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28 THE Marine REVIEW SCIENTIFIC LAKE NAVIGATION » HOW TO FORM A TABLE OF DEVIATION. CHAPTER VII. WHAT CONCERNS HIM MOST. The finding of the compass deviation and its tabulations are probably of more real and practical benefit to the lake mariner than any other one branch of lake navigation. How to ascertain, name and apply the compass deviation concerns him far more than its causes and effects. THE VARIOUS METHODS, There are several methods in use for ascertaining and tabulating the deviation. These methods are known by the follow- ing names. ."By the known correct mag- netic bearing of a range (such as two lighthouses, a lighthouse, church or chimney, or other object, in line); "By the known correct magnetic bearing of a distant object;' "By reciprocal simul- taneous bearings," By amplitudes of the sun," and "By time azimuths of the sun, moon and stars." _ TIME, AZIMUTHS. The method of time azimuths is of genuine benefit and is especially adapted for use on the great lakes. It is a handy and convenient method, and one that answers every ,purpose, you are, inside or outside of port, or out of sight of land. By its method the de- viation can be ascertained and tabulated and a vessel put on the right course. You cannot do this with any of the other methods. In fact the method of time azimuths can be' employed where the compass is of no use at all. GOING IN AND COMING OUT OF PIERS. The azimuth method is a simple one, and a knowledge of it is easily acquired, and-is more easily learned than many may believe. | Where a knowledge of time azimuths is once acquired all other methods will be discarded for it, and hardly, if every thought of let alone used,. unless, with the possible exception of noting and veri- fying the deviation on river and harbor pier ranges,-etc., which for this purpose alone, are of the greatest possible con- "venience to the lake navigator. THE SIMPLEST METHOD. First, we will explain the simplest method of finding and forming the de- viation; second, the method or methods employed being especially adapted for the need of the lake navigator; third, that a practical knowledge of compass deviations is easily acquired by any master or pilot on lake vessels, and that the finding and tabulation of the devia- tion, as well as its application in deduc- ing one kind of course or bearing to an- - other kind of course or bearing, is much no matter where. By Clarence E.. Long more simple than it at rst looks, and much more easily learned than many have been led to believe. We will first explain how to find and. name the deviation going out and com- ing into piers (being parallel thereto) and when on with a range, ahead or astern:- This is the first step to take, and it will lead the student gently and gradually to the azimuth method, which is the creany of them all, and by,far the © best, quickest and safest method known for ascertaining the deviation, the true course or bearing, the correct magnetic course or bearing, etc., all of which will be fully explained in its right place. TO GET THE NECESSARY INFORMATION. 'The arguments necessary to the solu- tion of the pier range problem, are the true bearings, the variation at place of ship, and the compass bearing, that is, the bearing as shown by the compass on board. The first thing to be done in this work is to find the true bearing of the piers to be used; this can be taken off the chart (from the true compass diagram) or from the government light- house book; which gives all bearings, etc., as being true, while all those taken from the Canadian book are correct for variation, and represent correct magnetic bearings. To get the true bearings of piers it is much-easier and more accu- rate to take them from the lighthouse hook than to measure them from the chart, especially a small scale chart. To the true bearing apply the variation at place; if Ely. to the left of the true bear- ing to get the correct magnetic bearing; and if Var. is Wly. to the right of true bearing to get the correct magnetic -bear- ing; then the difference between the cor- rect magnetic bearing, or what we know it to be, and the bearing as shown by compass (called compass course or bear- 'ing, im this 'case it is course) is the Dev. and is named Ely. if the correct magnetic bearing is to the right of the compass bearing; and if the correct magnetic hearing is to the left of the compass bear- ing then itis named Wly. Dev. This has been so thoroughly explained in a former chapter, that it hardly seems necessary to say more on the subject, but when- ever and wherever we deem it necessary we will sandwich in more explanations. Note.--If the Var. was not applied to the true bearing and the compass bearing compared with it the difference would be the Var. and Dey. combined. This would not do because the Var. is different at different places, being Ely, in some places and Wly. in others.. By applying the Var. to the true course or bearing it is just the same as getting rid of the Var., so that it cannot get into the Dev. Remember this. As we have already learned, applying the Dey. to the correct magnetic bearing gives the corresponding compass bearing, therefore the difference between the correct .magnetic bearing and the compass bearing must be. the deviation. Where there is no deviation the compass bearing will also be the~ correct magnetic bearing and vice versa. HOW TO FIND THE DEV. BY PIER RANGES. Now, we will take an example just as it comes up in every-day practice: Example:--How had your compass ought to read if it were right coming out parallel with the piers at Milwaukee? But supposing that your compass read ENE34E; how much is the deviation and which way is it, i. e, its name Ely. or Wly.? By looking in the lighthouse book we find that the true bearing of these piers is east, artd the Var. for Milwaukee on the chart says 3° 04 Ely. So,' here is the way it would be put on paper: True bearing of piers--E=8 pts. r, of n. Var. 3° o4' Ely (to the left)--= 1% pts 1, Corr. Mag. bearing of piers 734 " r.ofn. Com. bearing ENE34E (--)634 © r. of n. Dev" on, EUAN (core: mag.) 1" r. Ely: 'The deviation on correct magnetic E44N is I point Ely., because the corr. mag. bearing [E%4N] is to the right of the compass bearing [ENE34E], consequently the Dev. is named Ely. Here is another proof: Dev. Ely is vallowed to the left of the correct magnetic course to get the compass course; hence if you move to the Jet of: E44N (e.m. b.), 1 point: (the amount of the dev.) it brings you to ENE34E (the compass bearing). Or, again, to find the c. m. c., the compass course and dev. given, allow easterly de- viation to the right; hence 1 point to the right of compass bearing ENE34E, is EWN. the c. m. b. Do you see? Just try this on the portable compass card. Let E1%4N on outer card represent the c. m. bearing of piers, then swing the inner card (which is to represent the compass on board) so that ENE3%E comes or coincides with EI4N on the outside or fixed card. Does not the in- side card representing the compass board have to swing to the right for ENE34E to come where EN is? This is just the manner in which the compass card moves on board ship; you cannot see it move, but it does just the same, for we know that where ENE34E is EY N should be. If there were no deviation on EN, then in going out: of the piers your compass would read precisely the