PER 
n)eter of surfaces or figures are lines, tliose 
of bodies are surfaces. In circular figures, 
instead of perimeter, we say circumference, 
or periphery. 
PERINdEUM, or Perineum. See Ana- 
tomy. 
PERIOD, in astronomy, the time taken 
up by a star or planet in making a revolu- 
tion round tire sun ; or the duration ot its 
course till it return to the same point of its 
orbit. See Astronomy. There is a won- 
derful harmony between the distances of 
the planets from the sun, and their periods 
round him ; the great law whereof is, that 
the squares of the periodical times of the 
primary planets, are to each other as the 
cubes of their distances from the. sun ; and 
likewise, the squares of the periodical times 
of the secondaries of any planet, are to each 
other as the cubes of their distances from 
that primary. This harmony among the 
planets is one of the greatest confirmations 
of the Copernican hypothesis. 
Period, in chronology, denotes a revo- 
lution of a certain number of years, or a 
series of years, whereby, in different na- 
tions, and on different occasions, time is 
measured ; such are the following. 
Period, Calippic, a system of seventy- 
six years. The calippic period compre- 
hends 48 common years, and 28 intercalary 
ones, 940 lunations, and 22,759 days. See 
Chronology. 
Period, Dionyaian, or Victorian Period, 
a system of 532 lunae solar and Julian years, 
which being elapsed, the characteis of the 
moon fall again upon the same day and 
feria, and revolve in the same order, accord- 
ing to the opinion of the ancients. This 
period is otherwise called the great paschal 
cycle, because the Christian church first 
used it, to find the true time of the pascha, 
or easter. The sum of these years arise by 
multiplying together the cycles of the sun 
and moon. See Easter. 
Period, Hipparchus's, a system of 304 
years, both lunar and solar, which being 
elapsed, Hipparchus thought that the reckon- 
ing by the lunar motion wpuld coincide 
again with the solar measures. This period 
comprehends 3760 lunar months, or lit ,039 
days ; the sum of which arises from the miil- 
tipUc’ation of the calippic period by 4, sub- 
tr; cting unity from the product. 
Period, in grammar, denotes a small 
compass of discourse, containing a perfect 
sentence, and distinguished at the end by a 
point, or full stop, thus (.) ; and its mem- 
PER 
bers or divisions marked by commas, colons, 
die. 
Period is also used for the character 
(.) wherewith the periods of discourse are 
terminated, or expressed, Veing commonly 
called a full stop or point. See Punctua- 
tion. 
Period, in numbers, a distinction made 
by a point, or comma, after every sixth 
place or figure ; and is used in numeration 
for the readier distinguishing and naming 
the several figures or places, which see un- 
der Arithmetic. 
PERIOECI, in geography, such inhabi- 
tants of the earth as have the same latitudes, 
but opposite longitudes; or live under the 
same parallel and the same meridian, but 
in different semicircles of that meridian, or 
in opposite points of the parallel. These 
have the same common seasons throughout 
the year, and the same phenomena of the 
heavenly bodies ; but w’hen it is noon-day 
with the one it is midnight with the other, 
there being twelve hours between them in 
an east or west direction. These are found 
on the globe, by the hour-index, or by 
turning the globe half round, that is 180 
degrees either way. 
PERIOSTEUM, or Periostium, in 
anatomy, a nervous vasculous membrane, 
endued with a very quick sense, immedi- 
ately surrounding in every part both the 
internal and external surfaces of all the 
bones in the body, excepting only so much 
of the teeth as stand above the gums, and 
the peculiar places on the bones in which 
tlie muscles are inserted. 
PERIPATETIC philosophy, that system 
taught and established by Aristotle, and 
maintained by his followers, the Peripate- 
tics, called also Aristotelians. 
The philosophy of Aristotle may be di- 
vided into three distinct branches ; instru- 
mental, theoretical, and practical. Under 
the first head are included his doctrines 
concerning logic; under the second, his 
principles of physics, pneumatology, onto- 
logy, and mathematics ; and under the third, 
his system of ethics and policy. Upon all 
these we cannot enlarge ; but shall refer tp 
his doctrine concerning the human mind 
and animal life. 
Aristotle, having undertaken to teach a 
new system of philosophy, was desirous of 
receding as far as possible from former phi- 
losophers, and particularly from Plato ; and 
in treating upon any subject on which he 
had no new doctrine to offer, he gave old 
