Geographical Maps and Sailing Charts. 355 



rhumb courses. Moreover winds and ocean currents, although 

 they may not be counted on as proceeding along great circles 

 for any considerable distance, are likely to follow them, while 

 they can have absolutely nothing in common with the direc- 

 tions of rhumb lines on a Mercator's chart. Wherever a 

 navigator may be, it certainly would be to his advantage to be 

 able to find the great circle, or shortest course, and the dis- 

 tance, to his port of destination, and also his bearings at any 

 point along the route ; all of which may be determined with 

 ease and exactness with a stereograph ic chart and a protractor. 

 Moreover, none of them may be determined directly by means 

 of a Mercator's chart, although the bearings for rhumb courses 

 may be found on the latter. There are some who strongly 

 recommend the use of two charts for purposes of navigation, 

 one gnomonic, the other Mercator's. On the former, all great 

 circles are represented by straight lines, hence the shortest 

 course between any two points is easily found, and may then 

 be transposed to a Mercator's chart ; but still this combination 

 of the two charts does not give a much desired factor : the 

 distance from any point where the navigator may be to his 

 desired point of destination. 



In order to demonstrate the practicability of the stereo- 

 graphic projection for purposes of navigation, a chart of the 

 North Atlantic Ocean has been prepared, shown much reduced 

 in figure 24. The original chart measures 17 by 27 inches, 

 and is based upon a sphere of 1*2 meters, 3 feet 11 J inches, 

 diameter, regarding the chart as made on a tangent plane at 

 39° N., 45° W. Thus, although the scale is smaller than that 

 employed in making the map of the United States, much the 

 same data were available, as the point of tangency for both is 

 on the thirty-ninth parallel. 



To really appreciate the merits of the chart it must be 

 studied in connection with its accompanying protractor, figure 

 25, which, for the purpose, need be but a small portion (that 

 near the center) of a protractor such as shown in figure 10. 

 As the protractor is on a large scale (agreeing with the chart) 

 both great and small circles corresponding to every degree may 

 be shown on it without causing confusion. With a single 

 illustration it is possible to show the protractor in only one 

 position, and that chosen in figure 25 indicates the way it may 

 be applied to the problem of sailing from New York to the 

 English Channel along the arcs of great circles. It is impos- 

 sible to sail along one great circle ; and to show this with 

 the protractor it would be necessary to turn the latter about its 

 center until one of its great circle arcs extended from New 

 York to the Channel. The great circle which does this runs 

 from New York City, just south of Boston, along the northern 



