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.* 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

*Reports of the Juries,' Exhibition, 1851. 'Reports' by Dr. Gibson, Conservator of Forests, Bombay Presidency. 'Reports' by Dr. Cleghorn, Conservator of Forests, Madras Presidency. 'Reports' by Mr. H. B. Baden Powell, Inspector-General of the Forest Department, India, 1875. 'Reports' on the Teak Forests of Tenasserim, Calcutta, 1852. Papers by Mr. Mann and Mr. Heath on 'Decay of Woods in Tropical Climates,' Inst. C.E., 1866. Paper on 'The Ravages of the Limnoria Terebrans,' by Mr. R. Stevenson, Royal Society, 1862. 'Account of the Bell Rock Lighthouse,' by Robert Stevenson, 1824. Stevenson's 'Design and Construction of

Harbours.' Smeaton's 'Reports.'

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.

WOODS WHICH RESIST SEA WORMS.

Australia, Western.—Jarrah, beef-wood, tuart.

Bahama.-Stopper-wood.

Brazil.-Sicupira, greenheart.

British Guiana.—Cabacalli, greenheart, kakarilly, silverballi (yellow). Ceylon.-Halmalille, palmyra, theet-kha, neem.

Demerara.-Bullet, greenheart (purple heartwood), sabicu.

India.-Malabar teak, sissoo, morung sál, dabu, than-kya, ilupé, anan, angeli, may-tobek. (Teak resists the teredo, but is not proof against barnacles.)

Jamaica. -Greenheart.

North America.-Locust.

Sierra Leone.-African oak, or tortosa.

South America.-Santa Maria wood.

Philippine Islands.-Malacintud, barnabá, palma-brava.

Tasmania.-Blue gum.

West Indies.-Lignum vitæ.

Second. The chemical, viz. Kyau's process of corrosive sublimate; Payne's process of sulphate of iron and

muriate of lime; pitching and arring; Burnett's process of chloride of zinc; and arsenic, or other mercurial preparations, have all failed, with the exception of Bethell's process of oil of tar. The failure must proceed from one of two causes; either that the sea-water decomposes the poisonous ingredients contained in the wood, or that these poisonous compounds have no injurious effect on the worms; it appears, however, that both these causes have been in operation, principally the latter.

Without a series of the most minute experiments, it is impossible to form any general notion of the action of sea-water on timber. Common salt, chlorides of calcium and magnesium, sulphate of soda, iodides and bromides of the same metals, are known to exist in sea-water, and in great abundance in the torrid zone. What effect these different ingredients may have upon saturated timber it is difficult to say, but it is extremely probable that they do have an effect.

With regard to the different poisonous compounds having no injurious effect on the worms, it should be remembered that all cold-blooded animals are much more tenacious of life than those of a higher temperament, and in descending the scale of animal creation, the tenacity of life increases, and this principle is more developed. A frog, which though cold-blooded, is an animal of a much higher order than the teredo, will not only live in hydrogen gas, but also in a strong solution of hydrocyanic acid, while at the same time a single drop placed on the nose of a rat, or in the eye of a rabbit, would produce instant death. A somewhat similar occurrence is noticed in the

'British and Foreign Medical Review,' for July, 1841, showing the slow effects of prussic acid on the common snake and turtle.

It may therefore be inferred, that as it requires a large quantity of the most virulently poisoned matter to destroy animals of a much higher order than the teredo, it would take a still greater quantity to affect those animals as they exist in their own element.

The preserving property of soluble salts, such as corrosive sublimate, sulphate of copper, &c., was considered to be founded upon their power of coagulating the albumen, and the sap of wood, thereby rendering that sap less liable to decay; but that very quality of combining with the albumen, destroyed the activity of the poison of the salts. A given quantity of corrosive sublimate of mercury, which if administered to a dog would kill it, would, when mixed with the white of an egg, become coagulated, and if swallowed in that state would be perfectly harmless; so a piece of wood, saturated by those salts, could be eaten by a worm without injury.

A French naturalist, M. de Quatrefages,* in 1848, suggested that a weak solution of mercury (corrosive sublimate) thrown into the water will destroy the milt of the teredo, and consequently prevent fecundation of the eggs, thus exhausting the molluscs in the bud. proposed that ships should be cleared of this terrible pest by being taken into a closed dock, into which a few handfuls of corrosive sublimate should be thrown and

He

* See 'Sur un Moyen de Mettre tous les Approvisionnements de Bois de la Marine de la Piqûre des Tarets' (Compte. rend., Janv. 1848).

well mixed with the water. He considered that about 1 lb. of sublimate would be sufficient for 20,000 cubic metres (metre = 39.37 English inches) of water; but on account of the cost it would be advisable to use salts of lead or copper. This proposition of de Quatrefages reminds us of Chapman's suggestion, in 1812, to get rid of dry rot in ships, viz. by sweeping out the hold, laying from two to four tons of copperas in her bottom, and as much fresh water let in upon it as would make a saturated solution to soak into the wood.

M. de Quatrefages placed the four salts he used in his experiments in the following order, according to merit: 1st, corrosive sublimate; 2nd, acetate of lead; 3rd, sulphate of copper; and 4th, nitrate of copper.

In America, white oxide of zinc is used as a marine paint for ships and piles. In the United States Navy Yard at Gosport it is spoken well of, and very frequently employed. It is said to be much superior to white-lead, red-lead, verdigris, or coal-tar, and that timber covered with two coats of white zinc is neither attacked by the worm, nor do barnacles attach to it when immersed in salt water.

We can only find one instance of timber impregnated with water-glass having been tested against this subtle foe. Water-glass is certainly worth a further trial.

The instance we refer to occurred about forty years ago. In 1832, Dr. Lewis Feuchtwanger, of New York, was permitted by the Ordnance Department, under the direction of Commodore Perry, to perform experiments with water-glass on piles in the Brooklyn Navy Yard, and