In designing a shearing specimen it is necessary to take into consideration the proportions of the area of shear, since, if the length of the portion to be sheared off is too great in the direction of the shearing face, failure would occur by compression before the piece would shear. Inasmuch as the endwise compressive strength is sometimes not more than five times the shearing strength, the shearing surface should be less than five times the surface to which the load is applied. This condition is fulfilled in the specimens illustrated. Shearing specimens are frequently cut from beams after testing. In this case the specific gravity (dry), proportion of late wood, and rate of growth are assumed to be the same as already recorded for the beams. In specimens not so taken, these quantities are determined in the usual way. The sheared-off portion is used for a moisture section. _Adjusting specimen in machine_: The test specimen is placed in the shearing apparatus with the tenon or lip under the sliding plate, which is centred under the movable head of the machine. (See Fig. 39.) In order to reduce to a minimum the friction due to the lateral pressure of the plate against the bearings of the slot, the apparatus is sometimes placed upon several parallel steel rods to form a roller base. A slight initial load is applied to take up the lost motion of the machinery, and the beam balanced. [Illustration: FIG. 39.--Making a shearing test.] _Log of the test_: The load is applied continuously and at a uniform rate until failure, but no deformations are measured. The points noted are the maximum load and the length of time required to reach it. Sketches are made of the failure. If the failure is not pure shear the test is culled. The shearing strength per square inch is found by dividing the { P } maximum load by the cross-sectional area. { Q = --- } { A } IMPACT TEST _Apparatus_: There are several types of impact testing machines.[59] One of the simplest and most efficient for use with wood is illustrated in Figure 40. The base of the machine is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500 pounds. Two upright columns, each 8 feet long, act as guides for the striking head. At the top of the column is the hoisting mechanism for raising or lowering the striking weights. The power for operating the machine is furnished by a motor set on the top. The hoisting-mechanism is all controlled by a single operating lever, shown on the side of the column, whereby the striking weight may be raised, lowered, or stopped at the will of the operator. There is an automatic safety device for stopping the machine when the weight reaches the top. [Footnote 59: For description of U.S. Forest Service automatic and autographic impact testing machine, see Proc. Am. Soc. for Testing Materials, Vol. VIII, 1908, pp. 538-540.]