for the channel way where the rock was sound, but | sloping embankments of earth were then constructed where soft a brick arch was built over the channel on each side. way, and the space between its extrados or outer surface was filled with earth closely rammed around it. In some instances, where the perforated rock was at first quite hard, the natural roofing became, by exposure to the air, soft and insecure, so as to render it necessary to turn an arch for its support; a difficult operation after the tunnel had once been formed. Fig. 57 is a section of the aqueduct in earth tunnel Fig. 57. cuttings. When the earth was dry and compact, the excavation for the bottom and sides was made of the proper form to receive the masonry built closely against it. The top of the excavation was made sufficiently high to give room to turn the arch, and the space above was filled with earth closely rammed in. Where the earth was wet, the excavation was made larger, and props of timber and plank were used to support the top and sides until the masonry was completed; the whole exterior space was then compactly filled with earth. In carrying the aqueduct across valleys, it was supported on a foundation wall of stone laid dry, and Over the aqueduct were erected ventilating shafts of stone, rising about 14 feet above the ground. One of these was erected every mile, and every third shaft was furnished with a door to afford entrance to the aqueduct for the purposes of inspection and repairs. Openings 2 feet square were also made in the top of the roofing arch every quarter of a mile. Each opening was covered with a flag-stone, and the spot marked by a small stone monument projecting above the surface. The object of these openings is to obtain entrance or to increase the ventilation if necessary. In places where streams intersect the line of aqueduct, culverts or stone channel ways were erected, to allow the water to pass under, and to pursue its natural course without injury to the work. At the head of the aqueduct is a tunnel and a gate-chamber in connexion with the Fountain Reservoir, as shown in the longitudinal section, Fig. 58. This gate-chamber is not connected with the dam itself, but stands at some distance from it, and the water reaches it by means of the tunnel, T, which leaves the reservoir, S R, above the dam, and passes through the solid rock of the hill against which the masonry is built, a distance of upwards of 200 feet. This tunnel descends into the Reservoir, so that the centre of it at the mouth is about 12 feet below the surface of the water. This prevents floating substances from entering it, and during winter, when the water is frozen over, there is no obstruction to the flow into the aqueduct, A, and during summer the water is drawn from a level where it is cooler than at the surface. The gate-chamber has two ranges or sets of gates; one set called regulating gates, R G, and the other guard gates, G G. The regulating gates are made of gun-metal, and work in frames of the same material, which are fitted to stone jambs and lintels; the guard Fig. 58. SECTION OF THE TUNNEL AND GATE CHAMBER OF THE CROTON AQUEDUCT. gates are of cast-iron, and work in cast-iron frames, | directly over the mouth of the tunnel, and from this also attached to stone jambs and lintels. The gates are managed by means of wrought-iron rods attached to them, having a screw formed on the upper part on which a brass nut works, being set in a cast-iron socket cap. The accompanying view, Fig. 59, is taken above the dam, showing the position of the entrance to the tunnel leading from the reservoir to the gate-chamber. The entablature on the left against the rock is built the tunnel extends through the rock to the gate-house seen on the right of the view, and at some distance from the dam. In the centre, on the ridge of the dam, is a gate-house over a culvert which extends through the body of the dam. This culvert is 30 feet below the surface of the water when the reservoir is full, and has gates opening by rods rising into the house. When the river is low, the water which is not drawn off by the aqueduct can pass through this culvert, and allow none to pass over the | 2,244 feet from the chamber, and diminishing to the dam. The bottom of the water-way of the aqueduct, Fig. 59. THE CROTON LAKE AND DAM. where it leaves the gate-chamber, is 11.40 feet below the surface of the Fountain Reservoir, and 154.77 feet above the level of mean tide at the city of New York. The length of the aqueduct as it is divided into different planes of descent, from the gatechamber at the Croton Dam to the gate-chamber at the Receiving House on the Island of New York, is as follows: Feet. The first plane of aque- Length of pipes across " 28.053 0.261 30.69 2 29 head of the second plane, where it is cf the dimensions above mentioned, and continues the same throughout, except in tunnels, where the dimensions are those already stated. The curves used to change