At sea, Nov. 12th, 1918. Departure taken from noon position in Latitude 39° 40' N, Longitude 33° 20' 04" W. Log registered at noon 1. Course p.s.c. was 294° until about 3:30 P.M., Deviation 1° W, Variation 24° W, at which time observed altitude Circle with line under 13° 55' 10" and bearing by pelorus S 79° W. WT 3h 22m 18s. C-W 2h 13m 20S. CC + 1m 10s. IE 1' 10" off arc. HE 32 ft. Log registered 46.
Course was then changed to 290° p.s.c. until about 6:30 P.M. when observed altitude Star Polaris 40° 15' 40" N. WT 6h 32m 18s. C-W 2h 13m 20s. Same HE, IE, CC. Log registered 90.
Ship steamed on same course until 1:30 A.M. when log registered 196. At 1:30 A.M. sighted sub. on port bow. Ordered full speed ahead and made 17 knots per hour until 8 A.M. when observed altitude Circle with line under 8° 40' 00". WT 8h 01m 30s A.M. C-W 2h 13m 20s. Same HE, IE, CC.
Ship then steamed a true course of 272° at a rate of 15 knots per hour until noon, at which time observed meridian altitude Circle with line under 32° 35' 40" S. Same HE, IE, CC. Log registered 362.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a factor).
3. Deviation of Compass at 3:30 P.M.
4. Watch Time of Local Apparent Noon.
TUESDAY LECTURE
Day's Work
At sea, Dec. 10th, 1918. Departure taken from Latitude 19° 50' N, Longitude 20° 01' 20" W. Noon position. Log registered 20. Course p.s.c. N 16° E. Deviation 2° E, Variation 18° W. Ship steamed on this course until 8 P.M. when changed course to N 18° E p.s.c. and observed altitude Star Polaris 22° 33' 14" N. WT 8h 09m 10s. C-W 1h 20m 05s. CC 2m 00s fast. IE none. HE 39 ft. Log registered 104. Ship then steamed at 14 knots per hour until midnight. At midnight changed course to N 14° E p.s.c. and steamed at 12 knots per hour until 4 A.M. At 4 A.M. slowed down to 9 knots per hour and steamed at that rate until 8:30 A.M. when course p.s.c. was changed to N 17° E and observed altitude Circle with line under 22° 40' 30". WT 8h 34m 16s A.M. C-W 1h 20m 05s. Same IE, HE, CC. Sun bore by compass S 65° E, Variation 18° W. Continued on this course p.s.c. for two hours, speed 12 knots. Thence steamed a true course of 4° at same speed to noon when observed meridian altitude Circle with line under 42° 36' 50" S. Same IE, HE, CC.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a factor).
3. Deviation of Compass at 8:30 A.M.
4. Log reading at noon.
WEDNESDAY LECTURE
Day's Work
At sea, July 19th, 1918. Departure taken from Latitude 40° 30' N, Longitude 45° 00' 10" W. Noon position. Log registered at noon 68. Steamed until 2:30 P.M. on a course p.s.c. 115°. Deviation 1° W. Variation 25° W. Log registered 125. Changed course to 118° p.s.c. (Same Variation and Deviation) and steamed until about 5 P.M. At about 5 P.M. observed altitude (.) 22° 40' 20" and bearing by pelorus N 55° W. WT 5h 01m 16s. C-W 3h 00m 02s. IE 2' 20" on arc. CC 3m 32s slow. HE 32 ft. Log registered 168. Course p.s.c. was then changed to 113° until about 8 P.M. when observed meridian altitude of Star Vega 88° 15' 10" S. WT 8h 02m 26s. C-W 3h 00m 02s. Same HE, IE, CC. Log registered 210. Continued on same course p.s.c. until about 2 A.M. when observed altitude of Star Vega 47° 19' 20" West of meridian. WT 2h 04m 24s A.M. C-W 3h 00m 02s. Same IE, HE, CC. Log registered 299. TZ of Star N 86° W. Continued on same course until 4 A.M. when log registered 329.
