4433 HYDROGRAPHIC MANUAL PaGE 344 



A still smaller model is recommended for launches less than 50 feet in length. The 

 motor, clutch, and electric control equipment are all combined in one compact unit to 

 be attached directly to the steering wheel or somewhere in the steering system. The 

 compass unit is separate. 



4433. Azimuth Circle 



An azimuth circle is used on a compass or pelorus for the purpose of measuring 

 bearings of terrestrial or celestial objects. It is essentially a metal ring which fits 

 snugly around the compass in such a manner that it may be rotated horizontally with 

 respect to the lubber's line. The ring is equipped with (a) a pair of sight vanes whose line 

 of sight passes through the vertical axis of the compass; (6) a system of mirrors and 

 prisms by which the point of the compass card cut by the vertical plane through the 

 line of sight — in other words the compass reading — is brought into the field of view of 

 the observer; and (c) another system of mirrors and prisms by which a pencil of the 

 sun's rays is reflected on the compass card in such a manner as to indicate the bearing. 



An azimuth circle should be tested occasionally for accuracy. This may be done 

 by mounting it on a standard compass ashore at a place where the magnetic variation 

 is accurately known. If the compass is otherwise unaffected a compass bearing of the 

 sun should be the computed magnetic bearing ^t any instant and the difference between 

 the two, if any, will be equal to the combined error of the compass and the error of the 

 azimuth circle. There should be little measurable error in the compass, but any doubt 

 in the matter may be eliminated by the use of two or more compasses. 



A more accurate method is to test the azimuth circle on a planetable on shore, no 

 compass being used. 



On a piece of paper draw a circle slightly larger than the outside diameter of the azimuth circle. Two diameters of this circle are 

 drawn at right angles to each other. Place the paper on the planetable and level the table carefully. Set the azimuth circle on the 

 paper with the line joining the mirror and the prism coinciding with one of the penciled diameters. See that the bubble on the circle is 

 in the center. If not, adjust it. Turn the planetable so that the sun's reflection from the mirror is directed through the prism and 

 clamp the table. Adjust the table by slow motion until the pencil of light from the prism is reflected directly down on the penciled 

 diameter. Now tilt the mirror slightly and see if the pencil of light follows along the penciled line. The azimuth of the sun will change 

 very little in the time ntcessary to make this test. If the pencil of light follows the penciled diameter as the mirror is tilted, the 

 azimuth circle mirror and prism are as nearly correct as any field test will show. 



Now test the direct-vision vanes. First see if the line between them coincides with the penciled diameter at right angles to the one 

 joining the mirror and the prism. By rotating the planetable, repoint the mirror toward the sun, then rapidly shift the azimuth circle 

 90° in azimuth on the penciled circle. If the direct vision vanes point accurately toward the sun, you may assume they are at right 

 angles to the line joining the mirror and the prism. 



If the azimuth circle, by this test, is found to be out of adjustment, it should be returned to the Washington Ofl3ee for adjustment 

 and another one requisitioned, unless the error is caused by a minor maladjustment which can be remedied in the field. Small errors 

 in the direct-vision vanes can be corrected, but any displacement of the prism, caused by its being chipped or having been knocked 

 out of adjustment, can scarcely be corrected satisfactorily e.xcept by a skilled instrument maker. 



4434. Pelorus 



The pelorus, also called the Dumb Compass, consists of a circular flat metallic 

 ring, mounted in gimbals on a vertical stand. One is usually installed on each wing 

 of the bridge and on the flying bridge of the ship for use in taking bearings. The line 

 between the center of the ring and the index mark must be pq/"allel with the fore- 

 and-aft line of the ship (see 4437). The metallic ring is graduated from 0° through 

 360°, the same as the compass card, and can be revolved so that it can be set to corre- 

 spond with the ship's course. Bearings are observed by means of a sighting vane which 

 can be revolved horizontally by hand. A bearing by pelorus must be taken when the 

 ship is exactly on course, or the ship's heading by compass must be read at the instant 

 of observation and the pelorus bearing corrected for any deviation from the course. 



