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30 will show a method that not only simpli- fies this entire operation, but does away with about 90 per cent of the work in- volved under the above named processes. THE AZIMUTH METHOD THE BOSS. Note.--Since by the azimuth method it is possible to determine the deviation of ship's head correct magnetic in the first instance, it is- not intended to do away - with the graphic methods of plotting and drawing a curve of deviations. These graphic methods are invaluable and should be used in conjunction with the azimuth mode, as it will answer as a check to this work. If the master has a table of deviations from azimuth obser- vations he will want to verify the cor- _rectness of same by finding the devia- tion from ranges, or land objects. A curve drawn from these observatioris will - tell the story. The beauty of a curve lies in the fact that you:can find the devia- tion for any fractional point of the com- pass with as much ease as from the full ° point. A table of deviation does not af- ford this facility. WHEN THE DEV, IS ON THE FULL POINT. Finding the deviation by river and harbor ranges by heading on them, it will, of course, be impracticable, in a ma- jority of cases at least, to get the devia- tion exactly on a cardinal or intercardi- nal point, as is oftentimes desired. In such cases. two or more observations may have been taken with the ship's head cor- rect magnetic near such point. For ex- ample: Supposing it were desired to know the true deviation on South, from the following observations: S 15° E, tie Wey, was 9; S 7) E was 10°, and for S 8° W was.16°. In case no blank form, or deviation diagram, is available, proceed as follows: HOW TO GET AROUND A DIFFICULT PROBLEM. Draw a horizontal line to represent the line of no deviation. ular line to this to represent the point of the compass you desire the deviation for. Adopt a scale of equal parts, say an eighth of an inch to a degree, and mark off the horizontal line from the point cut by the perpendicular, the number of eighths of an inch that agrees with the number of degrees the ship's head was from the cardinal point at the time the 'several deviations were determined, these distances or stations being taken to the right or left of the first perpendicular ac- cording as the ship's head is to the right or left from the cardinal, or whichever point decided upon. At each of these stations draw a perpendicular, and on these perpendiculars lay off by the same scale of equal parts distances from the horizontal line proportioned to the devia- tions observed when the sh:p's head was in the direction corresponding to the sta- tions respectively. Now draw a curve Draw a perpendic-" THE Marine. REVIEW freely through the points marking these distances, and the distances above the horizontal line at which this curve cuts the first drawn perpendicular will mea- sure the true deviation with the ship's head on the cardinal point. _ The foregoing example, when laid down is shown in Fig. 37, shows that with the: ship's head South the deviation will be 131°. BAXTER'S DEVIATION DIAGRAM, Baxter's deviation diagram is without doubt the handiest device for plotting the deviations thus far introduced, since it requires no measuring with dividers or other instrument. All that is necessary is to find the compass point for which the deviation is known. Follow this out to the central line, and with the number of degrees of the deviation count a square for each degree and make a dot at the eee ° ° Se se Te) ° ~ so SiS%E 87°. SOUTH sew. iG. 47, last one. Do this with each one of the points of observation after which draw the curve. WHEN THE DEVIATION IS LARGE. To obtain a fair deviation curve a suf- ficient number of observations for devia- tion should be taken while the ship swings through an entire circle. In gen- eral, observations made on every alter- nate point of the compass are sufficient to establish a good curve, but in cases where the maximum deviation amounts to 40° or more, it often becomes neces- sary to observe on every po:nt. There are cases where the deviation is so great that a curve could not be drawn of it until the compass is first adjusted. Recollect that the dotted lines in Na- pier's deviation diagram represent com- pass courses or compass bearings, and the plain lines correct magnetic courses or bearing; or thus, Compass Courses. Correct Magnetic Courses. THE VALUE OF THE DEV, CURVE. Now, that compass adjustment is so universally resorted to, these deviation diagrams are hardly wanted; all the saine, itis well to know and understand them. In fact, the deviation after ad- justment, can be drawn to a curve and be of much more utility than the same data in tabular form. _ needle. CAUSES OF HEELING DEVIATION. The three principal causes of the heel- ing efror are vertical induction in trans- verse iron (such as iron deck beams), induction in iron vertical to the ship's : deck, and the vertical component of the sub-permanent magnetism. The part | arising from vertical induction: in trans- verse iron is due to the fact that such iron as beams, by departing from the hori- zontal position, and inclining to the ver- tical as the ship heels, acquires polarity | in its ends by induction from the earth, of the same sign as that of vertical iron in that hemisphere. This polarity tends to draw the north point of the compass to one side or the other. The part arising from induction in iron vertical to the ship's deck is due to the fact that such iron is not vertical to the earth when the ship heels. The amount of magnetism induced therein is less in quantity, but by the poles becoming on one side of the compass, instead of ver- tically under it, the north point is drawn to one side or the other. The part arising from the vertical com- ponent of the ship's sub-permanent mag- netism, arises from the fact that although the force remains the same in amount, it becomes on one side or the other as the ship heels, and so deflects the compass needle. There would also be a deflection from the ship's sub-permanent transverse magnetism, unless it had been corrected by magnets. NEEDLE DRAWN TO HIGH SIDE, In the northern hemisphere, the north point of the compass is usually drawn to the high side, because the higher ~ends of the beams, and the upper ends of ver- tical iron which terminates below the 'compass, and so go toward the high side when the ship heels; both have blue magnetism by induction in the northern ~ hemisphere, and so draw the point of the compass to the high side. It is plain to be seen that on northerly courses the de- viation will be westerly with a starboard heel and easterly with-a port heel. On southerly courses the reverse is a fact. If the compass is near vertical iron, such as the funnel, having its uppet end above the compass, the force to the high side is diminished. When a ship heels a magnetic force previously vertical is no longer so, and the position of the iron about a compass may be so changed with reference to the earth's magnetic force, that new forces may arise from induction. The heeling deviation. usually has a minimum value on courses near east or west, because the disturbing force is then acting in the direction of the compass Just imagine the ship heading east with a starboard list. The upper ends of the deckbeams near the compass,