Timber is sawed in countries producing, or using it, in great quantities in saw-mills, in which the tools are worked by water or steam, as described in the last chapter; and it is also sawed into battens, laths, &c., by circular saws, turned by machinery, like a lathe; but when timber is sawed by hand, it is done by two men acting in concert in the following manner:—A pit is generally chosen, round the margin of which a stout frame is laid. The beam to be sawed is laid along the centre of this frame, in the direction of the length of the pit. One man stands on the beam while another is in the pit below him, and each alternately raises or pulls down a large vertical saw, with which the beam is cut lengthwise into planks. Wedges of wood are placed in the fissure as the work proceeds, to keep the cut open, and thus allow the saw to play freely. This is very hard labour, especially to the upper man, who has not only to raise the weight of the saw in the up-stroke, but to guide it correctly along the chalked line on the beam. This man gets higher wages, and is called the top-sawyer, a term technically given in jest to any one who is, or fancies himself, of superior importance.
Scarfing or Joining Timber.
When timber is wanted in lengths exceeding those that can be procured from the tree in one piece, it must be joined by what is called scarfing; that is, the ends of the two lengths that are to be united into one, are cut so that a portion of the one may lap over and fit into a portion of the other, which is cut so as to receive it. The timber, when united, is thus of the same uniform size. The joined ends are secured together by bolts or spikes. The following figures show the more usual modes of scarfing timber for different purposes.
The last is a mode of scarfing invented by Mr. Roberts, of the Royal Dock Yards.
Trussing or Strengthening.
When a beam of timber is long in proportion to its breadth and thickness, it will bend by its own weight, and will be incapable of supporting much additional load; it may be strengthened by trussing, in different modes, of which we will only describe that usually adopted for girders, intended for floors. The beam is sawed longitudinally into two equal beams, each, of course, half the thickness of the original: these halves are reversed, end for end, so that if there were any weak part in the original beam, this may be divided equally between the ends of the compound beam made up of the two halves when bolted together. A flat truss, usually of oak, with iron king-bolts and abutting plates, resembling in form and principle a timber roof or bridge, is placed between the two half beams, and let into a shallow groove cut in each half to receive it; the compound beam, with this truss in the middle, is then bolted together again by means of iron bolts, with washers and nuts, and consequently becomes rigid by the construction of the truss. The truss is not entirely let into the double beam, as the full effect of strength may be obtained without the necessity for cutting the groove in each half beam of half the thickness of the oak truss; consequently, when the girder is completed, there is a slit all along it, through which the truss is seen lying in its place between the two sides.
Iron trusses are often used instead of oak, and beams are frequently strengthened by screwing a thin flat iron truss on one or both sides, let into the beam for about half the thickness of the metal.
This mode of strengthening a beam by trussing is only adopted in floors, where it is necessary to limit the depth of the truss to that of the beam, to obtain a level surface by means of joists laid across, and supported by, the beam. But it is obvious that much greater strength may be imparted to a long beam by making it the base of a triangular frame, as is done in roofs, in various manners, when the slanting sides of the triangular frame carry the battens or laths for supporting the tiles or other covering.