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- '•• '5'. That the compass-errors occasioned by the more permanent pa of a ship's magnetism may bo successfully compensated, and that this compensation equalizes the directive power of the compass-needle on the several courses on which a ship may be placed.
The first two points wo have already adverted to, and we fully agree with tho Committee in considering that they may now be accepted as well established. ..
The third point is one of the most important of tho results to which the making, registering, and discussing the observations of deviation in iron ships is at present leading us.
i It is clear that when an iron ship is first launched, her magnetic character depends almost entirely on her position in building, but that this magnetic state is extremely unstable; that very great changes take place within a few days, or even hours, after launching; but that, after no long time (the length of time depending no doubt, to a great extent, on the seryioc in which the vessel has been employed), what may be called tho temporary magnetism gets "shaken out" of her, and the magnetism of the ship acquires an extremely stable character. This is a matter on which exact and varied observations are much wanted; but we think it may be taken at present as the most probable result, that after about twelve months there is very little change in the magnetism of a ship which has made some voyages in tho interval. In some ships the stability is most striking. It must, however, be remembered that it does not follow from this that the whole of the magnetism which remains, and which affects the compass, is the permanent magnetism of hard iron. There is in all iron ships, as shown by the amount of the quadrantal deviation, a large quantity of soft iron, and consequently a large quantity of magnetism doveloped instantaneously (or nearly so) by induction; and the magnetism developed in the soft iron by vertical induction is not, in any given geographical position, distinguishable from the permanent magnetism of hard iron. The test of the kind of permanence which is acquired by the magnetism of an iron ship after the lapse of the period we had referred to is, that her table of deviation shall always bo the same when swung at the samo geographical position. If, in addition to this, her semicircular deviation in different parts of the globe is inversely proportional to the horizontal force of magnetism at the place, we infer that the vertically induced magnetism is so distributed as to produce a compensation of effects, and that the only cause which operates is the permanent magnetism of the hard iron. In some ships this appears to be the case. In H. M. S. 'Trident,' which has been particularly discussed by Mr. Airy, the magnetism is not only extremely stable, but nearly the whole of the semicircular deviation appears, from observations made in various latitudes, to be due to hard iron. The same is the case with H. M. S. 'Adventure' and with many other iron ships.
The practical conclusion which, it appears to Us, may be drawn from these facts, is the importance in all iron ships of having their magnetic history carefully recorded, and the observations discussed. We need hardly say that, to give any value to such a record, observations should be made with the compass in a fixed position in the ship, and not corrected in any way by magnets or soft iron.
On the fourth point we have, in fact, already expressed our opinion. We are not satisfied that the effects here referred to are in general of appreciable amount in so short a space, of. time as that occupied by the process of swinging a ship. There seems, however, no doubt that the cause operates sensibly in
many cases when a ship has been long sailing in one direction; and this remark might be taken as a qualification of what we have remarked as to the permanence of the magnetism of a ship.
On the fifth point we quite agree with the Liverpool Compass Committee, subject, however, to the qualification that this correction cannot be depended on in the case of a newly-built ship, and that when the correction is applied to compasses having large deviations, and placed near large vertical masses of iron, as a stern-post, there must always be great uncertainty as to the correction on a change of magnetic latitude. It is also right that we should not pass over this remark without protesting against the application of such correction to the standard compass (properly placed) of a ship which may be called on to make a voyage during which there is any great change in the dip or horizontal force.
The Committee notice as the principal points left for further discussion and inquiry, the effect of heeling on the compasses of iron ships, and the changes which occur on a change of magnetic latitude; and to these the Eeport is chiefly directed.
On the effect of heeling a considerable body of evidence is collected, but with the disadvantage that at that time the mathematical theory of the heeling error, and the formulae which express it, had not been fully investigated, and that consequently the comparison of theory with observation could not be precisely made; nor do the observations in all cases furnish sufficient data for the comparison.
"We think, however, that it may be said, with confidence, that the results of "observations agree with theory as to the connexion between the amount and direction of the heeling error and the coefficients of quadrantal deviation and of horizontal and vertical force; and that we may therefore feel assured that the heeling error may be predicted with sufficient accuracy from observations made on an even keel.
