The trees selected should be those that have most nearly attained their full maturity, a period which varies greatly with the different species. As a rule, the age and the rate of growth of a tree may be ascertained from the number and the width of the rings of annual increase which are exhibited in a cross section of the wood. (4) That timber should be felled or cut either in midsummer or in mid winter. In midsummer, the most suitable time is in the month of July. A tree should be cut as near to the ground as possible, as the lower part of the trunk furnishes the best timber. (5) That as soon as a tree is felled it should be by having its bark stripped off, then raised from the ground and the sap-wood re moved, and finally squared or reduced to the required dimensions. (6) That in the inspec tion of timber the quality of the wood may be ascertained by observing that its color, as ex hibited by a cross-section, is practically uniform in the heart. It may be a little deeper in color at the centre than near the white-colored sap wood next to the bark, but the gradation should be uniform and free from sudden transitions of color, or white spots, which are infallible signs of decay; that it is free from "wind-shakes" or circular cracks which sep arate the concentric layers of wood from each other, and constitute a very serious defect; that it is free from °splits," "checks" and °cracks," which extend very deeply toward the centre; and that it is free from large or de cayed knots, which tend to materially affect its strength.
Furthermore, the condition known as is generally consequent to the decay of the tree on account of age, and is characterized by a reddish color of the wood which becomes porous, and breaks off short without splintering; while "belted" timber is that which has been killed, or which has died from some unavoidable cause, before being felled, and is of a highly objectionable char acter. Other serious defects are indicated by the presence of many knots, which, although the timber may be sound, stamps it as being of stunted growth, and is commonly known as "knotty timber," and a spirally winding grain characteristic of "twisted wood" which is unfit for long pieces. Dry-rot is indicated by yellow stains; elm and beech are very quickly affected by it if left with the bark on after f elling.
The proper seasoning and preserving of timber is of the utmost importance in con nection with its use as a material of construc tion. Freshly cut timber contains from 35 to 50 per cent of moisture, which may be reduced to 17 or 25 per cent, by exposure to the air in seasoning one year,.andto less than 12 per i cent, by artificial drying in a comparatively short time.
There are various processes of seasoning. Natural seasoning requires a period ranging from two to eight years, according to the size and physical properties of the wood. Timber of large dimensions is not only improved in strength, but is rendered less liable to warp and crack in becoming seasoned, by being pre viously immersed in water for several weeks. When the seasoning is accomplished naturally by exposure to the air, the timber should be piled under a shed and kept dry, with a free circulation of air about it, but without exposure to strong currents. The bottom pieces should be placed upon skids raised about two feet from the ground, and a space of at least an inch should be allowed between the horizontal layers. Slats or piling strips should be placed between the layers at each end of the pile, and also at short intervals between the ends so as to prevent the timber from °winding.* It
is important that these strips should be placed directly one over the other and that they should not be less than one inch in thickness. Care should be taken to pile the heavy tim bers upon the ground floor of the shelter, and the light stuff upon the upper portion, with a clearance of at least two and one-half feet between the piles. The timber should he re piled from time to time, and all pieces showing any indications of decay should be removed, so as to prevent their affecting that which is sound and healthy. The gradual method of seasoning is undoubtedly the most suitable for preserving the strength and durability of the timber; but, as already stated in the preceding paragraphs, it has been very definitely ascer tained by tests that the results of artificial methods, properly applied, do not indicate that 'those qualities are materially affected by such processes, while other important advantages, such as the reduction in the time required for seasoning, and the uniformity of the percentage of moisture contained in the seasoned product, are unquestionably obtained. The hastening of seasoning by steaming the timber has been successfully accomplished, and the saturation of timber with a solution of corrosive sub limate, to secure it against dry-rot, and to protect it from the attacks of worms, has proved very satisfactory. Kiln-drying, how ever, is applicable only for boards and other pieces of small dimensions, and has a tendency to crack the wood and impair the strength in various ways, unless it is accomplished very slowly. Timber ought not to be seasoned by either charring or smoking, and should not be painted unless it has been thoroughly seasoned, as such methods and applications effectually prevent the drying of the wood in the interior of the piece, so that fermentation sets in and decay soon takes place.
The principal processes of impregnation are the following: •Kyanizing,° introduced by Kyan in 1832, consists in saturating the wood with a solution containing one pound of chloride of mercury to four gallons of water under a pressure of 15 pounds per square inch; gBur nettizing,* introduced by Burnett in 1838, by which the wood is first submitted to live steam at a pressure of 20 pounds for several hours; then exhausted by vacuum pumps for a few hours; then impregnated in vacuo with a solu tion of one pound of zinc chloride to 10 gal lons of water, at 160° F., and finally treated to air pressure at 150 pounds per square inch; "Creosoting,* introduced by Bethel, by which the wood is impregnated with the oil of creosote mixed with bituminous matter under an end wise pressure of 150 to 400 pounds per square inch. The Kyanizing and the Boucheri processes are applied to standing timber, that is, while the tree is still growing, the head is cut off and the top of the stem is hollowed into the form of a bowl and filled with the impreg nating solution which, being replenished from time to time, soaks down into the tree, killing It as it goes down, but thoroughly saturating the wood and imparting to it a remarkable degree of durability. Timber may be creosoted by simply steeping it in the oil of creosote, but the ((creosoting* and Thurnettizing* of tim ber of large dimensions is accomplished with the aid of special apparatus that gives the requisite pressures. Creosoting is the most satisfactory method of preserving timber used as piles for wharves against the attacks of the or ship-worm, but it is not an infal lible remedy.