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instead of the magnetic course, uninfluenced by the local magnetism of the ship or her cargo.

When an observation for the latitude is obtained at noon, this latitude enables the navigator to correct his longitude by dead reckoning, and also to work out his sights for the chronometer; but when a ship has been a day or two without obtaining an observation to determine her latitude, there may be a considerable error in the latitude by account. Now if sights be obtained for determining the longitude by chronometer, and the mean time for these sights be obtained by applying to the calculation a latitude which is not the latitude of the place where the observations were made, it is evident that the longitude obtained by such a calculation must be wrong.

H.M. Ship Challenger was wrecked on the coast of South America in consequence of placing too much confidence in calculations of the above description: she had not obtained an observation for her latitude for two days, her latitude by account was erroneous to the amount of thirty-four miles, and this latitude being used for working sights obtained for her chronometers, the computation gave a longitude one degree to the westward of the ship's place. (Vide sentence of Court Martial appointed to enquire into the causes of the loss of the Challenger.)

The makers as well as the managers of mariners' compasses should thoroughly understand the elementary principles of magnetism. The compass like chronometers, and other useful machines, should be submitted to some test of its efficiency, after it is made, and before it is offered for sale, or brought into use at sea. The compass although less costly than even a common watch, is infinitely more useful than the best chronometer. A compass is not subjected to any trial or test of its accuracy of manufacture, its magnetic intensity, or the amount of its friction on the pivot upon which the card traverses, and the amount of its directive power, compared with the weight of the needle and its card. Generally, we may remark, that neither the vendor nor purchaser of a compass knows much about these matters; the former being satisfied if he realise a good profit; and the latter being pleased if he purchase a handsome article.

The essential qualities of an efficient compass, are, great directive power combined with little weight or friction on the pivot, the compass bowl being freely slung in jimbals attached to the box, so as to preserve an horizontal position under all cases of the ship's rolling or pitching. The steel of the needle should be of pure metal of uniform hardness throughout, and magnetised to saturation, and the magnetic intensity of a compsss needle should be preserved by all possible care. We have said that a compass card should be submitted to some test. Now a very fair and efficient test of the magnetic energy of a compass needle is to try if two similar cards will by their mutual magnetism support each other's weight; that is to say, if the north point of the needle of one card, be applied to the south point of a similar one, and their mutual attraction be such as to support the weight of the card, the magnetic intensity of such needle may be regarded as sufficiently strong if they mutually support each other's weight, along with the cards they respectively carry. Now the magnetic intensity, or power of every compassneedle, should remain permanently a constant quantity, and this can

only be accomplished by each ship being supplied with a pair of artificial magnets of sufficient power to renovate the magnetism of the compass-needles whenever the test might indicate that they required re-touching. The magnetism of a single needle is probably best preserved by allowing it to traverse freely on its pivot, or else to be stowed in a direction parallel to that of the magnetic dip, for if it be placed, say, with its south pole (or north point) towards the south pole of the world, or vice versa its magnetic intensity will decrease; or, if a compass be placed near to a large mass of iron, in such a manner that the magnetic polarity of the iron may act so as to control the magnetism of the compass, to a certain extent, by acting in a contrary way to that of terrestrial magnetism, then will the magnetism of the compass-needle deteriobe rated. If, for example, the south point of a compass-needles were placed near to the upper end of a vertical iron pillar, the needle would be deprived of a portion of its magnetism; but if the north point of the card be placed near, or in contact with the upper end of the pillar (in north dip) the magnetic energy of the compass-needle would be augmented.

We have mentioned that the magnetic poles of the same name, or kind, in any two similar and equal artificial magnets, repel each other. There is a constant effort exerted between them to obtain the mastery, and so mutually destroy their magnetism; and if two such magnets be so situated for a considerable time, their magnetism will be nearly destroyed.

