dotted line, and by revolving the telescope in the axis sheath measure- 

 ments of this nature can be taken either in the vertical or horizontal 

 plane. If observations are to be taken between the targets on an 

 ordinary leveling rod, held vertically, the micrometer box is turned 

 with the graduated drum at the top; but if observations are to be 

 made at great distances, between specially prepared signals, the 

 whole telescope is to be turned until the graduated drum is at the 

 right hand and the focusing pinion underneath. 



To find the drum values in seconds of arc we give directions 

 from Mr. Sargent's article as follows: 



Place carefully measured test bases at 500, 1000, 1500 ?ind 

 2000 ft. from the center of the alidade and not from the anterior 

 focal point of the objective, as in the stadia method. At the 500-ft. 

 station two stakes should be set 5 ft. apart and another between 

 them at 3 ft. from one end, thus giving three bases one of 5ft. , a 

 second of 3 ft. and a third of 2 ft. At the 1000-ft. station, the ex- 

 treme stakes should be set 8 ft. apart with an intermediate stake 

 5 ft. from one end. At the 1500-ft. station the extreme pegs should 

 be 10 ft. apart with sub-bases at 4 ft. and 6 ft., and at the 2000-ft. 

 station the longest base should be 15 ft. with intervening bases of 

 7 ft. and 8 ft. 



Adjust the hair by means of the micrometer head until it covers 

 one end of a base, then revolve the head until the other end of the 

 base is covered, noting the number of divisions passed over on the 

 head, the length of base used, and the distance between it and the 

 instrument. At least ten readings on each of the bases should be 

 made and the mean of the results reduced to a hypothetical base of 

 1 ft. at a distance of 100 ft. For example, if at 1000 ft. distance, 

 on a base of 6ft., 365 spaces were turned on the micrometer head, 

 at 100 ft., on a base of the same length, theroretically, 3650 spaces 

 would be turned; but if the base were reduced to 1 ft. at 100 ft., 

 3650 -H 6, or 608.33 spaces, would be the result. After having thus 

 reduced all the readings and averaged them, the result is applied 

 in the following formula : 



in which 



d value in seconds of arc 0f one division of the 



micrometer head, 

 B length of reduced base, 



K number of divisions turned on micrometer head, and 

 H -- distance from instrument to base. 



Therefore d = 10 Q x 608.33 sin 1" 



1 log = .00000 



100 tolog. = 8.00000 

 608.33?% -- 7.21586 

 Co% sin 1" = 5.31443 

 Log. d = 0.53029 = 3. "391 



50 



