Details of the tests and results may be found in: Cir. 142, U.S.F.S.: Tests on vehicle and implement woods. _Cross-arms_ In tests by the U.S. Forest Service on cross-arms a special apparatus was devised in which the load was distributed along the arm as in actual practice. The load was applied by rods passing through the pinholes in the arms. Nuts on these rods pulled down on the wooden bearing-blocks shaped to fit the upper side of the arm. The lower ends of these rods were attached to a system of equalizing levers, so arranged that the load at each pinhole would be the same. In all the tests the load was applied vertically by means of the static machine. See Cir. 204, U.S.F.S.: Strength tests of cross-arms. _Other Tests_ Many other kinds of tests are made as occasion demands. One kind consists of barrels and liquid containers, match-boxes, and explosive containers. These articles are subjected to shocks such as they would receive in transit and in handling, and also to hydraulic pressure. One of the most important tests from a practical standpoint is that of built-up structures such as compounded beams composed of small pieces bolted together, mortised joints, wooden trusses, etc. Tests of this kind can best be worked out according to the specific requirements in each case. APPENDIX SAMPLE WORKING PLAN OF THE U.S. FOREST SERVICE MECHANICAL PROPERTIES OF WOODS GROWN IN THE UNITED STATES Working Plan No. 124 PURPOSE OF WORK It is the general purpose of the work here outlined to provide: (_a_) Reliable data for comparing the mechanical properties of various species; (_b_) Data for the establishment of correct strength functions or working stresses; (_c_) Data upon which may be based analyses of the influence on the mechanical properties of such factors as: Locality; Distance of timber from the pith of the tree; Height of timber in the tree; Change from the green to the air-dried condition, etc. The mechanical properties which will be considered and the principal tests used to determine them are as follows: Strength and stiffness-- Static bending; Compression parallel to grain; Compression perpendicular to grain; Shear. Toughness-- Impact bending; Static bending; Work to maximum load and total work. Cleavability-- Cleavage test. Hardness-- Modification of Janka ball test for surface hardness. MATERIAL _Selection and Number of Trees_