I am a buoy, placed to point out the way where beacons cannot be erected, and to prev

I am a buoy, placed to point out the way where beacons cannot be erected, and to prevent mariners from coming to danger. I am composed of marks which show different colours, that I may be distinguished from other buoys. I am lighted at night, to serve as a beacon to those who cannot see in the day-time. I am placed where channels are marked out, to prevent mistakes in the buoying of the beacons. I am set up a mark for