ON OBTAINING THE MERIDIAN 53 



Six months later the star Zeta (fl, in the tail of the Great Bear, 

 will supply its place, and will be used in precisely the same manner. 



The diagram, drawn to scale, exhibits the principal stars of 

 the constellations Cassiopeia and Great Bear, with Delta (5) Cas- 

 siopeiae, Zeta (f) Ursse Majoris (also called Mizar), and Polaris 

 on the meridian, represented by the straight Line; Polaris being 

 at lower culmination. 



In the above process, the interval of waiting time may 

 be found for the proper year from the following data : 



(1910 . 



* For Zeta Urs. Maj. { 1920 . 

 (1930 . 



1910 . 



For Delta Cass. 



!1930 



6.5 min. ( annual 



10.6 ' < increase 



14.7 " ( .41 min. 



7.1 min. ( annual 



11.0 " < increase 



14.9 " ( .39 min. 



* Data furnished by Prof. Robt. W. Willson. 



Instead of the transit the plumb line may be used for 

 this observation in much the manner described later on. 



At certain times of year it is inconvenient to observe 

 Polaris at culmination, and for other reasons as well it is 

 more usual to observe the star at elongation. The Land 

 Office instructions follow, and the table for azimuths of 

 the star and for time of elongation which are required. 



To ESTABLISH A MERIDIAN AT ELONGATION BY TELESCOPIC 

 INSTRUMENT 



Set a stone, or drive a wooden peg, firmly in the ground, and 

 upon the top thereof make a small, distinct mark. 



About thirty minutes before the time of the eastern or western 

 elongation of Polaris, obtained from the table, set up the transit 

 firmly, with its vertical axis exactly over the mark, and carefully 

 level the instrument. 



Illuminate the cross wires by the light from a suitable lantern, 

 the rays being directed into the object end of the telescope by an 

 assistant; while great care will be taken, by perfect leveling, to 

 insure that the line of collimation describe a truly vertical plane. 



Place the vertical wire upon the star, which, if it has not reached 

 its elongation, will move to the right for eastern, or to the left for 

 western elongation. 



While the star moves toward its point of elongation, by means of 

 the tangent screw of the vernier plate it will be repeatedly covered 

 by the vertical wire, until a point is reached where it will appear to 

 remain on the wire for some time, then leave it in a direction con- 

 trary to its former motion ; thus indicating the tune of elongation. 



Then while the star appears to thread the vertical wire, depress 



