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THE MARINE RECORD. GORRESPONDENGE. Bay-We do not hold ourselves responsible in any way for the views or apinions expressed by our correspondents. It is our desire that all sides of any question affecting the interests or welfare of the lake ma- tine should be fairly represented in THs Marine REcorD. OUR NEW LAKE CHARTS. To the Editor of The Marine Record: Probably the world cannot afford better charts of any ocean or locality than our government has fur- nished us of the lake region. In point of accuracy and in elegance of execution they are the peers of the best, and yet, there is one important condition or peculiarity in our navigation that is not provided for fully, and that is, ‘‘how to keep a vessel on her proper course.” It is generally supposed that when a vessel is cor- rectly started on her course, and is kept strictly to that course, as indicated by the ship’s compass, she makes a track that crosses the meridian in its run or voyage at the same angle. Such a track is called the ‘‘rhumb-track.”’ On the charts constructed on the ‘‘conic”’ projection this track would be a curve, concave toward the pole; but on a ‘‘Mer- cator”’ projection, such as our new lake charts, it would be a straight line. And for such a line it would be only necessary to ap- ply the variation due to place of ship in shaping course to any locality. Accordingly, the instructions given by our charts are to apply the variation of the lo- cality of ship to the true course, and also to apply the deviation, if any, to obtain the compass course that will take us on the true course. ‘Then, without further instruc- tions, we are supposed to hold ship’s head to such reading of compass till we reach our point of destination—and here our trouble begins. r ‘'The conditions necessary that a ship may sail on.a true rhumb-line, when steered toa coustant course, are that The change of variation between the ini- tial and terminal points of the voyage must be just equal to the change in the inclina- tion of the meridian for the voyage, z. ¢. to the convergency of the meridians pass- ing through the two points. Thus, if the change of the inclination, or the convergency of the meridians pas-— sing through two points is, say 5°, and the change of the variation between those two points be also 5°, then the track ofa ship sailed to a constant compass course be- tween those two points would be a true rhumb-line, and on a Mercator chart would be represented by a right line. This is a beautful theory, and if, as sup- posed and taught by all writers on naviga- tion, that the track of all ships were thus conditioned, then it would be an easy mat- ter to keep track of a ship in dark weather, and to conduct a ship to any part of the world. But, unfortunately, the idea is a most grievous error—a sorry mistake—with not a word, or not a syllable of truth in it—and for the simple reason that the conditions neces- sary to such a state of things do not exist. There is no known place on the face of the earth, where, for a given change of geographical position, the change in the inclination of the meridians is just equal to the corresponding change of variation, and only on a few small areas can such conditions be found to pre- vail, even approximately. In the entire Pacific Ocean, except for a small area in the Bering Sea and Straits, the change of variation for any difference of longitude, on any parallel of lati- tude, is less than the change in inclination of the meri- dians for the same difference of longitude, 7. ¢., less than the corresponding convergency of the meridians for the latitude. As a’consequence, the track of a ship steered to a constant compass course would lie south of the line in- dicating the course of starting, and be concave toward the equator, instead of being represented by a right line on a Mercator chart, the entire whereas on lake region the change of variation for any change of longitude is much greater than the corresponding change inthe convergency of the meridian. Thus, from Duluth to the Sault the diere ice of longitude is about or nearly 8°, the meridians of which two places converge nearly 6°, while the change of variation is nearly 10°—almost twice as great—so that the track of a ship steered to a constant compass course in the vicinity of the lakes would lie very much to the north of the right line representing such course on the Mercator chart, and be concave to the north, and utterly incapable of being represented by a right line between those places. On the Atlantic coast from Florida to the Gulf of St. Lawrence the same condition prevails, as the 200 to 300 wrecks, whose bones fringe the shores of Sable Island, off the Gulf of St. Lawrence, can testify. In sailing from the coast of Newfoundland to