A Treatise On The Principles And Practice Of Harbour Engineering

PILING. 67 seldom reaches to more than | inch. In appearance it is not unlike a grain of rice. It is mainly troublesome on account of the vast numbers in which it infests certain localities, and, as it is indifferent to the foulness or otherwise of the water, no harbour precincts can be considered free from its presence. The limnoriæ are active mainly about and below high water of neap tides, depredations proceeding rapidly until the whole of the timber-work is eaten away. Large balks of unprotected fir have been completely destroyed in three years, and even creosoted timber has perished within a decade, The Tanai vitalis belongs also to the Asellidai family. It preys upon vegetable fibre with powerful claws, rending it to pieces. The length is about i inch. The attacks of the white ant (Termes) in tropical countries do not call for detailed mention. They are not particularly associated with maritime situa- tions, and submerged timbers are, of course, not affected in any way. Decay of Timber. —Apart from the mechanical destruction of timber, there is the question of natural decay, which is due to one or other of two distinct forms of decomposition, known respectively as dry and wet rot. The former, which is a process of fibrous disintegration, accompanied by the growth of a parasitic fungus, is attributable to, and certainly accelerated by, the absence of adequate ventilation. The woodwork attacked is mostly that which is situated in confined and stuffy places, to which air has insufficient access— conditions not generally allied with harbour work. Wet rot, on the other hand, has a much more general and appropriate connection. It is the most characteristic disease, in fact, to which marine timbers are liable. It arises from, and is promoted by, frequent alternations of dryness and moisture, and these conditions are obviously prevalent along the water’s edge. Every time a log becomes immersed and dries again, a fresh portion of the fibre is converted into soluble matter, which, in due course, is abstracted and lost. Furthermore, the continuai evaporation of moisture from the pores of the wood results in putrefaction, the progress of which, once commenced, is often rapid. Wet rot will attack indifferently any part or substance of a log, whether it be heartwood or sapwood; whereas dry rot is generally to be found in the latter only. The disease, moreover, is contagious, and affects adjacent timbers which may not in themselves be exposed to the same predisposing causes. Préservation of Timber.—Having enumerated and described the inimical agencies, we corne now to the means used to combat them. Ex- pedients, as diverse as they are multitudinous, have been tried from time to time with a view to increasing the durability of timber, both as regards preserving it from internal decay and protecting it from external attack. The commonest preservative for structural work is paint. Applied to seasoned timber completely deprived of free sap, the method is one of the most efficacious which can be devised; but it calls for frequent and regular renewal, and this, in the case of submerged work, is an insurmountable obstacle to its adoption. Moreover, sea-water tends to soften paint, and the