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).
" A Dictionary of Thoughts: Being a Cyclopedia of Laconic Quotations from the Best Authors of the World, Both Ancient and Modern" by Tryon Edwards, New York, Cassell publishing company, (p. 134), 1891.