280 mk godfray, on a chart and diagram 



[This will always be found with sufficient accuracy by a mere inspection of the Chart, but 

 the exact point may be obtained by letting fall a perpendicular from the Pole on the track.] 



Now refer to the diagram, and, along the horizontal line corresponding to the highest 

 latitude, find the point where it intersects the curve corresponding to the latitude of the ship. 

 This point, which may be called the Ship's place on the Diagram, will fall in one of the light or 

 shaded spaces, and will indicate the course to be steered in points and quarter-points, from N, 

 or S. towards E. or W., as previously found ; and at the same time its position relatively to 

 the vertical lines will give the distance from the highest latitude in miles. 



We must, in the next place, determine how far the ship must run upon the course so found; 

 and this will be done by proceeding along the horizontal line which represents the ship's 

 course on the diagram (going towards the increasing or decreasing latitudes as the track on 

 the chart will indicate), until we reach the light or shaded space corresponding to the next 

 quarter-point. The difference between the two corresponding numbers at the top of the 

 diagram will be the distance to run on that first course, but it will be found easier to measure 

 the distance with a pair of compasses and apply it to the small scale of Nautical Miles at the 

 bottom of the diagram ; and in the same manner may the distances to be run on the successive 

 courses be known. 



This may be done for the whole track, but since it will be almost impossible to keep the 

 ship on the exact track during the entire voyage, the first two or three courses and distances 

 will be sufficient, and the whole operation being so simple and rapid, had better be repeated 

 each day with the new position of the ship. 



In just the same manner as the distance of the ship from the highest latitude has been 

 found, may the distance of the other place be determined ; and the sum of the two distances 

 when the highest latitude falls between the places, or their difference when not, will be the 

 distance of the one place from the other. 



The course curves are traced so as to give the nearest quarter-point. They are exact for 

 the middle of each space, and the separation between a light and a shaded space corresponds 

 accurately to the intermediate eighth. Thus the boundary line between the 5^ shaded and the 

 5f light spaces corresponds exactly to 5^ points. By multiplying the number of spaces there 

 would be no difficulty in further reducing the maximum error; but, as we have before stated, 

 such accuracy would be superfluous. 



I shall give one or two examples to illustrate the foregoing instructions. 



Ex. 1. To find the courses and distances on a great circle from Cape Agulhas, the 

 Southern extremity of Africa, to Perth in Australia. 



Having drawn the track on the chart it will be seen that the highest latitude is 44|°, and 

 the track is a practicable one. 



Now refer to the diagram and find the point where the horizontal line through 441^' is crossed 

 by the curve of 35", the latitude of Cape Agulhas. This point falls on the light space corre- 

 sponding to 5^ points. Hence the first course is S. 5\ points E. or S.E. by E. ^ E. ; and the 

 corresponding distance is 2130 miles from the highest latitude. 



