PLA 



I'LA 



the stones, plants, &c. in the water, and 

 never ventures out of it. Length about 

 three-tenths of an inch. 



PLANE, in geometry, denotes a plain 

 surface, or one that lies evenly between 

 its bounding lines : and as a right line is 

 the shortest extension from one point to 

 another, so a plain surface is the shortest 

 extension from one line to another. In 

 astronomy, conies, &c. the term plane, is 

 frequently used for an imaginary sur- 

 face, supposed to cut and pass through 

 solid bodies ; and on this foundation is 

 the whole doctrine of conic sections built. 

 See CONIC sections. 



In perspective, we meet with the per- 

 spective plane, which is supposed to be 

 pellucid, and perpendicular to the hori- 

 zon ; the horizontal plane, supposed to 

 pass through the spectator's eye, parallel 

 to the horizon ; the geometrical plane, 

 likewise parallel to the horizon, whereon 

 the object to be represented is sup- 

 posed to be placed, &c. See PERSPEC- 

 TIVE. 



The plane of projection, in the stereo- 

 graphic projection of the sphere, is that 

 on which the projection is made ; cor- 

 responding to the perspective plane. 



PLANE, in joinery, an edged tool, or 

 instrument for paring and shaving 1 of 

 wood smooth. It consists of a piece of 

 wood, very smooth at bottom, as a stock 

 or shaft ; in the middle of which is an 

 aperture, through which a steel-edge, or 

 chisel, placed obliquely, passes ; this 

 being very sharp, takes off the inequa- 

 lities of the wood it is slid along. Planes 

 have various names, according to their 

 various forms, sizes, and uses : as, 1. The 

 fore-plane, which is a very long one, and 

 is usually that which is first used .- the 

 edge of its iron or chisel is not ground 

 straight, but rises with a convex arch in 

 the middle ; its use is to take off the 

 greater irregularities of the stuff, and to 

 prepare it for the smoothing-plane. 2. 

 The smoothing-plane is short and small, 

 its chisel being finer : its use is to take 

 off the greater irregularities left by the 

 fore-plane, and to prepare the wood for 

 the jointer. 3. The jointer is the longest 

 of all ; its edge is very fine, and does not 

 stand out above an hair's breadth ; it is 

 chiefly used for shooting the edge of a 

 board perfectly straight, for jointing 

 tables, &c. 4. "The strike-block, which 

 is like the jointer, but shorter: its use 

 is to shoot short joints. 5. The rabbit- 

 plane, which is used in cutting the up- 

 per edge of a board, straight or square, 

 down into the stuff, so Uiat the edge 



of another cut after the same manner, 

 may join in with it, on the square; it 

 is also used in striking facias on mould- 

 ings; the iron or chisel of this plane 

 is as broad as its stock, that the angle 

 may cut straight, and it delivers its 

 shavings at the sides, and not at the 

 top, like the others. 6. The plough, 

 which is a narrow-rabbit plane, with 

 the addition of two staves, on which are 

 shoulders : its use is to plough a nar- 

 row square groove on the edge of a 

 board. 7. Moulding-planes, which are 

 of various kinds, accommodated to the 

 various forms and profiles of the mould- 

 ing ; as the round-plane, the hollow- 

 plane, the ogee, the snipe's bill, &c. 

 which are all of several sizes, from ha] 

 an inch to an inch and a half. 



PLANE tree. See PI.ATANUS. 



PLANET, a celestial body, revolving 

 round the Sun as a centre, and continu- 

 ally changing its position, with respect ! 

 to the fixed stars ; whence the name 

 planet, which is a Greek word signifying 

 wanderer. 



The planets are usually distinguished 

 into primary and secondary. The prima- 

 ry ones, called by way of eminencei 

 planets, are those which revolve round 

 the Sun as a centre ; and the secondary 

 planets, more usually called satellites, or 

 moons, are those which revolve round a 

 primary planet as a centre, and constant- 

 ly attend it in its revolution round the 

 Sun. See ASTROKOMT. 



The primary planets are again distin- 

 guished into superior and inferior. 

 The superior planets are those further 

 from the Sun than our Earth ; as Mars, 

 Jupiter, Saturn, and the Herschel : and 

 the inferior planets are those nearer the 

 Sun than our Earth ; as Venus and Mer- 

 cury : for the astronomy, and other pecu- 

 liarities, of which, see JUPITER, MARS, 

 &c. 



PJLAKETS, nature of the. That the plan- 

 ets are opaque bodies, like our. Earth, ap- 

 pears evident for the following reasons : 

 1. Since in Venus, Mercury, and Mars, 

 only that part of the disc illuminated by 

 the Sun is found to shine ; and again, 

 Venus and Mercury, when between the 

 Earth and the Sun, appear like dark 

 spots, or maculae, on the Sun's disc ; it 

 is evident that Mars, Venus, and Mercu- 

 ry, are opaque bodies, illuminated with 

 the borrowed light of the Sun. And the 

 same appears of Jupiter, frofn its being 

 void of light in that part to which the 

 shadow of the satellites reaches, as well 

 as in that part turned from the Sun ; and 



