236 TIMBER AND TIMBER TREES. [CHAP. 



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 Ibs., giving an average of 

 3,231 Ibs. 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.* 



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 ^ -j- 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 CXXVL), the 

 maximum strength lay in that of 12 inches in length, 

 making the proportion as 9:12 (V'^-L= 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" x 2" x 30" were each tested by hydraulic 

 machinery most carefully applied, the tabular values here given may, I 

 consider, be depended upon. 