The most important practical results as to the amount of the heeling error, are the very great amount to which it reaches in certain ships, and in certain positions in the ship. This heeling error is conveniently measured by the fraction of a degree or the number of degrees of error produced hy every degree of heel when the ship's head is North or South. Estimating it in this way, it will be seen that the error may have serious effects if it exceed •5 or -6, when an inclination of 10° may produce half a point of error.
Among the examples given we have—
Iron- S. S. City of Baltimore (built head North). heeling error.
Compass placed above the aft end of iron round-house.. +6-70
Port steering-compass compensated — "30
Starboard steering-compass compensated — '60
Standard compass 4-2-20
Azimuth compass 4-2
Dipping-needle compass +2*
Fore compass compensated 4- '80
Compass over fore hatch 4- '85
Aphrodite (built head East).
Compass under companion +2"
Compass near companion 4- 2"85
Admiralty standard compass 4-1-20
Dipping-needle compass 4-1-15
Simla (built head Vest). heeling error.
Compass over companion -fl-65
Dipping-needle compass + "£0
Standard compass + "73
Forward compass + •TO
Slieve Donard (built head S.E. to E.).
Aftermost steering-compass compensated + *40
Second steering-compass compensated + "12
Skylight-compass compensated + '33
Mast-compass + '23
Port skylight compass + '26
In other compasses of the 'Slieve Donard' the heeling error was almost imperceptible. In the case of the ' City of Baltimore,' the large heeling error is evidently due to the vertical force downwards near the stern, arising from the ship having been built head north. In the 'Slieve Donard,' the small heeling error is evidently due to the ship having been built with her head to the southward.
Before leaving the subject of the third Bcport, we must beg leave to mention one point which has made the duty of reviewing the Beport more difficult than it would otherwise have been, and which we fear will detract from its general utility, viz. that the mathematical formula; made use of in reducing the observations are nowhere given, and that we have been unable, in some cases, to verify or use them. We hope that the Admiralty Manual may be of some use to future investigators, as providing a uniform notation and mode of reduction, which will make the results derived by one investigator intelligible to all.
In concluding this notice, we think we may say that the principal desiderata at present are—
1. That in the construction of iron vessels, regard should be had to the providing a proper place for the compass. It is not difficult for any one who has studied the question to point out arrangements which would greatly mitigate the injurious effects of the iron of the ship; the difficulty is to reconcile them with the requirements of construction and of working the vessel.
2. That for throwing light on the points which are still obscure, what is chiefly required is, that the complete magnetic history of some iron vessels in various latitudes should be known. This, we think, might easily be accomplished by observations of deviations and horizontal and vertical force made at various fixed positions in an iron vessel in an extended voyage in both hemispheres. We need hardly add, that this should be a vessel of war of moderate size, and in which the magnetical observations would be made an object of importance.
Report on Tidal Observations on the Humber. Presented by James OldHam, C.E.; John Scott Russell, C.E., F.R.S.; J. F. Bateman, C.E., F.R.S.; and Thomas Thompson.
At the Meeting of the British Association held at Manchester last year a paper was read in Section G, on the Port of Hull, in which occurred the following remark, referring to the tides of the Humber: "I would notice here a singular tidal phenomenon which exists at the Port of Hull; I refer to the fact, that whenever the tide reaches the'18-feet mark" (over the dock-sill), "it is then three hours to high water, whether they be spring tides or neap tides. I am notaware that the same thing occurs at any other port; but such is the fact at Hull, that three hours after the tide has attained to the 16-feet mark, there is no more rise."
Theso remarks gave rise to an animated discussion on tho alleged phenomenon, and resulted in. the. appointment of the following members of the Association as a Committee to conduct a series of tidal observations on the Humber, and report on the same to the next Meeting to be holden at Cambridge, viz. Mr. James Oldham, C.E., Mr. John Scott Eussell, C.E., F.E.S., Mr. J. F. Bateman, C.E., F.B.S., and Mr. Thomas Thompson, with £25 at their disposal.
