J. L. Cowley, London, 1766, 2 vols. 4to. Familiar Introduction to the Theory and Practice of Perspective, by J. Priestley, London, 1770, 8vo. The Elements of Linear Perspective demonstrated by Geometrical Principles, by Edward Noble, London, 1771, 8vo. A Complete Treatise on Perspective, in Theory and Practice, on the Principles of Dr. Brook Taylor, by T. Malton, London, 1776, folio. Ware's Complete Body of Architecture contains a Treatise on Perspective, 1760, folio. A Practical Treatise of Perspective on the Principles of Dr. Brook Taylor, by Edward Edwards, second edition, 1806. A thin 4to., without the author's name, entitled A New Treatise on Perspective, Founded on the Simplest Principles, containing Universal Rules for Drawing the Representation of any Object on a Vertical Plane, 1810. The work of D. Cresswell, A. M, 1811, 8vo. Milne, in his Elements of Architecture, 1812, 4to., and Mr. Hayter's work, 1813, 8vo. Martin, Muller, and Emerson, have also given treatises in their mathematical courses.

Works on this subject, under the following denominations, have appeared in the German language:-Of Perspective, as it regards the Arts, 1509, folio, with thirty-seven wood-cuts. Gualt. Henr. Rivius, New Perspective, or The True Foundation of the Arts of Design, Nuremberg, 1547, folio. John Lautensak, Instructions on the Use of the Compass and Rule, particularly in Perspective, Franckf. 1567, folio. Perspectivum Corporum Regularium, &c., par Jamitzer, Nuremberg, 1564, folio. Lud Bruns. Practice of Perspective, &c., Leipsic, 1615, folio, Lenkart, Treatise on Perspective, Augs. 1616, folio. Alberti on Perspective and Shading, Nürnb. 1623-7, folio. Schubler, Instructions on Perspective, &c., Nürnb. 1719-20, 2 vols. folio, with fifty engravings. Lucidum Prospectivæ Speculum, by P. Heinecken, Augs. 1727, folio, with ninety-three engravings. Ibid. 1753, folio, with 126 engravings. Summary Instructions on Perspective, by John Christopher Bischof, Halle, 1741, 8vo. Instructions on the manner of tracing all Elevations in Perspective, without having regard to a plan, by J. H. Lambert, Zurich, 1759, 8vo., and 1774, 8vo. A French translation appeared in 1759, 8vo. Manner of learning to draw by means of Geometry and Perspective, by Werner, Erfort, 1764, 8vo. Detailed Instructions on Perspect.ve, after an easy and clear method, by C. Phil. Jacobz, Amst. 1767, 8vo., with sixty plates. Treatise on Perspective, by Luc. Voch, Augs. 1780, 8vo. Elements of Perspective for the ase of Painters, by Burja, Berlin, 1793, 8vo.

The reader may also turn with advantage to Leçons de Perspective, par L. le Bicheur, Ludovico Cigoli, on Perspective. Perspectiva Practica, by Franc. De Breuil. The work of Albert Durer on the Proportions of the Human Body, Nürnberg, 1528, folio. The second book of the Architettura of Seb. Serlio, Paris, 1545, folio. The fifth book of Trattato dell' Arte della Pittura, of Lomazzo, Milan, 1585, 4to. Museo Pittorico, by Velasco, Madrid, 1715, folio. Remarques sur les Tableaux en jeu d'Optique, in the French Mercury for the year 1763, &c.

&c.

PERSPECTIVE, AERIAL, is the art of giving a due diminution or degradation to the strength of the light, shade, and colors 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 color, 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 color 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 colors, 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 color of an object is meant that color 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 color receives an alteration from many causes, the principal of which are the following: 1. From the object's being removed to a greater distance from the eye, whereby the rays of light which it reflects are less vivid, and the color 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 color 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 color of the light itself which falls upon it, whether it be from the reflection of colored light from any adjacent object, or by its passage through a colored medium, which will exhibit a color compounded of the original color of the object, and the other accidental colors 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 colors 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 colors which reflect less light.

PERSPECTIVE MACHINES, or contrivances for designing or drawing in perspective, are of various kinds. We have described the construction of the camera obscura, and shall here add only the machine suggested originally by Dr. Bevis, and another by Mr. Kirby.

