Lines 21-30 Summary

Get out the microscope, because we’re going through this poem line-by-line.

but now my eyes flicker
from the in-flight movie
to the stewardess's pantyline,
then back into my book,

We're back in the present and our speaker is definitely distracted. He glances at the in-flight movie, checks out the stewardess, and then turns back to his book.
We get the feeling our speaker isn't very impressed by straight lines anymore. He might even be bored by them.
Maybe he'd like it better if the plane flew closer to the ground, or if he could open the windows and feel the wind on his face, or at least if there were a little bit of turbulence to remind him that he was actually hurtling through the air at speeds he could never reach on the ground.
That word "flicker" has a cool way of getting across our speaker's distraction while also connecting us with the image of the in-flight movie (the way it flickers on the movie screen).
We get a sense of his lack of fulfillment here. Our speaker looks to the movie but doesn't seem to find anything worthwhile there.
He has a sexual desire (as we see when he scopes out the stewardess), but that goes unfulfilled as well.
Finally, his attention settles on his book, which we're guessing (and it's a good guess) is Moby-Dick.

where men throw harpoons at something
much bigger and probably
better than themselves,

In his book, men are throwing harpoons at whales, and our speaker throws out the idea that the whales may just be better than the men who are hunting them.
Ah, it looks like we've finally arrived in the pages of Moby-Dick, which we've been waiting for since we read the title.
There's a definite sense of a contrast between the life of the men in the book and that of our speaker. We mean, those men are actually doing something, instead of just sitting on a plane, reading about it.

wanting to kill it,
wanting to see great clouds of blood erupt
to prove that they exist.

The reason the men are throwing harpoons at the whale is because they want to kill it, and anyone who's read Moby-Dick knows that's an accurate summary.
They want to see the whale's blood as a sort of proof of their own existence. Well now we're getting dramatic.
That sense of contrast is only getting bigger. The boredom of sitting in the plane is almost a complete opposite of this novel's scene of violence and wildness on the deep blue sea.
There's a weird relationship between life and death being suggested here. It seems as though the death of the whale in some way invigorates or proves the lives of the hunters.
We're not really sure how our speaker feels about the lives of these men. We know that those lives are very different, but we can't tell if our speaker would condemn them or join them.