7,2; 8,1; all parallel to W V. In this instance all the intervals, R, 9; 9, 10; 10, W, and W, V, being equal, the proximate superior ray will always give that parallel: thus the ray of R cuts X W exactly at the point 6, which gives the line 6, 3, parallel to 7, 2; and so of all in succession. The lines Y Z and W V are parallel; they determine the height of the front standards, and by means of the lines Y X and Z X cut the other standards at their proper heights. Their descent towards X shows them to be above the line II, which is level with the spectator's eye. The summits of the couples are ascertained by the line ▲ X. They will all have their centres over the centres of the lines 5, 4; 6, 3; 7, 2; and 8, 1; ascertained by drawing a line from B to X. Fig. 5. exhibits the wall of a monastery, supported in some parts by reinforcements, or pillars, between which the wall is less substantial. The measurement of the pillars and of the intervals is given on the base-line A B, while G F shows the horizon and line of sight. The rays from a, c, e, and g, show the places where the several divisions take place on the ground-line A E, and show the projections of b, d, f, h. The upper line is also determined by C D, and the crosses, in like manner, are made to diminish towards the vanishing point F. The small mark at A, in the middle of the wall's thickness, as shown by the shaded part, gives rule for each projection of the several pillars, as shown by the shaded parts: their summits and bases will, however, have their fronts, i. e. the parts parallel with C A, terminated by horizontal lines parallel with A B. We must once more impress, that all fronting horizontals in nature must be so represented in perspective, provided they do not extend beyond 60°; also, that in every instance perpendiculars, in nature are so delineated in perspective.

The reader will have seen, that the base-line and the depth below it, give the measure of the figure when obliqued. To render this more perfectly intelligible, let us say that it were necessary to place the square W, V, 13, T, in perspective be tween W, V and X (fig. 4). This being a square is readily done; the more so, as it is proximate to the line; because the quadrant T W is so readily acted upon; W V being equal to V T. But say that it were needful to place the line TK (fig. 4.) in perspective on the line V X. Draw the quadrant TW; and the quadrant K, VOL. IX.

10; the line W, 1, drawn to , will show the place T, and the line 8, 2, will show the place of K: therefore the line TK will be found in perspective between the points 1 and 2 on the line V X. Thus any line or object may be represented; observing that the distance at which it stands below the base-line must be measured on the base-line; when by drawing rays to the horizontal line (whereon all the vanishing points must rest), its place on the oblique line or site will be determined. Some authors on this subject have directed that the back ground should be limited by a semicircle, describing the half-horizon, and that all the vanishing points ought to be placed thereon. This, however well it may answer in a panoramic point of view, can never be so appropriate as the horizontal line, in a picture which includes only the sixth part of a circle.

What has been said relates entirely to mathematical perspective, and forms the basis of architectural design, and governs (though rather occultly) every kind of landscape painting: with regard to the perspective of living objects, and of varied nature, that can only be acquired by attention to models, and to the real figures.

PERSPECTIVE, aerial, is the art of giving a due diminution or degradation to the strength of the light, shade, and colours of objects, according to their different distances, the quantity of light which falls on them, and the medium through which they are seen.

As the eye does not judge of the distance of objects entirely by their apparent size, but also by their strength of colours and distinction of parts, so it is not sufficient to give an object its due apparent bulk according to the rules of stereography, unless at the same time it be expressed with that proper faintness and degradation of colour which the distance requires. Thus if the figure of a man, at a distance, were painted of a proper magnitude for the place, but with too great a distinction of parts, or too strong colours, it would appear to stand forward, and seem proportionally less, so as to represent a dwarf situated nearer the eye, and out of the plane on which the painter intended it should stand.

By the original colour of an object is meant, that colour which it exhibits to the eye when duly exposed to it in a full open uniform light, at such a moderate distance as to be clearly and distinctly seen. This colour receives an alteration from many

causes, the principal of which are the following.

