On The Heavens   

dusty materials which throng the air. With regard to these

questions, it is wrong to accept the explanation offered by

Democritus. He says that the warm bodies moving up out of the water

hold up heavy bodies which are broad, while the narrow ones fall

through, because the bodies which offer this resistance are not

numerous. But this would be even more likely to happen in air-an

objection which he himself raises. His reply to the objection is

feeble. In the air, he says, the 'drive' (meaning by drive the

movement of the upward moving bodies) is not uniform in direction. But

since some continua are easily divided and others less easily, and

things which produce division differ similarly in the case with

which they produce it, the explanation must be found in this fact.

It is the easily bounded, in proportion as it is easily bounded, which

is easily divided; and air is more so than water, water than earth.

Further, the smaller the quantity in each kind, the more easily it

is divided and disrupted. Thus the reason why broad things keep

their place is because they cover so wide a surface and the greater

quantity is less easily disrupted. Bodies of the opposite shape sink

down because they occupy so little of the surface, which is

therefore easily parted. And these considerations apply with far

greater force to air, since it is so much more easily divided than

water. But since there are two factors, the force responsible for

the downward motion of the heavy body and the disruption-resisting

force of the continuous surface, there must be some ratio between

the two. For in proportion as the force applied by the heavy thing

towards disruption and division exceeds that which resides in the

continuum, the quicker will it force its way down; only if the force

of the heavy thing is the weaker, will it ride upon the surface.

We have now finished our examination of the heavy and the light

and of the phenomena connected with them.


THE END