points ee / , wy ce THE BOOM STYLE MARINE LEG. in the vicinity of Kngland and the North Sea, the change of variation is less, for any difference of longi- tude, than the corresponding change in the inclination of the meridians, so that the track of a ship steered to a constant compass course between any two places in that area of ocean would lie to the south of the line on the Mercator chart joining the two places. From Cape Horn to the Cape of Good Hope the ship’s track would lie altogether on the south side of the right line on the Mercator chart, joining those places, but it would be a reversed curve, like an elongated letter S. This condition would result from the fact that fora part of the way it is less than the corresponding change in the convergency of the meridians. From the above considerations we see that we must abandon the idea that there is any such thing as a “rhumb-track’’? available for practical purposes—that any ship’s track can be represented by a right line on a lg) | \ = i, cy re 4 rp Mercator chart, or any other for that matter—fo: cannot, if we try, sail a ship in a right line by kee; her head to the same compass course any more we can make acurve of our radius fit to coincide wit the curve of another radius, and the quicker we fo this sorry mistake, the quicker will we be in the way o; knowing how to keep out ship to her proper course. - This lamentable mistake that a ship’s track whe steered to a constant compass course may be repre- sented by a right line on the Mercator chart is an ow growth from dnother mistake equally untenable, 3 that isthe idea that ‘there is one individual point, tha asa center, controls and gives direction to the comp i needle in the entire magnetic. hemisphere.” There no such thing. ; : From well- known laws of navigation it is easy show that the near portions of a magnetic field ill control and. give direction to the compass needle, i spite of a larger and stronger positio of l]_° the field that may be more remote. So by changing the place of the compass we : x bring it within the influence of other po tions of the magnetic field, when we find that we have anew pole, and so we ma continue to do indefinitely, z.¢, to say, ‘stead of haying merely one magnetic po we may have as many as there are possible ~ positions for a compass in the Hee hemisphere. A method recently adopted of overcom. ing the difficulty above pointed out is to ap ply the means of variation of the initia and the terminal points of a route to the true course between them, instead of merely the variation at point of departure, as. are told to do by the instructions of th chart. : An example will illustrate: In shaping a course, say from Whites to Copper Harbor, on Lake Superior, th master finds the true course to be N. 70° W and the distance, say 125 nautical miles, th variation at the. Point being 1° to the left, ‘and that at Copper Harbor being 3° E. o: ' to the right,’the mean of which is 1° KE. or to ‘the right, giving a compass course of N. 71° * W., according to lake usage. 2: By following the instructions of the chat: _we would apply the variation due to the lo- “cality of ‘the Point only, giving a compass ~ course of N..69° W. or 2° to the right of he ' other course. By the former course the ship would sail i in the curved line AB, attaining the point of: destination at B, (see Fig. 1) while by the second course, as per instructions per chart the ship would sailin the curve AC, attain- “ing a point C, 4.4 nautical miles, or five stat ute miles north of the ie ot 5 point B.: Again: : From Caribou Island, Var: 0, to Grand Marais, Var. 6° E., “distance 183 nautical miles, course truie, N. 82° W. The lake sailor * would take his compass course, half the ' change of variation, or.3° to the left of the true course, giving him N. 8.° W. for com- pass course. “This would lead him to his objective point B, by the curved line AB. (See Fig. 2.) The salt water sailors, following the instruction of the chart, and the practice at sea, would assume the true course as his compass course, there being no varia- tion to apply. This would lead his ship in the curve AC, to the point C, 10 nautical miles north of his objective point, for it uitust be remembered that we cannot sail a ship ona straight line if we try by keeping her head to a con- stant compass course. From: the above considerations we see that the instructions per chart may get us into difficulty in shaping courses on the lakes. This failure of the. instructions on our new lake charts, to meet our requirements in the’ shaping of courses, does not appear to be the result of any lack of care or painstaking in their preparation, but rather of a peculiar feature of our. navigation; for the mistake of supposing that the track of a ship steered to a constant aS = = === -ss