north: but I see nothing to prevent these buildings being made of concrete. I have shown that with hydraulic mortar we obtain a strength approaching that of common bricks. The walls of houses constructed of concrete containing this kind of mortar would possess great solidity, and wherever arenes were at command and building materials bad, would be erected with great comparative advantage. The angles of the building, and the casements of doors and windows, should be made of factitious stone, and the walls should be covered with a coat of hydraulic mortar, coloured to resemble stone. The precautions indicated above, against too rapid drying, should not be omitted. It would be easy to make a trial of this kind, on some small building, or a guard room, kitchen, &c.

We often have to construct bomb proof buildings, for powder magazines, barracks and other establishments. In countries where the mortars are of bad quality, I am of opinion that it would be wrong to make them of concrete. When made of stones, or bricks, I have already said, that it seems to me necessary to use hydraulic mortar. I will add that when it is thought proper to make the arches of stones or bricks, I regard as indispensable, after the centre has been struck, to make all the masonry above these arches of concrete; this being, in my opinion, the only means of securing their future impermeability. This mass of concrete might, in fact, enable us to dispense with a plastered cap; but, for greater security, it will be best to put on a coat of plaster of about one inch in thickness. It is very important when these caps are in progress, to shield them from the sun. They are sometimes covered with a sail cloth; but this does not preserve them from hot air which causes them to dry too quickly. It would be better and cheaper, to cover them, as they advance, with moistened straw. When the caps have acquired sufficient consistence, they should be polished, and again covered with straw, which should be kept moist all summer. If the arches are to be covered with earth, this earth, at least in part, should be put on as soon as the caps have been polished.

There is still another use to which concrete may be applied with great advantage, namely in the abutments of suspended bridges. When there is not rock to which to attach the chains, it is necessary to build abutments; but these chains acting in an angle of about 45°, a great part of the tension is in a horizontal direction. If the mass of the abutment is of cut stone; or rubble masonry, the mortar not fastening the stones together strongly, this force may disjoin them horizontally. This lately happened to the suspended bridge built opposite the Invalids at Paris. I do not doubt that the masses of masonry forming the abutments would have been competent to resist a much greater force than they were subject to, if those abutments had been of a single piece each: but as they were constructed of cut stone and rubble masonry, and there had not been time for the mortar to dry thoroughly, the masonry separated into two parts near the middle of the abutment, so that the hand might be introduced between them. This I consider the explanation of the accident which made it necessary to demolish the bridge before it had been entirely uncentered. If the masses to which the chains were fastened had been of concrete, they would have formed a single, homogeneous, inseparable mass: it would have been necessary for the chains to have drawn it off entirely, or to have broken it, while, in the actual case, it was only necessary to disjoin stones that had

but recently been united by mortar. When bridges of this kind are built it seems to me proper to make the abutments, to which the chains are to be attached, of concrete: and they should be erected a year in advance so that the substance may acquire sufficient solidity.

The beginning of autumn is the most favorable season for making concretes that are to be exposed to the air.

I will conclude by observing that the expense necessary to the composition of good mortars for concretes, must be encountered. In countries where the arenes are not very hydraulic, they may be mixed with hydraulic cement in proportions depending on the energy of the materials. said above, it is more economical to encounter the requisite expense of good masonry, at once, than to execute it cheaply of bad materials.

As I have

I have thought it proper to enter into some details as to the advantageous use to be made of concrete in places where building materials are of bad quality; and to indicate some new modes of construction to be substi tuted for those now in use.

CHAPTER XVI.

Summary of the Second Section.

The experiments reported in the second section of this memoir, lead me to the following conclusions, as to mortars exposed in the air.

When I made mortar by using fat lime that had been lying slaked and moist for a considerable time, and sand only, I could obtain no satisfactory result, whatever were the proportions. In making mortars with sand, and fat lime just from the kiln, I could obtain only mediocre results. The lime I used was got from a carbonate of lime containing only a minute quantity of iron. It will be important to ascertain, whether, as I anticipate, similar results will be produced in other countries. Should the experiments in other places afford similar results, it will be necessary to abandon the practice of keeping fat limes lying slaked and wet for a long period, and to adopt that of using them fresh from the kiln.

The process of slaking lime announced by Mr. Lafaye, and that given by Mr. Loriot, afford no sensible amelioration with fat lime.

Hydraulic limes mixed with sand, produced very good mortars in the air, as well as in water. These limes must be used soon after the calcination, as they lose a great part of their energy. Mortars made of hydraulic limes afford a greater resistance when they are mixed with sand and hydraulic cement, than when mixed with sand only. Hydraulic cements, or other analogous matters, mixed with limes, restore the energy of those which had been too long exposed to the air; and augment the energy of those which are but moderately hydraulic.

Hydraulic mortars made of fat lime, sand, and hydraulic cements, or other analogous matters, are excellent when used in the air. When sand and cement, in equal parts, are added to lime, it is almost a matter of indifference whether the lime be used fresh from the kiln, or slaked to powder with a little water, and for some time exposed to the air in this state, or, having been slaked and lying wet for a considerable time. But if only a small quantity of cement is to be added to the mortar, it will be best to use the fat lime shortly after its calcination.

In countries where good natural hydraulic limes are to be had, they may