Laudator Temporis Acti

The Journal of Sir Walter Scott, ed. W.E.K. Anderson (Oxford: Clarendon Press, 1972), p. 354 (September 24, 1827):

Some things of the black dog still hanging about me but I will shake him off. I generally affect good spirits in company of my family whether I am enjoying them or not. It is too severe to sadden the harmless mirth of others by suffering your own causeless melancholy to be seen. And this species of exertion is like virtue its own reward for the good spirits which are at first simulated become at length real.

Id., p. 445 (March 18, 1828):

I was sorely worried by the black dog this morning, that vile palpitation of the heart—that tremor cordis—that histerical passion which forces unbidden sighs and tears and falls upon a contented life like a drop of ink on white paper which is not the less a stain because it conveys no meaning.

Hester Lynch Piozzi, Diary (October 19, 1790):

The Black Dog is upon his Back; was a common saying some Years ago when a Man was seen troubled with Melancholy.