This attachment can be used when the sun is partly obscured 

 by clouds, when the lens-bar of the Burt model fails altogether. 

 The telescopic sight also has a great advantage in favor of accurate 

 centering. The sun's amplitude in the celestial sphere is equal to 

 about 32'. An attempt to center an image of this angular value 

 between the cross lines on a lens bar oilers unusual opportunities 

 for error in declination.! 



Fig. 92 Equatorial Solar Attachment with Swivel Adapter Base 

 Pat. by G. N. Saegmuller, May 3, 1881 ; Apr. 13, 1909. 



The following table, prepared by the late Prof. J. B. Johnson 

 of Washington University, St. Louis, Mo., will show the effect of 

 such errors. He says:- 



"This table is valuable in indicating the errors to which the 

 work is liable at different hours of the day and for different latitudes; 

 as well as serving to correct the observed bearings of lines when it 

 afterwards appears that a wrong latitude or declination has been 

 used. Thus, on the first day's observations I used a latitude in the 

 forenoon of 38 37', but when I came to make the meridian obser- 

 vation for latitude I found the instrument gave 38 39'. This was 

 the latitude that should have been used, so I corrected the morning's 

 observations for two minutes error in latitude by this table." 



| See Engineer's Surveying hut > umetits, f. O. 



158 



, Jut!. Edn, 1906, p. 62. 



