Find the deviation corresponding to the given correct magnetic course in column 3 of Table of Deviations (see Table B, below), and apply the deviation to the correct magnetic course the reverse way to that given in Rule XXXVI, page 104, to find the correct magnetic course, having given the compass course; namely, apply easterly deviation to the left of the correct magnetic course and westerly to the right as in the following example.

NOTE.-When the correct magnetic course proposed to be made does not correspond to any of the courses in column 3 of Table of Deviations, nor near midway between them; then, if the deviation is large, the corresponding deviation may be found by proportion, as in the following:

Ex. Correct magnetic course by chart S. 45° E.: required the course to be steered by compass (deviations as annexed Table A).

To find the Course.-Lay a pair of parallel rulers over the points indicated by A and B, and join these points by a straight line. Then, taking care to preserve the direction of the parallels, move them to the centre of the compass, when the True or Correct Magnetic Course is found according as the compass on the chart is a True or Magnetic compass.

If the compass on the chart is a True compass you must seek on the chart the variation nearest the place of the ship, and apply it

Now apply the Deviation-same way as variation—viz. :-
Easterly to the left and Westerly to the right.

This will give the Compass Course.

If the compass is a magnetic compass having found the magnetic course you proceed to apply the deviation as directed above.

To find the Distance.-Take half the distance between the two points (A and B), and placing one leg of the dividers on the graduated meridian at the latitude of the point midway between the two points measure on each side of the same, and the number of degrees between the extreme points brought into miles is the distance required.

1o. If the compass on the chart is a True Compass.-Find the True Bearing of each point by allowing

but the deviation must be that due to the direction of the ship's head at the time of making the observation.

2o. If the compass on the chart is correct magnetic.-Correct the compass bearings for deviation only (as above directed). Having found the true bearings or correct magnetic bearings (as the case may be) of each point; then from each point draw lines opposite to the bearings, and the point of intersection is the position of the ship.

1o. If the compass on the chart is a True Compass.-Find the true bearings of the point by allowing the deviation and variation as in Question II.

2o. If the compass on the chart is a Correct Magnetic Compass.Find the correct magnetic bearings of the point by correcting the compass bearings

for deviation only, as in Question II. Then, the ship's course having been set off, from the given point, or lighthouse, draw a line opposite to the first corrected bearing cutting the line AB in a point which call a. Draw another line running

in an opposite direction to the second corrected bearing, which name b. From the spot where the first bearing crosses the course lay off the distance run, taken from the graduated meridian; the extremity of this distance, which name d, may not reach to, or it may reach beyond, the crossing spot of the second bearing, in either case draw a line parallel to the first bearing, such that it shall pass through the extremity of the distance run and cut the line of the second bearing; the ship's position is where the last line crosses the second bearing, thence measure the distance from the point, and take off the ship's position.

QUESTION IV.

Find the course to steer by compass from

to

(see at the miles per

Question I) to counteract the effect of a current which set
at the rate of miles per hour, the ship making by log
hour; also, the distance the ship would then make good in
towards

RULE LXXXV.

hours

Draw a line on a chart to represent the course to be made good; from the ship's place on the chart lay off a line in the direction of the set of the current, on which mark off from the ship's place the rate of the current per hour; then take in the compasses the distance the ship sails in an hour by log, and put one foot on the last-named mark, and from the point where the other foot reaches the first line draw a line to the mark on the line representing the direction of the current. The course to be steered is represented by the line last drawn, and the parallel ruler being placed to it, and moved to the centre of the compass on the chart, will give the course of the ship; and that portion of the first line drawn, intersected by the last line drawn, will be the distance the ship will make good per hour.

On a chart, suppose A to be the place of the ship, B the point of destination; also A C the set of the current, the rate per hour being taken from the scale

of miles and laid off in the direction of the line. Take the distance sailed by the ship per hour from the scale of miles, and with one foot of the dividers at C, make an arc cutting A at D. Join CD, and move the parallel ruler from CD to A, drawing A E parallel to CD; then A E will be the direction of the ship's head; and the parallel ruler being moved to the centre of the compass on the chart, will give the course of the ship on the chart; and AD will be the distance the ship will make good.

EXAMPLE I.-(True Chart-Chart used for working Sumner's.)

