CHAP. VIII.] SHIP SOUNDING 213 



head can be observed as soon as it appears above the horizon, 

 is a plan sometimes employed. These need not see one 

 another. As long as their relative positions on the chart are 

 known, and the true bearing of the zero employed has been 

 established, the angles to the ship can be plotted. The 

 smoke from the ship when she herself is below the horizon 

 will often enable a valuable angle to be observed. A single 

 theodolite line is often of great value, as it can be utilised in 

 conjunction with angles, bearings, or observations from the 

 ship herself. 



It is scarcely necessary to say that an observer will be on surveyo 

 the topgallant yard of the sliip, as he may, from atmospheric Aloft, 

 or local causes, be able to see something on the land before 

 the theodolite observers catch sight of the ship. 



True bearings come in useful again. The angular distance 

 between the sun and the mountain or other object seen from 

 aloft, will be taken by the observer aloft, while the sun's 

 altitude is taken from deck for the azimuth. 



Another method is to have one or two ships anchored as Tenders 

 far from the land as they can fix, which observe and are 

 observed from the sounding-ship, as she rups in and out 

 on her lines. A ship can easily in light winds anchor in 

 100 fathoms, and even in deeper water. 



When land stations are employed, heliostats are useful, as use of 

 informing the running ship that she is seen from the station. Heiiostat 

 A flash will tell the officer aloft that a sounding can be taken, 

 with the certainty of an angle being got to the ship, for which, 

 perhaps, she has been waiting. 



If not able to return and pick up the land before nightfall, Position 

 blue lights and rockets are useful, both from stations and p^*®^ 

 moving ship. 



A true bearing of a light or mountain, if visible, as it often use of 

 is a great distance on moonlight nights, can be obtained in stars. 

 the Northern Hemisphere very conveniently by the angular 

 distance from the Pole Star, as described on p. 360. This 

 angular distance can again be taken from aloft ; but in the 

 case of Polaris we require no altitude. 



If Polaris is not available, a time azimuth of a star near 

 the prime vertical wiU give a good result. Should the re- 

 sulting longitude differ much from the assumed, it may be 



