Abstract: A liquid driven pump has a driving-liquid accelerating nozzle which opens into a pump chamber having an outlet part positioned coaxially with the nozzle. Opening into the pump chamber is an evacuation line which is intended for connection to a space to be evacuated. The outlet part of the pump chamber discharges into a coaxial drainage chamber and has an abrupt increase in cross sectional area in relation to the outlet part. The drainage chamber has downstream net-like elements which are passed through by the liquid. The relationship between the smallest cross sectional areas of the nozzle and the outlet part respectively and the distance between the nozzle orifice and the drainage chamber are such that essentially the whole of the diffusion process takes place through the agency of the net-like elements. The smallest cross sectional area of the outlet part is from 1.4 to 4.