Marine Collimating-Compass 



Table 50. — Values of the Scale Inclinations, m. 



189 



Method of Sea Observations. 



If the instrument has just been mounted, or if the vessel has changed her course since 

 the last observation, the cylinder is oriented some 5 minutes before observations, so as 

 to avoid producing currents in the liquid by a sudden or large turn of the cylinder at the 

 beginning of the observations. Determinations at sea may be made by one observer, 

 assisted by one recorder (see PI. 11, Fig. 3). But it is desirable to have another observer 

 to measure the altitude of the object sighted in the middle 

 of a set of 10 or 11 readings, and some one to keep the 

 cylinder oriented when the vessel yaws more than 2°. 



Figure 12 represents a projection of a scale upon a 

 plane perpendicular to the collimator axis, so that, if held 

 in a vertical position at arm's length in the direction of 

 the selected cardinal point, it would give a fau- perspec- 

 tive of the scale. As the sextant is rotated about its 

 line of sight, the star's (Sun's) unage is seen in successive positions indicated by the 

 circles. A rapid swing to and fro tlirough a small arc, by persistence of vision, produces 

 a bright line or bar, so that the scale can be read at any instant, even when its motion is 

 quite rapid, provided, of course, that it is not lost to view. Herein lies the difficulty for 

 the novice, especially if there is much rolling or pitching. But, having once acquired the 

 skill necessary to preserve a continuous view of the scale, he has no difficulty in projecting 

 the image of the star upon it, by rotating the sextant as above described. 



The star's (Sun's) image is observed for one or two oscillations to determine mentally 

 the amplitude. It is then quickly moved by the index arm so that it oscillates to equal 



Fig. 12. 



