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. 
