Of all the navigational instruments now in practical use, there are few, if any, that exceed the mariner’s compass in usefulness to mankind. The part it has played in the development of the world has been most important, and its utility is no less to-day than in the past, for the intercourse of nations is still guided by the compass needle. With such a responsibility depending on this instrument, it would naturally be supposed that its indications must be very accurate, but, on the contrary, the needle is swayed by the slightest magnetic influence and points North only on rare occasions; and in steel vessels only by mere chance.

The needle is drawn from true north first by the direction of the earth’s magnetic force which is not coincident with the meridian owing to the position of the magnetic poles. The north magnetic pole being in the extreme northern part of Canada, all the lines of force in the northern hemisphere converge toward this locality. The needle when otherwise undisturbed lies in the direction of these lines of force and takes an angle with the meridian depending on the locality.

The amount of divergence from the true north, or variation, as it is called, differs in different localities but is readily obtained by a glance at the chart where each compass rose shows the amount of variation at that place. From a magnetic course, or bearing, the true course, or bearing, is readily found by the proper application of this variation, which may be either easterly or westerly. The true course is to the right of the magnetic course, when considered from the center of the compass, in easterly variation; T. R. E.—True-Right-Easterly. Remember these three words and the whole lesson is learned, for if true is to the right in easterly variation it must be to the left in westerly; and if true is to the right in easterly, the magnetic course must be to the left of true course in easterly and to the right in westerly. In this way the true and magnetic courses are converted one to the other at will.

If we were to sail always in an entirely wooden ship, our compass troubles would be very few, for the above would include every phase of the situation. As wood is non-magnetic the compass would be uninfluenced by outside disturbances. Wood, as a ship-building material, having been so much displaced by iron and steel, the use of these metals has brought many problems to solve in connection with the deflection of the compass needle.

The effect on the compass needle of the magnetism in a vessel and her cargo is known as deviation and is very complicated, owing to many influences which are at work at all times giving an ever-varying value to this element of error.

The causes of deviation and its treatment in the way of compensating the compass are subjects much too extensive for this little book; furthermore, they are carefully dealt with in a half dozen of the well-known works on navigation, so we will touch only on the every-day side of compass work.

The deviation changes with every alteration of the vessel’s head, owing to the change in position of prominent parts of the vessel’s hull relative to each other, to the compass, and to the terrestrial lines of force (magnetism).

As a result of these influences on the compass needle, the mariner has three courses to deal with. The first is the true course, which is based on a compass whose needle points true north. The second, the magnetic course, is taken from a compass affected by variation alone and therefore pointing to the magnetic pole. The third is the compass course, or that course actually shown by an ordinary standard compass in a steel ship, affected by the error of variation combined with the error of deviation.

The combination of the deviation and the variation is the compass error and is obtained by adding the deviation and variation if both are of the same name, the compass error taking that name; for instance suppose we have a variation of 2° W. and deviation of 10° W., the combined error is 12° W. If, however, the variation and deviation are of different names, it becomes necessary to find the difference between the two and name the result after the greater quantity; thus, with a deviation of 4° E. and a variation of 10° W., the error is 6° W.

The compass error is applied to compass course to obtain true course and vice versa by the same rule as for variation.

The navigator in planning his course between two positions lays the parallel rulers on these positions on the chart and carries this direction to the nearest compass rose. This may be a true rose, in which event he remembers his T. R. E. rule, reversing it in this case, and with the variation given on the chart secures the magnetic course. In an iron or steel vessel, the deviation for that course must be ascertained from the deviation card by trial or from a Napier Diagram direct and applied to the magnetic course in order to obtain the compass course. This is accomplished precisely as in finding the magnetic from the true course (to the left if deviation is easterly and to the right if westerly). The course by standard compass is now at hand by which we can steam from one selected point to the other.

The deviation as has been said is an ever-varying error, and consequently it is quite impracticable to depend wholly on a fixed deviation card. We may take aboard some magnetic cargo or change our latitude to a great extent, the vessel may be pounded excessively by heavy seas, a stroke of lightning or by stranding; all these are causes liable to affect the deviation more or less.

In order to forestall the serious consequences that are liable to attend such a derangement of the normal and expected deviation, the careful navigator takes azimuths or amplitudes on every course when practicable. Azimuths and amplitudes are nothing more nor less than astronomical bearings of heavenly bodies; they indicate the true bearing of the body, and the difference between this bearing and the bearing taken simultaneously by standard compass is the compass error.

