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On the outer side of the arch dnl is a sliding piece ? (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 6 and 7: and there is such another slider o on the arch cm, which may be set to any part between c and 7-A thread cpn (CPN) is stretched tight from the centre c (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 thread being fastened to these centres and sliders.

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 the open space within the arches. In the groove k (K) is a straight sliding bar i (I), which may be drawn further out, or pushed further in at pleasure.

To the outer end of this bar I (fig. 46.) is fixed the upright piece HZ, in which is a groove for receiving the sliding piece Q. In this slider is a small hole r for the eye to look through, in using the machine; and there is a long slit in HZ, to let the hole r be seen through when the eye is placed behind it, at any height of the bole above the level of the bar I.

How to delineate the perspective figure of any distant object or objects, by means of this machine.

Suppose you wanted to delineate a perspective representation of the house q srp (which we must imagine to be a great way off, without the limits of the plate), place the machine on a steady table, with the end EF of the horizontal board ABEF, 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 at 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.

Set the arch upright, as in the figure; which it will be when it comes to the perpendicular side t of the upright piece s t fixed to the horizontal board behind D. Then place your eve 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 point q: then put down the arch flat upon the paper on the board, as at ST, 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 come 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 p q of the house.

Proceed in the same manner, by bringing the intersection of the threads successively between your eye and other points of the outlines of the house, as r s, &c. and put down the arch to mark the like points on the VOL. XVI. Part I. +

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paper, at the intersection of the threads: then connect Perspecthese points by straight lines, which will be the perspective outlines of the house. In like manner find points for the corners of the doors 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 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. And as this makes every thing in perspective equally easy, without taking the trouble to learn any of the rules for drawing, the operations must be very pleasing and agreeable. Yet as science is still more so, we would by all means recommend it to our readers to learn the rules for drawing particular objects; and to draw landscapes by the eye, for which, we believe, no perspective rules can be given. And although any thing may be very truly drawn in perspective by means of this machine, it cannot be said that there is the least degree of science in going that way to work.

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, 10 inches 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 60 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 14 inches 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 45 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 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 hole r may delineate the objects upon the glass which he sees at a distance through and beyoud it, and then transfer the delineation to a paper put upon the glass, as mentioned in the beginning of the article PERSPECTIVE.

Mr Peacock likewise invented three simple instrument for drawing architecture and machinery in perspective, of which the reader will find sketches and descriptions in the 75th volume of the Philosophical Transactions. These descriptions are not inserted here, because we do not think the instruments superior to that described by Ferguson, and because we wish that our readers who have occasion to draw may make themselves so much masters of the art of perspective, as to be above A a the

Perspective.

the aid of such mechanical contrivances. But for the sake of those whose opportunities of improvement in the art do not enable them to practise it without such helps, we annex the following description of an instrument invented for this purpose by Dr Wollaston, and to which he has given the name of Camera Lucida.

"Having a short time since (says the author) amused myself with attempts to sketch various interesting views, without an adequate knowledge of the art of drawing, my mind was naturally employed in facilitating the means of transferring to paper the apparent relative positions of the objects before me; and I am in hopes that the instrument, which I contrived for this purpose, may be acceptable even to those who have attained to greater proficiency in the art, on account of the many advantait possesses over the camera obscura.

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"The principles on which it is constructed will probably be most distinctly explained by tracing the successive steps, by which I proceeded in its formation.

"While I look directly down at a sheet of paper on my table, if I hold between my eye and the paper a piece of plain glass, inclined from me downwards at an angle of 45°, I see by reflection the view that is before me, in the same direction that I see my paper through the glass. I might then take a sketch of it; but the position of the objects would be reversed.

"To obtain a direct view, it is necessary to have two reflections. The transparent glass must for this purpose be inclined to the perpendicular line of sight only the half of 45°, that it may reflect the view a second time from a piece of looking-glass placed beneath it, and inclined upwards at an equal angle. The objects now appear as if seen through the paper in the same place as before; but they are direct instead of being inverted, and they may be discerned in this manner sufficiently well for determining the principal positions.

