To disappear altogether. Exactly when it was known that the higher one goes the thinner the air (owing to less available oxygen) is not certain. Shakespeare, however, wrote of ghosts that “Melted into air, into thin air” in 1610 (The Tempest, 4:1). A twentieth-century version of this cliché is the vanishing act, said of a person who unexpectedly disappears. It comes from the magician’s trick of making something disappear (hence “act”). The essayist Logan Pearsall Smith used it poignantly in All Trivia (1933): “I cannot forgive my friends for dying; I do not find these vanishing acts of theirs at all amusing.”