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TAE Marine Review the route, and that the variation at departure will not agree all along any one' route; nor will the variation at destination agree with the variation at departure. The only correct and safe way this can be done, and should .be done,' is to take the mean of the vari- ations at departure and destination, and apply the amount to the true course to get the correct magnetic course. While the foregoing example may be an extraordinary one, it goes to show that the method of taking off the correct magnetic course from the magnetic rose is not only incor- rect, but a dangerous one, and one. that may lead to disas- trous results if practiced. As it is absolutely necessary to get the mean of the variations at each end of the route to work correctly, the question naturally suggests itself, of what use and good are these magnetic roses on the chart? Their accuracy, aS we have seen, is more apparent than real, and therefore should be shunned by the navigator. No careful man would take the chance of its giving accurately the cor- rect magnetic course, unless he first compared and proved it; and as it becomes necessary to do this each time, the mag- - netic roses render no service whatever in arriving at the truth of the matter and, therefore, should be dispensed with. They are more harmful than good; they look nice and that is all. MAGNETIC VARIATION, It is very important that the navigator should have a thorough knowledge of the magnetic variation, The amount of allowance and its direction, and annual change are in- dicated on the charts. On all mercator charts, with the ex- ception of harbor charts, will be found irregular lines run- ning from the top to bottom of the paper in a northwesterly and southeasterly direction, and having beside them such in- scriptions as 5° Wly., 8° Ely. This means that along this line the variation of the compass from true north is W. 5°, 8° E. There are certain lines, which have no variation, and here no allowance is to be made. There is one such line in the region of the great lakes. It passes through the east end of Lake Superior, thence across the Straits of Mackinac down through central Michigan. This line is called the "line of no variation." At all places to the east of this line of no variation the variation is westerly, and at all places to the west it is easterly. Westerly variation is increasing and east- erly variation is decreasing, or in other words, the line of no variation is moving west. On harbor charts the variation is shown by the compass card printed on the chart. The north point of it will be found slewed a little to the eastward or westward of a meridian line. On all mercator charts, in- side of each compass rose will be found an inscription such as "Var. 8° Ely. in 1898; decreasing 5' annually.' On the charts published by the Engineers' Dept., you will find a table of magnetic variation or declination, giving the variation for 'the different localities with the annual increase and decrease. The latest lake survey charts now contain, not only the magnetic compass diagrams or roses, but tables of the mag- netic variation, and in many places the, magnetic meridian is shown by a line about three inches long drawn to show the angle of the variation, or, in other words, the magnetic needle uninfluenced at that place. The variation tables are corrected for the year of publication of the chart, and are of great benefit to the navigator if he understands how to apply them. ANNUAL CHANGE IN THE VARIATION, A knowledge of this is very important, as in many parts of the world it is very rapid, and after a few years the cor- rection becomes quite a consideration. Masters are not al- ways acquainted with this peculiarity of terrestrial magnet- ism, and, in consequence, give courses which were the cor- rect thing years ago, but are so no longer. It is evident, from what has just been stated, that correct magnetic courses given in sailing directions need revision in a comparatively 15 short time. A man with a chart, and possessed of the above knowledge, does not require such dry nursing. , _TO ALLOW FOR VARIATION BETWEEN DEPARTURE AND DESTINATION, To apply the variation to a true course, to obtain a cor- rect magnetic course, between a point of departure and a point of destination 'add the two variations together if they have like names, that is, both easterly or both westerly, and take the mean of the sum by dividing by 2. The quotient will be the proportional variation to be applied to the true course between the two points. For example: Variation at Chicago ....... Viessere 2° 49' Ely, Variation at Twin River Pt jie) eo ee 2).§ O4 Variation to be(applied .. 720 vie ee The above method of applying the variation may be used -- where there is ample room, but on long routes, where the variation changes rapidly, to apply a single correct magnetic course to be steered all along the line, might lead to dan- gerous results. Long, straight routes should be subdivided into parts and the correct magnetic course given for each part. However, on most lake routes it is practicable to take the mean of the two variations. Remarks:--If the variation at departure was easterly and westerly at destination what would that indicate? It would indicate that in order to reach your point of destination you will have to cross the line of no variation. In a case of this kind just take one-half of the variation at the departure and apply it to the true course and this will give the proportional variation between the starting point and the line of no variation. When you have crossed the line of no variation take one-half of the variation at destination and apply it to your true course. TO CORRECT FOR ANNUAL CHANGE IN VARIATION, To correct the variation for the annual change. multiply the number of years that have elapsed since the chart was corrected. Remember, that easterly variation is decreasing and westerly is increasing; therefore, when correcting easterly variation subtract the annual change, and westerly add it. For example: The variation at Chicago in 1898 was 2° 49' E, annual change 4.7'. What will the correct variation be in 1906? . Variation at Chicago 1898 ...... 2. S64 Ely Decreasing annually, 4.7', 8 years 38.4' (subtract), Correct variation at Chicago, 1906 2° 20.0' Ely. Example :--If the annual change in the variation is 5', how many years will it take to effect the variation 1°? Ans. 12 years, because in 1° there are 60', and 5' is contained in 60' 12 times, or I2 years. _ OPPOSITE AND REVERSE. Do not get the terms "opposite" and "reverse," as applied to the compass mixed up with each other. Mariners, gen- erally, are a trifle out of their latitude on this seemingly "knotty" subject; but there is no reason for confusion as there is a marked distinction between the two terms. The average person will use reverse and opposite as meaning one and the same thing in speaking of the directions on the com- pass, or horizon; as for instance, they will tell you that SE is opposite NW, but this is not so, the opposite of SE. is SW and NW is the reverse of SE. Example:--Supposing the true or chart course (the course found by a true compass) is north, and the chart tells us, where the ship is situated, there is one point of westerly vari- ation, which means that the north end of the compass needle: is inclined one point to the west of the true or geographical north. Under these circumstances it will be necessary to steer N by E by the ship's compass in order to make a true