Meteorology

disperse in the infinity of space and are lost. To explain
this we must suppose either that it is not all air from 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
35 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
340^b 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
5 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 in degree
10 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
15 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 quali-
ties 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
20 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 commonly call
fire. It is not really fire, for fire is an excess of heat and
a sort of ebullition; but in reality, of what we call air,
25 the part surrounding the earth is moist and warm, because
it contains both vapour and a dry exhalation from the