for their adoption. The argument rests mainly on a matter of 

 personal preference, but the fixed stadia have been heretofore over- 

 whelmingly preferred. 



It is generally understood that the fixed stadia are not intended 

 for precise work, and if it is desired that the error shall be reduced 

 below 1:800* it is a generally accepted requirement, quite apart 

 from the original accuracy of spacing, that a special rod should be 

 graduated for each instrument ** or the interval factor should be 

 occasionally checked up and revised. There is much to be said in 

 favor of the adjustable stadia and, while condemned by several 

 authorities, they might be more widely and successfully employed. 

 Recently we made for export a diaphragm with a fixed interval of 

 1 : 200 below the central cross wire and an adjustable interval above, 

 ranging between 1:200 and 1:1000. 



To Regulate the Adjustable Stadia, level the telescope, as 

 nearly as may be, and place a rod exactly at 100 ft. plus (c + f ) 



63 



* 'I'lif I'. .V. l-'.m:>'!>. on the. Survey of the Gt. iMkrs tunr secured an 

 accut ,'hf Idaho Bottndry Survey reached I : .'.^tf -t'hile. others 



have attained even bitl,r results; but l\'. Navbrnugh <>f J-.i'anston, Wyo. 

 reports that for filling in details in mountainous districts a greater accuracy 

 (Han 1 : 400 is ti/ >/><< mlly attempted. Relative accuracy tends to increase with 

 the. length of six lit. 



** / May 25. 191?. J. IV. H'nrrtnann, f r . S. Ass't. Eng. reports 



the use of 13 transits of different makes, each with a specially frac 



108 



niULitcd rod. 



