In our period the existence of water in various forms underneath the surface of the land was well understood. Bernard Sylvester says:[902] “A watery humor is diffused all through the lap of the land and makes streams and rivers, swamps and lakes.” William of Conches attributed[903] the origin of the water in springs and wells to (1) underground streams, or, as he called them, “cataracts,” which pass through wells en route from one part of the earth to another, and (2) the sweat of the earth (_sudor terrae_), or minute particles of water percolating through small holes in the earth much as human sweat percolates through the pores of the body. William maintained that the existence of underground watercourses as a source of well water was proved by the fact that wells near rivers are constantly full and that whatever happens to the water of a well in a given district is likely to happen to the water of all the other wells in the vicinity, showing that there must be some intercommunication between them. That springs and wells were constantly replenished in dry times was proof to William—as to modern geographers—of the existence of the sweat of the earth, or what we now call “ground water,” which permeates the interstices in rocks, gravel, and sand alike. It is possible, however, that William believed that the _sudor terrae_ was actually generated by the earth. This was undoubtedly the opinion of Adelard of Bath, who discusses the subject in terms very similar to those of William.[904] THE SEA AS THE SOURCE OF THE WATERS OF THE LAND Most of this subterranean water, as we have already seen, was supposed during the Middle Ages to come from the sea, whence it made its way inland either through the atmosphere in the form of rain or directly through the land. We need cite but two texts to show how firmly this idea was rooted in the medieval mind. One is from a sermon of Bernard of Clairvaux, the other from a questionnaire prepared by the Emperor Frederick II. It would perhaps be hard to find two men who stood at more diametrically opposite intellectual poles, and yet both, in this case, shared the same conviction.