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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 case. Then such wounds must be scrutinized closely for the 

 presence of a lightning ring. Two or more of these wounds, which 

 resemble sapsucker wounds, occurring on the same annual ring 

 and connected by a lightning ring, are sometimes found. Figure 4 

 shows one of these wounds on an interplane strut, in this case not of 



Fig. 4. — Section from a finished interplane strut, showin 

 a small lightning injury in Sitka spruce. 



