DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. or 
as liable to ony as before. To be sure, kiln drying is much better 
than air drying, since the high temperatures employed i in the former 
process are probably fatal to the hyphe of some decay-producing 
fungi, while under the latter conditions the fungi may merely re- 
main dormant until suitable moisture conditions are again restored. 
However, since wood-destroying fungi are common around and in 
ards and wood-working factories, the chances are that kiln-dried 
umber will be reinfected, and if it becomes moist again decay will 
begin. 
Shipping green or even partially air-dried lumber on long voy- 
ages through tropical seas in the hold of a vessel offers a chance 
for a heavy loss through decay. The close humid air of the ship’s 
hold becomes a perfect forcing chamber for wood-destroying fungi 
when warm latitudes are reached. Shipments of Douglas fir leav- 
ing the Pacific coast perfectly sound have contained a considerable 
percentage of decayed lumber when unloaded at a South African 
port (36, p. 36). Indirect reports indicate that the same condition 
resulted during the World War in some shipments of Sitka spruce 
ped to Europe through the Panama Canal and the Mediterranean 
ea 
Durability of wood.—Resistance to decay, or as it is termed 
“durability,” is a factor that should no longer be neglected in 
selecting woods for airplane construction. Airplanes are being 
more and more exposed to unfavorable weather conditions as their 
use extends, conditions which in some instances are highly favorable 
to decay. Furthermore, certain conditions created by the construc- 
tion of an airplane promote decay. For example, in the interior of 
the wings the relative humidity may be much higher than that of 
the surrounding air, and there is often considerable condensation 
of moisture. In addition, the temperature is slightly higher. All 
pee. factors are favorable to the development of wood- destroying 
ngi. 
Within any species durability increases with the increase in 
specific gravity. Consequently, the fact that only wood with high 
specific gravity is used for aircraft not only increases strength but 
serves to increase durability. However, it is well known that differ- 
ent species vary widely in their durability. Unfortunately, spruce 
is not at all durable. Neither are basswood and birch. Douglas fir 
is fairly durable, as is also white oak. But the cedars are remarkable 
for their inherent durability, and among these Port Orford cedar 
compares favorably with spruce in all its streneth properties and is 
only slightly heavier. Consequently, this wood can not be too 
highly recommended for use in aircraft where resistance to decay 
must be considered. Sapwood must not be used under such cir- 
cumstances, for no matter what the species is it decays easily. 
Contrary to existing belief, the resin conan: of ie is of slight 
importance in relation to durability (74, p. 153-154; 75, p. 66-68). 
Resin itself has no poisonous effect, on the aia. “of fungous 
hyphe, and its only beneficial effect in increasing durability is its 
waterproofing action on wood. This is so slight, however, if the 
normal resin content of softwoods is considered, as to be practically 
negligible. If wood is rendered more durable through a sufficient 
increase in its resin content to have a decided waterproofing effect, 
