336 SURVEY OF THE COAST 



vertical axes there is no bad influence to be apprehended 

 from them. 



The vertical circle being supported in this kind of instru- 

 ments between two columns, it is subject to less spring, and 

 has much more stability than in the circle with two tele- 

 scopes, where the double weight of the circle hangs upon a 

 short axis, upon which equilibrium is maiistained between 

 the circle and its counterpoise. The verification of the 

 verticality by the pole star can be made with the greatest 

 nicety ; and the large detached level itself can be adjusted 

 upon it. 



Method of Ohsei'viiig Horizojital Angles with the Repeating 



Theodolite. 



The adjustments of this instrument are evidently the same 

 as those of the two-feet theodolite, and grounded upon sim- 

 ple and well known principles, which are self evident from 

 a view of the instrument, and need therefore not to be ex- 

 plained here. 



The use of common theodolites with double axes for the 

 repetition of angles is also so well known, that the mere de- 

 scription and view of the instrument would lead to the use 

 of these double repeating theodolites, yet it may be proper 

 to describe here the mode of taking a regular series, in or- 

 der to render more intelligible the other pecularitios which 

 are to be mentioned as means leading to the greatest accu- 

 racy. 



When the instrument is carefully levelled, the riding level 

 is placed upon the arm t, parallel to the vertical ciiclc, and 

 there adjusted. 



Then the vernier A being placed upon 0° or any other di- 

 vision, which may also be the last reading of a foregoing sc- 



