velocity of, 22; undulatory and musical, 26; chemical force of, 30; experiments with, 37; approach and departure of a light-giving body measured, 51; aberration of, 199. LIMB, the edge of the disk of the moon, sun, or a planet. LONGITUDE. If a perpendicular be dropped from a body to the ecliptic, its celestial longitude is the distance of the foot of the perpendicular from the vertical equinox, counted toward the east; mode of ascertaining terrestrial, 72. MAGELLANIC CLOUDS, 208. [Page 282] MARS, 159; snow spots of, 160; satellites of, 161. MASS, the quantity of matter a body contains. MEAN DISTANCE OF A PLANET, half the sum of the aphelion and perihelion distances. MEASUREMENTS, celestial, 57. MERCURY, 138. MERIDIAN, terrestrial, of a place, a great circle of the heavens passing through the poles, the zenith, and the north and south points of the horizon; celestial, any great circle passing from one pole to the other. METEORS, 119; swarm of, meeting the earth, 118; explosion of, 120; systems of, 123; relation of, to comets, 124. MICROMETER, any instrument for the accurate measurement of very small distances or angles. MIND, origin of force, 252; continuous relation of, to the universe, 252. MILKY WAY, 210, 215. MIRA, the Wonderful, 221. MOON, the, 151; greatest and least distance from the earth, 10; telescopic appearance of, 155. MURAL CIRCLE, 61. NADIR, the point in the celestial sphere directly beneath our feet, opposite to zenith. NEBULÆ, 217. NEBULAR HYPOTHESIS, not atheistic, 182; stated, 182; confirmatory facts, 183; objections to, 185. NEPTUNE, elements of, 175. NODE, the point in which an orbit intersects the ecliptic, or other plane of reference; ascending, descending, line of, 107. OCCULTATION, the hiding of a star, planet, or satellite by the interposition of a nearer body of greater angular magnitude. OPPOSITION. A superior planet is in opposition when the sun, earth, and the planet are in a line, the earth being in the middle. ORBIT, the path of a planet, comet, or meteor around the sun, or of a satellite around a primary; inclination of, 106; earth's, seen from the stars, 70. OUTLINE FOR STUDENTS, 276. PARALLAX, the difference of direction of a heavenly body as seen from two points, as the centre of the earth and some point of its surface, 69. PARALLELS, imaginary circles on the earth or in the heavens parallel to the equator, having the poles for their centre. PERIGEE, nearest the earth; said of a point in an orbit. PERIHELION, the point of an orbit nearest the sun. PERIODIC TIME, time of a planet's, comet's, or satellite's revolution. PERSONAL EQUATION, 65. PERTURBATION, the effect of the attractions of the planets or other [Page 283] bodies upon each other, disturbing their regular motion; of Saturn and Jupiter, 11; of asteroids, 13; of Uranus and Neptune, 176. PHASES, the portions of the illuminated half of the moon or