For coasting purposes and for crossing narrow seas such as the Channel, and the North Sea, and even for a voyage to the Baltic, sextants, chronometers, and other instruments of navigation proper are not required, and the following is a complete list of all that is needed for the purpose of finding one’s way. 1. The necessary charts. 2. A mariner’s compass. 3. A parallel rule. 4. A pair of compasses or dividers. 5. A log ship and line. 6. A hand-lead and line. 7. One of the small nautical almanacs which give the tide tables for the principal harbours of Europe. Pearson’s almanac, which only costs sixpence, is one of the best of these and, in addition to the tides, imparts a great deal of useful information.
Charts.—Whatever method is employed for representing a portion of a globe on a plane surface, the result can only be approximately correct. Charts are drawn on what is known as Mercator’s projection, which, though it greatly distorts the relative magnitudes, is the most convenient for purposes of navigation.
The parallels of latitude on the globe are everywhere equidistant. On the other hand, the meridians of longitude are furthest apart at the equator, and gradually approach till they meet at the poles.
In Mercator’s chart the meridians of longitude are drawn parallel and equidistant. The distortion in the direction of longitude resulting from this projection is counterbalanced by distorting the latitude to a proportionate extent; that is, the parallels of latitude are drawn further apart as the poles are approached.
Thus, on a chart of the world, the polar regions are greatly magnified when compared to the regions near the equator; still, as the degrees of latitude and those of longitude are proportionately magnified, the general shape of any particular tract is preserved.
The reason why this projection is the most adapted for purposes of navigation is that it produces no errors of direction; that is, if one place is north-west of another, it appears so on a Mercator’s chart.
It follows, from what has been said, that one scale of miles will not serve for a chart; as the scale is ever varying. The method of overcoming this difficulty while measuring the distance from one place to another on the chart will now be explained.
On inspecting an ordinary chart it will be found that the top of it is true north the bottom true south.
Along the top and bottom are graduated parallels, which are divided into degrees and minutes, and which enable us to distinguish the longitude of any position on the chart.
So, too, on the right and left sides of the chart we find graduated meridians also divided into degrees and minutes and which enable us to measure the latitude of any position on the chart.