At the present time we divide the study of the atmosphere into the sciences of meteorology, devoted to the investigation of individual and local atmospheric phenomena, and climatology, devoted to the investigation of the geographical distribution of weather conditions throughout the world as observed during long periods of time. We may make the same arbitrary division in dealing with the theories current in the twelfth and thirteenth centuries. Meteorology and climatology, however, merge into each other. Some understanding of one is absolutely essential to an understanding of the other, and hence we must take certain meteorological theories into consideration before attempting to deal with the more truly geographic subject of climatology. _METEOROLOGY_ Probably the most complete and satisfactory extant treatment of meteorology from our period is to be found in the writings of William of Conches, whose interest in physics and in the natural sciences led him to study carefully the views of Seneca and also to express at great length opinions of his own about the atmosphere.[730] COMPOSITION OF THE ATMOSPHERE In the first place, William had very definite ideas concerning the composition of the air. The aerial and aqueous spheres, he said, act as intermediaries between the spheres of fire and earth.[731] The qualities of the two latter are opposite; but the atmosphere partakes more or less of the qualities of each, for neither sphere is made up exclusively of one element. William was an atomist: he thought that matter is composed of minute atoms and that each atom is the smallest conceivable particle of one of the four elements.[732] He explained that the atmosphere, which extends up as far as the moon’s orbit, contains in addition to the aerial atoms a certain number of aqueous particles in its lower levels and of fiery atoms higher up. Hence its density and humidity decrease progressively from the earth’s surface upward; the higher air is clear and lucid, the abode of good demons or angels, messengers of God to man, whereas the lower air is full of clouds and constitutes the abode of evil spirits.[733] These parts of the atmosphere formed two out of five concentric regions into which William divided the entire universe.[734] TEMPERATURE