ON OBTAINING THE MERIDIAN 51 



From the table for change of declination, and for the 

 locality eastern New York, the values 5 28' and 11 31' 

 are obtained, showing that the needle in the 110 years 

 swung 6 03' to the westward. The desired bearing 

 therefore should prove to be N 6 E nearly. 



SECTION VII 

 ON OBTAINING THE MERIDIAN 



When for any reason it is necessary to determine a true 

 meridian, that is best obtained from the north star. This 

 star, easily identified by the range of the " pointers," is not 

 exactly at the pole of the heavens, but in 1908 was 1 11' 4" 

 from it. This angle is called the " polar distance " of the 

 star. It is decreasing at the rate of about one third of a 

 minute yearly. 



The north star, like other stars, is thus circling around 

 the pole once in about 24 hours. When directly over or 

 under the pole it is said to be in culmination, upper or 

 lower as the case may be. The star is then in the meridian, 

 and bringing it down with plumb line or transit gives the 

 meridian directly. 



\Vhen the north star is farthest from the meridian it is 

 said to be in elongation, east when the star is east of the 

 meridian, west when on the opposite side. A plane through 

 the observer, the zenith, and the north star when at elonga- 

 tion, prolonged downward to the horizon, makes an angle 

 with the meridian which is called the azimuth of the star 

 at that time. This angle may be obtained for any time and 

 position from tables, and setting off the angle, the true 

 meridian is found. Upon this meridian the needle can be 

 read or marks can be left for reference at any future time. 



The operation of bringing down the star may be per- 

 formed either with the plumb line or, more accurately and 

 conveniently, with a well-adjusted transit. When the 

 transit is used it is necessary to illuminate the cross wires. 

 This may often be done by holding a lantern or candle 

 in front of the transit tube and a little to one side, when 

 the field should appear light with the cross hairs show- 