(I.) Using Deviation Card No. J: find the course to be steered by compass from A to B, also the distance (variation 11° E.)

Latitude A 52° 5' N.

Longitude A 176° 50′ E.

Lay an edge of a parallel ruler over the positions A and B, then move the parallel rule (strictly preserving the direction) until an edge pass through the centre of the (true) compass, then the point of the compass corresponding with the edge of the ruler shows the true course from A to B-it will be found to be S. by E.; then proceed as follows:

The variation being 11° E., this

S. 22 E. (L. of S.) = S.S.E.

6 W., allow to right (R.)

S. 16 E. S. by E. E.

allowed to the left makes correct magnetic course S. 22° E., or S.S.E. (Rule LXXXII). Then entering Deviation Table, Card I, we take out the deviation for S.S.E., nearest to S. 22° E. (note that deviation has to be applied as in correcting courses in Day's Work), and proceed thus:Compass Courses

Next, enter Table with compass course S. by E.

It is evident that the correct mag. course S. 22° E. lies between S. 14° 15' E. and S. 33° 30′ E., and that the required deviation will, therefore, lie between the deviations for these two last corr. mag. courses, that is the deviation lies between 3° W. and 11° W., and may be found by proportion as follows:

Corr. mag. course S. 14°15' E. gives Dev. 3° W.
S. 33 30 E.

Diff. for I point

II W.

Given Corr. mag. course S. 22° 0' E.
Next less
S. 14 15 E.

745

70.7

Whence the deviation for correct magnetic course S. 22° E. is 6° W.; allowed to the right (opposite way to that when correcting courses in Day's correct magnetic course, making compass course S. 16° E., or S. by E. } E. this compass course, and also the distance from A to B as measured in the usual way on the graduated meridian.

Answer.-Compass course S. by E. E., distance 61 miles.

(II.) With the ship's head on the above-named compass course, D bore by compass N. 54° E., also, E bore S. 85° E. by the same compass: find the ship's position.

The compass on the chart being a true compass, and as in this question (II) the bearings are given by compass, with the ship's head on the course from A to B, as found in the previous question (I); the compass bearings must, therefore, be turned into true bearings by allowing the deviation proper to the course and the given variation; that is, the same corrections (deviation 6o W., and variation 11° E.) applied to the compass bearings of D and E (but in a contrary way to that in Question I) give their true bearings, N. 59° E. = N.E. by E. E., and S. 80° E. — =E. S.; thus:—

Having found the true bearings of the points D and E, we now proceed to lay off these bearings on the chart, thus:

Place the parallel rules on the compass on the chart, having an edge over N.E. by E. E. and S.W. by W. W., then work the parallels (preserving the direction) till an edge lie over D; draw a line from D, trending S. W. by W. W., toward the line A B (always take care that the lines of bearing are drawn in a contrary direction), so that as the ship is somewhere on AB, the point shall bear N.E. by E. E. Again, place the parallel rules on the compass over E. 7 S. and W. N., then move the parallels (being careful to preserve the direction) till an edge lies over E, then draw a line trending W. N. from E towards the line AB; the point where these two lines intersect is the ship's position, the latitude and longitude of which, found in the usual way, is

(III). With the ship's head on the above compass course, C bore N. 85° E., and after continuing on the same course 13 miles it bore N. 39° E.: find the position of the ship and her distance from C at the second bearing.

First, the two compass bearings are to be turned into true bearings, by applying the same corrections as in II.

Place the parallel rules on the compass on the chart, over the first true bearing which lies East and West, and working the parallels (strictly preserving the direction) till an edge lies over C. Draw a line trending West from C (i.e., in a contrary direction to the bearing from the ship), cutting the line A B in a point which call a. Again, proceed to lay off the second true bearing, N.E., by laying the parallels over N.E. and S.W. on the compass on the chart (and being careful to preserve the direction), move them to the point C, and draw another line running S.W. (opposite the bearing) toward the ship, and cutting the line A B in a second point, which mark b. Next, with a pair of dividers, take the distance sailed, 13 miles, from the graduated meridian and lay it off from a towards b, and mark the point to which the foot of the dividers extend d. Through d draw a line parallel to the line of first bearing, and where this parallel line cuts the line of second bearing is the ship's position