In this manner, in about an hour's time, they passed safely over the central stream, and began descending the bank on the other side, passing without difficulty two or three of the narrower channels. But their progress through the tangled underwood, which in some places had to be cut with the axe before it would yield a passage, was necessarily slow, and it was past noon before they came to the edge of the last and broadest of the tributary channels--a stream too wide and deep to be forded, even if there had not been fear that the overhanging banks contained holes in which crocodiles might lurk. "We must fell a tree," said the missionary. "We shan't get across in any other way. One of the longest of these pines will answer our purpose, if it is dropped in the right place; but we must go to work without delay, for I fear before nightfall there will be rain. It seldom gives long notice of its coming in this country, and when it does fall, it falls in a perfect deluge. It is lucky we have the axe, or we must have gone back to the other bank again. Hand it to me, Ernest. I think I can contrive to drop this fir exactly into the fork of that large projecting yellow-wood there." He took the axe as he spoke, and went to work with a will, the others relieving him at intervals, and labouring under his directions. But the edge of the instrument had unfortunately become blunt from use, and made its way but slowly into the tough wood. It was nearly three hours before the task was accomplished, and the long trunk dropped skilfully into the hollow of the tree opposite. "Now then, we must not lose a minute," said De Walden. "We are fortunate that the rain has held off so long, but it must come soon." He mounted the trunk as he spoke and crawled along it, observing the same precautions as before. They had just reached the further end, when suddenly there came--from a considerable distance it seemed--a dull hollow roar, accompanied by a rush of chilling wind.