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the direction of the magnetism of the needle or needles is parallel to the longitudinal N. and S. line of the card; (3) that the pivot and centre of the cap working on it are in the centre of the graduated (or divided) circumference of the card; and (4) that the horizontal position of the card is preserved, for which purpose the needles must be furnished with sliding weights to counteract the tendency to dip. These subjects are further described in connection with “compass adjustment.”
The COURSE.—The meaning of the term course is direction; and in reference to a ship it is the point of the compass upon which she sails,or in other words, the direction in which the helmsman is ordered to keep the ship's head by compass. To shape a course is to determine the direction in which a ship is to be steered in order to prosecute a voyage; when the wind is foul, she cannot lie her course, if free, she steers her course. When the course is neither on a meridian (N. or S.) nor on a parallel (E. or W.), it is said to be oblique.
RUMBS or RHUMBS.-- The points of the compass were formerly called Rumbs, and later Rhumbs, after the points of the horizon of which, as already explained, the compass card is the representation.
The STEERING COMPASS.-- When the compass card, with magnetised needles attached to it, rests on the pivot in the bowl, and the bowl is placed in the binnacle with the lubber's line directed forward, i.e., towards the ship's head, and at the same time in the midship line, the instrument for steering purposes is complete. The point of the compass close to the lubber's line is said to be the direction of the ship’s head by compass; and if the ship is under way, it is the COURSE by compass.
An AZIMUTH COMPASS is an instrument similar to the steering compass, but of superior make; the card is more accurately divided, and the outside of the compass bowl is fitted with a movable ring, to which are attached (exactly opposite to each other, and in a line with the centre of the compass card) two sight-vanes that can be turned down when not in use. Sightvanes are merely oblong pieces of brass with a vertical slit in each ; the slit of the eye-vane is very narrow, and the wider slit of the other (and opposite or object) vane is fitted with a single horse-hair for a vertical line ; when you look through the vanes and turn them in the direction of an object, you take the object's bearing, which is indicated by the compass card. In the Prismatic Azimuth Compass a prism is attached to the sight-vane for the purpose of more accurate observation ; and thus the divisions of the card below it are read by reflection, at the same time that the bearing is taken ; but be careful not to read the card-indication the wrong way—for example 62° instead
The azimuth compass is placed in a higher binnacle than is the steering compass, or sometimes on a tripod removable at pleasure, and in the latter case the compass is kept in a box (for safety) when not in use. The position of the azimuth compass should be such as to command a clear view in every direction around it. And it is of the first importance in taking bearings,(1) that the compass card is perfectly horizontal ; (2) that the surface of the prism is horizontal; (3) that the two vanes are vertical ; and (4) that the line of sight passes directly over the pivot of the card.
NOTE.—The terms azimuth and bearing are synonymous. When observing bearings, if the compass is slightly vibrating take the mean of two or more bearings, read off as quickly as convenient. Never stop the card to read.
To reverse a bearing or course, by which you get the opposite point, simply reverse the letters which compose it ; thus the opposite to N.W. I N. is S.E. I S. ; similarly the opposite to N. 37° E. is S. 37° W.
The STANDARD COMPASS is that to which all the indications of the other compasses on board are referred, and by which the course is set. It should be fixed in a firm binnacle, or on a wooden pillar—the card not less than four, nor more than five, feet high, and well within direct vision-in a well selected part of the ship both as regards the ship's magnetic character and for the purpose of a clear unobstructed view around in every directionto facilitate the taking of bearings rapidly. In this sense, and with the various appliances of a good compass, as already indicated, the standard compass is the azimuth compass, and should be fitted as such in every respect. The errors of this compass receive all the necessary attention. When you set a course, as for instance S.W. by S. by standard compass, you say to the man at the wheel—“How is her head ?”—he may reply ‘S.W. by S. 1 S.”; and that is the reading of the steering compass, corresponding to S.W. by S. by standard ; you say, “Keep her so "; the officer of the watch refers from time to time to the standard compass to see that the proper course is kept ; thus you know the course is being made good that you intend.
DEAD RECKONING.—The position of a ship when determined from the distance run by log, and the courses steered by compass, subsequently rectified, is the position by dead reckoning; it is generally expressed by the initials D.R.
THE SEXTANT The first known instrument with which the early navigators tried to measure the altitude of celestial objects and from thence obtain the latitude was the Astrolabe.
It was a very crude instrument consisting of a graduated disc with a ruler (or alidade or a bable, as it was called) that moved on a centre and carried a sight.
About 1590 Captain John Davis invented the Back Staff. It consisted of two concentric arcs and three vanes; the longer radius had an arc of 30°, the shorter radius had an arc of 60°.
This instrument was in common use until 1730, when John Hadley invented a reflecting instrument called the Quadrant.
The Sextant of to-day is simply an extension and improvement of Hadley's Quadrant; the principle of construction remains the same.
The first idea of a reflecting instrument, with only a single mirror attached to a movable radius, is due to Hooke, but it was not adapted to the purposes of the navigator. It is not easy to understand why so much pains have been taken to deprive Hadley of the merit of his invention. Sir Isaac Newton, it is said, had previously invented a reflecting instrument with two mirrors ; this, however, was not known to Hadley, whose discovery was published in 1731, that of Newton in 1742. Lalande mentions Godfrey, of Philadelphia, as having anticipated Hadley ; but it was not known in England until after the publication of Hadley's paper in the Philosophical Transactions.
The SEXTANT derives its name from the extent of its limb, which is the sixth part of a circle, or 60°, but being an instrument of double-reflection it is divided into 120°. It is used for measuring angles--as the altitudes of, and distance between, heavenly bodies and also altitudes of, and angles between, terrestrial objects.
BRIEF DESCRIPTION OF THE PARTS OF THE SEXTANT.—The form of the Sextant is that figured on Plate VIII.; but the student, in reading the description of the various parts, should place the instrument itself before him.
The flat upper surface of the Sextant is the plane of the instrument, and the following are the principal parts
A A' is the arc (or limb) which is graduated from right to left, from o (the zero point) to 120° or 150°; this is the arc proper, and the subdivisions of each degree are by 10', 15', or 20', according to the size and perfection of the instrument. To the right of o—i.e., from left to right of the arc, there is also a small portion of graduated arc which is called the Arc of Excess.
IR V is the radius, or index-bar, movable along the arc and round a centre, and having a dividing scale V (called the vernier) close to the arc, by which the subdivisions of the arc are read off.
I is the index-glass, a reflector or mirror which moves with the indexbar, and is fixed on it in such a manner that its plane is over the centre of motion of the index, and perpendicular to the plane of the instrument.