Meteorology   

Some say that what is called air, when it is in motion and flows, is

wind, and that this same air when it condenses again becomes cloud and

water, implying that the nature of wind and water is the same. So they

define wind as a motion of the air. Hence some, wishing to say a

clever thing, assert that all the winds are one wind, because the

air that moves is in fact all of it one and the same; they maintain

that the winds appear to differ owing to the region from which the air

may happen to flow on each occasion, but really do not differ at

all. This is just like thinking that all rivers are one and the same

river, and the ordinary unscientific view is better than a

scientific theory like this. If all rivers flow from one source, and

the same is true in the case of the winds, there might be some truth

in this theory; but if it is no more true in the one case than in

the other, this ingenious idea is plainly false. What requires

investigation is this: the nature of wind and how it originates, its

efficient cause and whence they derive their source; whether one ought

to think of the wind as issuing from a sort of vessel and flowing

until the vessel is empty, as if let out of a wineskin, or, as

painters represent the winds, as drawing their source from themselves.

We find analogous views about the origin of rivers. It is thought

that the water is raised by the sun and descends in rain and gathers

below the earth and so flows from a great reservoir, all the rivers

from one, or each from a different one. No water at all is

generated, but the volume of the rivers consists of the water that

is gathered into such reservoirs in winter. Hence rivers are always

fuller in winter than in summer, and some are perennial, others not.

Rivers are perennial where the reservoir is large and so enough

water has collected in it to last out and not be used up before the

winter rain returns. Where the reservoirs are smaller there is less

water in the rivers, and they are dried up and their vessel empty

before the fresh rain comes on.

But if any one will picture to himself a reservoir adequate to the