A question about "On Time" by Milton

[breadcatcher101]

I am sure all have heard it, a very fine writing. To put ones thoughts to pen and paper is truly a gift, yet there is one line I have trouble understanding and wonder if some of you can help.

"Whose speed is but the heavy Plummets pace"

What is he referring to with the word "Plummet" ?

Today it means a device to use on the end of a cord, to plumb, if you will. I am not sure if he is referring to this or maybe a variation of the word "plum", like for example "It was plum crazy".

For years I thought it meant either some type of a bird or maybe even the hooves of a horse on a cobblestone path.

Anyway, "On Time" has always been one of my best loved writings. I was pleased to find it on the opening movie of SH4.