Analysis of Twenty Years Hence

Twenty years hence my eyes may grow
If not quite dim, yet rather so,
Still yours from others they shall know
Twenty years hence.

Twenty years hence though it may hap
That I be called to take a nap
In a cool cell where thunderclap
Was never heard,

There breathe but o'er my arch of grass
A not too sadly sighed Alas,
And I shall catch, ere you can pass,
That winged word.