MLK was a master of metaphor. He didn't give a sermon or a speech without at least a few of 'em sprinkled in, especially when he was driving toward a concluding crescendo.

In "Letter from Birmingham Jail," he uses metaphor for a variety of effects, both to paint the painful picture of life in the segregated south and to point to the bright possibilities for racial harmony.

For instance, he compares unjust laws with dangerous dams, and social progress with a river:

I had hoped that the white moderate would understand that law and order exist for the purpose of establishing justice and that when they fail in this purpose they become the dangerously structured dams that block the flow of social progress. (20)

This isn't just pretty language. There's an important point being made here, and the metaphors just illustrate it as clearly as possible. In other texts, "social progress" can sit on the page as a mere abstraction, but in Dr. King's writings, it's something you can see in your mind as a river.

Another set of metaphors that he used many times during his career compared the dire social situation in America with an "elegy, and the potential future as a "creative psalm"; racial injustice to "quicksand," and the ultimate goal as a "solid rock":

Now is the time to make real the promise of democracy and transform our pending national elegy into a creative psalm of brotherhood. Now is the time to lift our national policy from the quicksand of racial injustice to the solid rock of human dignity. (21)

An elegy is a mournful poem or speech, typically about the dead. A psalm, on the other hand, is a hymn in praise of God and creation. Quicksand, of course, is a dangerous kind of sinkhole that is very difficult to escape, while solid rock is commonly understood (especially by Jesus in the Christian Scriptures) to be a good place to build, both literally and spiritually.

In typical MLK style, he concludes his "Letter" with a poetic tumble of metaphors to leave his readers uplifted and hopeful:

Let us all hope that the dark clouds of racial prejudice will soon pass away and the deep fog of misunderstanding will be lifted from our fear drenched communities, and in some not too distant tomorrow the radiant stars of love and brotherhood will shine over our great nation with all their scintillating beauty. (39)

This is the kind of thing we all need after we've been given a dose of the dreary and pain-filled truth: an inspiring vision that we can actually see in our mind's eye, powered by accessible metaphors. Everyone knows what dark clouds and deep fog are like. Everyone understands what fear and being drenched feels like. And who doesn't love a clear starry sky?