Bernard O'Donoghue's poem 'What's the Time, Mr Wolf?' in his collection Gunpowder (1995) opens with a snatch of hearsay about the amorphous inorganic material that casts a lustre on our lives:

Glass, someone once told me, is a liquid
Of such density that its sluggish
Downward seep takes centuries to work,
So medieval windows are thicker
At the bottom than the top.

To imagine that old glass flows imperceptibly, much as we imagine time itself flowing, is a beautiful conceit; alas, like most poetic ideas, it isn't true. It resembles the belief, which persisted into the twentieth century, that as pressure increased with depth the sea became more solid: in the deeps of the oceans there were 'floors' on which sunken objects gathered according to their weight. O'Donoghue goes on to reflect - more plausibly - that our flesh is subject to the same gravitational pull as cathedral windows, sagging as the years pass: 'It creeps for the earth...'. The German poet Gottfried Benn said the same thing, more laconically still, in one of his expressionist poems: 'Earth calls'. Our lives aren't time-reversal symmetric; and our bodies not even especially shapely. They perdure a few years, but with ever less buoyancy. And though we can never perceive time with our senses, we notice from the evidence of change around us that we must being living in it. So O'Donoghue's poem is actually a memento mori, an ancient and perfectly respectable theme ...