1. From the objects being removed to a greater distance from the eye, whereby the rays of light which it reflects are less vivid, and the colour becomes more diluted and tinged, in some measure, by the faint bluish cast, or with the dimness or haziness of the body of air through which the rays pass

2. From the greater or less degree of light with which the object is enlightened; the same original colour having a different appearance in the shade from what it has in the light, although at an equal distance from the eye, and so in proportion to the strength of the light or shade.

3 From the colour of the light itself which falls upon it, whether it be from the reflection of coloured light from any adjacent object, or by its passage through a coloured medium, which will exhibit a colour compounded of the original colour of the object, and the other accidental colours which the light brings with it.

4. From the position of the surface of the object, or of its several parts with respect to the eye; such parts of it appearing more lively and distinct than those which are seen obliquely.

5. From the closeness or openness of the place where the object is situated; the light being much more variously directed and reflected within a room, than in the open air.

6. Some original colours naturally reflect light in a greater proportion than others, though equally exposed to the same degrees of it; whereby their degradation at several distances will be different from that of other colours which reflect less light.

From these several causes it happens that the colours of objects are seldom seen pure and unmixed, but generally arrive at the eye broken and softened by each other; and, therefore, in painting, where the natural appearances of objects are to be described, all hard or sharp colouring should be carefully avoided.

A painter, therefore, who would succeed in aerial perspective, ought carefully to study the effects which distance, or the different degrees or colours of light, have on each particular orig nal colour, to know how its appearance or strength is changed in the several circumstances above mentioned, and represent it accordingly so that, in a picture of various coloured objects, he may be able to give each original colour its own proper dimi

nution or degradation according to its place.

Now, as all objects in a picture are proportioned to those placed in the front; so in aerial perspective the strength of light, and the brightness of the colours of objects close to the picture, must serve as a standard; with respect to which all the same colours, at different distances, must have a proportional degradation in like circumstances.

In order, therefore, to give any colour its proper diminution in proportion to its distance, it ought to be known what the appearance of that colour would be, were it close to the picture, regard being had to that degree of light which is chosen as the principal light of the picture. For if any colour should be made too bright for another, or for the general colours em. ployed in the rest of the picture, it will appear too glaring, seem to start out of its place, and throw a flatness and damp upon the rest of the work; or, as the painters express it, the brightness of that colour will kill the rest.

PERSPECTIVE glass, in optics, differs from a telescope in this: instead of the convex eye-glass placed behind the image, to make the rays of each pencil go parallel to the eye, there is placed a concave eye-glass as much before it; which opens the converging rays, and makes them emerge parallel to the eye. The quantity of objects taken in at one view with this instrument does not depend upon the breadth of the eye-glass, as in the astronomical telescope, but upon the breadth of the pupil of the eye.

Reflecting perspective glasses, called by some opera-glasses, or diagonal perspectives, are so contrived, that a person can view any one in a public place, as the opera or play-houses, without it being possible to distinguish who it is he looks See OPERA glass.

at.

PERSPECTIVE plane, is the glass, or other transparent surface, supposed to be placed between the eye and the object, perpendicular to the horizon. It is sometimes called the section, table, or glass.

PERSPIRATION, in medicine, the evacuation of the juices of the body through the pores of the skin. Perspiration is distinguished into sensible and insensible. See PHYSIOLOGY.

The skin of man and of animals is pierc ed with an infinitude of pores, through which, by means of the transpiration, the parts of the aliments escape which do not contribute to nourishment. Independently of the sensible perspiration, which is

