ON OBTAINING THE MERIDIAN 55 



The peep sight will now be secured in place by a clamp or 

 weight with its exact position marked on the rest, and all further 

 operations will be deferred until the next morning. 



By daylight, place a slender rod at a distance of two or three 

 hundred feet from the peep sight, and exactly in range with it and 

 the plumb line ; carefully measure this distance. 



Take from the table on page 56 the azimuth of Polaris cor- 

 responding to the latitude of the station and year of observation ; 

 find the natural tangent of said azimuth and multiply it by the 

 distance from the peep sight to the rod ; the product will express 

 the distance to be laid off from the rod exactly at right angles to 

 the direction already determined (to the west for eastern elonga- 

 tion or to the east for western elongation), to a point, which with 

 the peep sight, will define the direction of the meridian with suffi- 

 cient accuracy for the needs of local surveyors. 



Example: Sept. 10, 1915, in latitude 45 N, longitude 

 71 W, it is desired to obtain the declination of the needle. 



From the table giving times of elongation it is found that 

 Polaris is at eastern elongation on Sept. 1st at 53.2 minutes past 

 8 P.M. 



Correction A is not required in this case. 



Correction B, for the 9 days elapsed since Sept. 1st, is 35.3 min., 

 to be subtracted. 



Correction C, for 71 longitude, is 16 min., to be subtracted. 



Correction D, for 45 latitude, is 0.85 min., to be added. 



Correction E is 0.2 min., to be added. 



8 hrs. 53.2 min. >35.3 min. 16 min. + .85 min. + .2 min. 

 = 8 hrs. 3 min., time of elongation by the watch. 



The star having been observed at the time indicated and brought 

 down to the horizon, its azimuth is ascertained from the table of 

 azimuths. For 1915 and latitude 45, this value is 1 37.4' and 

 there is no appreciable correction for apparent place. The merid- 

 ian then is that much to the west of the line determined. In this 

 case, with the instrument on the azimuth line the needle was 

 allowed to settle and a reading of N 17 50' E obtained. 17 50' - 

 1 37.4' = 16 12.6'. 16 12.6' is therefore the magnetic declination 

 for the place and time, or 16 15' as near as a needle can be 

 read. 



In practice corrections D and E may usually be neglected. 

 Using the table for time of elongation with corrections A, B, and C 

 applied to it, the surveyor will ascertain when to be on hand for 

 the observation. Then, watching the star, when satisfied by its 

 motion that it has reached elongation he will bring his instrument 

 down without regard to time. In fact, Polaris traverses less than 

 4' of azimuth in the hour before and the hour after elongation. 