the direction of the line of the aqueduct are generally formed with a radius of 500 feet; some have a radius of 1,000 feet; and in a few instances even larger ones have been adopted. The velocity of the water in the aqueduct is about a mile and a half per hour, when it is 2 feet deep. This was determined by floating billets of wood from the Croton Dam to Harlem River, and noting the time of their passage. This would give the surface velocity, which is greater than that of the whole body of water in the aqueduct; but, as the working depth is 4 feet, there is a corresponding increase in the velocity of the body of water: hence, the velocity of a mile and a half an hour may be taken in general terms as the velocity of the water in the aqueduct. The Receiving Reservoir, into which the aqueduct discharges its water, occupies an elevated part of the island of New York. It is 1,826 feet long, and 836 feet wide, from outside to outside, at the top of the external walls of the embankment, making altogether an area of 35 acres. In this, as in almost every instance of excavation, the rock was found above the proposed bottom of the excavation; and the difficulties of preventing leakage along the surface of the rock were great. The natural veins and fissures of this gneiss formation, and the partial unsoundness of the rock, rendered leakages still more probable; but all this was overcome by the skill of the engineers. The embankments of the reservoir were made of good assorted earth, and a portion of the 2.25 bank was puddled, or made compact and impervious by wetting the earth and using a spade to force it into a compact state. The embankments were about 20 feet wide on the top, and increased in thickness towards the base by sloping on both sides. The outside face was protected by a stone wall, 4 feet thick, with the face laid in mortar; the inside face was protected by a sloping wall of stone, 14 feet thick, laid without mortar. The top of the bank is 4 feet above the top water-line, and the inside sloping wall terminates 2 feet above the top water-line, leaving the remainder of the face to be covered with grass, so as to present a belt of green above the water on the bank all round the reservoir. A neat fence bounds the outside and the inside of the top bank, forming a walk of a mile in length round the entire reservoir. The reservoir is formed into distinct divisions: the greatest depth of the water in the nort? division is 20 feet, and in the south division 30 feet; but the whole of the rock to these depths was not excavated. The capacity of the reservoir, when both divisions are full, is 150,000,000 imperial gallons. The aqueduct enters a gate-chamber in the south division, where there are regulating gates for discharging the water into either division by a continuation of the aqueduct within the reservoir. There is also a connexion-pipe of cast-iron, for allowing the 3.86 1.60 43.63 The descent on the first plane is about 7 inches per mile; on the second and third planes about 134 inches per mile; and on the fourth plane about 91 inches per mile. The bottom of the water-way of the aqueduct at the gate-chamber, where it enters the receiving reservoir, is 7.86 feet below the level of the top waterline in the reservoir; so that when the reservoir is full, the water will rise to within 74 inches of the top of the interior of the aqueduct at that place, and the height from the top water to the top of the interior will increase according to the plane of the aqueduct grade, until it reach the surface level of the flow of water in the aqueduct. The height of the interior of the aqueduct is 8 feet 5 inches, and the greatest width is 7 feet 5 inches. The sectional area of the interior is 53.34 square feet. On the first plane the aqueduct is larger, being 2.05 feet higher at the gate-chamber, 2.31 feet higher at water to flow from one division into the other, so as to equalize the level. There is also a waste-weir for draining off the surplus water into a sewer. Mr. Towers remarks truly, that this beautiful lake of pure water resting on the summit of the island is truly a pleasing object, and, considering its size, is what no other city can boast of having within its limits. From the Receiving Reservoir the water flows, by means of large pipes, a distance of two miles, into the Distributing Reservoir, situated within the city, the object being to have a sufficient head of water near the densely-populated parts. Fig. 60 is an isometrical view of this reservoir. The pipes from the Receiving Reservoir enter it at the base of the central pilaster. The flow of water is regulated by stopcocks; and a door in the pilaster affords an entrance to the vault in which the stop-cocks are situated. This reservoir is divided into two parts by means of a wall. On the south side of the reservoir a pipe, 3 feet in diameter, proceeds from each division to the supply pipes, so arranged as to draw from one or both divisions. The