ments are separated by means of aqueous sulphuric acid, consists of fluorine united to hydrogen, derived from the water of the sulphuric acid. It has therefore received the name of hydrofluoric acid. It possesses the property of dissolving silica, and consequently of corroding. glass. Fluor spar is an abundant mineral production. The hydrofluoric acid is also found in the cryolite, and a few other rare minerals. Fluorine is a negative electric, Its combining proportion is 18. REPORT OF MAGNETICAL EXPERIMENTS, TRIED ON BOARD AN IRON STEAM VESSEL, BY ORDER OF THE RIGHT HONOURABLE THE LORDS COMMISSIONERS OF THE ADMIRALTY. BY EDWARD J. JOHNSON, ESQ., COMMANDER, R.N., F.R.S. THE very extensive use of iron in the construction of modern vessels, and still more recently, the formation of steam vessels entirely of that material, has rendered the compass, notwithstanding its successive improvements, little more than a piece of useless lumber: or, more properly speaking, it is become unworthy of confidence, and, consequently, where it is trusted, a deceptive and frivolous apparatus. Indeed, the compass in its rudest form, even the Chinese, or the early European, with ships built as ships were then built, was worthy of far greater confidence than the most improved compass is on board a vessel of modern construction. There is no doubt, indeed, that were the weather always clear, the compass might be advantageously dispensed with in maritime affairs; but, as during not only days but weeks of the most tempestuous weather, when not a single lunar can be taken, nor any kind of complete observation made, the vessel is driving about amongst known or unknown dangers, it cannot but be viewed as a most perilous condition, when the direction itself upon which she is sailing, is a matter of almost total uncertainty. If, indeed, she be out at sea, and far from land, she is safe, provided her strength be sufficient to ride out the storm: but every year, we are well assured that numerous cases of the heart-rending scenes of wreck and desolation would be escaped, had they the power to ascertain the course upon which they were bearing. This, however, is utterly impossible as ships are now built, by the use of the compass simply. Mr. Barlow's correcting-plate is, on this account, one of the greatest boons conferred on modern navigation. This term, as most of our readers are aware, is something of a misnomer; since the plate, instead of correcting the error produced by the iron of the ship, doubles it: but we would not quarrel with names—as it is with things we have to deal. It enables us, by a very simple numerical process, to ascertain approximately the effect of the iron of the ship upon the direction of the needle, and to make allowance for it in our reckoning. It is strange, however, to witness the apathy with which the reckless seaman, in time of security, looks upon the possible danger of a future, and not remote, period. A single hour in port would enable the master to find the effect of his iron with considerable precision: and yet this single hour he thinks it too much to give to his own and his crew's future safety. Strange infatuation !—but infatuation almost always follows close upon the heels of familiarity with danger. We do not require to be told that Barlow's plate is not perfect. This we are as fully aware of as any one: and we do not urge implicit reliance upon it, in any sense of the word, under all possible circumstances. Still, if it enable us under all conditions to ascertain the amount of the effect of iron approximately, and often within very narrow limits, surely we must be determined upon rushing into danger, if we do not avail ourselves of it to the degree in which it can assist us. We are utterly opposed to the use of the compass at all, in those cases where it can be dispensed with: but as cases so perpetually occur where it is our only guide, and those cases precisely those of the greatest danger, it is surely worthy of the most serious attention, from every practical navigator, as well as from men of science*. At the period when Mr. Barlow proposed his plan of the correctingplate, he had in especial view the effect of the immense masses of iron which the guns on board men of war contained. Of course, cæteris paribus, the same circumstances would occur on board the smaller vessels in the merchant service, and require correction accordingly. The recent introduction of iron steamers has, however, given a new and important interest to this contrivance. These are chiefly designed for passengers, and, in some cases, more than five hundred are crowded on board a single steamer. We do not indeed, just now, know to what extent the iron steamers have been introduced: but as they have many advantages, in respect to security and convenience, over those of wood, they will most likely supersede them entirely, provided they can be rendered as safe for the purposes of navigation by the compass,-circumstances giving rise which must inevitably occur in all voyages of any considerable extent. The inquiry into this possibility, it was the main object of Captain Johnson's experiments to satisfactorily answer; and we proceed to give a brief analysis of them. The Dublin Steam Navigation Company placed at the disposal of the Lords Commissioners of the Admiralty a fine new vessel, built en tirely of iron, the Garryowen, for the purpose of investigating the effect of the vessel upon the indications of the compass, in any way that their Lordships may think propert. They appointed Captain Johnson to make the requisite experiments; and he repaired in her to the port of Limerick, in the autumn of last year, to carry them into execution. The results have been printed for the Royal Society's