Sivut kuvina
PDF
ePub

EXPLAINING THE

RELATION OF POINTS AND

ANGLES TO COMPASS COURSES.

The circumference of the compass is divided into 32 points (which will be seen by reference to the diagram), 8 points to a quadrant. That is, from N. to E. is a quadrant, and there are 8 points contained in it. Now as there are four quadrants (quarters) to the card, we multiply the 8 points of one quadrant by the number of quadrants (4) and receive an answer of 32 points. It will be seen that each course has a numerical value, and that the counting commences at the north and south (the Poles of the compass) and increases towards the east and west points (the Equator of the compass). North and south are considered zero (0) and east and west are called 8 point courses.

North-east, south-east, south-west and north-west are all 4 point courses, because they are equi-distant from the cardinal points, and as 8 points is the complement (the highest number), 4 points must be one-half the complement.

North-by-east, north-by-west, south-by-east and south-by-west are all 1 point courses, being the same distance from the poles of the compass.

South-south-east, south-south-west, north-north-east and northnorth-west are all 2 point courses.

North-east-by-north, south-east-by-south, south-west-by-south and north-west-by-north are all 3 point courses.

And so the counting is continued until the east and west points are reached, when those courses receive a value of 8 points.

A little study of the compass card will make the subject very clear.

We will now take up the question of angles :

An angle is the difference in the direction of two lines from the point where they join.

The circumference of any circle is equal to 360° (three hundred and sixty degrees).

The compass card is divided into 360° in the aggregate, but, like the points, the degrees are contained in the four quadrants

-north to east being 90°, north to west 90°, south to east 90° and south to west 90°-the four counts being equal to the cir cumference, 360°.

Now as there are 360° to the circumference, and 32 points to the compass, it stands that 1 point of the compass will be equal to the number of times 32 is contained into 360, which is 111. Hence 1 point of the compass is eleven and one quarter of a degree (11° 15').

The north and south poles of the compass are called zero, and the degrees are counted from there exactly the same as the points. Thus north-by-east, north-by-west, south-by-east and south-bywest are all angles of 11° 15'-being the same distance from the angle poles.

East and west are 90° courses (the complement). North-east, south-east, south-west and north-west are all 45° courses (half the complement), and the angles the intermediate points make with the horizon will be seen by reference to the diagram.

It is imperative that the navigator should be thoroughly familiar with the names of the points, and the value of the same in angles, for the compass is the foundation upon which the science of navigation rests.

SHORT TREATISE ON THE COMPASS.

It is generally conceded by historians that the Chinese were the discoverers of the pointing of the magnetic needle north and south, and that it was first employed by them for the purposes of navigation.

The power of the lodestone to attract small pieces of iron was known many centuries before the Christian era, and the names magnet and magnetism probably originated from the discovery of magnetos, or magnetic iron ore by the Greeks.

The compass was not used as an instrument of navigation by European nations till the latter part of the fifteenth century. The compass needle points to the magnetic poles of the earth, and

as the magnetic poles do not coincide with the true poles of the earth, which are at the extremities of the axis of revolution, and between which the true meridians trend, it is seen that the magnetic meridian forms an angle with the true meridian, and the difference is called the Variation of the Compass.

The phenomena of the development and distribution of magnetic force over the earth's surface have never been satisfactorily explained (although good theories are not lacking), but it is sufficient for practical purposes to know that such a force exists and that its properties are understood.

There are two magnetic north poles and two magnetic south poles, but one of each is comparatively weak in magnetic force; consequently it is only of the two great magnetic poles that science generally treats, and in speaking of the sources of compass attrac· tion (magnetism) I allude only to them.

The magnetic north pole has been reached, and is situated in the latitude of 70° North, and the longitude of 97° West. The south magnetic pole is situated in the latitude of 70° South and the longitude of 145° East.

The magnetic equator is not the same as the earth's equator, but an irregular line running round the globe, not greatly distant from the earth's equator, which it crosses in two places, one near the west coast of Africa and the other about the middle of the Pacific Ocean.