house standing across the dividing wall is situated directly over the mouth of the effluent pipes, which leave the reservoir at the base of this pilaster. The water is distributed over the town, through 134 miles of pipe, of all sizes, between 36 and 4 inches bore. By this means also several public and private fountains are supplied. Feedpipes, from half-an-inch to 1 inch bore, are led from the main-pipes into the basement of every house, and in many cases a pipe rises to the bed-rooms, for supplying baths, &c. The mains are also furnished with muzzles, to which the engine-hose of fire-engines can be screwed, for extinguishing fires; and at the harbour pipes are branched out, terminating at the bulwarks, for supplying ships, and filling the watercasks on board, by means of a hose. The Distributing Reservoir is 425 feet square, measuring from the top of the corners of the main wall, and 436 feet square at the base. It covers a little more than 4 acres. Its height is 45 feet above the neighbouring streets, and about 50 feet above the foundations. Its capacity is 20,000,000 gallons. The outside walls are constructed with openings in them, so that by entering a door one may walk entirely round the reservoir, within the walls. The object of this construction was to obtain a greater breadth with a given quantity of material, and also to afford an opportunity of examining the work, so as to guard against leakages, and to prevent any moisture from finding its way through the exterior, so as to cause injury to the wall by freezing. This kind of open-work rises to within about 8 feet of the top of the water-line. Inside these walls is an embankment of puddled work, formed with a suitable breadth of base to give security to the work, and the face of this embankment next the water is covered with a wall of hydraulic masonry 14 foot thick. The top of the embankment is covered with a stone flagging, forming a walk round the reservoir, and is bounded by an iron railing. The exterior of the reservoir is built on a slope of onesixth its height, and an Egyptian cornice projects at the top of the main wall and pilasters. Terraces are formed at the foot of the walls, covered with grass, giving a rich finish to the work. This reservoir may be considered as the termination of the Croton Aqueduct. Its distance from the Fountain Reservoir on the Croton River is 40 miles. The cost of this noble work amounted to 8,575,000 dollars, including the purchase of land and extinguishing water rights and some unfinished works. This amount is within 5 per cent. of the estimate of the chief engineer, Mr. John B. Jervis. To this sum must be added 1,800,000 dollars, as the cost of the distributing pipes. The first two millions were raised at interest of 7 per cent., and is payable from 1847 to 1857: 5 per cent. interest was charged for the remainder, to be redeemed from 1858 to 1880. A discount of 647,157 dollars was charged for issuing the loan, which, together with the interest paid during the construction of the work, brings the total expense to 12,500,000 dollars. The annual interest for this capital amounts to 665,000 dollars, which is raised by a direct water-tax and some indirect taxes. By means of an existing sinking-fund the capital will be gradually redeemed. The water-tax amounts to 10 dollars for a house of middle size, and there are more than 33,500 houses in the city. Manufactories, hotels, bathing houses, distilleries, livery-stables, breweries, slaughter-houses, &c., and ships, pay according to the rate of consumption. The complete success of the Croton Aqueduct has stimulated other cities of the United States to adopt similar plans for obtaining a supply of pure water. The Boston Aqueduct is a worthy successor of the Croton. Our chief authority for the details respecting the Croton Aqueduct is an excellent work, entitled, "Illustrations of the Croton Aqueduct," by F. B. Towers, of the Engineer Department; New York and London, 1843. We have also consulted Schramke's "Description of the New York Croton Aqueduct." ARCH. [See BRIDGE.] ARCHIL. ORCHIL. CUDBEAR. A violet dye obtained from several species of lichen, the most important of which are Roccella tinctoria and fusiformis, the dye of which makes litmus, and is largely used by manufacturers under the name of Archil or Orseille des Canaries, and Lecanora perella or Orseille de terre, and L. tartarea or Cudbear. Other species may be similarly employed, and the mode of testing is as follows:-The lichen whose properties are to be ascertained is placed in a glass bottle, and moistened with equal parts of ammonia and lime-water. A little hydrochlorate of ammonia is also added, and the bottle corked. In three or four days the small portion of liquid which will run off on inclining the bottle, will perhaps be tinged with crimson, and the plant itself will assume the same colour. If so, the lichen will yield a dye similar to that of Roccella tinctoria. The last named lichen