Prism, in geometry, a body or solid whose two-thirds are any plane figures which are parallel, equal, and similar, and its sides parallograms

Prism, in optics, a triangular bar of glass, well known from the effect it produces on a ray of light transmitted through it: this effect is a decomposition of the light into its component emanations, consisting of the three primary colours and the secondary tints arising from their intermixture, which together form what is termed the solar spectrum. The lensic prism is a new optical glass, in which the powers of the lens and prism are combined.

Prison, an edifice, unfortunately mostly

of large dimensions, for the confinement of persons warring against society

Profile, the outline of a series of

mouldings, or of any other parts, as shown by a section through

them

Profile of an Order, in architecture, an assemblage and arrangement of essential and subservient parts. That profile is preferable wherein the parts are few, varied, and fitly applied. Some member should predominate in each division, which it should appear the office of the other parts to fortify, support, or shelter. In a cornice the corona is supported by modillions, dentils, ovolos, &c., and sheltered and covered from the effects of the weather by its cyma or cavetto. Projectile, in mechanics, a body put in motion by an external force Projectiles, in mechanics, that branch

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which considers the mass, velocity, range, &c. of a heavy body projected into void space by an external force, and then left to the free action of gravity

Projection, in geometry, drawing, &c., a plan or delineation; in chemistry, the crisis of an operation Projecture, in architecture, the outjutting or prominence which the moulding and members have beyond the plane of a wall or column Prolate, in geometry, an epithet ap

plied to a spheroid produced by the revolution of a semi-ellipsis about its long diameter Pronaos, the area immediately before a temple. The term is often used for the portico in front of a building. The posticus in one front corresponds to the pronaos in the other in some temples, the cella was entered through both. The generality of Grecian temples had two approaches. Proportion, in architecture, the magnitude of one part as compared with some other. The term ' 'proportion' is used absolutely in the sense of 'good proportion,' although every thing that has shape has proportions of some kind or other. The subject of proportion has been greatly mystified by writers, who have laid down certain fixed proportions as the best of all on every occasion, and as the ne plus ultra of artistic taste. But fixed proportions can be followed mechanically by every one alike; whereas it requires ability to deviate successfully from routine measurement, and apply the poco più or the poco meno as the particular occasion or the

particular effect aimed at may require at least, justify. It is the eye that takes cognizance of proportions; and the architect's own eye ought to be quite as correct as that of other people. Proportion, that branch of mathematical science which defines the ratio of numbers or quantities to each other Proportions of rooms should be suited

to the purposes for which they are used: all figures, from the square to one and a half the breadth of the room, may be employed for the plan. Some have extended the plan to a double square. Galleries may be from five to eight times their breadth. The height, if with flat ceilings, is not required to be so great as in those that are coved. The height of square apartments should not exceed five-sixths the side of the square, nor be less than four-fifths; but in rooms that are oblong, the height ought to be equal to the breadth. The height of square rooms that are coved should be equal to one of the sides of the square; but coved oblong rooms require a height equal to the breadth, added to one-fifth, onequarter, or, at most, one-third of the difference between their length and breadth. The height of galleries should be from one and threefifths, at most, to one and one-third, at least, of their breadth. Cornices and dressings in the interior of houses are always to be kept more delicate than those on the outside.

Propylæum, in Greek architecture,

the porch of a temple or great hall Propylaa: the entrance to a Greek temple, a sacred enclosure, consisted of a gateway flanked by buildings, whence the plural of the word. The Egyptian temples generally had magnificent propylæa, consisting of a pair of oblong truncated pyramids of solid masonry, the faces of which were sculptured with hieroglyphics. The word, however, is

