William the Breton wondered at the tides but, like Neckam, refrained from trying to explain their cause and said that God alone understands this and no man can comprehend it either now or ever.[878] He was amazed that such a wide, deep, and powerful stream as the Seine at Rouen could be forced back upon itself by the waters of the sea and made to flow in the opposite direction through a space of land across which its normal current could scarcely pass in three days. Was this due to the fact that fresh water is less powerful than salt? Or does the fresh water find the salt water odious and recoil before it? Or does the stream do reverence, as it were, to its mother, the sea, falling back before her and then when the tide turns following behind her respectfully? None of these explanations was William ready to accept as true. “For us who live our human lot here below, it is sufficient to know the fact; it is not allowed to us to know the cause.”[879] _Giraldus Cambrensis’ Tidal Studies_ The most elaborate tidal studies of our period are in the pages of Giraldus Cambrensis’ _Topographia Hiberniae_, where we find a combination of the astrological theory of Abū Maʿshar, the whirlpool theory of Paul the Deacon, and the ocean-current theory of Macrobius. Giraldus said that when the moon passes the meridian the waters begin to recede from the coasts of Britain and to retire into hidden submarine reservoirs.[880] The moon, being the heavenly body that controls all things humid on the earth, when full causes the tides to rise to unusual heights. A little further on in his discussion, Giraldus explains that at the four opposite parts of the ocean there is a force that violently attracts the sea water, producing a sort of periodic swelling and sinking; this is connected in some manner with a belief in Giraldus’ mind that greater quantities of fresh water enter the sea at the extremities of the earth and in the vicinity of the poles than elsewhere, though on what he based this supposition and how it produced the results which he ascribes to it, he does not explain. Giraldus’ theory also owes much to Macrobius’ hypothesis of the effects of the collision of ocean currents on the tides, as well as to Paul the Deacon’s whirlpool theory, for he explains elsewhere[881] that philosophers mention the existence of four whirlpools at the opposite ends of the earth and that some people attribute to these the causation of tides and storms of wind. Each of the whirlpools resembles a great vortex in the northern seas towards which the waters of the sea rush together, to be absorbed in secret caverns as if in an abyss; ships approaching too near are sucked in and destroyed.