Popular Science Monthly/Volume 34/January 1889/The Guiding-Needle on an Iron Ship
By Lieutenant-Commander T. A. LYONS, U. S. Navy.
THERE is an agency that pervades the earth and is peculiarly resident in all its iron. It is magnetism. This force is akin to electricity, though not identical with it, and the manifestations of both are often similar.
The small steel wire, scarcely larger than a sewing-needle, which constitutes the mariner's compass — every iron vessel, even the huge steamship City of New York, and the earth itself — all have certain properties in common that warrant classing them as magnets; and, as the ship sails the earth and is guided by the compass, there is a very intimate though varying relationship between these three that should deeply interest those who traverse the ocean. To describe this relationship, its contentions, and the constant struggle of each member for mastery, rather than their amicable companionship, is the object of this article; and it will render our ideas of the subject clear if we begin by stating the properties of the ordinary bar-magnet. The needle, the ship, and the earth are but magnets of different size.
The Steel Bar-Magnet. — Fig. 1 represents a steel bar which has been magnetized. Its centers of power are located close to each extremity, while near the middle is a neutral ground over which the influence of neither end predominates. If fine iron filings be sprinkled around the magnet, they will form into curved lines emanating from each center, and eventually trending toward a union.
These centers are called poles. The magnetism in one is opposite in kind and equal in degree to that in the other; there is a mutual attraction between these opposite magnetisms, and this tendency to rush across the neutral ground, and, by combining, yield up every distinctive feature of the magnet, is successfully opposed by the hardness of the steel bar.
The lines are called lines of magnetic force, and the area over which their influence is felt is known as the magnetic field. If a compass-needle, suspended by a silk thread and free to move in any plane, be brought into this field, it will assume a direction
Fig. 1. — The Steel Bar-Magnet. N and S, poles; n n, neutral ground.
parallel to the lines of force, as at e, e', e" . . . evi. The strength of the field, and hence the force that tends to give the needle steadiness and direction, varies greatly at different points — at e it is powerful, at e"' feeble.
If two magnets similar to that of Fig. 1 be brought into proximity, so that poles of the same name touch, they will repel each other; if, on the other hand, the north pole of one be approached to the south pole of the other, both bars, as if instinct with life, will fly into contact and cling one unto the other with the tenacity of a hundred arms. And this intense affinity of opposite magnetisms is a general characteristic.
The Earth a Magnet. — Now, to show that the earth has magnetic features entirely analogous to those of the bar-magnet, we will examine Fig. 2. Here the parallels of latitude and meridians of longitude appear as regular curves. But from a focus at N radiate a series of curves which take sinuous forms and finally
Fig. 2. — The Earth a Magnet. P P, geographical poles; N S, magnetic poles; m m, magnetic equator; r v and a b, lines of equal magnetic dip; x z and o y, lines of equal magnetic variation.
converge toward another focus at the antipodes. These foci are the magnetic poles of the earth toward which the compass-needle ever points, not indeed directly, but parallel to the lines of force. These poles are not coincident with the geographical poles, but, on the contrary, are far removed from them.
There are other, but minor, magnetic foci on the earth, just as there are secondary poles in a bar-magnet, but so overtopped in prominence by the two grand foci that they scarcely deserve mention.
The lines issuing from one pole to meet again in the other are called lines of equal variation; that is, a compass carried along one of them from north to south would always point at the same angle from the geographical meridian, and this angle may vary from 1° to nearly 90° in different parts of the globe.
Thus, then, the variation may be defined as the angle between the geographical meridian and the direction of the compass-needle.
Another set of interesting magnetic lines are those of equal dip. They gird the earth in circles concentric with the magnetic poles, just as the parallels of latitude do the geographical poles. There is a magnetic equator along every point of which the compass-needle is horizontal. As we travel from the magnetic equator toward the north magnetic pole, the needle begins to incline, the north end tending downward until, when we reach the vicinity of the pole, the needle becomes vertical.
