Selection cutting consists in removing the more mature trees of a given species or of all species down to a certain diame ter limit. On large tracts a valuation sur vey is made at the time to determine the quantity of timber in board feet above a certain diameter limit. In measuring the diameter it is always taken at breast height (Fig. 474), or four feet and four inches from the ground, to avoid the usual expansion at the base. The diameter limit is any that may be determined on, but is usually twelve, fourteen or sixteen inches ; the lower the limit the greater the harvest at the time and the longer the period that must elapse before another equal harvest can be gath ered from the same land. Usually 2 to 6 per cent of the timber is measured, and from this the re mainder is estimated. If the tract is small, a higher percentage or even all the trees may be measured. As this method implies making calcula tions for another crop, the diameter of all species down to two inches is frequently taken.
Calipers are used for measuring the diameter and a hypsometer f o r determining the height, althoughthe height may be ocularly estimated for all practical purposes in that particular kind of timber. If a hyp someter is not at hand, the height of a tree or any point on it may be determined by triangulation, according to the following diagram (Fig. 475): AB X at._ DE; or, in figures, suppose AB equals 90, AC and ab equals 20 ; then AB by ab equals 1800 ; divided by AC, supposed to equal 22, it gives nearly 82 feet as the height of the tree, or DE in the diagram.
In practice, a gang of four men is frequently engaged in making the survey. A half chain of thirty-three feet -is fastened to the belt of the chain-man, who is guided by a man with a compass in order to make as straight a line as possible through the woods. A man on either side of the chain-man calipers the trees for a lateral distance of thirty-three feet, and calls out the result to the chain-man, who makes record of it on a sheet especially prepared for the purpose. The chain-man also makes note of the direction and size of streams, of hills and of inclines that may be of interest or use in the harvest. The gang , proceeds for twenty half chains, when an acre has been covered. The measured acres are equal distances apart to the right and left of a base line through the tract. Sometimes circular areas of such radii as to contain a certain fraction or a whole acre, considered to be an average of the whole stand, are measured.
The volume is approximated by multiplying the area of the base of the tree at stump height by one-half the height. Each cubic foot of saw tim ber will cut out five to seven board feet. About eighty-five cubic feet of wood will pile up a stand ard cord of 4 x4 x 8 feet, or about thirty solid cubic feet will pile up a cord of sixteen-inch wood.
As a further means of determining the most prof itable procedure, stem analyses are made by de termining the increase through decades by measur ing the thickness of each ten annual rings, begin ning at the bark. By deducting from the present
volume that at any year previous, the increment during that period is obtained. From the average increment of a sufficiently large number of trees, a reasonably accurate account of what the whole area has been doing can be given and a working plan laid out. This method will determine for the owners whether the area being exploited is large enough to keep the mill running indefinitely.
The capacity of mills is usually far too large for the area, so that after a few years' cut a move must be made or the mill go out of the business.
Felling. (Figs. 476, 477.) In felling. the tree is chipped with the axe on the side in the direction in which it is to fall, in order to direct its course. Considerable skill in this matter is often necessary in order to place the tree where it is wanted on the ground. The cross cut saw is used for the remainder of the cut, beginning on the opposite side from the chipping. When the tree is about to fall the workmen should step off at right angles to the direction the tree is taking, in order to avoid falling limbs that are often thrown in the line with the tree. No attempt should be made to drive farm animals from danger after the tree begins to fall. Failure to heed one or the other of these precautions costs numerous lives every year. Care is taken to avoid breakage as much as possible and to have the logs in a con venient place for loading. When wood is frozen, it is much more brittle than at other times. When trees are small enough to permit of it, they are cut close to the ground, which makes a saving of timber. In felling the large trees of the West, the choppers stand on a scaffolding. (Fig. 477.) Sawmills. (Figs. 478-480.) The location of the mill is one of the most im portant factors in the harvesting of a forest crop. The large mills are always located on a pond, stream or lake, in order to provide water for the steam boilers and to have water into which the logs may be rolled before they are taken into the mill. The logs are taken into the mill by means of a jack-ladder,—a heavy, endless chain that runs in the bottom of a V-shaped groove extending into the water, over which logs are floated,—or other suit able conveyance. The small portable mill, which is moved about to gather up what the larger mills do not take, is located in a position convenient to most of the timber, the water for the l.eiler being supplied from a tank, p. I or small stream and the logs are rolled on to the carriage from a skidwac. The capacity of mills varies from one furnished with circular and band saws and which runs night and day, cutting in twenty-four hours one or two hundred thousand feet, to one that runs for a longer or shorter period daring the day; according to the demand of the customer and the will of the sawyer, cutting a few hundred or a thousand feet per day.