 other directions. In the radial direction—i.e., in the direction of the medullary rays—it may shrink from 1 to 7·5 per cent of its measurement when first felled; and in the direction vertical to this—i.e., parallel to a tangent to the cylindrical stem—the variation is from 0·8 to 10·6 per cent. Of course, green oak shrinks much more than seasoned and older wood, the process of seasoning being, in point of fact, the period of chief shrinkage. It is said that wood from the variety sessiliflora shrinks more than that of the variety pedunculata, but it may be doubted how far the difference would hold if sufficiently numerous comparisons were made.
(5) Swelling may be regarded as complementary to shrinkage. It has been found that if oak wood is allowed to absorb water until thoroughly saturated it will increase from 0·13 to 0·4 per cent in length, and be distended radially from 2·66 to 3·9 per cent, or tangentially 5·59 to 7·55 per cent, according to age and condition, young wood swelling more than old. It has also been found that the total volume increased from 5·5 to 7·9 per cent, and the weight from 60 to 91 per cent, on complete saturation.
(6) Elasticity and Tenacity.—Oak is very elastic, and easily bent if steamed, and it does not readily splinter. When pulled in a direction parallel to the length of the structure the absolute tenacity = 2·23 to 14·51 kgr.—i.e., it took a pull equal to this weight per 1 sq. mm. of section to pull the wood asunder.
The limit of elasticity corresponds to a load of 2·72 to 3·5 kgr., according to various authorities, the specimen lengthening th in the former case.
The modulus of elasticity is given as 826 to 1,030 kgr., and the breaking limit as 4·66 to 6·85.
When the pull is in a direction across the length of the fibers, the results differ according as the load is applied so as to act radially or tangentially.
When acting radially the modulus of elasticity is given as 188·7 kgr., and the breaking limit as 0·582 kgr.
When acting parallel to a tangent the modulus of elasticity = 129·8 kgr., and the breaking limit 0·406 kgr.
The absolute tenacity in the transverse direction is given as 0·44 to 0·61 kgr.
In the case where pressures are applied in the direction of the length of the fibers the limit of elasticity = 2·09 to 2·22 kgr.; the modulus of elasticity, 933 to 1,250 kgr.; and the absolute resistance, 2·58 to 3·64 kgr.
Flexibility.—The limit of elasticity = 1·77 to 2·71 kgr.; modulus of elasticity, 620 to 735 kgr.; resistance to bending, 4·53 to 6·18 kgr.
Torsion.—Oak warps considerably unless carefully seasoned. Limit of elasticity = 0·4 to 0·54 kgr.; modulus of elasticity, 612·5 to 785 kgr.; resistance to torsion, 0·75 to 0·97 kgr.
Resistance to shearing-stress, in the direction of the fibers = 0·61 to 0·97 kgr.; perpendicular to them, 1·9 to 3·49 kgr.
(7) Resistance to Splitting.—Oak is easily split into tolerably smooth and even staves, and is much employed for this purpose.
(8) Hardness.—Oak is neither the hardest and heaviest nor the most supple and toughest of woods, but it combines in a useful manner the average of these qualities. Good oak is hard, firm, and compact, and with a glossy surface, and varies much; young oak is often tougher, more cross-grained, and harder to work than older wood. According to Gayer, if we call the resistance which the beech offers to the saw, applied transverse to the fibers, 1, then that of freshly felled oak = 1·09.
(9) Durability.—A mild climate and open situation produces the most durable oak, and it is extraordinarily durable under water, in the earth, or exposed to wind and weather, or under shelter; in the latter case it becomes more and more brittle as years roll by.
The alburnum becomes rotten usually in a few years if exposed, and is the prey of insects if under cover. The heart, if sound, may last for centuries under cover and well ventilated, and even in earth or water will endure for several generations. There are, for instance, in the museum at Kew, a portion of a pile from old London Bridge which was taken up in 1827, after having bee