Figure of the Earth, 69 LETTER VIII. Diurnal Revolution, 81 LETTER IX. Parallax and Refraction, 89 LETTER X. The Sun, 101 LETTER XI. Annual Revolution.--Seasons, 111 LETTER XII. Laws of Motion, 126 LETTER XIII. Terrestrial Gravity, 134 LETTER XIV. Sir Isaac Newton.--Universal Gravitation.--Figure of the Earth's Orbit.--Precession of the Equinoxes, 143 LETTER XV. The Moon, 157 LETTER XVI. The Moon.--Phases.--Harvest Moon.--Librations, 172 LETTER XVII. Moon's Orbit.--Her Irregularities, 180 LETTER XVIII. Eclipses, 195 LETTER XIX. Longitude.--Tides, 208 LETTER XX. Planets.--Mercury and Venus, 225 LETTER XXI. Superior Planets: Mars, Jupiter, Saturn, and Uranus, 243 LETTER XXII. Copernicus.--Galileo, 254 LETTER XXIII. Saturn.--Uranus.--Asteroids, 274 LETTER XXIV. The Planetary Motions.--Kepler's Laws.--Kepler, 291 LETTER XXV. Comets, 312 LETTER XXVI. Comets, 334 LETTER XXVII. Meteoric Showers, 346 LETTER XXVIII. Fixed Stars, 365 LETTER XXIX. Fixed Stars, 383 LETTER XXX. System of the World, 392 LETTER XXXI. Natural Theology, 406 LETTER XXXII.