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260 Patroclus, 94. Peirce, no, 125, 126. Perrine, 15. Perrotin, 116. Perturbations, in motion of planets, heralding a catastrophe, 28, 30. reflected, 63. mass of planet determined by, 68. of asteroids by Jupiter, 98, 99. restrictive action of, 99. the fashioning force of planetary orbits, 99, 100. of rings of Saturn by satellites, in, 112. of Uranus lead to discovery of Neptune, 121-126. Petersen, Dr., 123. Photometric determinations, 92, 93. background, the fundamental factor in, 92, 93. Piazzi, 96. Pilgrim Star, 5, 6. Planetary astronomy, advance in, 59, 60. Planetology, 203. defined, 173, 174. Planets, 61. knots in spiral nebulæ, 25, 139. developed by agglomeration, 143, 149, 151, 152. Pliny, 5. Plutonic rocks, 160. Pluvial eras, contemporaneous with glacial, 200. Polyp corals, in paleologic times, 186. Pristine motion of planetary particles, retrograde, 144. superfluous energy in, 145. unstable, 145. Ptolemaic system, 58.

R Refrigeration, tempered by loss of cloud, 196. Revolutions, of shooting stars, 39. of asteroids, direct like planets, 100. planetary, in same sense, 129, 130. outermost satellites, retrograde, 132. of satellites explained 146, 147, Notes 252. Ritchey, 14. Roberts, Dr., 20. Roche, Edouard, 110. Rosse, Lord, 17. Rotation of planets, 131, 132. systematic righting of axes, 132. initially, retrograde, 146. Rotation period, of Venus, spectrographically determined, 83, 85-90. of Mars, spectrographically determined, 88, 89. of Jupiter, spectrographically determined, 89. of Uranus, 116. Royal Observatory, Edinburgh, 7.

S Satellites, 61. of Mars, 92. of Saturn, 108, 112. of Uranus, 116. solid, 117. of Neptune, 118 turn same face to primaries, 134, 147, 148, 225. latest discoveries in regard to motions of, 146. origin of, 147. death of, before planet, 233. impact of interplanetary particles on, Notes 246-250. capture of, Notes 251, 252. Saturn, 108-114. belts of, 109, 168. inherent light, 109, 152. Saturn's rings, 109-114. mechanical marvel of, not early appreciated, 110. discrete particles, 110, 135. knots upon, 110-113. not flat, but tores, 111-114. show devolution—not pristine state of solar system, 138, 139. once a congeries, 139. Schaeberle, 9. Schiaparelli, 34, 36, 64-66, 69, 76, 77, 221. Schroeter, 65, 77. Seasons, loss of, 71, 83, 217, 218. begin with clearing of sky, 185. fully developed, 189. See, Notes 251.