Transactions; but they are not yet published. We avail ourselves, however, of a copy of the Memorial, with which we have been favoured, to lay before our readers a succinct account of them, and a few reflections on the results they bring to light. There being no wet dock in the port of Limerick in which the Garryowen could be conveniently swung round, to make the observations in different azimuths, point by point, he fixed upon a position in Tarbert Bay, well adapted to his purpose. His operations were commenced on the 19th of October, and continued, as circumstances permitted, till the 18th of November. In order, however, to show the positions in the vessel at which the several observations were made, the following diagram is given. The line y z is the keel, Y being the stern, and z the stem; v is the place of the chimney, and w the axle of the paddle-wheel. The positions of the other points will be easily judged of, from the accompanying scale and table: and these designate the positions of the compasses named by the several letters themselves. Quarter deck, 5 feet 9 inches above the deck. B Forecastle, 5 feet 11 inches above the deck. C Bowsprit, on glass-legs. D On the fore-part of the temporary poop, above deck 8 feet 5 inches. F On a stage level with the taffrail. GOn a plank 4 feet below the main-gaff end, and above deck, 20 feet 5 inches. I On the centre of the temporary poop, above deck 13 feet 4 inches. L On the poop projecting over the stern. MMM Three stations on the stage over the stern, level with the taffrail. Between the paddle-boxes. P Two-thirds up the fore-topmast, above the deck 40 feet 2 inches. Q (On glass legs) in the fore-hold. S In the iron sphere a-midships, above deck 7 feet. T In the cabin. All made in the middle of the vessel, 9 feet 11 inches from each side. After the preliminary operations of fixing stations, &c. were gone through, Captain Johnson saw it to be necessary to put all the iron which the vessel carried, in the places which it usually occupied during the voyage, such as the anchors, cables, &c. He then tried the effect of the whole in that direction of the vessel where, in the generality of cases, the deviation had been found to be a maximum, so as in some degree to guide him in the selection of a place for the principal observations, or that, which his orders especially directed, in which to place a steering compass, and where the effect of the plate may be successfully tried. VOL. II. 2 G 12 - With this view, the vessel's head was warped to the true magnetic east, and the deviations at several parts of her ascertained: At B, the deviation of the marked end of the needle = Near the centre, and before the funnel, it was At A, when the boats' davits were out were in board 16° E. 26° 20' E. 11° 40' E. 6° 20' E. The preceding facts, showing the influence of iron in the vicinity of the compass, may be worthy of the attention of the practical navigator: for they show, at a position not far from the binnacle, a difference of deviation of no less than 5° 20′ under the mere circumstance of swinging the quarter boats' davits in board, from their usual position where boats are hoisted up, to that place in which they would be secured in stormy weather at sea. Though the principal object of these experiments was to determine the best position for a steering compass on board the iron steamer (which induced him to select A and B as especial places of observation), Captain Johnson tried the effect of the vessel at all the places in the table given above. We need not follow him through all the details of the precautions which he used for insuring the elimination of all foreign influences of an accidental kind, nor of those for securing good observations. A brief statement of the positions is, however, necessary. A fixed station x* was selected on the south-west side of Tarbert Bay, a mile distant from the vessel, from which the cone of the mountain Dicomede, in the county of Clare, distant about nineteen miles, was distinctly visible. The bearing of it was determined by the compass which was afterwards used at station a on board the vessel; the magnetic meridian was also then determined by this compass, and a distant object in that meridian, on land, was noted. The theodolite, in the succeeding observations, was placed at x in the same position as the compass, thereby rendering the simultaneous observations at a and x virtually identical with those which would have been determined by one compass only. The vessel was then taut moored in the line between the station x and the cone of the mountain,-in the line of direction of which, was also a remarkable heap of stones on Kilkerran Point,—so that the vertical wire of the theodolite at x bisected these objects and the instrument on the forecastle of the vessel. The bearings of the cone of the mountain from the positions A and B as well as the simultaneous bearings between A and x, were observed when the vessel's head was at each point, and from these bearings between ▲ and x the deviation of the compass produced by local attraction was deduced. Of course we cannot give the tables entire, and we mention the circumstances under which the observations were made, to show that every requisite care was used to ensure correct results. We are, however, still under the necessity of giving one, referring our readers to the paper itself for the others. This contains the simultaneous observations made with nine compasses in different parts of the vessel, the bell being struck as a signal for observation. See opposite page. In looking at this table, it is impossible not to be struck with the No figure is given to these descriptions; nor is any necessary, as the reader can easily sketch it from the verbal statement. |