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sideways in the direction of least resistance. The same thing

happens to the next part, and the next, and so on, till the series

becomes one, that is, till a circle is formed: for if a figure is

described by a single motion that figure must itself be one. This is

how eddies are generated on the earth, and the case is the same in the

clouds as far as the beginning of them goes. Only here (as in the case

of the hurricane which shakes off the cloud without cessation and

becomes a continuous wind) the cloud follows the exhalation

unbroken, and the exhalation, failing to break away from the cloud

because of its density, first moves in a circle for the reason given

and then descends, because clouds are always densest on the side where

the heat escapes. This phenomenon is called a whirlwind when it is

colourless; and it is a sort of undigested hurricane. There is never a

whirlwind when the weather is northerly, nor a hurricane when there is

snow. The reason is that all these phenomena are 'wind', and wind is a

dry and warm evaporation. Now frost and cold prevail over this

principle and quench it at its birth: that they do prevail is clear or

there could be no snow or northerly rain, since these occur when the

cold does prevail.

So the whirlwind originates in the failure of an incipient hurricane

to escape from its cloud: it is due to the resistance which

generates the eddy, and it consists in the spiral which descends to

the earth and drags with it the cloud which it cannot shake off. It

moves things by its wind in the direction in which it is blowing in

a straight line, and whirls round by its circular motion and

forcibly snatches up whatever it meets.

When the cloud burns as it is drawn downwards, that is, when the

exhalation becomes rarer, it is called a fire-wind, for its fire

colours the neighbouring air and inflames it.

When there is a great quantity of exhalation and it is rare and is

squeezed out in the cloud itself we get a thunderbolt. If the

exhalation is exceedingly rare this rareness prevents the

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