13. You have steered the following courses by the standard compass; find the magnetic courses from the curve of deviations. 14. You have taken the following bearings of two distant objects by your standard compass with the ship's head at E. by N.; find the magnetic bearings. Compass bearing N.E. by E. and S. by E. S. 11° W. 15. Do you expect the deviation to change; if so, state under what circumstances? I should expect the deviation to change. First, from lapse of time, especially in a new ship, also when she is kept or steered in a direction opposite to that in which she was built. The deviation may also be expected to change with change of geographical position, from sustaining strains from heavy seas, or shocks, such as from a collision. If a ship is steered for some time on one course, especially if near the east or west point, when the course is altered the north point of the compass may be drawn towards that part of the ship which was previously towards the south. 16. How often is it desirable to test the accuracy of your table of deviations? For the reasons before given, the compasses of a new ship require to be more frequently tested than those of an old one, and those of a ship proceeding to the opposite hemisphere than those of a ship sailing on the same parallel. But under all circumstances the deviation should be tested for the course on which the ship is sailing as frequently as is practicable; and if a change in the deviation on such a course is observed, the accuracy of your table should be tested, at the earliest opportunity and, if necessary, corrected. 17. What is meant by variation of the compass; what is it caused by; and where can you find the variation for any given position? Variation of the compass is the angle, at any place, between the true and magnetic meridians and is caused by the magnetic poles not coinciding with the geographical poles. The variation for any given position can be found on the Admiralty variation chart. 18. The earth being regarded as a magnet, which is usually termed the blue, and which the red magnetic pole? The north magnetic pole is usually termed the blue and the south the red. Which end of a magnet (or compass needle) is usually termed the red or "marked" end, and which the blue? That end of a magnet or compass needle which points towards the north when it is suspended so as to move freely in the horizontal plane is usually termed the red, or marked end; the end which points towards the south is termed the blue 20. What effect has the pole of one magnet of either name on the pole of another magnet? The pole of one magnet of either name will repel the pole of the same name, and attract the pole of the contrary name of another magnet. 21. What is meant by transient induced magnetism? Transient induced magnetism is magnetism which is instantly produced in soft iron when it is exposed to any magnetic force, such as that of the earth, and is parted with or changed immediately the inducing force is removed or changed. The near poles of the induced magnetism and of the inducing cause are always of contrary names. 22. Which is the red and which is the blue pole of a mass of soft vertical iron, by induction, and what effect would the upper and lower ends of it have on the compass needle (a) in the northern hemisphere, (b) in the southern hemisphere, (c) on the magnetic equator? The red pole is at the lower end in the northern hemisphere, and the blue pole is at the upper end; in the southern hemisphere the red pole is at the upper end, and the blue pole is at the lower end. Therefore the upper end attracts the north end of the compass needle, and the lower end repels it in the northern hemisphere; and in the southern hemisphere the upper end repels the north end of the compass needle, and the lower end attracts it. On the magnetic equator one side is red and the other blue, so that the compass needle should not be affected by either end. 23. Describe what is usually termed the sub-permanent magnetism of an iron ship, and state when and how it is acquired, and which is the red and which the blue pole, and why it is called sub-permanent magnetism. The sub-permanent magnetism of an iron ship is first acquired from the earth's inductive influence while the ship is being built, but remains after she is launched, although it undergoes a considerable reduction, especially when she proceeds to sea or is subjected to concussion with her head in any other direction than that in which she was built. The red sub-permanent pole of the ship is that which was directed towards the blue or north magnetic poie of the earth, and the opposite extremity of the ship is the blue subpermanent pole. It is termed sub-permanent magnetism to distinguish the magnetism which is thus acquired by a ship while building, from the magnetism of a magnetised steel bar, which is of a much more permanent character. 24. Describe the meaning of the expression co-efficient A. Co-efficient A represents a deviation of the same sign and amount, ol all points of the compass. It has the sign + when that deviation is easterly, and the sign when westerly. It should have no value when the iron is symmetrically situated beside the compass. The term "sub-permanent " magnetism in these questions is used in the original sense, as proposed by the late Sir G. B. Airy, to denote the character of the permanent magnetism of an iron ship as distinguished from the permanent magnetism of a magnetised steel bar The terms sub-permanent and permanent throughout these questions may, therefore, be considered as synonymous. 25. Describe the meaning of the expression co-efficient B, its signs and effects. The expression co-efficient B represents a magnetic force in the foreand-aft line of the ship. It has the sign + when the north point of the compass is attracted towards the ship's head, and the sign — when the north point of the compass is drawn towards the ship's stern. The effects of a + B are easterly deviations with ship's head in the eastern semicircle of the compass, and westerly deviations in the western semicircle, attaining a maximum value on the east and west points, decreasing to zero on the north and south points, by compass. Bare The effects of a B are easterly deviations with ship's head in the western semicircle, and westerly deviations with the ship's head in the eastern semicircle, with a maximum value on the east and west points, decreasing to zero on the north and south points, by compass. 26. Describe the meaning of the expression co-efficient C, its signs and effects. The expression co-efficient C represents the athwartship component of sub-permanent magnetism. It has the sign + when the north point of the compass is attracted towards the starboard side, and the sign when the north point of the compass is attracted towards the port side. +C causes easterly deviations with ship's head in the northern semicircle, and westerly deviations in the southern semicircle, attaining a maximum value on the north and south points and decreasing to zero on the east and west points, by compass. - Ccauses westerly deviations with ship's head in the northern semicircle, and easterly in the southern semicircle, attaining a maximum value on the north and south points and decreasing to zero on the east and west points, by compass. B and C are termed co-efficients of semicircular deviation, because the deviation they represent is always easterly in one semicircle and westerly in the other. 