Meteorology   

neither appreciably near to the heat of the stars, nor to the rays

relected from the earth. It is these that dissolve any formation by

their heat and so prevent clouds from forming near the earth. For

clouds gather at the point where the reflected rays disperse in the

infinity of space and are lost. To explain this we must suppose either

that it is not all air which water is generated, or, if it is produced

from all air alike, that what immediately surrounds the earth is not

mere air, but a sort of vapour, and that its vaporous nature is the

reason why it condenses back to water again. But if the whole of

that vast region is vapour, the amount of air and of water will be

disproportionately great. For the spaces left by the heavenly bodies

must be filled by some element. This cannot be fire, for then all

the rest would have been dried up. Consequently, what fills it must be

air and the water that surrounds the whole earth-vapour being water

dissolved.

After this exposition of the difficulties involved, let us go on

to lay down the truth, with a view at once to what follows and to what

has already been said. The upper region as far as the moon we affirm

to consist of a body distinct both from fire and from air, but varying

degree of purity and in kind, especially towards its limit on the side

of the air, and of the world surrounding the earth. Now the circular

motion of the first element and of the bodies it contains dissolves,

and inflames by its motion, whatever part of the lower world is

nearest to it, and so generates heat. From another point of view we

may look at the motion as follows. The body that lies below the

circular motion of the heavens is, in a sort, matter, and is

potentially hot, cold, dry, moist, and possessed of whatever other

qualities are derived from these. But it actually acquires or

retains one of these in virtue of motion or rest, the cause and

principle of which has already been explained. So at the centre and

round it we get earth and water, the heaviest and coldest elements, by

themselves; round them and contiguous with them, air and what we