If we travel toward the south magnetic pole, the same occurs with the south end of the needle, now tending downward. An entirely similar experience will result from carrying a small needle through the magnetic field of the steel bar. At the neutral ground it will be parallel to the bar, while, as we approach either end, the dip toward the pole becomes more and more until it stands vertical at the pole. And as it was stated, regarding the steel bar, that the intensity of its magnetic field varies from point to point, so with the earth, it also has a magnetic field which is powerful near its poles and steadily moderates in strength as we approach the magnetic equator.
A third set of lines are those of equal intensity. They are not drawn in Fig. 2. In general contour they follow those of equal dip, though, in point of fact, they are not identical with them.
All these different systems of magnetic lines—variation, dip, and intensity—have not on the earth that symmetry and regularity which they would present around a steel bar; on the contrary, they are often bent, looped, and turned into many a devious path—wherefore, none can tell. The fact alone is well established, while theories fail to account satisfactorily for the earth being an irregular magnet.
The observations that have determined the various magnetic features of the earth have been made with delicate instruments in stationary observatories in every country, and also on ships-of-war in every sea.
The magnetism of the earth is not fixed either in locality or amount; but the different systems of lines just described, and by which it has been found convenient to represent this magnetism, are ever varying—ever migratory. The hourly, daily, and other periodic changes are all small, it is true; but, however minute, they are the object of inquiry at every magnetic observatory, with the hope that in time, by the accumulation of data, a satisfactory theory of the earth's magnetism may be deduced.
As the part of the earth's magnetism which affects the compass-needle is the important factor of this article, since the needle, as an instrument of navigation, is specially treated, it will be necessary to dwell further on that element. It has already been stated that poles of opposite name attract and those of the same name repel each other. Now, on the earth, the pole nearest the geographical north is commonly known as the north magnetic pole, and the end of the needle pointing to it is also spoken of as the north pole, whence repulsion would, of necessity, seem to result; but this is an unfortunate use of terms that has grown up in daily life. The real state of the case is, that whichever of the two—the earth's pole, or that of the compass—we agree to designate as north, the other, having magnetism of the opposite kind, must be called south, and hence attraction naturally takes place. To show the variability of this attraction in direction and amount in various parts of the globe, we will examine Fig. 3.
Fig. 3.—Variability of the Earth's Magnetic Force. P, geographical pole; N, magnetic pole; m m, magnetic equator; Z, zenith; s t, K K, d f, etc., earth's lines of magnetic force.
Let us conceive the air filled with iron particles as it is with congealed vapor on a wintry night: they will not float about, listless and without form, but, like the frosty foliage on a window-pane, will seem projected from a parent stem, shooting up and out in graceful, wavy filaments. They are the earth's magnetic lines of force permeating space (how far, I do not presume to say), and coming to a focus at the poles; the mariner's compass is everywhere subject to their influence, and it is this influence that gives steadiness and direction to the needle.
At e, e', e", and e'" is a magnetic needle, represented as suspended at the middle by a thread from the zenith, and assuming, as it always will, a direction parallel to a line of force. At the magnetic equator (m m) this line is parallel to the horizon, and so is the needle e'"; we go north, and the line becomes bent, so the needle inclines as at e"; proceeding further, the line bends more and the needle inclines accordingly; finally, at e it is all but vertical in the vicinity of the pole. In all these cases the force or intensity of the magnetic field steadily increases from the first toward the last position of the needle, so that, if at e'" it be made to oscillate, the motion will be slow, extend over a wide sweep, and the needle will take some little time to come to rest; at e' the vibration will be quicker, the arc smaller, and the time less; while at e we will have but a few quick, jerky movements, and then a stop, as if checked by a powerful strain.
Now, a needle dipping thus at every remove from equatorial regions is of no value to guide a ship; it must always be horizontal, and this is practically obtained by placing a small sliding counterpoise on the needle to overcome the downward pull of magnetism; it is easily adjusted with every change. In this constantly horizontal direction of the needle, however, the portion of the magnetic intensity that gives it steadiness is materially changed—lessened and more diminished as we proceed from e'" to e.
