The outer graduated circle is intended to represent the correct Magnetic Points of the horizon. The inner graduated circle is intended to show Ship's Head" by Compass," and the lines leading from it to the outer circle indicate the corresponding direction of Ship's Head "Correct Magnetic" for each point of the Compass. 60115040302019 40 50 60 70 800 80 FIG. 19. Art. 59. Napier's Method is a combination of a curved line and a straight line, the deviation between which shows at once the amount of compass deviation for each point. Its chief merit is that the deviation may be determined for a few points, and a symmetrical curve traced through them will give a graphical illustration of the deviation for each point and fractional part of a point of the compass. The diagram (see figure) consists of a vertical line of any convenient length and divided into 32 equal parts representing the points of the compass from N. at the top downward; or it may be, as in the figure, divided into two equal parts, one from N. to S. by the way of E., and the other from S. to N. by way of W. The line is also divided into 360°, and also from o at N. and S. to 90° at E. and W., similarly to the compass-card. The vertical line is intersected at each of the 32 points by two straight lines inclined to it at an angle of 60°; one dotted line and one plain line. These lines therefore form equilateral triangles; the scale on each being the same, the amount of deviation may therefore be taken from the vertical scale of degrees. Easterly deviations are laid down to the right hand and westerly deviations to the left. If the deviation has been determined on an exact point of the compass, lay down the deviation on the dotted line passing through that point; if not on an exact point, then lay it down on a line drawn parallel to the dotted line, the direction of the ship's head being taken from the vertical line. Mark the points of deviation and draw through them as nearly as possible a curve. Should the deviations be determined on correct magnetic courses, the plain line is used instead of the dotted, but this is very rarely the case in practice. To find the Magnetic Course from the course steered. I On the vertical line take the given compass course. Move in a direction parallel to the dotted lines till the curve is reached, and then parallel to the plain lines until the vertical line is reached again. The point arrived at on the vertical line is the magnetic course required. EXAMPLE. What is the magnetic course corresponding to course by compass N. E. by E.? N. 74° E. To find the Compass Course, having given a Magnetic Course. On the vertical line take the given magnetic course. Move in a direction parallel to the plain lines till the curve is reached, then back parallel to the dotted lines to the vertical line. The point there arrived at will be the compass course required. |