"The pencil, however, and any object, which it is to trace, cannot both be seen distinctly in the same state of the eye, on account of the difference of their distances; and the efforts of successive adaption of the eye to one or to the other would become painful if frequently repeated. In order to remedy this inconvenience, paper and pencil may be viewed through a convex lens of such a focus, as to require no more effort than is necessary for seeing the distant objects distinctly. These will then appear to correspond with the paper in distance as well as direction, and may be drawn with facility, and with any desired degree of precision.

the

"This arrangement of glasses will be best understood from inspecting fig. 47. a b in the transparent glass; be the lower reflector; bd a convex lens (of 12 inches focus); e the position of the eye; and fg he the course of the rays.

"In some cases a different construction will be prefe rable. Those eyes, which without assistance are adapted to seeing near objects alone, will not admit the use of a convex glass; but will on the contrary require one that is concave to be placed in front, to render the distant objects distinct. The frame for a glass of this construction is represented at ik, (fig. 49.) turning up on the same hinge at h with a convex glass in the frame I m, and moving in such a manner, that either of the glasses may be turned alone into its place, as may be necessary to suit an eye that is long or short sighted.

Those persons, however, whose sight is nearly perfect, Perspecmay at pleasure use either of the glasses.

"The instrument represented in that figure differs moreover in other respects from the foregoing, which I have chosen to describe first, because the action of the reflectors there employed would be more generally understood. But those who are conversant with the science of optics will perceive the advantage that may be derived in this instance from prismatic reflection; for when a ray of light has entered a solid piece of glass, and falls from within upon any surface, at an inclination of only twenty-two or twenty-three degrees, as above supposed, the refractive power of the glass is such as to suffer none of that light to pass out, and the surface becomes in this case the most brilliant reflector that can be employed.

"Fig. 58. represents the section of a solid prismatic piece of glass, within which both the reflections requisite are effected at the surfaces a b, b c, in such a manner that the ray fg, after being reflected first at g, and again at h, arrives at the eye in a direction he at right angles to fg.

"There is another circumstance in this construction necessary to be attended to, and which remains to be explained. Where the reflection was produced by a piece of plain glass, it is obvious that any objects behind the glass (if sufficiently illuminated) might be seen through the glass as well as the reflected image. But when the prismatic reflector is employed, since no light can be transmitted directly through it, the eye must be so placed that only a part of its pupil may be intercepted by the edge of the prism, as at e, fig. 48. The distant ob jects will then be seen by this portion of the eye, while the paper and pencil are seen past the edge of the prism by the remainder of the pupil.

"In order to avoid inconvenience that might arise from unintentional motion of the eye, the relative quantities of light to be received from the object, and from the paper, are regulated by a small hole in a piece of brass, which by moving on a centre at c, fig. 49. is capable of adjustment to every inequality of light that is likely to occur.

"Since the size of the whole instrument, from being so near the eye, does not require to be large, I have on many accounts preferred the smallest size that could be executed with correctness, and have had it constructed on such a scale, that the lenses are only three-fourths of an inch in diameter.

"Though the original design, and principal use of this instrument is to facilitate the delineation of objects in true perspective, yet this is by no means the sole purpose to which it is adapted; for the same arrangement of reflectors may be employed with equal advantage for copying what has been already drawn, and may thus assist a learner in acquiring at least a correct outline of any subject.

"For this purpose the drawing to be copied should be placed as nearly as may be at the same distance before the instrument that the paper is beneath the eye-hole, for in that case the size will be the same, and no lens will be necessary either to the object, or to the pencil.

"By a proper use of the same instrument, every purpose of the pentagraph may also be answered; as a painting may be reduced in any proportion required, by pla

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ing which it receives many streams and rivulets, falls into the Tay at Abernethy.

Perspee- cing it at a distance in due proportion greater than that of the paper from the instrument. In this case a lens becomes requisite for enabling the eye to see at two unPerth equal distances with equal distinctness, and in order that one lens may suit for all these purposes, there is an advantage in carrying the height of the stand according to the proportion in which the reduction is to be effected.

