Sâl, or Saul, is a close-grained, straight-fibred wood, possessing the same useful characteristics as the deodar, but much stronger and more durable. The wood is heavy and coarse in grain.

Teak, perhaps the best known of Indian trees, is endowed with considerable strength and durability. It has been designated the Indian oak, but it is also found in Burmah, Siam, and Java. The grain is fine and straight, the wood light and easily worked, with a tendency to splinter. Teak contains an aromatic oil of a resinous nature, which coagulates to such a degree of hardness as to spoil the edges of cutting tools. The oil is further reputed to be a preservative from the white ant and from seaworms. Marketable logs do not exceed 40 feet in length, with a quartergirth of 15 inches downwards. Teak is mainly used in small scantlings.

Elm is a wood of great strength and toughness, found generally on the continent of Europe and in North America. The grain is smooth, close, and fibrous, offering great resistance to crushing. English elm is brownish in colour, with a curly grain not easy to split, and it is a noteworthy feature that the sapwood is equally durable with the heartwood, provided the conditions be those of total immersion or complete dryness. Alternations of wet and dry bring about speedy decay. In American elm, which is lighter in colour, stringy in the grain, and liable to split under exposure, the heartwood alone is durable, and that only when kept constantly under

water.

Beech is a light, compact, fine-grained wood, grown in Europe and the United States. It is readily cleavable and easily worked. Like elm it is subject to decay under changes of condition, but is fairly durable if maintained in either the wet or the dry state.

Oak is possibly the most valuable timber of northern latitudes, and English oak is particularly renowned for its strength and toughness. It is unfortunately liable to the attacks of insects, and it contains an acid which has a tendency to corrode iron fastenings. American oak is considered somewhat inferior to the English and European varieties.

Pitchpine is a product of the Southern States of North America. It is a strong, heavy resinous wood, extremely durable, when not exposed to marine insects, but difficult to work, and subject to cupshakes. It is procurable in logs, reaching up to 70 or 75 feet in length, with 10 to 20 inches quarter-girth.

Pine, Deal, Fir, and Spruce are terms covering a large variety of timber of the same generic character, which it is not necessary to discuss here at any length, more especially as the wood plays no part of unique importance in dock work. Its uses are confined to purposes common to most structures. It is a very handy material, with a considerable range of strength and toughness among the various species. It will be sufficient to remark that wood from the Baltic is generally superior to that from North America. Red pine from Scandinavia makes the best timber for framing, and spruce deals from the same locality make admirable sheeting piles. The former is

imported in logs, 12 to 14 inches square, and the latter in scantlings of 9 inches by 3 inches and in lengths up to 16 feet. Oregon pine is noted for the great length and girth of its logs, but it is not a very strong or durable wood. Signs of decay have been observed in a very short time. It is, however, very useful for temporary shoring, and can be obtained from 30 to 130 feet in length with 12 to 30 inches quarter-girth. Hemlock, from the Southern States of South America, is in demand for deals and sleepers.

TABLE XIII.-WEIGHT AND STRENGTH OF TIMBER.

NOTE. The transverse strength given above is the concentrated central breaking weight of a beam 1 inch wide, 1 inch deep, and 1 foot clear span.

Selection of Timber.—A thorough insight into the merits and defects of different logs can only be obtained by much experience and close personal investigation. The selection of timber for important marine works should, accordingly, only be entrusted to a competent and reliable man. It would be a difficult matter to enumerate all the indications of weakness in logs, and many defects are quite latent to the inexperienced eye. Shakes or splits should be looked for and their extent gauged by tapping. Discoloration is a bad sign, as also are sponginess and the appearance of wormholes on the surface. Timber with large or dead knots is unsuitable. The heart should be central. Rankine states the following general indications of strong and durable timber:

:

"In the same species, that specimen will in general be the strongest and the most durable which has grown the slowest, as shown by the narrowness of the annual rings.

"The cellular tissue, as seen in the medullary rays (when visible), should be hard and compact.

"The vascular or fibrous tissue should adhere firmly together, and should show no woolliness at a freshly cut surface, nor should it clog the teeth of the saw with loose fibres.

* A Manual of Civil Engineering, p. 441.

DECAY AND DESTRUCTION OF TIMBER.

151

"If the wood is coloured, darkness of colour is, in general, a sign of strength and durability.

"The freshly-cut surface of the wood should be firm and shining, and should have somewhat of a translucent appearance. A dull, chalky appearance is a sign of bad timber.

"In wood of a given species, the heavier specimens are, in general, the stronger and the more lasting.

66

Among resinous woods, those which have the least resin in their pores, and amongst non-resinous woods, those which have least sap or gum in them, are, in general, the strongest and most lasting."

Decay and Destruction of Timber.-Timber is subject to dry and wet rot and to the depredations of worms and insects. Dry rot is a disintegration of the fibres accompanied by the growth of a fungus, due to, and accelerated by, inadequate ventilation. It attacks woodwork in confined situations free from moisture, and reduces it to the condition of a fine powder. The disease is infectious, and spreads with startling rapidity. Once attacked, no remedy can save the affected parts, and the only efficient preventive is thorough ventilation. Wet rot is a decomposition of the fibres under the influence of moisture, resulting in putrefaction and decay. It is not infectious like dry rot, but is communicable to sound timber by actual contact.

Of worms and insects which attack timber, impair its strength, and in some cases bring about its utter destruction, the most important are the Teredo navalis, the Limnoria terebrans, the Chelura terebrans, and the Termes or white ant.

