A Treatise On The Principles And Practice Of Dock Engineering

152 DOCK ENGINEERING. Above ground, timber is subject to the déprédations of ants—particu- larly, in tropical climates, the white ant. Even the hårdest woods succumb to its attacks. The boring is most insidious, the whole of the interior being eaten away, while the surface remains intact. Préservation of Timber.—Of all artificial means available for the protec- tion 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 antiseptie substance, which excludes moisture, repels worms and insects, and prevents dry rot. Greosote 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. Oreosote at a temperature of about 120° F. is then introduced into the containing cylinder under considérable pressure. The liquid is absorbed by the wood to an extent ranging between 3 and 16 Ibs. per cubic foot. The former figure applies to oak and other hard woods, which are rather unsuitable subjects for treat- ment. Soft, and even green, woods are better adapted on account of their higher power of absorption. Other substances have been advocated for the imprégnation 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