Let the length A B at e'" and F B at each of the other points e", e', and e, represent the total force of the magnetic field at those places, then the portion of this intensity that is effective in horizontal planes will be represented by the length of the line A B, which is h, h,' h" at each point, and it is readily seen that these lengths are shorter and shorter. And the motion of the needle at h, h,' and h" successively will become slower, more sluggish and uncertain. Thus the seeming paradox is explained that, as we proceed from the magnetic equator toward its pole, the compass becomes less steady and reliable, while, at the same time, the total intensity of the magnetic field increases.
If a wooden ship with no metal other than copper in her frame were to sail round the globe, her compass, adjusted for dip, would experience only those magnetic phases that have already been described as peculiar to the earth—more or less steadying force and a variation of larger or smaller amount according to location; the ship herself would exert no influence whatever. But on board an iron or steel ship, with all her metal equipment and armament, the case is far otherwise; there, contention—unceasing strife—is ever active, as we shall see hereafter.
The Mariner's Compass.—To relate what is known or conjectured regarding the origin, history, and development of the compass would not be pertinent to this article, and, besides, such information is readily accessible in any encyclopædia; to impart in a general way a knowledge of its construction is more to the point, especially in view of the object of this paper, which is to treat of the behavior of the instrument in an iron ship; and this kind of knowledge is neither easily obtained nor generally free enough from technical terms to be readily intelligible to the non-professional reader.
Like almost every other instrument, the compass has representatives of many a type; to explain the mechanical and magnetical principles of construction, however, in their general application, it is necessary to have reference to some particular type, and for this purpose I shall select the one that in my opinion is the most trustworthy for steering a ship from her port of departure to her haven of destination—the Ritchie liquid compass. This is an American invention, Mr. E. S. Ritchie, of Boston, having many years ago taken out a patent for a liquid compass to be used at sea. From time to time it has been improved, until to-day, in the seven-and-a-half-inch compass supplied our navy, is probably realized the most accurate and complete instrument afloat. So generally has the excellent workmanship of the manufacturer been appreciated, that his compasses now guide the ships of many a nation in every sea. In the wheel-house of the latest large floating structure—the British steamship City of New York—will be found a Ritchie liquid compass.
The compass and its several parts are represented in Figs, 4 to 7, and the reference-letters in every instance pertain to the same parts: E is a copper bowl, with two short arms, D, D (one only being visible), which rest in the grooves of an outer ring,jB; this, in turn, has two short arms, C, C, which repose in the sockets of the binnacle, as the case for holding the compass on a ship is called. This method of suspension, termed gimbals, allows the bowl to swing freely in two planes, so that really it partakes but little of the rolling and pitching motion of the ship. A slender brass spindle known as the pivot (P, Figs. 4 and 7) is screwed into the bottom of the bowl, and on it rests the card. The bowl is filled with a liquid composed of nearly equal parts of pure alcohol and distilled water, and is hermetically sealed with a plane glass cover, which permits the card to be distinctly seen without distortion. The card (Fig. 6) consists of an outer rim (M), a central bulb (K), and four tubes, H, H, H, H—all made of very thin sheet-brass. The rim has double curvature—circular around the pivot, and semi-cylindrical from the inner to the outer edge, as shown in section in Fig. 5. The card is painted white, and has two systems of graduation traced upon its outer edge—degrees and points. The bulb is an air-tight ellipsoid, with a conical depression on its nether surface; in this depression is a small brass cap (T, Fig. 7),
|Figs. 4, 5.—United States Navy Compass.|
with four tiny set-screws (R, R)—only two are shown, however—which press and hold steady in place the jewel (Y)—a sapphire hollowed out and smoothed to the utmost degree, so that the highly polished pivot-point upon which it rests may encounter the least friction possible.
The little set-screws are for adjusting the jewel to the very center of the graduated rim. The tubes, which are two of one length and two of another, are about the diameter of a lead-pencil; their extremities are soldered to the under edge of the rim, and in addition the two inner ones are soldered to the bulb K; this gives the rim rigidity, as of itself it is both light and flexible.
In the tubes are placed the magnets—the vital part of the compass. These magnets are prepared with the most scrupulous
6, 7. — United States Navy Compass.
these wires large enough to enter the tubes are made up and tempered to the degree that experience has proved best for holding captive the magnetic charge. This is imparted to each bundle between the poles of a powerful electro-magnet. Of course, every one of the many slender wires that compose a bundle is itself a magnet; they lie together with their north poles in contact, and likewise their south poles: hence repulsion—a mutually deteriorating influence—is the result; and indeed oft-times a bundle of wires loses much of its magnetic strength because the steel is not of a quality and temper to resist the destructive force.
