46 



HYDROGRAPHIC MANUAL 



ute of arc. This type of sextant is preferred 

 by most observers ; however, it is sometimes 

 difficult to measure an angle accurately with 

 this sextant when the angle is changing 

 very rapidly. 



A navigating sextant shall always be used 

 for astronomic observations, for measure- 

 ment of angles to locate a hydrographic sta- 

 tion, and for observing small vertical angles. 



3-10 Adjustment of sextant. — The ad- 

 justments of all sextants used in hydrograph- 

 ic surveying should be verified, and readjust- 

 ments made if necessary, each morning 

 before the start of the day's work. They 

 must be verified each night at the close of 

 the day's work. The fact that this has been 

 done at both times and the amount of the 

 index correction found at the end of the day, 

 if any, must be entered in the sounding rec- 

 ord (see 5-90 and 98). 



A sextant should be adjusted as follows: 

 the index mirror (the large mirror on the 

 movable arm) must first be made perpen- 

 dicular to the plane of the instrument. To 

 make this adjustment, set the index arm 

 near the middle of the arc and hold the 



Figure 7. — Micrometer drum sextant with endless 

 tangent screw. 



sextant with the eye close to the index mir- 

 ror and as nearly as possible in the plane of 

 the sextant. Observe the graduated arc di- 

 rect and its reflection in the index mirror, 

 moving the arm slowly back and forth. The 

 arc and its reflection should form an ap- 

 parently continuous unbroken arc if the mir- 

 ror is perpendicular to the plane of the arc. 

 If it does not, correct the position of the 

 mirror by adjusting the screw at the back 

 of the frame. 



The horizon mirror should now be adjusted 

 perpendicular to the plane of the arc. With 

 the index mirror in adjustment, set the in- 

 dex arm near zero and hold the sextant so 

 its plane is vertical and sight at the sea ho- 

 rizon. Bring the horizon and its image 

 into coincidence with the tangent screw. 

 Rotate the sextant slowly and see if the 

 horizon and its image are still in coincidence. 

 If not, adjust the position of the mirror by 

 means of the screws at the back of the 

 frame until coincidence is obtained. 



Finally, the two mirrors should be parallel 

 to each other when the index arm is set at 

 zero. After making the two adjustments de- 

 scribed above, set the index arm exactly at 

 zero and, with the sextant in a vertical posi- 

 tion, sight at the horizon. The observed and 

 reflected horizon should coincide. If they do 

 not, adjust the mirror by means of the 

 screws at the side of the mirror until coin- 

 cidence is achieved. If this last adjustment 

 is required, it may disturb the vertical ad- 

 justment. Both should be repeated until the 

 horizon mirror is adjusted for any position 

 of the sextant. 



3-11 Index error of sextant. — The index 

 error of a sextant is due to the fact that the 

 reflecting surfaces of the mirrors are not 

 parallel with each other when the index arm 

 is set at zero. The amount of the index er- 

 ror, if any, can be determined by one of 

 the following three methods : 



(a) With the sextant held vertically and 

 pointed at the sea horizon bring the direct 

 and reflected images into coincidence and 

 read the setting of the index arm. Repeat 

 several times, bringing the reflected horizon 



