EXPLANATION OF SCIENTIFIC TERMS.

YEAR. The period of time in which the earth performs her revolution round the sun, or that in which the sun apparently moves from a point in the ecliptic until he returns to the same point, is the Solar Year. It is also termed the Tropical Year, and consists of 365 days, 5 hours, and nearly 49 minutes. The Silereal Year, that between the departure of the sun from any fixed star to his return to the same star, is about 17 minutes longer. The Anomalistical Year is the time that elapses from the sun's leaving his apogee till he returns to it, and is 365 days, 6 hours, and about 14 minutes. See Calendar.

ZENITH.-See Nadir.
ZENITH DISTANCE. The complement
of the altitude of a heavenly body is its
Zenith distance.

ZODIAC. The ecliptic (as is stated under
that head) crosses the equator at an angle
of about 23 degrees. It is the earth's
orbit, and the apparent path of the sun.
The orbits of the other planets cut the
ecliptic at different angles; but (until

lately, that Juno, Pallas, Ceres, and Vesta
were discovered), the orbits of all passed
within seven degrees on either side of the
A zone of sixteen degrees
earth's course.
in width, having the ecliptic for its central
line, was, therefore, conceived to include
the whole of the planetary orbits. It was
called the Zodiac (Greek Zodiacos, from
Zoon, an animal), because it contained
the figures of all the animals, &c. which
formed the twelve signs.

ZONE (Latin Zona, a girdle), in Geography,
is the denomination given to each of the
five parallel belts into which the earth is
imagined to be divided in respect to tem-
perature. The Torrid Zone includes all
the space which lies between the tropics,
being nearly 47 degrees, or 23 degrees
on each side of the equinoctial line. Two
Frigid Zones occupy those parts which lie
between the poles and the polar circles;
and two divisions that lie between those
circles and the torrid zone, are called the
Temperate Zones.

ZOOLOGY (Greek_zöon, an animal, and logos, a discourse), is that division of Natural History which treats of animals, and

ZOOPHYTES (Greek phyton, a plant) are such natural productions as are supposed to participate of the qualities both of plants and animals.

GENERAL INDEX.

The references are given to the treatise or treatises in which the article is found. Of the
contractions, HIST. OF ASTRO. Stands for History of Astronomy; MATH. GEOG. for Mathe-
matical Geography; and PHYS. GEOG. for Physical Geography.

Action of gravity explained

Afternoon of the day, why warmest

Albinoes, description of that variety of man

Alexandria, the ground of its site did not exist in the time of
Homer

Alexandrian school, astronomers of

America and Europe, comparative temperature of
South, atmospheric appearances in
has no indigenous species of heath
particularly abundant in venomous reptiles
Amount of parallax, how ascertained
Anaximander said to have invented the gnomon
Ancient opinions concerning the figure of the earth
Animals of New Holland, account of

ASTRONOMY

HIST. OF ASTRO.
MATH. GEO.
PHYS. GEOG.

ASTRONOMY

153,170-174
.95-97
97

elements of

Aratus, his astronomical poem

.

23,24

Arctic regions, red coloured snow of, accounted for

Arcturus, proper motion of that star

his heliacal rising in the age of Hesiod

Aristarchus, his method of ascertaining the distance of the sun

and moon from the earth

another account of his method of ascertaining the

sun's distance

Aristotelian philosophy overthrown by Galileo

Aristotle supposed that the earth was shaped like a drum
Arithmetic, binary, account of a Chinese work on
Art of observing, improvements in

Aryabhatta, an ancient Indian astronomer, taught the diurnal
motion of the earth

Asteroids proposed designation for the planets Juno, Ceres,
Pallas, and Vesta

Astrolabium of Ptolemy, construction of

Astrology originated in Chaldæa

not known in Greece before the time of Alexander
prevalent at Rome under Augustus

infected some of the greatest astronomers of modern
Europe up to the 17th century

Astronomers of Chaldæa, their method of calculating eclipses
of the Alexandrian school, account of

only one of eminence during the Roman republic
of Europe were astrologers up to the beginning of
the 17th century.

Regiomontanus, an account of his works

Copernicus, account of his system and discoveries
accused of impiety

Tycho Brahé, account of his discoveries
Huygens, account of, and of his inventions
Kepler, account of his theories and discoveries

Galileo, account of his discoveries

accused of impiety by the church

.

43-47
-44,64-69
49-54

54-59

57

Hevelius, account of his observations

(early) description of their quadrants

63,64

67

Römer, account of his inventions and discoveries

70,75

Descartes, account of his system

Newton, account of his system

Bradley, account of his discoveries

La Caille, account of his astronomical labours

Mayer, account of his labours

Astronomical day distinguished from the civil

twilight distinguished from the popular

observations, correct the variation of the magnetic

Tables of the Chinese, account of

of the Hindoos, account of

theories of Hipparchus, account of

poem by Aratus, account of

of Manilius, account of

discoveries of Hipparchus, account of

doctrines of Ptolemy, account of

observatories, when first established

system of Newton, account of

clocks regulated by sidereal time

Astronomy the immediate results of observation contrary to its

82,83

needle

255

253-255

HIST. OF ASTRO.

1-7

7-11

observations on

telescopes of immense length, account of

ASTRONOMY

HIST. OF ASTRO.

Astronomy, progress of Eratosthenes in
of the Arabs, history of

of the Persians, history of
of the middle ages, history of

patronised by Alphonso, King of Castile

by Charles the Second

physical, origin and history of

principal periods in its history

of the Hindoos, tabular view of its elements
of the Arabs, elements of, from El-Batani
how applied to navigation

Atmosphere, shooting stars are meteors engendered in

of the moon different from that of the earth

has no clouds

of the sun, its density

of Venus, similar to that of the earth

of Jupiter, on the clouds in

currents in, account of

appearances of, in South America

Attraction of the planets disturbs the elliptical figure of the earth's

NAVIGATION
ASTRONOMY

26-32

18

86

86

125

126

126

PHYS. GEOG.

30

41

. 194

105

• 36

30

10

27

why there are no tides in

Barlow's method of finding the deviation of the compass
Bearing, magnetic, explained

Beccaria and others measure a degree of the meridian in Italy
Belief in uniformity, a general law of the mind

Betelgeuse, the brightest star in Orion.

Birds, each species confined to one quarter of the globe

account of their migrations

Black Sea, its surface higher than that of the Caspian
Blood-colour in the lunar eclipse of 1830 accounted for
Bode's remarkable guess respecting the planetary orbits

law of the distances of the planets from the sun, confir-
mation of
Bodies, lightest at the equator, increasing in weight towards the
poles

Boiling Fountain, description of

Bones of known and unknown quadrupeds found buried in the
earth

Books on mathematical geography, referred

HIST. OF ASTRO.

Canada, its climate compared with that of England

Cape of Good Hope, measure of a degree of the meridian at, by
La Caille and others