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The Examiner will then place the zero of the vernier on the arc, not near any of the marked divisions, and the candidates will read it.
Note. In all cases the applicant will name or otherwise point out the screws used in the various adjustments. Additional for Only Mates, First Mates, and Masters.
How do you find the index error by the sun?
13. The readings being
what is the index
error, and how do you apply it?
14. What proof have you that those measurements or angles have been taken with tolerable accuracy?
ANSWERS TO THE FOREGOING QUESTIONS, numbered with the same numbers as the Questions.
To set the index glass perpendicular to the plane of the instrument.
Place the index about the middle of the arc, hold the sextant face up and arc from me, and look into the index glass, and see if the true and reflected arcs are in one line. If they are, the index glass is perpendicular to the plane of the instrument; if not, then it is made so by the screws in the frame on which the index glass stands.
3. To set the horizon glass perpendicular to the plane of the sextant.
Set the index to o, hold the sextant nearly horizontal, look through the telescope and the horizon glass at the horizon. If the true and reflected parts of the horizon are in one straight line, the horizon glass is perpendicular to the plane of the sextant; but if not, it is made so by a screw at the back of the horizon glass.
5. To set the horizon glass parallel to the index glass when the index is at o.
6. Set the index at o, hold the sextant vertically, look
through the telescope and the horizon glass at the horizon, and see if the true and reflected parts of the horizon are in one straight line; if they are, then the horizon glass is parallel to the index glass; if not, it must be made so by a screw at the back of the horizon glass.
I would find the index error.
8. Set the index near o, hold the sextant vertically, and look through the telescope and the horizon glass at the horizon; if the true and reflected parts of the horizon are not in the same straight line, move the tangent screw until they are. The reading is the index error.
9. To be added if the reading is off the arc, but to be subtracted if on the arc.
No answer to be written. The Examiner will see that
it is correct.
No answer to be written.
The Examiner will see that
it is correct. Place the index to about 30' on the arc, hold the sextant vertically, and look through the telescope and the horizon glass at the sun on doing so, two suns will appear; bring them edge to edge, one above the other, by the tangent screw, read off, and mark it down. Then place the index to about 30' off the arc, bring the two suns again into contact, read off, and mark it down. Subtract the less reading from the greater, and divide the remainder by two. The result is the index error.
31'. 10" on
2) 0.20 difference.
0. IO to be subtracted.
14. By adding the two readings together, and dividing the sum by 4, the result should be the sun's
semidiameter for the day, as given in the Nautical Almanac. If it does not agree, the observations must be repeated.
QUESTIONS ON THE DEVIATION OF THE COMPASS.
The Candidate is to answer correctly at least eight of such of the following questions as are marked with a cross by the Examiner. The Examiner will not mark less than twelve.
What do you mean by deviation of the compass?
(a.) When in port?
(b.) When at sea?
3. Having determined the deviation with the ship's head on the various points of the compass, how do you know when it is Easterly and when Westerly? Why is it necessary, in order to ascertain the deviations, to bring the ship's head in more than one direction? For accuracy, what is the least number of points to which the ship's head should be brought?
6. How would you find the deviation, when sailing along a well-known coast?
In the following table give the correct magnetic bearing of the distant object, and thence the deviation.
8. With the deviation as above, give the course you
would steer by the standard compass to make the
Correct magnetic courses.
9. Supposing you have steered the following courses by the standard compass, find the correct magnetic courses made from the above deviation table
You have taken the following bearings of two distant objects by your standard compass as above; with the ship's head at find the bearings, correct
II. Name some suitable objects by which you could readily obtain the deviation of the compass when sailing along the coast of the English Channel.
Do you expect the deviation to change; if so, state under what circumstances?
How often is it advisable to test the accuracy of your table of deviations?
State briefly what you have chiefly to guard against in selecting a position for the compass.
15. The compasses of iron ships are more or less affected by what is termed the heeling error; on what courses does this error vanish, and on what courses is it the greatest?
16. State to which side of the ship, in the majority of cases, is the North point of the compass drawn in the Northern Hemisphere; and what effect has it on the assumed position of the ship when she is steering on Northerly and also on Southerly courses? 17. The effect being as you state, on what courses would you keep away, and on what courses would you
keep closer to the wind, in order to make good a
Does the same rule hold good in both hemispheres
N.B.-The Candidate is required to construct a deviation curve upon a
THE ANSWERS TO THE FOREGOING QUESTIONS.
Deviation is that error of the compass caused by the attractive force of the iron in the build of the ship, or in her as cargo.
When in port, by reciprocal bearings or by bearings of a distant object.
When at sea, by azimuths and amplitudes of the sun or other celestial object.
3. The deviation is Easterly when the correct magnetic bearing is to the right hand of the bearing by compass, and Westerly when to the left hand.
Because the deviation changes as the direction of the ship's head is altered.
5. Eight equi-distant points.
6. By bringing two known beacons or two known lights in one whose correct magnetic bearing is known
by the chart, and comparing that bearing with the bearing taken by compass.
Note.-For answers to Nos. 7, 8, 9, and 10 questions, see further on, under the head of "NAPIER'S DIAGRAM."
The South Foreland Lights, The Portland Lights, and the Lizard Lights.
Yes; with the change of lat. from a high North to a
high South; with the ship heeling over; after