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this force is determined through three magnetic elements, (1) the intensity, (2) the declination, or, as it is more generally called, the variation, and (3) the inclination or dip. But the three elements are separable though intimately connected with each other.
The magnitude of the magnetic force, called its intensity, is determined by the vibrating needle.
The variation, as shown by the horizontal needle, is the angle through which that needle declines from the geographical meridian, and is the outcome of the magnetic poles of the earth not being at the ends of its axis.
The dip, as shown by the dipping needle, is the angle through which that needle is inclined to the horizontal plane.
These elements having been determined for any given place, the effective action of terrestrial magnetism at that place is known and defined. But all are subject to disturbances or change, classed as secular, periodic, and irregular; none of these changes, however, affect the compass considered as an instrument of navigation, except in time.
With the variation of the compass the mariner is well acquainted, as it comes under his cognisance every day.
Bat the dip is not so apparent to him, and yet it plays an important part in connection with the changes in the compass which are due to the iron of the ship. The globe is divided into two magnetic hemispheres— northern and a southern one-by the magnetic equator, or line of no dip, along which the magnetised needle rests horizontally. This line cuts the terrestrial equator in two points (not equi-distant)-one being on the African side of the Atlantic-the other in the Pacific, in about long. 164° W. Receding from the magnetic equator—where the dip is 0° and consequently the horizontal force greatest—the red pole of the magnetised needle inclines downwards in the northern hemisphere, and the blue pole similarly inclines in the southern hemisphere; the inclination continues to increase more and more until the earth's magnetic poles are attained—where the dip is 90°, and the needle stands vertically; and where the vertical force is at its greatest, the horizontal force is nil. The lines of equal dip in low
magnetic latitudes are not very irregular, but as they approach the magnetic poles they become more and more ansymmetrical in consequence of these poles being neither diametrically opposite to each other, nor coincident with the geometrical poles of the magnetic equator.
The globe is also divided into two hemispheres by an undulating line, called the equator of force, which passes through all those places where the magnetic intensity is weakest; it coincides with neither the terrestrial, nor the magnetic, equator. The intensity everywhere increases in receding from this line, on either side, towards the higher latitudes. This is also an element to be considered in connection with the compass in an iron ship.
The iron of which a ship is constructed is neither wholly hard, nor wholly soft, but of an intermediate character ; and the more nearly, it approximates to hard the more capable is it of the retention of magnetism when it has been submitted to any process of magnetisation. This is accomplished while the ship is on the stocks, building; when magnetism more or less intense is developed by the bending, twisting, and hammering to which the iron is subjected; and so far as the hull is concerned it is of that kind which is now usually called sub-permanent, because it has not the precise persistency of a steel bar-magnet. The magnetic direction in which the hull lies while on the stocks determines the quality or character of the impressed sub-permanent magnetism, and the amount of hammering and straining of the iron while in that particular direction determines its quantity,
Previous remarks have already shown that position plays an important part in connection with magnetic action, and since the dip of the needle varies between 671° and 72° in the latitudes where our building yards are situated, it follows from the direction of the ship on the stocks conjointly with the hammering of the iron, that red magnetism (i.e., of the North-seeking kind) is developed in that part of the ship which is below and towards the North, and blue magnetism in that part which is above and towards the South.
Fig. 5 represents the magnetic character of a ship built head North; the nearly vertically line halving the length of the ship is
the line of the dip, and the irregular dotted line may be taken as an
equatorial plane at right angles to the line of dip and passing through the body of the ship. This equatorial plane separates the red (R) from the blue (B) magnetism, the red being below and towards the North, and the blue being above and towards the South ; the blue magnetism is consequently well developed towards the after part of the ship. When the compass is aft this distribution of magnetism causes the North end of the needle to be strongly attracted towards the stern, but less so as the compass is carried forward, until, on approaching the bow, the North end of the needle is repelled, and the South end is attracted.
Fig. 6 represents the magnetic character of a ship built head Sonth, and is the reverse of Fig. 5; here blue magnetism is
developed towards the bow, and red towards the stern, causing repulsion of the North end of the needle in a compass placed aft, and attraction towards the bow when placed forward.
Fig. 7 represents the magnetic character of a ship built head East, in which the whole of upper part of the ship has blue magnetism, and the whole of the lower part red; but owing to the
B greater development of the blue on the starboard-side, the North end of the needle is attracted to that side, and this occurs on every part of the deck from forward
Fio. 7. to aft.
SHIP Built HEAD East. Fig. 8 represents the magnetic character of a ship built head
West. It is the reverse of Fig. 7 in this, that owing to the greater develop
ment of blue magnetism being now on R
the port side, the North end of the needle is attracted to that side, and
such is the effect on every part of the Fig. 8. SHIP BUILT Head West. deck from forward to aft.
This disposition of magnetism and its results as here described, appertain to the Northern hemisphere.
The equatorial plane rising to the deck—forward in ships built head North, and aft in ships built head South-might lead to the supposition that if a compass were placed in one or the other position the needle would not be affected by the magnetism of the ship; it is well to place a compass aft in ships built head to South, and as far forward as possible in ships built head North ; but the line or space along which the magnetism of the ship has no influence on the needle is not easily discovered, and if it were ascertained for any given latitude it would be found to have changed on reaching another magnetic latitude.
An iron ship built in the Southern hemisphere would be affected somewhat differently from what has been stated above, to the extent of blue magnetism being below, and red above.
To what end is it necessary to study the distribution of magnetism in an iron ship ?–To ascertain and understand its effect on the compass.
You know that variation is the amount by which, at any given ice on the globe, the compass needle points East or West of the
true North, and that it does this owing to the magnetic poles not being co-incident with the geographical or true poles of the earth; bat deviation is the amount by which the compass needle deviates from the magnetic North; and it differs from variation in this that it is not of the same amount or name for every point of the compass. If an iron ship be considered as a magnet which acts on the needle as the earth does, then the needle will be acted upon by the combined forces of the earth and the ship, and will assume a direction depending upon the mechanical resultant of those forces. Thus, take the case of a ship's magnetism acting in such manner that the needle is attracted towards the bow ; so long as the ship's head is directed to magnetic North, the action of the ship and the earth being in the same azimuth, the needle will not deviate, but on turning the ship's head East, the needle will be drawn by the earth's force towards the North, and by the ship's force towards the East ; hence it must assume a position between North and East depending upon the relative amount of the two forces. This is what is called the deviation of the compass, and it changes its amount and name with change in the azimuth of the ship's head.
After launching, much of the sub-permanent magnetism that the ship acquired during building is shaken out of her, and within two or three years it obtains a certain stability ; but it is a stability in which, though the quality of the magnetism does not alter, it is liable to change in quantity, according to the magnetic character of that part of the globe in which the ship is navigated.
Intimately connected with every iron ship—and more especially with a steamer-are iron balkheads, beams, stringers, and longitudinal strengthenings ; besides which, there are deck-houses wholly or partly of iron, the engine, funnel, cowls, stanchions everywhere, cranes, davits, &c.; of this iron, all that is in a more or less vertical position, and subject to vertical induction, affects the compass as would a magnet,--acts with or against the subpermanent magnetism of the hull of the ship, increasing or diminishing its effect; were this the only result, the problem, although there must be change, and change with a difference, would not present any serious difficulty in the investigation ; but, on the other hand, it becomes more complicated when the effect on