These worms, it must be remembered, do not live except where they have the action of the water almost every tide, nor do they live in the parts covered with sand. The wooden piles of embankments and sea locks suffer very much from their depredations, and in the sea dykes of Holland they cause very expensive annual repairs.
The Dutch used to coat their piles with a mixture of pitch and tar, and then strew small pieces of cockle and other shells, beaten almost to powder, and mixed with sea sand, which incrusted and armed the piles against the attacks of the teredo. We believe it was a frequent practice in London, about half a century ago, to place small shells in the wooden pugging between the floor joists to deaden sound.
Having described the chief peculiarities of these worms, shown their mode of working, and the extent to which their destructive powers may be carried, it will now be necessary to consider the various schemes which have been proposed and tried to prevent their desolating ravages. These may be divided into three classes, viz. the natural, chemical, and mechanical.
1st. By using woods which are able to resist the attacks of sea worms.
2nd. By subjecting piles to a chemical process.
3rd. By adopting a mechanical process.
First. We have not any English woods which resist their attacks. Elm (used for piles in England) or beech (used for piles, if entirely under water, in France) cannot withstand the teredo; while oak cannot battle successfully against wood-beetles in carvings. It is therefore necessary to inquire whether foreign woods are any better.[20] Unfortunately the great expense of importing them into England prevents their use for piles.
Nearly all our foreign woods used for engineering and building purposes come from the Baltic or Canada: they are fir and pine. Memel timber from the Baltic is comparatively useless unless thoroughly creosoted; and Canadian timber is not so good as the Baltic wood. At Liverpool and some of the western ports of England Canadian timber is preferred to Baltic, although we believe the reason to be that they cannot get the latter, except in small quantities at a time.
The following is a list of timber woods which, according to good authorities, resist for a long period of time the attacks of sea worms. It should be borne in mind, however, that the timber should be cut, during the proper season, from a large and full-grown tree; and, to prevent splitting, it should be kept from the direct action of the sun when first cut; it should have all the bark and sapwood removed, and allowed to dry a certain time before being used.