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30 COMPASSES.ON LAKE STEAMERS. _ BY CLARENCE E. LONG The chief difficulty the lake master has' in the use of his compass is in finding and allowing for the changes "TAE Marine. REVIEW bor ranges, etc. Azimuths offer the best means of setting the course, but when the sky is overcast they are of no account and other means must be made to supply this deficiency. Harbor and river ranges must take in ,the deviation -that take place the place of the azimuth. These t \ { ' , : A ea MAGNETIC CIRCUIT OR FIELD Cee a Se ee oe et to ee ae | 0 = peortPass -- a L So 7 7 . Noon : / oe wt _ a. So > ae _.. LINE OR DIRECTION OF SHIPS MAGNETISM -- WATER LINE - _ --_ - FIG, 1.--VESSEL LOADED---ON AN EVEN KEEL NEARLY, from the various trims of his boat, such as when either loaded or light, water bottoms filled, of empty. is owing to the fact that, the forces of the ship's magnetism 'change their lines of direction on the compass needle at every such change of the hull in the water. A simple example will illustrate this: Suppose the ship's magnetism to be in the fore-and-aft line of the hull with its plane below the deck. See Figs. 1 and 2. Fig. 1 shows the boat loaded and. Fig. 2 the same boat in light trim. Now in both cases the line or plane of the ship's magnetism remains the same, that is, the magnetic poles of same will remain in the same part of the hull in either position. "When the _ boat is light her bow rises high in the air, and the line of the ship's magne- tism rises with it, and the direction of its lines of force undergo a cor- responding change. Although the magnetic field of the ship's magnetism encompasses the same extent in both cases, the field has shifted its posi- tion, and the force that acted on the compass needle in the one case is al-' together different in the second case. It must be borne in mind that the compass, as it hangs in its gimbals is always on an even. keel no matter what position the ship's hull is in. When the ship is loaded and about On an eyen, keel the' card of the compass and the keel are about paral- lel with each other; but when the ship is light the compass card and keel ate no longer parallel. <A careful study of Figs. 1 and 2 will make this all very plain. Owing to this peculiarity of lake steamers' (bows high out of the wa- ter when light) all courses steered or to be steered must be checked by~ .frequent azimuths and river and har- This, ranges should be used anyway as a check on the azimuths. The deviation curve offers the great- est facility in, this work, both in find- ing the deviation from azimuths and from:-ranges. The practice now pre- vails in setting the course by an azi- muth each time the course is shifted. This answers very well so long as the sun is out, but when the 'sun cannot be observed he must rely on harbor ranges. Coming out of piers, etc., does not afford the facility for finding the deviation as does a good range ahead, and the deviation as found in the former cases is. not al- ways to be relied upon. Where the ship's head can be put on a mark ahead and steadied for some time the result is more reliable and conse- quently more satisfactory, than where one has to line up parallel with a pier or cut, as is oftentimes the case. te) _----- 'Erie | port, time before arriving at the Soo. ' ee them. Using a table of deviation it is necessary to find the actual devia- tion on every full point. If, while the master is setting his. course by an azimuth, he would swing his boat and steady her on the cardinal and intercardinal points, he could, . by means of the curve ascertain the de- viation and compass courses for any other point of the compass with per- fect. ease and accuracy. For an' ex- ample: Suppose the case of a steamer. bound from Duluth to some Lake Instead. of only., setting the course to Devil's island. by an azi- © muth, the deviation on' NE) 2, SE, S and SW was ascertained and then the same set of azimuths used to set the Devil's island course, the master would then have in his possession sufficient _data to make the: proper allowance for all the courses he will need to steer to his destination. He can draw his deviation curve at any The ranges in the Soo river afford him another means of verifying the devia- tion found by the azimuths. Every course should be checked by an azi- muth when the opportunity is offered. Supposing the case where no azimuth could be taken to set the course, then the pier range must be utilized in the best possible manner, also the courses steered on previous courses, or the course that agreed for the ship in her present trim. As soon as the sun is available azimuths should be taken. Verify or check your course first and then swing your boat on those points of the compass that you will need the deviation for to make courses for your destination. To draw it to a curve .the deviation for the nearest cardinal and intercardinal points is all eS eres ie ee a ces: WATER LINE _-- -- -- --_ PIG. 2.-_VESSEL LIGHT BOW HIGH UP, Drawing the deviation to a curve the deviations necessary for same are only required on a few equidistant points of the compass. This is the beauty of the deviation curve, you only need "to know the deviation on two points, say four points separated, to find the deviation for the points lying between - but: that is necessary. Any other point will do just as well so long as they are about equally distant from each. other. Such curves can be easily con- structed without loss of time and with little effort. The | numerous ranges in the rivers and cuts and the correct variation of the locality as