The azimuth of a body is the angle at the zenith between the meridian and the vertical circle passing through the body. It is customary, however, to consider the azimuth as measured by the arc of the horizon rather than by the angle at the zenith. It is measured from the north or south point according to the latitude, toward either the east or west point, through 180°.

An amplitude, unlike an azimuth, is restricted as to time of observation, for the body must be on the horizon either rising or setting; and should be observed when the sun is about its own diameter above the horizon and with a not excessive height of eye. The amplitude is measured from the east or west point through 90° to the north or south point. If the body observed has a south declination and is rising, the amplitude will be East so much South; if declination is north, East so much North, for a body rises in the East point when its declination is 0°—on the equator.

The principle of the amplitude lies in the solution of a right-angled spherical triangle, whose sides are the body’s polar distance, the co-latitude, and the zenith distance which is 90°. We desire the complement of the angle at the zenith. It is unnecessary to compute an amplitude, for in Table 39, Bowditch, will be found the desired bearings for different latitudes and declinations. The sun will be found the most satisfactory of the heavenly bodies to utilize for amplitudes.

There are two methods of calculating an azimuth, one known as the time azimuth and the other as the altitude azimuth. The former is the most popular owing to the tables that have been compiled, an inspection of which facilitates the navigator in quickly obtaining the true azimuth of a body. Before entering the tables, it is necessary to have as arguments the latitude and declination, and, if using the sun, the local apparent time, or for stellar work the hour angle. Should the star’s hour angle exceed 12 hours, 12 hours should be subtracted from it, and the remainder used as P.M. time. A planet may be employed precisely in the same manner as a star.

One of the simplest and most expeditious methods of securing the azimuth is by means of a diagram. Upon this convenient invention the bearing of a body can very quickly be taken off with a pair of rulers. Weir’s Azimuth Diagram is sold by the Hydrographic Office for a very small sum. The only argument that can be used against its use is that it requires a small table to lay it upon. Simple and complete directions are printed on the diagram.

The altitude azimuth is often computed at the same time as the ordinary A.M. and P.M. time sights, utilizing the altitude of the body for both operations. The principle involved in computing both an altitude azimuth and a time azimuth is the solution of the same astronomical triangle for the same angle, but in the case of the altitude azimuth three sides are given (the co-latitude, the zenith distance, and polar distance) to find the angle at the zenith. In the time azimuth, two sides and the included angle are given (the polar distance, co-latitude and local apparent time or hour angle) to find also, the angle at the zenith.

The azimuth found by computation should be named North if in north latitude, or South if in south latitude.

It has been customary to add up the logs, divide by 2 and the cosine will be half the azimuth named from the elevated pole, but a more expeditious way is after adding the logs seek in the log haversines and find the azimuth directly but named from the opposite pole to the latitude.

With the correct bearing of the sun, and its simultaneous bearing by standard compass at hand, the compass error is found by merely taking their difference. Now this error, as said before, is composed of the sum or difference of the deviation and the variation, so, if either is subtracted from their sum, or added to their difference the remainder is the other quantity. The variation being always known is subtracted from or added to the compass error to obtain the deviation, thus checking the deviation card for that particular course the vessel was steering at the time of observation.

With the compass error at hand, many students become perplexed as to the proper manner of dealing with this error, and finding from it the deviation. The compass error is first named, by considering the two bearings (compass and true) from the center of the compass; if the true is to the right of the compass bearing, the error is easterly, if to the left, westerly.

Now should the variation happen to be identical with the compass error, both in amount and in name, there is no deviation; if the variation is 0°, then the whole error is deviation. If by chance the compass error is 0°, it indicates that the variation and deviation are equal in amount and opposed to each other in their influence on the needle. The deviation, in such a case, naturally takes the opposite name from the variation.

In separating the variation from the compass error, it is necessary to exercise a little thought and to consider what deviation applied to the given variation will produce that compass error. This will be readily seen after a little practice. There are, however, some rules which are here given, by which the deviation can be obtained mechanically.

The deviation is the difference between the variation and the compass error if they are of the same name or adding them if of different names. It is given the same name as the compass error unless the compass error is subtracted from the variation, when the deviation takes the opposite name.

Or a diagram in which the error is shown by its particular number of degrees east or west of the true north line may be drawn and the variation likewise properly shown east or west of true north. If the error is to the left of the variation the deviation is west and if to the right the deviation is east.