64b 



tions for refraction, using the tables given in this catalogue, or such as are gi\ -n 

 in Prof. J. B. Johnson's work on surveying. Careful attention should be paid i<> 

 signs. 



We will assume for the present that the latitude is known, and proceed to the 

 description of the 



FIELD OPERATIONS. 



1. Lay off on the vertical arc the declination setting for the time of observation, 

 tipping the telescope in such a direction that the small telescope will point above 

 or below the equator according as the declination is N. or S. 2. Level the small 

 telescope by means of the striding level, and then clamp it. 3. Next change 1 1m 

 setting of the vertical circle so that it reads the co-latitude of the place. 5. Using 

 both the horizontal and the equatorial motions, point the small telescope at the 

 sun, making the four segments cut off by the cross hairs equal. The main i It-- 

 scope is now in the meridian. To be certain that the settings are correct wait a 

 few moments and see if the disc follows the equatorial wires perfectly. Both 

 plates should be clamped while the image is in the center of the field. The line 

 may then be brought down to the ground and marked. 



EXAMPLE OF COMPUTATION. 



Long. 5h. West., Lat. + 40. Jan. 10, 1900. 

 Decl. for Gr. Mean Noon = 21 59' 04". 

 Diff. for Ih. = + 22".25. 



TIMK. DECLINATION. REFRACTION SETrnra. 



7h. A.M. 21 59' 04" 



8 " 58 42 5' 40" 21 53' 02" 



9 " 58 20 2' 51' 21 55 29 



10 " 57 57 2' 07" 21 55 50 



11 57 35 1' 51" 21 55 44 



12 M. 57 13 (V 47") (21 55 26) 



1 P.M. 56 51 1' 51" 21 55 00 



2 ' 56 28 2' 07" 21 54 21 

 x " 56 06 2' 51" 21 53 15 

 4 55 44 5' 40" 21 50 04 



The co-latitude may be found by measuring the altitude of the sun's lower 

 limb at noon, i.e. by measuring the maximum altitude. This angle must be cor- 

 rected for refraction, semi-diameter and declination. The result is tin- co-latitude. 

 The co-latitude may also be found, very nearly, as follows: Make the angle be- 

 tween the telescopes equal to the declination setting at noon in the same way as for 

 any other hour. Bring the telescopes into the same vertical plane, and point 1 he 

 small telescope at the sun. By varying the elevation angle of the main telescope 

 keep the small telescope pointing at the sun until a maximum elevation is reached. 

 This angle is the co-latitude, already corrected for refraction, semi-diameter and 

 declination. This method is not quite as accurate as the former. 



A TEST. 



The following observations were made by the writer with the Berber Solar 

 Attachment. The plates were clamped at zero degrees and tho nn>ridian found l>y 

 solar observation. An angle was then turned to a mark ^ mile away. The re- 

 sults are as follows : 



TIM I \/. A NCiLK 



A.M. 



8:30 240 07' 



8:40 05^ 



8 :50 05^ 



9 :00 06 



P.M. 



3:23 240 05' 



3 :30 03 



Clouds j>n'v.-nt'd further observations. 

 The true azimuth as found afterward by an observat ion on Polaris was 240 05' 



