hear the whistle blow:
Every paddlewheeler needs at least one steam whistle. Whistles with three different sounds were preferred. Some of the boats had up to five whistles. Of course, the whistles run with steam. The high pressure directly from the boiler makes the whistles sound.
Before the invention of radar, the whistles could warn other boats when there was fog or by night. Also, the whistles announced the boat to the next landing. Similar to the ships bell, the pilot could use the whistles to give signals and orders to the engineers at the steam engines.