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28 TAe MarRINE REvIEw SCIENTIFIC LAKE NAVIGATION BY CLASENCE E. LONG PROCESS BY RECIPROCAL BEARINGS, Should there be no suitable object visi- ble from the ship, and at the requisite distance, the deviations must be ascer- tained by the process of reciprocal bear- ing. A careful observer must go on shore with a second compass and place its tri- pod in some open spot, but strictly re-- moved from local magnetic influences and where it may be distinctly seen from the standard compass on board. Then by means of preconcerted signals the mutual bearings of these two compasses from each other are to be observed at the mo- ment when the ship's head is steady on each of the thirty-two pointS successively, as before directed. The mode of regis- tering the observations, when this process with the two compasses has been adopt- ed, is shown in the accompanying speci- men form: FORM FOR REGISTERING THE-PROCESS BY RE- CIPROCAL BEARINGS. Simultaneous Bearings. Ship's Head From Stand- From the Deviation _by the ard compass shore com- of the standard on board. pass. compass. Compass. ° Otth 2. <0. Ne 209-5 S150 W 5 Wly N by &....N 25 § $19 W 6 Wly NNE cs N-3i) S 24 W 7 Wily etc. etc. ete. etc. To ensure the success "of this opera- tion the compass on shore should not be more distant from the ship than is con- sistent with the most distinct visibility with the naked eye of both compasses from each other. The observations should be made as strictly simultaneous as pos- sible and, to guard against any mistake, such as might be occasioned by a signal being misinterpreted, the time at which each bearing is' taken should be noted, both on shore and on board by compared watches. For instance, a signal is made on board for the observer on shore to take a bearing of the observer on board. At this instant the observer on board takes a bearing of the observer on shore, and at the same instant both observers take the time by watch. The observer on board, of course, keeps tab as to the direction of the ship's head at the time of each observation. ~ Tt has been found a very au practice for the observer on shore to chalk each observation upon a blackboard. By this process, or by the use of signal flags or semaphore, if there should be any apparent inconsistency the observa- tion can be immediately repeated and the' necessity for again swinging the ship thereby prevented. The student should remember that the bearings of the shore compass, (which, in each case, is the correct magnetic bearing) are to be reversed before the deviation can be found. For instance, in the foregoing example, when the ship heads north by standard compass, the ob- server on shore (his compass) bears N 20° E, but the bearing of the standard compass on board from the compass on shore at the same instant, is S 15° W. -This reversed is N 15° E, which is the corréct magnetic bearing, or the bearing it would have been from the ship had not the ship's magnetism influenced the compass. The difference 'between N 15° E and N 20° E is 5°, and as the correct magnetic bearing is to the left of the compass bearing the deviation is named westerly. INDEX ERROR OF THE COMPASS, The vessel should be made to describe two circles, one with a port and one with 'a starboard helm. Two sets of deviations will be obtained; the mean of these will be a correct table. The best results will be obtained by steaming slowly in a circle as large as the station will permit; it is also advisable to frequently tap the com- pass to overcome any friction on the pivot. . This has reference to the index error of the compass, and is frequently the re- sult of magnetism, induced when the ship's head was on one point of the com- pass, being retained when her head is swung to some other point. The value of this error, of course, is greatest when the ship is swung quickest. Ordinarily this error should be the same on each observation, but where friction enters the problem, and the ship not swung and steadied uniformly, this can not be. How- ever, this error is small in amount. Usually it is easterly when the ship is swung against the sun, and westerly when swung with the sun. In case of easterly deviation and the ship swung with a star- board helm the actual deviation would be increased; if swung with a port helm decreased. When selecting a range which is taken from the chart, and which is to be used for finding the deviation, pick out objects which are distant from each other at least two miles and in a locality where the ves- sel can pass within a mile of the front object of the range. A good rule to fol- low in selecting a range is to pick out one, where the front and rear objects will be two or more miles apart, and where the vessel will be over one-half as far from the front object, when she is passing it, as the 'front object is from the rear object. IN A WOODEN SHIP. If the observations for deviation had been made in a ship built entirely of wood and other non-magnetic materials the compass needle would point in the direction of the magnetic meridian, then the correct magnetic bearing as computed compared by the bearing as found on board would be precisely. the same thing; or the difference between the true bear- ing and the observed eeatne will be the Variation. : Now-a-days all vessels have iron and. steel entering into their construction and - equipment to such an extent as to pro- duce an independent magnetic effect on the compass by which the needle is at- tracted to one side or the other of the magnetic meridian on the various head- ings of the ship by an angular amount that is called the deviation of the com- pass, so that when the true bearing of a celestial body is compared with its com- pass bearing, as observed in such a ves- sel, the difference between the two bear- ings, called the total correction of the compass, shows the combined effect of the variation of the compass and the de- viation of the compass, or the combined. influence of the. earth's magnetism and of the ship's magnetism upon the pointing of the compass. The rela- tion between the total correction, devia- tion and variation will be seen by the formula: Total correction=variation + devia- tion, in which easterly variation and devi- ation have the sign +, and westerly vari- ation and deviation the sign --. AN UNCERTAIN QUANTITY. The variation of the compass, as laid down for the open lakes, is of an uncer- tain quantity and-should not be depend- ed upon to the degree; hence, ships can- not be headed truly on account of a lack of accurate knowledge of the variation of the compass. Every mariner knows - there are other causes why a ship cannot be held tfue to her course, but no one error can be any worse than the one of wrong variation. Navigation, at best, is a rough process as compared with sur- veying. All first-class charts are made, from the beginning of the survey to the final execution of the chart, with a nicety much greater than is attainable in the processes of navigation. A vessel, in 'pursuing her way, is subject to many un- certainties of the distance and the course not clearly made known by the instru- ments used in navigation. However ac-_ curate a chart may be, it is of no avail