. Department bulletin. Agriculture; Agriculture. DECAYS AND DECOLORATIONS IN AIRPLANE WOODS. 19 any importance, since the injury occurred alone with only a very short lightning ring and on the stream lining where high strength is not requisite. Lightning rings are more readily detected on ;i member before it is sanded. In some cases the seriousness of the defect can be determined after planing but before the piece is shaped. This is usually possible when the defect runs entirely through the piece. In white fir the lightning rings are easily detected both on cross section or on the radial face.
. Department bulletin. Agriculture; Agriculture. DECAYS AND DECOLORATIONS IN AIRPLANE WOODS. 19 any importance, since the injury occurred alone with only a very short lightning ring and on the stream lining where high strength is not requisite. Lightning rings are more readily detected on ;i member before it is sanded. In some cases the seriousness of the defect can be determined after planing but before the piece is shaped. This is usually possible when the defect runs entirely through the piece. In white fir the lightning rings are easily detected both on cross section or on the radial face. The normal color of the summer wood is a light brown, while the lightning ring is a pronounced brown or purplish brown, which stands out strongly against the whitish sapwood or heartwood: Abundant resin ducts occur in these rings. Lightning rings in incense cedar are dark brown in color, standing out plainly in the white sapwood, but are not so apparent, although still recognizable, against the reddish brown heartwood. Resin ducts do not ac- comp any lightning rings in cedar. Sitka spruce wood is rather susceptible to the effects of elec- tricity. The light- ning rings appear as light to dark brown lines in the pale pink- ish heartwood or white sapwood. Rings are found which appear to be chiefly composed of resin ducts; in fact, when viewed on the end section, it is seen that the resin ducts are so numerous that they almost coalesce. This condition is illustrated in Figure 5. Furthermore, spruce wood is peculiarly susceptible to dis- coloration by lightning injury. Often in connection with a lightning- ring a reddish brown discoloration is found, somewhat tinged with purple. This discoloration rarely extends radially more than 3 or 4 inches from the lightning ring toward the pith, but m?ij extend 2 feet beyond the limits of the ring in a vertical direction. Wood so dis- colored is not weakened. Furthermore, the color is not sufficiently intense to detract from its value for an
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