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30 cated by compass, a mistake of nearly 4 degrees would be made in the course. TO GET IT ON THE EVEN POINTS. What we then desire is a table of de- viations corresponding to the even points of the compass with the ship's- head ,correct magnetic instead of the ship's head by compass. This cannot be done by the process of Pelorus set to the correct magnetic bearing of the range, for the reason that it is imprac- ticable to place the ship's head exactly on a full point by Pelorus, when the ship crosses the range for the simple reason that we do not know on which point it will come; but it can easily be determined by means of the following simple graphic method: A SIMPLE GRAPHIC METHOD. On a sheet of paper draw a vertical line down the center to represent the correct magnetic north and south points of the compass. Next adopt a scale of equal parts, say 1-16 of an inch to a degree (use a common foot rule); take the dividers and prick off 114 six- teenths, to represent the distance between pouts, aud mark {his off-on, the. ver-.| tical line. By means of the parallel rulers draw horizontal lines through each one of these marks that stand for compass points, at exactly right angles to the vertical line. After you have di- vided and ruled these various lines, write in rotation the points of the com- pass. Then with the number of. six- teenths of an inch that agrees with the |---- number of degrees that the ship's head was from the full point next to it, or from the nearest cardinal point (see Fig. 36 for full information), prick off with dividers each correct' magnetic direction of ship's head on which an observation was made. HOW IT IS DONE, Having determined these stations by pricking them off on the vertical line, draw horizontal lines through them in the same manner that you did the full points. Remember that these stations represent correct magnetic points of the compass. Next take from the Table of Dev. the amount of the Dev. corres- ponding to these various directions of the ship's head correct magnetic, and lay it off on each one of these horizontal lines, according to its amount and name--to the right of the vertical line if the Dey. is Ely., to the left if the Dev. is Wly. Thus, for example, take Fig. 36, which is an illustration of this process, and its data is the deviation obser#ations taken off Chicago. The first correct magnetic course, acording to this table, is N. 12 degrees W., the Dev. corresponding to it being 12 de- grees Wly. Take between the dividers 12 sixteenths of an inch, and prick it THE Marine. REVIEW off on the vertical line from North, (upwards, because it is west of north), and draw a horizontal line through it; then with this same extent between the dividers lay off 12 sixteenths of an inch to represent the number of degrees of deviation for this heading; lay it off to the left because it is Wly. The next observation is for N. 4 degrees E. and the deviation corresponding to it is 7 degrees Wly. Take between the points of the dividers 4-16ths and lay it off from North toward East (downwards) and draw a horizontal line through it; next take 7-l6ths between the dividers and lay this off on the horizontal line to the left of the vertical line because the Dev. is Wly. The next correct mag- netic course on which an observation was taken is N. 21 degrees E., and the Dev. corresponding to it is 2 degrees Wly., which lay off in the same man- ner as already explained, 'and do the oe Li W le rth } WEP E go ee eee £4 ese ee Te o 94% |X E. a. ease ee nares we ee a an ee ee HS ZCE LH oe ct ee eww ee me nel re a a ok pee ere DELE| §° \ °, exn| 72%? Re ee nee fae fast] : «xh ee SR th FIG. 30; sane with each of the other correct magnetic courses and the proper Dev. corresponding thereto. Remember that Ely deviations are laid off to the right of the vertical line. After completing the work of the Dev. -draw a curve freely through the points marking these distances. 'Then to determine the Dev. corresponding to each full point merely: measure from the full point to the curve on the horizontal Jine drawn through the full point, and the measure of sixteenths of an inch will give the number of degrees of deviation corres- ponding thereto,. Thus, in Fig. « 36, from North to the curve on the horizon- tal line is 844 sixteenths, or 814 degrees; on N. by E., 434 sixteenths, or 434 de- grees, and so on. The Dev. as found for each full point of the compass should be tabulated as follows: Ship's head correct Deviation. magnetic. NOL as Che ees 814° Wily. Ne by Ee A II eo ee ae og WE, By Ne ee 2%4° Ely No ea oe NE bya Boe 2s 38? : BIN Sn a eras 1 ae Bo oy Ny 62 iy Bast A a a ek, 14%4° etc. CLC, We now have the deviations corres- ponding to the ship's head correct mag- netic for each even point, which com- pletes the work. To determine the Dev. for the halves and quarters lying between the full points is performed in the same way as for the whole point. A deviation curve is'a handy and con- venient device for handling this perplex- ing problem, for after the curve is once drawn it becomes an easy matter to compute and tabulate the deviations for | any desired course or ship's head. OTHER METHODS. While the methods of determining the deviation "By the Known Correct Mag- netic Bearing of a Distant Object," and "By Reciprocal Simultaneous: Bearings of Two Compasses," are of little real and practical value on the lakes, due to our "hurry-up" navigation, and the length of time that would be consumed in per- forming the work by these methods, -we will, nevertheless, explain their prin- ciples. It is well to know how to use them, for the navigator may be so sit- uated that he can employ one or the other to the best of advantage. DEV. BY THE KNOWN CORR. MAG, BEARING OF A DISTANT OBJECT, First determine the correct magnetic bearing of the object to be employed by any method you deem best. This ean be done by finding the ship's position by cross bearings of two objects, and plot- ting the ship's position on the chart, and then establishing the correct magnetic bearing by drawing a line from the ship's position through the object. This line transferred to a compass rose by means of the parallel ruler will give the true bearing, to which apply the variation. TAKE THE COMPASS ON SHORE. If this cannot be done then the coni- pass may be carried on the shore and the ship's bearing taken. The bearing of the ship from the shore will evidently be the same as its correct magnetic bear- ing from the ship by that.compass. For instanee, supposing on shore there was a church with a lofty steeple, not given on the chart; the compass carried on the shore and set up in a direct line be- tween the steeple and ship, must give the correct magnetic bearing of the steeple from the ship, or vice versa, for the