NA VIGA TION. 29 



of navigation must be laid down on charts. 

 Though useful auxiliary drawings may be done on 

 the round surface of a blackened globe, you cannot 

 draw a straight line on a globe, and for accurate 

 drawing with existing mathematical instruments, a 

 flat surface is necessary. 



25. The various kinds of projections to be found 

 in different maps and atlases would take too long 

 to describe, but except for polar regions, the only 

 one of them used in navigation is that very 

 celebrated one called Mercator's projection, and I 

 shall therefore limit myself to describing it to you 

 this evening. It has the great advantage, that it 

 shows every island, every cape, every bay, every 

 coast line, if not too large, sensibly in true shape. 

 Every course, every direction, at any point of the 

 earth's surface, is shown precisely in its true 

 direction on Mercator's projection. Imagine a skin 

 of paper to coat this globe before you as the skin 

 of an orange coats an orange. Imagine a hole 

 made at the north pole, and another at the south 

 pole, and the skin to be stretched out without 

 altering the length from equator to either pole. Or 



