211 



SAILINGS. 



SAILINGS. 



242 



per hour. The terrestrial deviation on the compass to be 2 points 

 westerly. The ship's local attraction upon the average course to be 

 16 westerly : while, moreover, the wind being north-easterly (true), 

 and the ship .being nearly close-hauled, we should allow about 1 point 

 leeway. It is obvious that with so many elements of disturbance an 

 approximate course must be selected before starting. Now, the un- 

 scientific ship-master would guess at his course, and if he found himself 

 by subsequent observation or otherwise, setting either to leeward or 

 the reverse, would so vary his course as to attempt by time-losing ex- 

 periments to finally reach his destination ; while the complete navigator 

 would probably proceed thus : He would either roughly calculate his 

 true course, having in his mind's eye the following figure, or would 

 construct the figure itself, as under : 



**,/ 



Let o represent the ship's position at starting, and A the intended 

 port. It in usual to take the current as a course and distance. We get 

 Ui.- Approximate distance by saying in this case 8 : 80 : : 3 : 30. oz 

 will, therefore, represent in position and magnitude the current 

 course and distance ; compounding the ship's bearing and distance 

 from her intended port, and the set of current, atfarttt [COMPOSITION], 

 we get the parallelogram o z A y, and the L A o y (composed with the 

 N.\V. line o A) will be the course, which measured from north would 

 be about N. S2J w -. or by calculation, thus : o A being given as 80, 

 and bearing N.W., and oz being given 30, bearing W.N.W., the 

 . would 2 points : hence in the triangle A oy we have given 

 the two sides and included angle OAy(=zOA) to find" the rest by 

 trigonometry. From this it would be seen that starting from o, in the 

 direction oy, would, as influenced by the current, carry the ship 

 :tl'.i,_- the resultant o A to its intended port at A, were no other in- 

 fluence at work. But perhaps the moat difficult portion of a mariner's 

 duty U to clear his course from compass errors. To continue the 

 examplr: aft.-r projecting the 2 poinU variation westerly from B to c, 

 and the 10 W. by local attraction [LOCAL ATTRACTION] from c to D, 

 :ni I the 1 point leeway (westerly) from D to K, we find that the 

 correction for leeway and compact error auiounU in thU case to the 

 correction H >:, which, measured to the eastward of oil, gives the true 

 course which must be steered by compass to be o r, or instead of about 

 X. .>\ \V., it should be about N. 18 E. 



Whatever method of sailing we adopt, the above mode of correction 

 w indispensable ; but with the exception of terrestrial deviation, the 

 .mi'. unt of correction to be applied depends chiefly on the judgment, 

 and perception, and vigilance of the navigator. 



Miilille latitude sailing has already under the article RECKONINGS AT 

 SEA been partially explained as regards it* application to practice ; 

 iU principle* 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 U one of only relative value, depending on the latitude at 

 which it is situated : and when we estimate longitude by turning (as 

 the phrase U) departure into longitude, the assumption of its latitude 

 u either obtained by using middle latitude sailing, which adopts a 

 mean between the latitude left and that arrived at, or by Mercator's 

 wiling, which in most cases is more accurate. The questions will be 

 better understood from the following figure : 



Loi.1. 13 



48 1 l^.t. 



herself at B. By middle Latitude sailing the mean between the paralle 

 left and that reached would be If J?_ = 45, but if we measure 



along the track sailed, it will be found that one-half the distance 

 sailed would actually fall to the southward of 45, or at c ; hence the 

 inaccuracy of middle latitude sailing in finding the longitude. Of 

 course the remedy for this would be to divide the track into portions, 

 and find the difference of longitude for each. When near the equator 

 where the meridians are not so convergent, middle latitude sailing may 

 be used with very trifling error; but Mercator's sailing is at such 

 places less accurate, because, as the following figure will show, a small 

 error in the course would make a large error in longitude. 



In this,c.z would be the distance run, and the angle ACB the course, 

 rf.c 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 ia 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, alt, equal to the parallel of 40, and assume the 

 points thereon at c and rf, the nearest distance between them will 

 appear to be cd. This may be shown to be incorrect if the parallel of 

 40 be drawn in fy. 2 on gnomonic projection, where the chord ced 

 connects the two points and is their nearest distance ; hence the 

 curve c e d, in fig. 1, is the nearest distance between the points, c and il, 



Fig. 1. 



Fi?. 2. 



' in a portion of an ordinary map, let a ship start from A, and after a 

 d nj'n run (say of 200 miles) in various directions, suppose she finds 



ART* AND SCI. DIV. VOL. VII. 



because its plane would pass through the centre, while the plane of c </ 

 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 

 Mercator's; as already fully explained under GREAT CIRCLE, OR 

 TANUENT SAILING. 



Windward sailing in a term used in connection with great circle 

 sailing, 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, hi determining on 

 which " tack " to sail with reference to the great circle track itself. 

 For example, suppose that a ship starting from B towards A on a great 



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 

 tack, as at D, 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- 



R 