or Archil plant is abundant in the Canaries and Cape Verd islands, and in the Levant. Pliny describes it as growing on the rocks of Candia and Crete, and says that the first tint or ground colour of the costly purple was given with the dye obtained from this plant. So in the present day, archil is chiefly used to improve other colours, and give richness and brilliancy to them, being too fleeting to be employed alone. The use of this dye was revived in Italy in the fourteenth century by a Florentine, who enriched himself and his country by making it a branch of commerce. The Spanish name of the lichen was Orciglia, and this became Oricella or Orchella, and the family who had revived the manufacture Oricellarii, afterwards corrupted into Rucellari and Rucellai, a name still found among the first families in Florence. This lichen has an upright growth of about two inches, and when old is crowned by flat, round disks. The colour varies from white to dark grey. The several lichens employed in this dye are largely exported from the places where they grow, and are in considerable request at the ports of London, Amsterdam, and Marseilles. One hundred and thirty tons of the Lecanora tartarea are annually exported from Sweden. These lichens are principally collected on the rocks near the sea, and are frequently packed without any previous preparation. But it is necessary before using them, to clear and grind them into a pulp with water. Ammoniacal liquors derived from gas-works, or occasionally from urine, are gradually added, and the mass is frequently stirred so as to expose it to the action of the air. The colouring matter is thus evolved, and is afterwards pressed out and made into a paste with chalk and plaster of Paris. This is the archil of commerce. Archil readily yields its colouring matter to both water and alcohol. A total exclusion of air from this substance destroys its colour as effectually as too much exposure. Thus in spirit thermometers in which the liquid is usually tinged with archil, if the exclusion of air is complete, the colour gradually fades away, but upon breaking the tube, the colourless spirit soon resumes its colour, and this fading and reviving may be carried on a number of times in succession. One of the principal sources of colouring matter in lichens is a chemical substance called orcine, which reddens on exposure to the air. Orcine has a sweet but somewhat repulsive taste; its properties are perfectly neutral; it crystallizes in flat four-sided prisms. In the dyeing of silks archil is frequently employed for lilac colours, hence their usually fleeting character; but with other hues this dye is merely used to modify or brighten, the silks being passed through a bath of archil to receive the peculiar bloom of that substance. The beauty imparted by this dye is a temptation to manufacturers to employ it too largely. Archil is employed with indigo in the woollen cloth manufacture, and produces a saving of indigo, while it gives a rich appearance to the blue or black cloth dyed with it, but this requires caution, and the bloom imparted is often deceptive. Archil cannot be made more durable by the ordinary means; a solution of tin appears to be the only substance capable of fixing it, and this changes the colour from violet to crimson. A beautful violet colour is imparted to marble by archil, and this is less fugitive than other forms of this dye. The preparation called Cudbear, (Cuthbert,) is so named from Dr. Cuthbert Gordon, who took out a patent for this mode of preparation, and connected it with his own name. ARCHIMEDEAN SCREW. [See SCREW.] ARGAND LAMP. This valuable invention has been named in honour of the inventor Ami Argand, from the fleshy roots of Arum maculatum, which are acrid when raw, but when roasted or boiled become wholesome, and yield a starch resembling arrow-root, common in the Isle of Portland; and East Indian arrow-root, which is obtained from some species of Curcuma. Arrow-root is prepared for use by first mixing it with a small quantity of cold water to the smooth consistency of cream; then adding boiling water, or boiling milk under constant stirring, until it becomes a uniform pasty mixture, which may be flavoured with sugar, nutmeg, and wine. A table-spoonful of arrow-root is sufficient to make a pint of this food. a native of France, who in 1789 contrived a burner | the starch of Tacca pinnatifida; Portland arrow-root, consisting of two metallic cylinders one within the other, the annular space between them, which is closed at the bottom, containing oil and a cylindrical wick, the latter being attached to a short brass holder which is regulated by a screw. The space within the inner metallic cylinder is open both at top and bottom. When the wick is lighted it burns with a dull, wavy, smoky flame, but on suspending over it, as Argand did, a cylinder or chimney of sheet-iron, a powerful draught is excited both within and without the ring of flame, and