Who wrote the lines of poetry "quoted" by Lincoln at the Soldiers' Home?

[ELCore]

I reflect from time to time on the unbelievable power of the Internet as a source of knowledge.

One thing I think of in this regard is "Mortality", Lincoln's favorite poem. He spent, literally, most of his adult life wishing he knew who had written that poem. He did not find out until he was president, and one of his generals heard Lincoln refer to, or quote from, the poem. Recognizing it, the general (I forget whom) sent Lincoln the book from which the general knew the poem.

The poem is by William Knox. For three decades, Lincoln didn't know that. Nowadays, a few words from the first line would reveal its author in less than a minute.