WHEEL AND AXLE. One of the mechanical powers, used under various modifications. Cranes for raising weights, the wheels and pinions of clocks and watches, windlasses, &c. are all applications of this power. ZODIAC. A broad belt in the heavens, extending nearly eight degrees on each side of the ecliptic; the planes of the orbits of all the planets are included within this space. This belt is divided into twelve parts or signs, each containing 30 degrees. These signs are: _Aries_; the Ram. _Taurus_; the Bull. _Gemini_; the Twins. _Cancer_; the Crab. _Leo_; the Lion. _Virgo_; the Virgin. _Libra_; the Scales. _Scorpio_; the Scorpion. _Sagittarius_; the Archer. _Capricornus_; the Goat. _Aquarius_; the Waterer. _Pisces_; the Fishes. The first six are called northern signs; because the sun is in them, during that half of the year, in which he is vertical to the north of the equator; the last six, are called southern signs; because, during his journey among them, he is vertical to the south of the equator. The sun enters _Aries_, at the time of the _vernal equinox_; _Cancer_, at the _summer solstice_; _Libra_, at the _autumnal equinox_; and _Capricornus_, at the _winter solstice_. The sun is said to enter a sign, when the earth in going round in its orbit, enters the opposite sign. Thus, when the sun appears in the first degree of _Libra_, it is in consequence of the earth having arrived opposite to the first degree of Aries. A line then drawn from the earth, and passing through the centre of the sun, would, if extended to the fixed stars, touch the first degree of Libra. ZONE. The earth is divided into zones, or belts. See FRIGID, TEMPERATE, and TORRID ZONES. INDEX. A. Air, 11, 15, 28, 50, 136. Air-pump, 31, 145. Angle, 44. acute, 44. obtuse, 44. right, 44. of incidence, 45, 154, 160, 173. of reflection, 45, 154, 160, 173. visual, 168, 169, 170. Angular velocity, 171. Antarctic circle, 92. Aphelion, 75. Arctic circle, 92. Atmosphere, 28, 104, 129, 136, 144, 150, 163. colour of, 193. reflection of, 193. refraction of, 182. Attraction, 10, 14, 23, 25, 179. of cohesion, 15, 19, 118. capillary, 18. of gravitation, 18, 23, 29, 70, 80, 96, 116, 136. Avenue, 170. Auditory nerve, 151. Axis, 78. of motion, 48. of the earth, 22, 99. of mirrors, 176. of a lens, 184. B. Balloon, 30. Barometer, 140. Bass, 155. Bladder, 138. Bodies, 10. elastic, 40. fall of, 23, 26, 30, 36. luminous, 157. opaque, 157. sonorous, 152, 155. transparent, 157. Bulk, 16. C. Camera obscura, 184, 197, 201. Capillary tubes, 18. Centre, 48. of gravity, 48, 51, 52, 115. of magnitude, 48, 53. of motion, 48, 55, 115. Centrifugal force, 49, 72, 95, 115. Centripetal force, 49, 72. Ceres, 84. Circle, 44, 94. Circumference, 94. Clouds, 129. Colours, 23, 185. Comets, 86. Compression, 42. Concord, 155. Constellation, 86. Convergent rays, 175, 177. Crystals, 12.