j,et. vi. KeplEr. and Galileo. 89

peared to him as an idle conjecture; and fromthe observations of Tycho, and his own in-dustry, he soon proved it to be erroneous andill-founded.

That the orbits of the planets were not circu-lar, might, indeed, have been easily conjecturedfrom many circumstances. Their conjunctions,oppositions, and other mutual situations, not re-turning again in the fame time; and their dis-tances from the fun appearing to be greater orless in different parts of their orbits, were suffi-cient indications of the fallacy of this doctrine.But so firmly were astronomers persuaded thattheir motions must be circular, that they attri-buted these irregularities to an optical delusion,and invented cycles and epicycles without num-ber, to account for every appearance,

Kepler was the first who perceived, that allmotion is naturally performed in a straight line;and that when a body moves in a circle, or anyOther regular curve, it must be acted upon bytwo forces, one which sets it in motion, and ano-ther which opposes this motion, and changes itsdirection. From these principles, and a numberof calculations equally difficult and laborious, heproved, that the planets must revolve in elliptical,orbits the fun being placed in one of the foci ;and that their velocities are such, that a linedrawn from the sun to a planet, and supposed to

move