the course. They are entered in the traverse table in the north column for north courses and in the south column for south courses. It is also the north or south distance between any two points on the earth's surface. Departure is the quantity taken for each corrected course and distance from Tables 1 or 2 and entered in the traverse table in the east column for east courses and in the west column for west courses. It is also the east or west distance between any two points on the earth's surface expressed in true or latitude miles. Latitude is the distance any point on the earth's surface is north or south of the equator, and is expressed in degrees, minutes and seconds. Parallel of latitude is a circle parallel to the equator and equidistant from the north or south poles. The equator is a great circle around the globe equidistant from the poles. From it the degrees of latitude are counted north or south toward the poles. Longitude is the distance any point on the earth's surface is east or west of the meridian of Greenwich measured on the equator and is expressed in degrees, minutes and seconds in this work, but for other purposes in hours, minutes and seconds. A meridian is the arc of a great circle 180° extending from pole to pole, its plane cutting that of the equator at right angles. Meridian of Greenwich is the prime meridian from which longitude is counted east or west. Middle latitude is half the sum of the two latitudes between which a course and distance may be wanted. It is used for converting difference of longitude into departure or departure into difference of longitude, using Table 2. Meridional parts is the distance any point on the earth's surface is north or south of the equator measured on a Mercator chart with a degree of longitude at the equator and is expressed in miles and tenths. Meridional parts are found in Table 3 for any degree or mile of latitude to latitude 79° 59′. Meridional difference is the difference of the meridional parts for the latitudes of any two points on the earth's surface. HOW TO USE A DEVIATION TABLE Never take deviation from the table for a bearing; but when a bearing is taken, carefully note the point the ship was heading at the time the bearing was observed. No matter what the bearing may be, take the deviation for the ship's head. Example: Compass course W. by S. Ans., dev. 22° 00' E. Example: Compass course S. Compass course S. Ans., dev. 3° 00' E. Example: Compass course S. 1⁄2 W. At examinations it is usual to give the tables for whole points. When the deviation is wanted for a half point, the mean of the nearest point on each side will be the deviation for the half point. To find the deviation for S. 1⁄2 W., take deviations from the table for S. and S. by W. and half their sum, both being east, will be the deviation for S. 1⁄2 W. Both of which being east, it takes the same name. Example: Compass course S. 1⁄2 E. In this case the deviations for S. and S. by E. are of different name. To find the deviation for S. 1⁄2 E., take the deviations from the table for S. and S. by E. and half their difference named after the greater will be the deviation for S. 1⁄2 E. |