Part 97

PRISM. The instrument usually so called, is employed in optics to decompose the solar ray: it consists of a piece of solid glass, 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 passing 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 passage 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 circumference; 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 passing 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 glass 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 pass 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 masses 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 passage in its orbit.

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 pass 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 space, 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 mass. Solid bodies, are not absolutely so, all undoubtedly containing pores, or spaces 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.