(106)
He refers to his Cometography these Disquisitions: whether all Comets (in their innate Motion) move equal Spaces in equal Times? which is the swiftest, and which the slowest Motion they are capable of? what the cause of this acceleration and retardation of their true Motion?
He puts it out of doubt, that they are in the Sky it self, producing Reasons for it that are very considerable, and alledging among others, That the Parallaxes doe clearly evince it, which he finds far less in Comets, then in the Moon, yea then sometimes in the Sun it self. Where he also represents, That he hath deduced the Horizontal Parallax of this very Comet from one onely Observation, made Febr 4. st.n. by which he found, That then it was distant from the Earth 5000 Semidiameters of the same, or 4300000 German miles. From this its distant from the earth, he deduces, That on that Day when it was so remote from the Earth, its true Diameter was 2560 German miles, which is three times bigger then the Diameter of the Earth, and almost fix times bigger then that of the Moon, whose Diameter, according to his Theory, is 442 German miles.
He finds the Matter of Comets to be in the Æther it self, making the Æther and the Air to differ onely in purity, and esteeming, That the Planets do emit their Exhalations, and have their Atmospheres like unto our Earth. Where he affirms, That the Sun alone may cast out so much Matter at any time in one Year, as that thence shall be produced not one or two Comets, equalling the Moon in Diameter, but very many; which if so, what contribution may not be expected from the other Planets?
Of this Cometical Matter, he thinks, That first it is by little and little gathered together, then coagulated and condensed, and thereby reduced to a less Diameter; but then, after a while, it resolves again, and grows dilute and pale, and at last is dissipated. And accordingly he affirms, That he hath observed the Head of this Comet at first more confused, thin and pale, afterwards clearer and clearer.
