What was the cause?
Gravity, of course.

The heavier portion of the Projectile gravitated towards the Moon's centre exactly as if they were falling towards her surface.
But _were_ they falling?
Were they at last, contrary to all expectations, about to reach the goal that they had been so ardently wishing for?
No! A sight-point, just discovered by M'Nicholl, very soon convinced Barbican that the Projectile was as far as ever from approaching the Moon, but was moving around it in a curve pretty near concentric.
M'Nicholl's discovery, a luminous gleam flickering on the distant verge of the black disc, at once engrossed the complete attention of our travellers and set them to divining its course.

It could not possibly be confounded with a star.

Its glare was reddish, like that of a distant furnace on a dark night; it kept steadily increasing in size and brightness, thus showing beyond a doubt how the Projectile was moving--in the direction of the luminous point, and _not_ vertically falling towards the Moon's surface.
"It's a volcano!" cried the Captain, in great excitement; "a volcano in full blast! An outlet of the Moon's internal fires! Therefore she can't be a burnt out cinder!" "It certainly looks like a volcano," replied Barbican, carefully investigating this new and puzzling phenomenon with his night-glass.

"If it is not one, in fact, what can it be ?" "To maintain combustion," commenced Ardan syllogistically and sententiously, "air is necessary.