What is one literary device in Macbeth, Act 4, scene 1, and how is it used by William Shakespeare?

A literary device can be described as a particular method that an author uses to either enhance his/her writing or to effectively convey a message or to emphasize a particular word, phrase, sentence or situation, amongst others. The use of such literary devices enables the reader to critically analyze and appreciate the writer's work. There is a large range of such devices available to the writer. Comparison, for example, is a particularly useful device to clarify an author's purpose. When the writer calls a character 'a snake in the grass,' for instance, it is clear that he/she means that such a person is not to be trusted and means harm.

In Act 4, scene 1 of Macbeth, the most obvious literary device is irony. In this instance, the irony is both verbal and dramatic. Verbal irony involves a character saying one thing, but meaning another, as in the following example from lines 79-81:

Second Apparition
Be bloody, bold, and resolute; laugh to scorn
The power of man, for none of woman born
Shall harm Macbeth.

Macbeth has come to consult the witches and demands that they provide him with more predictions. They call up an apparition from a cauldron and the above extract conveys its response. The apparition tells Macbeth to be fearless and to mock man's power for no man born of a woman will ever harm him.

What Macbeth does not understand is that the prediction should not be interpreted literally. The witches deliberately use ambiguity to further encourage him into committing evil, and therein lies the irony. He thinks that the prediction means that no one will be able to harm him, for all humans are given birth to by women. He believes that he is untouchable.

When the tyrant is later confronted by Macduff in Act 5, scene 8, he learns to his dismay that he had been deceived:

MACBETH
Thou losest labour:
As easy mayst thou the intrenchant air
With thy keen sword impress as make me bleed:
Let fall thy blade on vulnerable crests;
I bear a charmed life, which must not yield,
To one of woman born.

MACDUFF
Despair thy charm;
And let the angel whom thou still hast served
Tell thee, Macduff was from his mother's womb
Untimely ripp'd.

MACBETH
Accursed be that tongue that tells me so,
For it hath cow'd my better part of man!
And be these juggling fiends no more believed,
That palter with us in a double sense;
That keep the word of promise to our ear,
And break it to our hope. I'll not fight with thee.

When the two men start fighting, Macbeth warns Macduff that he should cease the battle since he cannot be killed by anyone born of a woman. Macduff then tells him that he was not naturally born, i.e. by normal childbirth, but that he was prematurely cut out of his mother's womb, i.e. by Caesarean section.

Macbeth is obviously shocked and denounces the witches for having misled him. He, however, has only himself to blame, for he heard only what he wanted to since it suited his purpose.

As an example of dramatic irony, the scene conveys what the audience already knows: the witches had, from the outset, deliberately meant to deceive Macbeth. He, however, did not realize that and became a mere tool in their malicious claws.

Shakespeare cleverly uses irony throughout the play to enhance the dramatic impact of what the characters say and do. This allows the audience to be enthralled, entertained, and fully engaged in the drama that unfolds.