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 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.* 
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 = 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" x 2" x 30" were each tested by hydraulic 
machinery most carefully applied, the tabular values here given may, I 
consider, be depended upon. 
