54 Proceedings of Indiana Academy of Science. 
tive in preventing loss of balance or change of shape under the strains 
of service and in varying humidities. 
The species which proved to best combine the properties desirable in 
a propeller wood—i.e., relative freedom from checking, warping and 
splitting; good glueing qualities; moderate hardness; and ability to be 
pierced by a bullet without being split or shattered—are the Central 
American and African mahoganies and the black walnut. These are 
used on combat planes, where motors of great power are employed and 
the demands upon the propeller are particularly heavy. For training 
planes, however, white oak (quartered), cherry, birch, and the various 
species known in the trade as Philippine mahogany were found suitable. 
There fell also to the Forest Products Laboratory the task of train- 
ing many of the inspectors essential in the carrying out of the airplane 
program. It was necessary for these men to inspect material and parts 
after various steps in manufacture, such as kilning, glueing and fin- 
ishing. Some of them must also identify wood species and discern 
defects in the wood, often very difficult to detect, but, if overlooked, 
sufficient to cause the destruction of a costly machine or even the loss 
of an aviator’s life. Decay, knots and brittleness or brashiness are rela-, 
tively easy to detect; but pitch pockets well below the surface are very 
difficult to make out, as are also the so-called heart breaks. The origin 
of the latter defect is still somewhat obscure, but it is probably due to 
injuries to the tree by high winds while still standing, or to damage 
in felling. In planing, the fibers are sometimes broken in such a way 
as to closely simulate a heart break, and thus it becomes more difficult 
for the inspector to detect this source of weakness. Diagonal and spiral 
grain are important sources of weakness in airplane stock. Spiral grain 
is due to a peculiar development of the tree itself, but diagonal grain is 
due to mistakes in sawing, a tapering log being cut not parallel to its 
outer surface but to the center line. In some woods the direction of the 
grain is easily detected, but in others it can be made out only with diffi- 
culty. For some purposes wood with a greater divergence than one inch 
in thirty must be rejected. 
The extensive research carried out by the Forest Products Labora- 
tory in connection with the airplane program which has been briefly 
summarized above does not constitute the only war work of the labora- 
tory. Investigations undertaken in co-operation with the Chemical War- 
fare Section of the War Department had important results, the confi- 
dential nature of which prevent their publication. Work was also con- 
ducted bearing on wooden ship building, gun stock manvfacture, and 
the construction of artillery wheels and various military vehicles. Thus 
the laboratory was called upon to investigate the seasoning of the tree- 
nails or wooden spikes employed in large numbers in fastening parts of 
