ON OBTAINING THE MERIDIAN 
53 
Six months later the star Zeta (f), 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- 
siopeue, 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: 
i l920 . . . 10.5 min. (annual 
1930 ... 15.1 “ {change 
1940 ... 20.2 “ (.48 min. 
1 1920 . . . 11.7 min. (annual 
1930 . . . 16.4 “ {change 
1940 . . . 21.6 “ (.5 min. 
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 time of elongation. 
Then while the star appears to thread the vertical wire, depress 
