MERIDIONAL DlerAxcz, in naviga tion, is the same with the departure, east ing or westing, or the difference of lon gitude between the meridian under which the ship now is, and any other me ridian she was before under.
MaRIDIONAL PARTS, MILKS, or anivirras, in navigation, are the parts by which the meridians in Mr. Wright's chart (com monly though falsely called Mercator's) do increase as the parallels of latitude de crease : and as the cosine of the latitude of any place is equal to the radius or se mi-diameter of that parallel, therefore, in the true sea-chart, or nautical plansiph ere, this radius being the radius of the equi noctial, or whole sine of the meri dional parts at each degree of latitude must increase, as the secants of the arch, contained between that latitude and the equinoctial, decrease. The tables, there fore, of meridional parts, which we have in books of navigation, are made by a con tinual addition of secants ; they are calcu lated in some booksfor every degree and minute of latitude • and they will serve either to make or iTaduate a Mercator's chart, or to work the Mercator's sailing. To use them, you must enter the table with the degree of latitude at the head, and the minute on the first column to wards the left hand, and in the angle of meeting you will have the meridional parts. Having the latitudes of two places,
to find the meridional miles or minutes between them, consider whether one of the places lies on the equator, or both on the same side of it, or, lastly, on different sides. 1. If one of the proposed places lies on the equator, then the meridional difference of latitude lathe same with the latitude of the other place, taken from the table of meridional parts. 2. If the two proposed places be on the same side of the equator, then the meridional differ ence of latitude is found, by subtracting the meridional parts answering to the least latitude from those answering to the greatest, and the difference is that required. 3. If the places lie on different aides of the equator, then the meridional difference of latitude, is found by adding together the meridional parts answering to each latitude, and the sum is that re quired.