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The deflection of the magnetic needle from the true North, or, as it is usually called, the variation of the compass, is found at sea either by computing the true bearing of the sun from an observed altitude, the compass bearing being noted at the time of the observation; or, without taking an altitude, determining the true bearing of the sun when in the horizon, its compass bearing being observed at the same instant. The difference between the compass bearing and true bearing thus found is the variation of the
Sometimes it is necessary to correct the variation of the compass determined as above for the deviation of the compass itself, arising from the following local causes. The iron on board draws the needle to the east or west of the magnetic meridian, and this effect is greater or less on the needle according as the iron is distributed more or less unequally on different sides of the magnetic meridian. The deviation of the compass due to this cause is discovered, previously to the ship going to sea by swinging her round and noting the deflection of the needle from the magnetic
meridian on different points; a table is then formed similar to the one in p. 244, from which the correction of the compass for different positions of the ship's head may be readily found (see p. 16). The method of determining whether the variation of the compass is east or west will be best seen by means of the following examples.
1. Suppose the true bearing of the sun was found by observation to be N. 100° 10' E., when the compass bearing was N. 90° 42′ E.; required the variation of the compass, the ship's head being N.E.
Let N represent the true north point of the horizon, and Ns the true meridian, measure (roughly) 100° 10′ from north towards east as the angle NZT; then T represents the place of the sun when the observation was taken. From T measure back towards N the compass bearing 90° 42', as TZ c; then z c is the direction of the magnetic needle, and the angle NZC is the variation of the compass, which is evidently easterly, since the compass north is to the east of the true north: hence in this example the variation is said to be east: thus,
Now if the iron on board had no effect on the needle, this would be the true variation, but referring to the table it appears that the needle itself is drawn or deflected 10° to the east, in consequence of the disturbing effects of the
iron when the direction of the ship's head is N.E.: placing the 10° 0' E., under the above 9° 28' E., and subtracting we have the true variation of the compass corrected for local deviation: thus,
2. The true bearing of the sun was found by observation to be S. 60° 42′ E., when the compass bearing was S. 50° 10' E.: required the variation
of the compass, the ship's head being N.E.
Let N z s represent as before the true meridian, draw z T, making the angle sz T, equal to 60° 42' (roughly), then T represents the true place of the sun; from T measure back towards s, the compass bearing Tz c', equal 50° 10', then
z c' is the direction of the magnetic needle, and the angle s zc or N z c is the observed variation of the compass, to be corrected for deviation (if any).
For by the table it appears that the needle, by the effects of the iron, is drawn 10° to the eastward; if there had been no iron on board the needle would have been directed 10° to the westward of its observed place. Hence may be deduced the following rule to find the variation of the compass.
DEVIATION OF THE COMPASS OF H.M.S.
(Caused by the local attraction of the Ship) for given positions of the
Given the true bearing and compass bearing and deviation, to find the variation of the compass.
1. Reckon the compass bearing and the true bearing from the same point, north or south.
2. Take the difference of the two bearings when measured towards the same point, but the sum when measured towards different points; the result is the apparent variation of the compass; east when the true bearing is to the right of the compass bearing, west if the true bearing is to the left of the compass; the observer being supposed to be placed in the centre of the compass, and looking towards the heavenly body.
NOTE. The name of the variation, whether east or west, may also be readily found by making a figure similar to those in the preceding examples.
3. If there be no deviation to be allowed for local attraction, the above is the true variation.
4. To correct for local deviation (if any). Under the apparent variation just found, put the correction from the table of deviation, with the opposite letter to that given in the table.
5. When the names put down are alike add, putting the common letter to the result: if the names put down be unlike, subtract the less from the greater, putting to the remainder the name of the greater. The result will be the variation of the compass corrected for deviation, and therefore the true variation.
The true bearing of the sun is N. 117° 32′ E., and compass bearing S. 71° 10' E.: required the true variation. The ship's head being S.b.E., and therefore the deviation.