brick, are of the best quality; those from near the top and sides, are generally underburnt, and are denominated soft, pale, or sammel brick; they have neither sufficient strength, nor durability, for heavy masonry, nor the outside courses of walls, which are exposed to the weather.

165. The quality of good brick may be improved by soaking it for some days in water, and re-burning it. This process increases both the strength and durability, and renders the brick more suitable for hydraulic constructions, as it is found not to imbibe water so readily after having undergone it.

166. The size and form of bricks present but trifling variations. They are generally rectangular parallelopipeds, from eight to nine inches long, from four to four and a half wide, and from two to two and a quarter thick. Thin brick is generally of a better quality than thick, because it can be dried and burned more uniformly.

167. Fire-brick. This material is used for the facing of furnaces, fireplaces, &c., where a high degree of temperature is to be sustained. It is made of a very refractory kind of pure clay, that remains unchanged by a degree of heat which would vitrify and completely destroy ordinary brick. A very remarkable brick of this character has been made of agaric mineral; it remains unchanged under the highest temperature, is one of the worst conductors of heat, and so light that it will float on water.

168. Tiles. As a roof covering, tiles are, in many respects, superior to slate, or metallic coverings. They are strong and durable, and are very suitable for the covering of arches, as their great weight is not so objectionable here, as in the case of roofs formed of frames of timber.

Tiles should be made of the best potter's clay, and be moulded with great care to give them the greatest density and strength. They are of very variable form and size; the worst being the flat square form, as, from the liability of the clay to warp in burning, they do not make a perfectly water-tight covering.

WOOD.

169. This material holds the next rank to stone, owing to its durability and strength, and the very general use made of it in constructions. To suit it to the purposes of the engineer, the tree is felled after having attained its mature growth, and the trunk, the larger branches that spring from the trunk, and the main parts of the root, are cut into suitable dimensions, and seasoned, in which state, the term timber is applied to it. The crooked, or compass timber of the branches and roots, is mostly applied to the purposes of ship-building, for the knees and other

parts of the frame-work of vessels, requiring crooked timber. The trunk furnishes all the straight timber.

170. The trunk of a full-grown tree presents three distinct parts: the bark, which forms the exterior coating; the sap-wood, which is next to the bark; the heart, or inner part, which is easily distinguishable from the sap-wood by its greater firmness and darker color.

171. The heart forms the essential part of the trunk, as a building material. The sap-wood possesses but little strength, and is subject to rapid decay, owing to the great quantity of fermentable matter contained in it; and the bark is not only without strength, but, if suffered to remain on the tree after it is felled, it hastens the decay of the sap-wood and heart.

172. Trees should not be felled for timber until they have attained their mature growth, nor after they exhibit symptoms of decline; otherwise, the timber will be less strong, and far less durable. Most forest trees arrive at maturity between fifty and one hundred years, and commence to decline after one hundred and fifty or two hundred years. The age of the tree can, in most cases, be ascertained either by its external appearances, or by cutting into the centre of the trunk, and counting the rings, or layers of the sap and heart, as a new ring is formed each year in the process of vegetation. When the tree commences to decline, the extremities of the old branches, and particularly the top, exhibit signs of decay.

173. Trees should not be felled while the sap is in circulation ; for this substance is of a peculiarly fermentable nature, and, therefore, very productive of destruction to the wood. The winter months, and July, are the seasons in which trees are felled for timber, as the sap is generally considered as dormant during these months; this practice, however, is in part condemned by some writers; and the recent experiments of M. Boucherie, in France, support this opinion, and indicate midsummer and autumn as the seasons in which the sap is least active, and therefore as most favorable for felling.

174. As the sap-wood, in most trees, forms a large portion of the trunk, experiments have been made, for the purpose of improving its strength and durability. These experiments have been mostly directed towards the manner of preparing the tree, before felling it. One method consists in girdling, or making an incision with an axe around the trunk, completely through the sapwood, and suffering the tree to stand in this state until it is dead; the other consists in barking, or stripping the entire trunk of its bark, without wounding the sap-wood, early in the spring, and allowing the tree to stand until the new leaves have put forth and fallen, before it is felled. The sap-wood of trees, treated by both

of these methods, was found very much improved in hardness, strength, and durability; the results from girdling were, however, inferior to those from barking.

175. The seasoning of timber is of the greatest importance, not only to its durability, but to the solidity of the structure for which it may be used; as a very slight shrinking of some of the pieces, arising from the seasoning of the wood, might, in many cases, cause material injury, if not complete destruction to the structure. Timber is considered as sufficiently seasoned, for the purposes of frame-work, when it has lost about one fifth of the weight which it has in a green state. Several methods are in use for seasoning timber: they consist either in an exposure to the air for a certain period in a sheltered position, which is termed natural seasoning; in immersion in water, termed water seasoning; or in boiling, or steaming.

