to make its way through the wood longitudinally, or along the grain, as is the case with the common ship worm, but directly, or obliquely, inward. Neither does it appear to make its way by means of any hard tools or instruments, but rather by some species of dissolvent liquor furnished by the juices of the animal itself. The rate of progression is, that a three inch oak plank will be destroyed in eight years by action from the outside only." For resisting the effects of these worms, Smeaton recommends the piles to be squared, to be fitted as closely as possible together, and to fill all openings with tar and oakum, to make the face smooth, and cover it with sheathing.

1744. The destructive effects of the white ant are so little known here, that it is unnecessary to make further mention of them, than that in India they are the most inveterate enemies with which timber has to contend. From Young's Annals we extract the following curious statement of experiments made upon inch and a half planks, from trees of thirty to forty-five years' growth, after an exposure of ten years to the weather.

Whence we may be led to some inference of the value of different sorts of timber in resisting weather; though we must not be altogether guided by the above table, inasmuch as it is well known that the soil on which timber is grown much increases or deteriorates its value, and that split timber is more durable and stronger than that which is sawn, from the circumstance of the fibres, on account of their continuity, resisting by means of their longitudinal strength; whereas when severed by the saw, the resistance depends more on the lateral cohesion of the fibres. Hence whole trees are invariably stronger than specimens, unless these be particularly well selected, and of a straight and even grain; but in practice the results of experiments are on this account the more useful.

DECAY OF TIMBER.

1745. If timber, whatever its species, be well seasoned, and be not exposed to alternate dryness and moisture, its durability is great, though from time it is known to lose its elastic and cohesive powers, and to become brittle if constantly dry. On this account it is unfit, after a certain period, to be subjected to variable strains: however, in a quiescent state it might endure for centuries. Dryness will, if carried to excess, produce this category. The mere moisture it absorbs from the air in dry weather is not sufficient to impair its durability. So, also, timber continually exposed to moisture is found to retain for a very long period its pristine strength. Heat with moisture is extremely injurious to it, and is in most cases productive of rot, whereof two kinds are the curse of the builder, the wet and the dry rot, though perhaps there be but little difference between the two. They appear to be produced by the same causes, excepting that the freedom of evaporation determines the former, and an imperfect evaporation the latter. In both cases the timber is affected by a fungus-like parasite, beginning with a species of mildew; but how this fungus is generated is still a vexata quæstio; all we know is, that its vegetation is so rapid, that often before it has arrived at its height, a building is ruined. From our inquiries on the Continent, we believe the disease does not occur to the extent that it does in this country; a fact which we are inclined, perhaps erroneously, to attribute to the use of the timber of the country, instead of imported timber. Our opinion may be fanciful, but there are many grounds on which we think that is not altogether the case. Our notion is, that our imported timber is infected with the seeds of decay long before its arrival here (we speak of fir more especially), and that the comparative warmth and moisture of the climate bring more effectually the causes of decay into action, especially where the situation is close and confined. Warmth is, doubtless, known to be a great agent in the dry rot, and most especially when moisture co-operates with it, for in warm cellars and other close and confined situations, where the vapour which feeds the disease is not altered by a constant change of air, the timbers are soon destroyed, and become perfectly decomposed.

1746. The lime, and more especially the damp brickwork, which receive the timbers of a new building, are great causes of decay to the ends of them; but we do not think that the regulations of the 19 Car. II. cap. 3., which directed the builders after the fire of London, to bed the ends of their girders and joists in loam instead of mortar, would. if followed out in the present day, be at all effective in preventing the decay incident to the ends of timbers. Timber, in a perfectly dry state, does not appear to be injured by dry lime; and indeed, lime is known to be effectual in the protection of wood against worms. Timber in contact with masonry is constantly found to decay, when the other parts of the

beam have been sound. This will be entirely obviated by inserting the wood in an iron shoe, or by placing a thin piece of iron betwixt the wood and the stone. Cases are known in which the iron shoe appeared to have proved a complete protection against dry rot and decay; a hard crust being formed on the timber in contact with the metal. The system of grouting must contribute to the early decay of wood bond; but at Manchester, where it was used very generally, it appeared to answer well, for the high temperature kept up in the buildings may cause the walls to dry very soon. Sea-sand, used for outside and inside purposes, in a spirit of economy, soon shows the result by inducing the appearance of rot in timber. Wood laid in sandy soil is well preserved, as was found to be the case in the specimens lately dug up at Birkenhead from depths varying from 8 feet to 32 feet; they were considered to have been buried for centuries.

