GREAT CIRCLE SAILING. A great circle is a circle or a sphere the centre of which coincides with the centre of the sphere. Thus, the equator, the meridian and the ecliptic cir cles are such 'great circles" on the globe. Be tween any two points on the surface of a sphere the shortest distance lies along the arc of a great circle which passes through those two places. Such a great circle cuts the suc cessive meridians at different angles whenever the two places are not on the equator or on different meridians. The point of the curve nearest the pole is called the "vertex," the point most remote from the rhumb line is designated as the 'point of maximum separa tion." 'Great circle sailing' is merely a method of navigating vessels by the arc of a great circle. This method was first mentioned by one John Davis (1594) in a book called 'Seaman's Secrets.) It was much elucidated subsequently —by Townson (1847), by Godfrey (1858), by Captain Bergen (1880), by Goodwin (1894), and by Captain Lecky (1903). The great circle course, as followed by mariners, is that on which a vessel sails straight for her distant port, which she would follow if heading for a heavenly body directly overhead. That this
heavenly body is only imaginary does not in the least rob the method of practicability. Of course difficulties of computation in great circle sailing are numerous, but the invention of sev eral ingenious devices has greatly minimized these difficulties. Charts are prepared on what is known as the gnomonic projection, and the use is made of great circle protractors, the sphereograph and other instruments. The gnomonic chart is made by a projection of the earth's surface upon a plane, tangent to the earth at some point on its surface, taking as the point of sight the centre of the earth. By this construction, all planes cutting great circles on the earth, pass through its centre and cut the plane of projection in straight lines, so that a straight line joining any two points on the chart will be the projected arc of the great circle. Charts have been constructed in this way by the hydrographic office of the United States Navy Department, and greatly facilitate the work of the navigator.