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32 the. result is: the compass course, and, similarly if we apply the deviation to a compass course the result is the correct magnetic course. The two rules of ap- plication, are of course, the reverse of | each other. Af there 18: no deviation the correct magnetic course is also the compass course, and vice versa, the compass course can also be the correct magnetic course, If there is neither variation nor devia- tion the true course is likewise the com- pass course, and vice versa. A few examples will explain the rules of applying variation and deviation from true course to compass course, and com- pass course to true course: os The true course is N 50° E, variation 6° westerly, deviation 12° easterly, what is the compass course? True course ...... N 50°°R Variation Wly. | Correct magnetic coOurse........ 56? i Deviation Ely. (sub.) 12° (allow to the left.) Compass course .. N 44° E From compass course to true course: The compass course is N 44° E, devia- tion 12° easterly, variation 6° westerly, what is the true course corresponding to the above compass course? = Compass course.. N 44° E Deviation Ely, (add) 12° (allow to "he right.) Correct magnetic PeCOUISE oy he. N 56° E Variation Wly. (sub.) 6° (allow to the left. ) True course Sees N 50° E Supposing that you were steering N 44° E with the above variation and. devi- ation and you took the bearing of a light- house by compass, and say it was S 12° E, what would be its Se eons true bearing? _Compass bearing... S12° FE Deviation Ely, (sub.) 12° (allow to the right.) Correct Magnetic Wearing: see. eae S028 = South.< Variation Wly. (add) 6° (allow to the left.) EUG DEATING. 24.000, S362 fe: 5S OE, Note.--In bearings of this kind always remember that the deviation to be ap- plied is due to the direction of the ship's head at the time the bearing was ob- served, and not the deviation for the point of bearing. : When the variation and deviation are 'of the same amount, but unlike in name (one counteracting the other) the true course is likewise the compass course. For example: The true course is NE, variation us point easterly, deviation Y% point wester- ly, what is the compass course? An- swer NE, the same as the true course started with. - Variation is measured Com the true meridian to the magnetic meridian, When the variation is applied to the true course the result is the correct magnetic course, hence their difference is the vari- ation. Deviation is measured from the mag- (add) 6° (allow to the right.) ° TAE Marine REVIEW netic-meridian.to the line passing through the north and south points of the com- pass. card. When the deviation is ap- plied to the correct magnetic course the result. is the compass course, hence, their difference is the deviation, Variation. is the same for all directions of the ship's head in the same locality. This is because the variation is due to the earth's magnetism which controls the needle, and since the compass needle points to the north magnetic pole, and this pole has practically the same relative bearing for any one position on the earth, it makes no difference which course we may desire to steer on from this one position the variation must remain the same. Therefore, the variation is out- side of the ship's hull and is independent of the deflection of the compass needle known as deviation. Deviation of the compass is the angle through which north and south on the compass card is deflected from its natural magnetic position by the disturbing force of iron near it. Every ship is a magnet having magnetic poles of precisely the same character (only greater) as the compass needle, and also of the same character as the magnetic poles of the earth's magnetism, that is a north mag- netic pole and a south magnetic pole. The ship's magnetic poles are located some- where within the hull, depending on the magnetic latitude in which she was: built, and. the direction of her. head while building. - Deviation is due entirely to the mag- netism of the ship itself, and is therefore independent. of all influences' outside of the ship such as the variation and. local attraction, The deviation unlike the variation, is of a different amount on nearly all courses of the compass and is of an op- posite name in reverse semi-circles of the compass: that is to say, if the natural total deviation on NE is easterly the natural total deviation on S W would be westerly; one end of the compass needle being attracted in the one case and the opposite end in the other case. The rea- son that the' deviation is of a different amount on different courses is due to the fact that the ship's magnetism (the mag- netic poles) changes its line of direction with respect to the compass needle (which is supposed to remain stationary point- ing always to the north magnetic pole of the earth) at every change of the ship's head. When the magnetic poles of the ship's magnetism are in the same line with the compass needle's length the de- viation will be least since the magnetism is pulling in the same direction as the needle is pointing, the tendency being to hold it in its natural position. But when the ship is heading in such a direction that the poles of her magnetism are at right angles to the needle's length, the deviation, or deflection will be greatest. The corrections of the 'compass (not compass errors) are those quantities which must be applied to the indica- tions of that instrument to obtain the reading that would be given it if the north point of the compass card always corresponded to the true north point of the horizon. These quantities are the variation and deviation. They are cor- rections and not errors of the compass, since they are not faults of the compass but are due to well defined laws ¢ of nature. They are just what any good navigator would expect his compass to indicate when brought under the influ- ences that cause them. _ The idea is to find the course to be steered by compass in order to make good the true course, which is the angle the real track of the ship makes with the true meridian. 'We know that the com- pass does not indicate true directions, but on the contrary is deflected from this meridian by the influences known as variation and deviation. -Therefore, when the cofrections for these quantities are known and applied to the compass we "get the direction that would correspond to the true direction were the compass always to point true instead of otherwise. The same thing is true when we apply these quantities to what we know the true directions to be in the first place, and while these directions according to the compass will be of a different name. than the true course started with, it is nothing more than the true course, be- cause in order to find out where the ship is, when out of sight of land, her real course must be measured from the true meridian. : The "true" meridian is in the -direc- tion that the imaginary needle of the true compass points. We 'learn that all magnetic bodies have north and south magnetic poles and that unlike magnetic poles attract each other and like poles repel. each other. This being the case the compass needle being a magnet it must have a north and south magnetic pole arid so it. has, likewise the earth which is a huge mag- net has a north and south magnetic pole. The magnetic poles of the earth are what cause the compass needle to point to them, and the ship's magnetic poles are what cause the compass needle to point to them and away from the magnetic poles of the earth. The deflection of the compass needle caused by the ship's mag- netic. poles is called deviation 'and the deviation is measured from the magnetic meridian. To Determine the Deviation when on with a Range--To the true bearing of the range apply the local variation, so as to make a correct magnetic bearing of it. The difference then between what