27. Describe the meaning of the expression co-efficient D, its signs and effects. The expression co-efficient + or - D represents a magnetic force, caused by induction in horizontal soft iron, either fore-and-aft or athwart ships, which so changes in direction, as the ship goes round, as to cause a deviation alternately easterly and westerly in successive quadrants, hence the name quadrantal deviation. + D causes easterly deviations with ship's head between N. and E., and S. and W.; and westerly deviations between S. and E., and N. and W. D gives results exactly the reverse to + D. Both D and D have a maximum value on the four quadrantal. points, and become zero on the cardinal points, by compass. Co-efficient D very rarely has a sign. 28. Describe the meaning of the expression co-efficient E, its signs and effects. The expression co-efficient + or - E represents a magnetic force, caused by induction in horizontal iron, diagonal to the fore-and-aft line, or unsymmetrically distributed about the compass, which so changes in direction as the ship goes round, as to cause a deviation alternately easterly and westerly in successive quadrants. + E causes easterly deviations with ship's head. between N.E. and N.W., and S.E. and S.W.; and westerly deviations between N.E. and S.E., and N.W. and S.W. Both + E and E have a maximum value on the cardinal points, and become zero on the four quadrantal points, but are usually very small in amount in compasses placed in the middle line of the ship. D and E are termed co-efficients of quadrantal deviation. But as - D is very rare, and E has no value when the iron is symmetrical beside the compass, the quadrantal deviation is nearly always due to + D. 29. Would you expect any change to be caused in the error of your compass by the ship heeling over either from the effect of the wind or the cargo, &c. ? Yes, because forces which were vertical when the ship was upright and caused no deviation, act obliquely when she heels, and cause deviation. 30. The compasses of iron ships being more or less affected by what is termed the heeling error, on what courses is this error usually at its minimum, and on what courses at its maximum ? The heeling error usually has a minimum value on courses east or west by compass, because the disturbing force is then acting in the direction of the compass needle and causes no deflection. It usually has a maximum value on courses near north or south, because the disturbing force is then acting at right angles to the compass needle and causes the greatest amount of deflection. 31. Describe clearly the three principal causes of the heeling error on board ship. The three principal causes of the heeling error are vertical induction in transverse iron, induction in iron vertical to the ship's deck, and the vertical component of the sub-permanent magnetism. The part arising from vertical induction in transverse iron is due to the fact that such iron as beams, by departing from the horizontal position, and inclining to the vertical as the ship heels, acquires polarity in its ends by induction from the earth, of the same sign as that of vertical iron in that hemisphere. This polarity tends to draw the north point of the compass to one side or the other. The part arising from induction in iron vertical to the ship's deck is due to the fact that such iron is not vertical to the earth when the ship heels. The amount of magnetism induced therein is less in quantity, but by the poles becoming on one side of the compass, instead of vertically under it, the north point is drawn to one side or the other. The part arising from the vertical component of the ship's sub-permanent magnetism, arises from the fact that although the force remains the same in amount, in all latitudes and both hemispheres it becomes on one side or the other as the ship heels, and so deflects the compass needle. 32. State to which side of the ship in the majority of cases is the north point of the compass drawn when the ship heels over in the northern hemisphere. To the weather or high side, because the higher ends of the beams, and the upper ends of vertical iron which terminate below the compass, and so go towards the high side when the ship heels, have blue magnetism by induction in the northern hemisphere, and draw the north point of the compass to the high side. If the compass is near vertical iron, such as the funnel, having its upper end above the compass, the force to the high side is diminished. 33. Under what conditions (that is, as regards position of ship whilst building, and the arrangement of iron in the ship) is the north point of the compass needle usually drawn to windward or the high side of the ship in the northern hemisphere, and if not allowed for, what effect has it on the assumed position of the ship when she is steering on northerly also on southerly courses in the northern hemisphere? In the northern hemisphere, if a ship is built head to the northward, and the compass is aft, the north point will be drawn downward by sub-permanent magnetism. If the compass is forward this downward force is much diminished. As the force downward in the ship becomes in part a horizontal force towards the high side when the ship heels, continuous athwartship iron would also draw the needle to the high side. In the northern hemisphere the error thus caused will put the vessel to windward of her assumed position on northerly courses, and to leeward on southerly courses. 34. Under what conditions (as in question 33) is the north point of the compass needle usually drawn to leeward or the low side of the ship in the northern hemisphere, and, if not allowed for, what effect would it have on the assumed position of the ship, when she is steering on northerly also on southerly courses, in the northern hemisphere? In the northern hemisphere, if a ship is built with head to the southward, and the compass is aft, the north point of the needle will be pushed upward by sub-permanent magnetism. If the compass is forward, this force is diminished. As an upward force in the ship becomes in part a horizontal force towards the low side when the ship heels, if the compass were placed between divided beams they would also repel it to the low side. In the northern hemisphere the error thus caused, if not allowed for, will put the vessel to windward of her assumed position on southerly courses and to leeward on northerly courses. 35. The effects being as you state, on what courses would you keep away, and on what courses would you keep closer to the wind in the northern hemisphere in order to make good a given compass course (a) when north point of compass is drawn to windward or the high side of ship; and (b) when drawn to leeward or the low side? When the north point of the compass is drawn to windward, keep away on northern courses and closer to the wind on southern courses. When the north point of the compass is drawn to leeward, keep closer to the wind on northern courses and keep away on southern courses. 36. Does the same rule hold good in both hemispheres with regard to the heeling error? Yes. |