88 POPULAR LECTURES AND ADDRESSES. 



ship at the time of observation was somewhere 

 on a certain circle of the earth at every point of 

 which the sun's altitude was the same. To draw 

 this circle on a drawing globe, such as the black 

 globe before you, you must find first at what 

 point of the earth the sun was overhead at the 

 instant of observation. This you do immediately 

 by aid of the Nautical Almanac, which gives you 

 the instant of the sun's being due south at Green- 

 wich every day of the year. Thus on the 27th of 

 August 1874 the sun "southed" at Greenwich at 

 I2H. IM. 238. P.M.; therefore in a place in west 

 longitude 20, he was due south at the instant 

 of observation. His declination was 10 N., hence 

 he was overhead in lat. 10 N., Ion. 20 W. Put 

 one point of the compasses at the corresponding 

 point of your drawing globe, and draw by aid 

 of the compasses a circle running at 40 of the 

 earth's surface from this point. The ship was 

 somewhere on this circle at the instant of the 

 observation. The chart before you shows this 

 circle drawn on Mercator's projection not a true 

 circle as you see, because circles on the earth's 