The plane of the former machine is represented fig. 1, PERSPECTIVE. Plate III. fig. 2 is a representation of it when made use of in draw

ing distant objects in perspective. a bef, fig. 1, is an oblong square board, represented by ABEF in fig. 2; x and y (X and Y) are two hinges on which the part cld (CLD) is moveable. This part consists of two arches or porvions of circles clm (CL M) and dnl (DNL), joined together at the top (L), and at bottom to the cross bar dc (DC), to which one part of of each hinge is fixed, and the other part to a flat board, half the length of the board a bef (A BEF), and glued to its uppermost side. The centre of the arch cm l is at d, and the centre of the arch dnl is at c. On the outer side of the arch dnl is a sliding piece n (much like the nut of the quadrant of altitude belonging to a common globe), which may be moved to any part of the arch between d and 7: and there is such another slider, o, on the arch cm l, which may be set to any part between c and l. A thread cpn (CPN) is stretched tight from the centre (C) to the slider n (N), and such another thread is stretched from the centre d (D) to the slider o (O); the ends of the threads being fastened to these centres and sliders.

the

Now it is plain that, by moving these sliders on their respective arches, the intersection p (P) of the threads may be brought to any point of open space within the arches. In the groove (K) is a straight sliding bar i (I), which may be drawn farther out, or pushed farther in, at pleasure. To the outer end of this bar I, fig. 2, is fixed the upright piece HZ, in which is a groove for receiving the sliding piece Q. In this slider is a small holer for the eye to look through in using the machine: and there is a long slit in H Z, to let the hole r be seen through when the eye is placed behind it any height of the hole above the level of the bar I.

In delineating a representation, i. e. of the house qsrp, a great way off, place the machine on a steady table, with the end E F of the horizontal board, A BE F, toward the house, so that when the Gothic-like arch DLC is set upright, the middle part of the open space (about P) within it may be even with the house when you place your eye on Z, and look at the house through the small hole r. Then fix the corners of a square piece of paper with four wafers on the surface of that half of the horizontal board which is nearest the house; and all is ready for drawing. Now set the arch upright, as in the figure; which it will be when it comes to the perpendicular side t of the upright piece st, fixed to the horizontal board behind D. Then place your eye at Z, and look through the hole r at any point of the house, as q, and move the sliders N and O till you bring the intersection of the threads at P directly between your eye and the pointq: then put down the arch flat upon the paper on the board, as at S T, and the intersection of the threads will be at W. Mark the point W on the paper with the dot of a black lead pencil, and set the arch upright again, as before: then look through the hole r, and move the sliders N and O till the intersection of the threads comes between your eye and any other point of the house, as p: then put down the arch again to the paper, and make a pencil mark thereon at the intersection of the threads, and draw a line from that

mark to the former one at W; which line will be a true perspective representation of the corner pq of the house. Proceed in the same manner by bringing the intersection of the threads successively between your eye and the other points of the outlines of the house, as r, s, &c., and put down the arch to mark the points on the paper at the intersection of the threads: then connect these points by straight lines, which will be the perspective outlines of the house. In like manner find the points of the corners of the door and windows, top of the house, chimneys, &c., and draw the finishing lines from point to point: then shade the whole, making the lights and shades as you see them on the house itself, and you will have a true perspective figure of it. Great care must be taken, during the whole time, that the position of the machine be not shifted on the table; and, to prevent such an inconvenience, the table should be very strong and steady, and the machine fixed to it, either by screws or clamps.

In the same way a landscape, or any number of objects within the field of a view through the arch, may be delineated, by finding a sufficient number of perspective points on the paper, and connecting them by straight or curved lines as they appear to the eye. The arch ought to be at least a foot wide at bottom, that the eye at Z may have a large field of view through it; and the eye should then be, at least, ten inches and half from the intersection of the threads at P when the arch is set upright. For if it be nearer the boundaries of view at the sides near the foot of the arch will subtend an angle at Z of more than sixty degrees, which will not only strain the eye, but will also cause the outermost parts of the drawing to have a disagreeable appearance, To avoid this, it will be proper to draw back the sliding bar I, till Z be fourteen inches and a half distant from P; and then the whole field of view, through the foot wide arch, will not subtend an angle to the eye at Z of more than fortyfive degrees; which will give a more easy and pleasant view, not only of all the objects themselves, but also of their representations on the paper whereon they are delineated. So that, whatever the width of the arch be, the distance of the eye from it should be in this proportion : as 12 is to the width of the arch, so is 14 to-the distance of the eye (at Z) from it. If a pane of glass, laid over with gum water, be fixed into the arch, and set upright when dry, a person who looks through the holer may delineate the objects upon the glass which he sees at a distance through and beyond it, and then transfer the delineation to a paper put upon the glass. Ferguson's Perspective, ch. iii.

