96 POPULAR LECTURES AND ADDRESSES. 



above the horizon, the meridian altitude is lost, 

 and Sumner's method, or something equivalent to 

 it, must be put in requisition. When the meridian 

 observation is lost, but instead of it the altitude 

 within half an hour before or after the sun crosses 

 the meridian is observed, it is usual to employ a 

 a table, which is given in the books on Navigation, 

 for computing what is called " reduction to the 

 meridian," that is to say, the addition which must 

 be made to the observed altitude to find the true 

 meridian or highest altitude ; but in practice it 

 is really much better to draw the Sumner line of 

 the actual observation on the chart, and judge from 

 it what the observation has really told you as to 

 the ship's position. 



55. The chart before you (Fig. 15) illustrates 

 Sumner's method by an actual case of its use in 

 ordinary navigation, in a voyage from Falmouth to 

 Madeira, made by the sailing yacht Lalla Rookh, 

 from the 3rd to the Qth of May, 1874. The times 

 marked on the several Sumner's lines are the 

 Greenwich mean times of the observations. Look 

 carefully at the positions of the Sumner day-lines, 



