nd not touching the container while the scales are
again balanced. The weight required to balance is the weight of
water displaced by the specimen, and hence if in grams is
numerically equal to the volume of the specimen in cubic
centimetres. A diagrammatic sketch of the arrangement of this
apparatus is shown in Fig. 51.

[Illustration: FIG. 51.--Diagram of specific gravity apparatus,
showing a balance with container (_c_) filled with water in
which the test block (_b_) is held submerged by a light rod
(_a_) which is adjustable vertically and provided with a sharp
point to be driven into the specimen.]

Air-dry specimens will be dipped in water and then wiped dry
after the first weighing and just before being immersed for
weighing their displacement. All displacement determinations
will be made as quickly as possible in order to minimize the
absorption of water by the specimen.

STRENGTH VALUES FOR STRUCTURAL TIMBERS

(From Cir. 189, U.S. Forest Service)

The following tables bring together in condensed form the
average strength values resulting from a large number of tests
made by the Forest Service on the principal structural timbers
of the United States. These results are more completely
discussed in other publications of the Service, a list of which
is given in BIBLIOGRAPHY, PART III.

The tests were made at the laboratories of the U.S. Forest
Service, in cooperation with the following institutions: Yale
Forest School, Purdue University, University of California,
University of Oregon, University of Washington, University of
Colorado, and University of Wisconsin.

Tables XVIII and XIX give the average results obtained from
tests on green material, while Tables XX and XXI give average
results from tests on air-seasoned material. The small
specimens, which were invariably 2" X 2" in cross section, were
free from defects such as knots, checks, and cross grain; all
other specimens were representative of material secured in the
open market. The relation of stresses developed in different
structural forms to those developed in the small clear specimens
is shown for each factor in the column headed "Ratio to 2" X
2"." Tests to determine the mechanical properties of different
species are often confined to small, clear specimens. The ratios
included in the tables may be applied to such results in order
to approximate the strength of the species in structural sizes,
and containing the defects usually encounte