Everbach replies: Leonard Finegold’s points are sound on several counts. The “sounding machine” used by the ancient Greeks and for thousands of years thereafter consisted of a lead weight that was thrown over-board, into the sound, tied to a knotted rope. The “propagation time” was the time for the weight to fall at its terminal velocity to the sea bottom; that time was proportional to the length of rope paid out—that is, distance from the source. Thanks for sounding me out on this question, Len!