. A text book of physics, for the use of students of science and engineering . uld otherwise be troublesome. Passing through the axisof support is the mid line of the ship, indicated by two lines ormarks, one fore and the other aft of the card, thus enabling the bearingof the ship according to the compass to be observed directly. It follows, of course, that in order to know the geographicalbearing of the ship, the declination (or magnetic variation) mustbe added to, or subtracted from, its magnetic bearing. The valueof this at all parts of the world is given on charts supplied by theAdmiralty.
. A text book of physics, for the use of students of science and engineering . uld otherwise be troublesome. Passing through the axisof support is the mid line of the ship, indicated by two lines ormarks, one fore and the other aft of the card, thus enabling the bearingof the ship according to the compass to be observed directly. It follows, of course, that in order to know the geographicalbearing of the ship, the declination (or magnetic variation) mustbe added to, or subtracted from, its magnetic bearing. The valueof this at all parts of the world is given on charts supplied by The magnetic compass has also great importance in the steering Lxm THE MAGNETIC COMPASS 805 of air-craft. An aeroplane compass of the Creagh-Osborne type isshown in Fig. 742. The bowl is of spherical form and is mountedin such a manner as to minimise vibration. The compass card hasseveral steel magnets, and is provided with a vertical mica ring, onwhich the points of the compass are marked in luminous radiumpaint. A window at the back of the bowl enables the pilot to read. Fig. 742.—The Creagh-Osborne aeroplane compass. the course by means of the scale on the compass card. The bowlcontains liquid to buoy up the card, part of which consists of ahollow float. The liquid thus takes part of the weight off theagate point suspension, and also helps to damp the vibrations ofthe card. Ships magnetisation.—Masses of iron or steel situated near thecompass will, of course, affect its reading; and since modern shipsare built almost entirely of these materials, the errors introducedand the corrections necessitated are considerable. These errors are of many kinds and cannot all be considered herein detail, but the three principal errors will now be dealt with. Semicircular deviation.—Most iron ships are permanently mag-netised, thus the ship itself is a large magnet, and as it takes different 806 MAGNETISM AND ELECTRICITY CHAP. courses it will have different effects upon its compass.
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Keywords: ., bookcentury1900, bookdecade1910, bookpublishe, booksubjectphysics
