The conservation of moment of momentum remains true in a system, even though there may have been actual collisions between the several parts. This is included in the proof already given, for collisions are among the mutual actions referred to. It may, however, be instructive to give a direct proof of a particular case. Let two particles collide when meeting in the directions A P and B P (Fig. 63) respectively. Whether the particles be elastic or inelastic is quite immaterial, for in both cases the action and reaction must be equal and opposite, and take place along some line P Q. The action on the particle moving along A P will give to it an acceleration of moment of momentum which is equal to the moment of the action around O. The acceleration of the moment of momentum coming along B P will be equal and opposite. Thus the total acceleration of the moment of momentum is zero. Hence the collision has no effect on the total moment of momentum. § 16. FRICTION AND TIDES. We have shown that such actions as collisions cannot affect the moment of momentum of the system, neither can it be affected by friction of one body on another. Here, as in the former case, the actions and reactions are equal and opposite, and consequently the accelerations of moment of momentum are zero. Nor is it possible for any tidal action to affect the total moment of momentum of the system. Every such action must be composed of the effects of one particle in the system on another, and as this must invariably produce an equal and opposite reaction the total moment of momentum is unaltered. ------------------------------------------------------------------------ INDEX. Acceleration of moment of momentum, 377 Aldebaran, 27, 28 Anderson, Dr., 356 Andromeda, Great Nebula in, 43, 204 Antinous, Cluster of stars in, 353 Apteryx, Rudimentary wing-bones of, 364, 366 ——, Skeleton of, 364, 366 ——, The, 363, 365 Arcturus, Spectrum of, 85 Argon, 265 Argus and surrounding stars, 103 Ariel, 338 Boring, The great, 123 Brooks’ comet, 89 Bunsen burner, The, 283 Butterfly and the oak-tree, The, 15 Calcium, 274 Capella, Spectrum of, 61–64 Carbon, 280 Ceres, 311 Change of moment of momentum, 375 Cluster, Nebulous region round a, 33 Clusters of stars, 53–60, 203 —— —— of 17th magnitude, 353 Coal-unit, 110 Collisions, 219, 380 ——, Cause of formation of nebulæ, 356 Comets, 37 Comet, Brooks’, 89 —— of 1882, 119 ——, Spectrum of, 290 Common, Dr. A. A., 44 Concord, The first, 294–307 ——, The second, 308–323 ——, The third, 324–336 Conglomerates, 159 Conservation of moment of momentum, 374 Corona of the sun, 117 Crab nebula, The, 19, 44 Crossley Reflector, The, 45, 46, 48, 49, 50, 67, 199 Cygnus, Nebula in, 329 Dark bodies in universe, 355 Darwin, Professor G. H., 153, 254, 332 Darwinian theory, 10, 268, 362 Dewar, Professor, 144, 272