The author has a suspicion that earthquakes may be due to volcanic action, but he offers another explanation, though he does not give it as his own belief. Down in the bowels of the earth there is probably a large number of caverns and empty passages. “At times it may happen that these passages and cavities will be so completely packed with air either by the winds or by the power of the roaring breakers, that the pressure of the blast cannot be confined, and this may be the origin of those great earthquakes that occur in that country.”[35] In this theory there is nothing new or original: the belief that the earth is of a spongy constitution and that earthquakes are caused by air currents is a very old one, which can be followed back through the writings of Alexander Neckam,[36] the Venerable Bede,[37] and others, at least as far as to Isidore.[38] The elder Pliny, who wrote his _Natural History_ in the first century of the Christian era, seems to have held similar views: “I believe there can be no doubt that the winds are the cause of earthquakes.”[39] Footnote 35: _Ibid._ Footnote 36: _De Naturis Rerum_, 158. Footnote 37: _De Natura Rerum_, c. xlix: Migne, _Patrologia Latina_, XC, 275-276. Footnote 38: _De Natura Rerum_, c. xlvi: Migne, _Patrologia Latina_, LXXXIII, 1015. See also _The Christian Topography_ of Cosmas (written about 547), 17-18; Cosmas scoffs at the theory. Footnote 39: _Naturalis Historiae_, I, 201 (ii, c. lxxix). The chapters that deal with the northern lights are interesting because they seem to imply that these lights were not visible in those parts of Norway where the _King’s Mirror_ was written. The editors of the Christiania edition of this work call attention to the fact that there have been periods when these phenomena were less prominent, and suggest that there may have been such a period in the thirteenth century.[40] The author discusses these lights as one of the wonders of Greenland, and the natural inference is that they were not known in Norway. But it is also true that he speaks of whales as if they were limited to the seas about Iceland and Greenland, which is manifestly incorrect. It is likely that the author merely wishes to emphasize the fact that the northern lights appear with greater frequency and in greater brilliance in Greenland than anywhere in Norway. He gives three theories to account for these phenomena: some ascribe them to a girdle of fire which encircles the earth beyond the outer ocean; others hold that the lights are merely rays of the sun which find their way past the edges of the earth while the sun is coursing underneath; but his own belief is that frost and cold have attained to such a power in the Arctic that they are able to put forth light.[41] In his opinion cold is a positive force as much as heat or any other form of energy. To the men of the author’s time there was nothing strange in this belief: it seems to have been