176. For natural seasoning, it is usually recommended to strip the trunk of its branches and bark, immediately upon felling, and to remove it to some dry position, until it can be sawed into suitable scantling. From the experiments of M. Boucherie, just cited, it would seem that better results would ensue, from allowing the branches and bark to remain on the trunk for some days after felling. In this state, the vital action of the tree continuing in operation, the sap-vessels will be gradually exhausted of sap, and filled with air, and the trunk thus better prepared for the process of seasoning. To complete the seasoning, the sawed timber should be piled under drying sheds, where it will be freely exposed to the circulation of the air, but sheltered from the direct action of the wind, rain, and sun. By taking these precautions, an equable evaporation of the moisture will take place over the entire surface, which will prevent either warping or splitting, which necessarily ensues when one part dries more rapidly than another. It is farther recommended, instead of piling the pieces on each other in a horizontal position, that they be laid on castiron supports properly prepared, and with a sufficient inclination to facilitate the dripping of the sap from one end; and that heavy round timber be bored through the centre, to expose a greater surface to the air, as it has been found that it cracks more in seasoning than square timber.

Natural seasoning is preferable to any other, as timber seasoned in this way is both stronger and more durable than when prepared

by any artificial process. Most timber will require, on an aver

age, about two years to become fully seasoned in the natural

way.

177. The process of seasoning by immersion in water, is slow and imperfect, as it takes years to saturate heavy timber; and the soluble matter is discharged very slowly, and chiefly from the

exterior layers of the immersed wood. The practice of keeping timber in water, with a view to facilitate its seasoning, has been condemned as of doubtful utility; particularly immersion in salt water, where the timber is liable to the inroads of those two very destructive inhabitants of our waters, the Limnoria Terebrans, and Teredo Navalis; the former of which rapidly destroys the heaviest logs, by gradually eating in between the annual rings; and the latter, the well-known ship-worm, by converting timber into a perfect honeycomb state by its numerous perforations.

178. Steaming is mostly in use for ship-building, where it is necessary to soften the fibres, for the purpose of bending large pieces of timber. This is effected by placing the timber in strong steam-tight cylinders, where it is subjected to the action of steam long enough for the object in view; the period usually allowed, is one hour to each inch in thickness. Steaming slightly impairs the strength of timber, but renders it less subject to decay, and less liable to warp and crack.

179. When timber is used for posts partly imbedded in the ground, it is usual to char the part imbedded, to preserve it from decay. This method is only serviceable when the timber has been previously well seasoned; but for green timber it is highly injurious, as by closing the pores, it prevents the evaporation from the surface, and thus causes fermentation and rapid decay within.

180. The most durable timber is procured from trees of a close compact texture, which, on analysis, yield the largest quantity of carbon. And those which grow in moist and shady localities, furnish timber which is weaker and less durable than that from trees growing in a dry open exposure.

181. Timber is subject to defects, arising either from some, peculiarity in the growth of the tree, or from the effects of the weather. Straight-grained timber, free from knots, is superior in strength and quality, as a building material, to that which is the reverse.

182. The action of high winds, or of severe frosts, injures the tree while standing: the former separating the layers from each other, forming what is denominated rolled timber; the latter cracking the timber in several places, from the surface to the centre. These defects, as well as those arising from worms, or age, are easily seen by examining a cross section of the trunk.

183. The wet and dry rot are the most serious causes of the decay of timber; as all the remedies thus far proposed to prevent them, are too expensive to admit of a very general application. Both of these causes have the same origin, fermentation, and consequent putrefaction. The wet rot takes place in wood exposed, alternately, to moisture and dryness; and the dry rot is occasioned by want of a free circulation of air, as in confined,

warm localities, like cellars and the more confined parts of vessels.

Trees of rapid growth, which contain a large portion of sapwood, and timber of every description, when used green, where there is a want of a free circulation of air, decay very rapidly with the rot.

184. Numberless experiments have been made on the preservation of timber, and many processes for this purpose have been patented both in Europe and this country. Several of these processes have yielded the most satisfactory results; and nearly all have proved more or less efficacious. The means mostly resorted to have been the saturation of the timber in the solution of some salt with a metallic, or earthy base, thus forming an insoluble compound with the soluble matter of the timber. The salts which have been most generally tried, are the sulphate of iron, or copper, and the chloride of mercury, zinc, or calcium. The results obtained from the chlorides have been more satisfactory than those from the sulphates; the latter class of salts with metallic bases possess undoubted antiseptic properties; but it is stated that the freed sulphuric acid, arising from the chemical action of the salt on the wood, impairs the woody fibre, and changes it into a substance resembling carbon.

185. The processes which have come into most general use, are those of Mr. Kyan, and of Sir W. Burnett, called after the patentees kyanizing and burnetizing. Kyan's process is to saturate the timber with a solution of chloride of mercury; using, for the solution, one pound of the salt to five gallons of water. Burnett uses a solution of chloride of zinc, in the proportion of one pound of the salt to ten gallons of water, for common purposes; and a more highly concentrated solution when the object is also to render the wood incombustible.

186. As timber under the ordinary circumstances of immersion imbibes the solutions very slowly, a more expeditious, as well as more perfect means of saturation has been used of late, which consists in placing the wood to be prepared in strong wrought-iron cylinders, lined with felt and boards, to protect the iron from the action of the solution, where, first by exhausting the cylinders of air, and then applying a strong pressure by means of a force-pump, the liquid is forced into the sap and air-vessels, and penetrates to the very centre of the timber.

187. Among the patented processes in our country, that of Mr. Earle has received most notice. This consists in boiling the timber in a solution of the sulphates of copper and iron. Opinion seems to be divided as to the efficacy of this method. It has been tried for the preservation of timber for artillery carriages, but not with satisfactory results.