A Treatise On The Principles And Practice Of Dock Engineering

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. “ Among resinous woods, those which have the least resin in their pores, and amongst non-resinous woods, those which have least sap or guni 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 décomposition 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 'iiavalis, 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 faet that it avoids fresh, sewage- polluted, 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 ot 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 high- water 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 sea- water, 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.