Each bundle is weighed and its magnetic strength tested, and, in placing them on a card, due care is had to the equal distribution of weight and force on each side of the center, for the characteristic of symmetry is ever kept in view. The magnets, except one, are rigidly set in the tubes, and the latter sealed; the movable magnet has screws at one end for the purpose of adjusting the magnetic axis of the whole system to that diameter of the card which passes through the north and south points: it would be the imaginary dotted line (Z . . . Z) shown in Fig. 6.
The card as above described, with tubes, magnets, bulb, rim, brass cap, and jewel all in place, weighs many ounces—a heavy weight for the delicate force of terrestrial magnetism to turn about on a pivot, how highly soever both this and the jewel may be polished.
The essential principle observed in the manufacture of the instrument is to reduce the friction on the pivot to a minimum and increase the moving power—the strength of the magnets—to a maximum; and this object is greatly furthered by the introduction of the liquid: its buoyant effect upon the card reduces the pressure of several ounces to that of a few grains.
The liquid has another advantage—it steadies the card, prevents all those small oscillatory movements that characterize a dry or air compass, while at the same time enabling the magnetic power to cope more efficiently with its burden. The liquid must fill the bowl completely, otherwise an air-bubble would gather and impede the free motion of the card.
The same compass may guide a ship into all climates—polar seas and tropical oceans; but as every change of temperature causes a varying expansibility of the copper bowl and the liquid in it, the former, when filled to complete fullness, would soon burst, were no provision made for expansion. To prevent this, an air-tight case of thin flexible metal is placed in the bottom of the bowl, which contracts or expands with every changing pressure. The alcohol in the liquid is to guard against its freezing in cold weather.
It has been stated that a single needle suspended by a thread would dip more and more as one proceeded from the equator toward the pole; and that in the dry compass this is prevented and the card always maintained horizontal by an adjustable counterpoise on the needle: no such contrivance is needed in the liquid compass; any downward pull of the earth's magnetism is at once met by such opposite pressure of the liquid on the rim of the card as to neutralize it. Magnetic attraction and liquid pressure counterbalance, and the card remains horizontal.
On the inside of the bowl is traced a fine black line—the lubber's-point, or, as it has of recent years been more appropriately designated in the navy, the keel-line (L, Fig. 4). It is this line toward which the point of the card indicating the ship's course is always directed. The binnacle which holds the compass is screwed down to the deck, so that the keel-line, as its name indicates, is in the vertical plane through the keel of the ship, or in a plane parallel to that one. This plane extends from bow to stern, and divides the ship into two equal and symmetrical parts.
Now, let an observer look at the compass-card and keel-line while the ship's bow swings through a portion of a circle: as each point passes the keel-line, it will seem that the card itself is moving, but this is an illusion; the card is still—ever pointing to the magnetic pole, in obedience to the attraction that there exists for the magnetism in the steel wires it carries. But it must not be understood that this attraction is of a nature to pull the card off its pivot: on the contrary, there is no tendency to motion of translation, but merely of direction—to turn the magnets on their pivot and place them parallel to the earth's lines of magnetic force.
To illustrate this, let us examine Fig. 8: C, C, is a steel arrow free to move upon a pivot P; from the extremities of the arrow light threads t . . . t', extend and pass over revolving wheels at N and S; small, equal weights Q and Q' are attached to the ends of the threads. Under the strain communicated to the arrow by the weights, it will, of course, lie in the straight line joining the points N and S.
Now, with the fingers, turn the arrow into the position C, C, the threads will assume the positions t, t', and both weights will be equally raised. Release the arrow suddenly, both weights will descend, and alternately rise and fall as the arrow makes a series of short and constantly diminishing vibrations, as shown in the positions C', C' and C", C", until it finally comes to rest and all is still. Let us replace the arrow by a magnet, and the threads, wheels, and weights by that mysterious agency we call magnetism, and the oscillations of the magnet, when drawn out of parallelism to the lines of force, will be entirely similar. Such are the efforts of the compass to regain its normal direction when disturbed; and the test of a good compass is the sensitive quickness with which it will turn aside from the magnetic meridian when another magnet is brought near, and the celerity of its return thereto when the intruder is removed.
