522 DEFENCE OF ESTUARIES, ETC., AGAINST TORPEDO-BOAT ATTACK. 
Fig. 1. 
These formalee are, of course, not strictly accurate, but sufficiently 
so for all practical purposes. 
It will be seen that the particular form of beam to be employed 
depends very much on the width of the estuary to be defended, and 
the dispersed beam as suggested by the author could only be used with 
effect in very narrow channels. On a clear dark night, with every¬ 
thing favourable to the defence, a boat could be discovered by aid of 
the dispersed beam at a distance not much exceeding three-quarters of 
a mile, while, by employing the parallel beam, this distance is increased 
to between 3000 and 4000 yards. 
Secondly, with regard to the particular arrangement of the beams 
as suggested in the paper, the following objection may be urged:— 
That neither of the operators at the projectors would be able to see 
anything in the diamond-shaped area on account of the opposite light 
shining in his eyes ; also, it is a well-known fact that it is impossible 
to see anything on the further side of a beam of light, so that the look¬ 
out men would have to be stationed outside, and on both sides, of the 
illuminated area. 
Taking into consideration that, though narrower, the parallel beam 
gives the more intense light, with greater distance, than the dispersed 
beam, it would, perhaps, be better to employ it, and two methods of 
fixing the beams could be resorted to : one by forming a single parallel 
beam, the two lights overlapping more or less (Fig. 3) ; the other by 
Fig. 3. 
throwing the beams parallel to each other and touching, and overlap- 
