Brent Dalrymple (born 1937), US geologist. Dalrymple is adjusting a mass spectrometer vacuum flight tube. Mass spectrometers accurately detect the mass of molecules of a sample. Dalrymple and others worked in the 1960s on geomagnetic reversals, developing the timescales allowing magnetic changes in rocks to be used for geological dating. This work was carried out at the US Geological Survey, Rock Magnetics Laboratory, Menlo Park, California, USA, where this photograph was taken in 1971. Dalrymple received the National Medal of Science in 2003 for his work. This photograph is part of the Historic American Engineering Record (HAER).