Imágenes de páginas

chief forces may be explained as follows : Let B B’ (Fig. 5) represent a beam when the vessel's head is at N. or S. Being at right angles to the meridian it has no disturbing effect, but when the beam is inclined one way or the

[ocr errors][merged small][merged small][merged small][merged small][merged small][merged small]


Northern Hemisphere

Fig. 5 Showing upper and lower ends of beams in Northern Hemisphere other the upper ends at C become blue and attract the N. end of the needle towards the high side in the Northern Hemisphere and repel the N. end of the needle towards the low side in the Southern Hemisphere where the upper ends at C are red (see Fig. 6).



Red 172/22T

[blocks in formation]

Southern Hemisphere

Fig. 6. Showing upper and lower ends of beams in Southern Hemisphere Vertical Iron.–Vertical iron that was under the binnacle when the vessel was upright has no disturbing power, but when the vessel heels the relative positions of compass needle and the vertical iron are altered and a new horizontal force is developed whose action depends upon the colour of the pole nearest the compass.

The induced magnetism in vertical iron varies as the tangent of the dip, and is, therefore, greatest at the magnetic poles and nil on the magnetic equator. In the Northern Magnetic Hemisphere the upper ends of vertical iron are blue and the lower ends are red, therefore when the ship heels in the Northern Magnetic Hemisphere the blue ends of vertical iron below the compass coming out on the high side will draw the needle to the high side.

On the magnetic equator the vertical iron is blue on the South side and red on the North side, and the effect on the compass is nil.

In the Southern Magnetic Hemisphere the upper ends of vertical iron are red and the lower ends blue. Therefore when the ship heels in the Southern Magnetic Hemisphere the red ends of vertical iron below the compass coming out on the high side will repel the north end of the needle to the low side. Induction in vertical iron causes semi-circular deviation of a different name in each hemisphere.

The disturbance caused by the beams B B', (Figs. 5 and 6) is largely compensated automatically for all positions by the spheres used in the D correction. The beams B B' cause a + D, the spheres a -D; these two forces vary together both in the upright and inclined positions. The heeling error from vertical iron is compensated by a permanent magnet. As induced magnetism in vertical iron is a N. P. M. force this is a contradiction in compensation; it is the only case where a P. M. magnet is used to correct

a N. P. M. disturbance. To be consistent a soft iron corrector should be placed vertically above the compass, but this would be so inconvenient that it is never done ; instead, a permanent magnet is used, which must be moved up or down as the dip increases or decreases. On the magnetic equator where the N. P. M. force reaches zero the heeling error is caused by the vertical component of sub-permanent magnetism only, and when the ship proceeds into the Southern Hemisphere if the magnetic induction in vertical iron exceed that from sub-permanent magnetism the correcting vertical magnet would have to be placed with the other end up.

Heeling.--The adjustment for the heeling error is made either by heeling the vessel, or by the dipping needle.

The vessel is placed heading N. or S. by compass, then heeled about 10 degrees. If the north end of the compass needle wanders towards the high side of the vessel there is a blue vertical force beneath the compass. If the north end of the compass needle wanders towards the low side there is a red vertical force beneath the compass.

The vertical magnet beneath the compass is moved up or down so as to counteract the vertical disturbance, red end up if the north end goes to windward and blue end up if the north end goes to leeward.

With the Dipping Needle.—The dipping needle (see p.507) is adjusted on shore. It is placed in the magnetic meridian with its north end towards the N. pole; a small sliding weight in a slot at the other (blue) end is moved along until the needle is horizontal. It is then taken on board, the vessel is placed with her head E. or W. magnetic, the compass card is unshipped, and the dipping needle fixed vertically on the pivot, the north end, as before, towards the N. magnetic pole. The unweighted end dipping downwards indicates a blue force bencath the compass; if it is repelled upward a red force is indicated.

This applies to the Northern Hemisphere. For the Southern Hemisphere the dipping needle would have the red end of the needle weighted.

The adjustment is made by means of a vertical magnet the same as when heeling

If the north end of the needle dips, make it horizontal by placing a magnet vertically under the centre of the compass, red end uppermost ; if the north end is repelled upwards the above-mentioned magnet must be placed with its blue end uppermost.

This adjustment holds good only for places of the same magnetic dip; for greater dip the magnet should be moved up; for lesser, down.

The heeling error can also be corrected by the method of vertical vibration carried out in a similar manner to that explained in finding coefficient 1. (Lambda).

Retained Magnetism.-So long as there is any vibration in the hull of the vessel there is an ebb or flow of magnetism more or less temporary in nature, because the vessel never stays in any one fixed position long enough to secure permanency. If a vessel is laid up in dock for a long time with her head in one direction, or for a lesser time with winches going, or most commonly when she has been kept on one course for a long time, a red polarity is developed in that part of the vessel directed towards the north magnetic ! pole. This polarity decreases the attractive power of the earth’s magnetic pole and causes the compass to become less and less sensitive while continuing on the meridian course. When the course is changed the red

[ocr errors]

polarity will have at first a strong repelling effect, which rapidly diminishes n force and eventually disappears.

