Anaximenes affirms that the circumference of heaven makes the limit
of the earth's revolution. Empedocles, that the heaven is a solid
substance, and hath the form and hardness of crystal, it being composed
of the air compacted by fire, and that in both hemispheres it invests
the elements of air and fire. Aristotle, that it is formed by the fifth
body, and by the mixture of extreme heat and cold.