Mr. Kirby's instrument is seen in figs. 4, 5, 6. The ruler A B, fig 3, nineteen inches long, is graduated into nineteen equal parts; it has a dovetail groove on its upper edge to receive the perpendicular ruler G, which has one end fitted to it, so as to slide very easily; this ruler is fourteen inches long, and is divided into fourteen equal parts, and upon the back side of it F is a line drawn exactly in the middle, to which is fixed a silken line with a small plummet at the end. The ruler A B is fixed by two screws a, c,

to two pieces of thin brass; and these pieces of brass are fixed at the other ends by two screws d,e, to a stronger piece of brass bf, which goes close to the ruler A B, and has a joint at X turning upon a screw; below this joint is a piece of round brass about six inches long, which goes into a hole made in the top of the staff, and may be raised higher or lower, by means of a screw S: CDE represents part of this staff, the whole length of which is about three feet, and at the bottom is a rank screw made of iron and fixed to the staff. HI is a wire twenty-two inches long, with a screw at h to go into the hole b; the piece of brass wire bent into the form ik is fixed to the wire HI by the screw k; and the part i goes into the hole ƒ in the brass piece bf. The small wire K L is about twelve inches long, and flatted at K, at which place is a little hole above one-eighth of an inch in diameter; this wire K L is fitted to the holes l, m, n, o, which are made in the larger wire H I, and it may be placed higher or lower by means of a small screw. This instrument is used in the following manner: fix a paper upon a drawing board, as in fig. 4, and divide the paper lengthways into nineteen equal parts, and perpendicularly into fourteen equal parts; making these divisions greater or smaller according to your design. Then take the staff, and fix it strongly in the ground by means of a screw at bottom, and at a convenient distance from the prospect which you intend to take.

Now put the instrument together as in fig. 5, and fix the ruler A B exactly horizontal by means of the plummet on the perpendicular ruler and

the brass joint X; then fix the wire K L, so as to have the eye-hole exactly level with the horizon or equal to the height of the eye, and take care to have the greatest distance of the eye-hole from the ruler equal to the whole length of the longest ruler A B, and never less than the distance hl. Having thus fixed the instrument, proceed to make the drawing; look through the eye-hole, and then move the perpendicular ruler in the groove, till you get one edge exactly against some principal object; then will the parts upon the ruler show how high the object is from the bottom of the ruler, i. e. from the bottom of the picture, and you will also know its apparent height; therefore transfer this to the paper in those squares which correspond with the divisions upon the rulers. For the breadth of objects, move the perpendicular ruler so as to be even with the sides of an object, and the divisions upon the lower ruler will show their apparent breadths. After the same manner, get the places and apparent sizes of as many principal objects as are necessary for assisting you, in completing the whole drawing, which may be done by this method with great exactness. When the drawing is finished, the instrument may be taken to pieces and put into a box, which may serve as a drawing board; the top M may be screwed into the staff, which will serve as a walking-stick, and the stool to sit on may be made very portable; so that every part of this apparatus may be carried by one person without any inconvenience. Kirby, b. ii. p. 65, &c. This last instrument has been found particularly convenient in taking views of extensive landscapes, or buildings.

PERSPIRATION, in medicine, is the evacuation of the juices of the body through the pores of the skin. Perspiration is distinguished into sensible and insensible; and here sensible perspiration is the same with sweating, and insensible perspiration that which escapes the notice of the senses. This last is the idea affixed to the word perspiration when used alone. The secretory organ is composed of the extremities of the cutaneous arteries. The smell of the perspirable fluid, in a healthy man, is fatuous and animal; its taste manifestly salt and ammoniacal. In consistence it is vaporous or aqueous; and its specific gravity in the latter state is greater than that of water. For the most part it is yellowish, from the passage of the subcutaneous oil, and sebaceous matter of the subcutaneous glands. Whatever form it takes, the liquid that escapes from the skin is composed, according to Thenard, of a great deal of water, a small quantity of acetic acid, of muriate of soda and potassa, a small quantity of earthy phosphate, an atom of oxide of iron, and a trace of animal matter. Berzelius considers the acid of sweat not the same as acetic acid, but like the lactic acid of Scheel. The skin exhales besides an oily matter, and some carbonic acid.