Fig. 8—Mechanical Illustration of the Magnetic Force that gives the Compass-Needle Direction. The dotted parallel lines represent the earth's magnetic lines of force.
The Iron Ship a Magnet.—It is a characteristic of every mass and particle of iron on the earth's surface to acquire in varied degree the terrestrial magnetism that surrounds it; and this agency enters naturally, without effort or force: it is gently induced in the material so congenial to it, by the mere fact of the material quietly lying in its midst—the magnetic field, which pervades all space. And the word iron is not here used in a specific sense, but as a general term to include wrought-iron, cast-iron, and steel, which are all susceptible to magnetism.
The steel rails that afford transit from seaboard to interior, the trestle-work upon which the elevated trains traverse the metropolis, the heavy castings in a foundry, the massive forgings in a machine-shop, even the little scraps upon a neglected heap, have one and all magnetic features that distinguish them from other metals, and point out the common kindred among themselves. And these features are entirely analogous to those of the steel magnet already described—two poles, one at each end of the mass, with a neutral belt between.
Let us conceive a metallically pure cylinder of wrought or cast iron that has not been hammered, and let us further conceive it entirely free from magnetism: hold it vertically, and instantly the upper end becomes a south, and the lower a north pole (in this latitude). Reverse it as quickly as we may, and the magnetism also reverses, so that the upper and lower ends are still as they were before—a south and a north pole respectively.
Hold it horizontally in the meridian, and the end toward the north becomes a north pole, while that toward the south becomes a south pole. Revolve it slowly or rapidly in azimuth, and the foci of magnetic polarity also move with the fidelity of a shadow, until, when the cylinder points east and west, all the side facing the north is pervaded by north magnetism, and all facing the south by south magnetism. Again: let us conceive the hull of a ship to be like our cylinder of metallically pure wrought-iron, and as susceptible of magnetic induction in its ever-changing courses as the cylinder is when turned round. Then, as the ship steers north (in this latitude), the bow will become the center of north polarity, and the stern that of south polarity. As she gradually changes course to the eastward, so will the north focus shift to the port bow, the south focus to the starboard quarter, and the neutral line dividing them, which while the ship headed north was athwartship, will now become a diagonal from starboard bow to port quarter. When the ship heads east, all the starboard side is pervaded with south polarity, the port with north, and the neutral line takes a general fore-and-aft direction. Continuing to change course to the southward, the poles and neutral line continue their motion in the opposite direction, until at south the conditions at north are repeated, but this time it is the stern that is a north pole, while the bow is a south pole. At west the conditions at east prevail, only that it is now the starboard side that has north polarity, and the port side south polarity. And this transitory induction in both the cylinder and the ideal ship is solely due to the mild effect of the earth's magnetic field in which they move.
Now, to consider it in connection with an actual ship. The hull of no vessel is metallically pure, nor has it acquired shape and stability without much hammering; moreover, it can not be made an abstraction from a magnetic state. By hammering in the process of construction, it has been made as permanent and well defined a magnet as the steel bar, with poles and neutral line as in the bar, but located according to the magnetic direction in which the ship lay on the stocks, in strict conformity to the places they occupied in the ideal vessel just described. Therefore, it is not as susceptible of the mild magnetic induction of the earth as the cylinder and ideal hull, although the straining while on a passage and the buffeting of the waves do assist the inducing tendency; besides, once that the induced magnetism becomes lodged, it does not move and shift with the freedom and facility that it did in the cylinder; and finally, as it already finds a tenacious occupant of the vessel in its permanent magnetism, hammered into it while building, it must adapt itself to the greater power, and thus it is the resultant of both we always find, and not the individuality of either.
Time is a chief element in the acquisition and efficacy of this induced magnetism; for the longer a ship steers on a given course, or lies in the same general direction, the greater will be the magnetic charge, and the more slowly will it move and shift with the changing courses of the vessel.
