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the centre, where they run together in a blaze. The more condensed part is projected on a ground of irregularly scattered stars, which fills the whole field of the telescope. There are few stars near this cluster.
NOTE 236, p. 420. Plate 8 shows five nebulæ as seen in Sir John Herschel's 20-feet telescope.
1. An enormous ring seen obliquely with a dark centre and a small star at each extremity.
2. The ring in the constellation Lyra.
3. The dumb-bell nebula in Vulpicula.
4. The spiral nebula or brother system in the 20-feet telescope.
5. A spindle-shaped nebula.
Plate 9 represents some of the same objects as seen by Lord Rosse.
1. Nebula in the girdle of Andromeda.
2. The circular nebula of Lyra.
3. The dumb-bell nebula in Vulpicula.
The spiral nebula of 51 Meissier, as seen by Lord Rosse, 1 in plate 10, represents fig. 4 of plate 8; and fig. 2 in the same plate is part of the great nebula in Orion, for the whole has never been seen, on account of extreme remoteness.
NOTE 237, pp. 32, 427. The motion of the earth is visibly proved by M. Foucault's experiments. If a pendulum be left to oscillate quite freely, the forces producing the oscillations being in the vertical plane, there is no cause that can produce an absolute change in its position with regard to space; but the motion of the earth changes the position of a spectator with respect to the vertical plane, and he refers his own motion to it, which seems gradually to turn away from its position, precisely as a person in a boat refers his own motion to that of the land, and thus the motion of the earth is truly and visibly proved.
ABERDEEN, high water at, 94. Absorption, influence of, on temperature, 239; difference of sea and land in power of, 242; gradually decreasing, in transmission of radiant heat, 259; of radiant heat, varying with substances, 268; a transfer of force, 275, 276. Acceleration of the moon's mean motion, 37, 38.
Adams, Mr., perturbation in Uranus's motion computed by, 22; discovery of Neptune, 62.
Aërolites, theory of, 420, 423.
annual equatorial temperature in,
Air, comparative velocity of light in water and, 202. See Atmosphere. Airey, Professor, periodic inequality in the solar system worked out by, 26; phenomenon observed by, during an eclipse, 41; mass of Jupiter ascertained by, 55; experiments ascertaining its density, 57; astronomical tables improved by, 63; discoveries in polarization of light, 192, 193.
Aldebaran, an optically double star, 401.
Aleutian Islands, the, vegetation of, 252.
Alexandria, arc of the meridian mea
sured between Syene and, 49. Algæ, districts of distinct species of,
252; banks of, in the Atlantic, 253. Algol, fluctuations in lustre of, 390, 391.
Alhazen, effects of refraction observed by, 155.
Alkalies, resolved into metallic oxides, 307.
Alpha Antaris, "Coal Sacks" between a Centauri and, 386.
Alpha Aquila, an optically double star, 401.
Centauri, the parallax of, 54; its rank, 384; the Milky Way near, 386; parallax, as determined by Henderson and Maclear, 387; distance from the sun, 388; orbit and mass of, 399, 400; colour, 401; amount of light emitted by, 404; rate of its proper motion, 404, 405; globular nebulous cluster, 414.
Crucis, zone of stars passing through, 385; zone between Argûs and, 390; nebulous cluster round, 415,
Lyræ, the polar star of the northern hemisphere, 82; parallax of, 388; distance from the sun, 389; an optically double star, 400; amount of light emitted by, 404.
- Orionis, a variable star, 393, 394. Alum, experiments on the crystallization of, 106, 107; heat transmitted through, 261, 262.
Amazons, the river of, distance from its mouth where tides are perceptible, 98; area occupied by forests on, 243.
America, course of the tidal wave along its coasts, 93, 94; mean annual equatorial temperature in, 245; separation of isothermal lines in high latitudes, ib. ; number of known species of plants indigenous in, 249; number of species of trees, 252; shooting stars over the continent of, 421.
South, area of country raised by an earthquake in, 234. Ampère, M., his discovery in electricity, 316; theory of magnetism, 317, 318; experiment testing his theory, 319, 320.
