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202
HAKBOUR ENGINEERING.
boats in the English and French services have lengths of over 100 feet, and
one boat in the German service has a length of 134| feet. The depth and
draught of these vessels manifests a proportionate increase, but the beam has,
if anything, tended to diminish, or, at least, to remain stationary, as will be
evident from an inspection of the accompanying table, which classifies the
leading dimensions of certain representative vessels of all three nationalities.
Suspension of Floating-lights.—In order to maintain verticality, the
illuminating apparatus of a lightship is supported on gimbals. In catoptric 1
lights, the mirror and lamp are suspended in this manner from above. The
dioptric1 apparatus is generally hung in the form of a pendulum swinging
about a horizontal axis located immediately beneath the lamp. The pendulum,
or rod, is weighted and counterweighted above and below, the weights being
adjusted in such a way that the period of oscillation of the lamp is consider-
ably longer than that of the vessel, so that the maximum inclination of the
former may not exceed 5 or 6 degrees. Manifestly, the apparatus must not
only be sufficiently sensitive to maintain its verticality, but it must also admit
of free and ready response to change of direction, and this is secured by
attaching the gimbals to a horizontal circle rotating on steel balls. In the
event of exceptioually heavy rolling on the part of the vessel, the possibility
of collision between the pendulum and the lower part of the lantern may be
guarded against by the provision of a thick annular pad of india-rubber on
the weighted portion of the latter, or by restricting the swing of the pendulum
with the aid of check chains and flexible guys.
Lightship Attendance.—The reliability and automatic continuity of
the compressed oil gas illuminating apparatus has very largely done away
with the necessity for crews on board lightships. In many cases now these
vessels are unattended, except at long intervals for the purpose of supplying
fresh gas. This has effected considerable economy in maintenance expenses,
and extended the scope of utility.
The liability, however, of all floating objects to displacement, is the
inherent weakness of the lightship, as also of the light-buoy. A displaced
signal is much worse than none at all. Beacons and lighthouses, therefore,
from their very fixity, possess uncontrovertible merits as regards accuracy
of alignment, and it is usual to rely mainly upon them in so far as they
happen to be available for this purpose.
Lighthouses and Luminous Beacons. —The earliest type of the
lighthouse was the lighted beacon, usually situated upon a natural eminence
or upon a tower. It was an iron-barred grate, or receptacle, for wood and
coal, which was ignited at night-time. These signals were, therefore, most
crude and primitive, and often gave out more smoke than light. Moreover,
they lent themselves to easy reproduction and imitation for illicit ends. In
this form they have long since disappeared into the lumber of the past.
Their present development is the harbour light, a lantern attached to the
top of an upright mast, which is used at the entrances of minor ports.
1 See p. 268.