What is a metaphor, and why might it be used?

A metaphor is the literary technique used to compare one thing with another by saying that it is that thing. For example, Alfred Noyes's poem, 'The Highway Man', begins with three metaphors:

'The wind was a torrent of darkness among the gusty trees.

The moon was a ghostly galleon tossed upon cloudy seas.

The road was a ribbon of moonlight over the purple moor,'.

In this extract, metaphors are used to compare the wind to moving water, the moon to a ship at sea, and the road to a ribbon.

  The most important question to ask when you see a metaphor is, 'Why is it being used?' Often, you will be able to think of many answers to this question, and all of them could be correct. In the example above, the wind could be compared to a 'torrent' because it is fast and strong (fast-flowing water would push with more force than fast-flowing air, so Noyes could be suggesting that the wind is so strong that it feels as though it is water moving, rather than air). A torrent also pushes in only one direction (as in a river), so there could be the suggestion of inevitability. Just as the flow of water is unstoppable, so the tragic ending of the poem is unavoidable.

  On a more complex level, a metaphor can be used to highlight the difference between the two things it compares. For example, a road and a ribbon (compared by a metaphor in the quotation above) have few similarities. Though a reader might understand, from this extract, that the road is beautiful and shining, like a ribbon, the differences between a ribbon and a road could also aid their understanding of the poem. Though the road is substantial and often used by men (for trade, or crime), the ribbon is small, able to be ruined, and associated with women. The image of the ribbon, then, is seen in the wrong place, where the reader could imagine its being trampled on and ruined by the vehicles that travel on the highway. In the same way, the heroine of this poem will be killed because her beautiful, fragile presence is placed in a man's world. The difference between the two things compared by the metaphor, therefore, reflects a theme of the poem that could make the reader doubt that the beauty of the setting is safe, and possibly lead to a sense of foreboding.

  If you want to mention the use of a metaphor in an essay, it is important not only to identify the metaphor, and the things it compares,  but why it is comparing these things, and what effect it will have on the reader.