VOL. I.] PHILOSOPHICAL TRANSACTIONS. J07 



the moon being at C, and the earth at c: and the Hke at the new moon. But 

 if the moon be in the first quarter at A, and the earth at a; Mars will be seen, 

 not at <y, but at a, too slow; and when the moon is at B, and the earth at b. 

 Mars will be seen at |3 yet too slow; till at the full moon, the moon at C, the 

 earth at c, Mars will be seen at -y, 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 y but at §, too forward, and yet more, when the moon, at the last quarter, is 

 at E, the earth at e, and Mars seen at f. If therefore Mars, when in opposi- 

 tion to the sun, be found (all other allowances being made) somewhat too back- 

 ward 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 earth's 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 earth's epicycle is so small, as not to 

 make any discernible 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 itself, or about IJ- 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 discernible parallax of Mars, even in his nearest 

 position to the earth ; it is very suspicious, that here it may prove so too. An^ 

 whether any of the other planets will be more favourable in this point, I can- 

 not say.* 



Animadversions of Dr. JFallis, upon Mr. HoBBEs's'f late BookX, De 

 Principiis et Ratiocinatione Geometrariim. Written to a Friend. 

 N' 16, p. 289. 



Since I saw you last I have read over Mr. Hobbes's book Contra Geometras, 

 or De Principiis et Ratiocinatione Geometrarum, which you then showed me. 



* Although Dr. Wallis did not strike out the true cause and theory of the tides ; yet tlie numer- 

 ous arguments and reflections here employed are so ingenious in other respects^ as to render the paper 

 on the whole a very important composition. 



+ Thomas Hobbesj the author of this book, was bom at Malmsbury in 1588, and died in 1679, 

 being 91 years of age. He studied at Oxford, and afterwards travelled through Europe several 

 times, as governor to different young noblemen 3 on which occasions, holding a distinguished rank 

 as a philosopher and a general scholar, he cultivated an intimate personal acquaintance with Des- 

 cartes, Mersenne, Gassendi, Galileo, and other eminent philosophers;, with whom, after his 



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