LOCAL DISTURBANCE OF SHIP'S COMPASSES.

It will be found that most ship's compasses are now more or less affected. In iron-built ships the necessity of understanding the compass disturbances is imperative, and a knowledge of the laws of magnetism should be acquired by every navigator. Every iron ship should have a compass placed high enough aloft to be out of the influences of the ship's iron, and furnished, of course, with means of ready access to it.

A mechanical method for correcting the local disturbances of a ship's compass is by the adjustment of permanent magnets in such parts of the deck as just to counterbalance the error of the compass-needle from the magnetic north. The adjustment of magnets is done by professional men, so it is unnecessary to enter into detail concerning this branch of magnetic science. It may be remarked, however, that this arrangement is liable to be affected on merchant vessels owing to the magnets losing their power from various causes; as this would necessitate a re-adjustment of the compass, attended by considerable loss of time and expense, it will be acknowledged, I am sure, by men of experience that the idea of an elevated compass is by all means the most reliable, convenient, and economical.

Tables of Errors show deviation due to each point of the compass, constructed by swinging a ship's head around to every one of the thirty-two points, and observing the bearing of some distant object (whose true magnetic bearing is known) when the ship's head comes to each respective point.

Every ship whose compasses are disturbed by deviation must be furnished with a table of errors, and these tables should be subjected to a careful checking when at sea by the employment of amplitudes.

TO CORRECT A COMPASS COURSE FOR VARIATION.

A magnetic (compass) course is never a geographical (true) course, unless the ship is sailing in latitudes and longitudes where no variation exists. At any other time the course made by compass must have the variation applied to it, in order to deduce the true angle made across the meridians and parallels. As this' subject was pretty thoroughly explained in the treatise on the compass, in the preceding pages, unnecessary space will not be given here, and with the introduction of a few examples we will leave the question.

A ship sails by compass, north, with one point of westerly variation; what is the true course she has made? Answer: north by west.

A ship sails by compass, east, with a half point of westerly variation; ; what is the true course she has made? Answer: east half north.

A ship sails by compass, south, with one point of easterly variation; what is the true course she has made? Answer: south by west.

A ship sails by compass, west, with one point of easterly variation; what is the true course she has made? Answer: west by north.

DIAGRAM

SHOWING HOW TO ALLOW FOR EASTERLY AND WESTERLY VARIATION.

To correct a course a ship has sailed, allow around the compass in the direction indicated by hand No. 1.

FOR EASTERLY VARIATION.

To correct a course a ship has sailed, allow around the compass in the direction indicated by hand No. 2.

THE ECLIPTIC AND DECLINATION.

WHY AND HOW THE LATTER IS CORRECTED FOR HOURLY CHANGE.

The axis of the earth is not perpendicular to its orbit, but is inclined to it 23° 27′ 30′′-in round figures generally called 234°. In other words, the earth leans toward the right on its axis, and its inclination from the perpendicular is 234°. As the earth moves round the sun, a path is described by this luminary passing over the earth, and this path is called the "sun's track" or "Ecliptic." The ecliptic and equator are inclined to one another at an angle of 231°, owing to the earth's inclination.

By reference to the Nautical Almanac it will be seen that on March 20th the sun crosses the equator on its summer journey to the northern hemisphere; and by following it along we will find that on June 21st it has reached the limits of its northern declination (23°), and has begun its backward track towards the equator, which, we will see, it again crosses on September 23d. It now continues its southern course until the southern limit of its declination is reached, which is on December 21st, then goes back over its old track.

It will be understood, then, by the above explanation, that the sun's declination is continually changing, and as its exact position must be calculated when finding the ship's position by observation, the following additional explanations are given :

The sun does not move; but we suppose for convenience sake that it does, and when we say that the sun has so much declination, we mean that,

owing to the earth's position, the sun is over such a point of latitude. The declination of the sun means in reality the latitude of the sun; consequently, if we look in the almanac and see that on a certain date the declination of the sun is 20° 19' South, we know that it is over the parallel of latitude 20° 19' South at 12 o'clock at Greenwich, because the almanac shows declination reckoned for noon each day at Greenwich. We also observe that opposite to the column of declination is what is called the "hourly difference of declination "meaning the change of latitude the sun is making each hour.

Now it may readily be perceived that if at 12 o'clock at Greenwich on a certain day the declination is 10° 20' North, and the hourly difference 30" (half a mile), in one hour from noon the sun will have changed its declination 30", either to the north or to the south, according as the sun is moving, and this correction will be added or subtracted accordingly.

On the equator there is no declination, as there is no latitude.

The chronometer used in navigation is set to Greenwich time, and when working an observation we must observe the time shown by chronometer in order to know how many hours from noon it is at Greenwich when the sight is taken.

In west longitute the chronometer is always ahead of local time, and in east longitude it is always behind local time; that is, in west longitude, when it is noon at ship, it is always later than noon at Greenwich, and in east longitude, when it is noon at ship, it is always before noon at Greenwich.

Suppose we should take an observation of the sun on December 1st, at noon at ship, and at that time the chronometer should show 5 o'clock P.M. at Greenwich. In order to find the true declination of the sun at the time we took the sight, it would be necessary to multiply the "hourly difference of declination" by 5, and add the correction to the declination given for that day, because the declination is increasing, and after noon at Greenwich the declination must in this case be more than it was at noon.

NOTE.-See page under heading of "Chronometer Rate" for an understanding of the declination record, as given by the nautical almanac for each day of the year.