On the questions of time and its divisions the author of the _King’s Mirror_ seems to have had nearly all the information that the age possessed. He divides the period of day and night into two “days” (_dægr_) of twelve hours each. Each hour is again divided into smaller hours called _ostenta_ in Latin.[28] Any division below the minute he apparently does not know. The length of the year he fixes at 365 days and six hours, every fourth year these additional hours make twenty-four and we have leap year.[29] The waxing and waning of the moon and the tidal changes in the ocean are also reckoned with fair accuracy.[30] Footnote 28: C. vi. Footnote 29: _Ibid._ Footnote 30: _Ibid._ Medieval scientists found these movements in the ocean a great mystery. Some ascribed the tides to the influence of the moon;[31] others believed that they were caused by the collision of the waters of two arms of the ocean, an eastern arm and a western; still others imagined that somewhere there were “certain cavern-like abysses, which now swallow up the water, and now spew it forth again.”[32] The author of the _Speculum_ has no doubts on the subject: he believes that the tides are due to the waxing and waning of the moon.[33] Footnote 31: The Venerable Bede held that the moon is in some way responsible for the tides. _De Natura Rerum_, c. xxxix: Migne, _Patrologia Latina_, XC, 258-259; see also _ibid._, XC, 422-426 (_De Tempore Ratione_, c. xxix). Footnote 32: Alexander Neckam, _De Naturis Rerum_, 138. Footnote 33: C. vi. In his discussion of the volcanic fires of Iceland he shows that on this subject he was completely under the influence of medieval conceptions. He has heard that Gregory the Great believed that the volcanic eruptions in Sicily have their origins in the infernal regions. Our author is inclined to question, however, that there is anything supernatural about the eruptions of Mount Etna; but he is quite sure that the volcanic fires of Iceland rise from the places of pain. The fires of Sicily are living fires, inasmuch as they devour living materials, such as wood and earth; those of Iceland, on the other hand, consume nothing living but only dead matter like rock. And he therefore concludes that these fires must have their origin in the realms of death.[34] Footnote 34: C. xiii.