4825 HYDROGRAPHIC MANUAL PaGE 426 



even 5 meters, distances are scaled on the appropriate horizontal line ; for an intermediate 

 velocity, measurements are made at a proportionate distance between lines. 



The scales were formerly inscribed on German silver. They are now made at the 

 Washington Office by a lithographic process on strips of grained aluminum, coated with 

 transparent varnish to protect the markings, and glued to thin wood backs. They are 

 furnished for the same velocity range at scales of 1:40,000, 1:80,000, 1:100,000, and 

 1:120,000. 



For scaling and drawing distance arcs, a beam compass is used, the right-hand 

 fixture of which is fitted with a chisel-edged pencil and the left-hand fixture with a steel 

 point. Distances are scaled horizontally at the point on the vertical scale which repre- 

 sents the velocity of sound in the locality. The steel point should be set on the line 

 representing the whole number of seconds and the pencil point adjusted to the correct 

 distance in tenths and hundredths on the scale at the right. 



The lines markmg second intervals may be engraved on the scale if desired by using 

 a wedge-point engraving tool guided by a straightedge. This can be done accurately 

 only with care and a steady hand. 



4825. R.A.R. Chronograph Scale 



The R.A.R. chronograph scale is used to scale to the nearest hundredth of a second 

 the time of an event recorded on the tape of a Gaertner chronograph (6731) on which 

 time has been marked in seconds. It is constructed at the Washington Office by photo- 

 graphic methods on plate glass. The lines are on the underside of the glass and are 

 protected by a thin layer of transparent lacquer. 



The scale has 11 equally spaced converging lines that divide a distance of about 1.5 

 cm at the top and about 4 cm at the bottom into 10 equal intervals. The middle line is 

 parallel with the edge of the glass and is about 8 cm long. The converging lines are 

 crossed by six equally spaced parallel horizontal lines perpendicular to the centerline, 

 by use of which the scale is held parallel to the record on the chronograph tape when 

 in use. 



4826. Graphic Speed Scales 



The rate of speed of a survey vessel, or the distance traveled in a given length of 

 time, may be determined quickly and accurately from a speed scale. In hydrographic 

 surveying it is especially useful in small-scale offshore surveys controlled by dead 

 reckoning or R.A.R. The scale may be drawn on a separate sheet of paper or, prefer- 

 ably, on -the boat sheet in a space that will not otherwise be used. Two types of 

 speed scales are in general use. 



Cpnstrudion.— One type (fig. 93) is constructed for the scale of the boat sheet, principally for use in the field during the progress of 

 the survey. It is usually drawn on the boat sheet with its ordinates and abscissas superposed on the meridian and parallel lines of the 

 projection. It is best adapted for use at comparatively small scales. 



To construct this scale, choose a convenient meridian line to represent a speed of 12 knots. Each nautical mile, or minute of lati- 

 tude, on this line will equal 5 minutes of time. At an arbitrary distance to the right or left choose another meridian line to represent a 

 speed of 6 knots. Each nautical mile on this line will equal 10 minutes of time. The distance between the meridians is subdivided 

 into si.x equal parts by vertical lines drawn to represent the intermediate speeds between 6 and 12 knots. These are intersected by a 

 series of diagonal lines representing time in minutes, which are drawn to connect the corresponding points on the two meridians first 

 mentioned. 



The scale can be extended in either direction to provide for other speeds, or meridians can be first selected to represent other speeds, 

 if the principle of construction described is followed. It is to be noted that the diagonal lines converge at a point representing zero 

 speed. It is frequently more convenient to find this point, at a horizontal distance from the first meridian line equal to twice the 

 distance between it and the second meridian, and draw the diagonal lines from it to the points representing intervals of time on the 

 first meridian. 



