He recalled that when he and his friend were coming up the Xingu, far below the last rapids, they observed a dark cloud resting in the western horizon.

There was no thought at that time that it was caused by a burning mountain, but such must have been the fact.

The most singular fact was, that while on his way across the lake to the tunnel, he had failed to notice and remark it.
There was a steady draft in the direction of the flaming cavern.

He had observed it while paddling through the tunnel where it was strong enough to assist in the propulsion of the canoe.

It was caused by the ascent of the vapor through the chimney of the fiery mountain, and averted the intolerable heat that otherwise would have been felt over every portion of the lake.