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Volume 6, 1873
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Art. II.—On Observed Irregularities in the Action of the Compass in Iron Steam Vessels.

[Read before the Otago Institute, 11th November, 1873.]

It would be extremely difficult to say how many or how few of the casualties which have occurred on the New Zealand coast to iron-built steamers, which have been engaged in its navigation, may be attributed to compass errors. I have little doubt, however, that a great proportion of them are attributable to that cause, and I have less doubt that, if so, the cause is preventable. How? let us consider. As I intend this paper to be of an entirely practical character, I shall not introduce into it any of the investigations of the subject which have been made by Professor Airy and other eminent men, with a view to discover such general laws of the magnetic disturbance in iron ships as enable us to correct the local attraction. I will simply enumerate a few observations of the action of compasses on board two of our coasting steamers, taken by myself during the present year, and offer a few remarks with a view to the removal of what I consider to be a source of great danger to life and property, viz., the navigating of our iron-built screw steamers by means of tabulated cards of deviations.

On the deck of one of the vessels on board of which I took notes, two binnacles were placed, in each of which was suspended an apparently well-made compass; one was placed immediately in front of the wheel, the other about thirty feet forward. Upon no course steered during the voyage did the compasses indicate alike; there were continual, though not constant, differences, varying with every change in the direction of the ship's head. The least difference which I observed amounted to about 12 degrees—a little over one point: this occurred when the vessel's head was N.N.E.; the greatest, about 27°, or nearly 2-½ points, when the direction steered was W.N.W. These notes were taken when making the northward voyage. The southward voyage I made in another vessel; the compasses on board were placed somewhat similarly to those in the first ship, the distance between them being, however, not quite so great. The difference between the indications of the compasses were much greater than in the former case. I noted them carefully, as follows:—

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1st. Stern compass, W. ¾ S. Difference, 78° 45′, or 7 points.
Forward compass, S.S.W. ¼ S
2nd. Stern compass, W. Difference, 67° 30′, or 6 points.
Forward compass, S.S.W.
3rd. Stern compass, W.N.W. Difference, 61° 52-½′, or 5-½ points.
Forward compass, S.W. ½ W.
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4th. Stern compass, W. by N. ¼ N. Difference, 73° 07-½′, or 6-½ points.
Forward compass, S. W. by S. ½ S.
5th. Stern compass, W.N.W. Difference, 78° 45′, or 7 points.
Forward compass, S. W. by S.

The last-mentioned difference was noted during a dense fog, and differs 1-½ points from No. 3, taken when the stern compass indicated the same course, viz., W.N.W. The observations were taken when the ship was on an even keel. The use of a card of deviations in this case is dangerous in the extreme, as the officer on watch not only has to apply the correction to the forward, or standard compass, but, in giving the course to the man at the wheel, has to apply a second correction, both corrections being different on each course: on some courses +, on others —. The liability to err in applying these corrections is therefore great. It is not to be wondered at if the captain, after having, at a late hour of the evening, given a course in order to clear a certain headland, should at daybreak find his ship three or four miles nearer the land than he reckoned upon; or that, having at night given a course which he expected would keep his vessel running parallel to the land, he should in the morning find himself out of sight of land altogether. After considerable experience in the adjustment of the compasses of iron vessels, I have no hesitation in saying that I believe that the local attraction in vessels navigating in these latitudes can be easily and effectually neutralized, and that, by this being done, the safety of passengers and property would be secured, and the officers in charge relieved from a mental strain to which they ought not to be subjected. Let us see what are the inferences drawn by Professor Airy from the elaborate investigations made by that eminent philosopher on this subject. He says:—

1st. At any place the deviation of the compass in any ship, whether wood-built or iron-built, may be accurately represented as the effect of the combination of two forces, one of which alone would produce a disturbance, following the law of polar-magnet-deviation; and the other alone would produce a disturbance, following the law of quadrantal deviation. In northern magnetic latitudes, the nature of the effect of the first will usually be the same as if the boreal magnetism were towards the ship's head; in southern magnetic latitudes, it will be usually the same as if the austral magnetism were towards the ship's head—the quadrantal deviation will be the same in all magnetic latitudes—and whatever the magnitude of the earth's directive force. These are the disturbances that are produced by transient induced magnetism only. The polar magnet deviation will, however, be affected in a greater or less degree if the iron which enters into the composition of a ship possess independent polar magnetism, similar to that of a magnetized steel bar, i.e., not depending on the terrestrial magnetism at the

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present moment for its existence; and not changing its amount, or quality, or direction in regard to the ship's keel, while the ship is swung round in different positions. From the slowness of its changes, this has been designated by Professor Airy, “sub-permanent magnetism.”

I will now proceed experimentally to show the difference between what is termed transient induced magnetism and the sub-permanent magnetism above mentioned. I take this piece of bar iron, and place it in the line of the magnetic meridian; parallel to the bar, near its north end, I place this small compass; the north end of the compass needle is repelled. I now place the compass similarly near the south end of the bar; the south end of the compass needle is now repelled. Reverse the bar end for end; the result is the same— the end which formerly, when pointing north, repelled the north end of the compass needle, now attracts it and repels the south end, showing that, although the bar has reversed its position, the magnetic current maintains the same direction. This may be termed “horizontal induced magnetism.”

