PHYSICAL ANI) MATHEMATICAL SCIENCES. 173

duke. These dialogues consist chiefly of illustrationsof the author’s various previous discoveries, and adevelopement of the arguments deducible from them;together with a full discussion of the several objectionsusually brought against the Copernican system, and thereasons by which it is supported. The dispute is car-ried on with infinite spirit between a Copernican, anAristotelian, and a man of wit and acuteness, who ishalf a convert, and whose province it is to draw outthe arguments of the other two. The tendency infavour of the Copernican system is obvious, though allthe arguments on the other side are stated in the mostforcible manner of which they are capable.

The difficulties as to the solar system derived fromthe Aristotelian notion of the perfection of the celestialbodies are cleverly combated. The analogy of thesystem of Jupiter and his satellites is forcibly urged.The phases of those planets which are so situated as toexhibit them, and the transits of the inferior planetsover the sun, appearing as dark spots, are alleged asstriking proofs that they shine hy reflected light only.In particular, the author comments largely on the vari-able stars of 1604 and 1572, and on the spots on thesun, as entirely at variance with the unchangeable natureof the celestial bodies.

The physical objections which had been urged by thePtolemaists against the earth’s motion were, as we havealready seen, chiefly grounded on certain alleged dif-ficulties which it was asserted must follow from it: thata stone, dropped from a height, would fall far to thewestward, from the rapidity with which the earth wouldmove away from under it; birds in flight, in like man-ner, would be left behind; and even the atmosphere, andindeed all detached bodies, would he carried away. Tothese objections, the answers given before the time ofGalileo were all grounded, more or less, on the samekind of metaphysical distinctions, by which it was at-tempted to be shown that these bodies are all parts ofthe earth, and therefore partake in its motion. The