The Teredo is found in all British waters, and, indeed, frequents the majority of seaports. It has a preference for clear salt water, and the available evidence seems to point to the fact that it avoids fresh, sewagepolluted, and muddy water with equal impartiality. Its depredations take the form of tunnellings or excavations into the timber, generally along the grain, and these it lines with a chalky secretion. It is no uncommon experience to find holes inch or inch in diameter. Some specimens of the Teredo are very large, measuring as much as 2 feet in length.

The Limnoria is a small insect, which is troublesome on account of the vast numbers in which it infests certain localities. It appears to be indifferent to the foulness of the water, provided it be saline. Its ravages are confined to the range of the tide, and it generally works about highwater level of neap tides.

The Chelura is a shrimp, which undercuts woodwork and causes it to fall away in flakes. This insect manifests a decided partiality for pure seawater, and is, consequently, more often found along the open coast than in enclosed harbours.

The Pholas dactylus, while principally regarded as an enemy of masonry, has also been known to attack wood. It bores a number of holes close together.

Above ground, timber is subject to the depredations of ants-particularly, in tropical climates, the white ant. Even the hardest woods succumb to its attacks. The boring is most insidious, the whole of the interior being eaten away, while the surface remains intact.

Preservation of Timber.-Of all artificial means available for the protection of timber, alike from destruction and decay, by far the most satisfactory is the process of creosoting. It coagulates the albumen and fills the pores with an antiseptic substance, which excludes moisture, repels worms and insects, and prevents dry rot.

Creosote is an oily liquid contained in the second distillation of tar. Its composition is somewhat variable; but, in order to be effective, it should contain over 40 per cent. of naphthaline, about 4 or 5 per cent. of carbolic acid, and as little pitch as possible. The process is as follows:-The timber to be treated, after being dried, is placed in a vacuum, and there heated to vaporise the sap and expel all traces of moisture. Creosote at a temperature of about 120° F. is then introduced into the containing cylinder under considerable pressure. The liquid is absorbed by the wood to an extent ranging between 3 and 16 lbs. per cubic foot. The former figure applies to oak and other hard woods, which are rather unsuitable subjects for treatment. Soft, and even green, woods are better adapted on account of their higher power of absorption.

Other substances have been advocated for the impregnation of timber, notably solutions of sulphate of copper (Boucherie's process), corrosive sublimate (kyanizing), and chloride of zinc, but they do not give such good results as oil of tar. A Commission appointed by the Dutch Government some time ago, for the purpose of investigating the claims of various preservative agencies, reported that "the only process which could be relied upon for the protection of wood from the attacks of the Teredo was that of creosoting."

Apart from internal treatment, various superficial applications have been tried, with more or less success. Paint is a very usual agent and an effective preservative, provided it be applied only to well-seasoned timber and periodically renewed. If applied to green timber, it imprisons the sap and induces decay. In sea-water the coating is liable to be softened and eroded. Tar, verdigris, and paraffin have also been employed as external coverings.

The extremities of timber posts let into the ground are frequently charred to a height of a few inches above the ground level.

For open woodwork in marine situations the following measures have been adopted, with generally favourable results, more particularly in regard to the attacks of worms :

Metallic Sheeting.—A thin covering of copper-plate has proved to be a most satisfactory protection for piles, but it must extend from below the surface of the mud to somewhat above high water mark, otherwise the insect may intrude itself between the metal and the wood. The drawback

GRANITE.

153 to its extensive use is its expense. A zinc covering has been tried, but it is soon corroded by sea-water. Muntz metal is another substitute.

Pipe Casings.--Piles encased in earthenware pipes, such as drain pipes, with the intervening space filled in with sand or cement grout, make a durable combination in situations free from shocks and erosion. A coating of Portland cement will often answer the same purpose, but it is more likely to crack. Tubes of steel wire netting, embedded in concrete on the Monier principle, have been found very effective.

Compound Coverings.-The following method, used on the Pacific coast, has attracted attention :--" After removing the bark, the surface of the pile is covered with a prepared compound, some of the ingredients of which are paraffin, powdered limestone, and kaolin. The pile is then wrapped in jute burlap, and another application of the compound is made. Wooden battens are then nailed along the surface, which receives a final coat of the paint. Piles thus protected have been in use for ten years. The coating protects the piles from the teredo, limnoria, and similar animals, but its duration is not known."

"

Close Nailing. The driving in, very closely together, of broad-headed scupper-nails is an expedient of some antiquity. The heads are apt to rust, and though this is sometimes held to be a further protection from worms, the statement lacks confirmation. The method has been applied to dock gates, but it is troublesome and expensive.

A natural protection is very often provided by the accumulation of barnacles, mussels, and other shellfish upon the surface of the wood. Sea thorns act in the same way when the surface has been covered with their discs.

STONE.

There are many varieties of stone suitable for constructive work, but the dock engineer confines his attention to a comparative few, which, by long experience, have gained a reputation for durability and strength. The principal of these is granite.

Granite is a very hard and extremely durable rock, of igneous origin, crystalline in structure, and of great value in dock work on account of its heavy and massive proportions. In its true form it is composed of crystals of quartz, felspar, and mica; but there are other-so-called-granites containing hornblende (syenitic granite), quartz diorite, &c.

The quartz is a very hard substance, with a vitreous lustre, and practically indestructible. It renders the granite very difficult to work. The felspar is lustrous and granular, and, being present in greatest volume, gives the granite its distinctive colour, which may be white, grey, pink, red, or brown. It is less hard and less durable than quartz. Mica is a thin, flaky substance, with a bright, metallic lustre. It is easily decomposed.

* Snow on

"Marine Woodborers,” Engineering, Oct. 7, 1898.