"Beneath this colossal fortress, fires glared and furnaces smoked in the darkness of early dawn, and from many miles away Ruta Skadi heard the clang of hammers and the pounding of great mills. And from every direction, she could see more flights of angels winging towards it, and not only angels, but machines too: steel-winged craft gliding like albatrosses, glass cabins under flickering dragonfly-wings, droning zeppelins like huge bumble bees — all making for the fortress that Lord Asriel was building on the mountains at the edge of the world."

from The Subtle Knife (chapter 6: Lighted Fliers) by Philip Pullman