Analysis, boundless dominion of, 427, 428.
Andes, the, proportion of, to the earth's mass, 6; increasing rarity of the air experienced in ascending,
Andromeda, nebula in, 413; nebulous region of, 417.
Angström, the electric spark defined by, 303.
Animals, specific diversity of, laws regulating their distribution, 254, 255. Annual equation, the, of the moon, 35, 36.
variations in mean values of the magnetic elements, 343. Annular nebulæ, 409; in the northern hemisphere, 410, 411.
Antarctic Ocean, tidal wave rising in, 93; period of its passage to the Thames, 94; depth of the stratum of constant temperature in, 101; depression of the barometer observed in, 120.
Antilles Islands, hurricanes beginning at, 126.
Antinori, Cav., experiments of, in electricity, 333.
Antinous, comet observed in the con-
Aphelion of a planet's path defined, 16.
rings, 67; apparent and mean time
Apsides of an axis defined, 9; direct, variable motion of, 14; cause of their advance, or recession, 16. Apures, the mission of the, Humboldt's observations on sound at, 135. Aquæous vapour, proportion of, in the atmosphere, 117.
Ara, nebula in, 414.
Arabian Gulf, the, monsoons blowing over, 124.
Arabs, the, their observations on planetary irregularities, 26; lunar eclipses observed by, 38; their division of time, 85; the pendulum used as a measure of time by, 90.
Arago, François, experiment by, in proof of the undulatory theory of light, 200; decisive experiment suggested by, 202; observations in photography, 213; observations on the moon's atmosphere, 226; increase of temperature below the earth's surface calculated by, 230; slow communication of temperature from the earth, observed, 244; source of magnetism discovered, 330; theory of his magnetic experiments, 332; divergent flames of a comet described by, 364; his treatise on comets, 368; nature of comet's light determined by, 380, 381; numbers of comets computed, 381, 382; remark of, on fixed stars, 405.
Arc, the Voltaic, 303-305. Arcet, M. d', vibration of fibres of the retina according to, 178. Archer, Scott, stimulus given to photography by, 207.
Arcs of the meridian, mode of measuring, 47.
Arcturus, comet bearing comparison with, 379; rank of, 384. Areas, described by the radii vectores
of planets, a test of disturbing forces, 10; unequable description of, 15. Argelander, M., period of a comet calculated by, 370; his mode of estimating distance of fixed stars, 389; periods of fluctuation in stars computed by, 390, 391; sun's motion proved, 405.
Argentine preparations in photography, chemical energy varying with, 207, 208; changes effected by washing with alkalies, 210, 211. Argo, variable star in, 393. Aries, season of the sun's entrance
into, in Hipparchus' age, 80. Arseniate of soda, its crystals, 109. Artesian wells, mode of sinking, origin of the name, 230.
Artic Sea, depth of the zone of constant temperature, 101.
- regions, vegetation found in, 249. Asia, indigenous productions of, 249. Assyrians, the, division of time by, 85.
Astronomers, fruits of their labours,
Astronomical distances, method of
Astronomy, its rank in the physical
waves in, 93; conditions modifying
telegraph, 325, 326; terrestrial
of planets, 226, 227.
of the sun, its constitution, 42;
terrestrial, solar rays bent by,
changes in its strata, 155-156;
pressure, effect of, on electricity,
Atoms, qualities of, determining the
nature of substances, 110; dif-
celestial bodies, 4; determining the
static and in voltaic electricity, 317;
Aurora, the, affecting the compass,
Australia, evidence of deserts in the
interior of, 124; species of plants
Axes, change in form of masses re-
-, major, length of, in orbits, in-
-, optic, of crystals, 183.
major, of a planet's orbit, dis-
-, magnecrystallic, 349.
Baily, Mr., compression of the terres-
Bali, volcanic eruption in, 233.
Baltic, the, a tideless sea, 98; de-
Barlow, Mr., observations supporting
magnetic, constructed by Dr.
Batsha, port of, tides neutralised in,