+---------+--------------++---------+--------------+ |Jupiter’s|Periods round || Saturn’s|Periods round | | Moons. | Jupiter. || Moons. | Saturn. | |---------+--------------||---------+--------------+ | N^o | D. H. M. || N^o | D. H. M. | |---------+--------------||---------+--------------+ | 1 | 1 18 36 || 1 | 1 21 19 | | 2 | 3 13 15 || 2 | 2 17 40 | | 3 | 7 3 59 || 3 | 4 12 25 | | 4 | 16 18 30 || 4 | 15 22 41 | +---------+--------------+| 5 | 79 7 48 | +---------+--------------+ CHAP. III. _The_ COPERNICAN SYSTEM _demonstrated to be true_. [Sidenote: Of matter and motion.] 99. Matter is of itself inactive, and indifferent to motion or rest. A body at rest can never put itself in motion; a body in motion can never stop nor move slower of itself. Hence, when we see a body in motion we conclude some other substance must have given it that motion; when we see a body fall from motion to rest we conclude some other body or cause stopt it. 100. All motion is naturally rectilineal. A bullet thrown by the hand, or discharged from a cannon would continue to move in the same direction it received at first, if no other power diverted its course. Therefore, when we see a body moving in a curve of whatever kind, we conclude it must be acted upon by two powers at least: one to put it in motion, and another drawing it off from the rectilineal course which it would otherwise have continued to move in. [Sidenote: Gravity demonstrable.] 101. The power by which bodies fall towards the Earth is called _Gravity_ or _Attraction_. By this power in the Earth it is, that all bodies, on whatever side, fall in lines perpendicular to it’s surface. On opposite parts of the Earth bodies fall in opposite directions, all towards the centre where the force of gravity is as it were accumulated. By this power constantly acting on bodies near the Earth they are kept from leaving it altogether; and those on its surface are kept thereto on all sides, so that they cannot fall from it. Bodies thrown with any obliquity are drawn by this power from a straight line into a curve, until they fall to the Ground: the greater the force by which they are thrown, the greater is the distance they are carried before they fall. If we suppose a body carried several miles above the Earth, and there projected in an horizontal direction, with so great a velocity that it would move more than a semidiameter of the Earth, in the time it would take to fall to the Earth by gravity; in that case, if there were no resisting medium in the way, the body would not fall to the Earth at all; but continue to circulate round the Earth, keeping always the same path, and returning to the point from whence it was projected, with the same velocity as at first. [Sidenote: Projectile force demonstrable.]