Such being the divisions of the atmosphere, I may observe that in 1 its lowest layer it is most variable, unstable, and changeful. It is near the earth that the air is, so to speak, most enterprising and most long-suffering, as it tosses or is tossed. But withal, it is not all affected in the same way, but at different times at different points its different parts are in unrest and turmoil. The reasons of the changefulness and inconstancy are in part derived from the earth: her position turning hither and thither is a potent factor in determining the quality of the atmosphere. Other reasons are due to the heavenly bodies, chiefly the sun, whose course directs the year, whose solstices determine winter and summer. Next in importance is the moon’s influence. But even the other stars produce an effect alike on 2 the earth and on the air that rests upon the face of the earth. Their rising or their corresponding setting and their disturbances cause now cold, now rain, now other damage such as earth is subject to. It was necessary for me to make these preliminary remarks before going on to speak of thunder and thunderbolts and lightnings. For as these phenomena occur in the atmosphere, I had to explain the nature of the latter, that it might more readily appear what active or passive capacities it possessed. XII There are, I have just said, three phenomena--lightnings, 1 thunderbolts, thunderings: the last is simultaneous in occurrence with the others, but its sound reaches us subsequently. Lightning (_i.e._ sheet) merely reveals fire, the thunderbolt (forked lightning) actually despatches it on its mission. The former is, so to speak, a threatening and feint without a blow, the latter a stroke and a blow. There are some of the facts connected with the phenomena of thunder and lightning on which there is general agreement, others on which there is much diversity of opinion. For example, there is agreement that they 2 occur in the clouds and issue from the clouds; further, it is agreed that lightning of both kinds is either composed of fire or at any rate presents the appearance of fire. But to pass on to the points which are disputed--some authorities believe that the fire is actually resident in the clouds, some that it is merely produced for the occasion, and that it does not exist until it issues out. But yet there is no agreement as to what brings out the fire. One explains it as due to 3 light. Again, a certain author says that the sun’s rays accumulate through recurrent intersection, and kindle the fire. Anaxagoras asserts that it is distilled from the ether, that from such heat in the sky many sparks fall which the clouds enclose and retain for a long time.