north-in other words the compass needle is pointing north-bywest instead of north. Now in order to make a north course we must head one point to the right, and so find that the compass course of north-by-east will agree to a true north course.

Again, the true course given is east, and there is one point of westerly variation. The ship must head east-by-south in order to make her east course good.

Before leaving this question let me impress the fact that when shaping a course by the parallel rules, it is only necessary to consider variation for charts not laid out with variation diagrams.

It will be found that most ship's compasses are 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, 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 local 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 local attraction should be furnished with a table of errors, whether compensation magnets are used or not, and these tables should be subjected to a careful checking when at sea by the employment of amplitudes.

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 pernpedicular 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 2340, 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 (234), 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, the same 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 longitude 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.

THE QUADRANT.

The quadrant contains an arc of 45°, but owing to its double reflection it measures 90°, reading from right to left. The arc is divided into degrees, and these are subdivided into 3 parts of 20 minutes each, and the vernier on the sliding limb of the instrument is divided into single minutes. The sliding limb is moved from right to left in measuring altitudes, and the screw on the back is used for clamping it against the arc when the altitude is noted. The screw on the forward part of the limb is called the "tangent screw," and it is used for making a perfect contact after the sliding limb has been clamped. The colored glasses are for shading the eye when obtaining an altitude.

TO READ OFF AN ALTITude.

Ascertain by the zero on the vernier (sometimes marked thus, (0), and sometimes as an arrow) how many degrees and thirds of a degree it has passed on the arc, and then look along the vernier until one of its lines coincides with one of the lines on the arc, and the number of minutes given will be added to the last 20 minutes division the vernier zero has passed over, and the whole answer will be the required altitude.

MANNER OF ADJUSTING A QUADRANT.

FIRST.

Place the index at about 45° on the arc, and look into the mirror so as to see both the arc and its reflection. If they show in one line the glass is perpendicular to the plane of the instrument; but if they show on a broken line, move the screws in the frame upon which the glass stands, slackening one and tightening another, until the desired effect is realized.

SECOND.

Now make the zero on the vernier coincide with zero on the arc, and hold the instrument with its face upwards and look at the horizon. If the reflected part and the horizon itself (shown through the clear part of the horizon glass) show in one line, this adjustment is perfect; but if not, they must be brought in contact by moving the screw at the back of the glass.

THIRD.

With the two zeros meeting, hold the instrument vertically and look again at the horizon, and if the reflected part and the horizon itself show in an unbroken line, the instrument has a correct adjustment; but if otherwise, move the screw at the back of the horizon glass until one line shows straight across the glass.

TO FIND THE INDEX ERROR.

When it is impossible to secure a correct adjustment, proceed as follows:

By the aid of the tangent screw on the vernier, gently move the limb of the instrument until the image of, and the horizon itself, coincide; then the difference between the zero on the vernier and zero on the arc will be the "index error," and the amount will be added to the observed altitude if zero on the vernier is to the right hand of zero on the arc, but it will be subtracted if to the left hand.