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For Second Mate.
3. In latitude 54°, the departure made was 60m.; required the difference of longitude by parallel sailing. 4. Sept. 23, 1854 ; long. 80° 15'E.; the observed meridian
. altitude o 57° 20' 30"; observer S. of the sun ; index error +1' 20"; cye 21 feet; required the latitude.
Additional For Only Mate. 5. Determine by Mercator's sailing, the course and distance from Halifax, lat. 44° 39'N.; long. 63° 37'W; to Cape Town, lat 33° 56'S.; long. 18° 28'E.
6. Dec. 16, 1854, at 7h 24" 288 P.M. apparent time at ship; lat. 40 4'S. ; long. 126° 7'E. ; the sun's setting amplitude was observed to be W. 27° 30' 30"S. ; required the variation of the compass. 7. Dec. 16, 1854; required the A.M. and P.m. tides at Shields.
Additional for Chief Mate. 8. Dec. 16, 1854, at 8h 2" A.M. mean time at ship; latitude 48°58'S.; long. 149° 30'W.; the observed altitude o 37° 4' 30"; bearing by azimuth compass N. 74° oʻE. ; index error 0”; eye 17 feet; determine the variation of the
compass. 9. Dec. 15, 1854, A.M. at ship; lat. 30° 49'N.; the observed altitude o 17° 49'0"; index error + 34"; eye 18 feet; time by chronometer 15d 5h 40m 44', which had been found 46" 10% slow on mean time at Greenwich July 13, and was gaining 4:68 daily; determine the longitude.
10. Dec. 16, 1854, A.M. at ship; lat. by account 33° 48'N.; long. 25° 19'E.; the observed altitude o near the meridian 32° 30' 40'S.; index error +22"; eye 17 feet; time by watch 160 ob 2" 54', which had been found 22 22 fast on apparent time at ship, but the ship had made 11 diff. of long to the E. since the error for apparent time had been determined ; required the true latitude.
Additional for Master Ordinary 11. Correct the following courses for local attraction, as given at page 35, E. į S.-N.E. E.-N. by E. } E —N.N.W:
pl 12. Dec. 16, 1854 ; long. 147°W.; the observed meridian altitute of Aldebaran, (a Tauri, being 50° 13' 13'; the zenith N. of the star; index error — 1'17''; eye 18 feet; required the latitude.
DEVIATION OF THE COMPASS.
LOCAL ATTRACTION is a term used to denote the influence of iron in disturbing the direction of the magnetic needle, whereby according to the extent and position of that metal in respect to the compass, a greater or less amount of deviation from the magnetic meridian is the result. This derangement of the compass, some 30 years ago scarcely noticed, must have been the source of numberless accidents to vessels, often accompanied by a great sacrifice of human life ; for since that period, the careful researches of scientific observers have proved that it would be extremely hazardous to allow the ships of H. M. navy, steamers, or iron-built vessels to proceed to sea, without the amount of errors arising from the local attraction being previously determined; it may suffice to mention, in illustration, that in some screw-steamers the deviation with the ships's head South has amounted in the binnacle to more than fifteen points.
It must not be supposed that the compasses on board merchant vessels, are wholly free from the effect of local attraction: under ordinary circumstances, with no undue proportion of iron in the ship, the deviation may not be so great as to produce any serious error on the courses made, but the case is materially altered when the cargo consists in part, or wholly, of iron; or in fact, when any quantity of that metal is placed in the vicinity of the binnacle.
The amount of deviation on a given point of the compass is by no means a constant quantity, but it differs in different
vessels, each having a local attraction peculiarly its own : it also varies with change of position on the globe; in north magnetic latitude the north pole is attracted, in south magnetic latitude, the south pole,-increasing or decreasing as the dip of the needle increases or decreases : nor is it by any means the same in all parts of the same ship, and it alter from circumstances connected with the vessel itself, or the needle,—or even both. It is therefore necessary that masters should be able to ascertain, as well as know how to apply, the errors arising from the local magnetic disturbance : for this purpose two methods may be adopted, the first being the most approved.
Method 1.– With Two Compasses. The ship must be placed in such a position that she may be gradually swung, and the two compasses being compared together to note their agreement, let one of them be placed in the binnacle in its usual position, and the other taken on shore beyond the influence of the attractive force; adopt such means that a good bearing of each may be taken, and as the ship’s head is brought to each point in sucession of the compass on board, at that instant let observations be made, thus,—the person on shore must take the bearing of the compass in the binnacle, and the person on board must take the bearing of the compass on shore, proceeding in this manner through the 32 points ; these bearings must be tabulated after the following method :
of Ship’s Head.
Bearing of Shore Bearing of Compass
Compass, from on Board, from
bearings, or Deviation.
The difference between the bearings will be the amount of deviation due to the local attraction of the ship, and is named East, when the north point of the needle is drawn to the eastward or right hand,—West, when it is drawn to the westward or left hand; and must be applied to the ship's courses in the same manner as the variation of the compass.
Method 2.-With One Compass.—Having determined the true bearing of a conveniently distant object, let the ship be carefully swung to each point of the compass, and on each occassion let the bearing of the object be taken ; the difference
. between the true and observed bearing will be the error of the compass, to be named and applied as in the former case.
From the annexed table correct the courses given in the previous papers.
E. by S.
W. by N.
S.E. by E.
N.W. by W.
S.E. by S.
0 20 E.
N.W. by N.
0 56 W.
1 40 W.
S. by E.
N. by W.
1 10 E.
THE ADJUSTMENTS OF THE SEXTANT.
1.-The Index Glass should be perpendicular to the plane of the Instrument. To determine if it be so, bring the vernier to the middle of the arc, and with the limb turned from the observer, look obliquely into the mirror, then if the reflected and true arcs appear as one continued arc of a circle, the index glass is in perfect adjustment.
2.—The Horizon Glass should be perpendicular to the plane of the Instrument. With 0 on the vernier coinciding with o on the arc, hold the sextant horizontally, and looking at the horizon, observe if the reflected and true horizons are in one line: or, the instrument being held perpendicularly, look at any convenient object, as the sun, sweep the index glass, along the limb, and if the reflected image pass exactly over the direct image without any lateral projection, the horizon glass is perpendicular.
3.-The Horizon Glass should be parallel with the Index Glass, when 0 on the vernier exactly coincides with 0 on the arc. To ascertain this, hold the instrument vertically, and direct the sight through the telescope or sight vane, to the horizon, and if the reflected and true horizons form one continuous line, the horizon glass is parallel with the index glass.
4.-To adjust the Line of Collimation, or to set the axis of the telescope parallel to the plane of the sextant. Fix the telescope in its place, taking care that two wires are parallel with the plane of the instrument; select two objects, as the sun
ti and moon, or moon and star, which are more than 90° distant from each other, bring them into contact on the wire nearest to the instrument; then by slightly moving the sextant, see how they appear on the other wire; if they are still in contact, the Line of Collimation is in adjustment ; but if the bodies