Retained magnetism always has the effect of carrying the vessel towards the last course steered. The deviation should be obtained directly the course is changed. There is no possible compensation for this error.

The great object in compensating a compass is to entirely eliminate the magnetism of the ship by introducing co-efficients of the same amount and of opposite names by means of magnets and soft iron correctors.

For detailed information read the “Elementary Manual,” edition revised by Captain Craik, R.N., etc., and Towson's Deviation.



1. State briefly the essentials of an efficient compass.

It is essential that the card should have the greatest possible magnetic power in its needles, combined with the smallest possible weight in the whole card. The jewelled cap should be sound—that is, not worn nor cracked, and the pivot sharp and free from rust. If the card is placed on the pivot and deflected through a small angle from its position of rest, it should always come back exactly to the same point. The card should be accurately divided and centred, and the point of the pivot should be in the same plane as the gimbals of the bowl.

Standard compasses must be furnished with the means of taking the bearing of an object at any elevation, and of reading it off within a degree. When a compass is placed on board ship, the lubber line should be vertical and exactly in the fore-and-ast line from the centre of the card, and the bowl should swing freely in its gimbals. It is desirable that all compass bowls should be made of pure copper.

2. State briefly the chief points to be considered when selecting a position for your compass on board ship, and what should be particularly guarded against.

The standard compass should be in the midship line, in a convenient position for constant watching by the officer of the watch, and for comparison with the steering compass, and should have a clear view all round for taking bearings. It should not be less than five feet from iron of any kind, and the proximity of vertical iron, and of iron which is liable to be changed in position, such as davits, derricks, ventilating cowls, &c., should be particularly guarded against. Where electric lighting is used, the position of the dynamo has also to be considered, as it may disturb a compass at the distance of fifty or sixty feet.

Steering compasses must be so placed that the card can be clearly seen hy the helmsman, and should be as far from any iron as circumstances will allow.

3. What do you mean by deviation of the compass, and how is it caused ?

The deviation of the compass is the angle between the magnetic meridian and the direction of the compass needle, and is caused by the iron of the ship, whether used in her construction, in her equipment, or in her as cargo.

4. Describe how you would determine the deviation of your compass: (1) by reciprocal bearings ; (2) by figures on the dock walls ; (3) by bearings of a distant object ; (4) by the bearings of the sun or other celestial body. To determine the deviation of the compass by reciprocal bearings, a

compass should be placed on shore where there is no iron in the vicinity, and where it can be conveniently seen from the standard compass. the ship is swung round, observe the bearing of the shore compass from the standard compass when the ship's head is steady on each point; and, by signal, have the bearings of the standard compass taken by the shore compass at the same instant. The difference between the bearing taken on board and the opposite of the shore bearing is the deviation on the respective points.

By figures on the dock wall. Where they are available, as at Liverpool, the difference between the bearing of the object in the background and the bearing marked on the wall exactly in line with the object is the error for any point the ship's head may be on. Apply the variation and get the deviation.

[ocr errors]

By bearing of distant object. When the magnetic bearing is known, the difference between it and the bearing observed by the standard compass with head on any point is the deviation. When the magnetic bearing is not known it may be taken from the chart, or it may be found by getting the difference of bearing between it and the sun, and applying that difference to the sun's true bearing, computed or found from Azimuth tables, and allowing the variation; the magnetic bearing can be found and thence the deviation.

By bearings of the sun or other celestial object. The exact time must be noted and the bearing of the object taken when the ship's head is on each point. The true bearing of the object at each observation can be computed or found by Azimuth tables. By applying the variation with its proper sign to the true bearing, the magnetic bearing is found. The difference between the magnetic bearing and the compass bearing is the deviation on each point.

5. 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 ?

If the correct magnetic bearing be to the right of the bearing by the compass on board, the deviation is Easterly; if to the left, Westerly.

6. Why is it necessary, in order to a scertain the deviations, to bring the ship's head in more than one direction ?

Because the deviation changes in amount, as well as in the direction in which it is to be applied, being easterly on some points and westerly on others, when the direction of the ship’s head is changed.

7. For accuracy, what is the least number of points to which the ship’s head should be brought for constructing a curve or table of deviations.

For an accurate table the deviation must be obtained with the ship’s head on the four cardinal points, and on the four quadrantal points,

8. How would you find the deviation when sailing along a well-known coast ?

By taking the compass bearing of any two objects which are marked on the chart, when they are in line. The magnetic bearing can be obtained from the chart, and the difference between it and the compass bearing is the deviation.

« AnteriorContinuar »