Page 429 equipment and instruments 4828 



is needed. It may be constructed to resemble the log-factor scale, except that hori- 

 zontal distances on the scale represent elapsed intervals of time run, instead of log 

 distances, and the horizontal lines represent various rates of speed, instead of log factors. 

 The rate of speed is determined and distances are taken off the scale in the same manner, 

 except the elapsed time between positions is used instead of the log distances. This 

 time-distance scale is constructed according to the principle used in constructing the 

 first speed scale described in 4826. 



As used in hydrographic surveying, the true distance is the actual distance over 

 the ground, and such a log factor not only takes into account the error of the log but also 

 the effect of current and wind in the direction of progress. Thus the log factor is con- 

 stantly changing. 



4828. R.A.R. Plotting Scale 



The R.A.R. plottmg scale is a triangular rule about 20 mches long, similar in appear- 

 ance to an engineer scale. It is graduated in seconds of travel time for a velocity of 

 sound of 1,460 meters per second, and each rule has six graduated edges for the scales of 

 1:40,000, 1:60,000, 1:80,000, 1:100,000, 1:120,000, and 1:200,000. It is for use in 

 plottmg smooth sheets on which distance circles have been drawn, based on a velocity 

 of sound of 1,460 meters per second (see 7341). 



Each second is numbered, except on the 1:200,000 scale where the even seconds 

 oiAj are numbered, and each scale carries two series of numbers starting at zero at 

 opposite ends, so that either end may be used as an initial. Each scale is completely 

 graduated for its entire length. The scale at 1:40,000 has a graduated length of 14 

 seconds, the smallest division being 0.02 second. The 1 : 200,000 scale has a length of 70 

 seconds with a smallest division of 0.1 second. The other scales have proportionate 

 lengths in seconds, their smallest intervals having approxunately the same linear length. 

 To plot an R.A.R. position with the scale the mcrements or decrements of distance 

 in seconds from the nearest distance circles (see 7631c) are plotted normal to the circles. 

 The length of the rule facilitates this. In many cases the rule will extend to the plotted 

 position of the R.A.R. station itself, but where it does not, its length permits 'alinement 

 by eye accurate enough for all practical purposes. 



483. Miscellaneous Drafting Instruments 



4831. Straightedges 



Straightedges, usually of stainless steel, are available in lengths from 6 to 72 inches. 

 They can be furnished with one edge beveled. 



The fiducial edge of a straightedge must be a truly straight line, and tests to verify 

 this should be made prior to use in constructing projections or any operation requiring 

 a straight line. To test a straightedge, two fine dots should be pricked on a thick sheet 

 of paper at a distance apart slightly less than the length of the straightedge. The 

 fiducial edge of the straightedge is accurately centered through these two dots and a fine 

 pencil line is carefully drawn along it with a chisel-edged pencil. The pencil should be 

 held firmly against the edge at a constant angle with the paper. The straightedge is 

 then turned end for end and again centered through the pricked points. If the fiducial 

 edge coincides with the pencil line throughout its length, it is straight, unless it happens 

 to have a symmetrical reverse curve. A magnifying glass wOl be found helpful in 

 making the test. 



