346 SURVEY OF THE COAST 



tact being made by the tangent screw at C, when this has 

 been clamped. The time of this observation is written in 

 the first column, before the readings of A and B. 



4. Kead the verniers C, D, and write the result in the fifth 

 and sixth columns of the Day-Book, one line lower down 

 than the foregoing. 



5. If a night observation, place the stop q on the light cir- 

 cle near the outside of the projccthig piece r of the alhidade 

 A, leaving it some freedom of motion, and the other piece 

 q upon the opposite side of the parallelism, upon the same 

 number of degrees, ^c. 



6. Invert the instrument, either from right to left, when 

 in a vertical observation, or upside down in other positions, 

 and unclamping the alhidade of A ^ B, move it up to the 

 opposite stop on the small circle, or in general near the place 

 which will be indicated by the reading on the small circle, 

 and after clamping there, make again an observation, bring- 

 ing the objects in contact by the tangent screw at A. 



7. The time corresponding to this observation being ob- 

 served, it is written in the first column below the former 

 time, and opposite to the second reading. 



8. If the stand of the alhidade of A 6j B is now read off, 

 a result is obtained of the first angle of this series, to which 

 the two times observed would correspond ; and in a vertical 

 observation on the mercury horizon, the angle indicated 

 would be the double altitude corresponding to the mean of 

 the two times. 



9. Invert the instrument again, to bring it in the same po- 

 sition as for the first observation, unclamp the alliidade C, D 

 and move the circle with the large mirror, (the alhidade of 

 which remains clamped to it) near to the first posifion or 

 place of the stop, where C is again clamped. 



10. In this posifion, make again an observation effecting 

 the contact by the tangent screw at C, and writing the time 

 under the second time. 



H. In reading the verniers C ^ D, the first angle of the 



