P L A 
PLA 
P L A 
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PITTOSPORUM, in botany, a genus of 
the monogynia order, in the pentandria class 
of plants. The calyx is pentaphyllous, in- 
ferior, and deciduous. The petals are five 
in number; the style thread-shaped; the 
capsule somewhat angular, trilocular, and 
containing three or four angulated seeds, 
-adhering to the capsule by means of a liquid 
resin in the loculaments. Of this there are 
three species, viz. 1. Tenuifolium. 2. Um- 
bellatum. 3. Coriaceum. The first and 
second are natives of the Cape of Good 
Hope; the third grows in Madeira, and 
flowers in May and June. 
PITUITARY Gland. See Anatomy. 
PLACENTA. See Anatomy and Mid- 
wifery. 
PLAGTANTHUS, a genus of the class 
and order monadelphia dodecandria. The 
calyx is five-cleft; petals five; berry. 
There is one species, a native of New Zea- 
land. 
PLAGUE, Pestilence, or Pestilen- 
tial fever. See Medicine. 
Plague-water, one of the compound 
waters of the shops, distilled from mint, rose- 
mary, angelica-roots, &c. 
PLAIN, in general, an appellation given 
to whatever is smooth and even, or simple, 
obvious, and easy to be understood; and, 
consequently, stands opposed to rough, en- 
riched, or laboured. 
A plain figure, in geometry, is an uniform 
surface ; from every point of whose peri- 
meter, right lines may be drawn to every 
other point in the same. 
A plain angle is one contained under the 
two lines or surfaces, in contradistinction to 
a solid angle. The doctrine of plain tri- 
angles, as those included under three right 
lines, is termed plain trigonometry. See 
Trigonometry. 
Plain chart. See Chart. 
Pl^in sailing. See Navigation. 
Plain scale. See Scale. 
Plain table, in surveying, a very simple 
instrument, whereby the draught of a field 
is taken on the spot, without any future pro- 
traction. It is generally of an oblong rect- 
angular figure, and supported by a fulcrum, 
■so as to turn every way by means of a ball 
and socket. It has a moveable frame which 
serves to hold fast a clean paper; and the 
sides of this frame, facing the paper, are di- 
vided into equal parts every way. It has 
also a box with a magnetical needle, and a 
large index with two sights; and, lastly, on 
the edge of the frame, are marked degrees 
and minutes. See Surveying. 
Plain number, is a number that may he 
produced by the multiplication of two num- 
bers into one another; thus 20 is a plain 
number produced by the multiplication of 
5 into 4. 
Plain place, locus planus, or locus ad 
planum, among the antient geometricians, 
denoted a geometrical locus, when it was a 
right line or a circle, in opposition to a solid 
place, which was an ellipsis, parabola, or hy- 
perbola. These the moderns distinguished 
into loci ad rectum, and loci ad circuium. 
Plain problem, in mathematics, is such 
a problem as cannot be solved geometri- 
cally, but by the intersection either of aright 
line and a circle, or of the circumferences of 
•two circles ; as, given the greatest side, and i 
VOL. II. 
j the sum of the other two sides of a right- 
I angle. 1 triangle, to find the triang’e, as also 
to describe a trapezium that shall make a 
given area of four given lines. Such pro- 
blems can only have two solutions, in regard 
a right line can only cut a circle, or one cir- 
cle cut another, in "two points. 
PLAISTER of Paris. See Sulfhat of 
LIME. 
PLAN, in general, denotes the represen- 
tation of something drawn on a plane; such 
are maps, charts, ichnpgraphies, & c. See 
Map, Chart, &c. 
1 he term plan, however, is particularly 
used for a draught of a building, such as it 
appears, or is intended to appear, on the 
ground; shewing the extent, division, and 
distribution of its area, or ground-plot, into 
apartments, rooms, passages, &c. See Ar- 
chitecture. 
PLAN ARIA, a genus of vermes intestina ; 
the generic character is, body gelatinous, 
fiattish, with a double ventral pore, mouth 
terminal; this genus contains many species, 
and is divided into sections. 
