4467 HYDROGRAPHIC MANUAL PaGE 358 



measurement is made between only two stations or between the last two stations of a 

 traverse, the azimuth of the far station should be measured when the ship is abeam the 

 first station and the far station kept on this azimuth until it is approached. This will 

 ensure the maintenance of a straight course between the two stations. 



Where long distances are measured between two stations which are not intervisible, 

 the course between them must be carefully computed and allowance made for the current 

 indicated at the first station by the lead of the anchor cable. In R.A.R. surveys, posi- 

 tions may be obtained at regular intervals along the line so that the course may be 

 altered slightly to maintain the ship on line. If a long distance between two sono- 

 radio buoys is being measured, bombs fired at intermediate positions, to measure the 

 travel time of sound, give valuable data for the study of the velocity of sound (see 

 6351). When the far station is finally sighted the course should be altered immediately 

 to pass the station at the desired distance. If the course has been carefull}' deter- 

 mined, or if the ship has been maintained on line by fixes, a change in course of no more 

 than 2° or 3° should be necessary to head toward the station. The direction of the 

 current at this station may be assumed to be the same as at the first station, until the 

 ship is close enough for the direction to be observed, when a further change in course 

 must be made, if necessary, to pass the station on the down-current side. 



The wire should be cut immediately after passing the last station in a taut-wire 

 measurement. A straight section should be selected, preferably aft of the measuring 

 apparatus. It may be cut by holding a piece of metal under the wire, while it is tapped 

 with the sharp edge of a chipping hammer or other sharp tool. The cyclometer wheel 

 revolves rapidly after the wire has been cut, and in order to keep a record on the revolu- 

 tion counter of the total amount of wire used, so as to estimate the amount remaining 

 on the reel, the wheel should be stopped immediately by a slight pressure against its side. 



A taut-wire measurement should never be started so late in the afternoon that there 

 is a chance of not completing it before dark. Neither should a measurement be started 

 when there are indications of fog, which may close in to obscure the far station from 

 view. Taut-wire measurements may be made during comparatively rough weather, 

 but it should not be so rough as to prevent an even tension being maintained on the 

 wire. 



4467. Calibration of the Taut-Wire Apparatus 



The cyclometer wheel, of the taut-wire apparatus is made with a circumference 

 approximately equal to 1.853 meters, so that 1,000 revolutions are equal to 1 nautical 

 mile. This circumferential length cannot be accepted as exact, since it varies slightly 

 with each taut-wire apparatus and changes with use as the wheel wears slightly. It 

 must be accurately calibrated to determine the correct factor to reduce the distance, 

 measured in revolutions, to meters. The apparatus should be calibrated at least once 

 annually and at any other time when there is reason to believe that the factor being 

 used is incorrect. The factor may be determined either by direct measurement of 

 the circumference of the wheel or by the indirect method of determining the number of 

 revolutions of the wheel required to measure with the wire a sufficiently long distance, 

 the length of which is accurately knoMn or may be determined from simultaneous 

 observations. 



The circumference of the cyclometer wheel may be measured directly by stretch- 

 ing a piece of the wire around the wheel. About 1 8 inches of each end of the wire should 

 be annealed and a sufficient length should be used so that one end may be made fast 



