A foreign writer has asked for assistance with locating the original
work containing a Twain quote.  He writes:

"I am currently faced with a problem in a translation I have to
make from German into English - which involves finding the original words
to something a German author quoted [from Twain]

What he said approximately was:

And as soon as they had lost their goal, they doubled their efforts.
(German: Und als sie ihr Ziel aus den Augen verloren, verdoppelten sie
ihre Anstrengungen)"

Any ideas on the original source of such a passage?