If we discard fanciful ideas like that of Richard, prior of St. Victor in Paris (died 1173), to the effect that the tides are produced by the breathing of some great submarine monster or spirit,[861] we find two distinct groups of tidal theories prevalent in the twelfth century: as Duhem defines them, the physical and the astrological. The astrological theories, which explained the tides by the influence of the moon, had been expounded before the period we are studying by Posidonius, Pliny, Bede, and the Moslem Abū Maʿshar. The physical theories had been set forth by Macrobius, who had believed that the tides were due to the impact of ocean currents, and Paul the Deacon, who had attributed them to the action of great whirlpools. Although twelfth-century students added little to these earlier opinions, they made some remarkable combinations of them, and their observations were distinguished by a few close records of actual tidal phenomena.[862] _Lunar Causation_ Bernard Sylvester explained the tides by lunar causation alone[863] and attributed to the moon the power of attracting and repelling not only the waters but also terrestrial substances,[864] inasmuch as the moon is the nearest planet to the earth, the largest, and consequently the most powerful.[865] In the following century we find that Robert Grosseteste saw in the effects of lunar rays upon the bottom of the sea sufficient cause for the ebb and flood. If in their broad outlines the ideas of the bishop of Lincoln are plain enough, the individual steps of his argument are neither clear nor coherent. They are of sufficient interest, nevertheless, to justify an attempt at interpreting them.[866] We have already alluded to Robert’s theory of rays emanating from the celestial bodies in the shape of cones or “pyramids” and to his principle that the power of these rays is inverse to the obliquity of the angle at which they meet the earth’s surface and to the length of the pyramids.[867] When the moon is rising, Grosseteste explains in the _De natura locorum_,[868] the rays are very oblique and the pyramids long: hence the power of the rays is much too weak to disperse vapors that have accumulated on the sea floor or to draw these vapors up into the air. The result is that the vapors tend to displace the waters in the depths, to rise in bubbles to the surface, and thus to produce flood tides. As the moon approaches the meridian the rays become less oblique, the pyramids shorter, and the lunar power consequently greater. The moon now disperses and consumes the vapors and draws them up into the air from the depths of the sea. By the time our satellite reaches the meridian, the vapors are entirely consumed, “and, since the cause ceases, the effect also ceases; and the waters of the sea naturally flow back into their proper place in order not to create a vacuum.” Hence the ebb begins.