generally used to signify the entrance to the acropolis of Athens, which was the last completed of the great works of architecture executed under the administration of Pericles. Pausanias relates that "there is only one entrance to the acropolis, it being in every remaining part of its circuit a precipice, and fortified with strong walls. This entrance was fronted by a magnificent building, called the propylæa, covered with roofs of white marble, which surpassed for beauty, and the dimensions of the marble, all that he had before seen." The building was commenced during the administration of Pericles, and finished in five years, Mnesicles being the architect, at the expense of 2012 talents, or nearly £464,000 sterling. There were five gates to the propylæa, and before it stood two lofty piers, on each of which was placed an equestrian statue, supposed to be the sons of Xenophon. On the right of the propylæa was the temple of Victory without wings, whence is a prospect of the sea; and from this place it was said that Ægeus threw himself down headlong, and died. On the left of the propylæa was an edifice adorned with paintings, the work of Polygnotus, of which, says Pausanias, though some were effaced by time, there still remained those of Diomedes and Ulysses, the one bearing off the bow and arrows of Philoctetes from Lemnos, the other, the Palladium from Troy. There were those also of Orestes slaying Ægisthus, and Pylades encountering the sons of Nauplius, who had come to succour Ægisthus; Polyxena, at the sepulchre of Achilles, about to be sacrificed, and Ulysses addressing himself to Nausicaa and her maidens, as described by Homer. Several other pictures in the same place are described by Pausanias. These three contiguous buildings originally formed one front, occupying

the whole breadth of the rock from side to side, at its western end, so that the only admission into the acropolis was through the middle building, the five gates of which are still remaining, and prove it to have been the propylæa. It may be supposed that the Hermes Propylæus was here placed, and perhaps the Graces, a piece of sculpture by the hand of Socrates, in which that celebrated philosopher, deviating from the practice of the sculptors who preceded him, had represented them not naked but clothed. Other sculptors are also mentioned by Pausanias who seem to have decorated this stately entrance. Proscenium, the area in front of the

scene of a theatre, which was perceived when the pulpitum was removed, and when it is probable the temporary scenes were taken away in order to exhibit the front of the permanent scene Prostyle, a temple which has a portico in one front, consisting of insulated columns with their entabla

tures and fastigium. When the temple had a portico in both fronts, it was termed amphi-prostyle, or prostyle in all parts. Protractor, in surveying and trigo

nometry, an instrument by which angles taken in the field with a theodilite-circumferentor are represented on paper

Prow, in navigation, the head or forepart of a ship, in opposition to the poop or stern

Prussian Blue, otherwise called Berlin blue, Parisian blue, Prussiate of iron, cyanide of iron, or, in language more pedantically chemical, per-ferro-cyanate of iron, with alumine, &c., is rather a modern pigment, produced by the combination of the prussic or hydrocyanic acid, iron, and alumina. It is of a deep and powerful blue colour, of vast body and considerable transparency, and forms tints of much beauty with white-lead, though they are by no means equal

in purity and brilliancy to those of cobalt or ultramarine, nor have they the perfect durability of the latter.

Prussian Brown is a preparation of Prussian blue from which the blue colouring principle has been expelled by fire, or extracted by an alkaline ley: it is an orange brown, of the nature and properties of Sienna earth, and dries well in oil

Prussian Green. The pigment celebrated under this name is an imperfect prussiate of iron, or Prussian blue, in which the yellow oxide of iron superabounds, or to which yellow tincture of French berries has been added, but is not in any respect superior as a pigment to the compounds of Prussian blue and yellow ochre. A better sort of Prussian green is formed by precipitating the prussiate of potash with nitrate of cobalt. Prussiate of Copper differs chemically from Prussian blue only in having copper instead of iron for its basis. It varies in colour from russet to brown, is transparent and deep, but being very liable to change in colour by the action of light or by other pigments, it has been very little employed by artists Pryan, in mining, that which is pro

ductive of ore, but does not break in large stones, but only in pebbles with a mixture of clay Pseudo-dipteral, a temple which has a single range of columns in the flanks, at the same distance from the walls of the cella as though the temple had been dipteral Pteroma, the spaces between the

walls of the cella of a temple and the columns of a peristyle; called also ambulatio Puddling, in metallurgy, a process in the refining of iron which consists in stirring it actively about Pulley, one of the six mechanical powers. The pulley is a small wheel turning on an axis, with a rope or chain passing over it. The