In Act 4 Scene 1 of Macbeth, what are some literary/poetic devices that are used, and why?They could be very subtle devices, or they could be...

In Act 4 Scene1 of Macbeth, Macbeth goes to
see the witches to get another prediction of events to come.  At the beginning of the scene, the
witches are making a spell to help conjure the apparitions that will show Macbeth his future. 
Just before Macbeth arrives, the second witch says, "By the pricking of my thumbs, something
wicked this way comes" (IV.i.44-45).  This is an element of symbolism
and foreshadowing:  the witches already know that
Macbeth is on his way, so the witch's statement foresees his arrival.  In addition, she calls him
"wicked," and by this point in the play, Macbeth has already shown that he is an evil, selfish
character who will stop at nothing to have his way.  As the witches show Macbeth the apparitions,
the foreshadowing and symbolism continues, and the appartitions appear as symbols of future
events:  the armed head which represents Macbeth's duel with Macduff, the bloody child which
represents Macduff's having been ripped from his mother's womb, and the crowned child holding the
tree which represents Malcolm's ordering the English army to use the trees of Birnam wood to
disguise their approach.