1747. Nothing is more injurious to the floors of a building than covering them with painted floorcloth, which entirely prevents the access of atmospheric air, whence the dampness of the boards never evaporates; and it is well known that oak and fir posts have been brought into premature decay by painting them before their moisture had evaporated; whilst in the timber and pewing of old churches, which have never been painted, we see them sound after the lapse of centuries. Semple, in his Treatise on Building in Water, notices an instance of some field gates made of the fir of the place, part whereof, near the mansion, were painted, and had become rotten, while those more distant from the mansion, which had never been painted, were quite sound.

1747a. According to Baron Liebig, the decay of wood takes place in the three following modes:-I. The oxygen in the atmosphere combines with the hydrogen of the fibre, and the oxygen unites with the portion of carbon of the fibre, and evaporates as carbonic acid; this process is called decomposition. II. The actual decay of the wood which takes place when it is brought in contact with rotting substances. And III. The inner decomposition of the wood in itself, by losing its carbon forming carbonic acid gas, and the fibre under the influence of the latter is changed into white dust; this is called putrefaction.

PREVENTION OF DECAY.

1748. After timber is felled, the best method of preventing decay is the immediate removal of it to a dry situation, where it should be stacked in such a manner as to secure a free circulation of air round it, but without exposure to the sun and wind, and it should be rough squared as soon as possible. When thoroughly seasoned before cutting it into scantlings it is less liable to warp and twist in drying. The ground about its place of deposit should be dry and perfectly drained, so that no vegetation may rise on it. Hence a timber yard should be strewed with ashes, or the scales from a foundry or forge, which supply an admirable antidote to all vegetation. It is thought that the more gradually timber is seasoned the greater its durability; and as a general rule, it may be stated, that it should not be used till a period of at least two years from its being felled, and for joiners' work at least four years. Much, however, is dependent on the size of the pieces. By some, water seasoning has been recommended; by others the steaming and boiling it; smokedrying, charring, and scorching have also been recommended. The latter is, perhaps, the best for piles and other pieces that are to stand in the water or in the ground. It was practised by the ancients, and is still in use generally for the posts of park paling and the like. 1749. In Norway the deal planks are seasoned by laying them in salt water for three or four days, when newly sawed, and then drying them in the sun, a process which is considered to be attended with advantage; but it does not prevent their shrinking. Mr. Evelyn recommends the water seasoning for fir.

1749a. The effectual seasoning obtained by Davison and Symington's patent process of forcing heated air in a continued current through timber under pressure, effectually dries it, and coagulates the albumen. The timbers for the flooring of the Coal Exchange at London have been sɔ treated, and show no signs of shrinkage. The wood was taken in its natural state, and in less than ten days it was thoroughly seasoned; in some cases from 10 to 48 per cent of moisture was taken out of it. The air when heated to about 110° or 120° is sent through the timber at a rate of about 48 miles per hour; the heat being regulated according to the quality of the timber. Honduras mahogany exposed to a heat of 300°, would have the whole of the moisture taken from it in 48 hours. This process, however, sometimes splits the timber. Out of a hundred specimens of wood experimented upon, varying from one inch to twelve inches square, not one of them split: even some openings which were visible before the process was applied, were found to be closer after it. Perhaps 9 inches square is the limit to which the operation can be successfully applied.

1750. Notwithstanding, however, all care in seasoning, when timber is employed in a damp situation it soon decays; and one of the principal remedies against that is good drainage, without which no precautions will avail. It is most important to take care that earth should not lie in contact with the walls of a building. for the damp is quickly communicated, in that case, by their means to the ends of timbers, and rot soon follows. No expedient to guard against this contingency is so good as what are cailed air drains.

