Geographical Maps and Sailing Charts. 



365 



in constructing them. This is a grave error, which should cer- 

 tainly be corrected. Every map which is published should be 

 accompanied by a statement of the method of projection used 

 in making the same. As it is at the present time, maps are 

 plotted in various ways, and if one is called upon for an expres- 

 sion of opinion concerning any map, it is highly .probable that 

 the projection on which it is based would not be recognized with 

 certainty. Let it be assumed that one of the simplest projec- 

 tions, the conical, is recognized ; the map might have been 

 made upon a tangent cone, with either central projection, 

 figure 28, or projection at right angles to the conical surface 



Tangent Cone, with Central Pro- 

 jection. 



Secant Cone, with Projection 

 at right angles to the Conical 

 Surface. 



as in figure 29 ; or the map might have been made on a secant 

 cone, with projection at right angles to the conical surface, 

 figure 29, or central projection, as in figure 28. How then may 

 one pass judgment on a map, based on the conical projection, or 

 some other which is not recognized with certainty, and tell, for 

 example, whether a measurement, such as from New York to 

 San Francisco is reasonably exact, or perhaps one hundred 

 miles out of the way? The present writer has long entertained 

 the belief that measurements made on maps with the scales of 

 miles accompanying them are only approximately correct, 

 and this has been confirmed by numerous trials. Take, for 

 example, the stretch from New York to New Orleans : This 

 has been repeatedly measured by the present writer with 

 results varying from 1170 to 1236 miles, while the calculated 

 distance is 1166|- miles. The results of map measurements made 

 with accompanying scales of miles are almost invariably too 

 great, according to the writer's experience. This is conspicu- 