At 4 A.M. heavy fog and rain forced ship to slow down to 5 knots per hour until about 9 A.M. when weather cleared and observed altitude Circle with line under 51° 52' 40". WT 9h 03m 18s A.M. C-W 3h 00m 02s. Same HE, IE, CC. Thence ship steamed a true course of 88° at a rate of 13 knots per hour to noon, when log registered 394 and observed meridian altitude Circle with line under 69° 52' 20" S. Same IE, HE, CC.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a factor).
3. Deviation of Compass at 5 P.M.
4. Watch Time of Local Apparent Noon.
THURSDAY LECTURE
Day's Work
At sea, Nov. 25th, 1918. Departure taken at noon from Latitude 25° 05' N, Longitude 37° 10' 40" W. Log registered at noon 32. Course p.s.c. was 119°, Deviation 2° E, Variation 19° W until twilight when log registered 110 and observed altitude Star Polaris 25° 30' 40" N. IE 1' 10" off arc. HE 28 ft. WT 5h 40m 18s. C-W 2h 28m 11s. CC 4m 15s slow. Changed course to SE x E ¾ E, same Variation and Deviation, and steamed on this course until about 8:30 P.M. when observed altitude of Star Markab, West of meridian, 59° 48' 10". Log registered 157. WT 8h 34m 48s. C-W 2h 28m 11s. Same HE, IE, CC. Steamed on same course until midnight when log registered 210. Changed course to 110° p.s.c. (same Variation and Deviation), and steamed at 12 knots speed until about 8 A.M. when observed altitude Circle with line under 23° 05' 10" and bearing by compass S 33° E. Variation 19° W. WT 8h 04m 10s, A.M. C-W 2h 28m 11s. Same HE, IE, CC. Continued on same course p.s.c. at a speed of 15 knots per hour until noon when observed meridian altitude Circle with line under 44" 30' 50" S. Same IE, HE, CC. Log registered 366.
Required 1. D. R. position at noon.
2. Position by observation at noon (corrected for Longitude by a factor).
3. Deviation of Compass at 8:04 A.M.
4. Watch Time of Local Apparent Noon.
ADDITIONAL LECTURE
Compass Adjustment
The aim of this lecture is to give you a very few facts about magnetism in general and compass adjustment in particular. The reason for including the lecture in this book is because of repeated requests on the part of graduates who have been consulted about the adjustment of the compass on their ships and who have realized that their advice might have been more helpful if they had learned more about the matter The earth is a huge magnet. It is the effect of the magnetism in the earth upon the compass needle which causes the compass error and makes it necessary to correct it. How can it be corrected? To know that we must first know the fundamental law of magnetism, namely, that opposite poles of two magnets attract each other and similar poles repel each other. From which it follows that if we decide to color red, for instance, that end of a magnetic needle which points to North, the magnetism of that part of the earth must be considered blue, i.e., of opposite magnetism to the north-seeking end of the red magnetic needle.
Now, there are various kinds of magnetism which affect a ship's compass. One is from the earth, another from the iron in the ship, etc. To discuss them and, the theoretical cause of them in detail is beyond the scope of this lecture. To correct them, four sets of magnets are necessary, two of which are usually found in the binnacle of the compass itself. One is a fore and aft magnet or set of magnets, the other an athwartship magnet or set of magnets. The third set consists of the two globes of cast iron placed on either side of the compass bowl (called Quadrantal Correctors). The fourth magnet, or set of magnets, is to correct the compass in case of severe heeling by the ship.
If you are ordered to adjust the compass the first thing to do is to choose a fine day with smooth water. Take your ship to a certain spot, the exact location of which you have found from the chart, and where you are certain you will have plenty of sea-way in which to swing. Set your watch to local apparent time (which you have calculated before coming out). Take from the Azimuth Tables the sun's true bearing for every four minutes of the time during which you will be occupied adjusting, and convert it into the magnetic bearing by applying the variation at the place selected (secured from the chart). Write down in a small book these times and corresponding magnetic bearings.
