PLANE. 90 level, llaf. plane, plain). A tool useil for roniler- ing the .surface of vood smooth ami level. It consists of an oblong bloek of wood or metal with an opening through the centre; this open- ing is .sipiare on the u|)pcr side, and is always large enough to admit the cutting instrument: it diminishes down to a mere slit on the under side, just wide enough to allow the cutting edge of the plane-iron and the sliaving of wood which it cuts off to pass througli. The essential part of the tool is the idaiie-iron, a piece of steel with a chisel-shaped edge, and a slot in its centre for a large-headed screw to work and to attach to it a strengthening plate. In the older wooden planes it was held in place by the liard-wood wedge. By driving in the wedge, the irons are held very firmly in their iilace. and they are so adjusted that only the line sliarp chisel-edge of the cutting-to(d ])rojccts tlinnigh the slit in the bottom of the bdciy of tlic plane, so that when the tool is pushed forward by the force of the hand the cutting edge pares otf all irregularities, until the wood is as smooth as the under surface of the plane. In the iron jdani-s now used the iron is held in the bloek and adjusted by a screw. There are many modifications in this tool, which can have its cutting edge and under surface made to almost any contour, so that moldings of all kinds may be made. The two commonest are the jack-plane, for rough work, and the smoothing- plaiie, for finishing otV plane surfaces. For iihin- iiig iiiacliiiics, see !Met.l Working Machinery ami WoDIl-WoiiKl.NO MACHINERY. PLANE CUKVE. See CiRVE. PLANE OF POLARIZATION. See Light. PLANER TREE (named in honor of .J. S. Planer, a (icrman botanist of the eighteenth cen- tury). A moderately sized elm-like tree (Pla^ neni tuiudticii), found on swampy lands in the Southern United States. It supplies a useful hard timber. Zelkova acuminata and Zclkova crenata, tall, handsome trees which furnish ex- cellent timber, were formi'rly included in this genus, which is now consiilcred monotypic. PLANE SAILING. See Sailings. PLANE-TABLE. An instrument used to fill in the details of topographical work (see Sur- veying), and. on account of the ease and rapidity with which it can be o|icratcd. popular where only a moderate approach to accuracy is re- quired. The plane-table consists essentially of a mounted drawing-l)(iard bearing an alidacje or ruler suniiouiited by a telescope whose line of sight is parallel in azimuth to the ruler. In orienting the fable a declinator, or compass box, and a plumbing arm serve to place a given point on the table over that on the ground which it represents, and to cause a line of the drawing on a paper mounted on the board to lie in or jiarallel to the vertical plane through its counter- part on the grouml. After the paper has been attached to the table it is then leveled and ori- ented as already described. The observer then sights successively at other difTeient objects in the area under surey, rev(dving the tele- scope and ruler in azimuth and drawing corre- sponding lines along the edge of the ruler at each position. All of these lines pass through the given jioint and give the angular direction of the diflVri'iit objects in the field. The plane- table is then taken to a second station located at some place previously sighted and whose dis- PLANETOIDS. tance from the first can be measured bv stadia, chain, or tape, and is there set up and oriented as before. The distance from the first station to the second is .set off at the desired scale on the line connecting the two points, and the telescope is directed at the same objects as were sighted at from the first station, as well as PLANE-TABLE. others not visible from the first point, and new lines are ruled. The instrument is then taken to other jioints which are given bv the intersection of the various lines ruled on the paper, and the positions of the various objects in the area under survey are determined. In this w.ay it is possible to fill in the detail work of a topographical sur- vey with great rapidity and sutlicient exactness for most map work. See Surveying. PLAN'ETA'RIXJM (Xeo-Lat., from Lat. pUt- tichi, from Gk, TrXaKiJrijs, plani'tfs. wanderer, wan- ilering star, planet, iroimrap'^s,pl(int's, wanderer, jilanet, from 7rXd«), phinc, a wandering; connect- ed with Litli. plonas, thin. Lat. plamts, level, flat, plane, plain). A machine much employed by astronomers in the seventeenth and eighteenth centuries, and first constructed by lluygens and Riinier, for the purpose of exhibiting clearly the motion of the heavenly bodies in conformity with the Coperniean doctrine. The planetarium exhibited only the orbital motions of the planets about the sun. either in circles or ellipses, and with constant or varying motions, according to the perfection of the machine. It was later im- proved by the addition of a combination of the tellurian and the lunarian, which exhibited both the motion of the moon about the earth and the earth round the sun. with the princi])al ]ihe- noniena (such as the succession of day and night, the varying length of each, eclipses, and the mo- tion of the moon's apogee and nodes) which ac- company these motions. A .satellite machine was also inventeil to illustrate the motions of .Jupi- ter's satellites. All these machines were later combined in the orrery (q.v. ), which exhiliits in the best manner possible the varied motions and phenomena of the bodies in the solar S3'stem ; but this whole class of instruments have ceased to possess practical usefulness, and have been rele- gated to astronomical museums. PLANETOIDS (from Gk. TrXa»i}r7)5, planHes, wanderer, wandering star, planet --|- eT5o$, eidos, form), or Asteroid.s. A numerous group I I i
