Every second or two, terrible concussions leaped toward the zenith; and the earth, hidden somewhere down there below the fog-blanket, seemed flaming upward like a huge volcano.

One by one the search-lights, whipping the sky, went black; and now the glow of them was fast diminishing, only to be replaced by a ruddier and more intermittent glare.
"The plant's burning, at last," thought Gabriel.

"Heaven grant the fire may spread to the oxygen-tanks! If we can only get _those_--!" Again he launched a projectile, and again he circled over the doomed plant.
A swift black shape swooped by him.

He had just time to exchange a yell of warning, when it was gone.

The near peril gripped his heart, but did not shake it.
"Close call!" said he.
If that machine and his had met, good-bye forever! But after all, the danger of collision in mid-air, or of being struck by a projectile from some other machine, above, was no greater than his comrades on the ground were facing.