called sweat, and which is accidental, there is, moreover, one that is insensible, acting more or less at every instant, and which none could conceive to be so abundant as it is, before the experiments of Sanctorius. This celebrated philosopher had the resolution to pass a part of his life in a balance, wherein he weighed himself, in order to determine the loss occasioned by the effects of the insensible perspiration. He has found that this kind! of evacuation causes us to lose, in the space of twenty-four hours, about fiveeighths of the nutriment which we have taken. Dodard, in repeating afterwards the same experiments, has had regard to the difference of age, and is convinced that a person perspires much the most in his youth. But the philosophers who have directed their attention to this object have not sufficiently distinguished the effect of the perspiration or transpiration which is performed by the lungs, and of which the matter escapes by expiration, from the effect which is attributable to the cutaneous perspiration, or to that which obtains through the intermediation of the skin Seguin has undertaken, in conjunction with Lavoisier, to determine these two effects separately; and after having sought, in the usual manner, the total result of the transpiration, has suppressed that which is performed by the skin, by applying upon that organ a cover impermeable to the humour which it transmits outwardly: thus has been obtained the quantity of the pulmonary transpiration; and the mean between the results of these experiments gives sevenelevenths for the ratio between this quantity and that of the cutaneous perspiration; that is, the effect produced by the pulmonary transpiration is more than the third of the total effect.

PERUKE. It appears that this term was originally applied to describe a fine natural head of long hair; but whatever may have been the ancient use or meaniug of the word, it has now almost become obsolete, though it was for more than a century in constant application to those artificial heads of hair, made probably at first to conceal natural or accidental baldness, but which afterwards became so ridiculously fashionable, as to be worn in preference to the most beautiful locks, absurdly shaved off the head to make room for them.

Ancient authors might be quoted to prove, that the great and luxurious of that time had recourse to this mode of concealing defects, and of decorating the

head; nay, it might perhaps be proved, that the peruke of the Emperor Commodus was more absurdly composed than any modern peruke has ever been; and indeed it must be admitted, that a wig powdered with scrapings of gold, in addition to oils and glutinous perfumes, must have made a more wonderful appearance than our immediate ancestors ever witnessed. It was in the reign of Charles the First, that perukes were in. troduced throughout Europe, when the moralists attacked them without mercy, as they perceived that the folly of youth even extended to the cutting off nature's locks, to be replaced by the hair of the dead, and of horses, woven into a filthy piece of canvass. Admonition and ridicule was, however, of little avail, and the clergy began to be affected by the general mania. Those on the continent, being almost universally Roman Catholics, were so completely subject to their superiors, that the peruke was soon routed from their body; but as the dignified clergy of England conceive that their consequence is increased by the enormous bushes of hair upon their heads, and the judges have adopted their sentiments in this particular, it is probable many years will elapse, before the shape and absurdity of two particular species of perukes are forgotten.

About the close of the seventeenth century the peruke was made to represent the natural curl of the hair, but in such profusion, that ten heads would not have furnished an equal quantity, as it flowed down the back, and hung over the shoulders, half way down the arms. By 1721, it had become fashionable to tie one half of it on the left side into a club. Between 1730 and 1740, the bag-wig came into fashion, and the peruke was docked considerably, and sometimes plaited behind into a queue, though even till 1752 the long flowing locks maintained their influence. After 1770 those were rarely seen and since that time persons wearing perukes have generally had substantial reasons for so doing, from baldness and complaints in the head. At one time, indeed, when the stern virtues of Brutus were much in vogue, the young men of Europe wore perukes of black or dark hair, dressed from his statues. Many particulars on this subject have been preserved by Mr. Malcolm, in his" Anecdotes of the Manners and Customs of London," from which we learn, that a young country woman obtained 601. for her head of hair in the year

1700, when human hair sold at 31. per ounce; and in 1720, the grey locks of an aged woman sold for 50%. after her decease, as did wigs at 401. each, of peculiar excellence.

A petition from the master peruke. makers of London and Westminster, presented to the King in 1763, points out the final decline of their use to have taken place at that time. In this they complain of the public wearing their own hair: and say, "That this mode, pernicious enough in itself to their trade, is rendered excessively more so by swarms of French hair dressers already in those cities, and daily increasing."

