Peridotntes

3674

Peristalsis

mous Aspasia. Pericles was a man of the
highest principle, integrity, nobility, and dig-
nity.

Peridotites, a group of crystalline igneous
rocks, of which olivine is an abundant in-
gredient.

Perigueux, chief tn. of French dep. Dor-
dogne. The old town, with many Renaissance
houses, contains the remarkable cathedral of
St. Front in the Byzantine style (984-1047).
Perigueux was the Vesunna of the Romans,
and has a large amphitheatre and the circular
tower of Vesone, 89 ft. high. It is famous
for its pdtds de joie gras and truffled par-
tridges; p. 33,144-

Perihelion, the point of its orbit at which
a planet or comet makes its nearest approach
to the sun.

Perihelion,

Perim, isL in Strait of Babel-Mandcb, at s.
entrance of Red Sea. It is sYz m. long and
iJ4 ni- wide. The British took possession in
1857. It is a telegraph and coaling-station.

Perimeter, in any figure, the sum of the
lengths of the bounding lines. See RECTIFICA-
TION'.

Perineum, the soft external floor of the
pelvis, plays an important part in childbirth,
and is frequently ruptured in primiparous pa-
tients. In the male the perinaeum derives its
importance chiefly from the various forms of
permeal lithotomy.

Period and Periodicity. The most famil-
iar example of a periodic process is the alter-
nation of day and night, brought about by
the rotation of the earth upon its axis in
presence of the sun. Perhaps the simplest type
of periodicity is the oscillation of a pendulum
or the vibration of a tuning-fork. Periodicity
is one of the most widely spread phenomena
in nature. All kinds of wave motion are peri-
odic; and there seems to bo little doubt that
the molecules of matter are all capable of
vibrating in definite periods. In the organic
world many of the ordinary vital processes
are periodic—such as the pulsations of the
heart and the arteries. From the dynamical
point of view periodicity means stability, and

instability is associated w4th motion or ten-
dency to motion which has no periodic char-
acter.

Periodicals. See Magazines.

Periodic Law, in chemistry. About iS6&
Newlands, Lothar Meyer, and Mendelceff dis-
covered that if the elements are arranged in
the order of their atomic weighty those ot
similar properties arc separated by regular in-
tervals—a fact summarized by Mendelecff as
follows: The properties of the elements are a
periodic function of the atomic weight.

Periosteum. See Bone.

Periostitis, inflammation of the perios-
teum, the tough, fibrous membrane which in-
vests the bones. Its chief causes art' a syphi-
litic taint, rheumatism, and tuberculosis, but
its occurrence is often due to injury of the
part.

Peripatetics, a philosophical school found-
ed by Aristotle. The name is supposedly de-
rived either from Aristotle's custom of walk-
ing about (penpalein} during the delivery of
his lectures, or from the place in which they
were delivered. Sec ARISTOTLE.

Periscope, an instrument by means of
which an observer may view his entire sur-
roundings through a fixed eyepiece. It is of
especial value in submarine warfare as by its
use observations may be made without com-
ing to the surface, only the top of the instru-
ment projecting above the water. Many mod-
ifications exist, but the essential features are
the same. They include a heavy steel tube
which, when not in use, can be lowered into
the hull of the boat; a. series of prisms and
lenses, and an eyepiece. The rays of light
enter the periscope horizontally, arc reflected
by the first prism into a second prism, from
which they pass through an object glass, and
thence through a third prism to the eyepiece.
The first prism inverts the imago, but this
effect is counteracted by the second prism.
The image is again inverted by the object
lens and again restored by tho third prism.
Other applications of the periscope arc to
gun sights, and to trench warfare, the field
or trench periscope being used behind earth-
works and parapets for obtaining, unob-
served, a view of the surrounding terrain.

Perissodactyla, the odd-toed ungulates,
as the horse and rhinoceros, in which the
third toe is larger than the others, and is
symmetrical upon itself.

Peristalsis, the wormttke movement of the
intestine, which presses forward the food by
muscular contraction behind it. See INTE&'-

TINES.