How at last the sand so gently grinds the glass
-Merrill
He's talking about an hourglass, which over the course of years runs faster as the sand erodes the neck between the two lobes. An apt metaphor for the way time seems to speed up as we grow older. I like the delicate assonance/consonance, and the subtle metronomic effect (in comparison to the more thumping "When I do count the clock that tells the time"). And it reminds me of a passage from Herbert's Church Monuments:
That flesh is but the glass, which holds the dust
That measures all our time; which also shall
Be crumbled into dust.