"The principles on which the height of the stem is adjusted will be readily understood by those who are accustomed to optical considerations. For as in taking a perspective view the rays from the paper are rendered parallel, by placing a lens at the distance of its principal focus from the paper, because the rays received from the distant objects are parallel; so also when the object seen by reflection is at so short a distance that the rays received from it are in a certain degree divergent, the rays from the paper should be made to have the same degree of divergency in order that the paper may be seen distinctly by the same eye; and for this purpose the lens must be placed at a distance less than its principal focus. The stem of the instrument is accordingly marked at certain distances to which the conjugate foci are in the several proportions of 2, 3, 4, &c. to 1, so that distinct vision may be obtained in all cases, by placing the painting proportionally more distant.

"By transposing the convex lens to the front of the instrument and reversing the proportional distances, the artist might also enlarge his smaller sketches with every desirable degree of correctness, and the naturalist might delineate minute objects in any degree magnified."

PERSPECTIVE Gluss, or Graphical Perspective. See DIOPTRICS.

PERSPIRATION, in Physiology, the excretion of a fluid 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. PERSPICUITY, properly signifies the property which any thing has of being easily seen through; hence it is generally applied to such writings or discourses as are easily understood.

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PERSPICUITY, in composition. See ORATORY, N° PERTH, a county of Scotland, including Menteith, Braidalbin, Athol, Stratherne, part of Gowrie, and Perth Proper; is bounded by Badenoch and Lochaber on the north and north-west; by Marr on the northeast; by Argyle and Lennox on the west and southwest; having Clackmannanshire, part of Stirlingshire, and the Forth to the south; the shires of Kinross and Fife to the south-east, and Angus to the east. It extends above 70 miles in length, and near 60 at its greatest breadth, exhibiting a variety of Highlands and Lowlands; mountains, hills, dales, and straths, diversified with pasture grounds, corn fields, and meadows; rivers, lakes, forests, woods, plantations, inclosures, towns, villages, and a great number of elegant seats, beautifully situated, belonging to noblemen and gentlemen. The chief rivers of Perthshire are the Tay, the Teith, and the Erne, besides a great number of subordinate streams. The river Tay is famous for its salmon-fishery. The river Erne rises from Loch Erne, a lake seven miles long, in the mountainous country of Stratherne: this river, after a course of 34 miles from west to east, dur

Perth

"The Tay (says a late traveller), on the southern Heron's bank of which the city of Perth stands, is truly a noble Tour, 1792. river. It rises in Braidalbin, on the frontiers of Lorne. Before it has advanced many miles from its source, its stream is considerably augmented by the accession of several small vills. Soon after, it diffuses its waters into a small lake called Loch Dochart; and indeed the river itself there bears rather the name of the Dochart. Continuing its course from Loch Dochart, it soon again expands into another lake, Out of this it proceeds to Killin, still bearing, if I remember right, the name of the Dochart. Here it meets with another river which flows hither by a more north-easterly course. The waters are diffused into the famous Loch Tay, 16 miles in length. Issuing from this spacious lake at Kenmore, the Tay is soon after increased by the accession of the Lyon. It proceeds onward in an eastern direction through Athol, receiving as it advances all the waters in the country, till at Logierait it is joined by the large river of Tummel. Here it bends to the south, and advancing about eight miles reaches Dunkeld; whence taking a more northern direction, it continues its course towards Perth; being as it advances still augmented by the accession of various tributary streams, the most considerable of which is the Almond. At Perth it turns to the south-east, and receiving as it proceeds the waters of the Erne, passes by Abernethy, once the capital of the Pictish kingdom. Soon after this, it expands itself to the breadth of three miles. Contracting its breadth, as it approaches Dundee, it there opens into the German ocean.

"Such is the noble river; on the southern bank of which, where it has increased into a vast body of water, and not a great many miles above where it discharges itself into the ocean, Perth is advantageously situated. A person acquainted with the general character of great rivers, and with their influence in determining the aspect and the fertility of the districts through which they pass, might readily, without farther knowledge of the local circumstances than what is conveyed in this account of the course of the Tay, and of the situation of Perth upon it, conclude the city to stand amid delightful scenery, and to enjoy most of the advantages which natural circumstances afford, for the promotion of trade and industry."

