G As runs the Glass,
Our Life doth pass.

Glass here seems to mean hour glass.

Well, I'm sure you're right, but I just thought I'd add that glass (as in window glass) DOES run. If you look at old (say, more than 100 years old) window panes (where the glass has not been replaced) you can clearly see the ripples in the glass, and the bottom edge will be significantly thicker than the top edge. I have heard glass referred to as an extremely viscous liquid, rather than a solid.

Also,

X XERXES did die,
And so must I.

Y While youth do chear
Death may be near.

No wonder the puritans were so dour, eh? Learning to read on stuff like this would put a damper on any kid's spirit.