Page:Timber and Timber Trees, Native and Foreign.djvu/78

 CHAPTER X.

ON THE TENSILE STRENGTH, OR DIRECT COHESION, AND VERTICAL STRENGTH OF BRITISH OAK.

tensile experiments are somewhat difficult to carry out, and therefore only specimens Nos. I to 6, Table VII., were tested from the log referred to at page 53. They varied from 2,240 to 5,320 lbs., giving a mean strength of 3,837 lbs. the square inch, the wood next to the pith or centre proving to be the strongest, as with the transverse test. The gradations of strength, taking No. 1 as unity or 1.00, give No. 2 as .82 ; No. 3, .785; No. 4, .81; No. 5, .475 5 and No. 6, .42, the tensile strength of the inner wood of this tree being therefore about 58 per cent, greater than the outer.

Instances of weakness, both transversely and tensilely, similar to those which are given in Table VII., are not unfrequent, and may occur, as before stated, in good-looking specimens of any species of timber : and this, again, serves to show that it would be unsafe to arrange the various parts of any construction according to the highest calculated strength of any timber to be employed.

Further tensile experiments were made on six specimens of British Oak saved from the pieces experimented upon, and referred to in Tables V. and VI. They appear