the daymark to prevent their whole surface from being filmed with sand and dust. The Scotchlite was installed in other District Areas, mainly Coos Bay, with hearty approval of the mariners in the Bay.
A serious problem confronted the Aids to Navigation Section in the Upper Columbia where it was impracticable, due to the impossible terrain to establish the conventional range lights but where it was necessary to provide channel markings for safe navigation. After two years, consideration, C. E. Sherman, Nautical Scientist of the Aids to Navigation Section, devised a means to provide this marking and the initial experiment proved most successful. Sherman's "channel limiting group" lights provided positive protection for a width of 198 feet and were so arranged that the center light showed flashing red and the lights on either side showed fixed white until the navigator departed from his course and reached the edge of the safe channel, at which time the white light marking the channel side became red. Should he continue into the red light, it appeared to be extinguished, which indicated a deep penetration into the outer side of the safe channel. The openings in the side lights, although but 1/8" wide, could be seen for a distance of 1.4 statute miles with a light of sufficient brilliance that it could be seen by a person having normal vision looking within 30° from the light. These openings were provided by the arrangement of two opaque screens spaced 10 feet apart along the axis line of the light, one having its left edge and the other its right edge on this axis. (See photographs and sketch on following page.)
(image) EXAMPLE OF PRECIPITOUS SHORE MAKING INSTALLATION OF CONVENTIONAL RANGE IMPOSSIBLE
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