After a further flight of half a mile, this variable sigh changed to a continuous murmur.  There was now before the voyagers a straight course of nearly two miles, at the end of which lay hid the unseen power which gave forth this solemn menace.  The river, perfectly clear of rocks, was a sheet of liquid porphyry, an arrow of dark-red water slightly flecked with foam.  The walls of the canon, scarcely fifty yards apart and more stupendous than ever, rose in precipices without a landing-place or a foothold.  So far as eye could pierce into the twilight of the sublime chasm, there was not a spot where the boat could be arrested in its flight, or where a swimmer could find a shelf of safety.

“It is a rapid,” said Thurstane.  “You did well, Captain Glover, to get another paddle.”

“Lord bless ye!” returned the skipper impatiently, “it’s lucky I was whittlin’ while you was thinkin’.  If we on’y had a boat-hook!”

From moment to moment the murmur came nearer and grew louder.  It was smothered and then redoubled by the reverberations of the canon, so that sometimes it seemed the tigerish snarl of a rapid, and sometimes the leonine roar of a cataract.  A bend of the chasm at last brought the voyagers in sight of the monster, which was frothing and howling to devour them.  It was a terrific spectacle.  It was like Apollyon “straddling quite across the way,” to intercept Christian in the Valley of the Shadow of Death.  From one dizzy rampart to the other, and as far down the echoing cavern as eye could reach, the river was white with an arrowy rapid storming though a labyrinth of rocks.

Sweeny, evidently praying, moved his lips in silence.  Glover’s face had the keen, anxious, watchful look of the sailor affronting shipwreck; and Thurstane’s the set, enduring rigidity of the soldier who is tried to his utmost by cannonade.

CHAPTER XXVI.

The three adventurers were entering the gorge of an impassable rapid.

Here had once been the barrier of a cataract; the waters had ground through it, tumbled it down, and gnawed it to tatters; the scattered bowlders which showed through the foam were the remnants of the Cyclopean feast.

There appeared to be no escape from death.  Any one of those stones would rend the canvas boat from end to end, or double it into a wet rug; and if a swimmer should perchance reach the bank, he would drown there, looking up at precipices; or, if he should find a footing, it would only be to starve.

“There is our chance,” said Thurstane, pointing to a bowlder as large as a house which stood under the northern wall of the canon, about a quarter of a mile above the first yeast of the rapid.

He and Glover each took a paddle.  They had but one object:  it was to get under the lee of the bowlder, and so stop their descent; after that they would see what more could be done.  Danger and safety were alike swift here; it was a hurry as of battle or tempest Almost before they began to hope for success, they were circling in the narrow eddy, very nearly a whirlpool, which wheeled just below the isolated rock.  Even here the utmost caution was necessary, for while the Buchanan was as light as a bubble, it was also as fragile.