Commissioned and published by Baen Books in 1987, from the book by Bob Shaw.
The first book in the series, the story is about 2 worlds, sharing a common atmosphere, like an hourglass. The only way for these 16th century level explorers, is to fly through a narrow passage of air, using hot air balloons.
The second book "The Wooden Spaceships" is also in this gallery.
18" x 27" oil on masonite.