The section A is without eyes ; planaria 
gulo, body long, pellucid, and truncate be- 
fore ; inhabits stagnant water, under duck- 
weed ; body grey ; the margin all round is 
tesseiate with very fine stria; : it swallows the 
cyclidia which inhabit the same waters, and 
after a time discharges them again. Planaria 
operculata inhabits the sandy shores, and 
among fuci in the bays of Norway; something 
resembles a coffee berry ; and moves by 
bending its margin, and by means of its mar- 
ginal folds fixes itself to and ascends other 
bodies. The section B, with a single eye : 
G with two eyes; D with three eyes, and 
E with four eyes. 
PLANE. See Geometry. 
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 chisel, placed obliquely, 
passes, which 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 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 smooth- 
ing-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 a 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 .upper edge of 
a board, straight or square, down into the 
stuff, so that 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 
mouldings ; 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. 
3 K 
0. The plough, which is a narrow rabbit- 
plane, with the addition of two staves, on 
which are shoulders; its use is to plow a 
narrow 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 moulding ; as the round- 
plane, the hollow-plane, the ogee, the snipe’s 
bill, &c. which are all of several sizes, from 
half an inch to an inch and a half. 
PLANET. See Astronomy. 
PLANETARIUM, an astronomical ma- 
chine, made to represent the motions of the 
plant ts, and their satellites, as they really are 
in nature. We have in some degree ex- 
plained the theory of the planetarium, under 
the article. Orrery; we shall now describe 
a perspective view of the machine itself, and 
shew the nature and structure of the wheels 
by which the motion of the whole is produ- 
ced. See Plate Planetarium. 
In the planetarium exh bited in the plate, 
A represents the Sun, which is fixed firmly 
to a wire a, and has no motion ; B is the 
planet Mercury, revolving round the Sun; 
1) is the planet Venus; E represents the 
Earth, and e the Moon revolving round it ; 
f is a segment of brass called the Earth’s 
terminator, which shews that all the parts of 
the Earth behind it are not illuminated by r 
the Sun ; F is the planet Mars ; G , Jupiter and 
his four satellites ; H, Saturn, with his Ring 
and seven satellites; Iv the Hcrschel, and 
six satellites. L is a small winch, which 
when turned gives motion to Mercury and 
\ enus, and shews the Earth’s annual motion 
round the Sun, its diurnal motion, and the 
Moon’s motion round the Earth. I'he pro- 
jection in the middle of the circular board 
M, consists of the following parts : a steel 
wire a, whose lower end is screwed to a 
bridge under the board, and which carries 
the Sun; over this is put a tube, on whose 
lower end a worm-wheel, worked by a worm 
on the arbor of the winch L above-mention 
ed, is fixed; and to the upper end the frame- 
of wheels N, with the Earth and Moon. 
Over these is a conical tube, which has a 
flaunch at its lower end, and is fastened to 
the board M by three screws ; the arms car- 
rying the planets Mars F, Jupiter G, Saturn. 
H, and the Iierschel K, are fitted stiffly upon 
this tube, so as not to turn unless they are 
moved. These planets do not move by- 
turning the winch, but are to be set by hand ; 
as also their satellites. In the frame of 
wheels N (figs. 1 and 2) g is the first wheel ; 
which is fixed to the wire a (fig. 1), and is 
without any motion : this works into another 
wheel h of the same size, fixed to the spindle 
i. The wheel h works another wheel k of 
the same size, on whose spindle y (fig. 1) the; 
Earth is fixed. Besides the wheel h, the 
spindle i has three other wheels l m n, fixed 
on it. The wheel l turns o, which works 
a pinion beneath the wheel g, carrying the 
planet Mercury B (fig. 1) : this pinion has a 
hollow spindle, and goes over the wire cl 
T he wheel m on the spindle i works into p ; 
which gives motion to the pinion g (whose 
spindle goes over the spindle of ’the pinion 
which carries Mercury), and has the planet 
Venus (D fig. 1) fixed to it. The large wheel 
n on the spindle i turns, by intermediate 
wheels, the pinion S, whose arbor goes over 
the spindle carrying the Earth; this has ati 
arm for the Moon fixed to it. The wire t, 
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