. Elements of plane and spherical trigonometry . are markedN. and S., corresponding to the north and south poles, or ends, of themagnetized bar. The diameter W. E., at right angles to the diam,eterN. S., point out the west and east points ;these four are called the cardinal points,and the others are marked as in the sub-joined diagram. Thus one point from the north towardsthe east is north by east; two points, north,north east; three points, north-east by north; jand so on. (See the table of Rhumbs atthe end, p. 300.) The card thus furnished being now sap-pended horizontally, and so as to allo
. Elements of plane and spherical trigonometry . are markedN. and S., corresponding to the north and south poles, or ends, of themagnetized bar. The diameter W. E., at right angles to the diam,eterN. S., point out the west and east points ;these four are called the cardinal points,and the others are marked as in the sub-joined diagram. Thus one point from the north towardsthe east is north by east; two points, north,north east; three points, north-east by north; jand so on. (See the table of Rhumbs atthe end, p. 300.) The card thus furnished being now sap-pended horizontally, and so as to allow theneedle to settle itself freely, will point outthe four cardinal points of the horizon, asalso the several intermediate points, provided only that it is the propertyof the magnetic needle to point due north and south. Such, however,is not strictly the case, as the needle is found from accurate observa-tions, to deviate from this position, and at some places very consider-ably, and this deviation is itself subject to variation. But the true. OS PLANE AND SPHERICAL TRIGONOMETRY. direction of the compass, or the angle it makes at any place with a linepointing duly north and south, maybe ascertained at any time by astro-nomical observations, and thus the deviation of the compass-points,from the corresponding points of the horizon, may always be found andallowed for. The compass is so placed on ship-board that the vertical p)lane,cutting the ship from stem to stern, may pass through the centre of thecard, so that that point of the compass which is directed to the shipshead shows the coynpass-course, and the proper correction for variationbeing applied the true course will be obtained. 11. A ships rate of sailing is determined by means of an instrument,called the Log, and an attached line called the log-line. The log is apiece of wood forming the sector of a circle, audits rim is so loadedwith lead that when heaved into the sea it assumes a vertical position,with its centre barel
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Keywords: ., bookcentury1800, booksubjectnavigation, booksubjecttrigonometry
