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.