We have found that the constant movement and change manifested in the circulation of the waters of the globe impressed the mind of Thales and largely determined the course of his speculation. When his great successor, Heracleitus, passed from water to fire, in his search for the _Welt-stoff_, he by no means became insensible to the mystic appeal of running water. "All things are flowing." Such was the ancient expression of the universal flux; and it is plainly based on the analogy of a stream. If Heracleitus was not its author, at any rate it became his favourite simile. "We cannot step" (he said) "into the same river twice, for fresh and ever fresh waters are constantly pouring into it." And yet, in a sense, though the waters change, the river remains. Hence the statement assumed a form more paradoxical and mystical--"We step into the same river, and we do not step into it; we are, and we are not." Moving water, then, has the power of stimulating emotion and prompting intuition; and this power is manifested in exceptional degree when the source from which the water issues, and the goal to which it flows, are unknown. These conditions are best satisfied in the case of streams that flow in volume through subterranean caverns. The darkness contributes its element of undefined dread, and the hollow rumblings make the darkness to be felt. What more calculated to fill the mind of the child of nature with a sense of life and will behind the phenomena? The weird reverberations are interpreted by him as significant utterances of mighty, unseen powers, and the caves and chasms are invested with the awe due to entrances into the gloomy regions where reign the monarchs of the dead. True, it may be said, for the child of nature. But are such experiences possible for the modern mind? Yes, if we can pierce through the varied disguises which the intuitional material assumes as times and manners change. Coleridge, for instance, is thrown into a deep sleep by an anodyne. His imagination takes wings to itself; images rise up before him, and, without conscious effort, find verbal equivalents. The enduring substance of the vision is embodied in the fragment, "Kubla Khan," the glamour of which depends chiefly on the mystical appeal of subterranean waters. We are transported to where "Alph, the sacred river, ran Through caverns measureless to man, Down to a sunless sea." These three lines make a deeper impression than any others in the poem, and form its main theme. Nor is the feeling of the supernatural unrecognised. Spirits are near with prophetic promptings. From a deep chasm the sacred river throws up a mighty fountain, and for a short space wanders through wood and dale, only to plunge again into its measureless caverns, and sink in tumult to a lifeless ocean: "And mid this tumult Kubla heard from far Ancestral voices prophesying war."