Book 10
1.111
And this happens at certain parts either because a current has forced its way in from without or because the heat has gained possession of certain passages in order to effect this.
1.111
Comets arise either because fire is nourished in certain places at certain intervals in the heavens, if circumstances are favourable; or because at times the heaven has a particular motion above us so that such stars appear; or because the stars themselves are set in motion under certain conditions and come to our neighbourhood and show themselves. And their disappearance is due to the causes which are the opposite of these.
1.112
Certain stars may revolve without setting not only for the reason alleged by some, because this is the part of the world round which, itself unmoved, the rest revolves, but it may also be because a circular eddy of air surrounds this part, which prevents them from travelling out of sight like other stars; or because there is a dearth of necessary fuel farther on, while there is abundance in that part where they are seen to be. Moreover there are several other ways in which this might be brought about, as may be seen by anyone capable of reasoning in accordance with the facts. The wanderings of certain stars, if such wandering is their actual motion, and the regular movement of certain other stars,