Freestone, lead, iron, and copper ores, with some lapis calaminaris, are found in different parts of Perthshire. The soil, being generally rich and well manured, produces excellent wheat, and all kinds of grain. The hilly country abounds with pasture for the black cattle, horses, sheep, goats, and deer. The heaths, woods, and forests, are stored with variety of game; the rivers teem with salmon and trout; the gardens and orchards are stored with all kinds of herbs, roots, apples, pears, cherries, plums, and almost every species of fruit found in South Britain. The houses and attire, even of the commonalty, are neat and decent; and every peasant can produce a good quantity of linen, and great store of blankets, made in his own family. Indeed, this is the case through all the Lowlands of Scotland. Flax is reared by every husbandman; and being dressed at home, is spun by the females of his family into thread for linen; this is woven by country weavers, of whom there is a great number through all the low country, and afterA a 2 wards

Perth.

wards bleached or whitened by the good-wife and her servants; so that the whole is made fit for use at a very small expence. They likewise wash, card, spin, and weave their wool into tartan for plaids, kersies, and coarse russet-cloth, for common wearing, besides great part of it which is knit into caps, stockings, and mitts. Plaids, made of the finest worsted, are worn either plain or variegated, as veils, by women of the lower, and even of the middle rank; nay some years ago, ladies of fashion wore silken plaids with an undress: this is a loose piece of drapery, gathered about the head, shoulders, and waist, on which it is crossed, so as to leave the hands at liberty, and produces a very good effect to the eye of the spectator. The Lowlanders of Perthshire are civilized, hospitable, and industrious: the commerce of the country consists chiefly in corn, linen, and black cattle: there are, moreover, some merchants who trade to fo- . reign countries. -For an account of the different divisions of this country above mentioned, see the articles as they occur in the order of the alphabet.

The population of this county in 1801 amounted to 126,366 (A), and in 1811 to 135,093.

The following table shows the state of the population, according to its parishes at two different periods.

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Methven

1790

1786

Parishes.

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1320

1 Aberdalgy

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60 Monivaird

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Aberfoil

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Monzie

1192

1136

Abernethy

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1749

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Muckhart

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5 Alyth

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Muthil

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Arngask

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65 Perth

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Auchterarder

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Port

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Auchtergaven

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Rattray

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Balquhidder

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Redgorton

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10 Blackford

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Blair Athol

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70 St Madoes

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Blairgowrie

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1083

1090

Bendothy

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1442

Callander

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1280

45 Caputh

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Cargill

1897

1720

75 Tulliallan

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76 Weem

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Collace

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Comrie

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20 Crieff

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118,903

Culross

1695

1442

Cupar

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Dron Dull

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25 Dumbarnie

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Dumblane

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Dunkeld

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Dunkeld, Little

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Dunning

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30 Errol

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Forgandenny

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Fortevoit

1164

Fortingal

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Fossaway

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35 Foulis, Wester

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970

Increase, 14,371

See PERTHSHIRE, SUPPLEMENT.

PERTH Proper, stretching 20 miles in length, and at some places 15 in breadth, is bounded on the north-east, by the Carse of Gowrie; on the east, by Angus; on the west, by Stratherne; on the north, by Athol; and on the south, by the frith of Tay. This is likewise a fruitful country, populous and well cultivated, abounding with gentlemen who possess opulent estates; with farmers who understand agriculture; and with manufacturers who turn their industry to great account. Northeastward from Perth to Brechin lies the vale of Strathmore, one of the most fertile districts in Scotland, which gives the title of Earl to the noble family of Lyon. PERTH,

(A) It is supposed that there is some error in the statement of the population in 1801, by, which it appears to be less than in 1790 and 1798. But by the return of the population of the town of Perth, the amount in 1801 is only 14,878; and in 1791 it was nearly 20,000, which will account for the difference.

