Grosseteste does not make clear what generates the vapors, though he probably meant us to assume that they were produced by the heat due to the reflection of the moon’s rays upon the sea floor. In another treatise, the _De impressionibus elementorum_,[869] he explains how reflected rays, though not necessarily the rays of the moon, in passing through a transparent body of water may create heat at the bottom. The problem of the flood tide when the moon is in the opposite hemisphere of the heavens still remained. Grosseteste’s obscure explanation of this runs about as follows: “Many try to give a reason for this difficult circumstance on the grounds that opposite quarters of the universe are of the same composition (_commixtionis_) and consequently produce the same effects. But this explanation falls short, since it is false to assert that there are any actual replicas of the stars of one quarter of the heavens in another quarter, inasmuch as the earth interposes its bulk between a planet in one quarter and the quarter opposite. Moreover, even were this explanation true, an explanation of the original cause would be required. That is to say, it would be necessary to ask why the opposite quarters are of the same composition and consequently exert the same effect. The fact is that the reflection of rays solves this problem, since the rays of the moon are multiplied on the stellar heaven. Because the stellar heaven is an opaque body, we are consequently not able to see it, though it nevertheless is very luminous according to Alpetragius and Messalahe [_sic_]. Other reflected rays fall on the opposite quarter at right angles.”[870] _Terrestrial Causation_ Most writers found that the astrological, or lunar, theory alone was insufficient to explain all the peculiarities of the tides and made appeal, as well, to physical theories—in particular to that of Macrobius. This is given in varying terms by Adelard of Bath, Lambert of St. Omer, William of Conches, and Giraldus Cambrensis. Macrobius, as we have already observed, had conceived of four ocean currents issuing out of the great equatorial ocean and flowing north and south in the girdling ocean which includes the poles.[871] These currents run together somewhere in the polar regions; the waters rebound on themselves (_ex repercussione ingurgitur retro mare_) and in this way cause the ebb and flow. Lambert of St. Omer in his _Liber floridus_ seems to have accepted the Macrobian theory much as it stands,[872] but Adelard harbored doubts as to the sufficiency of the impact of the waters against each other to produce a tidal rebound and thought that some mountain or other mass of land must interpose to produce such an effect.[873]