POLES. The extremities of the axis of motion either of our earth, or of any other revolving sphere. The poles of the earth have never been visited; the regions by which they are surrounded, being obstructed by impa.s.sable barriers of ice.

POWER. That force which we apply to any mechanical instrument, to effect a given purpose, is denominated power, from whatever source it may be derived. We have the power of weights, of springs, of horses, of men, of steam, &c.

PRISM. The instrument usually so called, is employed in optics to decompose the solar ray: it consists of a piece of solid gla.s.s, several inches in length, and having three flat sides; the ends are equal in size, and are of course triangular.

PRECESSION OF THE EQUINOXES. Every equinox takes place a few seconds of a degree, before the earth arrives at that part of the ecliptic in which the preceding equinox occurred. This phenomenon is called the precession of the equinoxes. There is consequently a gradual change of the places of the signs of the zodiac: a fact, the discovery of which has thrown much light on ancient chronology.

PROJECTION. That force by which motion is given to a body, by some power acting upon it, independently of gravity.

PULLEY. One of the six mechanical powers. A wheel turning upon an axis, with a line pa.s.sing over it. It is the moveable pulley only, which gives any mechanical advantage.

PUMP. An hydraulic, or pneumatic instrument, for the purpose of raising water, or exhausting air.

QUADRANT. A quarter of a circle. An instrument used to measure the elevation of a body in degrees above the horizon.

QUADRATURES OF THE MOON. That period in which she appears in the form of a semicircle. She is then either in her first, or her last quarter; and exactly half way, between the places of new, and of full moon.

RADIATION. The pa.s.sage of light or heat in rays, or straight lines; these being projected from every luminous, or heated point, in all directions.

RADIUS. The distance from the centre of a circle, to its circ.u.mference; or one half of its diameter. In the plural denominated radii.

RAINBOW. An appearance in the atmosphere, occasioned by the decomposition of solar light, in its refraction, and reflection, in pa.s.sing through drops of rain. The bow can be seen, only when the sun is near the horizon, when the back is turned towards it, and there is a shower in the opposite direction.

RAY. A single line of light, emitted in one direction, from any luminous point.

REACTION. Every body, whether in a state of motion, or at rest, tends to remain in such state, and resists the action of any other body upon it, with a force equal to that action. This resistance, is called its _reaction_.

RECEIVER. This name is applied to gla.s.s vessels of various kinds, appertaining to the air pump, and from which the air may be exhausted.

They are made to contain, or receive, any article upon which an effect is to be produced, by taking off the pressure of the atmosphere.

REFRACTION, of the rays of light, is the bending of those rays, when they pa.s.s obliquely from one medium into another of different density. A stick held obliquely in water, appears bent or broken at the surface of the fluid.

REFRANGIBILITY. Capacity of being refracted. Light is decomposed by the prism, because its component parts are refrangible in different degrees, by the same refracting medium.

REPULSION. The reverse of attraction. A tendency in particles, or in ma.s.ses of matter, to recede from each other. The matter of heat within a body, appears to counteract the attraction of its particles, so as to prevent absolute contact.

RETINA. That part of the ball of the eye, upon which the images of visible objects are formed; and from which, the idea of such forms, is conveyed to the mind.

REVOLUTION, of a planet; is either diurnal, or annual; the former, is its turning upon its own axis; the latter, is its pa.s.sage in its...o...b..t.

SATELLITES. Moons, secondary planets.

SEGMENT OF A CIRCLE. A portion, or part of a circle; called also, an arc of a circle.

SEMI-DIAMETER. Half the diameter. The semi-diameter of the earth, is the distance from its surface, to its centre.

SIDERIAL. Belonging to the stars. A siderial day, is the time required for a star to reappear on a given meridian. A siderial year, the period in which the sun appears to have travelled round the ecliptic, so as to have arrived opposite to any particular star, from which his course was calculated.

SIGNS, or CONSTELLATIONS. Collections, or groups, of stars. Those of the zodiac are twelve, corresponding with the twelve months in the year. In the centre of these the ecliptic is situated. The sun appears to pa.s.s in succession through these signs; entering the first degree of Aries, which is accounted the first sign, about the 21st of March.

SKY. That vast expanse, or s.p.a.ce, in which the heavenly bodies are situated. Its blue appearance is supposed to arise from the particles of which the atmosphere is composed, possessing the property of reflecting the blue rays, in greatest abundance.

SOLAR. Appertaining to, or governed by, the sun: as the solar system, the solar year, solar eclipses.

SOLID. Not fluid. Having its parts connected so as to form a ma.s.s. Solid bodies, are not absolutely so, all undoubtedly containing pores, or s.p.a.ces void of matter.

SOLSTICES. The middle of summer and the middle of winter; those two points in the orbit of the earth, in which its poles point most directly towards the sun.

SONOROUS BODIES. Those bodies which are capable of being put into a state of vibration, so as to emit sounds.

SPECIFIC GRAVITY. The relative weight of bodies of different species, when the same bulk of each is taken. Water has been chosen as the standard for comparison. If we say that the specific gravity of a body is 6, we mean, that its weight is six times as great as that of a portion of water, exactly equal to it in bulk.

SPECTRUM. That appearance of differently coloured rays, which is produced by the refraction of the solar ray, by means of a prism, is called the prismatic spectrum; it exhibits most distinctly, and beautifully, all the colours seen in the rainbow.

SPHERE. A globe, or ball.

SPHEROID. Spherical; a body approaching nearly to a sphere in its figure. The earth, is denominated an _oblate spheroid_; it not being an exact sphere, but flattened at the poles, so as to cause the polar diameter to be upwards of thirty miles less than the equatorial. Oblate, is the reverse of oblong, and means shorter in one direction, than in another.

SPRING TIDES. Those tides which occur at the time of new, or of full moon. The tides then rise to a greater height than at any other period.

SQUARE. A figure having four sides of equal length, and its angles all right angles.

In numbers; the product of a number multiplied into itself; thus, the square of 3 is 9, and the square of 8 is 64.

STAR. The _fixed_ stars are so called, because they retain their relative situations; while the planets, by revolving in their orbits, appear to wander amongst the fixed stars.

SUBTEND. This term is applied to the measurement of an angle; when the lines by which it is bounded recede but little from each other, they are said to subtend; that is, to be contained under, a small angle.

SUPERFICIES. The surface of any figure. s.p.a.ce extended in length and width.

SYSTEM. The mutual connexion, and dependance of things, upon each other.

The solar, or Copernican system, includes the sun, the planets, with their moons, and the comets.

TANGENT. A straight line touching the circ.u.mference of a circle; but which would not cut off any portion of it, were it extended beyond the touching point, in both directions.

TELESCOPE. An instrument by which distant objects may be distinctly seen; the images of objects being brought near to the eye, and greatly magnified.

TEMPERATE ZONES. Those portions of the surface of the earth situated between 23-1/2 and 66-1/2 degrees of lat.i.tude. Within these boundaries, the sun is never vertical; nor does he ever remain, during a whole day, below the horizon.

THERMOMETER. An instrument for measuring the temperature of the atmosphere, or of other bodies.

TORRID ZONE. That portion of the earth which extends 23-1/2 degrees on each side of the equator, to the tropical circles; within this limit, the sun is vertical, twice in the year.

TRANSIT. Mercury or Venus, are said to transit the sun, when they pa.s.s between the earth and that luminary. They then appear like dark spots, upon the face of the sun.