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same as what you know the c. m. b. to be; in this case, EIN. Bear in mind that the Dev. just found is good only for that one course, and as soon as the vessel's head is changed to another course the deviation will be dif- ferent in amount and probably different in name, We will now go on and show in a sim- ple and practical way how a full table of deviation for an entire swing of the compass can be determined from the eight principal points of . the compass right in the harbor of Milwaukee. After getting clear and outside of the piers turn your boat around and head in for the center of the piers, and be careful to be in line with them. Now, if your compass is correct it will read W14S, the reverse of EN, or the true . bearing is W and the Var. %4-pt. Ely, allowed to the left gives us W%4S. But supposing that when you get your boat lined up with the pier the compass says W by N; how much is the Dev. which way is the compass card drawn? (It is the direction of the swing of the card that gives us the name of the de- viation). Corr. Mag. bearing W14S=734 pts. r. of s. Comp. bear'g WxN (+)=7. pts.lofn. 14% 16 Dewiation 22... 1% pts. Wly. Because the c. m. bearing (W14S) is to the left of the compass bearing (W by N), the difference between the two being 114 points. Try this and prove it on the portable card. The southerly end of the Milwaukee breakwater runs true N by E and S by W (see lighthouse book) Var. the same © as before (%4-pt. Ely). Either head on and in line with the breakwater, or lay alongside of it, and parallel thereto (the stem and stern timbers the same distance from the outside facing of the break- water). If your compass is correct how had it ought to read both ways on the breakwater? True bearing NxE= 1 pt. r. of 1. Var 32° 04 Ely. = 4 ot 1. Corr. Mag. bearing 3% pt. r. of n, or N34E, ; The reverse of N34E is 'sw two correct magnetic directions oe com- pass ought to read if correct. But sup- posing that when you head on it to the -- 'north that your compass reads NYE and to the south'ard S by W. How much is the Dev. in each case and which way t& it? - Corr. Mag. bearing N34 Compass. bearing NYZE=%pt.r.ofn. % pt. Ely. : Deviation ..... Corr, Mag. bearing mA and . E=3 ptr. of n. = % pt.r. of s. 'TAE Marine. REVIEW . Compass bearing S by W=1 pt.r.ofs. Deviation -- 4 pt. Wly. Thus, the Dev. on c. m. N34E is % pt. Ely.; and the Dev. on c. m. S34W is 4 pt. Wly. Remember that the Dev. is always the difference between the c. m. bearing and the comp. bearing. The lighthouse on the end oi the breakwater ranges with the one on the end of the north pier true NE%E and SWY4W (see lighthouse book for this information), what are the correct mag- netic bearings of this range both ways? , 'True bearing NEYWE=4y pts. r.ofn. eee eee Vat.3° of) By. = y pts. 1. Corr. Mag. bearing 4pts. r. of n; or NE. The reverse of this is SW. Stipposing that you were coming in from outside and you had these two lighthouses in range (say when you were 'inside of North Pt.); when steady and directly on the range your compass says SW14W how much is the Dev. and which way is it? Corr. Mag. bearing SW =, pts. r.. of s. Comp. bear. SWY44W =--4% pts. r. of s. Deviation 1% pt. Wly. Now turn your boat around and head out getting these two lighthouses direct- ly over the stern (line up with the spars, or smokestack) and when you are steady on it your compass says NE34N; how much is the deviation and which way is ite Corr. Mag. bearing NE=4 pts.r. ofn. Comp. bearing NE34 N= 3% pts.rofn. Deviation 34 pts. Ely. Norte.--Be sure to keep a memorandum of the deviations for. the points already found; and bear in mind that it is only good for those points on which it was found. The black can buoy: off Milwaukee ~ South Pt. and the Milwaukee pierhead lighthouse ranges true NW%, N, and SES, Var. 3.4. Ely, what are the cor- responding correct magnetic courses? True bearing NW36N = 3% opts. 1. of n. Nato 40 = \&% pt.-1. Corr. Mag. bearing ae pis: of 4, NWKW. _ The reverse of this is SEE. The forcaode correct, magnetic bearings would be the readings of the compass on this range both ways if it were right. Supposing that in the one case it read NWN, and in the other case SESZE how much and which way is the Dey. in both cases? : Note.--In cases like the above place your boat directly between the two 'ob- jects, and about midway between them. Corr. Mag. bearing NW%W=4% pts. born. _named conditions, tions of intermediate points. 29 Compass hearing NW54N = 356 pts. LO -- Deviation 34 pt. Wily. Corr. Mag b. of range SERE= 4% pts. 1s 61s: Compass b. of range SEE (=45% pts, 1, OLS, Deviation Y pts. Ely. Thus, we have the Dev. for the 8 prin- cipal points of the compass, or enough to these points so that we,can determine the Dev. on them. Remember that in each 'case the Dev. is for the cor- rect magnetic points of the compass, and not for the compass points or true points, but the correct magnetic points; that is, when the vessel heads SESE, as in the above case she is really heading correct magnetic SE'%E, but. the compass needle carrying the card is drawn to the right hand or Ely of the magnetic meridian TICAL «* by the attraction of the ship's iron, which -- is the cause of the deviation. Now, to find from these deviations the Dev. corresponding to the remaining in- termediate points of the compass: Note.--When the deviations are not large (as in the foregoing examples) and increase and decrease regularly, as' they do above, and are taken for about equi- _ distant points, we can calculate the de- viation between these points by taking the mean of the deviations on two given points to find it on the point lying mid- way between them. This process is not precisely accurate only under the above- and that is, that the deviation is small and regular; but in a good many cases will be sufficiently cor- rect to. answer in practice, However, we give it here more to show what can be done with the limited means at hand, as found on some vessels for finding this compass correction; and also to lead the _ Student step by step to more important and necessary steps in the observations for compass deviations. In practice these luke-warm conditions are not frequently met with, hence the necessity of other and more accurate methods. When the deviations are large and in- crease and decrease irregularly, they are arrived at by a different method and pro-. cess, Now, to go on with finding the devia- The first thing to be done is to rule and make a blank. form like the following : | _ DEVIATION TABLE BLANK, Gree ee Oe ea ' Compass Ship's ce Ship's oo. Head Deviation Head Deviation Correct in Points. Correct in Points, '