You say you're glad I write—oh, say not so!
My fount of song, dear friend, 's a bitter well;
And when the numbers freely from it flow,
'Tis that my heart and eyes o'erflow as well.
Castalia, famed of yore,—the spring divine,
Apollo's smile upon its current wears:
Moore and Anacreon found its waves were wine,
To me it flows a sullen stream of tears.

Frances Anne Kemble

http://www.poemhunter.com/poem/impromptu-i/