494 HYDROGRAPHICAL SURVEYING [chap, xxi 



for not laying down too many rules between the ships ; and it 

 can be safely said that we cannot do wrong if we direct our 

 signals steadily divided between the ships engaged, who may be 

 experiencing like difficulties, and whose efforts may possibly 

 be concentrated at one time in other directions than the other 

 ships engaged. 



It will greatly help the observer, and save fatigue, if he is 

 prepared beforehand with the approximate value of the angles 

 required, whilst the assistant, with a watch, times the periods 

 of the lights, giving the observer sufficient warning beforehand 

 of their reappearance. 



The less difficult angles can be obtained from the bridge, also 

 the true bearing, by the other assistants. 



Should the searchhght be screened by shij)s' fixtures, etc., 

 obstructing the view in the direction it is required to use it, 

 the situation may be often saved by adapting the searchhght 

 as a heUostat, using the simple form of portable mirror upon 

 which to direct the beam, from which mirror rays can be directed 

 in the required direction with the aid of an extemporised 

 movable directing arm. 



It is often possible to observe rehable angles to the beam of a 

 searchhght when it is dipped beneath the horizon, if care is 

 taken, in the first place, to train the projector in the direction 

 it is required to be observed from. 



It should be the duty of an officer to control the searchlight 

 and ensure that it is bemg properly trained. This should not 

 be left to the crew, who soon become inattentive, especially 

 if the ship has motion and is yawing about and it is not possible 

 to see the horizon or the object upon which it is required to 

 train; or, again, on a dark night with no guide, such as a star, 

 etc. 



A boat's compass near at hand is often a help, and occasional 

 comparison -with a ship's compass should ensure the ray being 

 observed if a sufficient margin of training is allowed for. 



All data inserted in the deck-book should include the time 

 being entered against each observation, for should the angles 

 differ to any extent from various causes, it may be necessary 

 on a subsequent review to select those obtained simultaneously 

 at a particular period in order to best combine them with the 

 observations obtained by the other vessels engaged. 