There is at present a common steering compass, made by "John Syeds," in June 1810, in the binnacle of the Plymouth Breakwater light-vessel; it is the only compass in the vessel, and has been twentyeight years in her. No care has been taken of this card, which has remained in its box for so many years, and yet it retains a considerable amount of magnetism! If this card, instead of having one needle only, had been fitted with two or more, equal and similar parallel needles, we venture to assert from the principles we have explained, that the magnetic force of these needles would, in a comparatively short period of time have been reduced to the natural standard of the earth's magnetism. The spare compass cards, carried to sea in ships, should be stowed in boxes, and their opposite poles connected by pieces of soft iron, in the judicious manner recommended by Professor Barlow, and practised in the Royal Navy.

We have already shewn by experiment, how the changeable polarity of the inductive magnetism, in the metals within a ship, either draws the compass-needle quietly aside from its true magnetic bearing, in smoth water, or else causes the compass card to maintain a constant oscillation of a point or two, on each side of the course, when a ship rolls heavily from side to side in stormy weather. These troublesome oscillations give rise to the most serious obstacles to good steerage! There are but very few helmsmen to be found who can steer a ship in stormy weather, where the compass card is swinging about with every roll or lurch of the ship! And when

"High o'er the poop the audacious seas aspire,

Uproll'd in hills of fluctuating fire;

With labouring throes she rolls on either side,
And dips her gunnels in the yawning tide;
Her joints unhinged in palsied langour's play,
As ice flakes part beneath the noontide ray:

The gale howls doleful through the blocks and shrouds,
And big rain pours a deluge from the clouds;
From wintry magazines that sweep the sky
Descending globes of hail impetuous fly,

High on the masts with pale and livid rays

Amid the gloom portentous meteors blaze."-Falconer's Shipwreck.

It is under such circumstances as are described by Falconer, that we are taught to appreciate the worth of a good helmsman, and the value of an efficient compass! One card is exchanged for another, and weak needles are loaded with heavy weights in order to lessen the oscillations, but neither brass bars, nor brass rings, wax, paper, nor talc can cure the evil; for as we increase the weight and friction of the card, we only make it the more sluggish and unfit for the helmsman's use, who instead of being guided by the compass in the binnacle, must ever and anon, keep looking ahead at the clouds, the waves, or the stars, for he finds that a sluggish compass does not indicate a change in the direction of the ship's course, until some time after that change has taken place.

The brass box compass having been found to be more steady than wooden boxed compasses, the latter have been almost entirely laid aside without sufficient reason; for it is admitted that a compass-needle mounted in a wooden box, is more sensitive than one in a brass box, when all other things are equal. Let us enquire how this happens.* The comparative magnetic inductive susceptibility of the metals is very considerable, that of copper to mahogany as 29: 0,37, or as 78:1 nearly: now all magnets have a power of communicating a certain portion of magnetism to substances brought within their spheres of action; thus, a magnetic needle, enclosed in a mahogany box, would communicate a magnetism to the box; the north pole of the needle imparting a south polarity to the wood near it, and the south pole giving out a north polarity to that part of the wooden box opposite to the south pole of the magnetic needle. There would therefore result a certain amount of attraction between the ends of the magnetic needle, and those parts of the wooden bowl nearest to the needle; and as magnetism is taken up in less time than it is parted with, the induced magnetism of the box would tend to retard any oscillation of the needle. The magnetism of mahogany is very small indeed, and can only be detected by such delicate and elegant instruments as were used by Mr. Snow Harris, but the magnetic inductive susceptibility of copper or brass being about 80 times greater than that of wood, its effects become sensible and apparent. We see then that a copper or brass mounted compass, is more steady in a gale of wind, because its box is inductively magnetised from its enclosed magnetic needle, and therefore although it be really more steady in its vibrations, it is also more sluggish in its motion than it would be if mounted in wood instead of metal.

Compasses mounted in metal may also be more steady in stormy weather, by the screening influence of the metals surrounding the card; take for example a gun, on each side of the binnacle, where the polarity The Chinese avoid the use of Metal in the mounting of their compasses

changes at every roll of the ship; then the changeable magnetism given out by the guns may be in some measure screened by the surrounding box and rings of the compass. It is therefore prudent and proper that ships should be supplied with brass or copper boxed compasses for use in bad weather, but they should have at least one compass mounted in wood for use in light winds and smooth water, especially when the effect of local magnetism has not been corrected, or magnetic oscillations cut off by artificial means.