Experiments have been made to determine the quantity of transpiration which is formed in a given time, and the variations that this quantity undergoes according to circumstances. The first were those of Sanctorius, who, during thirty years, weighed every day, with extreme care, and an indefatigable patience, his food and his drink, his solid and liquid excretions, and even himself. Sanctorius arrived at no exact results. Since his time several philosophers and physicians have been employed on the same subject with more success; but the most remarkable labor in this way is that of Lavoisier and Seguin. These philosophers were the first who distinguished the loss that takes place by pulmonary transpiration from that of the skin.

Seguin shut himself up in a bag of gummed silk, tied above his head, and presenting an opening, the edges of which were fixed round his mouth by a mixture of turpentine and pitch. In this manner only the humor of the pulmonary transpiration passed into the air. In order to know the quantity, it was sufficient to weigh himself, with the bag, at the beginning and end of the experiment, in a very fine balance. By repeating the experiment out of the bag, he determined the whole quantity of humor transpired; so that, by

deducting from this the quantity that he knew had passed out from the lungs, he had the quantity of humor exhaled by the skin. Besides, he took into account the food that he had used, his excretions solid and liquid, and generally all the causes that could have any influence upon the of Lavoisier and Seguin are these:-1. The transpiration. By following this plan, the results greatest quantity of insensible transpiration (the pulmonary included) is 256 grs. troy per minute; consequently 3 ozs. 1 drm. 36 grs. per hour; and 6 lbs. 4 ozs. 6 drms. 24 grs. in twentyfour hours. 2. The least considerable loss is 8.8 grs. per minute; consequently 2 lbs. 2 ozs. 3 drms. in twenty-four hours. 3. It is during the digestion that the loss of weight occasioned by insensible transpiration is at its minimum. 4. The transpiration is at its maximum immediately after dinner. 5. The mean of the insensible transpiration is 14-4 grs. per minute; in the mean 14.4 grs. 8-8 depend on cutaneous transpiration, and 5.6 upon the pulmonary. 6. The cutaneous transpiration alone varies during and after repasts. 7. Whatever quantity of food is taken, or whatever are the variations of the atmosphere, the same individual, after having augmented in weight by all the food that he has taken, returns, in twenty-four hours, to the same weight nearly that he was the day before, provided he is not growing, or has not eaten to excess.

The cutaneous transpiration has various uses. It keeps up the suppleness of the epidermis, and thus favors the exercise of the tact and the touch. It is by evaporation along with that of the lungs, the principal means of cooling, by which the body maintains itself within certain limits of temperature; also its expulsion from the economy appears very important, for every time that it is diminished or suspended, derangements of more or less consequence follow, and many diseases are not arrested until a considerable quantity of sweat is expelled.

It cannot be doubted that carbon is emitted from the skin; but in what state, the experiments hitherto made do not enable us to decide. Cruickshanks found that the air of the glass vessel in which his hand and foot had been confined for an hour, contained carbonic acid gas; for a candle burned dimly in it, and it rendered lime-water turbid. And Jurine, that air which had remained for some time in contact with the skin consisted almost entirely of carbonic acid gas. The same conclusion may be drawn from the experiments of Ingenhousz and Milly. Trousset has lately observed that air was separated copiously from patient of his, while bathing.

The skin emits also a particular odorous substance. That every animal has a peculiar smell is well known: the dog can discover his master, and even trace him to a distance by the scent. Cruickshanks has made it probable that this matter is an oily substance, or at least that there is an oily substance emitted by the skin. He wore repeatedly, night and day, for a month, the same under waistcoat of fleecy hosiery, during the hottest part of the summer. At the end of this time he always found an oily substance accumulated in considerable masses on the nap

of the inner surface of the waistcoat, in the form of black tears. When rubbed on paper, it rendered it transparent, and hardened on it like grease. It burned with a white flame, and left behind it a charry residuum.

