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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 tempe-
rature, 239; difference of sea and
land in power of, 242; gradually
decreasing, in transmission of ra-
diant heat, 259; of radiant heat,
varying with substances, 268; a
transfer of force, 275, 276.
Acceleration of the moon's mean mo-
tion, 37, 38.

Adams, Mr., perturbation in Uranus's
motion computed by, 22; discovery
of Neptune, 62.

Aërolites, theory of, 420, 423.
Africa, tidal wave passing, 94; mean

annual equatorial temperature in,
245; indigenous productions of,
249, 250.

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, dur-
ing an eclipse, 41; mass of Jupiter
ascertained by, 55; experiments
ascertaining its density, 57; astro-
nomical tables improved by, 63;
discoveries in polarization of light,
192, 193.

Aldebaran, an optically double star,

Aleutian Islands, the, vegetation of,

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,


Alhazen, effects of refraction observed
by, 155.

Alkalies, resolved into metallic oxides,


Alpha Aquila, an optically double star,

Centauri, the parallax of, 54;
its rank, 384; the Milky Way near,
386; parallax, as determined by
Henderson and Maclear, 387; dis-
tance 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 Ar-
gûs 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 crystalliza-
tion of, 106, 107; heat transmitted
through, 261, 262.

Amazons, the river of, distance from
its mouth where tides are per-
ceptible, 98; area occupied by fo-
rests 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 conti-
nent of, 421.

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South, area of country raised by
an earthquake in, 234.

Ampère, M., his discovery in elec-
tricity, 316; theory of magnetism,
317, 318; experiment testing his
theory, 319, 320.

Alpha Antaris, "Coal Sacks" between Analysis, boundless dominion of, 427,

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Andes, the, proportion of, to the earth's mass, 6; increasing rarity of the air experienced in ascending, 118.

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 constellation of, 372; the Milky Way between Orion and, 386. Antithesis, the general character of magnetism, 339.

Aphelion of a planet's path defined, 16. Apogee, solar, its coincidence with the solstices, 86, 87.

April, 1833, disappearance of Saturn's

rings, 67; apparent and mean time coinciding in, 84. 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


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,
3; question still to be resolved by,
24; terrestrial orbit differently mea-
sured by, 36.

Astronomical distances, method of
measuring, 43; tables, method of
forming, 58-64.

Astronomy, its rank in the physical
sciences, an important office of, 1;
studies necessary to the study of,
2; the key to divers problems in
physical science, 3; the two greatest
discoveries in, 23; the three de-
partments of, 58; standards for
measurement afforded by, 83; ap-
plication of, to chronology, 87-89;
furnishing standards of weights and
measures, 89, 90; atmospheric ef-
fects connecting the laws of molecu-
lar attraction with, 102; progress
lately made by, 419, 420.
Atalanta, diameter of, 56.
Atlantic Ocean, direction of tidal

waves in, 93; conditions modifying
tides, 94; depth of, 96; currents,
100; origin of hurricanes, 126; su-
perficial temperature of, 244; dis-
tinct vegetation of the polar basin,
252; beds of algae in, 253; meteors
falling in, 421.

telegraph, 325, 326; terrestrial
magnetism disturbing, 346.
Atmosphere of nebulous stars, 411,

of planets, 226, 227.

of the sun, its constitution, 42;
indications of an absorptive sur-
rounding the luminous, 213; the
true, 224.

terrestrial, solar rays bent by,
in lunar eclipses, 40; influence of,
in solar eclipses, 41; its analysis,
pressure on the surface of the globe,
117; form of, gradual decrease in
density of its strata, 117, 118; in-
fluence of temperature on its density,
119; mean pressure of, variable,
120; the medium conveying sound,
129; sympathetic vibrations trans-
mitted by, 147, 148; its action on
light, falsifying vision, 153; phe-
nomena produced by accidental


changes in its strata, 155-156;
effects of increased density in the
stratum in the horizon, 157, 158;
lunar heat absorbed by, 227; cause
of the cooler air in higher regions of,
240, 241; sun's heat modified by,
244; action of electricity in, 284;
transmission of electricity by induc-
tion, 286; periodical variations of
electricity in, 291; accidental de-
velopments of electricity, 291, 292;
cause of variations in its magnetism,
344, 345; nebulous bodies made
visible by, 421-423.
Atmospheric air, extreme elasticity of,

pressure, effect of, on electricity,


Atomic constitution determining crys-
talline forms, 109.

