William of Conches cites two theories of tidal controls:[874] the first is that of Macrobius; the second, confusingly stated, suggests Adelard’s hypothesis of an interposing mass of land. William says, in effect, that the tides are due in part to the existence of mountains submerged beneath the sea, against which the waters are attracted forward and then repelled, producing an oscillating motion. As to this, we may well be led to inquire how Macrobius, Adelard, and William explained this oscillating motion, for certainly two steadily flowing currents meeting each other or running against submerged reefs would not create any such motion. Unfortunately in this we are left unsatisfied by our medieval writers, who characteristically here, as often elsewhere, were content, when stating that one phenomenon causes another, to leave entirely to the imagination the explanation of the manner in which such causation is actually effected. William did not rule out all lunar control over the ebb and flood but explained the spring and neap tides by variations in the moon’s power of heating and drying the atmosphere. This power, he thought, is at a minimum both when the moon is full and when it is new. Consequently we have high spring tides at these times, and vice versa. William’s theory is the reverse of Abū Maʿshar’s:[875] that the tides are caused by the active attraction by the moon of the humid elements on the earth’s surface. William fails to show us why the tides should be in flood when the moon is rising toward the meridian and why spring tides should occur when there is a full moon. Abū Maʿshar, on the other hand, fails to explain why there is a flood tide when the moon is on the other side of the earth, in the opposite celestial hemisphere. Alexander Neckam gives what is, to say the least, an unsatisfactory treatment of the tides.[876] After quoting the scientific opinions of others, he remarks that to explain the ebb and flow of the waters is a problem that cannot be solved. Then, in his customary vein, he adds the moral observation that the tides are like the persecutions of the Christians and that they should not fill one with too much despair, for after they have risen they always subside again in the due course of time.[877]