Søgning i bogen
Den bedste måde at søge i bogen er ved at downloade PDF'en og søge i den.
Derved får du fremhævet ordene visuelt direkte på billedet af siden.
Digitaliseret bog
Bogens tekst er maskinlæst, så der kan være en del fejl og mangler.
2^g HARBOUR ENGINEERING.
The minimum duration of a visible flash has been determined by
laboratory experiments as one-tenth of a second ; but under conditions obtain-
ing in marine navigation, the period will be increased to, at least, one-third of
a second for white lights and very much more for coloured lights. If the
duration were restricted to its exact experimental limit, it is quite conceivable,
and even likely, that the occurrence of a flash might escape détection by those
on board a vessel rolling in a heavy sea. The flash might also be eclipsed by
any intervening vessel.
Flashes are either uniformly regular or arranged in groups of two, three,
Fig. 246. —Bell-buoy.
or more. The interval between successive flashes need not exceed—and,
indeed, will be preferably limited to—five seconds, so as to afford frequent
opportunities for observation. In grouped flashes, eclipses of 2„ to 4 secon s
are common. Certain lighthouses signal a definite number, as, for example,
that at Minot’s Ledge, U.S.A., which constantly repeats the figures 143 in a
series of flashes separated from each other in the same group by an interva
of 2 seconds, the groups being separated by an interval of 3 seconds, and
the entire signal followed by an eelipse of 15 seconds. The whole period
covers 30 seconds, which is the time of a single revolution of the apparatus.
Sound Signals.— Lighting, while effective eiiough as a guiding agency
in darkness, is practically useless in fog, and reliance has then to be placed