PER 
we receive by perception are often altered 
by the judgment, without our taking notice 
of it ; so that we take that for the percep- 
tion of our senses, which is but an idea 
formed by the judgment: thus a man who 
reads or hears, with attention, takes little 
notice of the characters or sounds, but of 
the ideas excited in him by them. 
The faculty of perception seems to be 
that which constitutes the distinction be- 
tween tire animal kingdom and the inferior 
parts of nature. Perception is also the 
first step towards knowledge, and the inlet 
of all the materials of it ; so that the fewer 
senses a man has, and the duller the im- 
pressions that are made by them are, the 
more remote he is from that knowledge 
which is to be found in other men. 
PERCH, or Pearch. See Perca. 
Perch, a measure of length,, equal to 
five yards and a half. See Measure. 
PERCUSSION, in mechanics, the im- 
pression a body makes in falling or striking 
upon another, or the shock of two bodies 
in motion. See Motion. 
Percussion is either direct or oblique ; 
direct, when the impulse is given in a line 
perpendicular to the point of contact; and 
oblique, when it is given in a line oblique 
to the point of contact. The ratio which 
an oblique stroke bears to a perpendicular 
one, is as the sine of the angle of incidence 
to the radius. Thus, let a h (Plate XII. 
Miscel. fig. 13) be the side of any body on 
which an oblique force falls, with the direc- 
tion d a ; draw d c at right angles to d 6, a 
perpendicular let fall from d to the body to 
be moved, and make ad the radius of a 
circle ; it is plain that the oblique force d a, 
by the laws of composition and resolution 
of motions, will be resolved into the two 
forces d c and.6 d ; of which d c, being pa- 
rallel to a b, hath no energy or force to 
move that body ; and, consequently, d b 
expresses all the power of the stroke or 
impulse on the body to be moved : but d b 
is the right sine of the angle of incidence 
dab-, wherefore the oblique force d a, to 
one falling perpendicularly, is as the sine of 
the angle of incidence to the radius. 
PERDICIUM, in botany,, a genus of the 
Syngenesia Polygamia Superflua class and 
order. Natural order of Compositas Discoi- 
deEe. Corymbiferse, Jussieu. Essential cha- 
racter : corollets bilabiate ; down simple ; 
receptacle naked. There are six species. 
PERENNIAL, in botany, is applied to 
those plants whose roots will abide many 
years, whether they retain their leaves in 
PER 
winter or not : those which retain their 
leaves are called ever-greens ; but such 
as cast their leaves, are called deciduous. 
Some of these have annual stalks, which 
die to the root every autumn, and shoot up 
again in the spring. 
PERFECT, in arithmetic. Perfect num- 
ber is, that all whose aliquot parts added 
together, make the same number with the 
number whereof they are such parts. Thus 
six is a perfect number, being equal 1 -j- 
2 -j-3 : so also is 28 = 1 -j- 2 -j- 4-}- 7 -^14. 
Mathematicians have been at considerable 
pains to investigate the perfect numbers, 
but with no great success, the following are 
given as the first six perfect numbers ; 
6 
28 
496 
8128 
33550336 
8589869056 
PERGULARIA, in botany, a genus of 
the Pentandria Digynia class and order. 
Natural order of Contortae. Apocineae, 
Jussieu. Essential character: contorted; 
nectary surrounding the gentials with five 
sagittated cusps; corolla salver-shaped. 
There are five species. 
PERIANTHIUM, in botany, the flower- 
cup, properly so called, the most common 
species of calyx, placed immediately under 
the flower, which is contained in it as a 
cup. The flower-cup differs in point, num- 
ber, figure, proportion, and situation. 
PERICARPIUM. See Botany. 
PERICRANIUM, in anatomy, a thick 
solid coat, or membrane, covering the out- 
side of the cranium or skull. 
PERIGEE, in astronomy, that point of 
I the sun’s or moon’s orbit, wherein they are 
at their least distance from the earth, in 
which sense it stands opposed to apogee. 
PERIHELIUM, in astronomy, that 
point of a planet’s or comet’s orbit, wherein 
it is in its least distance from the sun ; in 
which sense it stands in opposition to aphe- 
lium. 
PERILLAj in botany, a genus of the 
Didynamia Gymnospermia class and order. 
Natural order of Verticellatae. Labiatae, 
Jussieu. Essential character : calyx, upper- 
most segment very short ; stamens distant ; 
styles two, connected. There is but one 
species ; viz. P. ocymoides, an annual plant, 
and a native of the East Indies. 
PERIMETER, in geometry, the bounds 
or limits of any figure or body. The peri- 
M 2 ■ 
