_Father._ Those things that you have now asked about do not all wax or wane with equal rapidity; for the tide, when it rises, completes its course in seven days plus half an hour of the eighth day; and every seventh day there is flood tide in place of ebb. For the tide rises one seventh part daily from the time when the rise begins; and after it turns and begins to fall, it ebbs in the same way during the next seven days but is retarded as much as half an hour of the eighth day,[167] which must be added to the seven days. As to how long an hour should be I can give you definite information; for there should be twenty-four hours in two days, that is, a night and a day, while the sun courses through the eight chief points of the sky: and according to right reckoning the sun will pass through each division in three hours of the day. On the other hand, the moon, while it waxes, completes its course in fifteen days less six hours;[168] and in a like period it wanes until the course is complete and another comes. And it is always true that at this time the flood tide is highest and the ebb strongest. But when the moon has waxed to half, the flood tide is lowest and the ebb, too, is quite low. At full moon the flood tide is again very high and the ebb is strong. But when it has waned to half, both ebb and flood are quite low. Merchants are, however, scarcely able to note these changes, as the course is too swift; for the moon takes such long strides both in waxing and waning that men, on that account, find it difficult to determine the divisions of its course. The sun, on the other hand, completes its course more slowly both in ascending and declining, so that one may easily mark all the stages of its course. The sun moves upward one hundred and eighty-two and one-half days and three hours and for a like period it recedes again; it has then completed its entire course, both ascent and decline, in three hundred days, by the twelve-count[169] {360}, plus five days and six hours. Every fourth year this becomes three hundred by the twelve-count and six days more {366}; this is called leap year, for it has one day more than the preceding twelvemonth, the additional hours being gathered into twenty-four, a night and a day. In Latin all hundreds are counted by tens, and there are, therefore, properly computed three hundred by the ten-count plus sixty-six days whenever leap year occurs, while the intervening years have only five days and six hours with as many additional days by the other reckoning as I have just stated. Footnote 167: The mean retardation is forty-eight minutes. Footnote 168: This is within twenty-two minutes of the length of the lunar half-month. Footnote 169: The Northmen in medieval times had two hundreds, the great hundred, or duodecimal hundred, which counted 120 (12 × 10) and the ordinary hundred (10 × 10).