Page 361 equipment and instruments 45 



The amount of resistance offered by the water to a side pull on the taut wire when 

 a change in course is made is not definitely known. Observation indicates a great 

 resistance and probably changes in course as large as 10° or 12° can be made at buoys 

 in a traverse line without introducing appreciable errors in the measurements. Some- 

 times it is practicable to pass outside of a buoy on a sharp turn to ensure that if the 

 wire is pulled sidewise it will be stopped by the buoy cable. This should not be done 

 at a sono-radio buoy, for the wire may foul the hydrophone. When appreciable changes 

 in course are made during a taut-wire measurement between two buoys, especially 

 where long distances are involved, the measured distance must be reduced to the 

 tinae distance between marks. 



The expansion, or contraction, of the wire after it passes into the water probably 

 does not cause any appreciable error. The coefficient of expansion of the wire is such 

 that a temperature dift'erence of 1° C. between the temperature of the air and water 

 will change the length only 0.024 meter in 1 nautical mile of wire. For the differences 

 in temperature ordinarily encountered and for the usual distances measured this may 

 be neglected. 



45. NAVIGATION AND POSITION-LOCATION INSTRUMENTS 



This section describes the most important navigation and position-location instru- 

 ments used on survey ships, auxiliary vessels, and launches of the Coast and Geodetic 

 Survey, and their care and use in navigation and hydrographic surveying. Included 

 are such instruments as sextants, protractors, and chronometers. Theodolites, alidades, 

 and similar instruments are not described in this Manual. 



451. Sextant 



The sextant is a portable instrument for measuring the angle between two objects. 

 It is universally used on shipboard by the hydrographic surveyor and is one of the 

 most important instruments used in marine navigation. With it the hydrographer 

 afloat makes most of the measurements for which a transit or theodolite is used ashore. 

 In hydrographic surveying the sextant is used principally to measure the horizontal 

 angle between two terrestrial objects or survey buoys. It is by this means that the 

 three-point problem is used in hydrographic surveying for locating the position of the 

 survey vessel at selected times during the sounding. The sextant is also used exten- 

 sively to measure the altitude above the horizon of celestial bodies; to measure inclined 

 angles to celestial bodies to obtain azimuths; and to measure small vertical angles 

 from which distances are obtained. 



A sexant consists of a light rigid framework DAF (fig. 76), usually of brass or some similar alloy, 

 which carries a limb or arc DF in which is inlaid a strip of silver on which the arc is graduated. On the 

 underside of^the sextant are usually three legs by which the sextant may be supported on a table, 

 and a wooden handle by which it is held when measuring angles. An index arm AE carries the index 

 mirror .4 which is perpendicular to the plane of the sextant. The index arm is pivoted at A so that 

 it can be rotated in the plane of the instrument from D to F. The lower end of the arm is provided 

 with a means for clamping it at any position on the limb, a tangent screw for slow motion, and a 

 vernier E for more accurate reading of the measured angles. A magnifying glass is attached to the 

 index arm for use in making the readings. 



At B is another mirror, called the horizon mirror, which is also perpendicular to the plane of 

 the sextant. At C is a small telescope with its axis fixed in the line CB. 



