Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

very important one, has been adopted with the most promising success. By the King's Order in Council, September 20, 1809, a superior class of ship-wrights’apprentices has been established at the dock-yard of Portsmouth. It consists of twenty-five young men of liberal education, who, before admittance, must be examined by the professor of the Royal Naval College and the instructor in the theory of naval architecture. Their mornings are passed in the study of mathematics and mechanics, and in the application of them to naval architecture ; in drawing the different parts of a ship, and making complete draughts and plans. The remainder of the day is employed under the master shipwright in the mould loft, and in all the various kinds of manual labour connected with ship-building, as well as in the management and conversion of timber, so as to make them fully acquainted with the detail of the duties of a practical shipwright. The last year of their apprenticeship is to be served at sea, to afford them an opportunity of acquiring some practical knowledge in the steering, sailing, trimming, and ballasting of ships, &c.—during which, the order directs,

they shall mess with the officers, and be treated in all respects as gentlemen.'

Nothing can be more judicious than such an establishment; and we understand that a number of young men of the highest promise have already been entered; among whom we may hope for future surveyors

of the
navy,

who will excel the French in the science of naval architecture as much as our shipwrights at present surpass theirs in the practice of the art. It is to this part, however, that the animadversions of Mr. Pering are chiefly directed.

By the present mode of ship-building, that noble structure, a firstrate man of war, becomes useless, from premature decay, in five or si.x years, and the average duration of the navy itself may be said to be limited to eight years-a short lived existence indeed, when we reflect on the immense expence and importance of our fleets !

This is indeed a gloomy prospect! but is it a faithful representation of the fact? We are inclined to think it is not, and that we shall come nearer the truth in stating that, except in some particular cases, the period of ten years may be assigned to ships of war built in merchants' yards, and fifteen to those built in the king's yards, and that twelve years and a half may safely be taken as the average duration of a fleet of modern built ships. Even this is sufticiently discouraging, when compared with the duration of ships in earlier periods of our naval history. The Royal William, for instance, a first rate, was built at Portsmouth in 1719, was among the ships sent to the relief of Gibraltar in 1782, and, at the age

of near a century, bears the flag of the port admiral at Spithead.

The

6

The Queen Charlotte, a first rate, was launched at Deptford in 1810, sent round to Plymouth under jury masts in 1811, found too rotten to be sea-worthy, and is now undergoing a repair which, we imagine, will cost at least £20,000. These, we admit, are extreme cases of durability and decay; but we could easily furnish a hundred instances of the superior quality of ships built in former days to those built in our time. The Sovereign of the Seas, afterwards named the Royal Sovereign, was built at Woolwich in 1637, and stood 47 years service. The Barfleur was built at Chatham in 1768, is still a good ship, and now under repair for further service. The Montague was launched at Chatham in 1779; after undergoing several repairs, she now carries the flag of Admiral Dixon at Rio de Janeiro: whilst the Ocean, the Foudroyant, the St. Domingo, the Rodney, the Ajax, the Albion, and many others, were falling to pieces within tive years after launching, and some of them in less than three. The principal cause of this extraordinary difference in their duration may, we think, be ascribed to the rapid manner in which ships have recently been run up, with timber not properly seasoned, or with a mixture of different kinds of timber; to which may also be added Mr. Pering's complaint of slovenly workmanship, and an injudicious mode of fastening.

It is well known that when timber, not duly seasoned, is put into certain situations, and more especially in those where there is much warmth and where a free circulation of air is wanting, it contracts a disease known, improperly enough, by the name of the dry-rot, the effect of which is a complete decomposition of the vegetable fibre, and consequently a privation of all strength. It is altogether different from the common rot, of which indeed Mr. Pering seems aware, though he constantly confounds the two processes of decay. Neither do we believe that the professed dry-rot doctors have accurately distinguislied the one from the other; we shall therefore submit the ideas which have occurred to us on this important subject.

The common rot in wood is a gradual decay of the fibre, more or less accelerated by the alternate action of wind, heat, and moisture on the surface; its progress internally being greatest when the wood is constantly exposed to the alternations of wet and drought, as exemplified in the rapid decay of that part of a post which is close to the surface of the earthi, while all above and below is perfectly sound :--and least, when constantly soaked in water, or constantly kept dry--when exposed to a free current of air, or excluded from all air.

The dry-rot, on the contrary, is a disease which commences its ravages internally, and is but little affected by any external circumstance, excepting that of heat. The application of a strong heat will kill the disease, but at the same time injure the wood; a moderate heat seems to be necessary to bring out the symptoms of the disease:-in dwelling houses, for instance, the dry-rot generally makes its approaches in the kitchen, or in a close warm cellar. At first, its appearance is that of fine fibres running on the surface iu endless ramifications, resembling the nervous fibres of leaves; presently the interstices are filled up with a spongy or leather-like substance, assuming the character of that order of cryptogamous plants distinguished by the name of fungus. It is well known that if a piece of green wood be laid across a fire, the air within, expanded by the heat, will drive out, at each extremity, a viscous fluid possessing the property of disposing itself on the surface in reticulated filaments. The same appearance of nervous foliation is not uncommon in the intermediate spaces of the concentric layers of the alburnum of wood; and the core or heart of trees, and particularly of the pitch-pine, after its passage in the heated hold of a ship, is often enveloped with a membranous corticle like that which lies immediately beneath the bark. All these appearances are certain indications of the dry-rot; and they point out, with sufficient clearness, that the sap, or principle of vegetation, brought into activity, is the cause of the disease; the effect, though infinitely more rapid, is the same as that of the common rot. It is still a problem in what manner this sap circulates; but there is no doubt that the tubes and cells of the alburnum, or sap-wood, are filled with it in the spring of the year, and that they are empty in the winter; that it is organized matter, developing itself by heat in all the various forms of new bark, leaves, and branches. The stem of a tree cut down will, on the return of summer, make an effort to. push out leaves; a more feeble effort of this organized sap ends in the production of fungus only.

derate

VOL. VII, NO. XV.

