Camping and woodcraft - Scoutmastercg.com

USE OF THE COMPASS 73of south. At present the agonic line runs fromMackinac Island, in Lake Michigan, loops west andthen diagonally through eastern Michigan to centralOhio, makes a big loop north toward Lake Erie andback, south to the Ohio River, makes two big loopseast and west in eastern Kentucky, runs souththrough western Virginia, loops west in easternTennessee and then far back east, goes down throughwestern North Carolina, loops east again, and thenruns diagonally down through Georgia and out intothe Atlantic. This line is not stationary, but hasa slow movement westward called the ''annualchange." Nobody knows the cause of these vagaries:magnetic variation is a mystery as yet unsolved.Now note this: at all places east of the agonic linethe north end of a compass needle points to the westof true north (more and more as the distance increases),and everywhere west of this line it pointseasterly.For instance, at New York City the compass nowpoints io°W; at Eastport, Me., 20°W; at Lincoln,Neb., io°E; at Helena, Mont., 20°E. This"declination" or "variation of the compass" must beallowed tor when running a true course, or whenplotting one by map.A line passing through all places that show thesame compass variation is called a "line of equalmagnetic variation." Such lines do not by any meansrun straight like meridians, but are wavy and loopedand run off at strange angles, like the agonic line,though none of them correspond to its meanders;and they, too, shift slowly westward from year toyear.Now for the practical application. Suppose youare on the line of magnetic variation that runsthrough Ogden, Utah, where the declination Isi8E°. To find true north, you set your compassso that the needle points i8°E, as In Fig. 10. Thenthe N mark on the dial points due north.To lay out a course by map ; spread the sheet out