not nearly so important as the
proportion of the late wood in the ring.

It is not only the proportion of late wood, but also its
quality, that counts. In specimens that show a very large
proportion of late wood it may be noticeably more porous and
weigh considerably less than the late wood in pieces that
contain but little. One can judge comparative density, and
therefore to some extent weight and strength, by visual
inspection.

The conclusions of the U.S. Forest Service regarding the effect
of rate of growth on the properties of Douglas fir are
summarized as follows:

"1. In general, rapidly grown wood (less than eight rings per
inch) is relatively weak. A study of the individual tests upon
which the average points are based shows, however, that when it
is not associated with light weight and a small proportion of
summer wood, rapid growth is not indicative of weak wood.

"2. An average rate of growth, indicated by from 12 to 16 rings
per inch, seems to produce the best material.

"3. In rates of growths lower than 16 rings per inch, the
average strength of the material decreases, apparently
approaching a uniform condition above 24 rings per inch. In such
slow rates of growth the texture of the wood is very uniform,
and naturally there is little variation in weight or strength.

"An analysis of tests on large beams was made to ascertain if
average rate of growth has any relation to the mechanical
properties of the beams. The analysis indicated conclusively
that there was no such relation. Average rate of growth [without
consideration also of density], therefore, has little
significance in grading structural timber."[18] This is because
of the wide variation in the percentage of late wood in
different parts of the cross section.

[Footnote 18: Bul. 88: Properties and uses of Douglas fir, p.
29.]

Experiments seem to indicate that for most species there is a
rate of growth which, in general, is associated with the
greatest strength, especially in small specimens. For eight
conifers it is as follows:[19]

[Footnote 19: Bul. 108, U. S. Forest Service: Tests of
structural timbers, p. 37.]

Rings per inch
Douglas fir 24
Shortleaf pine 12
Loblolly pine 6
Western larch 18
Western hemlock 14
Tamarack 20
Norway pine 18
Redwood 30

No satisfactory explanation can as yet be given for the real
cau