relative magnitudes and distances, the direction from one point to another is correctly represented; that is, if a place is N.N.E. of another place it appears so on the chart, and a straight line drawn on a chart between the two places would correctly represent the track of a vessel steering on a N.N.E. course. The mariner can therefore find his course from one place to another without any difficulty on this chart. The parallel rules are laid so that one edge touches both places on the chart; the rules are then slid over the surface of the chart (one of the rules only being moved at a time, so that the direction is preserved) until one edge is exactly over the centre of one of the compasses which are designed on the chart. The edge will then indicate on the compass the course to be steered.
Except in certain special plans, which, for convenience, are drawn on a diagonal scale, the top of a chart is the true North, the bottom is the true South, and the right and left sides are true East and West respectively; straight parallel lines drawn across the chart from right to left and from top to bottom representing the parallels of latitude and the meridians of longitude. I need scarcely explain that the magnetic needle, in most parts of the world, does not point due North, but to the West or East of it at an angle which varies with the locality. The difference between the true and magnetic North is called the Variation of the Compass, and is expressed in its angular value; thus, outside the mouth of the
