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moon, the Moon at C, the Earth at c, Mars will he seen at γ, its true place, as if the Earth were at T. But then, after the Full, the Moon at D, the Earth at d; Mars will be seen, not at γ, but at δ; too forward: and yet more, when the Moon (at the last Quarter) is at E, the Earth at e, and Mars seen at ς. If therefore Mars (when in opposition to the Sun) be found (all other allowances being made) somewhat too backward before the Full moon, and somewhat too forward after the Full-moon, (and most of all, at the Quadratures:) it will be the best confirmation of the Hypothesis. (The like may be fitted to Mars in other positions, mutatis mutandis; and so for the other Planets.)
But this proof, is of like nature as that of the Parallax is of the Earths Annual Orb to prove the Copernican Hypothesis. If it can be observed, it proves the Affirmative; but if it cannot be observed, it doth not convince the Negative, but only proves that the Semidiameter of the Earths Epicycle is so small as not to make any discernable Parallax. And indeed, I doubt, that will be the issue. For the Semidiameter of this Epicycle, being little more than the Semidiameter of the Earth it self, or about 123 thereof (as is conjectured, in the Hypothesis, from the Magnitudes and Distances of the Earth and Moon compared) and there having not as yet been observed any discernable Parallax of Mars, even in his neerest position to the Earth; it is very suspicious, that here it may prove so too. And whether any of the other Planets will be more favourable in this point, I cannot say.
ANIMADVERSIONS
Of Dr. Wallis, upon Mr. Hobs's late Book, De Principiis & Ratiocinatione Geometrarum.
These were communicated by way of Letter, written in Oxford, July 24. 1666. to an Acquaintance of the Author, as follows:
SInce I saw you last, I have read over Mr. Hob's Book Contra Geometras (or De Principiis & Ratiocinatione Geometrarum) which you then shewed me. A New Book of Old master: Containing but a Repetition of what he had before told us, more than once; and which hath been Answered long agoe.
In which, though there be Faults enough to offer ample mat-
