74 BULLETIN 
pose. Ash, mahogany, oak, and some other woods are used for these 
propellers at the present time in the United States. The propeller 
blades are built up of several plies of wood glued together. Extreme 
care must be taken in making propellers, for any slight warping or 
opening of a glued joint may render the propeller worthless. Pro- 
peller stock should be seasoned by special methods, and the different 
laminations should be approximately of the same density and the 
same moisture content. The laminations are kept under constant 
moisture conditions for several days prior to gluing. Propellers 
are first finished to the exact form and afterward are balanced. The 
ends of the blades are often tipped with a strip of sheet copper, and 
the propeller is covered with varnish or some other preparation to 
protect the wood. Walnut for airplane stock must be of the highest 
grade and entirely clear. Wide lumber, usually 1 inch thick, is de- 
manded. Statistics are not available on the amounts of black walnut 
consumed in the industry or on the average prices paid for what 
was used. The United States, through the Bureau of Aircraft Pro- 
duction, paid during the war as much as $310 a thousand board feet 
for black-walnut propeller stock, and this was considered a very high 
price at that time. 
EXPORT. 
Exportation has always played an important part in the walnut- 
lumber industry. European countries early recognized the value of 
the timber, particularly for cabinetwork. It is claimed that black- 
walnut timber was shipped to Europe as early as 1629. War con- 
ditions have interfered with its exportation during the past few 
years. During the year 1918 its shipment from the United States 
in the form of the log and for any but war uses was stopped, and, 
on account of the disturbed conditions abroad, exportation is still 
considered below normal. However, the amounts exported are now 
steadily increasing. 
Walnut is exported in the form of both logs and lumber. Table 24 
gives the amounts of logs and the total and average values for dif- 
ferent years, by countries. Similar data are not available for lumber. 
Records of walnut-lumber exports for the year 1912 show that ap- 
proximately 75 per cent of the total lumber output of the large wal- 
nut producers was exported during that year. According to the 
calculated amounts shown in the section on " Demand " this exporta- 
tion amounted to about 31 million feet, which would be distributed 
about as follows: Germany, 50 per cent, 15.5 million board feet: 
British Isles, 35 per cent, 10.85 million board feet; other countries 
(principally Denmark, Scandinavia, Holland, Belgium, and Spain), 
15 per cent, 4.65 million board feet. During the first six months of 
