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Same Adjustment by the Sun. Place the Index near 0°, (do not clamp it,) screw in a Telescope, and look at the Sun; move the Index backwards and forwards past 0°, the reflected Sun will pass and repass the direct Sun; if, in passing, they exactly cover each other, (that is, form one Sun,) the Horizon-glass is perpendicular; if not, they must be made to do so by the screws at the back of the Horizon-glass.
3rd. To set the plane of the Horizon-glass parallel to the plane of the Index-glass when the arrow or nonius is at 0.
Bring the arrow or nonius to 0 on the Arch, and, holding the Quadrant vertically, observe whether the real and the reflected horizons form one unbroken line; if so, the glasses are parallel ; if not, they must be made to form one line by means of the screw placed underneath the Horizon-glass.
Same Adjustment by the Sun.
Clamp the Index exactly at 0°, look through the Telescope at the Sun ; if the true and reflected Suns are exactly covering each other, (so that only one is seen,) the two glasses are parallel. Any error may either be corrected by the adjusting screw or Indesc Error, (which is the amount of error in this adjustment,) or may be measured as directed before, and applied to all observations.
4th. To set the axis of the Telescope parallel to the plane of the Sextant.
Screw in the Inverting Telescope, and set a pair of its wires parallel to the Sextant-plane; select two heavenly objects, such as the moon and sun, or the moon and a star, or two stars, but let them be from 90° to 120° apart; bring one object to touch the other on the lower wire of the Telescope, and, by moving the whole Sextant, quickly bring the objects to the upper wire. If the objects remain in contact with each other as before, the Telescope is parallel. The separating or overlapping of the objects shews an error, which must be corrected by the screws in the Telescope ring.
ᏟᎻᎪᎡᎢ . In looking at a Chart, it will be observed that there are lines which run up and down, and others across, the Chart; those which run up and down are due North and South, North being at the top, and South at the bottom; they are termed meridians. Those which run across are called parallels of latitude, and are due East and West, the right hand being East, and the left hand West.
(1) To find the Course and Distance from one Headland to another Headland.
Lay the edge of your parallel rulers on both the Headlands, and bring the rulers to the compass on the Chart, so that the edge passes through its centre; you will then see the course by the compass. In finding the distance, put one leg of your dividers on one Headland and the other leg on the other Headland, and measure your distance on the graduated scale, which runs North and South on the edge of your Chart. Care should be taken that the distance be taken equally on each side of the middle latitude between the two places or Headlands.
(2) Given the ship's position, that is, the latitude and longitude. Required the Course and Distance to a known Headland.
Mark the latitude off on the graduated scale running N. and S. on one side of your Chart, measure this on the nearest meridian of the ship's longitude, then bringing your parallel rulers up from the parallel of latitude, all places along the edge of the ruler will be in the same latitude; allowing your parallel rulers to remain, mark your longitude from the scale at the top or bottom of your Chart from the same meridian, East or West, as you require; running this along the edge of your rulers, will give you the position of the ship; then to find the Course and Distance, use Rule 1.
(3) To find the position of the ship by Cross Bearings.
Take the bearings of two known Headlands, differing as near 90° as possible, and not less than 45°; lay the edge of your rulers on the centre of the compass on the Chart to the bearing of the first Headland, and bring them up to it, marking with a pencil on the Chart a line from the Headland along the edge of your parallel rulers; do the same with the other Headland; and where the two lines intersect, that will be the position of the ship.
In the above three rules, if the magnetic points of the compass are not marked on the Chart, care should be taken to allow the variation to the left when Easterly, and to the right when Westerly, contrary to the usual way.
EXERCISES ON THE CHART.
FOR ONLY AND FIRST MATE. NORTH SEA. (1) Latitude 55° 28' N.
Longitude 0° 30' W. Required the Compass Course and Distance to Tynemouth. (2) Latitude 52° 15' N.
Longitude 2° 15' E.
(3) Latitude 53° 35' N.
Longitude 0° 55' E. Required the Compass Course and Distance to the Dudgeon. (4) Latitude 55° 5' N.
Longitude 0° ã E. Required the Compass Course and Distance to Hartlepool. ENGLISH CHANNEL (1) Latitude 50° 30 N.
Longitude 0° 55 E. Required the Compass Course and Distance to Dungenena, (2) Latitude 50° 10 N.
Longitude 3° lớy W.
Longitude 6 % W.
Latitude 45 5 X.
Longitude 115 W
(2) Latitude 50° 45' N.
Longitude 6° 20' W. Required the Compass Course and Distance to Longships. (3) Latitude 51° 10 N.
Longitude 6° 10' W. Required the Compass Course and Distance to Lundy Island Light.
(4) Latitude 50° 55' N.
Longitude 6° 55' W. Required the Compass Course and Distance to Trevose Head.
ORDINARY MASTER. NORTH SEA. (1) Sunderland Light bearing S.W. by Compass.
Coquet Island bearing N.W. I w. by Compass. Required the Compass Course and Distance to Hartlepool. (2) Kentish Knock Light bearing S.W. by S. by Compass.
Galloper Light bearing S.E. S. by Compass. Required the Compass Course and Distance to Shipwash Light
(3) Dudgeon Light bearing W. 1 N. by Compass.
Hasbro' Sand-end Lights bearing S. S.W. i W. by
ENGLISH AND BRISTOL CHANNELS, AND SOUTH COAST OF
IRELAND. (1) Lizard Light bearing E. & S. by Compass.
Longships Light bearing N. & W. by Compass. Required the Compass Course and Distance to the Seven Stones
Light, and Latitude and Longitude of the Ship. (2) Caldy Island Light bearing E.N.E. by Compass.
Lundy Island Light bearing S. by E. E. by Compass. Required the Compass Course and Distance to the Smalls Light. (3) Fastnet Light bearing N.E. I E. by Compass.
Skelligs bearing N. } W. by Compass. Required the Compass Course and Distance to the Old Head of
Kinsale ; also the Latitude and Longitude of the Ship. PROBLEM
TO FIND THE SHIP'S DISTANCE FROM A POINT
OF LAND, OR A LIGHT, WITHOUT A TABLE,
This Problem is of great service when sailing up or down channel, or along any coast, and is as follows :
Suppose Beachy Head bears two or three points on the port bow, then let the Ship keep on her course till it bears the same number of points to the left of the first bearing; the distance the Ship has run in the interval is her distance from the light at the time of taking the second bearing.
Let A be a Ship steering East; B a Light bearing three points on the port bow, that is, NE by E: she proceeds on her course till the Light bears three points to the left of NE by E, that is, NNE; then C B is equal to C A., by 6th Proposition, 1st Book of Euclid.
Communicated by J. Newton, F.R.A.S., Well Street Navigation Schools, London.