CHANNEL DEMARCATION.
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in a medium such as that which envelops the British coasts : often foggy,
only occasionally very clear. And as localities vary in their meteorological
expériences, no serviceable standard can be devised. The French have a
system of denoting the visible range of their lights during two percentages
Figs. 243, 244. —Single Flashing Apparates of 250 mm. focal distance, consisting
of 1 panel of 1611° horizontal angle and dioptrie mirror.
of the whole year, viz., 50 per cent, corresponding to clear weather only, and
90 per cent, corresponding to the inclusion of moderately misty and variable
weather. It is difficult to see how such a system could be successfully
applied to British pharology, where climatic changes are more frequent and
much more adverse.
18