Meteorology   

exhalation. That which rises from the moisture contained in the

earth and on its surface is vapour, while that rising from the earth

itself, which is dry, is like smoke. Of these the windy exhalation,

being warm, rises above the moister vapour, which is heavy and sinks

below the other. Hence the world surrounding the earth is ordered as

follows. First below the circular motion comes the warm and dry

element, which we call fire, for there is no word fully adequate to

every state of the fumid evaporation: but we must use this terminology

since this element is the most inflammable of all bodies. Below this

comes air. We must think of what we just called fire as being spread

round the terrestrial sphere on the outside like a kind of fuel, so

that a little motion often makes it burst into flame just as smoke

does: for flame is the ebullition of a dry exhalation. So whenever the

circular motion stirs this stuff up in any way, it catches fire at the

point at which it is most inflammable. The result differs according to

the disposition and quantity of the combustible material. If this is

broad and long, we often see a flame burning as in a field of stubble:

if it burns lengthwise only, we see what are called 'torches' and

'goats' and shooting-stars. Now when the inflammable material is

longer than it is broad sometimes it seems to throw off sparks as it

burns. (This happens because matter catches fire at the sides in small

portions but continuously with the main body.) Then it is called a

'goat'. When this does not happen it is a 'torch'. But if the whole

length of the exhalation is scattered in small parts and in many

directions and in breadth and depth alike, we get what are called

shooting-stars.

The cause of these shooting-stars is sometimes the motion which

ignites the exhalation. At other times the air is condensed by cold

and squeezes out and ejects the hot element; making their motion

look more like that of a thing thrown than like a running fire. For

the question might be raised whether the 'shooting' of a 'star' is the

same thing as when you put an exhalation below a lamp and it lights