Atoms, qualities of, determining the

nature of substances, 110; dif-
ferences in weight of, 111.
Attraction, modes of, in spheres, in the
celestial bodies, 4; determining the
forms of planets, 6; determining
the motions of planets, 7; solar,
compelling the elliptical revolutions
of planets, 8; mutual, of planets,
complicating their motions, 10; in-
terference of, disturbing the motions
of heavenly bodies, 11; disturbances
from the operation of reciprocal,
13; disturbances from inequality
of, 14; of satellites to primaries,
little disturbed, 26; disturbing force
of, in spheroids, 27; its effects on
Jupiter's satellites, 28; sun's, of
the moon, 34; principle modifying
the earth's, 37; local, affecting the
plumb-line, 48; comparative force
of the sun's, 57; of an external
body affecting a spheroid, 79; pro-
ducing tides, 91, 92; of particles
of matter, 103; capillary, 113;
producing annual atmospheric un-
dulations, 121; the lunar atmo-
sphere affected by, 226; expansive
force of heat overcoming, 271; of
electricities, 283; destruction of,
producing electricity, 284; laws of
electrical, 286-288; modes of, in



static and in voltaic electricity, 317;
action of planetary, on comet's or-
bits, 361-363; range of solar,

Aurora, the, affecting the compass,

Australia, evidence of deserts in the

interior of, 124; species of plants
common to Europe and, 251.
Auvergne, temperature of hot springs
in, 231.

Axes, change in form of masses re-
volving round, 6.

-, major, length of, in orbits, in-
variable, 20; of the orbits of Ju-
piter's satellites, cause of the direct
motion observed in, 28; position
of, in the solar system, 65; a
nutation in planetary, 66; of the
moon, 68, 69; mechanical law
affecting, 76.

-, optic, of crystals, 183.
Axis, greater, of the earth's orbit,
period of its revolution, 38; period
of the earth's revolution, 58; excess
of Jupiter's equatorial over his
polar, 66; of rotation, proof of its
being invariable, 76, 77.

major, of a planet's orbit, dis-
tance from the sun measured by, 8;
designation of its extremities, 9;
length of, determining the form of
the orbit, 10; periods of its revolu-
tions, 17; length of, not perma-
nently changed, 20; Jupiter's pe-
riodically diminished, Saturn's in-
creased, 26; of the solar ellipse,
period of its revolution, 86.

-, magnecrystallic, 349.
Azores, the, icebergs reaching, 100.

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Baily, Mr., compression of the terres-
trial spheroid calculated by, 50;
density of the earth determined,
57; fictitious antiquity ascribed to
Indian astronomical observations,

Bali, volcanic eruption in, 233.
Balloon, rarity of the air felt in a,
118; observations made from,

Baltic, the, a tideless sea, 98; de-
creased atmospheric pressure on
the shores of, 120.

Barlow, Mr., observations supporting
his theory of electric currents, 346.
Barometer, the, principles of cohesion
and attraction applied to the con-
struction of, 113; density of the
atmosphere measured by, 117;
mean heights of, varying with at-
mospheric densities, 118; mountain
heights measured by, 119, 120;
atmospheric phenomena affecting,
120; used to trace the course of
atmospheric waves, 121; cause of
sudden fall in, before hurricanes,
127; refraction varying with, 154.
Barrow, Cape, observations on mag-
netic storms at, 345, 346.
Battery, voltaic, construction of, 298,
299; Professor Daniell's improve-
ments, 299, 300; action of, charged
with water, 300; constant flow
of electricity obtained by means of,

magnetic, constructed by Dr.
Faraday, 324, 325; Mr. Henley's
magneto-electric, 325; Atlantic tele-
graph, 326; structure of, for land
telegraphs, 328; relation of heat
to power of, 329; thermo-electric,

Batsha, port of, tides neutralised in,


Bayle, comparative density of the at-

mosphere in interplanetary space
according to his law, 356.
Bear, Little, the, the polar star in,

Becquerel, M. E., unexplained pho-

tographic phenomenon observed
by, 213; phosphorescent property

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