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its the magnetic charseta dhe
atitudes are not very integrar
e is also divided into tro benesten
the equator of fore, with
. This is as
connection with the aim is equatorial plane at right angles to the line of dip and passing through of which s ship is constatari the body of the ship. This equatorial plane separates the red (R) ut, but of an intermedia caz. from the blue (B) magnetism, the red being below and towards the proximates to band than North, and the blue being above and towards the South; the blue svagnetism when it has been ina magnetism is consequently well developed towards the after part Dow usnally called sub-ame su repelled, and the South end is attracted. red magnetism (ic., of the lide that part of the ship skich i love
the line of the dip, and the irregular dotted line may be taken as an
Fig. 6 represents the magnetic character of a ship built head
con. This is accomplis ling; when magnetin Due the bending, tristi, al benes ted; and so far as the best
ich the hall lies while on the doors icter of the imprese che
of bammering and standa lirection determines its quantity arks have already skorz te pare in connection with amazet nad
needle varies betree hig'd var building yards are ship on the stocks conjuntaments
developed towards the bow, and red towards the stern, causing
Vue magnetism in that part de
vertically line balming the las
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 greater development of the blue on the
R starboard-side, the North end of the needle is attracted to that side, and this occurs on every part of the deck from forward
Fig. 7. to aft.
SHIP BUILT HEAD EASE. Fig. 8 represents the magnetic character of a ship bailt 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 latitudo 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 mag netism in an iron ship ?-To ascertain and understand its effect on
You know that variation is the amount by which, at any given place on the globe, the compass needle points East or West of the
a 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; but 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 bulkheads, 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
the compass is the resultant of the action of large masses and numberless pieces of iron in different positions, and liable also to change of position.
The formula generally accepted for the solution of the effect of a ship's magnetism on the compass is that deduced by the late Archibald Smith from the earlier investigations of Poisson, Sir G. B. Airy, and others; it is as follows:
d= A + B sin x + C cos z + D sin 2 z + E cos 2 : in which d is the deviation to be determined ; z is the azimuth of the ship's head by the disturbed compass; and A, B, C, D and E are co-efficients (or temporary or local constants) the value of which has to be determined by experiment or trial.
The interpretation of the formula is this, that knowing the value of the co-efficients in arc, the deviation on any given point of the compass, say N.N.E. (which is two points), is as follows :A (the constant), plus B times the sine of two points, plus C
times the cosine of two points, plus D times the sine of
four (i.e., twice two) points ;
W. I. R. (To be continued.)
To the Editor of the "Nautical Magazine."
that is taking place outside the little floating world
where our knowledge of past events is gleaned from old newspapers, magazines, &c., for our spare hours at home are too few and precious to waste in studying current dows or politics.
During nearly thirty-five years active seafaring life I have but once voted for a member of Parliament, and there are doubtless thousands of seamen who, like myself, are perforce content to see others think and work for us, realising the probability that those "others" by living on shore, and making our welfare their special study, may acquire a more general knowledge of our requirements as a class than we ourselves who work in such a mole-like
We have an effectual leading light in your esteemed magazine, and I have to thank it for my knowledge of the many efforts that are being made to conduce to our moral and physical welfare and safety. The perasal at sea of many of the articles therein contained has caused me to envy, as well as admire, the facility with which the writers “coin their thoughts into words,” and place vividly before us ideas and facts which are in strict accord with our own knowledge of the truth.
But, Sir, whatever may have been your motive in publishing an article by Captain Dawson, R.N., on the Merchant Seamen Bill, 1878 (in your November number for that year), it is plain that the views expressed therein were not such as generally characterize the articles appearing in your pages. I can hardly think that the author has succeeded in recording his convictions, when he conjoins legislators and employers with crimps and publicans, as the natural foes of seamen, and charges with annual wholesale manslaughter, if not worse, the class in whose interest he is writing. There can be no danger of your or constant readers" being misled by the article in question, as to the character of employers or seamen, but there are many non-nautical men deeply interested in the Merchant Navy, who may be led to form very erroneous conclusions through Captain Dawson writing from insuficient data, or being unfortunate in his choice of language. His own quotations from shipowners' evidence afford a sufficient refatation (were it needed) of the charges direct and implied ugainst them.
Figares are misleading, and though it would perhaps be advigable that reports of deaths at sea should be referred to the health officer of the port (if the custom does not exist), and an inquest