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EQUIPMENT AND INSTRUMENTS 



4513 



All navigating sextants now acquired by the Bureau are equipped with binocular- 

 type telescopes with a magnification of about 3K diameters. The telescope can be 

 focused and is removable but is provided with no adjustment. 



Each navigating sextant is provided with two sets of colored shade glasses installed 

 so that one or more of one set can be interposed in the direct line of sight and one or 

 more of the other set in the reflected line of sight. Their purpose is to reduce the bright- 

 ness of the rays of light from the sun and sometimes the horizon. The two mirrors used 

 on the navigating Sextant are slightly larger than those used on the hydrographic 

 sextant. 



Figure 77. — Principal parts of a sextant. A. Index |mirror. B. Horizon mirror. C. Telescope. DF. Graduated arc. E 

 Vernier. AE. Index arm. L. Index mirror adjusting screw. M. Screw to adjust horizon mirror perpendicular to plane of sextant. 

 A''. Screw to adjust horizon mirror parallel to index mirror when index arm is set at zero. P. Wooden handle. Q. Attached reading 

 glass. R. Clamp screw. S. Tangent screw. 



4513. Hydrographic Sextant 



To provide steadiness during observations, a sextant should be as heavy as practi- 

 cable without tiring the observer or causing him discomfort. Since a hydrographic 

 sextant is used continually in hydrograpliic surveying, it should be hghter in weight 

 than a navigating sextant. It may also be smaller since the accuracy of a navigating 

 sextant is usually not required in hydrography. 



Hydrographic sextants (fig. 77) used by the Coast and Geodetic Survey are similar 

 to the navigating sextant, except that they are smaller in size and weight, the arc 

 usually having a radius of about 5^4 inches and the sextant weighing about 2){ pounds. 

 The arc is graduated in half-degrees, but angles can be read to the nearest minute by 

 means of a vernier. Like the navigating sextant, each is fitted with a binocular tele- 

 scope of 3K diameters magnification. 



