Now, as a degree or a minute of latitude is the same length everywhere, we can use these graduated meridians as scales of miles. Each degree is divided into sixty minutes. A minute or nautical mile is about one-sixth longer than an English mile.
But as Mercator’s chart distorts the latitudes, each portion of the graduated meridians serves as a scale of miles only to that part of a chart immediately opposite to it, that is, on the same latitude. Consequently, to measure the distance between two places on the chart, for instance, Falmouth harbour in latitude 50° and Madeira in latitude 30°, we proceed as follows. The dividers are spread out till their points are over the two places on the chart. This distance is then measured off on the graduated meridian, by so placing the dividers on the scale that the mean latitude of the two places, that is, latitude 40°, is marked on the graduated meridian exactly halfway between the two points of the dividers.
The graduated parallels cannot, of course, be used as scales of miles for purposes of measurement.
While measuring distances on the large scale charts used for coasting purposes it matters little what portion of the graduated meridians be used as a scale, for the error will be imperceptible. It is only while using what are known as general charts, which extend across many degrees of latitude, that a considerable error will result from not measuring from the mean latitude.
Compasses are generally designed on various portions of a chart. These compasses usually indicate the magnetic and not the true bearings. (See Compass).
Various abbreviations are used on charts which are generally explained in a corner of the chart itself. Thus the nature of the bottom is indicated by sft for soft, c. for coarse, h. for hard, etc., while buoys are marked R. for red, H.S. for horizontally striped.
The soundings in fathoms or feet are indicated by numbers, the depth being calculated at low water ordinary spring tides. Where shoals sometimes dry, their height above low water spring tides is given by underlined numerals.
Roman numerals on a chart give the hour of high water at that spot at the full and change of the moon.
The velocity of the tide in knots is also indicated and its direction shown by arrows.
The yachtsman should provide himself with large scale Admiralty charts; for these alone can guide him into those shallow intricate channels frequented by small coasters only, which he will be so often tempted to enter.