The fact that, great as mountains may appear to men, they are in reality but insignificant in comparison with the size of the entire earth, was partially appreciated by the author of the _De imagine mundi_, when he remarked[976] that, if we could look down on the earth from the air above, the entire height of the mountains and depth of the valleys would seem less than the width of the fingers of one who holds a very large ball in his hand. We find occasional speculations regarding altitudes. Peter Comestor, followed by Gervase of Tilbury, asserted[977] that Olympus reaches up into a region of calm, windless air; and William of Conches held[978] that the presence of snow on mountains is due to the rarity of the air at high altitudes. Gervase stated,[979] on the authority of Posthumianus in the _Dialogue_ of Sulpicius Severus[980] (a historian of the fourth century of our era), that Sinai is so lofty that its peak is very near the heavens and that consequently it is impossible to go there.[981] _Miraculous Mountains_ Mountains and hills might have miraculous qualities. Giraldus Cambrensis told[982] of heights in Mona and in the northern part of Britain, beyond the Humber, over the crests of which no shouts could be heard; and Gervase of Tilbury described[983] Mount Cavagum (Canigou) in Catalonia, with a miraculous lake, the dwelling place of devils, on its summit; on one peak there lies perpetual snow and ice in a spot where the sun never shines, and a river of golden sand flows from its base. In the Romance of Alexander, the conqueror was said to have passed near a mountain which made brave men cowards and cowards brave.[984] On another occasion,[985] Alexander and his army became lost in a perilous valley among wild peaks; they could not find their way out unless one man sacrificed himself for the others by remaining in the valley. Alexander himself volunteered to remain, and the army escaped in the midst of fearful tempests; but subsequently Alexander was conducted out by a devil whom he found in the place and to whom he did a good turn. ACCURATE OBSERVATION OF OROGRAPHIC PHENOMENA