The canoe was swept over this singular place by the current, its keel and sides grinding and bumping on the stones, and sliding on the soft moss which here made the rock so slippery and black. The progress was aided by sundry pushes and jerks at proper times, but we advanced altogether in a clumsy, helpless style, until at length there came in sight the great white ridge of tossing foam where the din was great, and a sense of excitement and confusion filled the mind. I was quite conscious that the sight before me was made to look worse because of the noise around, and by the feeling of the loneliness and powerlessness of a puny man struggling in a waste of breakers, where to strike a single one was sure to upset the boat. From the nature of the place, too, it was evident that it would be difficult to save the canoe by swimming alongside it when capsized or foundered, and yet it was utterly impossible now to stop. Right in front, and in the middle, I saw the well-known wave which is always raised when a main stream converges, as it rushes down a narrow neck. The depression or trough of this was about two feet below, and the crest four feet above the level, so the height of the wave was about six feet. Though rather tall it was very thin and sharp-featured, and always stationary in position, though the water composing it was going at a tremendous pace. After this wave there was another smaller one, as frequently happens. It was not the _height_ of the wave that gave any concern; had it been at sea the boat would rise over any lofty billow, but here the wave stood still, and the canoe was to be impelled against it with all the force of a mighty stream, and so it _must_ go through the body of water, for it could not have time to rise. And so the question remained, "What is _behind_ that wave?" for if it is a rock then this is the last hour of the Rob Roy.[XXI.] [XXI.] I had not then acquired the knowledge of a valuable fact, that a sharp wave of this kind _never_ has a rock behind it. A sharp wave requires free water at its rear, and it is therefore in the safest part of the river so far as concealed dangers are concerned. This at least was the conclusion come to after frequent observation afterwards of many such places. The boat plunged headlong into the shining mound of water as I clenched my teeth and clutched my paddle. We saw her sharp prow deeply buried, and then (I confess) my eyes were shut involuntarily, and before she could rise the mass of solid water struck me with a heavy blow full in the breast, closing round my neck as if cold hands gripped me, and quite taking away my breath.[XXII.] [XXII.] See a faithful representation of this incident, so far as relates to the water, in the Frontispiece.