. Department bulletin. Agriculture; Agriculture. 18 BULLETIN L128, U. S. DEPARTMENT OF AGRICULTURE. portion. In a short time the cell formation returns to normal. Ulti- mately, as the growth of the tree proceeds, these lightning rings, always following one definite annual ring, arc dec]) within the wood. extending completely or partially around the circumference over a varying distance. When the tree is worked up for lumber certain of the boards may have such lightning rings extending completely through, both in width and length. Such a board then consists of two layers of wood held together b
. Department bulletin. Agriculture; Agriculture. 18 BULLETIN L128, U. S. DEPARTMENT OF AGRICULTURE. portion. In a short time the cell formation returns to normal. Ulti- mately, as the growth of the tree proceeds, these lightning rings, always following one definite annual ring, arc dec]) within the wood. extending completely or partially around the circumference over a varying distance. When the tree is worked up for lumber certain of the boards may have such lightning rings extending completely through, both in width and length. Such a board then consists of two layers of wood held together by a zone of abnormal structure forming a plane of cleavage. Checking often occurs along this line, since the continuity of the medullary rays may be interrupted. Such checks are striking, since they invariably are tangential, following an annual ring on end section or radial face but not visible on the tangential face. This is not at all an un- common defect in air- plane timber. An ab- normal number of resin ducts may be found in the annual ring following many types of mechanical injury, but for prac- tical purposes there is no difference between such so-called trau- matic resin ducts and the abnormal ducts formed as a result of lightning injury. It js self-evident that Avood with these lightning rings must be used with discre- tion. Even though the lightning ring- does not check on dry- ing, when a mem- ber with this defect is p u t u ncler severe strain and stress a serious check may develop. Of course, every member showing a lightning ring need not be considered valueless. Such a defect in the stream line of a strut, for example, would be trifling, while a much shorter ring in the butt or inner bay of a wing beam, particu- larly if in the same plane as the bolts, Avould be serious. The same ring in the tip of such a beam could be overlooked. The detection of lightning rings in rough lumber is exceedingly difficult, unless accompanied by small wounds, which is sometimes the
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