1751. When the carcass of a building is complete, it should be left as long as possible to dry, and to allow to the timbers what may be called a second seasoning. The modern practice of finishing buildings in the quickest possible period, has contributed more to dry rot than perhaps any other cause; and for this the architect has been blamed instead of his employer, whose object is generally to realize letting or to enjoy occupation of them as early as possible. After the walls and timbers of a building are once thoroughly dry, all means should be employed to exclude an accession of moisture, and delay is then prejudicial. 1752 Among the many inventions to preserve wood from decay, those of England have proved the most successful. In 1737 a patent was granted to Mr. Emerson to prepare timber with hot oil. This was followed by various recommendations early in the present century; those of later date consist of:-I. Kyan's process, 1832, who steeped the timber in a solution of bichloride of mercury, known as corrosive sublimate (par. 1742.) It appears to penetrate fir less than some other woods (Faraday). The wood thus treated becomes of less specific gravity, less flexibility, and more brittle. II. Sir William Burnett's patent of 1836, was for using the chloride of zinc. III. M. Bréant in 1837 suggested sulphate of iron, which was found not to alter the qualities of the timber as did the corrosive sublimate. IV. Margary's patent, 1837, is for steeping timber in a solution made of one pound of sulphate of copper with eight gallons of water. Wood impregnated with sulphate of copper (blue vitriol) will not last longer in sea water than any other wood. But wood so treated will last longer in the soil than if either tarred or charred. Its application for the prevention of rot is beneficial, and it might be used where not exposed to the action of water, on account of the solubility of the salts. The proportion of the sulphate should be one pound to four gallons of water; we have also met with the proportion of one pound to two gallons; perhaps the strongest is the best (par. 1752b.) V. Payne, 1841, patented a system for using two solutions; first, sulphate of iron, which would form an oxide of iron in the cells; and secondly, carbonate of soda: some very good results were obtained, but the process must be done under pressure and with the greatest care.

1752a. VI. Bethell's patent, 1838, consists in the injection of oil of tar, containing creasote and a crude solution of acetate of iron, commonly called pyrolignite of iron, after the air in the wood has been extracted. This process is effective to a great extent, and full particulars are given by G. R. Burnell in his paper read before the Society of Arts 1860, from which we have been quoting. It, however, can only be recommended for railways and other large works; the offensive smell and increased danger by fire should deter its use in house building. In the best creasoting works, the oil is injected at a temperature of 120° and under a pressure of 150 lbs. on the square inch, so that ordinary fir timber absorbs 10 lbs. weight of the creasote per cubic foot; the wood should be weighed to ascertain that it did absorb that quantity. For all engineering purposes, fir timber thus treated is far more durable than the best oak, teak, or other hard woods, and the cost of the operation is very small. Timber which has just been taken out of water contains so large a quantity that it resists the entrance of the oil; unless time, therefore, be given for it to be first dried, it would necessarily be badly prepared.

17526. VII. Dorsett and Blythe, 1863, patented the injection of heated solutions of sulphate of copper (par. 1752, IV.), a process said to have been adopted by French, Spanish, Italian, and other railway companies. Amongst its advantages, they state that wood so prepared is rendered to a great extent incombustible; and that for out-door purposes it has a clean yellowish surface, without odour, requires no painting, remaining unchanged for any length of time.

1752c. Experience of the English processes shows that creasoting is the most generally successful; the application of the sulphate of copper is satisfactory in many cases; while the other processes, although no doubt of occasional value, have been practically abandoned. They all depend for their success upon the skilful and conscientious manner in which the are applied; for as they involve chemical actions on a large scale, their efficiency must depend upon the observation of the minute practical precautions required to exclude any disturbing causes.

1752d. Carbolineum Avenarius is said (1887) to be an efficient preservative of wood against all external and internal injurious influence s, driving the moisture out of it by making it impervious to damp, and is stated to be a preservative against the attack of white ants in hot climat s. Being thin and liquid it soaks into the wood readily. One gallon will cover from 30 to 50 square yards.

1753. It is no easy matter to cure the dry rot where it has once taken root. If it be found necessary to substitute new timbers for old ones, every particle of the fungus, known as the Merulius lacrymans, must be removed from the neighbourhood of such new timbers. After scraping it from the adjoining walls and timbers, perhaps no better application than one of the washes above mentioned can be employed. About 300° of heat would effect the same purpose, but this is difficult in application. Coal tar has been found useful, but its odour, arising at a moderate degree of heat, is an objection to its use. A weak solution of vitriolic acid with water will generally stop the rot if it have not gone too far. Pyroligneous acid is recommended for preventing the spreading of the disease. The precautions indicated above for the prevention of decay, although not always successful, must be deemed preferable to the application of after remedies.

SECT. V.

IRON.

