146 PRACTICAL ASTRONOMY 



objective glass to illuminate the cross-hairs. Set both clamps, 

 and with either tangent screw set the vertical cross-hair exactly 

 on the star. The star will appear to be moving toward the left 

 or toward the right according as it is approaching upper or 

 lower culmination. Follow it in its motion by turning the 

 tangent screw until the exact time of culmination (which, 

 preferably, should be called out by an assistant). This com- 

 pletes the observation of the star. Now depress the telescope, 

 direct it to a point on the ground about 400 or 500 ft. from the 

 instrument, and have an assistant drive a tack in the top of a 

 stake in line with the line of sight; this completes the operation. 

 The line between the two stakes is a true north-and-south line, 

 or true meridian. 



Time of Culmination of Polaris. The accompanying table 

 contains the times of upper culmination of Polaris for the dates 

 given. The lower culmination occurs nearly ll h 58 m before 

 and after the upper culmination, and can be determined from 

 the latter. In the table the extreme right-hand column con- 

 tains the difference between the times of culmination for any two 

 succeeding days. Each difference applies to any day between 

 the date horizontally opposite that difference in the left-hand 

 column, and the following date. Thus, the difference 3.95 m , 

 which is horizontally opposite Jan. 1, indicates that, between 

 Jan. 1 and Jan. 15, the time of culmination decreases by 3.95 

 min. per day. For instance, the time of culmination on Jan. 8 

 is obtained by subtracting from the time of culmination for 

 Jan. 1 the product 3.95 m X7, or 27.65 m , the number of days 

 elapsed from Jan. 1 to Jan. 8 being 7. 



It should be borne in mind that the times given in the table 

 are mean local times counted in the astronomical way; that is, 

 from O h to 24 h , beginning at noon. 



EXAMPLE. Find the time of upper culmination of Polaris 

 on Sept. 6, 1913. 



SOLUTION. Referring to the table, 



Upper culmination, Sept. 1, 1913 = 14 h 45.3 m 



Difference for 1 da = 3.92 m 



Correction for 5 da = 3.92 m X5 = 19.6 m 



Time of culmination on Sept. 6 = 14 h 25.7 m 



