PL A 



PL A 



and the figure is trilateral, quadrilateral, 

 multilateral, &c., according as it has 

 three, four, or many sides. 



1. Plane, vertical. A plane which 

 passes through the zenith, or point which 

 is directly over our heads, and the centre 

 of the earth, is called a vertical plane, 

 and the circle in the heavens marked hy 

 such a plane, is a vertical circle, or 

 azimuth. 



2. Plane, Tangential. A plane which 

 touches a curvilineal solid. It is from 

 such a plane that angles of incidence are 

 measured, whether the impinging rays 

 be reflected or refracted. 



3. Plane, inclined. In Statics, this 

 term denotes a plane inclined to the hori- 

 zon. It is used as a mechanical power, 

 and the problem is to find the force 

 necessary to prevent a body placed upon 

 it from sliding down under the action of 

 its own weight. 



4. Plane of Floatation. The surface of 

 a heavy fluid at rest is a horizontal plane, 

 and the portion of this plane which we 

 may imagine to be within a floating body, 

 is called the plane of floatation. 



5. Plane of Vibration. It is assumed 

 that in polarized light the particles of 

 ether vibrate only in two opposite direc- 

 tions; the plane in which these excur- 

 sions take place is called the plane of 

 vibration; and, if a plane be conceived to 

 be situated at right angles to the plane of 

 vibration, and in the direction of the 

 ray, this will be its plane of polariza- 

 tion. 



PLA'NET (7r\avr}Tt)9, a wanderer). A 

 star which is found to change its relative 

 situation among the other stars. Planets 

 are distinguished, with reference to their 

 centres of revolution, into primary and 

 secondary. Primary planets are those 

 which revolve round the sun as a 

 centre ; secondary planets, more fre- 

 quently called satellites or moons, are 

 those which revolve round a primary 

 planet as a centre, and are carried with 

 it in its revolution round the sun. 



1. The Primary Planets are divided 

 into superior and inferior. The superior 

 are those more remote from the sun than 

 the earth, as Mars, Jupiter, Saturn, 

 Herschel, and Neptune. The inferior 

 are those which are nearer to the sun 

 than the earth, as Mercury and Venus. 

 See Asteroids. 



2. The following symbols are employed 

 to designate the planets in tables and on 

 globes. Mercury §, Venus $, the 

 Earth 0, Mars ^, Vesta S, Juno v, 



262 



Ceres ?, Pallas $ , Jupiter 2/., Saturn Vi, 

 Herschel or Uranus y. 



3. Two planets are in conjunction 

 when they have the same longitude ; 

 they are in opposition when their longi- 

 tudes differ by 180''. 



PLANETARY PERIOD. The period 

 in which a planet comes again into the 

 position which it occupied at first with 

 respect to the sun. The motions of the 

 planets being oscillatory, a planetary 

 period is not that in which a planet re- 

 volves round the heavens, but a repe- 

 tition of such periods. For instance, 

 when Mercury has just repeated the 

 circuit of the earth's visible heaven, he 

 is not in the same position with respect 

 to the sun as at the beginning. The 

 cycle of relative positions is not com- 

 pleted, for this planet, in less than thir- 

 teen years and three days, or two days 

 if there be four leap years in the period. 

 So, in a watch, the cycle of relative posi- 

 tions of the minute and the hour hand is 

 not repeated every hour, but in every 

 twelve hours. 



PLANIPE'NNES. One of the sec- 

 tions of Insects into which Latreille 

 divides the order Neuroptera. There 

 are four families: viz., the panorpidae, 

 the myrmeleonidae, the hemerobiidae, 

 and the perliriae. 



PLA'NISPHERE. An old term for 

 any representation of the sphere upon a 

 plane. It now denotes any contrivance 

 by which plane surfaces moving on one 

 another fulfil any of the uses of a ce- 

 lestial globe. 



PLANTI'GRADA {planta, the sole of 

 the foot, gradior, to walk). A division 

 of the terrestrial Carnivorous animals, 

 which, in walking, apply the entire sole 

 of the foot to the ground, as far back as 

 the end of the os calcis ; as the bear, the 

 badger, &c. See Digitigrada. 



PLASMA. A scarce green semitrans- 

 parent chalcedony, of a dark tint, sup- 

 posed to be coloured by chlorite. 



PLASTER OF PARIS. Sulphate of 

 lime, or gypsum, heated in an oven till 

 nearly anhydrous, and then reduced to 

 powder. On adding water, an artificial 

 hydrate, or stucco, is formed, which sets 

 in a short time, and has the same com- 

 position as native gypsum. 



PLASTIC CLAY (7rX«<r<rco, to fashion). 

 One of the beds of the Eocene Tertiary 

 Period, named from its being used in 

 the manufacture of pottery. The term 

 is applied to a series of beds, chiefly 

 sands, with which the clay is associated. 