I now hold the bar in the line of the magnetic dip, or nearly vertical; the upper end attracts the south end of the compass needle, the lower end of the bar attracts the north end of the compass needle. Reverse the bar by turning its lower end up, and the results are the same, shewing, as before, that the magnetic current maintains the same direction. This is termed “vertical induced magnetism.”

In the northern hemisphere the upper end of the bar would attract the north end of the compass needle, and the lower the south end—just contrary to the results obtained in this hemisphere. From this cause the north end of a ship's compass needle is drawn to windward when a ship heels over in the northern hemisphere, and the south end is drawn to windward in the southern hemisphere under similar circumstances. When sailing due east or west it is evident that no deviation will be caused by the vertical induced magnetism of the vessel.

I now take this small bar of steel, which has (probably in the process of rolling) acquired a certain amount of magnetism. If I hold the compass to the one end of the bar, the north end of the needle is attracted, no matter in what position the bar may be placed. This represents what is termed the sub-permanent magnetism of the ship. At any place the deviation of the compass may be accurately corrected by mechanical methods; namely, by a magnet in the athwart ship direction, fixed, at a distance determined by trial, for correcting the deviation when the ship's head is north or south; by a magnet in the head and stern direction, also at a distance determined by trial, for correcting the deviation when the ship's head is east or west; and by a mass of unmagnetized iron (a small box of chain is best) at the same level as the compass in the athwart-ship line, or in the head and stern line, according to circumstances,

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(usually in the former), also at a distance determined by trial, for correcting the deviation when the ship's head is N.E., S.E., S.W., or N.W. For the same ship the mass of unmagnetized iron, if adjusted at one port, will produce its due effect in all parts of the world, without ever requiring change or adjustment. The quadrantal deviation may thus be accurately corrected, leaving only the polar-magnet deviation uncorrected. The elements of polar-magnet deviation are liable to changes, but of very different amounts in different ships.

“It is therefore,” says Professor Airy, “imperatively the duty of every captain of a ship, particularly of an iron-built ship, to examine the state of the compasses at every opportunity. For the correctness of the compasses may be vitiated, not only by changes in the polar-magnetism of the ship, but also by changes in the intensity of the magnets used for the correction. But as the quadrantal deviation is not liable to any doubt whatever, it is sufficient, for ascertaining the existence and recording the amount of error of the polar-magnet deviation, to observe the error when the ship's head is N. or S., and when it is E. or W.”

From whatever cause the changes in the elements of polar-magnet-deviation may arise (whether from a real change in the sub-permanent magnetism of the ship, or from the variation of that part of induced magnetism which is similar to polar magnetism, but which, changes in different magnetic latitudes) they may be precisely corrected by re-adjusting the position of the magnets, leaving the unmagnetized iron undisturbed. And the change (if there is any) in the intensity of the correcting magnets will also be corrected, as to its effect on the compass, by the same re-adjustment of position. The re-adjustment can always be effected in harbour in a very short time. Or it may probably be done at sea by reference to a compass carried high up the ship's mast. It can also be done with the aid of astronomical observations, and of a knowledge of the local “variation” or “declination.” In all cases the mere adjustment of the magnets is an extremely rapid process.

Professor Airy denounces as dangerous any system of navigating a ship by forming a table of compass deviations at the starting port, and using that table until means of correction can be obtained from observations; and expresses an opinion that it ought at once to be discontinued. It does not, in the smallest degree, provide against the effects of possible change in the ship's sub-permanent magnetism during the interval in which no observations are obtained (which, with sometimes a minute change in the power of the magnets, is the only risk to which the method of mechanical correction is liable), and as it does not recognise the effect of the variation in the magnitude of terrestrial-horizontal magnetism at different places (which alters the compass

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deviation by changing the proportion of the ship's sub-permanent magnetism to the terrestrial-horizontal magnetism, upon which proportion the compass deviation depends), it gratuitously introduces a class of errors which are entirely avoided by correcting the compasses by magnets and soft iron.

The changes which occur in the magnetism of a ship sailing from one hemisphere to another, say from the Clyde to Port Chalmers (the difference of magnetic latitude being somewhere about 100 degrees), cannot take place in vessels traversing ten or twenty degrees in one hemisphere only; in fact, I believe that if one of our intercolonial steamers had the compasses on board accurately adjusted in a New Zealand port, by means of magnets and soft iron, the deviation from such changes as those to which I have alluded would be almost, if not altogether, imperceptible in a voyage to or from Melbourne.

I have perused with great pleasure the paper by Captain Edwin in the last volume of the “Transactions” (Trans. N.Z. Inst., Vol. V., p. 128), and, although I agree to a very great extent with his remarks, I am inclined to think that the smallness of the magnetic disturbance in the “Luna” is not so much owing to that vessel having been built of steel, as to the fact of her having wooden bulwarks and the very careful manner in which the position of her compass has been selected; and I think it probable that the difference in deviation observed on board that vessel between Auckland and the Bluff might be altogether eliminated by means of soft iron alone.