distances, parallelism between the sight lines, as well as the exact 

 amount of eccentricity, is of no material consequence if the con- 

 ditions expressed in the propositions on pp. 173-4 are observed. 



With the auxiliary applied to the side as in Fig. 105, 

 vertical angles may be observed without correction. They 

 should preferably be observed directly with the auxiliary first 

 applied to the right, then with the instrument reversed, the auxiliary 

 being at the left. If there is a discrepancy, the mean reading will 

 be exact within a very small correction which is insignificant. 



Kiif. 106 No. 47 4^-inch Tachymeter with Edge Graduated Vertical 



Circle, Reversion Bubble and Interchangeable Auxiliary 



Telescope Mounted for Mining and Slope Stake Work. 



Obviously such vertical angles should be determined with care, 

 for small errors have an appreciable effect in resolving the measured 

 distance into its vertical and horizontal components. Hori/ontal 

 angles, as from a steep shaft alignment into a drift, are to be doubled 

 by necessity- first from the right, then the left and averaged. The 

 errors of eccentricity and parallelism, whatever their amounts, will be 

 neutralized, and this process of reversion also tends to correct errors 

 in the instrument itself. 



182 





