NA VI G A TION. 



89 



surface are not shown as circles on Mercator's 

 projection. 



Suppose now that 2H. 4OM. later the altitude of 

 the sun is again taken and found to be 50. At 

 the moment of this second observation, the ship 

 was on this other circle which you see on the 



I2O IOO 80 60 40 2O O 2O 40 60 



FIG. 14. Mercator chart, showing Sumner circles. 



chart. What we learn from the two observations 

 then is, that at the time of the first observation 

 the ship was somewhere on that first circle, and 

 that at the time of the second observation she 

 was somewhere on that second circle. These 