1754. Iron is a metal found in almost all parts of the worla, and though not mentioned by Homer, and hence, we may suppose, in his time extremely scarce, it is now more abundant than any of the other metals, and is, at the same time, the most useful. Although, with the exception of tin, it is the lightest of all metals; yet it is, when pure, very malleable and extremely hard. Its malleability is increased by heat, whereas most other metals, as they are heated, become more brittle. It is the only known substance whereon the loadstone acts, and its specific gravity to water is as 7632 to 1000.

1755. The iron manufactured in Great Britain is obtained from three species of the ore. The Lancashire, which is very heavy, fibrous in texture, and of a dark purple colour inclining to black, and lodged in veins. The Bog ore, which has the appearance of a deep yellow clay, and is found in strata of from twelve to twenty inches in thickness. And lastly, Iron stones, of an irregular shape, frequently in beds of large extent, similar to other stony masses, and often intersected with seams of pit coal. It is principally from the argillaceous ore or clay iron-stone that iron is extracted in this country.

1756. After raising, the ores are selected and separated as much as possible from heterogeneous substances. They are then roasted in large heaps in the open air, for the purpose as well of freeing them from the arsenic and sulphur they contain as to render them friable or easy of reduction to a powder. The roasting is performed by means of bituminous coal, and the result is a substance full of fissures, friable, and a deprivation of all vitreous lustre. After this it is transferred to the crushing mill for complete pulverization, whence it is carried to the smelting furnace for conversion into iron. Herein it undergoes two separate processes: first, the reduction of the oxide to a metallic state; second, the separation of the earthy particles in the form of scoria. These operations are conducted by submitting the ore, ordinarily mixed with certain fluxes, to the action of carbon at a very high temperature, in what are called blast furnaces, which vary in height from twelve to Sixty feet, and are of the form of truncated cones, sometimes however of pyramids, terminating usually in cylindrical chimneys, whose internal diameter is from four to six feet. The interior of these furnaces is usually of a cylindrical form, whose internal diameter is from four to six feet. Their cavity is usually of a circular form, except at the crucible or hearth, where it becomes a right rectangular prism, oblong in a direction perpendicular to the blast orifices or tuyeres of the bellows. The sides of the crucible are most commonly formed of gritstone. The boshes, which are in the form of an inverted quadrangular pyramid approaching a prismatic shape, are placed above the crucible, and above them rises the conical body of the furnace, which is lined with fire-bricks, and, in ascending, is contracted similarly to the narrow end of an egg, until it terminates in the chimney. The furnace is of course constructed in the most solid manner, and strengthened by iron bands and bars. The bellows employed are mostly of a cylindrical form, and their pistons worked either by water or steam. The blast holes, which are in the upper part of the crucible, and frequently placed on opposite sides, but so that the two opposite currents may not impinge upon one another, are two in number. Openings are provided at the lower part of the crucible for the discharge of the metal and scoria, and are kept stopped by clay and sand upon the exterior when the furnace is in operation. The reduction is commenced by gradually heating up the furnace until capable of being entirely filled with fuel, and then, as its contents begin to sink, alternate changes of ore, mingled with flux, and of charcoal and coke, are added. The blast is now let on, and the metal in the ore, parting with its oxygen, flows by degrees, subsiding to the bottom of the crucible, covered with a melted slag, which is occasionally let off by removing the clay from one or more, if necessary, apertures in the crucible; and on the bottom of the furnace becoming filled with the metal, which generally occurs after nine to twelve hours, the iron itself is discharged by one of these openings into a fosse of sand mixed with clay. When the iron has flowed out the aperture is again closed, and by this method the furnace is kept in constant action.

1757. Limestone of the best quality is employed as a flux to assist the fusion of the ore, which it accomplishes by vitrefying the earths wherewith it is mixed up with the oxide of iron. The iron when run out from the blast furnace in the state of cast iron is far from being in a pure state, having a coarse grain, and being brittle. In its conversion to bar iron, it undergoes one of the two following processes, as charcoal or coke may be eployed. In the former case a furnace much resembling a smith's hearth is used, having a sloping cavity sunk from ten to twelve inches below the blast pipe. After the cavity has been filled with charcoal and scoria, a pig of cast iron, well covered with hot fuel, is placed opposite the blast pipe. The blast being introduced, the pig of iron lying in the very hottest part soon begins to melt, and runs down into the cavity below, where, being out of the influence of the blast, it becomes solid, and is replaced in its former position, and the

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cavity is again filled with charcoal. It is there again fused, and so on a third time, all these processes being accomplished in three or four hours. The iron, thus again solid, is taken out, and very slightly hammered, to free it from the attached scoria; after this it is returned to the furnace, in a corner whereof it is stacked, out of the action of the blast, and well covered with charcoal, where it remains gradually to cool until sufficiently compact to bear the tilt or trip hammer (to be shingled, to force out the cinders), which is moved by machinery, and whose weight is from 600 to 1200 lbs. Thus it is beaten till the scoriæ are forced out, and the particles cf iron welded together, when it is divided into several portions, which by repeated heating and hammering are drawn into bars, in which state it is ready for sale.

