Page 359 equipment and instruments 4467 



while a spring balance is attached to the other end. The wire is led one turn around 

 the wheel, lapped 3 or 4 inches and stretched with a tension of 36 pounds indicated on 

 the spring balance. A scratch, parallel to the axis of the wheel, is then marked across 

 the two lapped parts with a sharp tool. To measure the circumference, the wire is 

 stretched on a fiat surface with the same tension and the distance between marks 

 measured with a beam compass and meter bar. Since the distance is more than a meter, 

 an additional scratch must be made near the middle of the length and each part 

 accurately measured. The complete procedure should be repeated several times, using 

 a different piece of wire for each measurement, and the mean of the results is used as 

 the calibration factor. A factor so determined is sufficiently accurate if only short 

 di^ances are to be measured with the apparatus, but where long distances are to be 

 measured a calibration by indirect measurement should be made. 



The indirect method is the reverse of measuring a distance with the taut-wire 

 apparatus. Wire is laid by the apparatus along a distance of accurately Icnown length, 

 and the known distance divided by tlie nmnber of revolutions of the cyclometer wheel 

 gives the desired calibration factor. The measurement for calibration is made in ex- 

 actly the same manner a distance is measured (see 4466) but at a slightly slower 

 speed in order that the revolution counter may be read very accurately. If the dis- 

 tance is of sufficient length it need be measured in only one direction. No change in 

 course should be made during the calibration run. 



Any taut-wire measurement may be in error by one revolution of the cyclometer 

 wheel, because only whole revolutions are recorded on the counter; since the circum- 

 ference of the wheel is approximately 0.001 nautical mile, there may be an error of 1 

 part in 1,000 in a measurement of 1 mile. To reduce this error, the calibration factor 

 should be determined from a distance of at least 5 miles, if the apparatus is to be used 

 to measure long distances, such as those required in R.A.R. suiVeys and distances to 

 locate a station beyond the visibility of shore signals (see 2532). A measured mile 

 speed course should not be used because of its shortness; neither should the distance 

 between two buoys be used because of the error in distance which may be introduced 

 by the scopes of the anchor cables. The following methods of calibration have been 

 found satisfactory and one of them will generally be found practicable in most localities: 



(a) The wire may be laid along the straight course between beacons erected in the water, which 

 have been located by triangulation. Either an accurate distance between them is availaVjle or may 

 be derived from an inverse position computation. 



(6) The ends of a calibration run along a selected course may be determined by three-point 

 ■ sextant fixes observed to shore triangulation stations. The fixes must be strong and the sextant 

 angles must be accurately measured; each fix should be checked by a third angle. Three successive 

 fixes with simultaneous readings of the taut-wire revolution counter should be obtained at each end 

 of the line. The accuracy of the calibration may be increased if the run can be made where a range 

 may be steered. The positions fixed by sextant and the distances between them should be computed. 

 The accuracy of the position determinations may be gaged by studying the residual distances between 

 the three positions at each end of the line. 



(c) On a thickly settled coast it is occasionally possible to find four objects, located by triangula- 

 tion, which may be used as two ranges, normal to which a calibration run of sufficient length may be 

 made. The four objects should be selected so that the azimuths of the two ranges are approximately 

 the same. A buoy should be anchored with a short scope at a sensitive position on each range and 

 the position of each buoy determined by a three-point sextant fix, from which its distance from the 

 respective front range may be determined. The buoys are used as leading marks and from their 

 positions the azimuth of the course to be steered is computed. On the calibration run the revolution 

 counter is read as the range is crossed and a depression angle is measured to the buoy as it is passed. 

 The depression angle distance is applied to the distance of the buoy from the front range to obtain 



