The mitrailleuse was one of the early types of mechanical machine gun, along with the Gatling, Gardner, Nordenfelt, and others. “Mitrailleuse” is actually a general name for a volley gun – one with many barrels in a cluster, which are fired sequentially. The two most common types were the Montigny (a Belgian design fired by a lever) and the Reffye (a French design fired by crank).
The Reffye was a top-secret weapon used by the French in the Franco-Prussian War, which was expected to be a huge game-changer. However, there was little experience worldwide in how best to use a weapon like this, and the French commanders chose to use them like artillery, firing at long range where they were inaccurate and underpowered. In this role, they were utterly outclassed by the Prussian Krupp artillery, leading to a general European disdain for the effectiveness of machine guns that would last until the First World War.
This Reffye is a reproduction, here shown firing blanks. The footage comes to us courtesy of Julien Lucot, a writer for the French arms magazine Cibles. Thanks, Julien!
There is also a fellow on YouTube who has posted an excellent video showing how the Reffye functions, using a computer generated 3D model: