S.E. b E. on which is 10° E. Dev., the correct magnetic course made good would be about S.E., or S. 46° E.

say

Col. 3 is obtained by applying the Dev. in Col. 2 to the points of the compass in Col. 1; and Col. 3 indicates what course you should steer by compass to make good any required correct magnetic course;—thus, that the chart shows that you should steer a correct mag. course West; you look to Col. 3 and opposite it in Col. 1 is w. b N. your steering course. Similarly you wish to steer a correct mag. course N.N.E. † E.; you look to Col. 3 and find N. 30° E. (which is about N.N.E. E.) requires you to steer N.E. b N. as indicated by Col. 1.

Remember that whenever you take a bearing you correct it not for deviation on the bearing—but for the deviation due to the Direction of the ship's head, thus— Ex. Ship heading N.W. 6 W., a point of land bore by same compass E. b S. S. Required the cor. mag. bearing.—Ans. East; got by applying 17° W., the dev. on N.W. by W. (see card p. 53), to S. 73° E.

Note.—Iron ships, as a rule, lose much of their magnetism in the first two years; but it never wholly disappears.

I now give you a day's work with Deviation; apply it, as well as the leeway and variation, as already directed.

Thus, departure course S.E. by S. S. becomes, when reversed and turned into degrees, N. 25° W., to which apply 15° W. deviation, and 25° W. variation, and the resulting true course is N. 65° W.; N.W. by N. similarly corrected gives N. 74° W.; and so with the other courses: try to get the answer as given at the foot of the table.

LOG-BOARD. No. IV.

Remarks.

N.W. 6 N. W.S.W. 0 15°W. Noon. The De

6 5

7 5

8 5

5

10

10

9

5

parture is taken from Tory Island in Lat. 55° 16' N., Long. 8° 15′ W., bearing by compass S.E. b S. & S., distant 12 miles;

ship heading

N.W.N., with

18 miles in the

24 hours.

E.,

Course.

Dist.

D. Lat.

Dep.

Lat. in.

Long. in.

N.87°W. 104 m.

5.7' N. 103.4' W. 55° 22′ N. 11° 17′ W.

COURSE AND DISTANCE BY MERCATOR'S SAILING.

If the places are distant you must compute the Course and Distance by Logarithms as follows:

Then

For the difference of Latitude.--Both Lats. of the same name, subtract the less from the greater. If Lats. are of different names take their sum. multiply the degrees by sixty, and add in the miles. The result is the Diff. Lat. in miles.

For the meridional difference of Latitude.—From Table VI, take out the meridional parts for each Lat. If

both Lats. have the same name, take the difference of the Mer. Parts. If Lats. have different names, take the sum of the Mer. Parts. The result is the Mer. Diff. Lat.

For the difference of Longitude.—Both Long. of the same name, subtract the less from the greater. If Long. are of different names, take their sum. The result reduced to miles is the Diff. Long.

Note. Should the sum of the longitudes exceed 180°, take such sum from 360°; then, this remainder reduced to miles is the Diff. Long.

To find the Course between the two places.—From the Log. (Table I.) of the Diff. Long. (plus index 10) subtract the Log. of the Mer. Diff. Lat. The result is the Log. Tangent (Table V.) of the Course, which take out, and name N. or S. according to whether the Lat. of the place bound to is N. or S. of the point of departure, and also E. and W. according to whether the Long. of the place bound to is E. or W. of the point of departure.

To find the Distance.—To the Log. Secant (Table V.) of the Course, add the Log. of the Diff. Lat.; the sum (less 10 in the index) is the Log. of the Distance, which take from Table I.

Ex. Compute the Course and Distance from the Bishop's Rock, Lat. 49° 52' N., Long. 6° 26' W., to the Bermudas, Lat. 32° 26' N., Long. 64° 37′ W.

Hence, Course S. 68° 6' W. or W.S.W. (westerly), and Distance 2805 miles.

Parallel Sailing by Logarithms.For short distances and departures the method by Inspection is generally near enough, especially if you have interpolated between one degree and its next when using the Traverse Tables; but for long distances you will find Logarithms more accurate.

1. Given the Distance run on a parallel of Lat. to find the Diff. Long.

Rule.-To the log. secant of the Lat. add the log. of the Dist.; the sum (less 10 in the index) gives the log. of the Diff. Long., which take out. (Note. This rule is also applicable to Mid. Lat. Sailing, using the mid. lat. for the parallel, and dep. for distance.)

Ex. On the parallel of 50° 28' the distance run was 498 miles; find the difference of longitude made.

2. Given the Diff. Long. of two places on the same parallel, to find their Distance along that parallel.

Rule. To the log. cosine of the Lat. add the log. of the Diff. Longitude; the sum (less 10 in the index) is the log. of the Dist., which take out.

Ex. On the parallel of 42° 30′ S. it is intended to run from long. 20° E. to long. 112° E.; required the distance.

112°

Lat. 42° 30'

Diff. Long. 5520

Distance 4070 .

20° 92°5520' diff. long.

A few final words on correcting courses sailed and bearings taken :—

1. Remember that when correcting courses or bearings you must imagine yourself to be standing in the centre of the compass, looking in the direction of the course or bearing.

2. A compass course may be affected by Leeway, Deviation, and Variation.

3. When there is no Deviation, the course is correct magnetic, but may be affected by Leeway and Variation. 4. A compass bearing may be affected by Deviation (due to the direction of the ship's head, i.e. her course), and by Variation.

5. A correct magnetic bearing is affected only by

Variation.

6. To correct for Leeway, apply it to the course steered, in a direction from the wind; hence, to the right if on the port tack, but to the left if on the starboard tack.

7. To correct a compass Course for Deviation, apply Easterly Deviation to the right, and Westerly Deviation to the left, of the course.

8. To correct a compass course for Variation, apply Easterly Variation to the right, and Westerly Variation to the left, of the course.

The result of the application of the above corrections is the true course.

Note.—Remember that as regards the Variation at any given place, it is of the same amount and name every course; but each course has its own special

for

Deviation.

Note.—The Error of the Compass for any special course is the combined effect of the Variation at the place, and of the Deviation due to that course; hence, it is the sum or difference of the Variation and Devia