An Unfinished View of the West Gate, Canterbury; Paul Sandby
Link is to zoomable version on the Google Art Project; high-resolution downloadable file on Wikimedia Commons, original is in the Yale Center for British Art.
Here Paul Sandby, primarily known as a watercolorist, has started a landscape sketch in oil on paper. He’s laid in his drawing in pen and brown ink, and has started his application of oil.
I have no idea why the sketch — which seems to have been coming along nicely — was abandoned. It actually works remarkably well, even in its unfinished state, and has wonderful qualities of both painting and drawing.
I love coming across unfinished works like this; it’s like getting a tutorial from the 18th century.