Define dramatic irony and identify one line of Duncan's in Macbeth that uses dramatic irony.

In its simplest form, irony is a difference between what
appears to be and what really is.  More specifically, dramatic irony occurs when
audiences know something that a character does not. 

In the
case of Shakespeare's Macbeth, audiences are aware, toward the end
of Act 1, that Macbeth has been made the Thane of Cawdor by King Duncan and that
both Macbeth and his wife, Lady Macbeth, are power-hungry.  Driven by the witches'
prophecy, the two plot to kill the King so that Macbeth may assume that
title. 

When Duncan arrives at Macbeth's castle in Act 1,
scene 6, he observes,

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This castle hath a pleasant seat; the air

Nimbly and sweetly recommends itself
Unto our gentle senses
(1-3).

In these lines, Duncan
comments on the castle's fresh air and general pleasant atmosphere.  Obviously, readers
understand that as Macbeth is plotting to kill Duncan, the castle will certainly
not be a pleasant place for Duncan.