Sitting on a terrace high above the Kalverstreet in Amsterdam, I had a nice view at the street below. Usually this is one of the most crowded shopping streets, particular in the weekends. But at this moment there was not much public so the organ players had a coffee break. A barrel organ (or roller organ) is a mechanical music instrument consisting of bellows and one or more ranks of pipes housed in a case, usually of wood and mostly highly decorated. The barrel organ is activated by a person turning a crank, or by clockwork driven by weights or springs.
At a post in 2012 I showed some pictures of the barrel organs, have a look here