14 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 
latter, reported as being made from decayed or “dead” wood. This 
was characterized by the fact that the struts, after assembly, if 
struck sharply on the side near the middle, would “bow” and re- 
main in such position. If struck then on the opposite side the 
“bow ” might reverse, but the strut rarely straightened up. This 
condition results from such an excessive tightening of the drift 
and flying wires that the struts acting as posts between the wings 
are placed under such a heavy compression load 
that they are just on the point of failure, hence the 
inability to straighten up after bowing. Needless 
to state, such a condition is dangerous. 
Figure 3 shows a compression failure in the head 
of a vertical fuselage strut due to severe tightening 
of the tension wires in the fuselage. This failure 
occurred before the machine had left the factory 
floor. 
In the foregoing pages no attempt has been made 
to specify all the defects in airplane timber aside 
from decays or discolorations, or to describe fully 
those mentioned. The writer desires merely to call 
the attention of the reader to defects that can re- 
celve only passing mention or must be omitted 
entirely, so that, if interested, he can become con- 
versant with these through the references cited. 
COLOR COMPARISONS. 
still in the living tree. For the first few years after 
formation wood is white or nearly so, and finally 
when the sapwood is transformed to heartwood a 
Fic. 3—Head of a decided color change takes place in most woods, 
fuselage strut, > 
Color is a natural characteristic of wood while 
showing a com- While in some the change is negligible. In such 
eka spruce species as redwood (Sequoia sempervirens (Lam.) 
caused by an ex- Wndl.), incense cedar, Douglas fir, juniper (Juni- 
cessive tighten- ? y 
ing of the ten. perus), white ash, true mahogany, and white ocak 
sion wires In the there is a decided contrast between the light-colored 
3 sapwood and the dark heartwood, while in spruce, 
fir, western hemlock, and yellow buckeye (Aescutus octandra Marsh) 
the heartwood more nearly approaches the sapwood in color, and in 
some cases it is difficult to distinguish between the two. Color is not 
always uniform in the heartwood. It is necessary to be thoroughly 
acquainted with woods to be able to recognize normal color variations 
at a glance. 
Color should always be observed on a freshly cut surface and the 
surface (whether radial, tangential, or cross) recorded when making 
permanent observations. All woods change color on exposure to 
light and air (54), the most noticeable change occurring in the lighter 
colored woods, particularly of the conifers. The first change is a 
yellowing, then a graying, and finally in some conifers a decided 
browning. These color changes have no weakening effect on the me- 
chanical properties of wood, since the discolored portion is a very 
thin surface layer and microorganisms play no role in this change 
ibe Ye, « 4. 
