I would put this week's Sepia Saturday prompt in the category of unusual gadgets. Here we have a postcard of something very unusual. It's a whistling buoy This particular buoy guards the infamous and treacherous mouth of the Merrimack River in Massachusetts.
It's a buoy used in the same catagory as a bell buoy. It can be heard as well as seen. The buoy can be used navigating the channel at night or more usefully, when the weather is foggy.
It actually does whistle albeit a long low droning voice. Below the water line which can't be seen in this photo is a 10 foot long wide pipe heading straight downward. As the buoy moves up and down in the swells the water inside this pipe also moves up and down causing the column of air to change pressure and thus "blows" through the whistle at the top.
This buoy was always known as the "whistler," and locals know that if they can hear the whistler from their homes, which are sometimes a couple miles from the coast, that the ocean was too rough to go out fishing for the day. The east wind brings the sound inland and brings in rough seas.