I saw this portrait in the Huntington Library in San Marino (near Pasedena) California, USA. This man was the friend of a kite flyer, a pottery maker, and Erasmus Darwin. One of Ms. Sandwalk's direct ancestors worked for him.
He's very famous. You'll recognize the name instantly even if you don't recognize his face. Who is he?
I'll keep the comments invisible for 24 hours so you can post your guesses.