One example of a metaphor in the poem is "And now the pitcher holds the ball, and now he lets it go, And now the air is shattered by the force of Casey's blow." Here we can see that the speaker is comparing the force of Casey's blow to air "shattering," which tells us that Casey hit the ball very, very hard, which made it go very fast in the air. The metaphor the speaker used gives us a sense of imagery of how fast the ball was going.
