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DECAYS AND DISCOLORATIONS 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 Teadily detected on a 
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 j in the white sapwood, but are not so apparent, "although 
still recognizable, 
against the reddish 
brown heartwood. 
Resin ducts do not ac- 
company lightning 
rings 1n cedar. 
Sitka spruce wood 
is rather susceptible 
to the effects of elec- 
tricity. The light- 
ning rings appear as 
light to dark brown 
bnes in the pale pink- 
ish heartwood or white Fie. 5.—-Cross section from an unfinished elevator beam, 
p Wegmoe deities are eo criaa ces ecendime Giticels across the section can ne 
found which appear seen in the summer wood of the fifth annual ring from 
2 the bottom. The defect ran the entire length of the 
to be chiefly composed _ beam. 
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 hghtning 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 may 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 any purpose, particularly since 
the discoloration when varnished appears merely as a darker tone 
of the normal heartwood. 
The lightning rings found in Douglas fir are red-brown in color, 
darker than the summer wood and consequently are quite apparent 
in the white sapwood and orange-red or yellowish heartwood. These 
rings are practically composed ‘of resin ducts. The ducts are smaller 
than in Sitka spruce. 
The reader must not get the impression from what has been written 
that lightning rings are a feature of every piece of wood, but they 
do occur and must be taken into account. 
