PLA 
with a single eye : C with two eyes; D with 
three eyes : E with four eyes : and F with 
numerous eyes. Of the first division we 
may notice, P. quadrangularis ; body pale, 
ovate, very sliarp-pointed before, and 
winged with small curled longitudinal mem- 
branes. It is found in ditches among diick- 
wetd ; very soft, pellucid, of a changeable 
form, and moves like a slug, leaving a slime 
on the bodies it passes over ; when it meets 
another animal it draws itself in like a 
snail. 
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 an- 
other, so a plain surface is the shortest ex- 
tension from one line to another. In astro- 
nomy. conics, &c. the term plane, is fre- 
quently nsed for' an imaginary surface, sup- 
posed to cut and pass through solid bodies ; 
and on this foundation, is the whole doctrine 
of conic sections built. See Conic sec- 
tions. 
In perspective, we meet with the perspec- 
tive plane, which is supposed to be i)el!ucid, 
and perpendicular to the horizon ; the hori- 
zontal 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 repre- 
sented is supposed to be placed, '&c. See 
Perspective. 
The plane of projection, in the stereo- 
graphic projection of the sphere, is that on 
which the projection is made ; correspond- 
ing to the perspective plane. 
Plane, in joinery, an edged tool, or in- 
strument for paring and shaving 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 chissel, placed ob- 
liquely, passes, this being very sharp, takes 
off the inequalities 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 
edse Of its iron or chissel is not ground 
straight, but rises with a convex arch in the 
middle ; its use is to take off the greater ir- 
regnlarities of the stuff, and to prepare it for 
the smoothing-plane. 2. The smoothing- 
plane is short and small, its chissel being 
finer : its use is to take otF the greater irre- 
gularities left by the fore-plane, and to pre- 
pare the wood for the jointer. 5. The 
iointer is tlie longest of all ; its edge is very 
PLA 
fine, and does not stand out above an hair’s 
breadth; it is chiefly used for shooting tlse 
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 upper 
edge of a board, straight or square, dowm 
into the slntf, so that the edge of another 
cut after the same manner, may join in witli 
it, on the square ; it is also used in striking 
facias on monlclings ; the iron or chissel of 
this plane is as broad as its stock, that the 
angle may cut straight, and it delivers its 
sliavings at the sides, and not at tlie top, 
like the others. 6. Tiie plough, which is a 
narrow-rabbit plane, with the addition of 
two staves, on which are shoulders : its use 
is to plough a narrow square groove on the 
edge of a board. 7. Moulding-plane, which 
are of various kinds, accommodated to the 
various forms and profiles of the moulding ; 
as the round-plane, the hollow-plane, ilie 
ogee, the snipe’s bill, &c. which are all of 
several sizes from half an inch, to an inch 
and a half. 
PLANE free. See Platanus. 
PLANET, a celestial body, revolvin" 
round the Sun as a centre, and continually 
changing its position, with respect to the 
fixed stars ; whence the name planet, which 
is a Greek tvord signifying wanderer. 
The planets are usually distinguished into 
primary and secondary. Tlie primary ones, 
called, by way of eminence, planets, are 
those which revolve round the Sun as a cen- 
tre ; and the secondary planets, more usually 
called satellites, or moons, are those which 
revolve round a primary planet as a centre, 
and constantly attend it in its revolution 
round the Sun. See Astronomy. 
The primary planets are again distinguish- 
ed into superior and inferior. The superior 
planets are those further from the Sun 
than our Earth ; as Mars, Jupiter, Saturn, 
and the Ilerscliel: and the inferior planets 
are those nearer the Sun than our Eartli ■ 
as Venus and Mercury : for the astronomy’ 
and other peculiarities, of which, see Ju- 
piter, Mars, &c. 
Planets, nature of the. That the planets 
are opaque bodies, like our Earth, appears 
evident for the following reasons : l. Since 
in Venus, Mercury, and Mars, only that 
part of the disc illuminated by the Sun, is 
found to shine ; and, again. Verms and Mer- 
cury, when between the Earth and the Sun, 
appear like dark spots, or macnlse, on the 
Sun’s disc ; it is evident that Mars, Venus, 
