observations in regard to the tranquillity of the summit of Mount Olympus and the physiological effects of the rarity of the atmosphere.[742] So quiet and untroubled by winds is this peak that letters written there in the dust remain legible for a year. The air is too thin even to support the life of birds, and several philosophers who climbed the mountain would have been unable to remain on top if they had not held to their faces sponges soaked with water and in this way made it possible to breathe by attracting denser air to their nostrils.[743] CLOUDS In this connection a puzzling question seems to have occurred to William of Conches. If the general rule holds that the atmosphere is rarer higher up than on the earth’s surface, how then does it happen that the upper air so often becomes dense in the form of clouds? To this William gave the correct answer,[744] that clouds are not composed of air of greater density than the surrounding parts of the atmosphere, but that water vapor arising from below is turned into clouds by the cold. True as it may be, this idea does not fit in very well with William’s theory of the coldness of the higher altitudes. First he maintains that one of the main reasons why the upper air is cold is because it lacks dampness; then he goes on to explain that dampness rising to a great elevation is converted by the cold into clouds. Though there is no direct contradiction of two statements here, one cannot but sense inconsistency and looseness of thought of a sort that pervades all medieval natural science, though William of Conches on the whole was rather less illogical and less inconsistent than most of his contemporaries. Much the same explanation of the effects of cold on the condensation of water vapor is found in the _Dialogus_ of Peter Alphonsi,[745] where it is shown that the sun draws a damp vapor from the sea and a dry humor from the land. Out of a combination of these, clouds are formed which rise until they reach a height of about sixteen miles. Here, coming in contact with strata of cold air, they are prevented from ascending any higher, and the damp vapor may be precipitated in the form of rain. PRECIPITATION William of Conches also endeavored to explain rainfall.[746] This phenomenon may result, he said, from various causes: either from the conversion into drops of water of dense vapors arising from the earth, from the actual transformation of air into water through the influence of cold, from the tumbling back to earth of some of the water which the sun raises to itself for its own nourishment,[747] or, finally, from water swept up by the winds off the surface of streams, lakes, and swamps. That the last was possible he believed to be demonstrated by the fact that frogs sometimes fall with raindrops![748]