Such
a beam directed at an incoming ship gives effective illumination up to a
mile or more from the source of light, but has the disadvantage that it
must be moved so as to follow the ship's movements. Each beam thus
lights only one ship at a time, and the movements of several beams
crossing and recrossing have a very confusing effect, with the
consequent risk that a proportion of the attacking vessels may slip
through unnoticed.

An alternative method of using electric lights is to arrange the
projector so that the light comes out in a fan (generally of 30 deg.
divergence). Two or three such lights are usually placed side by side,
forming an illuminated fan of considerable divergence. These fans are
now used for the main defence, with in front of them one or more
search-lights to warn the defences of the approach of ships. There is
some loss of range when using these fans as compared with search-lights,
but by occupying both sides of a channel and placing the defences
against torpedo boats at the narrowest point, an effective illumination
can be obtained in moderate weather.

Heavy guns can, of course, be fired against torpedo boats, but their
rate of fire is relatively slow, and at first they had also the
disadvantage of using black powder, the smoke of which obscured the
lights.

A small quick-firing gun using smokeless powder was seen to be a
necessity. At first the 6-pounder was adopted as the stock size
supplemented by machine guns for close range, but soon afterwards it
became necessary to reconsider the scale of anti-torpedo boat defences,
owing first to the increased size of first-class torpedo boats, and
secondly to the introduction of a new type of vessel, the torpedo boat
destroyer. The increased size of torpedo boats, and improved
arrangements for the distribution of coal on board, made these boats
practically proof against 6-pounder guns and necessitated the
introduction of the 12-pounder. The torpedo boat destroyer, originally
introduced to chase and destroy torpedo boats, not only justified its
existence by checking the construction of more torpedo boats, but in
addition became itself a sea-going torpedo craft, and thus increased the
menace to defended ports and also the area over which this form of
attack would be dangerous.

This development was met by an increased number of 12-pounder guns,
assisted in the more important places by 4.7-in. (and latterly 4-in.)
guns, and also by an increased numb