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ciples and materials, the class is ready for the more specific consideration of each material by itself. It must not be forgotten in this connection that nearly all our computations regarding stresses in materials are based upon the principles of elasticity with stresses below the elastic limit. It is not sufficient, then, to consider only ultimate strengths. The elastic limit, and the more or less perfect elasticity of the material, as well as its behavior throughout the whole range of stress, should be made the subject of careful study. The student needs to be brought face to face with these principles and qualities, again and again, until they become a part of his unconscious possession, guiding his judgment, giving form to his expression.
Timber seems to offer many advantages as the material first to be considered. It is one of the most common materials of construction. The specimens are easily prepared and cheap. The deformations under stress are relatively great, and, therefore, readily measured. The strengths and elasticities vary in different directions with reference to the grain, thus requiring fuller investigation as well as clear ideas as to direction of stress. The strengths are subject to many variables, and yet, by controlling these variables, or by recognizing their presence, consistent and highly instructive results may be obtained.
Beginning the subject of timber, the first laboratory period is devoted to the examination of the minute structure of wood as revealed by the microscope in transverse, radial and tangential sections of the common hardwoods and softwoods; this being the basis for explaining so many of its strength, grain and shrinkage
properties. For the strength tests, a clear, straightgrained, well-seasoned, white pine plank, 16 inches broad and 15 inches thick, having a little sap-wood in two outer corners, was used (Fig. 4). Specimens from this plank were tested under three conditions of moisture, viz.: “Dry," specimens dried over a boiler one week; “Normal,” specimens tested as cut from the plank; and “Wet,” specimens kept under water one week. There being six laboratory divisions, the plank was cut so as to give six endwise compression specimens in its width (Fig. 4). Thus, specimens Nos. 1 Dry, No. 7 Normal, and No. 13 Wet, representing the three moisture conditions, and all from similar positions with respect to the center of the tree, were tested in the first laboratory division (Fig. 5). The next three specimens, in the second division (Fig. 6), and so on (Figs. 7, 8, 9 and 10). These three tests, with full data as to loads and shortenings, were easily finished within the hour. The shortening was measured with a Brown and Sharpe test indicator as shown in Fig. 11. This instrument reads directly to thousandths of an inch, but ten-thousandths may be estimated with fair degree of accuracy.
The method of measuring between compression blocks proves sufficiently accurate if a small initial load be applied before taking the first reading
In same manner, during the laboratory periods of the following two weeks, the remainder of the tests (Fig. 4), consisting of across grain compression (Figs. 12, 13 and 14), endwise tension, across grain tension, and along the grain shear, were completed; each student following a strip of the plank throughout its length through all these tests.