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).
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.52, St. Martin's Press