In treating the water element in his _De naturis rerum_, Alexander Neckam rushes over the problems of the four rivers of Paradise and of why the sea is salt to come to a discussion of springs,[922] about which he relates many marvels, appending to each a little moral lesson. For example, he tells of two founts in Italy, one of which turns the feathers of white birds black, and the other the feathers of black birds white. He suggests the analogy of the former to contemporary worldly knowledge that darkens minds glowing in the brightness of innocence; and of the latter to true wisdom that renders serene minds obscured by the shades of vice.[923] Then he goes on to discourse about springs that rise when some one throws a red cloth into them; a spring that boils up with much noise, as if in annoyance, when men talk near it; a spring that gives flame to an unburnt torch and puts out a lighted one; and a spring whose water, when thrown upon a certain rock in its neighborhood, causes a storm of wind, rain, and hail to arise. These are but a few of many remarkable sources that Neckam describes and places in various parts of the world, drawing on Solinus, Isidore, and the mass of medieval pseudo-science that flourished in all countries. More convincing is Saxo Grammaticus’ circumstantial account of certain water holes in Iceland. In these the water sometimes wells up in abundance and is thrown high into the air in a shower of drops. At other times the flow is quiescent, and the water seems to be sucked into the holes deep in the earth where it scarce may be seen. This description is obviously based on reports Saxo had received from eyewitnesses of the geysers of Iceland.[924] THE FOUNTAIN OF YOUTH The most remarkable and most sought-for of sources has always been the Fountain of Youth. In the first letter of Prester John we find the description[925] of a grove at the foot of Mount Olympus, not far from Paradise, in Central Asia. In this grove there is a spring that wafts forth odors of all kinds, varying from hour to hour, by day and by night. Its waters give eternal youth to any one who bathes therein, restoring him to the bodily strength and vigor that he possessed at the age of thirty-two. Closely parallel to this account, though probably not derived from it, is a description in the Romance of Alexander[926] of a fountain that receives its waters from the Euphrates, one of the four rivers of Paradise, and which four times daily has the power of rejuvenation. Two old men who jumped in emerged looking exactly as if they were thirty years old. Akin to the Fountain of Youth, but less powerful in its action, is a spring described by Gervase[927] in Staffordshire, England, to which he attributed the ability of restoring energy to men when weary. But this is true of any fresh mountain pool. RIVERS