Watercolour over Pencil, with scratching out.
Cooke visited Oxford in the 1830's and painted several of the local mills. At this time he was working mostly in watercolours. Cooke was a wonderful watercolourist and draughtsman, rarely using bodycolour.
Here he has used scratching out for added effect.
Cooke stopped painting watercolours in the late 1830's to work in oils for most of the rest of his life, except for a short period in the 1860's.