the law, and so named because that event took place on the fiftieth day after the escape from Egypt.

PENUMBRA, in astronomy, a partial shade observed between the perfect shadow and the full light in an eclipse, and arising from the magnitude of the sun. If that body were a mere point, the whole shadow would be perfect; but by reason of its diameter, it happens that a body which is not illuminated by the whole, still receives a part of its rays.

PEPPER, in natural history and commerce, an aromatic, dry, and hot berry. Black-pepper, the fruit of a shrub of the creeping-kind, growing in several parts of the East Indies. The berries are produced in clusters, and change, as they ripen, from a green colour to a red, and afterward to a black. White pepper differs from the black only in being stripped of its corticle or covering. To strip them, the black berries are steeped in seawater, and after they have been exposed to the sun for several days, the chaff is rubbed off with the hands. In this operation, the pepper loses much of its original warmth. Some persons assert, that there is also a species of pepper naturally white.

PERAMBULATOR, in mechanics, an instrument used by surveyors in measuring distances, and also known by the names pedometer, way-wiser, and surveying-wheel. It consists of a wheel, two feet seven inches and a half in diameter, and consequently, half a pole, or eight feet three inches in circumference; a carriage or handle; and a dial plate, marked with miles and subdivisions, with a

hand which, by means of the machinery, points out the progress of the wheel. The proper office of this instrument is that of measuring roads and large distances, where expedition and moderate accuracy are required.

PERCEPTION, in logic, the first act of the mind, which consists in the reception of ideas through the medium of the senses. The first objects that strike our senses, says the author of the Analyse de l'Homme, give us our first ideas'; and our wants are the cause of our attention: the repetition of these ideas, and the developement of new wants, give birth to our sentiments and thoughts: it is thus that nature creates our souls. The eyes convey the ideas of colours, the ears those of sounds, the nostrils those of odours, and the palate those of savours: these ideas have no connection with each other; they are separate ideas of different qualities of bodies; but the sense of touching unites the whole in one object, which may happen to be at the same time coloured, odorous, savoury, and

sonorous.

PERCUSSION, in mechanics, the impression a body makes in falling or striking upon another: or the shock of two bodies in motion. Percussion is 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.

PERENNIAL, a term applied to those plants whose roots abide many years, whether they retain their leaves in 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 number, is that, all whose aliquot parts added together, make the same number with the number of which they are such parts. Thus 6 is a perfect number being equal to 1+2+3. So also is 28 being equal to 1+2+4+7+14. The following are given as the first six perfect numbers: 6; 28; 496; 8128; 33550336; 8589869056.

PERIHELIUM, in astronomy, that point of a planet or comet's orbit, wherein it is at its least distance from the sun, in which sense it stands opposed to aphelium.

PERIOD, in astronomy, the time taken up by a star or planet in making a revolution round the

sun.

PERIOD, in chronology, a revolution of a certain number of years, and another word for a cycle; a century is a period or cycle.

PERIOD, in rhetoric, a speech or series of words, complete within itself. All the qualities which compose the excellence of a period may be arranged under the two heads of correctness and beauty. A period must contain one subject of discourse, the whole of that subject, and nothing extraneous.

In writing, the period and its parts, which in speech are marked by the inflections of the voice, are to be distinguished by certain characters appropriated to each. These characters are called points, and the act of using them pointing, or punctuation; an art the principles of which yield in importance to none which belong to literature, and yet appear to be little understood. The general idea respecting them is, that they are to be

placed according to the taste, that is the arbitrary choice of the writer; and particularly in such a manner as shall render the enunciation agreeable to his ear. This view of the subject, however, is

so far from being just, that, in reality, the points have nothing to do with the ear. The sense of which the ear is the organ, is to be satisfied by the harmony of the style, not by mere punctuation.

PERIOECI, in geography, such inhabitants of the earth, as have the same latitudes, but who live in opposite longitudes; or live under the same pa. rallel, and the same meridian, but in different semicircles of that meridian. These have the same common seasons throughout the year, but when it is noon-day with one, it is midnight with the other.

PERIOSTEUM, in anatomy, a nervous, vasculous membrane, endued with a very quick sense, immediately surrounding, in every part, both the internal and external surfaces of all the bones of the body, excepting only so much of the teeth as stand above the gums, and the peculiar places on the bones in which the muscles are inserted: it is hence divided into the external and internal periosteum ; and where it externally surrounds the bones of the skull, it is usually called the pericranium. The seeming sensibility of the bones is that of this membrane.

PERIPATETIC philosophy. See PHILOSOPHY, peripatetic.

PERISCII, in geography, the inhabitants of either frigid zone between the polar circles and the poles, where the sun, when in the summer signs, appears to move only round them, without setting, and consequently their shadows, in the same day, turn to all the points of the horizon.

PERISTALTIC motion, a vermicular spontaneous motion of the intestines performed by the contraction of the circular and longitudinal fibres of which the fleshy coats of the intestines is composed, by means of which the chyle is driven into the orifices of the lacteal veins, and the fœces driven forwards.

PERJURY, in law, the crime of swearing falsely, where a lawful oath is administered by one in authority, in a matter relating to the issue or cause in question, whether it be a person's own wilful act, or one committed at the subornation of another.

PERORATION, in rhetoric, the epilogue, or last part of an oration, wherein what the orator insisted on through his whole discourse is urged afresh with greater vehemence. The peroration consists of two parts: a rapid recapitulation of what has been said before; and a declamatory address to the pas sions. The qualities requisite in the peroration are, animation and brevity.

PERPENDICULAR, in geometry, a line falling directly on another line, so as to make the angles on each side equal.

PERPETUAL Screw, is one that is acted upon by the teeth of a wheel, and which continues its action for an indefinite length of time, or so long as the teeth of the wheel continue to act upon it.

PERPETUITY, is the number of years in which the simple interest of any principal sum will amount to the same as the principal itself: or it is the number of years purchase to be given for an annuity which is to continue for ever; and it is found by dividing 1007. by the rate of interest