Berthollet has concluded that the acid which is present is the phosphoric; but this has not been proved. Fourcroy and Vauquelin have ascertained that the scurf which collects upon the skins of horses consists chiefly of phosphate of 'ime, and urea is even sometimes mixed with it. According to Thenard, however, the acid contained in sweat is the acetous; which, he likewise observes, is the only free acid contained in urine and in milk, this acid existing in both of them when quite fresh.

His account of his examination of it is as follows:-The sweat is more or less copious in different individuals; and its quantity is perceptibly in the inverse ratio of that of the urine. All other circumstances being similar, much more is produced during digestion than during repose. The maximum of its production appears to be twenty-six grains and two-thirds in a minute; the minimum nine grains, troy weight. It is much inferior, however, to the pulmonary transpiration; and there is likewise a great difference between their nature and manner of formation. The one is a product of a particular secretion, similar in some sort to that of the urine; the other, composed of a great deal of water and carbonic acid, is the product of a combustion gradually effected by the atmospheric air. The sweat, in a healthy state, very sensibly reddens litmus paper or infusion. In certain diseases, and particularly in putrid fevers, it is alkaline; yet its taste is always rather saline, and more similar to that of salt than acid. Though colorless, it stains linen. Its smell is peculiar, and insupportable when it is concentrated, which is the case in particular during distillation. But before he speaks of the trials to which he subjected it, and of which he had occasion for a great quantity, he describes the method he adopted for procuring it, which was similar to that of Cruickshanks. Human sweat, according to Thenard, is formed of a great deal of water, free acetous acid, muriate of soda, an atom of phosphate of lime and oxide of iron, and an inappreciable quantity of animal matter, which approaches much nearer to gelatin than to any other substance.

Perspiration varies in respect to, 1. The temperature of the atmosphere. Thus men have a more copious, viscid, and higher-colored sweat in summer than in winter, and in warm countries, than in colder regions. 2. Sex.-The sweat of a

man is said to smell more acrid than that of a woman. 3. Age. The young are more subject to sweat than the aged, who, during the excessive heat of the summer, scarcely sweat at all. 4. Ingesta.-An alliacious sweat is perceived from eating garlick; a leguminous from peas; an acid from acids; a fetid from animal food only; and a rancid sweat from fat foods, as is observed in Greenland. A long abstinence from drink causes a more acrid and colored sweat; and the drinking a great quantity of cold water in summer a limpid and thin sweat. 5.

Medicines.-The sweat of those who have taken musk, even moderately, and assafœtida, or sulphur, smells of their respective natures. 6. Region of the body.-The sweat of the head is greasy; on the forehead it is more aqueous; under the axillæ very unguinous; and in the interstices of the toes it is very fetid, forming in the most healthy man blackish sordes. 7. Diseases. In this respect it varies very much in regard to quantity, smell, and color; for the sweat of gouty persons is said to turn blue vegetable juices to a red color. Some men also have a lucid sweat, others a sweat tinging their linen of a cerulean color.

PERSUADE', v. a. PERSUADER, n. s. PERSUA'SIBLE, adj. PERSUA'SIBLENESS, n. s. PERSUASION, n. s. PERSUASIVE, adj. PERSUASIVELY, adv. PERSUASIVENESS, n. s. PERSUA'SORY, adj.

Fr. persuader; Span. and Port. persuadir; Ital. and Lat. persuadere, persuadeo. To induce; bring to an opinion; influence by expostulation or argu

6

argument. Dr. Johnson says, persuasion seems ment; inculcate by rather applicable to the passions, and argument to the reason;' but this is not always observed: in an obsolete sense, to treat by persuasion: a persuader is one who thus treats or influences another; an officious adviser or importuner : persuasible is to be persuaded; the noun substantive that follows corresponding: persuasion is the act of persuading; state of being persuaded, or opinion to which one is persuaded : persuasive and persuasory, having the power to persuade or influence by argument; the adverb and noun substantive corresponding.

Let every man be fully persuaded in his own mind.

Romans. We are persuaded better things of you, and things Hebrews vi. 9. that accompany salvation.

Joy over them that are persuaded to salvation.

Esdras vii. They that were with Simon, being led with covetousness, were persuaded for money. 2 Mac. x.

Philoclea's beauty not only persuaded, but so persuaded as all hearts must yield: Pamela's beauty used violence, and such as no heart could' resist.