Page 369 equipment and instruments 4522 



marking the angle, because clamping the setscrew often causes the arm to jump a little, 

 introducing an error in the reading. 



As a general rule, after taking an angle and reading it, an observer should not 

 move the index arm until the position has been plotted, so that he can verify the reading 

 if necessary. In any event it is good practice, after an angle has been observed and 

 read, to verify the reading of the sextant before the arm is moved. If the sextant is 

 required immediately to measure a cut or another angle, the reading should be verified 

 at once. 



When the survey vessel is rolling and pitching considerably the greatest knack is 

 required to measure sextant angles accurately and at the required instant. The sextant 

 must be held in a horizontal plane for both the direct and reflected objects to be visible 

 and for the resulting angle to be correct. Under these conditions the observer should 

 station himself where the movement of the vessel is at a minimum and should learn to 

 counteract this movement by an opposite movement of the sextant so that it will 

 always remain as nearly in a horizontal plane as practicable. 



4522. Angles to Faint Objects 



Where the objects or signals are faint or where angles are being measured between 

 survey buoys out of sight of land, the clifSculties encountered may be extremely dis- 

 couraging to a beginner. The greatest difhcidty is usually in reflecting the right-hand 

 object, which sometimes cannot be found in the sextant even though it can be seen 

 fairly well when observed directly. In such cases it is of material assistance to set the 

 sextant in advance close enough to the expected angle so that the reflected image of the 

 right-hand object will be within the field of view of the telescope. 



There are several methods of finding approximate angles: 



(a) The angle may be scaled from the boat sheet with a protractor set at the approximate location 

 of the next position to be fixed. 



(6) If the angle is not changing too rapidly the rate of change between the two preceding angles 

 may be applied to determine the approximate angle at the next position. 



(c) The relation between the faint signal and a conspicnous object or peculiar cloud formation 

 on the skyline near the signal may be noted, and the conspicuous object or cloud formation reflected 

 first, from which a slight ■inovement of the sextant will bring the signal within the field of view. 



(d) With the sextant set approximately at zero the right-hand object may be observed direct 

 and then found in the index mirror. By slowly moving the index arm and at the same time rotating 

 the sextant to the left, the image of the right-hand object may be kept in view in the index mirror 

 until the left-hand object is seen directly above it. 



(e) Every observer should determine the angle subtended between the ends of the thumb and 

 little finger of one hand outstretched at arm's length, so that by sighting over his hand and moving 

 it along the horizon he can step off an approximate angle between any two objects. 



If the middle object of a three-point fix is difficult to reflect, the observers may 

 measure the right angle and the sum angle between the right and left objects, subtracting 

 the former from the latter to obtain the left angle. 



If the right-hand object is difficult to reflect but the left-hand one is very distinct, 

 the sextant may be held upside down to look directly at the right-hand object and 

 reflect the left-hand one. This is an unnatural and inconvenient procedure but may 

 have to be resorted to occasionally. 



Inexperienced observers have great difficulty in measuring angles between survey 

 buoys out of sight of land, particularly when the heading of the survey vessel is changed 

 frequently or by a large amount. The buoys are frequently so distant that they can 



465382—44 25 



