The younger _Pliny_, if I remember right, somewhere says, that there is, or ought to be, a wide Difference betwixt writing to a Friend, and writing to the Publick: I have indeed pleased myself with the one, but am far from thinking myself qualified for the other; I must therefore rather intreat you, though perhaps you cannot possibly overlook all my Faults as an Author, to excuse them at least in the Friend, and by such kind of unlimited Indulgence, you will give me a much greater Chance to do the Subject some Justice, though I own I despair in this first Attempt, to reconcile every thing I advance to your more cool and impartial Reasoning. But to the Business: As I have no Ambition to have the Substance of my Theory more admired by you than understood, which is too often the Case in Works of this Nature, I must beg leave to repeat to you Part of a former Discourse, which will refresh in your Ideas the principal Laws of the System of our Sun, and make you properly acquainted with such Things as are necessary to be known in the now-established Astronomy of [I]_Copernicus_, &c. before I proceed to any new Matter. [Footnote I: Nicolaus Copernicus, stiled by _Bulialdus_, _Vir absolutæ subtilitatis_, was a Native of _Thorn_ in _Polish Prussia_, and Canon of the Church of _Frawenburgh_; he was Scholar to _Dominicus Maria_ of _Ferrara_, to whom he was Assistant in his astronomical Observations at _Bologne_, and Professor of the Mathematicks at _Rome_, in his noble Work, _De Revolutionibus Orbium Cælestium_; he fortunately revived, happily united, and formed into an Hypothesis of his own, the several Opinions of _Philolaus_, _Heraclides Ponticus_, and _Ecphantus Pythagoreus_, _viz._ after the Opinion of _Philolaus_ he made the Earth to move about the Sun, as the Center of its annual Motion; and according to _Heraclides_ and _Ecphantus_, he likewise gave it a diurnal Rotation round its own Axis: Which System has withstood all Opposition; and as _Ricciolus_, (though a Dissenter from it) observes, _Per damna, per cædes, ab ipso sumit opes, animumque ferro._] The Sun, you are not to learn, is the reputed Center of our _Planetary System_, and may remember, that the Earth on which we live, and these five following _Erratic Stars_, viz. Saturn, Jupiter, Mars, Venus and Mercury, have been demonstrated to move round him in the Order and Manner following.