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CHANNEL DEMARCATION.
261
cussion. In certain instances, closely rivetted joints seem, nevertheless, to
have answered all requirements.
As regards the niethod of suspending the illuminating apparatns of a buoy,
it is evident that in order to ensure its remaiuing upright amid tidal currents
and storm swells, it should be attached in such a manner as to swing freely.
The principle of attachments of this kind will be explained more fully in
connection with lightships. Except for Wigham lamps and in boat-shaped
structures, ordinary attachments may, however, be made to serve.
Lightships.—Buoys are not invulnérable, and it is quite within the bounds
of possibility that a luminous buoy may be extinguished from one cause or
another, though the occurrence is by no means common. Where there are heavy
seas and strong currents, however, lighted buoys are exposed to undue risks,
and signals of a more reliable character are desirable. Lightships are much
steadier under these conditions: their oscillation is less, and they are not so
liable to be put out of action. Moreover, they have the additional advantages
of presenting a more conspicuous bulk and a more striking individuality.
Steadiness is one of the most essential qualities of a lightship, and the
attainment of it, so far as such a thing can be realised amid unstable
surroundings, involves the suppression of synchronism in the periods of
oscillation, respectively of the vessel and of the waves. Synchronism is more
likely to occur transversely under the action of rolling, than longitudinally
under the action of pitching. In order to avoid it, ballast should be located
at some distance from the centre of gravity of the vessel, so that the moment
of inertia of the latter about its longitudinal axis may be as great as possible,
and the metacentric arm reduced to the minimum consistent with stability.
Furthermore, the steadiness of lightships is promoted by deep central and
markedly protruding bilge keels, to all of which ballast may advantageously
be affixed. In some of the latest examples of lightships, the keels have a
depth exceeding 3 feet.
Dimensions ok Representative Lightships of Various Nationalitibs.
Nationality. Name or Locality. Length. Bearn. Deptk. Draugkt. Height of Focal Plane above Water.
ft. ft. ft. ft. ft.
British. . Longsand Talais . 60 24 12 65 30
French . . 61 20 9 6} 33
British.. Snouw. 65} 20 13 111 33
Gaspar Point, K. 75 28 12 65 35
French. . Hoogkly R. Mersey 103} 211 11 94 30
Sandettié 115 204 16} 15 39^
British . . R. Mersey H8ä 21 ni 94 30
German Fehrmarbelt . . 134} 24} 17
The dimensions of lightships have materially increased of recent years.
A length of 60 or 70 feet used to be considered a maximum, but now several