It was a twin piston reciprocating pump of continuous water flow that was used for fire-fighting and continued identically having the same use until recently.
It consisted of two pistons, that reciprocated oppositely with the help of a pivoted common hand lever within two vertical cylindrical containers immersed in the (probably wheeled) water tank. The non-return valves of the incoming water were in the raised bottom of the containers and the non-return outlet valves were at the base of the discharge pipes. The pipes converged to a common vertical pipe. At the end of the pipe there was an ingenious (horizontally and vertically) rotating pipe system with a nozzle that allowed the precise aim of the target.

SOURCE: Heron of Alexandria, Pneumatics.