Meteorology   

We see that motion is able to dissolve and inflame the air;

indeed, moving bodies are often actually found to melt. Now the

sun's motion alone is sufficient to account for the origin of

terrestrial warmth and heat. For a motion that is to have this

effect must be rapid and near, and that of the stars is rapid but

distant, while that of the moon is near but slow, whereas the sun's

motion combines both conditions in a sufficient degree. That most heat

should be generated where the sun is present is easy to understand

if we consider the analogy of terrestrial phenomena, for here, too, it

is the air that is nearest to a thing in rapid motion which is

heated most. This is just what we should expect, as it is the

nearest air that is most dissolved by the motion of a solid body.

This then is one reason why heat reaches our world. Another is

that the fire surrounding the air is often scattered by the motion

of the heavens and driven downwards in spite of itself.

Shooting-stars further suffix to prove that the celestial sphere

is not hot or fiery: for they do not occur in that upper region but

below: yet the more and the faster a thing moves, the more apt it is

to take fire. Besides, the sun, which most of all the stars is

considered to be hot, is really white and not fiery in colour.



4



Having determined these principles let us explain the cause of the

appearance in the sky of burning flames and of shooting-stars, and

of 'torches', and 'goats', as some people call them. All these

phenomena are one and the same thing, and are due to the same cause,

the difference between them being one of degree.

The explanation of these and many other phenomena is this. When

the sun warms the earth the evaporation which takes place is

necessarily of two kinds, not of one only as some think. One kind is

rather of the nature of vapour, the other of the nature of a windy