Since in large beams a great variation in rate of growth and relative amount of late wood is likely in different parts of the section, it is advisable to consider the cross section in three volumes, namely, the upper and lower quarters and the middle half.  The determination should be made upon each volume separately, and the average for the entire cross section obtained from these results.

At the conclusion of the test the failure, as it appears on each surface, is traced on the sketches, with the failures numbered in the order of their occurrence.  If the beam is subsequently cut up and used for other tests an additional sketch may be desirable to show the location of each piece.

Adjusting specimen in machine:  The beam is placed in the machine with the side marked a on top, and with the ends projecting equally beyond the supports.  In order to prevent crushing of the fibre at the points where the stress is applied it is necessary to use bearing blocks of maple or other hard wood with a convex surface in contact with the beam.  Roller bearings should be placed between the bearing blocks and the knife edges of the crosshead to allow for the shortening due to flexure. (See Fig. 29.) Third-point loading is used, that is, the load is applied at two points one-third the span of the beam apart. (See Fig. 30.) This affords a uniform bending moment throughout the central third of the beam.

[Illustration:  FIG. 30.—­Two methods of loading a beam, namely, third-point loading (upper), and centre loading (lower).]

Measuring the deflection:  The method of measuring the deflection should be such that any compression at the points of support or at the application of the load will not affect the reading.  This may be accomplished by driving a small nail near each end of the beam, the exact location being on the neutral plane and vertically above each knife-edge support.  Between these nails a fine wire is stretched free of the beam and kept taut by means of a rubber band or coiled spring on one end.  Behind the wire at a point on the beam midway between the supports a steel scale graduated to hundredths of an inch is fastened vertically by means of thumb-tacks or small screws passing through holes in it.  Attachment should be made on the neutral plane.

The first reading is made when the scale beam is balanced at zero load, and afterward at regular increments of the load which is applied continuously and at a uniform speed. (See SPEED OF TESTING MACHINE, above.) If desired, however, the load may be read at regular increments of deflection.  The deflection readings should be to the nearest 0.01 inch.  To avoid error due to parallax, the readings may be taken by means of a reading telescope about ten feet distant and approximately on a level with the wire.  A mirror fastened to the scale will increase the accuracy of the readings if the telescope is not used.  As in all tests on timber, the strain must be continuous to rupture, not intermittent, and readings must be taken “on the fly.”  The weighing beam is kept balanced after the yield point is reached and the maximum load, and at least one point beyond it, noted.