Note 504 — CANTO XVII.

A half score steps, etc.: Traversing the stone-built border which lies between the sand and the precipice. Had the brook flowed to the very edge of the Seventh Circle before tumbling down the rocky wall it is clear that they might have kept to the embankment until they were clear beyond the edge of the sand. We are therefore to figure to ourselves the water as plunging down at a point some yards, perhaps the width of the border, short of the true limit of the circle; and this is a touch of local truth, since waterfalls in time always wear out a funnel for themselves by eating back the precipice down which they tumble. It was into this funnel that Virgil flung the cord, and up it that Geryon was seen to ascend, as if by following up the course of the water he would find out who had made the signal. To keep to the narrow causeway where it ran on by the edge of this gulf would seem too full of risk.