134 jS. L. Penfield — Stereographic Projection. 



would indicate not only the distance from any point of danger 

 noted on the chart, but also the direction to any desired port. 

 The possibilities of mistakes in calculation, other than those 

 involved in determining the position of the vessel, would not 

 have to be taken into consideration. 



Still another method of projection which is in very general 

 use is that of Mercator, invented in 1569.* In this, figure 28, 

 the meridians appear as vertical straight lines equally spaced, 

 and the parallels as horizontal lines so distributed that the 



28 



Mercator' s Projection. 



points of the compass preserve the same direction over all the 

 map. In such a projection, areas are very much distorted, 

 especially those near the poles, and distances can not be meas- 

 ured directly. Although the writer does not consider himself 

 competent to pass judgment upon the relative merits of map 

 projections as employed by navigators, he does see many advan- 

 tages of the stereographic over all other kinds of projection. 

 It is interesting to note, for example, that the projection of 

 Mercator, designed especially for use in navigation, does not 

 give the navigator the information he most wishes to know, 

 except in a few cases. A straight line drawn on the map from 

 one point to another gives a possible sailing direction between 

 the two points. If the direction is due north or south on any 

 meridian, or east or west on the equator, the course is that of 

 a great circle ; hence the shortest possible. In all other cases, 

 however, a course thus plotted as a straight line on the map, 

 and sailed by compass without deviation from the direction 

 indicated, will not correspond to the arc of a great circle ; 

 hence will not be the desired shortest possible route. Take as 

 an illustration two ports on the same parallel, for example, Lis- 

 bon, 11° 30' W., 38° 30' K, and the mouth of Delaware Bay, 

 75° 0' W., 38° 30' N. ; how is a mariner to shape his course, 

 provided that no account is taken of ocean currents? To one 



* Germain, loc. cit., p. 205. 