These facts and appearances suggest an obvious preventive of the dry-rot: either by desiccation, or driving out the sap by artiticial heat—by destroying the principle of vegetable life inherent in the sap, by some chemical impregnation, or-by gradual seasoning. In all cases, the process will be considerably expedited upon timber felled in the winter months. There is reason to believe that our ancestors were particularly attentive to this point. They not only never allowed a piece of green timber to be put into ship, but were equally attentive to the time of felling, and the place of growth. The Sovereign of the Seas, already mentioned, was built of timber which had been stripped of its bark while growing, in the spring, and not felled till the second autumn afterwards, and it is observed by Doctor Plot,* in his discourse on the most seasonable time for felling timber, written by the advice

a

* Philosophical Transactions for 1691.

of

pores

of Mr. Pepys, secretary of the Admiralty, that, after 47 years, "all the ancient timber then remaining in her, was still so hard that it was no easy matter to drive a nail into it.' He farther observes, that trees felled in the spring are full of sap, which remains in the and putrifies ; leaving the wood full of cavities, which make the timber weak, subjects it to rifts or gapes, and causes it to shrink; that, in Staffordshire, therefore, they bark the trees in the spring, and leave them standing till winter, and fell them when the sap has subsided ; and he quotes the concurring testimony of Vitruvius and Pliny, that if the sap be let out of the oak while yet living, it will acquire almost a perpetuity of duration.

There can be no question that winter-felled timber is far superior to that which is cut down in the spring. The Montague, noticed before, was wholly built of winter-felled timber; but the advantages of stripping off the bark, while standing, are not so clearly established. It is thought in France, that timber acquires considerable durability by it. In England, the only experiment, we believe, is that of the Hawke sloop of war, which was built at Deptford in 1793, one half with timber stripped of its bark while standing, the other half with timber felled in the ordinary way. Ten years afterwards she was broken up, when no visible difference was apparent in the state of decay of the two kinds of timber.

From what we have premised, the rapid decay of many of the ships built since the recommencement of hostilities in 1803, admits of an easy explanation. It has generally been supposed that when Lord Spencer quitted the Admiralty in 1801, he left an efficient fleet; this was by no means the case. It was numerous, indeed; but many of the ships were nearly worn out. The avowed system of the new ministry being that of economy, Lord St. Vincent determined, in order to give the better effect to it in the naval department, to build no more ships of war in the merchants’ yards. The King's yards were almost wholly occupied in patching up those actually in commission, and those brought forward from the ordinary. The consequence was, that when Lord Melville in 1804 succeeded to the Admiralty, he found the navy wholly inadequate to the situation of the country, threatened as it then was with immediate invasion; scarcely one of the ships in commission had more than three years to run, most of them but two, and many only one: a few, and but a few, new ones were slowly coming forwards in the King's yards, and none in the merchants' yards. It therefore became absolutely necessary to have recourse to private builders, who were wholly unprepared with materials. Contracts, however, were entered into at advanced prices, the axe was set to work, and trees which were one year growing in the forest, were, in the next, floating on the ocean: and the demand since that time has trodden so closely on the heels of supply, that few, if any, ships have been built, either in the royal or private yards, with timber duly seasoned.

Another cause of this premature decay has been ascribed to the introduction of foreign timber into our dock-yards, and to the

practice which has very injudiciously prevailed of late, of intermixing it with native oak. The Queen Charlotte was nearly seven years in building: some of the timber of native growth was seasoned, some of it was quite green; some of it consisted of Canada oak, and some of pitch pine, both peculiarly susceptible of the dry-rot. This assemblage of heterogeneous materials was certainly not prudent. Different woods possess different kinds of oils or acids, and experiments have not yet determined what effects are produced by their being brought into contact. There are, no doubt, between woods, as well as between other bodies, certain sympathies and antipathies, the operations of which have sensible effects on their contiguous surfaces. Almost the whole of the Queen Charlotte was rotten from the water line upwards, and in many parts below it, and her timbers were covered with as many different species of the boletus as there were different kinds of-wood used in her construction. All this is perfectly accordant with our theory of the dry-rot; but, if our information be correct, we have much stronger 'proofs of its being the true one—for as soon as this unfortunate ship was closed in, stoves, strongly heated, were placed in various parts of her hold, so as to raise the thermometer to 90° of Fahrenheit; an excellent hot-house was thus formed for the growth of fungus on the surface of the unseasoned timber, and it did grow most luxuriantly.

The oak timber produced in the forests of Germany is remarkably subject to the dry-rot; and it is some little consolation to find that the enemy, in this respect, is no better circumstanced than ourselves. The ships in the Scheldt are known to be in a rapid progress towards rottenness. The Chatham, a 74 gun ship now in our navy, had the dry-rot in her timbers when taken from the stocks in the dock-yard of Flushing. The Rivoli, just off the stocks from Venice, is also infected with the dry-rot. This ship was built of timber from the forests of Dalmatia.' Of English oak, that of Sussex is the closest grained and the least subject to decay; hedge row oak, or trees growing singly, less so than trees from the midst of a forest. This too is perfectly consistent with our theory. The more exposed trees are to the wind and sun, the more compact and durable will the timber be, while that from the dense forests of Germany and Canada, into which the sun's rays never penetrate, is more porous, more abundant in sap, and more prone to the dry-rot. We may hence infer the great importance of using only well-seasoned timber in the construction of a ship.

The

« ΠροηγούμενηΣυνέχεια »