Her Majesty's government has determined on obtaining a set of superior steering compasses for the Royal Navy, to supercede the imperfect instruments formerly supplied by contract, and still in use in some ships. The "Compass Department" is placed under the superintendence of an intelligent and efficient naval officer, whose duty it will be to determine the local magnetism of ships on the Home Station; to supervise magnetic experiments, and recommend every known means for the improve ment or preservation of the compass. Every ship will have a Standard Compass of a superior description, fixed in some convenient part of the ship, and raised above the ordinary level of the binnacle, in order that bearings, amplitudes, or azimuths may be the more conveniently taken by it. The compass courses of the binnacle will be referable to the Standard Compass, and corrected accordingly, the local attraction of the ship upon each point being previously found on the Standard Compass, as the ship swings round the horizon.

These arrangements will prove highly advantageous and economical to the naval department, by lessening the number of casualties; for the money value of even a single steam vessel, or large frigate, would defray the expense of keeping up a set of compasses for the Royal navy for ever. The steering apparatus instead of being, as heretofore, consigned to the care of the boatswain, and stowed away in his store-room, with iron hooks and thimbles, chain cable gear, &c., and adjacent to the carpenter's and gunner's store rooms, crammed with all kinds of metals, will in future be placed in the master's charge, who being entrusted with the navigation of the ship, is of course the proper officer to have the care of the Mariners' Compass, the most important of all machines.

When compasses were served in by contract, the contractor's aim was the making of money! he made his needles, not of pure hard steel, but of soft iron pointed with steel! such needles were easiest made and easiest magnétised, and they required more frequent repair and cleaning, Being stowed away without care or attention, these needles soon lost their magnetic energy, and were returned from ships to the dockyards rusty and unserviceable, they were sent back to the maker for repair, for which there was a price, and also a price for re-touching weak needles, the consequence of all this was, that the expense of the compass department (imperfect as it was) was greater than it will be in

future.

The generality of sea-faring men are not so well informed about magnetism as they should be: how can they? since philosophers differ in opinion about their respective theories! We have touched but lightly on these theories, as our object has been rather to teach the navigator a few of the fundamental principles of terrestial and inductive magnetism, upon which the ractical utility of the mariners' compass

depends. These principles should form a part of the navigator's education; they are essential to the practice of his art.

(To be continued.)

ON MARINE SURVEYING.-By Lieut. Ryder, R.N.

A LATE perusal of a very interesting work, "William's Geodesey," has induced me to send you the solution of two problems in Practical Marine Surveying, which, I think, may prove useful to those of your readers, who take an interest in that branch of a naval officer's profession, so essential to the safety of our fleets. Both problems are, I believe, in print; but I never saw any solutions that did not involve long mathematical calculations, which, although 'rigidly correct, are unavailable in an open boat, with nothing at command but compass and ruler.

If, by any unfortunate accident (such as the fall of a beacon, &c.) part of a principal trigonometrical connection depended on the accurate position being known of spots of observation, circumstanced as in the following cases, true bearings and calculations would, of course, be imperatively necessary.

But the following solutions (by geometrical construction) are supposed to be useful, merely for fixing little coast features, &c., of no very essential importance.

Besides, the fact that the practical men to whom I have shewn them have agreed with me, as to their probable utility, I have yet another reason for presuming to occupy in your pages a space, that might have been filled with more valuable matter. Many of my professional acquaintance have in their possession valuable "wrinkles," connected either with surveying, astronomy, seamanship, or gunnery, &c., which, unless their possessors can be induced, by force of example or otherwise, to "haul out," for the benefit of the rising generation, will perish with the books that contain them, instead of proving useful to those who are interested in the subjects they refer to.

If, Mr. Editor, you approve of my motives for writing, assist them I pray you, with remarks, advice, &c., calculated, I have no doubt, to have more effect in opening the pages of those hitherto almost useless receptacles of hints, remarks, &c., y'clept "wrinkle books," than any thing likely to have that proceeds from my pen.

PROBLEM I.

There being only two known points in sight, all others having been unavoidably lost sight of, to find your position. If on shore one observer will be sufficient; if at sea two are necessary, and therefore two boats, unless floating beacons are available. (A boat-hook and the anchor will do.)

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