Oil pump. View of a pump, known as a jack pump or a nodding donkey", at an oil well. The pump draws oil out of the ground through a series of non- return valves (not seen here). Newly-drilled oil wells often contain oil at a high enough pressure to force the fluid out without pumping. As the oil is removed, the pressure falls. For a time, the pressure can be maintained by pumping water or air into the subterranean reservoir of oil. Eventually the pressure falls further and a jack pump is required to bring oil out at an economic rate. Even using jack pumps, generally only 30-50 per cent of the oil can be extracted. Photographed in Welton, Lincolnshire, England."