1758. There are various methods of procuring the blast; the first, and most ancient, is by means of bellows; the latest, which has been found in the mining districts to be a contrivance of great importance, is the placing a series of vanes attached to an axis, which, by machinery, are made to revolve in a box with great rapidity. A pipe passing from the outside of the box to the furnace conveys the air to it as the vanes revolve, a new portion continually entering by a hole at the axis. The air thus driven through at its natural temperature constitutes a cold blast in contradistinction to air heated by artificial means or hot blast. This latter system was discovered by J. B. Neilson, of Glasgow, about the year 1826; his patent expired in 1842. At the present day air is forced into the furnaces at a temperature of 600°, and even of 800° Fahr., although at the commencement it was rarely used above 300°. The irons obtained from the former process are considered to be tougher and stronger than those obtained from the latter process, and present a closer texture and a smaller crystallization than the latter irons. The Blaenavon, Coed Talon, Lowmoor, and Muirkirk irons are amongst the most esteemed varieties. Perhaps it may be laid down as a general principle that where pig-iron is remelte with coke in the cupola furnace, for the purposes of the ironfounder, or refined with coke in the conversion of forge pig into bar iron, it is of little consequence whether the reduction of the ore has been effected with the hot or the cold blast; but where large castings have to be run directly from the smelting furnace, the quality of the metal will, no doubt, suffer from the use of the hot blast.

1759. The proportion of pig or cast iron from a given quantity of ore varies as the dif. ference in the metallic contents of different parcels of ore and other circumstances, but the quantity of bar obtained from pig iron is not valued at more than 20 per cent.

1760. The other process for manufacturing bar iron, which is that chiefly employed in this country, is conducted in reverberatory furnaces, usually called puddling furnaces. The operation begins with the fusion of the cast iron in refinery furnaces, like the one above described. When the iron is fully melted, a tap-hole is opened in the crucible, and the metal and slag flow out together into a fosse covered with clay well mixed with water, by which a coating is formed that prevents the metal from sticking to the ground. The finer inetal forms a slab about ten feet long, three feet broad, and from two to two and a half inches in thickness. For the purpose of slightly oxidizing it, and to make it brittle, it is much sprinkled over with cold water. In this part of the process it loses in weight from 12 to 17 per cent. After this, it is broken up into pieces, and placed on the hearth of a reverberatory furnace, in portions heaped up to its sides in piles which rise nearly to the roof, leaving a space open in the middle to give room for puddling the metal as it flows down in streams. When the heat of the furnace has brought it to a pasty state, the tem perature is reduced, a little water being sometimes thrown on the melted mass. The semiliquid metal is stirred up by the workman with his puddle, during which it swells, and parts with a large quantity of oxide of iron, which burns with a blue flame, so that the mass appears ignited. As it refines, the metal becomes less fusible, or, as the workmen say, it begins to dry. The puddling goes on until the whole charge assumes the form of an incoherent sand, when the temperature is gradually increased to give it a red white heat, at which period the particles begin to agglutinate, and the charge, in technical language, works heavy. The refining is now considered finished, and the metal has only to be formed into balls, and condensed under the rolling cylinder. From this state it is brought into mill bar iron. After this last operation, several pieces are welded together, from which it acquires ductility, uniformity, and cohesion. A lateral welding of four pieces together now follows, and the mass passes through a series of cylinders as in the first case, and becomes English bar-iron.

1761. The lamination of iron into sheets is by a refinery furnace, with a charcoal instead of a coke fire.

1762. Malleable iron is often obtained from the ores directly, by one fusion, if the metallic oxide be not too much mixed with foreign substances. It is a mode of working much more economical than that above described, and from the circumstance of its having been long known and used in Catalonia, it is known by the name of the method of the Catalonian forge. The furnace employed is similar to the refiner's forge already described. The crucible is a kind of semicircular or oblong basin, eighteen inches in diameter, and