semper enim circumpositus res verberat aer; sed tali fit uti propellat tempore ferrum,  parte quod ex una spatium vacat et capit in se.
For the air all round is for ever buffeting things; but the reason why it propels the iron at that moment is that on one side there is empty space which receives it.
final_alignments\Lucretius_De_Rerum_Natura_Book6.json