Page:Cassells' Carpentry and Joinery.djvu/54

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The stiffest rectangular beam that can be cut out of a round log of timber is shown in Fig. 153, where the diameter is divided into four equal parts, but otherwise the construction and calculation will be on

Fig. 154.—Lining Log Timber.

similar lines to the above, resulting in $$ \frac {FB}{AF} = \frac {1}{\surd 3}$$; and with a log of diameter $$AB = D$$ the depth of the stiffest beam will be •866 and the breadth •5.

See that timber is free from sap, large or loose knots, flaws, shakes, stains or blemishes of any kind. A light portion near one edge indicates sap, and an absence of grain will be observed on it. This portion decays first and gets soft. The darker the natural

Fig. 155.—Lining Balk Timber.

wood, the lighter is the sappy portion usually when dry. Good timber should be uniform in substance, straight in fibre, and not twisted, warped, or waney. Diagonal knots are particularly objectionable in timber for piles. Good timber should smell sweet when fresh cut, and it has a firm, bright surface, and does not clog the saw. The annular rings should be fairly regular and approximately circular; the closer and narrower the rings the stronger the timber. The colour should be uniform throughout, and not become suddenly lighter towards the edges. Good timber is sonorous when struck; a dull sound indicates decay. In specimens of the same class of timber the heavier is generally the stronger.

Pit sawyers employ various methods for lining timber that is to be sawn; one reliable

Fig. 156.—Ochre Box.

method is shown by Fig. 154. The wedge boy (as he is termed) holds the string centrally at one end of the log, and the sawyer holds it at the other end. The string is then pulled tight, and one of the sawyers raises it at the centre (the string should not be raised exactly vertical, but pulled slightly at one side), then lets it go, so as to strike a line the whole length of the log. A plumb-line is then hung over the end of the log by one of the sawyers, and when perfectly plumb with the centre line, it is pulled tightly against the bottom edge of the log. The other sawyer pulls the string and strikes a central vertical line, as shown at. A similar line is struck at the other end of the log, after which the thickness

Fig. 157.—Wood for Pulling String.

of the planks that are to be sawn is pricked off with a pair of compasses, as indicated by the dots on the end of the log. The plumb-line is again hung over the end of the log in perfect line with the compass marks. Vertical lines, as before, are then struck, then corresponding longitudinal lines. When lining a square balk, a centre line is first struck, then the thickness of the planks is pricked off, as shown in Fig. 155, and the lines are struck. The plumb-line is then hung over the end, as shown, and the vertical