CHAP. XXL] OBSERVATIONS AT NIGHT 493 



SECTION XIV. 



OBSERVATIONS AT NIGHT— GENERAL CONSIDERATIONS. 



Endeavours should be made, if possible, to so arrange 

 matters that whilst the ships are observing they may be all 

 swung to the same general set of the tidal stream, and for this 

 purpose slack water occurring near about midnight would be 

 suitable, and before which all ships should have completed their 

 observations, say, in the summer months. 



It may be frequently necessary to obtain some of the angles 

 from aloft, for which some preparation should be made before- 

 hand. With no top or lookout platform available, it would be 

 well to extemporise this, together wdth electric control switch 

 for reading and writing, and thereby add much to the comfort 

 and expedition when observing, and minimise the chance of 

 injury to the instruments. An assistant should also be aloft 

 to record, and a signalman to work the cruiser arc lamp. 



No hard-and-fast rule should be laid down for working the 

 searchlights or arc lamps when directing them for other vessels 

 to observe, or when requiring their attention. 



It will generally suffice when the ships are at such a distance 

 as to render Morse signals imj)racticable, or their use involves 

 much delay, if the following procedure be adopted : 



1 . A succession of short flashes dii'ected from A to a vessel B 

 indicates that A wishes to observe B. 



2. B answers by a succession of short flashes, upon which A 

 shuts off her light to B (unless B also requires to observe A). 



3. B then maintains a steady light on A. 



4. When A no longer requires B, A makes a succession of 

 long flashes, which is answered by B shutting off her Hght. 



It is as well to remark here that in order to obtain precision 

 53ome practice is required in observing angles at night between 

 theUghts of lighthouses and hghtvessels of varjdng periods. 



It may frequently happen that their periods are such that 

 they may only synchronise momentarily after long intervals 

 of perhaps fully one hour, and then only for a comparatively 

 short duration. This feature, together with uncertain and 

 changing conditions of weather, motion of the ship, etc., in- 

 creases the difficulties at night, and affords additional reasons 



