I. On a Chart and Diagram for facilitating Great-Circle Sailing* 

 By Hugh Godfray, M.A. St John's College. 



[Read May 10, 1858.] 



The idea of navigating a ship on the arc of a great-circle, as being the shortest and most 

 direct route, is not one of modern origin ; it must, in fact, have presented itself to many 

 minds simultaneously with the knowledge of the Earth's being a sphere ; and we find that 

 very early in the progress of navigation, its principles were fully understood and acted upon, 

 and that before Mercator's invention it was employed for the guidance of vessels in distant 

 voyages in preference to sailing on a Rhumb. But if we consider that logarithms were not 

 yet invented, the amount of calculation must have been so great as very considerably to 

 restrict its use ; and it is not to be wondered at if the much simpler method given by 

 Mercator should at once have found favour among mariners, and entirely superseded the 

 laborious calculations which Great-circle sailing required. 



Among other causes which would also tend to the disuse of the method would be the 

 uncertainty of the ship's position in longitude, an element which ought to be known with 

 tolerable accuracy, but which after only a few days sailing could not be much relied upon, 

 even under the most favourable circumstances — absence of storms and of unknown currents. 

 This would not affect sailing on a Rhumb so long as the same Rhumb course could be kept; 

 but such would seldom be the case for any long distance, — and so the usual plan was to steer 

 a course which would bring the ship to the latitude of the port bound to, or of any other 

 point which it was considered desirable to make, and then sail due east or west on the parallel 

 until the place was reached ; and this method is even now frequently practised. 



But it is most probable that what, more than any other circumstance, contributed to the 

 neglect and ultimate rejection of Great-circle sailing, was the difficulty of ascertaining whether 

 obstacles in the shape of land or rocks lay in the path. In Mercator's charts the whole track 

 was seen on the map by merely drawing a straight line from the ship's place to her destination 

 or to such other point as it was found desirable to attain ; the mariner, by a mere inspection 

 of this line, could see at once whether the track was one on which he could navigate, and if 

 not, how far it was necessary to deviate from it. 



This great advantage combined with the simplicity of the calculation will fully account 

 for the universal adoption of the method, notwithstanding the greater distance which it was 

 well known the ship would have to go over. In Great-circle sailing all that could be done for 

 the same purpose was to determine with immense labour the position of a few points of the 

 track, and, having marked these on the chart, to connect them by a curved line, the eye being 

 made judge of the greater or less curvature that should be given in the intermediate spaces, 

 so that the line should curve in a regular manner. 



Of late there has been manifested a desire to revive the almost forgotten Great-circle 

 sailing : the great extension of our commerce, and the constantly and rapidly increasing inter- 



' Engraved by the Hydrographic Office, Admiralty. Published by J. D. Potter, 31, Poultry, Agent for the Admiralty Charts. 



Vol. X. Pakt. II. 35 



