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).
Peter Senge, Art Kleiner (2011). “The Dance of Change: The Challenges of Sustaining Momentum in a Learning Organization”, p.64, Nicholas Brealey Publishing