4814 HYDROGRAPHIC MANUAL PaGE 422 



Spacing dividers may be used to obtain quickly and accurately a vessel's rate of 

 speed. Knowing the elapsed time between positions, the dividers are set to indicate 

 minute intervals as described above. On a nautical mile scale constructed for the scale 

 of the sheet, the distance spanned between the zero point of the dividers and point 

 number 6 will give the distance traveled in 6 minutes. This distance multiplied by 10 

 will give the speed of the vessel in knots. 



4814. Beam Compass 



The beam compass (J in fig. 91) is used to measure distances that are too long to 

 be measured accurately with ordinary dividers. It is indispensable in making projec- 

 tions in the field and in swinging distance circles and arcs in R.A.R. plotting. A 

 beam compass with a short bar is often convenient for use in plotting geographic positions 

 by latitude and longitude (see 7411). 



The beam compass consists of a light, inflexible bar of wood, or metal, and two 

 compass fixtures which slide on the bar and may be clamped at any desired points. The 

 usual beam compass fixtures are made for use with wooden bars. Special fixtures 

 are made to fit tubular metal bars. 



It is important that beam compass bars be rigid. They should be made of hard- 

 wood and the cross-section shape should be designed to give maximum rigidity. A bar 

 with a T cross section is recommended. Bars are obtainable in lengths from 24 to 60 

 inches, in multiples of 6 inches. Extra long bars can be furnished for special purposes. 

 (See 3741.) 



Beam compass fixtures are constructed of metal and are designed to slide along the 

 bar guided by the small lip on its edge. A thumbscrew on the side of the fixture working 

 against a small bearing plate provides the means for keeping the fixture alined on the 

 bar and for clamping it in place. Each fixture is provided with a socket into which may 

 be fitted a needle point, pencil point, or ruling pen. One fixture of each pair is fitted 

 with a screw device which works against a spring, providing the point with a slow 

 motion parallel to the bar. 



It will be found convenient to assemble the fixtures on the bar so that the thumb 

 screw on the clamping device of each fixture is toward the user, with the slow-motion 

 fixture to the right. In use, the fixture with the fixed point should be kept clamped on 

 the bar while the slow-motion fixture is moved until it is at the approximate distance, 

 where it is clamped. The final adjustment is then made with the slow-motion thumb- 

 screw. The bar should be held lightly but firmly and no force should be applied that 

 might tend to bend it. 



The fixtures are furnished in a felt-lined case and they should be kept in the case 

 when not in use. 



482. Scales 



Scales used by the Coast and Geodetic Survey for constructing projections and for 

 plotting stations or distances on hydrographic sheets are graduated in the metric system. 



The standard scale for these purposes is the meter bar, which is made of German 

 silver and graduated by machine with great precision. 



Scales for special purposes may be constructed by the field party or will be furnished 

 by the Washington Ofiice upon requisition. Those prepared in the Office are usually 

 made by photographic reproduction and may be furnished on aluminum or glass; or on 

 plain, transparent, or metal-mounted paper if only for temporary use. 



