94 POPULAR LECTURES AND ADDRESSES. 



within so short a time one after the other that the 

 ship has not travelled far in the interval, we 

 get two Sumner lines on our chart, and know 

 that, at the time of the observations, the ship 

 was actually on the point in which the two lines 

 met. 



Thus on a clear night we can at any time find 

 the ship's actual place, as we can always choose 

 two good stars in good positions for the purpose ; 

 while by day, all we can tell, as we have only one 

 sun and no other visible body (except sometimes 

 the moon, which is not very convenient for such 

 observations), is that the ship is on a certain 

 line, viz., the Sumner line for the moment of 

 observation. If, then, we could observe the al- 

 titudes of stars with the same accuracy as the sun, 

 we could know the ship's place better by night 

 than by day ; but, alas, the observation of the star 

 altitude is rarely to be made with all the desired 

 accuracy, even by the most skilful observer, because 

 it is so difficult at night to see precisely where 

 the sea-horizon is. 



54. LATITUDE BY SUMMER'S METHOD. One 