the additional quantity of air thus brought into contact with the flame, greatly improves its brilliancy by ensuring the perfect combustion of the carbon, which without such a contrivance passes off in smoke. Soon after the original invention, Lange suggested a glass instead of an iron chimney, and altered its form by contracting the diameter of the cylinder a little above the burner. By this means the draught on entering the cylinder has its direction changed at the -point where the contraction begins, and is thus thrown upon the flame in a more advantageous direction for combustion. [See CANDLE. CHIMNEY. LAMP.] ARRIS, the intersection or line, on which two surfaces of a body forming an exterior angle meet each other. The term is used by all workmen engaged in building, as the arris of a stone, of a piece of wood, or of any other material. Although the edge of a body may in general language mean the same thing as its arris, yet, in building, the term edge is restricted to those two surfaces of a rectangular solid, on which the length and thickness be measured, as in boards, planks, doors, shutters, and other framed joinery. may ARROW-ROOT. A very pure form of STARCH, used as food. It is obtained from the tuberous roots of several species of Maranta, a family of herbaceous tropical plants, of which Maranta arundinacea is the most esteemed, and produces the best West Indian arrow-root. This starch is more nourishing than that from wheat or potatoes, and is obtained in the following manner :-The fleshy rhizomes, or roots of the plant, are dug up when a year old, and well washed in pure water; they are then either grated or pounded in wooden mortars, and the pulpy matter to which they are reduced is then thrown into a large quantity of water, and agitated. The fibrous parts are collected in the hand, squeezed, and removed, and the remaining milky liquor is strained through a hair-sieve, and left to settle. The white pasty mass, which sinks to the bottom of the vessel, is the arrow-root, which is sometimes subjected to a further washing, and again left to subside, before it is pronounced fit for the market. It is then dried and packed for exportation. The arrow-root of Bermuda is considered the finest. ARSENIC, (dpor evikov, masculine, so called from its masculine force in destroying man,) resembles the metals in its physical properties, but not so in its chemical, for, instead of forming oxides with oxygen, it forms acids, in which respect it resembles phosphorus. The term arsenic, in its popular signification, is one of the oxides of the metal, namely, arsenious acid, or white arsenic. This acid combining with bases forms arseniates, many of which are native. Arsenic also occurs as a sulphuret, and is frequently found in combination with other sulphurets, as with the sulphuret of iron, forming arsenical pyrites. During the roasting of the arsenical sulphurets of copper, iron, cobalt, and nickel, abundance of white arsenic is formed, from whence the commercial demand is supplied. Traces of arsenic are to be found in many minerals and in their products, as in sulphuric and sulphurous acids, in zinc, sulphuret of antimony, &c. Metallic arsenic may be obtained by gradually heating to redness white arsenic, with its weight of black flux,' in a small retort contained in a sand-bath: the metal sublimes into the neck of the retort. It is of a steel grey colour, of crystalline texture, very brittle, and of the specific gravity of about 5.8 Heated to a dull red, it sublimes without fusing, so that it would appear to be incapable of assuming the fluid state. According to Regnault, the reason for this is because the temperature at which it fuses is very near that at which it boils, under atmospheric pressure. Volatile bodies throw off vapours far below their boiling points, a property applicable to solids as well as to liquids. Arsenic gives off abundant vapours at a temperature a little below its boiling point, and can be completely sublimed without attaining its fusing point. But the distance between the fusing point and the boiling point of a body may be indefinitely increased. The boiling point of any substance is the temperature at which the elastic force of its vapour is in equilibrium with the pressure exerted on it. By increasing this pressure the boiling point is necessarily raised; but the fusing point is (1) Black flux is made from a mixture of 1 part nitre and 2 parts crude tartar, introduced in successive small quantities into a large earthen crucible, heated enough to cause feeble combustion. From the quantity of tartar used, this flux contains an excess of charcoal, resulting from the tartaric acid, and this frequently Several substitutes for arrow-root have been employed, such as Canna starch, or Tous les mois, obtained from the roots of Canna coccinea, extensively assists in converting metallic oxides into metals, by abstracting grown at St. Kitt's; Otaheite arrow-root, which is the oxygen. |