This induced magnetism has been dwelt upon at some length because of its prime importance to navigation.
The other magnetic qualities of a ship are comparatively stable, but this is treacherous and changeable to a degree that necessitates constant vigilance to prevent disaster. On the great fleet of trans-atlantic steamers it is more likely to lead into danger than on other routes: the ships steer a generally easterly course going to Europe, and a westerly one coming to New York; the magnetic influence on the outward trip is the opposite of that returning; the ships run at a high rate of speed, and the induction varies on different parts of the route, according to the intensity of the magnetic field passed over, the smoothness or roughness of the sea which affects the motion of the ship, and the warmth or coolness of the weather.
Instead of attributing the loss of vessels when approaching a coast to the magnetic effects of fogs and land, and other improbable influences upon the compass, it were much more reasonable to ascribe it to the changed conditions of her magnetism by induction during the passage, and which has not been discovered or kept account of by frequent azimuths previous to closing in with the land. Suddenly, a course the captain thought perfectly safe carries the ship upon a shoal or rock, and the fault is laid upon the compasses, whereas they but obeyed the magnetic influences that became altered, during a long passage, from what these influences were when the ship was last swung to determine the deviations of her compasses.
To illustrate the varied location of the poles and neutral line in an iron ship while building. Figs. 9 to 12 are drawn from actual cases. Imagine the ship cut in two by a vertical fore-and-aft plane, and both sections opened out from aft as if turned upon a hinge joining them at the bow; the outside of each half will then appear as on the paper. In Fig. 9, where the ship has been built head north, the whole upper after-body is pervaded by south polarity, while the lower forward portion has north polarity. In Fig. 10, where the ship was built head south, the whole upper forward body has south magnetism, and the lower after-body north magnetism—a condition of induction the opposite of Fig. 9. In Fig. 11, where the ship was built head northwest, we find the general magnetic features of a ship built head north, only that now the north magnetism predominates on the starboard side, and south magnetism on the port side. Finally, in Fig. 12, ship's head south-west, we have the general features of the ship's head south, but with the neutral line taking a more horizontal trend, and the south
Figs. 9, 10, 11, and 12.—The Varied Magnetic Features of Iron Ships, due to the Direction of their Heads while building.
polarity lessened on the starboard side and increased on the port side. And all the above is in close conformity to what theory requires.
The means taken for discovering the permanent magnetic character a ship has acquired in building, are a dock-survey, shown in Figs. 13 and 14. To simplify the explanation, let us suppose the ship and dock to lie parallel to the magnetic meridian. Stations numbered 1, 2, 3, etc., are established on the different steps of the dock, and the distance that each is from the line A, B and also from the ship's side is measured. A compass is taken successively to each station, and the direction in which its needle points is noted. Of course, if no disturbing mass were near, it would point to the north, at every station. But an iron ship is there: so at station 1 we find the north end of the needle repelled from the vessel; the same occurs at stations 3, 4, 16, and many others from bow to keel around the forward body of the ship. Now, only north magnetism can produce this kind of deflection: it varies in degree at each station, and where greatest there is its pole. Again: at stations 7, 8, 9, etc., we find the needle's north end attracted toward the ship; hence we have discovered the body of
Fig. 13.—Plan of a Dry-Dock with a Ship in it.
Fig. 14.—Vertical Section of Ship and Dock through the Line D D of Fig. 13.
south magnetism, for that alone can produce this phenomenon, and as with the other, so here, we locate its pole where the deflection is greatest. Finally, at stations 5, 6, etc., in an irregular path from bottom to rail we see that the needle points everywhere to the north: this is the neutral line, A sketch of each side of the ship is drawn on paper, and the degree of deflection at every station is plotted by means of the measurements from the line A, B and from the ship's side.
It has thus been shown that it is a huge magnet, the ship, that is guided around an enormous magnet, the earth, by a tiny magnet, the needle.
