The Eighth Ray, also known as the ray of propulsion, is an inherent property of all light regardless of the source from which that light emanates. It is the solar eighth ray that propels the light of the sun to the various planets, and it is the individual eighth ray of each planet that "reflects" or propels that light out into space once more. This ray is constantly streaming out from the planet constituting a force of repulsion of gravity which, when confined, is able to lift enormous weights from the surface of the ground.
Existence of this ray is widely known on Barsoom (where it is known as the eighth Barsoomian ray) and it is mainly used in airships as a medium of buoyancy. Large amounts of the ray are stored in buoyancy tanks, allowing a flier to rise up from the ground, and it was this ray that enabled the Barsoomians to perfect the science of aviation.
During the early years of the discovery of the eighth ray many strange accidents occurred before the Barsoomians learned to fully control this power. In one instance, the first great battleship to be built with eighth ray reservoirs was stored with too great a quantity of the ray and she had sailed up from Helium with five hundred men far into space, never to return. The ship can still be seen today centuries later by the aid of powerful telescopes, hurtling through the heavens ten thousand miles from Barsoom, orbiting the planet as a tiny satellite until the end of time.
Knowledge of the eighth ray eventually spread to Earth where it revolutionized travel on the planet.