How does the meter in this line from "The Destruction of Sennacherib" (provided below) emphasize the use of alliteration in the line?
"And the sheen of their spears was like stars on the sea."
What I've gathered so far: The alliteration in the line is the repeat of the letter "s," and the meter is an apestic tetrameter.
Have you tried reading this aloud yet? If not, doing so will provide you with your answer: there is a rise in the tone and sound of the human voice on every third syllable. Each third syllable is comprised of one "s" word -- sheen, spears, stars, and sea. The author has used this meter, rise, and rhythm to draw attention to the line's alliteration, which is the repetition of any initial consonant sound. It doesn't always have to be "s," just to clarify.
The effect accomplished by this line is tidal, meaning that it has the repetitiously soft sound of ocean waves. You'll notice the last word in the line is, in fact, "sea." The line itself has a sound like the body of water being mentioned.