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separate when brought to the far wire, the object end of the telescope inclines towards the plane of the sextant; if they overlap, it declines from the plane.
5.-To determine the Index error, measure the sun's diameter on the arc of the instrument, and on the arc of
excess, which is done by holding the sextant perpendicularly, and bringing the true and reflected suns in exact contact on each side of 0; half the difference of the two readings will be the index error, which is additive when the reading on the arc of excess is the greater, but substractive when the reading on the arc of excess is the less of the two.
Mercator's Chart which has been compared to a cylinder unrolled, is a convenient and ingenious method of representing the surface of the globe as if it were a plane. The lines drawn from the top (North) to the bottom (South) of the chart are meridians; those from left (West) to right (East) are parallels : the top and bottom are graduated parallels ; the extreme right and left are graduated meridians. The latitude of any place is measured on a graduated meridian and its longitude on a graduated parallel.
To find the course between two places, which is represented on this chart by a straight line, lay the edge of a parallel rule on the places, then slide it down until it comes exactly on the centre of one of the
compasses, and the course can be read off. To find the distance between two places, one general rule will apply:—Take half the distance between them; the point midway between the two places indicates the latitude to which one leg of the compasses is to be referred on the graduated meridian ; carrying the other leg first North and then South of that latitude, the degrees and minutes intercepted between the extreme points will be the approximate distance when the two places are on the same parallel or when they lie obliquely, and the true distance when they are on the same meridian.
It must be remembered that when the true course between two places is known, Easterly variation allowed to the left and Westerly variation to the right of this, will give the compass course,
To ensure accuracy in determining the place of a ship by cross bearings, the difference of the bearings should be as near as possible 90°.
The speed of a vessel is ascertained by means of the Logline and a sand glass running a given number of seconds. To determine the length of a knot on the log-line, we have the following rule: The length of the knot (in feet), must bear the same proportion to a geographical mile (in feet,) that the seconds of the glass used at the time of heaving the log, bear to the seconds contained in an hour; from which it follows that the number of knots and parts of an knot run during the interval indicated by the glass, will give the number of miles and parts of a mile the ship has sailed in an hour, supposing the rate of sailing to be uniform.
The geographical mile being about 6080 feet, we have for glasses running 284 and 30% respectively, the following proportions :
36005 : 285 :: 6080 ft. : 47.288ft.=47 ft. 3} in.
3600 : 30 :: 6080 ft. :50:666ft.=50 ft. 8in. the required lengths of the knot, but if 80 feet be rejected from the geographical mile, and the first and third terms of the proportion reduced, by dividing them by 600, the statements become
65 : 285 :: 10 ft. : 46.66 ft.=46 ft. 8 in. nearly
68: 30: : 10 ft. : 50 ft. hence the method very commonly adopted to ascertain the length of the knot: viz. annex a cipher to the number of seconds run by the glass, and divide this by 6.
The log-line must always have a sufficient quantity of what is termed “stray line,” in excess of the admeasured portion, in order to allow the log to get clear of the eddy of the ship's wake; this must be determined by the size of the vessel.
It is recommended to divide the knot to tenths.
THE LEAD LINE.
In nautical phrase, the Lead Line has “ nine marks and eleven deeps :"At 2 fathoms the mark is Leather, 3 ......
White Rag, 7
Red Rag, 10
Leather with a round hole in it 13
Blue Rag, 15
White Rag, 17
Red Rag, 20
A piece of cord with two knots.
The Deep Sea Lead Line is marked in a similar manner to the 20 fathoms, after which a piece of cord with an additional knot for every 10 fathoms is fixed in the line, and between the tens a piece of leather to denote five fathoms.
The Act of Parliament passed in the 10th year of the reign of Her present Majesty, entitled an “Act for the Regulation of Steam Navigation," requires that all British Steam Vessels whether (propelled by paddles or screws) shall, between sunset and sunrise, exhibit the following lights :
When under Steam, or Sail.
The Mast-Head Light is to be visible at a distance of at least five miles, in a dark night, with a clear atmosphere, and the Lantern is to be so constructed as to show an uniform and unbroken light over an arc of the horizon of 20 points of the compass being 10 points on each side of the ship, i. e. from right ahead to 2 points abaft the beam on either side.
Each Side Light is to be visible at the distance of at least 2 miles, in a dark night, with a clear atmosphere; the lantern of each is to be so constructed as to show an uniform and unbroken light, over an arc of the horizon of 10 points of the compass, but being fitted with an inboard screen, of at least three feet long, the light cannot be seen across the bows; hence it follows, the Green Light may be seen from right ahead to 2 points abaft the beam on the starboard side; and the Red Light from right ahead to 2 points abaft the beam on the port side.
When at Anchor, a common Bright Light is to be exhibited, the lantern of which is constructed to shew a good light all round the horizon.
LIGHTS FOR SAILING VESSELS.
Sailing Vessels when at anchor in Roadsteads or Fairways must exhibit a bright light at the mast head.
Sailing Vessels when under sail, or being towed, approaching or being approached by any other vessel must show between sunset and sunrise a bright light in such a position as can be best seen by such vessel or vessels, and in sufficient time to avoid collision.
THE RULE OF THE ROAD.
1. Sailing Vessels having the wind fair, give way to those on a wind.
2. Vessels close hauled on the starboard tack, always keep their wind.
3. Vessels close hauled on the port tack, must give way to those on the starboard tack.
N.B.-Steamers are considered as vessels with a fair wind.