4 BULLETIN 556, U. S. DEPARTMENT OF AGRICULTURE. 
as a guide to the use of data from tests on small specimens in estab- 
lishing working stresses and grading rules for structural timbers. The 
results of tests of this kind on a number of species have already been 
published. (See list of publications, p. 46.) 
PRECAUTIONS TO BE OBSERVED IN THE USE OF THE DATA. 
Careful attention must be given to the natural variability of timber 
in order to make correct use of timber-test data. The following sug- 
gestions are offered as a guide to the use of the data given herein. 
Definitions of the various technical terms, with illustrations, are given 
on pages 7 to 18. 
COMPARISON WITH DATA IN OTHER PUBLICATIONS. 
In comparing the data in this publication with those in other pub- 
lications, it must be kept in mind that scarcely any two series of 
tests have been made under the same conditions and that very fre- 
quently so little is specified concerning the character of the material 
and the methods of test as to make close comparisons impossible. A 
specific instance is furnished by the results of Sargent's tests * and 
those given in Forest Service Circular 15. These two publications 
are chosen as illustrations because of the numerous attempts which 
have been made to compare the figures in them with each other and 
with those obtained under the present series. Sargent made about 
2,700 tests on 300 species of American woods; but he did not take 
into account what may have been relative large variations in mois- 
ture content, and he selected his specimens from the lower end of the 
butt logs — in most cases the best although most variable portion of 
the tree. The lack of data upon moisture content is an insurmount- 
able barrier to comparison with the present series, since differences 
of moisture content between two groups of tests may be sufficient 
to cause as much as 100 per cent difference in the strength data. 
Circular 15, " Summary of Mechanical Tests on Thirty-two Species of 
American Woods," containing the results of about 30,000 tests, takes 
moisture into consideration, but allows of no comparison with the 
present series because of the selection of material with defects as 
found in the tree. Since no record of the extent or position of these 
defects in the test piece are now available, no estimate can be made 
as to the strength of the clear wood. 
Data from other publications of the Forest Service which are 
known to be strictly comparable to those obtained from the present 
series of tests are included in Tables 1 and 2. The reader is cautioned 
against any attempt at the comparison of the data in this publication 
with those in any previous one dealing with tests on small clear 
pieces. 
i Made for the Tenth United States Census, and results published in Vol. IX of the Tenth Census 
Eeports. 
