I am a light buoy, placed to warn mariners from danger. I exhibit two lights, which s

I am a light buoy, placed to warn mariners from danger. I exhibit two lights, which show white in front, and red behind, to mark the way, and to point out the danger. I am moored, as a beacon, over the sea marks, to render visible, in the night, the channels which are to be shunned, as well as those which are to be followed. I am exhibited, to show where buoys are to be placed, as well