Page:Small-boat sailing; an explanation of the management of small yachts, half-decked and open sailing-boats of various rigs; sailing on sea and on river; cruising, etc (IA smallboatsailing01knig).pdf/216

 English Channel, the variation is 25° W. The compasses designed on general charts, such as the chart of the North Atlantic, show the true points of the compass, but on the coasting charts employed by the owner of the small cruiser the compass designs are more generally magnetic. When laying down one's course on a chart of the latter description, one has but to use the parallel rules in the manner I have already explained, and the course to be steered will be read off on the compass design. On the other hand if one is using a chart on which the compass is true, one must correct for variation the course as shown on the chart. The variation is given in figures on most charts. When correcting a true course into a magnetic course, allowance to the right must be made if the variation is westerly, to the left if it is easterly. For example, if one's true course by the chart is S.W. and the variation is 25° W., one must steer 25°—that is, roughly, two points—to the right of S.W., which makes the course W.S.W.

Not only can one find one's correct course on a chart, but it is also possible, despite the distortion of magnitudes produced by Mercator's projection, to measure distances upon it. To do this with accuracy necessitates calculations beyond the scope of this book; these are explained in Norie's, Raper's, and other works on navigation. But distances can also be approximately ascertained on the chart by simple measurement, a method sufficiently accurate for all practical purposes. On looking at a chart it