4822 HYDROGRAPHIC MANUAL Page 424 



line at the proper estimated distance above the bottom horizontal line, and the fixed 

 point brought to the approximate setting at the left-hand end of the bar. Holding the 

 fixed point in the left hand the slow-motion screw is moved with the right hand until 

 the fixed point is felt to engage the groove at the correct point. The compass beam or 

 the dividers should always be held parallel to the horizontal lines on the meter bar 

 during a setting. 



Care. — Meter bars are expensive instruments which should be carefully handled 

 and stored. The finely ruled lines are easily marred by divider points and for this 

 reason particular care should be taken to handle the beam compass or dividers lightly. 

 Bars should be cleaned occasionally with a soft rag and a nonabrasive metal polish. 

 On board ship they should be stored where they will be least exposed to moisture and 

 where they will be secure during periods of rough weather. 



4822. Latitude and Longitude Scales 



Latitude and longitude scales (Cin fig. 91) are similar in appearance to triangular 

 engineer scales and are constructed so that any one of the three edges may be placed 

 in close contact with the paper. They are designed particularly for use at scales of 

 either 1:10,000 or 1:20,000 and are not especially useful for other scales. Each rule 

 contains six scales, one for use in plottmg latitude, and five for use in plotting longitude 

 at various latitudes. Each scale is slightly longer than 1 minute of arc at a scale of 

 1:10,000, and is graduated to read to 0.2 second of arc and may be estimated to 

 0.05 second. Small magnifying glasses are provided which can be attached to the scale 

 and slid along the rule to any desired position. 



The scales are designed for plotting or scaling geographic positions by seconds 

 without having to convert the seconds into meters. They are used diagonally across 

 the projection lines and plotting is performed by a proportional process in which any 

 distortion in the paper is automatically compensated for. An experienced operator can 

 use these scales somewhat faster than the beam compass or dividers, but the principal 

 economy in time occurs when distortion in the paper would make an adjustment of the 

 values in meters necessary when plotting with a meter bar. Tests in the Washington 

 Office indicate that geographic positions can be plotted with these scales with approxi- 

 mately the same accuracy as is attained with the beam compass. 



Points should be plotted adjacent to the edge of the scale by pricking a small hole in 

 the paper with a fine needle point. Best results are obtained by tilting the needle 

 slightly away from the edge of the scale, so that the needle point and the scale divisions 

 are not obscured by the shaft of the needle. 



Method of use. — For scaling seconds of latitude of a plotted position, place the 

 scale diagonally across the projection so that the latitude edge bisects the station point, 

 and the 0- and 60-second marks at the ends of the scale coincide with the lower and 

 upper minute lines of latitude. The value in seconds is read directly from the scale. 

 A similar procedure is followed for seconds of longitude, except that the edge bearing 

 the longitude scale corresponding to the latitude of the locality must be used and the 

 ends of the scale, of course, made to coincide with the lines of longitude. 



To plot a position it is necessary to plot the correct latitude twice near the meridians 

 east and west of the station, connecting these points with a fine straight line. The 

 value in seconds of longitude is then plotted twice near the adjacent parallels and 

 connected with a line. The correct position is at the intersection of these two lines. 

 Instead of plotting two points for longitude, the scale may be placed diagonally across 