The near approach of one magnet to another always excites contention and confusion in the field they occupy, and eventually the old, old story is told—the strongest alone survives. In order that the powerful ship may not paralyze its little guide, great care is taken to find a suitable place for it; and on every voyage, ceaselessly and without fail, a variety of observations have to be made and corrections applied to the courses indicated by the compass, that this may fulfill the object of its being. To explain how this is done would involve a mass of mathematical formulæ and astronomical and magnetical information that would but tire the general reader, besides being out of keeping with the character of this article. Let it suffice to state the problem in popular phrase; to solve it would necessitate the use of other language.
An iron ship—frames, plating, decks, beams, stanchions, carlings, engines, smoke-pipes, yards, masts, shafts, armament in a ship-of-war, and numberless other parts—is not like the steel bar, a simple magnet, but a network of magnetic entanglement; yet, how complex soever this may be, for the purpose of investigation, to the end that proper means may be devised for coping with it, its influence may be considered as taking place in three co-ordinate axes, namely, fore-and-aft, athwart-ships, and vertically downward, with the compass-pivot as the origin. To facilitate this conception, let us contemplate Fig. 15, and let T represent a bar of iron of such quality that when held upright it becomes instantly magnetic through the induction of terrestrial magnetism, and as instantly has its polarity reversed upon turning it end for end; in other words, what, in investigations of this kind, is technically known as soft iron.
Let this bar, supposed to be anywhere in the interior structure of the ship, take the most general position possible, namely, inclined to the plane of the deck, and also to that passing vertically through the keel.
As already stated, reciprocal action occurs between the magnetism of the bar and that of the compass-needle; the upper end of the former (in this hemisphere) attracts the north end of the needle and repels its south end, while, at the same time, the lower end of the bar repels the north end of the needle and attracts its south end. The difference in distance, however, between the near ends of the bar and needle and their remote ends, enters to such extent that the influence of the remote only modifies, not equals, that of the near ends; the net result may be stated as one of action between the near ends only.
We have thus to deal with but one kind of the bar's polarity; represent its force by a line of definite length, S T for example. This force is resolvable into two others, the horizontal S H, and the vertical S Z; and the former is further divisible into S B, parallel to the midship line, and B H, transverse to it.
The magnetic power of the bar is thus resolved parallel to the
Fig. 15.—The Magnetic Forces of a Ship concentrated in Three Planes.
three co-ordinate axes. Almost all the structural iron of the ship—beams, knees, engines, boilers, etc.—is symmetrically arranged with reference to the vertical plane through the keel; so that for a piece T, on the starboard side, we should generally find another similarly disposed on the port side.
The problem is now simplified to pairs of parallel forces, each pair having its resultant parallel to one of the co-ordinate axes; and the effect of every magnetic particle, whether of permanent or transitory magnetism, may be reduced to this condition. We may now with facility transfer into each co-ordinate axis the sum total of all the forces parallel to it, and concentrate the whole upon the north point of the compass, whence the final result, that we have reduced the entire magnetic power of the ship to that of three imaginary magnets—one laid horizontally in the axis of X; the second, also horizontal, in the axis of Y; and the third, vertical, in the axis of Z.
The individual and combined effect of these three imaginary magnets is the object of investigation; but, before entering upon it, it will be necessary to remark that each is not simple, but complex, and that, recognizing this, we shall have to consider all the component parts, leaving to every real case to determine which of the components reduce to zero, and which are prime factors.
The iron of a ship is of varied quality, from the "hard," which when hammered acquires and keeps its magnetism, to the "soft," which has absolutely no retentive power. It occupies every conceivable direction—vertical, longitudinal, transverse, and inclined at diverse angles; but, however varied the latter, it may be represented in the first three directions by pieces of equivalent effect. Finally, it may be symmetrical or unsymmetrical. To cover all the conditions of the problem, we shall choose representatives of quality and direction, of symmetry and singularity, and let each assert its power in the common struggle.
Fig. 16 represents the arena of these forces; they are arrayed in lines of attack upon the compass.
