Lectures in Navigation Part 25

ADDITIONAL LECTURE

COMPa.s.s ADJUSTMENT

The aim of this lecture is to give you a very few facts about magnetism in general and compa.s.s adjustment in particular. The reason for including the lecture in this book is because of repeated requests on the part of graduates who have been consulted about the adjustment of the compa.s.s on their s.h.i.+ps and who have realized that their advice might have been more helpful if they had learned more about the matter while at this school.

The earth is a huge magnet. It is the effect of the magnetism in the earth upon the compa.s.s needle which causes the compa.s.s error and makes it necessary to correct it. How can it be corrected? To know that we must first know the fundamental law of magnetism, namely, that opposite poles of two magnets attract each other and similar poles repel each other. From which it follows that if we decide to color red, for instance, that end of a magnetic needle which points to North, the magnetism of that part of the earth must be considered blue, i.e., of opposite magnetism to the north-seeking end of the red magnetic needle.

Now, there are various kinds of magnetism which affect a s.h.i.+p's compa.s.s.

One is from the earth, another from the iron in the s.h.i.+p, etc. To discuss them and, the theoretical cause of them in detail is beyond the scope of this lecture. To correct them, four sets of magnets are necessary, two of which are usually found in the binnacle of the compa.s.s itself. One is a fore and aft magnet or set of magnets, the other an athwarts.h.i.+p magnet or set of magnets. The third set consists of the two globes of cast iron placed on either side of the compa.s.s bowl (called Quadrantal Correctors). The fourth magnet, or set of magnets, is to correct the compa.s.s in case of severe heeling by the s.h.i.+p.

If you are ordered to adjust the compa.s.s the first thing to do is to choose a fine day with smooth water. Take your s.h.i.+p to a certain spot, the exact location of which you have found from the chart, and where you are certain you will have plenty of sea-way in which to swing. Set your watch to local apparent time (which you have calculated before coming out). Take from the Azimuth Tables the sun's true bearing for every four minutes of the time during which you will be occupied adjusting, and convert it into the magnetic bearing by applying the variation at the place selected (secured from the chart). Write down in a small book these times and corresponding magnetic bearings.

Now go to your compa.s.s and see that its lubber line is exactly fore and aft and in the keel line of the s.h.i.+p. Have another officer who is thoroughly familiar with the pelorus stand by it as the s.h.i.+p is swung.

All being ready, secure the lubber's point of the pelorus at North and clamp the sight vane to the sun's magnetic bearing at the time you have figured to take the first heading. Starboard or port your helm until at the time calculated the reflection of the sight vane on the pelorus dial cuts on the proper magnetic bearing. The vessel's head will then be pointing to magnetic North. If, now, the compa.s.s were correct it would agree with the pelorus in showing the s.h.i.+p's head to be North. If it does not do so, there is Deviation in the compa.s.s and its amount is the amount of Deviation on that particular course. Suppose the Deviation were to starboard, i.e., Easterly, and were due to magnetism in the s.h.i.+p's starboard side. Then, if the magnetism in the North end of the needle be considered red, the magnetism in the starboard side of the vessel, in order to attract the red end of the needle, would be considered blue and the s.h.i.+p's magnetism, with the compa.s.s needle included, would look like this:

[Ill.u.s.tration]

To counteract this blue attractive force on the starboard side, screw up the athwarts.h.i.+p magnet in the binnacle toward the compa.s.s dial. Its magnetism, if it were laid on the deck, would look like this:

[Ill.u.s.tration]

In other words, as this magnet is moved nearer the compa.s.s needle, by the law of magnetism just given, the red end of the magnet repels the red end of the compa.s.s needle from the starboard side and the blue end of the magnet attracts the red end of the compa.s.s needle toward the port side. When the compa.s.s needle points to North, as shown by the correct pelorus bearing, the Deviation on this heading (i.e., North) is corrected.

Now turn the lubber line of the pelorus to East. Steady the s.h.i.+p on this heading until the shadow from the pelorus vane at the proper L.A.T. cuts the circ.u.mference of the pelorus dial at the proper magnetic bearing.

The s.h.i.+p's compa.s.s should then show the s.h.i.+p's head pointing to East.

Suppose that it does not (as will usually be the case) but points to the right of East. Then the s.h.i.+p's magnetism and compa.s.s would look like this:

[Ill.u.s.tration]

To bring the compa.s.s needle back to North it would be necessary to move up nearer the compa.s.s dial the fore-and-aft magnet (shown below), whose magnetism would act on the compa.s.s needle on this heading of the s.h.i.+p exactly as the athwarts.h.i.+p magnet acted on the compa.s.s needle when the s.h.i.+p was headed North:

[Ill.u.s.tration]

Now the s.h.i.+p's compa.s.s has been corrected for the North and East headings respectively. The next correction is for the heading half way between, i.e., North-east. If there is any Deviation on this heading, adjust the cast iron cylinders (called Quadrantal Correctors), which are on each side of the compa.s.s bowl, by moving them toward or away from the compa.s.s until the s.h.i.+p's head by compa.s.s is North-east at the proper time and bearing by pelorus.

The s.h.i.+p's compa.s.s has now been corrected for one whole quadrant, namely, from North to East, and this will suffice for all four quadrants since the relations.h.i.+ps of the magnets themselves and the magnetism of the compa.s.s needle is the same for any of the other three quadrants as for the first. Compa.s.s adjustment, however, can never be absolutely accurate. For that reason, it is wise to steam the s.h.i.+p completely around, steadying on every fifteen degrees by pelorus to determine and keep a record of remaining errors.

There is one more correction to make, i.e., for the heeling error. This correction is necessary in case the s.h.i.+p is yawing in a sea-way so much that the relations.h.i.+p of the s.h.i.+p's magnetism to the compa.s.s needle is decidedly different from what it is when the s.h.i.+p is on a comparatively even keel. It is compensated by a vertical magnet directly underneath (or over) the binnacle, details in regard to which can be secured from Bowditch Art. 125, p. 53.

It must be borne in mind that compa.s.s adjustment is not an exact science, that an adjustment for one lat.i.tude is not correct for another, that anyone of a hundred different causes can affect the magnetism of the s.h.i.+p or of the compa.s.s needle, which in turn directly affects the Deviation. In this connection, it would be well to read Bowditch Art.

129, p. 55. You should also read Arts. 119-130 in which are given, more fully and in more scientific language, the contents of this lecture.