DAVID PARKER / SCIENCE PHOTO LIBRARY DAVID PARKER / SCIENCE PHOTO LIBRARY
A jack pump used for oil extraction. These pumps, also known as "nodding donkeys", are used where the natural pressure in the oil well is too low to force oil out at an economic rate. The pump uses a series of non return valves to draw the oil out as the head nods up and down. Such pumps are now becoming less frequently used as new recovery techniques are developed. This photograph was taken in North Dakota, USA.
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