In 1785, the Milanese provost Carlo Castelli described a hydraulic pump to which he gave the name "hydraulic fan" since it functioned in a similar manner to mine-ventilating machines. Its inventor claimed the device could be used for firefighting, extracting water from wells or ships' bilges, irrigation, and many other purposes.

The pump itself is mounted on a metal-lined wooden box that served as a water tank. The pump's brass body is cylindrical and topped by a spherical dome fitted with a long pipe. Inside the cylinder are four triangular chambers equipped with stacked pairs of valves. Two large tubes connect the lower chambers to the tank. The pump's body is divided vertically by a mobile watertight partition that is activated by the alternating motion of the metal handle hinged to the top of the machine. The movement of the panel across the sectors causes a suction effect in one segment of the cylinder, drawing water from the tank through the lower valves. In the other segment of the cylinder, the panel movement causes a compression effect that expels the water toward the dome through the upper valves. The machine is thus both a suction pump and a pressure pump. Thanks to the dome, which serves as a compressed-air chamber, the water gushes in a continuous, uniform stream. Castelli claimed that this type of hydraulic fan was not only more efficient that piston pumps, but also simpler to make.