PERULA, in botany, a genus of the Dioecia Polyandria class and order. Generic character: male, calyx; perianthum two-leaved, very small corolla; petal one, semi-globular, concave, hanging down; stamens, filaments very many; pistil, germs four, barren, very small; female on a distinct tree; calyx, perianthum as in the male, deciduous; corolla as in the male; pistil, germs four, fertile; pericarpium capsule, obovate, subtrigonal; seeds solitary, small. The number of species not known. P. arborea is a native of New Granada, about Mariquita, where it was found by Mutis.

PETALOMA, in botany, a genus of the Decandria Monogynia class and order. Essential character: calyx goblet-shaped, five-toothed; petals five, inserted between the teeth of the calyx; stamina on the margin of the calyx; berry onecelled: seeds one or four. There are two species, viz. P. myrtilloides and P. mouriri.

PETESIA, in botany, a genus of the Tetrandria Monogynia class and order. Natural order of Rubiaceæ, Jussieu. Essential character: corolla one petalled, funnel form; stigma bifid; berry manyseeded. There are three species.

PETIOLUS. See PEDUNCULUS. PETIT (PETER), a considerable mathematician and philosopher of France, was born at Montlucon, in the diocese of Bourges, in the year 1589, according to some, but in 1600 according to others. He first cultivated the mathematics and philosophy in the place of his nativity; but in 1633 he repaired to Paris, to which place his reputation had procured him an invitation. Here he became highly celebrated for his ingenious writings, and for his connections with Pascal, Des Cartes, Mersenne, and the other great men of that time He was employed on several occasions by Cardinal Richelieu; he was

commissioned by this minister to visit the seaports, with the title of the King's Engineer; and was also sent into Italy upon the King's business. He was at Tours in 1640, where he married; and was afterwards made Intendant of the Fortifications. Baillet, in his Life of Des Cartes, says that Petit had a great genius for mathematics; that he excelled particularly in astrono my; and had a singular passion for experi mental philosophy. He was intimately connected with Pascal, with whom he made, at Rouen, the same experiments concerning the vacuum, which Torricelli had before made in Italy; and was assured of their truth by frequent repetitions. He died August the 20th, 1667, at Lagny, near Paris, whither he had retir ed for some time before his decease. He published several works upon physical and astronomical subjects, also on chronology and theology.

PETITIA, in botany, so named in memory of Francis Petit, a genus of the Tetrandria Monogynia class and order. Natural order of Vitices, Jussieu. Essential character; calyx four-toothed, inferior; corolla four-parted; drupe with a twocelled nut. There is but one species, viz. P. Domingensis, a native of the island of St. Domingo.

PETITION, no petition to the King, or either house of parliament, for any alteration in church or state, shall be signed by above twenty persons, unless the matter thereof be approved by three Justices of the Peace, or the major part of the Grand Jury in the county; and in London, by the Lord Mayor, Aldermen, and Common Council: nor shall any petition be presented by more than ten persons at a time.

PETITION in chancery, a request in writing, directed to the Lord Chancellor, or Master of the Rolls, shewing some matter or cause whereupon the petitioner prays somewhat to be granted him.

PETIVERIA, in botany, Guinea-hen weed, a genus of the Hexandria Tetragynia class and order. Natural order of Holoraceæ. Atriplices, Jessieu. Essential character: calyx four-leaved; corolla none; seed one, with reflex awns at top. There are two species, viz. P. alliacea, common Guinea-hen weed; and P. octandra, dwarf Guinea-hen weed: both natives of the West-Indies.

PETEREA, in botany, so named in honour of Lord Petre, a genus of the Didynamia Angiospermia class and order. Natural order of Personatæ. Vitices, Jussieu. Essential character: calyx five

parted, very large, coloured; corolla wheel-shaped; capsule two-celled, at the bottom of the calyx; seeds solitary. There is but one species, viz. P. volubilis, a native of South America and the West Indies.

