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).
Wallace D. Wattles (2015). “The Science of Being Great: Personal Self-Help Book of Wallace D. Wattles (Unabridged): From one of The New Thought pioneers, author of The Science of Getting Rich, The Science of Being Well, How to Get What You Want, Hellfire Harrison, How to Promote Yourself and A New Christ”, p.17, e-artnow