In paper mills, the washing engine was used to grind the rags in water until they turned into a pulp, which would later in the process coagulate into sheets of paper. The words washing engine and beating engine seem to be sometimes used interchangeably*. The above device is also known as a Hollander.
The caption reads in the original French:
* In Dictionnaire technologique français-anglais-allemand, vol. 1 (Tolhausen, Alexandre and Louis, and Gardissal, Durand. Paris: A. Morel et Cie, 1864) Pile défileuse is translated as washing engine, or washer in English and Halbzeug Holländer, or Halbholländer in German. Pile raffineuse is translated as beating engine, or beater in English and Ganzzeug Holländer, or Ganzholländer in German.