« PoprzedniaDalej »
are subject by a change of temperature. If the makers were careful to introduce sufficient compensation, and the owners of ships were more anxious for correct than for cheap instruments, there would be no difficulty in obtaining proper chronometers for the mercantile marine. But the indifference of the merchant and ship-owner supports the apathy of the chronometer-maker, who will continue as long as possible to send his best instruments and improved compensations to Greenwich to be tested for the navy, and find a market for the inferior in the merchant service. To this the owners of small ships must submit till they have the energy and wisdom to establish or support, for their own purposes, an institution similar to that of Greenwich, to examine the qualities of the instruments they are requested to purchase. At the present time there is but one nautical observatory in the country, perhaps in the world. Ships enter every port except that of Liverpool, and the captains have no opportunity, if they desired it, of ascertaining the rates of their chronometers, or the correctness of those other instruments upon which the safety of their ships mainly depends. But let these facts be generally known, and if we have not overvalued the philanthropy as well as the commercial enterprise of our countrymen, observatories will be established in all the principal maritime towns, each having its time-ball and standard barometers and thermometers for the proof or correction of the instruments used in ships. In this matter, individual and national interests, and the obligations of humanity and religion, enforce the dictates of reason and the demands of science.
While all the resources of experimental science are engaged to improve the instruments upon which safe navigation depends, the engineer and ship-builder are projecting new ships of gigantic size and mighty power. When the Great Western' was launched it was an almost universal opinion that a larger steam vessel could not be safely carried over the ocean. But then came the Great Britain,' and she was condemned as a mad experiment by many an old sailor, who asked tauntingly where the owners would find a commander foolhardy enough to guide her over the Atlantic. But we have not yet, it appears, built a vessel large enough for the new colonial trade which has been opened within the last twenty years. Success sometimes makes men rash, and they only learn prudence when taught by some terrific accident. Are we now to be taught this lesson again by the experiments of the modern ship-builders ? or is it true that we have not yet reached those dimensions in naval architecture which it would be unsafe to exceed? Such are the questions many persons are now asking; and so satisfactorily have they been answered, that in spite of the doubts of the timid, and the forebodings of jealousy, we have confidence in the scientific knowledge and judginent of the professional men who not only admit the possibility of building good ships much larger than any now in existence, but are also making the trial.
Mr. Scott Russell, a gentleman well known in the annals of science, is now building, from the drawings of Mr. Brunel, a large iron ship for the Eastern Steam Navigation Company. This ship will be 675 feet long, 83 feet in breadth of beam, and 60 feet deep. It will carry 6000 tons of freight, 500 first-class passengers, 500 second-class, and 1000 third-class. The size of the vessel and the amount of freightage so largely exceed any dimensions and quantities ever before dreamed of, there can be no impropriety or impertinence in inquiring whether such a ship could be safely managed in a stormy sea ? and whether there are any advantages in the use of such a vessel to warrant the experiment?
The safety of such a vessel must depend, so far as its mechanical construction is concerned, upon its strength. Upon this point we have the testimony of two men eminently entitled to give an opinion. Mr. Scott Russell, when complimenting Mr. Brunel for the engineering skill and ingenuity he had displayed in designing large ships for our mercantile marine, said that he was perfectly convinced that the forebodings some people had indulged would be found fallacious. Mr. Fairbairn, an equally competent judge, acknowledged that he once thought such a ship would be too large, and that the necessary strength could not be obtained. But he had arrived at an opposite opinion by the examination of Mr. Brunel's drawings. I have no doubt now,' he said, 'that the ship will be perfectly strong, and be able to bear a gale of wind without bending. It is built on the same principle as the Britannia Tubular Bridge; and as that mode of structure is able to sustain a bridge without any support in the middle, there can be no doubt that supported, as the ship will be, by the water, it will, under all circumstances, be able to bear the strains to which it may be subjected. With these opini we must be content, for higher authority cannot be obtained.
The benefits expected from the use of the proposed large ships, in preference to those now in use for ocean voyages are, increased speed and greater profit,—the latter greatly depending upon the former. It has been discovered, and is now well known, that speed is governed by the length and entrance of the vessel.
'A fine concave entrance,' says Mr. Russell, “instead of a bluff round bow, is generally admitted to be the best; and in addition to the shape of the water-line, it has been found that the length of the body of a ship facilitates its passage through the water by allowing a longer time for the particles of the fluid to separate. A ship with a fino
concave bow, a long body, and a comparatively round stern, cleaves its passage through the water without raising a wave in front to obstruct its course. No steam ship that is not 180 feet long can be propelled at a speed of sixteen miles an hour without a great expenditure of power; and 400 feet is the shortest length for a ship that is intended to be propelled at so high a speed as twenty-four miles an hour. The * Himalaya,' which is 365 feet long, has a greater speed for the power employed than any other merchant ship.'
This settles the question how the greatest amount of speed is to be obtained from a given power; and as speed governs profit, the ship-owner has a great interest in facilitating the construction of vessels to secure that object.
The highest rates of freight, we are informed, do not pay the expense of small vessels employed on long voyages. The impossibility of carrying in such vessels as are now afloat a sufficient quantity of coal for a voyage to Australia (for example) compels the owners to establish coal depôts to supply the quantity which they require but cannot carry. This, of course, greatly enhances the value of the fuel ; and the unnecessary length of time occupied in the voyage increases the quantity consumed. Now, as by increasing the length of the ocean-going steamers less time will be occupied in the voyage, and all the coal necessary may be carried, with greater space both for passengers and freight, a good profit may be expected by the owner instead of a loss, and the commerce of the country with distant parts of the world will be supported and probably increased.
It is estimated that the ship now building will make a voyage to Australia in thirty, or, at most, thirty-three days, and this statement might lead the reader to suppose, that if so much time is gained in this voyage, a ship of the same dimensions would be suitable for any foreign trade. But this is a deduction which cannot be proved. The dimensions of a ship should have a proportion to the trade in which she is to be employed and the length of the voyage she is to make. The object of the shipbuilder should therefore be to supply the power required in the most convenient manner and at the cheapest rate. If this be made a subject of study, the number of the first or largest class steam vessels will always be few in comparison to the number of other vessels employed in the merchant service, however successful the present experiment may be. The Himalaya' is, perhaps, the best model yet produced for the ordinary traffic of the ocean, and it is not probable that shipbuilders will gain much advantage by constructing longer vessels, except when required for special purposes
The introduction of iron as a material for ship-building is one of the most important of the many invasions this age has made
upon old customs. The large vessels which it is proposed to construct could not have been built without it; for timber of sufficient size is not to be obtained, and joints cannot be so made as to have the same strength as a solid piece. But iron can be manufactured of any size, irrespective of shape; and when it is more convenient to unite one piece with another by a joint, a greater strength may, in some instances, be obtained than if the two parts were absolutely united. But although the change of material has increased the strength, improved the shape, and greatly increased the speed of the mercantile marine, there is still one disadvantage attending its use. When it was first proposed to build an iron ship, many reasons were given for calling it a visionary scheme; but of all these there was but one that had the semblance of truth. It would be impossible, it was said, to navigate au iron ship if it were made, for when surrounded by such a mass of iron the compass would be useless.
This was greatly overestimating the evil, as we now well know, for iron ships do take long voyages, and the compass is not altogether a useless thing, although it is subject to a deviation from local attractions, which it is most important to correct. There are risks at sea,' says the Council of the Astronomical Society, which no foresight can correct; but loss from defective compasses or ill-regulated chronometers should be treated as a crime, since common sense and common care will secure the efficacy of both these instruments. It is to be feared that life and property to a large amount are yearly sacrificed for the want of a little elementary knowledge and a small amount of precaution on the part of our seamen, who neglect the safeguards furnished by modern science.'
To this statement we give a ready assent, so far as the chronometer is concerned; but we should be sorry to treat as a criminal a commander who lost his vessel in consequence of a defective compass. It is notorious that compasses are liable to serious derangements in iron vessels, and that there is not at present any adequate means of correcting the error. The needle is always deflected from its position, more or less, in an iron ship; but supposing the displacement to be permanent, it might be corrected by placing it under the immediate counter-action of permanent magnets. This plan was adopted; but a long trial has proved it to be in no case more than partially successful, and, in some instances, absolutely dangerous. At the last meeting of the British Association this subject was discussed by Dr. Scoresby, and we should give an imperfect view of the practical character of the scientific investigations of the past year if we passed it without some notice.
The loss of the Tayleur' in the Irish Channel, and the mournful death of many of her passengers and crew, will be fresh in the memory of our readers. She was a new iron ship, of nearly 2000 tons burthen, and sailed from Liverpool for America with 528 persons on board. On the second day after leaving port the captain came in sight of land at an hour when he supposed he was sailing in the usual course, in nearly midchannel. A heavy sea was running at the time, and the attempt to wear the ship round having failed the anchors were thrown out, but the cables snapped, and the vessel was driven broadside upon Lambay Island. 290 persons were drowned. An investigation of the cause of the wreck followed, and some facts relative to the ship's compasses were made known, for which the public were quite unprepared. One compass, it appeared, was fixed before the helm, and another near the mizen mast. Both of these had been, according to custom, carefully adjusted at Liverpool, and the captain himself had been diligent in verifying their action. Previous to the wreck, however, it was discovered that the compasses differed in direction by as much as three points. Under these circumstances it was necessary to elect by which of the two the vessel should be guided, and that at the helm was chosen. This was the proximate cause of the fatal catastrophe.
To investigate this matter more fully, the Board of Trade instituted inquiries into the cause of the difference between the direction of the two compasses. The Marine Board of Liverpool reported that the “Tayleur' was brought into the dangerous position in which the wreck took place through the deviation of the compasses, the cause of which the Board cannot determine.' But,' it is added, 'numerous instances have been brought under the consideration of the Board, of compasses having proved greatly in error on board both wooden and iron ships in navigating the Irish Channel, which deviation is not accounted for by any theory hitherto propounded.'
Now if it were only in the Irish Sea that the deviation of the compass could be observed, the scientific investigation which this catastrophe still demands should be conducted in that locality, as the danger to be apprehended would in that case be limited to one troubled channel. But the cause is in the ship, and it matters but little where she may be, for the effect is everywhere the same under the same circumstances. Dr. Scoresby, however, thought the subject to be sufficiently important for the consideration of the mathematical and physical section of the British Association at its last meeting, and in introducing it, very properly explained those facts which must guide the inquiry. These facts it may be desirable to review.
The magnet, as everybody knows, has a directive force received from some invisible and subtle agent, which causes it to turn its