Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.24, St. Martin's Press
Quoted in Evan Esar, The Dictionary of Humorous Quotations (1949). Usually attributed to Dewar, but it should be noted that the line "mind like parachute - only function when open!" appears in the 1936 film Charlie Chan at the Circus (screenplay by Robert Ellis and Helen Logan).