Whatever method of sailing we adopt, the above mode of correction is indisrperusable ; but with the exception of terrestrial deviation, the amount of correction to be applied depends chiefly on the judgment, and perception, and vigilance of the navigator.
Middle latitude &ailing has already under the article RECKONINGS AT SEA been partially explained as regards its application to practice ; its principles will now be briefly considered. The meridians of the globe meeting at the poles, the parallels of latitude diminish in magni tude as they recede from the equator; but as each parallel circle must contain 360° of longitude, it evidently follows that the term degree of longitude is one of only relative value, depending on the latitude at which it is situated : and when we estimate longitude by turning (as the phrase is) departure into longitude, the assumption of its latitude is either obtained by using middle latitude sailing, which adopts a mean between the latitude left and that arrived at, or by Mercateee sailing, which in most cases is more accurate. The questions will be better understood from the following figure : In this, cx would be the distance run, and the angle ACR the course, dx the departure, and AB the difference of longitude.
Parallel sailing is used when the ship makes no difference of latitude, but sails upon a parallel of latitude. It is only preferred for its simplicity, because in " running along a parallel " the distance is the departure, and the true course is east and west : but it is at the expense of accu racy, for a ship thus sails along an arc of a circle instead of its chord, although at first sight the reverse appears to be the case. It is how ever certain, that the shortest distance between two points on the surface of a sphere is the arc of a great circle, the plane of which passes through the earth's centre. Now, if in the following Jig. 1, we draw on a right sphere, rib, equal to the parallel of 40°, and assume the points thereon at c and d, the nearest distance between them will aprear to ho c d. This may be shown to be incorrect if the parallel of 40 be drawn in fig. 2 on gnomonic projection, where the chord c e d connects the two points and is their nearest distance; hence the curve cc d, in jig. 1, is the nearest distance between the points c and d, because its plane would pass through the centre, while the plane of c d would be parallel to it. But these errors might be avoided altogether by the use of great circle sailing, and especially as Mr. Saxby has
rendered the finding of a great circle course more easy than even the Mercateen; as already fully explained under GREAT CIRCLE, OR TANGENT SAILING.
Windward sailing is a term used in connectiou with great circle hailing, by which is implied the advantage taken of the changes in the course of a ship when sailing upon the tangents of a great circle, and is such that a considerable saving of distance in a voyage may be effected when a ship is opposed by contrary winds, in determining on which " tack " to sail with reference to the great circle track itself. For example, suppose that a ship starting from n towards A on a great On a portion of an ordinary map, let a ship start from A, and after a day's run (say of 200 miles) in various directions, suppose she finds circle track, as drawn upon a Mercator's chart, meets at c with the wind N.W. ; if her captain puts her upon the port tack, he absolutely sails away from his proper course, while by keeping her on the starboard Lack, as at n, he sails in a direction nearly parallel to it.
Oblique sailing is merely a term applicable to those problems in which no right angle appears In the projected triangle. It is a mere term of oblique trigonometry, such as occurs in setting off one's posi Lion by crow bearings of objects whose relative bearing and distance from each other are well known.
Coteposate 'tailing was so called. and ably illustrated by Mr. Towson, but the subsequent invention of the Spherograph las rendered it as a quite unimportant.
having thus explained the usual resources of the navigator in his work of calculation under I ystema of progression, the subject may be viewed from another point. The comparative eteadinese of large steamer, ou the ocean, and the greater advantages they poaseas in their partial Independence as to the direction of the wind, might seem to be a relief to the ship-master ; such la however, counterpoised hy'the extreme difficulties inherent in steam navigation, and especially in iron ships. It is true the courses of stesuners are more direct and free from " traverses." but it is the question of local attraction, ever liable to vary, which needs all the vigilance of the commander. This important subject has been treated of under Local. Arreacetex, but we may add that Professor Airy has further illustrated his previous investigas lien by a very valuable communication read before the Institution of Nasal Architects on March 1st, 1860.