Now go to your compass and see that its lubber line is exactly fore and aft and in the keel line of the ship. Have another officer who is thoroughly familiar with the pelorus stand by it as the ship is swung. All being ready, secure the lubber's point of the pelorus at North and clamp the sight vane to the sun's magnetic bearing at the time you have figured to take the first heading. Starboard or port your helm until at the time calculated the reflection of the sight vane on the pelorus dial cuts on the proper magnetic bearing. The vessel's head will then be pointing to magnetic North. If, now, the compass were correct it would agree with the pelorus in showing the ship's head to be North. If it does not do so, there is Deviation in the compass and its amount is the amount of Deviation on that particular course. Suppose the Deviation were to starboard, i.e., Easterly, and were due to magnetism in the ship's starboard side. Then, if the magnetism in the North end of the needle be considered red, the magnetism in the starboard side of the vessel, in order to attract the red end of the needle, would be considered blue and the ship's magnetism, with the compass needle included, would look like this:
To counteract this blue attractive force on the starboard side, screw up the athwartship magnet in the binnacle toward the compass dial. Its magnetism, if it were laid on the deck, would look like this:
Compass Correction
In other words, as this magnet is moved nearer the compass needle, by the law of magnetism just given, the red end of the magnet repels the red end of the compass needle from the starboard side and the blue end of the magnet attracts the red end of the compass needle toward the port side. When the compass needle points to North, as shown by the correct pelorus bearing, the Deviation on this heading (i.e., North) is corrected.
Compass Correction
Now turn the lubber line of the pelorus to East. Steady the ship on this heading until the shadow from the pelorus vane at the proper L.A.T. cuts the circumference of the pelorus dial at the proper magnetic bearing. The ship's compass should then show the ship's head pointing to East. Suppose that it does not (as will usually be the case) but points to the right of East. Then the ship's magnetism and compass would look like this:
To bring the compass needle back to North it would be necessary to move up nearer the compass dial the fore-and-aft magnet (shown below), whose magnetism would act on the compass needle on this heading of the ship exactly as the athwartship magnet acted on the compass needle when the ship was headed North:
Compass Correction
Now the ship's compass has been corrected for the North and East headings respectively. The next correction is for the heading half way between, i.e., North-east. If there is any Deviation on this heading, adjust the cast iron cylinders (called Quadrantal Correctors), which are on each side of the compass bowl, by moving them toward or away from the compass until the ship's head by compass is North-east at the proper time and bearing by pelorus.
The ship's compass has now been corrected for one whole quadrant, namely, from North to East, and this will suffice for all four quadrants since the relationships of the magnets themselves and the magnetism of the compass needle is the same for any of the other three quadrants as for the first. Compass adjustment, however, can never be absolutely accurate. For that reason, it is wise to steam the ship completely around, steadying on every fifteen degrees by pelorus to determine and keep a record of remaining errors.
Compass Correction
There is one more correction to make, i.e., for the heeling error. This correction is necessary in case the ship is yawing in a sea-way so much that the relationship of the ship's magnetism to the compass needle is decidedly different from what it is when the ship is on a comparatively even keel. It is compensated by a vertical magnet directly underneath (or over) the binnacle, details in regard to which can be secured from Bowditch Art. 125, p. 53.d from Bowditch Art. 125, p. 53.
It must be borne in mind that compass adjustment is not an exact science, that an adjustment for one latitude is not correct for another, that anyone of a hundred different causes can affect the magnetism of the ship or of the compass needle, which in turn directly affects the Deviation. In this connection, it would be well to read Bowditch Art. 129, p. 55. You should also read Arts. 119-130 in which are given, more fully and in more scientific language, the contents of this lecture.
