Cone had hung his Dr. Dean Edell drug-store magnifying glasses off the magnetic compass and left his airplane for a few hours. When he got back his nose told him that something was wrong—that was before he saw the parallel slashes in the fabric of his pilot and copilot seat. After considering all the alternatives–such as renegade seat slashers, etc–he finally came to the conclusion that his reading glasses had focused the sun’s rays on the seat fabric, and as the sun traveled across the sky, the focused light left a trail of burnt fabric. He was very lucky.
The moral of the story–be careful with those magnifying glasses.