ON OBTAINING THE MERIDIAN 59 



subtract from the time of west elongation 0.16 min. for 

 every degree north of 40. For eastern elongations sub- 

 tract 0.10 min. for every degree south of 40, and add 0.16 

 min. for every degree north of 40. 



E. To refer to any other than the tabular longitude : Add 

 0.16 min.for each 15 east of the ninetieth meridian and sub- 

 tract 0.16 min. for each 15 west of the ninetieth meridian. 



The deduced time of elongation will seldom be in error 

 more than 0.3 min. 



For Evening Observation. Study of the tables will 

 show that at certain times of the year a choice of methods 

 is offered. Since, however, evening observation is usually 

 most convenient, the following directions have been ar- 

 ranged with that in view. The time limits for these 

 observations, it will be understood, vary somewhat with 

 the latitude. 



On the tenth of January observe western elongation 

 at midnight and for each fifteen days thereafter earlier 

 by one hour. This may be done until late March. 



From late March to early June, use lower culmination 

 with the help of Delta of Cassiopeia. On April 1st the 

 culmination occurs at 12.37 and after that for each fifteen 

 days earlier by one hour. 



From early June to early October use eastern elonga- 

 tion. On June 15th it occurs at 2 A. M. 



From early October to middle January use upper cul- 

 mination with Zeta of the Great Bear. 



