4538 HYDROGRAPHIC MANUAL PaGE 378 



Where a position is fixed by only two R.A.R. distances, the device may be used for 

 plotting the position directly. After it has been placed, as described above, so that its 

 center correctly marks one distance, it can be moved until its center correctly marks a 

 second distance referred to another set of distance chcles drawn about a second R.A.R. 

 station, while keeping it in its correct relationship to the first distance circle. The 

 center of the device will then mark the position fixed by the two R.A.R. distances. (See 

 76310.) 



4538. Metal R.A.R. Protractor 



A metal plotting device has been designed which can be used to advantage under 

 certain conditions to plot R.A.R. positions. The device consists of three arms pivoted 

 at a pricking device. With zero at the pivot, the arms are graduated to represent 

 distances (in meters or seconds of travel time) at the scale of the hydrographic sheet. 



Three index heads, or disks, are provided. These are fastened over the sheet, one 

 at each R.A.R. station, the index being vertically above the plotted position of the 

 station. The arms slide in grooves in rotatable inner parts of these heads. The index 

 is provided witli a vernier and the head with a setscrew to clamp the arm at its correct 

 setting. 



If distances are determined to two R.A.R. stations and the distances are set on the 

 arms passing through the respective stations, the pricker at the pivot will move me- 

 chanically to the position where the bomb was fired. Owing to inherent small errors 

 and distortions of the plotting sheet, the three arms cannot be used simultaneously, 

 but the two which give the strongest intersection or the most consistent results can 

 be used, with a third distance used as a check. Or the position can be plotted with 

 each pair of distances and the three resultmg positions compared. 



The instrument must be graduated for a mean velocity of sound or for a selected 

 plotting velocity, as 1,460 meters per second, the plotting velocity used on all R.A.R. 

 smooth sheets (see 763). In the latter case each R.A.R. distance must be corrected 

 arithmetically to the plotting velocity before bemg used, and in the former case similar 

 corrections can be made when the actual velocity differs enough from the velocity of 

 gi'aduation to warrant it. Corrections for distortion of the plotting sheet can be made 

 in the same manner. 



The instrument must be graduated for one scale of hydrographic sheet, although 

 it can be used for other scales by applying a factor to the distances before plotting. 

 Where the scales involved are double one another this is simple. 



4539. Special Types of Protractors 



Several forms of special transparent protractors, of advantage in particular circum- 

 stances, have been prepared in the Washington Office. A protractor printed on 

 tracing paper, merely a graduated cu'cle 3% inches in diameter with a marked center, is 

 available. This is particulary useful for plotting positions so close to the stations 

 that one or more of them are obscured by either the metal or transparent three-arm 

 protractors. For use in plotting, continuous lines are drawn from the center of the 

 protractor to represent the angles measured, after which the protractor is used in 

 the same manner as is the three-arm protractor. If other sizes are needed to meet 

 special conditions they will be furnished on request. 



Another form of protractor available at the Washington Office consists of a circular 

 piece of celluloid, similar in size and appearance to the solid part of the celluloid 

 three-arm protractor, throughout which numerous parallel lines have been printed 



