Otherwise it would he very easy tn incorporate a constant error 

 that mi^ht not he in evidence. 



A movable metal scale* which the surveyor can adjust himself 

 is otniously the best theoretical arrangement for the reverse side; 

 but the use of such a scale precludes the use of a cover guard which 

 is also important, and we are required, by preference, to graduate the 

 reverse side of the \ ial after it has been applied to the instrument. 



Hg. 37 Showing general equipment with Vertical Clamp, Telescope 



Bubble and Vertical Circle with Cover Guard and one Double Vernier. 



covered under the No. 045 4-in., No. 055 S-in. and No. 065 6-in. Transit. 



VII. The Index of the Vertical Circle. 



The index of the V. C. should read /no when the collimation 

 axis lies in the horizon. Let us be certain of the premises in the 

 case. The collimation axis is not a mere matter of chance, as we 

 have shown in V, (h-1). When the horizontal wire is indeed 

 collimated and the telescope bubble adjusted to parallelism with 

 this sight line, the V. ("., being permanently fixed to the telescope arid 

 revolving with it, should show no index error against its vernier scale. 



* Invented by , 



71 



