The experiments for determining the direct cohesive strength are, as before stated, somewhat difficult to carry out, even upon the hard woods, but they are infinitely more so on the soft woods, owing to the liability of the pieces to crush in the clamps holding them before the true tensile strain could be reached. The results given in Table CXXIII. were, therefore, only obtained after much perseverance and not a few failures. The five pieces subjected to the strain bore respectively 2,240, 2,800, 3,220, 3,416, and 4,480 lbs., giving an average of 3,231 lbs. as the direct* cohesion per square inch. Their specific gravities varied from 512 to 639, the average being 603, which is very near the average specific gravity of the pieces tried for the transverse strength.[1]
A great many experiments were made to ascertain the resistance of this wood to a vertical or crushing force, the details of which are given in Tables CXXIV. to CXXVIII. From Table CXXV. it may be deduced that the proportion of length to section best adapted for carrying the greatest weights is when the sectional area in inches is to the length in inches as 4:5 or = √45 L = side of square for the base. This confirms the opinion before given as regards English Oak; but the rule must rather be considered approximate than absolute, for in the experiments on pieces 3" X 3" (Table CXXVI.), the maximum strength lay in that of 12 inches in length, making the proportion as 9:12 ( = sectional area). If, however, the area of the base (or the sectional area) be too small for the length of the pillar, it will be liable
- ↑ The results of nearly all my experiments on the tensile strength of woods are lower than the values given by Rankine, Tredgold, and some others. But as the specimens 2″ × 2″ × 30″ were each tested by hydraulic machinery most carefully applied, the tabular values here given may, I consider, be depended upon.