PETRIFACTION. See ORYCTOLOGY. PETROCARYA, in botany, a genus of the Heptandria Monogynia class and order. Natural order of Pomaceæ. Rosaceæ, Jussieu. Essential character: calyx turbinate, five-cleft, with two bractes at the base; corolla five-petalled, less than the calyx; filaments fourteen, seven of which are barren; drupe inclosing a two-celled nut, with a stony shell. There are two species, viz. P. montana, and P. campestris, both found in the woods of Guiana, where they grow to the height of forty and eighty feet.

The

PETROLEUM, in chemistry. substances which mineralogists have distinguished by the names of asphaltum, maltha, petroleum, and naptha, are thought by Mr. Murray, and others, to be mere varieties of one species, and form a series which passes even into coal. Asphaltum forms the connection with pitch-coal: it is found in veins, and in small masses, and also sometimes on the surface of lakes. Maltha is softer, has a degree of tenacity, and a strong bituminous smell. Petroleum is semi-liquid, semi-transparent, of a reddish-brown colour, and fetid odour. Naptha is of a lighter colour, more or less transparent, perfectly thin and liquid, light, odoriferous, volatile, and inflammable. Naptha by inspis. sation becoming petroleum, and this passing into asphaltum. See ASPHALtum, BiTUMEN, &C.

In several parts of France petroleum is found floating on the water, and is known in commerce by the name of oil of Gabian. Wells are sometimes dug 100 feet deep, where the petroleum is found mixed with the soil, in such proportion, that ten pounds may be extracted from a hundred weight.

PETROMYZON, the lamprey, in natural history, a genus of fishes of the order Cartilaginei. Generic character: body shaped like an eel; mouth beneath, with numerous teeth, in circular rows; seven spiracles on each side the neck; no pectoral or ventral fins. Shaw notices nine species, and Gmelin only four. P. marinus, or the great lamprey, is usually of a brown olive colour, tinged with yellowish white. It is often three feet long; is an inhabitant of the seas, as its name indeed

implies; but in the beginning of spring ascends rivers, in which it resides for a few-months, then returning to the ocean. It is viviparous, and supposed to subsist almost entirely on worms and fishes. Its heart is enclosed not in a soft but in a cartilaginous pericardium, constituting thus a singular deviation from the general structure of animals. Its spine also possesses the peculiarity of being rather a soft cartilage than bone. These fishes fasten themselves with the jagged edges of the mouth to large stones, with the most extraordinary firmness, and may be lifted by the tail to a considerable height, without being made to quit a stone of the weight of even ten or twelve pounds. Their principle of vitality is extremely vigorous and persevering, various parts of the body long continuing to move for some hours after it is divided; and the head will adhere to a rock for hours after the greater part of the body is cut away. In some large rivers of Europe, these fishes are taken in vast numbers, and preserved with spices and salt as an article of merchandise. In England, the Severn is the most celebrated river for them, and they are much valued on their first arrival from the sea. They are considered a high luxury for the table, and the life of one of the Kings of England will be recollected to have been terminated by his excessive partiality to potted lampreys.

P. fluviatilis, or the lesser lamprey of Europe, is about twelve inches long, inhabits also the sea, but is found more frequently in the rivers than the former. It abounds in the Thames and Severn, and is preferred by many to the larger species, as being not so strong in taste. In some years half a million of these fishes have been sold from the neighbourhood of Mortlake, for the Dutch cod and turbot fishery, at the rate of two pounds per thousand. In many parts of Germany they are fried and packed up in barrels with spices and bay leaves, and are conveyed to very distant regions, where they are in high estimation, and sell for considerable prices. These fishes will live many days out of the water. In Russia they are taken from beneath the ice, packed in snow, and exported to great distances, and will generally recover themselves on being afterwards thrown into the water. The planer lamprey is ten inches long, will live immersed in spirits of wine for fourteen minutes, moving during that time with incessant violence. The leech lamprey is a native of the ri ver Seine, and will fix on the bellies of va