272 Mr GODFRAY, ON A CHART AND DIAGRAM 



course with Australia and other distant parts of the earth, have rendered it desirable to 

 shorten as much as possible these long voyages ; and it is especially in long voyages that the 

 advantages of sailing on a great-circle are most apparent. 



The improvements also which have taken place in our ships and in our means of navi- 

 gating them render this more practicable than in the days when a ship's longitude could be 

 determined by dead reckoning only ; the chronometer now has put it into the seaman's power 

 to calculate that element with as much accuracy as his latitude ; and so great is the confidence 

 placed in the results, that cases are known where, when once in the open sea, no other 

 reckoning has been kept but that of steering a course, the position of the ship being ascer- 

 tained by daily observations, and distances run, leeway, currents entirely disregarded. 



The introduction of steam vessels for Ocean navigation thereby enabling a mariner to 

 shape his course and lay the ship's head whichever way he pleases, independent, in a great 

 measure, of winds fair or foul, would again lead men to think of the direct course and shortest 

 route, although,, as we shall see when speaking of windward sailing, the advantages of the 

 method are still greater to. sailing vessels beating against a head wind. 



We find consequently that within the last ten or twelve years various methods have been 

 proposed for simplifying the calculations which determine the course to be followed, and most 

 of them also give directions for determining, with more or less accuracy, the latitudes and 

 longitudes of a few positions on the track generally at intervals of 5° of longitude. 



But still, it is not favourably received : — a few of the more intelligent captains occasionally 

 adopt it, but others equally intelligent, if they do not actually speak of it disparagingly, say 

 that the difficulty attending its use will always render it of little practical value ; and for the 

 great majority of our merchant captains, who have little time and still less inclination for 

 lengthy calculations or intricate manipulations, Mercator's sailing has advantages too obvious 

 to be overlooked. 



The fact is, that the main objection still exists in all its force: the track is not seen on. 

 the thart except in a greatly distorted form, and even this can be obtained only by a long and 

 tedious operation which after all gives only a few positions ; and, as the ship is nearly certain 

 to deviate from this track in the course of a few hours, the operation would have to be 

 repeated every day if not oftener. The course to be steered can be obtained with compara- 

 tive facility, but by itself it tells nothing of the dangers ahead, and without the chart-track 

 the ship may be blindly running to destruction. 



So strongly is this difficulty felt that even the advocates of great-circle sailing recommend 

 avoiding great-circle tracks which pass near groups of islands, whereas when using Mer- 

 cator's sailing and guided by Mercator's charts, no such recommendation is considered 

 necessary. 



While considering this subject it occurred to me that charts constructed on the Central 

 or Gnomonic Projection would exactly meet this difficulty, and would in Great-circle sailing 

 answer the same purpose that Mercator's charts do in Rhumb sailing. All great-circles would 

 be represented by straight lines, and to see the ship's track we should merely have to draw a^ 

 straight line from the ship's place to her destination. 



