Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 56

TRANSPARENT. Allowing the rays of light to pa.s.s freely through. The reverse of opaque. Gla.s.s, water, air, &c. are transparent bodies.

TROPICS. Two circles on the globe on either hemisphere, at the distance of 23-1/2 degrees from the equator. Beyond these circles, the sun is never vertical: and the countries within them, are denominated tropical.

TWILIGHT. That portion of the morning or evening, in which the light of the sun is perceptible, although he is below the horizon.

VACUUM. s.p.a.ce void of matter. Such is supposed to be the s.p.a.ce in which the planets revolve. We are said to produce a vacuum, when we exhaust the air from a receiver.

VALVE. A part of a pump, and of some other instruments, which opens to admit the pa.s.sage of a fluid in one direction, but closes when pressed in the opposite direction, so as to prevent the return of the fluid; a pair of bellows is furnished with a valve.

VAPOUR. Exhalations from fluid or solid substances, generally mixing with the atmosphere. The most abundant, is that from water.

VERTICAL. Exactly over our heads: ninety degrees above our horizon.

VIBRATION. The alternate motion of a body, forwards and backwards; swinging, as a pendulum.

VISUAL. Belonging to vision; as the visual angle, or that angle formed by the rays of light which enter the eye, from the extremities of any object.

UNDULATION. A vibratory, or wave-like motion communicated to fluids.

Sound, is said to be propagated by the undulatory, or vibratory motion of the air.

WEDGE. One of the mechanical powers; the form of the wedge is well known. It is of extensive use; serving to rend bodies of great strength, and to raise enormous weights.

WHEEL AND AXLE. One of the mechanical powers, used under various modifications. Cranes for raising weights, the wheels and pinions of clocks and watches, windla.s.ses, &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 s.p.a.ce. 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...o...b..t, 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 pa.s.sing 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.

THE END.