In commencing the arrangements for carrying out the wishes of the Association, application was made to the directors of the Manchester, Sheffield, and Lincolnshire Bailway Company for a month's observations to be taken at their self-acting tide-gauge at the Great Grimsby Docks, but it was not convenient to the directors to grant the request; they, however, permitted a gauge-pole to be fixed at their landing-pier at New Holland, on the Lincolnshire coast of the Humber, a little above Hull, and gave every facility in the progress of the operation of observing tho tides.:
The Hull Dock Company, through their secretary, Mr. W. H. Huffam, have complied with a request to have a month's observations from their selfacting gauge of the Victoria Docks; and the resident engineer of the com* pany, Mr. B. A. Marrillier, has furnished tho month's valuable tidal observations. - •• . Mr. Thomas "Wilson, of Leeds, an active member of the British Associa* tion, kindly offered a month's observations from the self-acting tide-gauge of the docks of the Air and Calder Works, at the Port of Goole, on the river Ouse, which have also been furnished by Mr. "W. H. Bartholomew, the resident engineer.
Those on the Humber were commenced at or about 11 A.m., July 9th, and terminated at 3 P.m., August 6th; but those at Goole, which were begun at 11 A.m. on the 9th July, were continued until twelve o'clock at noon on the 10th of August.
Tho gauge at New Holland is so fixed as to correspond with, and is on tho same level as, the Victoria Dock gauge at Hull, t. e. the zeros are made to coincide.
The observations were taken every five minutes at New Holland, but every fifteen minutes at tho Hull Dock gauge; the observations at Goole were taken at intervals of five minutes.
_ As a result of these tidal investigations it was seen, by the series of observations at both the stations on the Humber, how accurately the statement is borne out as to rise of tides for three hours after attaining the 16-feet mark, and also that the time which the tide is falling from the period of high water to the same level again of 16 feet is also found to average about three hours.
The observations are also important and valuable, as showing the general rate of the rising and falling of the tides at the various periods and places reported on.
Although little or no light may have been thrown on the phenomenon in question, yet the various tidal observations obtained on the Humber and tho river Ouse will no doubt prove valuable records on the question of tides.
From tho various observations the following are the results:—The observations made oil the Humber comprised 55 tides: the greatest variation at spring tides was 22 feet 3 inches flow; and the least variation at neap tides a rise only of 10 feet 7 inches. The lowest level of low water at spring tides was 3 feet 8 inches, and the highest rise 27 feet 11 inches; the highest at low water of neap tides 11 feet 2 inches. The mean rise of the 55 tides above low water was found to be 16-95 feet. The averago time of rising tide is about 5% hours, and tho falling tide about 6 J hours.
At the season of the year when the observations wero taken it is generally calm, and there is no undue influenco exerted on the riso and fall of the tides on tho Humber; but at tho time of the equinox, and in stormy winter seasons, particularly during north-westerly gales, there is a much greater riso and fall during spring tides than would otherwise occur.
The observations made at Goole (which port is about 30 miles above Hull) show on the 63 tides a mean rise of 11-67 feet,-—the greatest rise above low water being 15 feet 4 inches, and tho least rise from low-water line 7 feet 7 inches.
The tides at Goolo average about 3 hours in rising, and a little over 9 hours in falling.
The mean rate of the tidal wave on tho Humber is from 2£ to 3 miles at neap tides, and 4 to 5 miles per hour at spring tides.
On Rifled Guns and Projectiles adapted for Attacking Armour-plate Defences. By T. Aston, M.A., Barrister at Law.
[A communication ordered to be printed among the Beporta.]
As it is now an admitted fact that naval warfare will be carried on by iron-' clad navies, it has become an imperative necessity that the navy of England shall henceforth bo armed with artillery adapted for attacking the new1 armour-plate defences which all nations are hastening to adopt. The superiority which defence so suddenly acquired over attack, by simply putting on a coat of armour, threatened to upset not only the theoretical but tho practical tactics of modern warfare. The necessity of improving the means' of attack so as to restore, as far as possible, tho disturbed equilibrium was ebvious to every one; and the contest which has been carried on in this country for the last two or three years between the attack of improved artillery and the defence of improved armour-plates has been watched by all of us with the greatest interest. From a scientific point of view, with which we are on this occasion more immediately concerned, the Subject was one which engaged the attention of some of the keenest and most experienced intellects of the country,—these, on the one hand, giving practical aid on thd side of defence, those, on the other, devoting their best energies to restore attack to what must bo considered its normal position of superiority. For a long time—for too long a time—the defence-people had much the best of it. Under the energetic superintendence of tho Plate Committee (who in this matter de republica bene meriti sunt), armour-plate targets were erected by our able engineers which at fighting-ranges laughed to scorn the utmost Efforts of the artillery attack brought against them. Some of the targets combined the resistance of iron with wood; others, constructed with far-seeing