In different parts of the world the variation of the compass needle from the true or geographical north differs in both quantity and direction; that is, the north point of the compass inclines in some places to the westward of the true north, and in other places to the eastward of the true north, and these inclinations are called respectively east and west variation.

Over the North Atlantic Ocean, the greater part of the South Atlantic Ocean and the Indian Ocean, the variation of the compass is westerly, and over a part of the Atlantic Ocean and in the Pacific Ocean the variation is easterly.

There are places on the surface of the globe where no variation exists, and these points are termed "lines of no variation "-the north end of the compass pointing true north, or toward the geographical pole.

One of these lines of no variation runs through Eastern Europe, Asia and Australia, and another runs through North America, the eastern part of South America, near Rio de Janeiro, Brazil, and the south-western part of the South Atlantic Ocean.

Variation undergoes an annual change, hence the necessity of issuing new charts when this fluctuation amounts to a sufficient quantity to disturb the mariner in shaping his course from port to port, for it is the amount of variation given by the chart for different localities which governs the compass course over the trackless waste.

The parallels (lines of latitude) and meridians (lines of longitude) shown on charts, all coincide with the geographical cardinal points, North, East, South and West, but the diagram compasses marked on the charts (unless otherwise specified) allow for the variation of the compass in those particular localities. In other words, they are the reflection of the ship's compass for the latitudes and longitudes they represent. For example: if we look at a chart of Sandy Hook, New York, we will find that the compass drawn on the chart gives 7° 50′ of westerly variation, equaling about of a point of the compass. Hence we know that in the latitude and longitude of Sandy Hook, at the present time (1885), the north end of the compass deviates of a point to the westward of the geographical north. The idea of marking these magnetic indications on the chart is an excellent one, for it obviates the necessity of allowing the variation on a course found by the parallel rules -simply accepting the course angle given by the diagram being all-sufficient.

In order to emphasize this point, let me dwell upon the question a moment longer:

Desiring to run from one point to another, the parallel rules. are produced and the same edge of the rule is laid simultaneously upon the places in question; this edge is now moved to the dot in the center of the nearest compass marked on the chart (care having been exercised to preserve the angle made by the rules), and by reading that point of the diagram compass looking toward the place sought (the edge of the parallel rule cutting it), the magnetic compass course for the vessel to steer will be given. Without any consideration of the variation existing at the ship's

place the vessel will be headed the given course, because it is evident that if a course given by the chart compass will lead the vessel to the place sought, the sensitive compass aboard the ship will also lead her aright, for the chart compass and the sensitive compass are one and the same, being, as I expressed it previously, reflections of one another.

In dealing with variation on the preceding pages, I have given no recognition to the question of "local disturbances of the compass," occasioned by the metal used in the construction of a ship, in the machinery or in a cargo, preferring to deal with the latter under a separate and distinct head; but I will remark that local disturbances, as a rule, are seldom of vital importance in a wooden sailing vessel, unless they are produced by elements of magnetism in the cargo.

I suggested in the opening lines of this chapter that the diagram compasses shown on the charts allowed for the variation of the compass in different places on the earth, "unless otherwise specified;" and I will now explain this clause:

Suppose that by a diagram compass on a chart pointing true (that is, the four cardinal points of the picture compass agreeing to the four geographical cardinal points of the chart) a course was found between two places to be north-east. This will be the true angle across the meridians and parallels, and providing the compass was without variation, all that would be necessary to do would be to steer "north-east" by the ship's compass; but owing to the existing variation, we cannot steer the true course found, and we apply the compass variation to the true course in the following manner: our course is south-east, and there is one point of westerly variation given for the latitude and longitude of the ship. Now this one point of westerly variation has swung the entire compass card around one point to the left, consequently if we want to steer south-east true, we must sail southeast-by-south by the compass in order to make our south-east

course.

This will be readily seen by the following example:

A ship wants to head a true north course and there is one point of westerly variation to be considered. The north end of the compass card is inclined one point to the westward of the true

« EdellinenJatka »