Abstract: A reciprocating gas compressor in which a piston (3) reciprocates within a compression chamber (1) in order to compress gas. Water is injected through nozzle (6, 7) into the compression chamber to cool the gas during compression. A source of pressurised liquid (13) is arranged to accelerate liquid through the nozzle (6, 7). At least one duct (14, 17, 19) connects the pressurised source (13) to the nozzles (6, 7). The dimensions of the duct (14, 17, 19) are sized to define the inertia of the liquid to control the acceleration of the mass flow through the nozzles during compression such that the cooling capacity of the liquid in the compression chamber increases as the pressure therein approaches its final value.