P, Q, and R, represent hard iron, whose magnetism, the result of percussion, is of a permanent nature, like that of a steel bar; the hull itself of the ship is an example of this kind.
c,f, and h represent vertical soft iron; it becomes magnetic through the inductive agency of the earth's vertical force; c might represent the smoke-pipes; f, boat-davits; and k, stanchions on the deck below that on which the compass is located.
a, e, and g represent horizontal soft iron, the first and last, when in a longitudinal direction, and e, in a transverse direction; the power of this iron is derived from the inductive agency of the horizontal force of the earth; as examples of a may be cited the engines, boilers, and water-tanks; of e, a deck-beam cut amidships for a hatch or any other purpose; and of g (when below the compass), the shaft.
b, d, and h are substitutes for an isolated mass, like T, that hasno counterpart on the opposite side, and they proclaim T's
Fig. 16.—The Different Varieties of Iron in the Ship represented by Equivalent Rods.
horizontal and vertical induction, are T's characteristics.
In all cases of iron which becomes magnetic through the mild inductive influence of terrestrial magnetism, it should be remembered that this influence may be variously modified, if, indeed, not in some instances entirely superseded, by the inductive action of a powerful surrounding field of permanent magnetism in the hull itself.
According to the location of the bulk of each class of iron—the hard and the soft, the vertical, longitudinal, transverse, and unsymmetrical—its resultant or representative, which we may designate as a rod or a bar, will occupy a position relative to the compass, either forward or abaft, to starboard or to port; only one such position for each is shown in Fig. 16; there are, however, two possible positions for every rod, and four for some.
The problem has now been stated, so we will pursue it no further, as the vein of solution would introduce trigonometrical formulæ.
By swinging a ship at compass-buoys, or steaming in a circle on the open sea, the magnetic effect of the ship—that is, of the three imaginary magnets in the axis of X, Y, Z—is brought to bear at every point on the needle, causing it to deflect from the magnetic meridian by different angles at different points. These various deflections, being serially arranged, constitute what is known as "a table of deviations." Upon analyzing this, the numerical strength of each imaginary magnet is obtained, and further disintegration exposes to view their individual component parts. And thus it is that from effect we seek backward to an intelligent comprehension of the cause.
But as a ship sails the ocean she passes through ever-varying fields of terrestrial magnetism; also, her own magnetism is undergoing constant change, due to the wrenching and straining, the shock of waves, and the vibrations set up by firing her battery; from this mutability of cause naturally results a variety in the effect—the deviations. They are never the same.
Let a ship proceed to Havana, and she will find them different from the series determined at New York; at Hong-Kong they differ from those at Rio de Janeiro; in tropical seas they are moderate, in polar regions enormous; when a ship is upright, they have one value; when she heels, they have another. Their varying phases are a manifestation of the strife and successive domination of the three magnets whose intimate relationship has been pointed out; now it is the ship, as when she steers a certain course for many days and thus strengthens her forces; again, it is the earth, when the compass ventures into her frozen strongholds, where it but wavers sluggishly and totters about every course; and finally comes the little needle's turn — in the genial tropics, where it can point out steadily and safely the path to any port.
The necessity of frequent observations for determining corrections to be applied to the compass is, therefore, evident.
The series of total deviations is generally divided into two principal parts, the quadrantal and the semicircular: the first taking its name from the fact that it arises, reaches a maximum, and again reduces to zero, all within an angular space of 90°; and the second, for a similar reason, because of its origin, growth, and decline being confined to 180° of the circle.
Frequently, means are provided for opposing the magnetism of the ship by other powerful magnets, thus permitting the needle
Fig. 17. — The Compensating Binnacle.
to point in its natural direction, however the ship may head. Such a contrivance is known as a compensating binnacle, shown in Fig. 17. Before compensation, let the needle point in the direction N' S'. A portion of this deflection is the quadrantal deviation, due to the soft iron in the ship; it is overcome by placing two large cast-iron spheres, Q and Q', at suitable distances from the compass; the other portion of the deflection being due to the hard iron (that is, the semicircular deviation), a number of steel bar-magnets, M, are placed in a disk, which is turned to the requisite angle and then raised or lowered, until the needle returns to the magnetic meridian N S. The magnet H, to nullify the heeling deviation, is placed at a predetermined distance vertically below the compass-pivot.
In considerable changes of magnetic latitude the magnets have to be slightly moved to counterbalance the altered condition of the deviations, and sometimes, also, to correspond to a partial loss of power in the magnets themselves.