I begin also with thanks to Christoffer Wilhelm Eckersberg (1783-1853), whose painting appears above. The painting is titled (according to one source, at least) A View through Three of the Northwestern Arches of the Third Story of the Colosseum. Eckersberg painted it in 1815 or 1816.
And I thank John Nash as well for Canal Bridge, Sydney Gardens, Bath (circa 1927).
Do the paintings somehow relate to the phrase "First Known When Lost?" Not with any intention on my part. Things are pretty simple-minded around here.