I first wrote: "An example of the numerous illustrated UFO sightings in the XVIIIth century." But a doubtful visitor of the site made some research and has the final word on this picture. It is in the end not what we would call a UFO nowadays.
The image is an illustration to the "Gulliver's Travels" by Jonathan Swift. My visitor has found this color enlargement of the same picture.
The story has been illustrated many times. Here is another example I found. The imaginary Island of Laputa literally takes off from the seafloor and flies away in the air. The hero waves goodbye at it.