Page 379 equipment and instruments 454 



paralleling the 0°-180° line of the protractor. This protractor is especially useful 

 in plotting bearings or azimuths on a smooth sheet or boat sheet, since the protractor 

 can be centered over the point from which the bearing was measured and oriented by 

 means of the grid of parallel lines. 



Several refinements to the standard metal three-arm protractor have been devised 

 in recent years. These, however, are not in use in the Coast and Geodetic Survey. 

 The British have developed a metal protractor with micrometers substituted for the 

 verniers, and release clamps substituted for the setscrews, on the movable arms. It is 

 claimed for this instrument that the angles can be set more rapidly and with greater 

 ease to single minutes with the unaided eye. No magnifying glass is provided. The 

 micrometer is thrown in or out of gear by clamps which are released by the simple 

 grasp of the thumb and finger. After an approximate setting of the angle, the act of 

 releasmg the grip clamps the arm, and the final setting is made by the micrometer 

 tangent screw. 



Another European development of the metal protractor elevates the protractor 

 about 2 inches above the plotting sheet. Accessories to this development are station 

 holders or guides, one of which can be placed over each station to guide the arm auto- 

 matically. These guides are of practical value only where a large number of positions 

 are to be plotted from the same three stations. Each station holder is so constructed 

 that when set in place over the plotted position of a station, rollers guide the fiducial 

 edge of the arm through the plotted position of the station as the instrument is moved 

 with the arms undamped. When the center of the protractor is moved into approxi- 

 mate position, the roller guides force the movable arms to settings approximating the 

 observed angles; then as the observed angles are accurately set by tangent screws, the 

 center of the protractor automatically moves into the position to be plotted. After 

 a position has been plotted, the movable arms are undamped and the protractor is 

 moved ahead to the vicinity of the next position. An advantage of the elevation of the 

 protractor above the sheet is that it allows the draftsman to see almost all of the work 

 on the sheet and permits a limited amount of writing on the smooth sheet without 

 removal of the protractor. • 



454. Timepieces 



Accurate time is essential in- most survey operations and in navigation. Vessels 

 and field parties of the Coast and Geodetic Survey are provided with the necessary 

 chronometers, clocks, and stop watches for use in all operations. 



4541. Chronometer 



Clu-onometers and then- use on shipboard are described on pages 128 to 135 of 

 The American Practical Navigator (Bowditch). Methods of determining their rates 

 and errors are also described. Every hydrographer should be familiar with their use 

 and care. 



The break-circuit chronometer and the chronographs used in R.A.R. are described 

 in 673. Descriptive details which can be readily found elsewhere have been omitted 

 from this Manual. The information is limited to the most important precautions in 

 handling and use. 



a. Care, — The case in which the chronometers are kept should be located as nearly 

 as practicable on the centerline of the vessel and where it will be protected from shocks 

 or jars. Chronometers should not be exposed to rapid changes of temperature and 



