But as many of his Discoveries, such as relate particularly to the Laws of the planetary System, are but as so many Confirmations of the Conjectures and Imaginations of Astronomers and Philosophers before him, it perhaps will not be amiss to acquaint you a little with the Astronomy of the Antients concerning the Universe. And before I proceed to those of my own, shew you in the first Place how far their Speculations in the visible Creation have been carried; and with these I shall conclude this preparatory Epistle. The Universe, or mundane Space, by which the Antients comprehend all Creation, has, from time to time, according to the Progress of Science, come under a sort of Necessity of being variously modell'd agreeable to the Opinion of the several Authors, who have judged themselves wise enough to write upon it with a mathematical Foundation: And the cosmical System, by which is meant the Co-ordination of its constituent Parts has undergone almost as many Changes as its Elements are even capable of; every Age of the World, as Knowledge has increased, either from improved Imagination, or repeated Observations, producing something new concerning it. Milton, no doubt, had all this Diversity of Opinions in View, as appears from his supposed Pre-knowledge of _Raphael_, in the following Passage, _Book._ VIII. Hereafter, when they come to model Heaven, And calculate the Stars, how they will weild The mighty Frame! how build, unbuild, contrive To save Appearances, how gird the Sphere With centric and eccentric scribbl'd o'er; Cycle, and Epicycle, Orb in Orb. But the following Synopsis, I believe, will abundantly convince you that from certain Observations only, we ought to form all our Notions of it, if we either hope to arrive at Truth, or expect our Ideas should be supported by Reason. Aristotle was of Opinion, that the Universe, or Heaven, was all one World, and St. Chrysostom, Tertullian, St. Bonaventure, Tycho Brahe, Longomontanus, Kepler, Bulialdus and Tellez, were of an united Opinion, that this one Heaven, or Universe, was all sidereal and fluid. But Aegidius, Hurtadus, Cisalpinus, and Aversa, believing the same Heaven with them to be all one World, and that sidereal, yet on the contrary held it to be solid. Clemens, Acacius, Theodoret, Anastasius, Synaita, Procopius, Suidus, S. Bruno, and Claudianus Mamertus, supposed the universal mundane Space as divided into two Heavens, namely, The Empyræum created the first Day, And the Firmament created the second Day. Two Heavens were also held by Justin Martyr, the one sidereal, and the other aerial. The first supposed by St. Gregory Nyssene, to be that of the fixed Stars, and the last, that of the Planets. But _Mastrius_ and _Bellutus_, though agreeing in the Number of Heavens, calls one the _*Primum Mobile_, and the other, the Starry Heaven.