But the projectile was now describing in the shadow that incalculable course which no sight-mark would allow them to ascertain. Had its direction been altered, either by the influence of the lunar attraction, or by the action of some unknown star? Barbicane could not say. But a change had taken place in the relative position of the vehicle; and Barbicane verified it about four in the morning. The change consisted in this, that the base of the projectile had turned towards the moon's surface, and was so held by a perpendicular passing through its axis. The attraction, that is to say the _weight_, had brought about this alteration. The heaviest part of the projectile inclined towards the invisible disc as if it would fall upon it. Was it falling? Were the travellers attaining that much desired end? No. And the observation of a sign-point, quite inexplicable in itself, showed Barbicane that his projectile was not nearing the moon, and that it had shifted by following an almost concentric curve. This point of mark was a luminous brightness, which Nicholl sighted suddenly, on the limit of the horizon formed by the black disc. This point could not be confounded with a star. It was a reddish incandescence which increased by degrees, a decided proof that the projectile was shifting towards it and not falling normally on the surface of the moon. "A volcano! it is a volcano in action!" cried Nicholl; "a disembowelling of the interior fires of the moon! That world is not quite extinguished." "Yes, an eruption," replied Barbicane, who was carefully studying the phenomenon through his night glass. "What should it be, if not a volcano?" "But, then," said Michel Ardan, "in order to maintain that combustion, there must be air. So an atmosphere _does_ surround that part of the moon." "_Perhaps_ so," replied Barbicane, "but not necessarily. The volcano, by the decomposition of certain substances, can provide its own oxygen, and thus throw flames into space. It seems to me that the deflagration, by the intense brilliancy of the substances in combustion, is produced in pure oxygen. We must not be in a hurry to proclaim the existence of a lunar atmosphere."