PLANE 



6185 



PLANIMETER 



and by spiritualists as a medium 

 for communication with the un- 

 seen world. Two persons place 

 their fingers lightly on the plan- 

 chette, keeping their hands still 

 and their minds passive, and in a 

 few moments the instrument will 

 usually begin to move and often 

 trace words and sentences. See 

 Spiritualism. ' 



Plane (Lat. plamts). Word 

 meaning flat, level, or even. In 

 aeronautics, it is used as a contrac- 

 tion for aeroplane, and also for 

 the wing surfaces of an aeroplane. 

 In mathematics, plane geometry is 

 concerned with the geometry of 

 plane figures, i.e. figures which lie 

 wholly in surfaces determined by 

 any three points not in a straight 

 line. In joinery, a plane is a tool 

 used for smoothing the surfaces of 

 wood, or for cutting certain types 

 of grooves, etc. It is a chisel held 

 by a wooden or metal stock at 

 an angle to the surface being 

 smoothed. There are many varie- 

 ties, named according to their shape, 

 use, etc., e.g. jack plane, routing 

 plane, tonguing or grooving plane. 

 A plane is also a kind of trowel 

 used in various trades to smooth 

 or surface sand or clay. 



Plane (Platanus). Small genus 

 of large trees of the natural order 

 Platanaceae, natives of the N. 

 temperate regions. They attain a 

 height of 70 to 90 ft., with a trunk 

 circumference of 10-12 ft., which 

 is occasionally exceeded greatly. 

 The large alternate leaves are pal- 

 mately divided into five or seven 

 toothed and sharply pointed lobes, 

 and the dilated ba s e of the leaf- 

 stalk is hollow, fit- 

 ting over the resting- f 

 bud of next year's 

 shoot. 



The flowers are 

 simple, without 

 sepals or petals, and 

 the sexes separate ; 

 they are clustered 

 in spherical heads, 

 the male heads on 

 one branch, the 

 females on another. 

 The fruits are 

 closely packed in 

 spiky balls which 

 hang on long strings 

 through the winter 

 and disintegrate in 

 spring. A peculiar- 

 ity of the tree which 

 helps it to thrive in 

 soot-laden a t m o - 

 spheres is its habit 

 of throwing off the 

 outer layers of its 

 bark in large or 

 small thin flakes, 

 showing yellow 

 patches of newer 



bark. The Oriental plane (P. 

 orientalis) was introduced to Eng- 

 land from the Levant at some 

 date previous to 1548, and the 

 Western plane (P. occidentalis) 

 from Virginia about 1640. The 

 maple-leaved plane (P. acerifolia) 

 is believed to be a hybrid between 

 the two, originating in the Oxford 

 Botanic Garden about 1670. The 

 wood of P. orientalis is used for 

 carpentry and cabinet work, and 

 is susceptible of a high polish ; but 

 that of P. acerifolia is of less 

 value, being liable to warp. 



Planet (Gr. planetes, wanderer). 

 Name given to the chief of the 

 solid bodies that revolve around 

 the sun. Asteroids or planetoids 

 and meteors are not usually in- 

 cluded. The planets are Mercury, 

 Venus, Mars, Earth, Jupiter, 

 Saturn, Uranus, and Neptune. 



In the solar system the planets 

 are generally divided into two 

 groups, the inferior planets, those 

 between the earth and the sun, 

 and the superior planets, those 

 farther away from the sun than 

 the earth. All planets travel round 

 the sun in an anti-clockwise direc- 

 tion, looking down on the system, 

 and all orbits are elliptical. The 

 plane in which the earth revolves 

 is known as the plane of the eclip- 

 tic, and all the other planets re- 



Plane. Fine specimen o! this tree. Top, left, fruit 

 of London plane and, right, leaf 



volve in planes which only make 

 small angles with the ecliptic plane. 



The relative sizes and distances 

 of the planets have been clearly 

 drawn in analogy by Sir Norman 

 Lockyer. By this analogy the sun 

 is a globe 2 ft. in diameter, Mercury 

 a grain of mustard seed 164 ft. 

 away, Venus a pea in an orbit of 

 284 ft., the earth a pea 430 ft. dis- 

 tant, Mars a pin head 654 ft. 

 away, Jupiter an orange at a dis- 

 tance of half a mile, Saturn a 

 smaller orange four-fifths of a mile 

 away, Uranus a small plum 1 m. 

 away, and Neptune a larger plum at 

 2| m. The earth is the densest of 

 all planets, Saturn being only one- 

 eighth, and Neptune one-fifth as 

 dense. The density of the earth is 

 about 5 times that of water. 



In astronomical tables, almanacs, 

 etc., symbols are given to the 

 names of various planets and the 

 sun and moon for convenience of 

 tabulation, etc. These symbols 

 are: Mercury, ; Venus, : 

 Earth, ; Mars, <f ; Asteroids. 

 according to the order 

 of their discovery ; Jupiter, 2 ; 

 Saturn, ^ ; Uranus, $ ; Neptune, <J> 



There is considerable uncertainty 

 whether there are other planets 

 not yet discovered. The irregu- 

 larity of movements of Mercury 

 caused Leverrier to postulate an 

 inter-Mercurial planet, and the per- 

 turbations of Neptune have caused 

 many investigators to compute the 

 masses and distances of extra- 

 Neptune planets. These planets, 

 however, have not yet been 

 discovered. 



The planetismal hypothesis is 

 one which derives the origin of the 

 planets from the collision of two 

 stars, instead of from the conden- 

 sation of a disk-shaped nebula. The 

 hypothesis was suggested by T. C. 

 Chamberlain. See Asteroids ; Astro- 

 nomy; Cosmogony; Earth ; Jupiter ; 

 Nebula; Neptune ; Saturn; Solar 

 System ; Sun ; Zodiac. 



Planetarium. Machine de- 

 signed to exhibit the movements 

 of the planets and other bodies of 

 the solar system. It is practically 

 a working model, and was at one 

 time popular for the purpose of 

 demonstrating the relative move- 

 ments of the members of the 

 solar system. See Orrery. 



Planimeter (Lat. planum, level 

 ground ; Gr. metron, measure). 

 Instrument for measuring the 

 areas of plane figures. Used 

 practically only for measuring irre- 

 gular figures, the first planimeter 

 was invented in 1814 by Hermann, 

 a Bavarian engineer. The best known 

 modern planimeter is that known 

 as Amsler's. Its essential features 

 are two movable arms, A and B, 

 on the extremity of one of which 



