4526 HYDROGRAPHIC MANUAL PaGE 372 



bored to hold the battery or batteries. Small "pen" batteries approximately 2 inches 

 long by K inch in diameter should be used. 



On a vessel not provided with sextants equipped for night observations, or if the 

 number of such observations to be made does not warrant alteration of the sextants, 

 they should be read by illumination provided from a flashlight held by an assistant. 

 The flashlight must be held against the graduated arc but to one side of the vernier 

 to illuminate it diagonally, so as not to interfere with its being read. 



4526. Inclined Sextant Angles 



The sextant is used to determine the azimuth between two buoys or other objects 

 by the measurement of an inclined angle between that line of sight and a celestial 

 object, generally the sun. To measure such angles the sextant is held at an inclination 

 to the horizon. This is the most difficult type of angle to measure with a sextant, 

 because it has to be held at an unnatural angle with the horizon, this angle dift'ering 

 with almost every measurement. The prmcipal difficulty is in bringing the sun into 

 approximate coincidence with the objects on range. Frequently the method described 

 in 4524(a) for bringing a star down to the horizon has to be used. After approximate 

 coincidence has once been obtained the final observation is no more difficult than any 

 other. 



A vertical angle must be observed simultaneously with the incfined angle by a 

 second observer. The inclined angle may be reduced to a horizontal angle by one of the 

 formulas given m 3338. 



In measuring inclined angles for sun azimuths, when the sun must be observed 

 direct and the buoys on range reflected, the intense rays of the sun make the reflected 

 buoy difficult or impossible to see. An improvised shade may be used to advantage 

 to prevent the rays of the sun from passing around the shade glass and into the tele- 

 scope. Any stiff piece of cardboard or other material may be used, in which a circular 

 hole the size of the shade glass is cut, and through which the direct object may be 

 observed. This is temporarily attached to the sextant. A size 6 inches square wifl 

 suffice, with a horizontal piece cut out of its right edge, through which the ray from the 

 reflected object passes, and with slots cut m its lower edge by which it may be tem- 

 porarily fastened to the frame of the sextant. A simpler and smaller card containing 

 the same circular and horizontal openings may be made to rest on top of the frame of the 

 sextant and be temporarily attached with Scotch tape to the frame of the shade glass. 



453. Protractors 



The protractor in its simplest form is an arc or circle of flat material, graduated 

 in degrees, with which angles may be, plotted or scaled on a plane surface. In hydro- 

 graphic surveying the type most widely used is the thi-ee-arm protractor of metal or 

 transparent material. Horizontal angles, usuaDy forming three-point fixes, are plotted 

 graphically with the three-arm type. 



4531. Metal Three- Arm Protractor 



The metal three-arm protractor (fig. 78) consists essentially of a graduated circle 

 with one fixed arm and two movable arms pivoted at its center so that the extension of 

 each fiducial edge always passes through the precise center of the graduated circle. 

 The arms are about 18 inches long. The circle is usually about 6 inches in diameter, 