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PERTH, the capital of the county of that name, is an agreeable, populous town, situated 20 miles within land, on the south bank of the river Tay. It was otherwise called St Johnston's, from a church dedicated to St John, as the patron of the place. It is a royal borough, second in dignity to the metropolis, the seat of a large presbytery, and gave the title of Earl to the family of Drummond, which is now forfeited. James Drummond, 4th earl, was created duke of Perth by James II. for adhering to whose interests he was outlawed. His two grandsons were attainted in 1745. No less than 14 national councils have been held at Perth between 1201 and 1459. But the oldest was at Scone, A. D. 956. Perth, in the reign of Edward I. of England, was possessed by the English, who secured it with fortifications: but after an obstinate resistance, they were expelled by Robert Bruce. In the year 1715, the rebels made it a place of arms, and retired to it, after the battle of Dumblane; but they were in a little time dislodged by the duke of Argyle, and retreated northwards with the pretender. They possessed it also in 1745. The pretender was proclaimed king, new magistrates were appointed, and an attempt was made to fortify it. The town is populous and handsome; the streets are well paved, and tolerably clean at all times; and the houses, though not stately, make a very decent appearance. Both the streets and houses are, for the greater part, disposed in a regularity of plan, which proves them not to be of the most remote antiquity. It is indeed true that the level situation, being singularly favourable to regularity, might, even from the first, give this an advantage over many of our old boroughs. Several streets run in a direction parallel with the river, as far as a right line can bear this relation to a curve line, nearly between east and west: these are again intersected by others extending between north and south. It should seem that anciently particular streets were inhabited, each by a particular class of artisans. The names still preserved seem to indicate as much. The shop-keepers or merchants occupied one street; the hammermen a second; and other crafts occupied, in the same manner, each a separate street. Many of the houses in that street called the Water-gate, seem to be very old buildings. Towards the south end of the Water-gate stands the famous palace of the Gowrie family. The house, and the very room, where the attempt of the Gowries to seize or assassinate the king was supposed to have been made, is now converted into barracks for a train of artillery; but the back-stair, down which the Ruthvens were thrown, is pulled down. This strange event, however magnified or attested by contemporary writers, is made up of so many improbabilities, or circumstances for which no reason can be assigned, that Sir David Dalrymple, in republishing the account printed by authority, 1600, preparatory to his further observations on it, seems justified in absolutely discrediting a fact which passed for problematical with so many persons at the very time. Dr Robertson supposes it a plot of Elizabeth to get James into her power. Mr Cant having discussed the whole story of the conspiracy in his Muse's Threnodie, p. 185-261, concludes, "that as this would have been a very impolitic measure, the best way of accounting for it is by James's known hatred to the Puritans, and wish to get rid of two popular characters." The king had been seized and forced from his favourites by the father of the Ruthvens 12 years before

(1582), and though he affected to forgive him, took the first opportunity to condemn and execute him as a traitor, 1584. Mr Camden was too good a courtier to speak with impartiality of any part of this weak monarch's conduct. Though the name of Gowrie was abolished, the title of Ruthven was revived in the person of Sir Thomas Ruthven of Freeland, whom Charles II. 1651, created Lord Ruthven: but the honour, on the death of his son David in 1704, devolved on Isabel, surviving daughter of his second sister, who married Sir Francis Ruthven, and was succeeded, 1732, by his son James.

The castle of Perth stood near the red bridge, which terminated the narrow street called Skinner-gate. At the end of the Castle street another narrow street leads west to the Black-friars, called Couvre feu-row, where the curfeu bell was. The kings of Scotland before James II. were crowned at Scone, and resided at Perth as the metropolis of the nation. James resided and was educated in the castle of Edinburgh, and was crowned there 1437. The parliaments and courts of justice were removed from Perth to Edinburgh, but Perth kept its priority till 22 James III. 1482.

The church in which John Knox harangued is still standing, and is now divided into three; named the east, the middle, and the west kirks. The east kirk was lately very handsomely modernised within. There is an old hospital, a considerable building, the founding of which is ascribed to James VI. The townhouse shuts up the eastern end of the High-street. A monastery of Carthusians was here established by King James I. of Scotland, who lost his life on the very spot, by the treachery of Athol and his accomplices. The king was buried in a very stately monument in this place, which was called monasterium vallis virtutis, one of the most magnificent buildings in the kingdom, which with the rest was destroyed by the populace. James VI. created George Hay commendator of the Carthusian priory, giving him all its emoluments, with a vote and seat in parliament; but these not being sufficient to support the title, he surrendered it back to the king. The only remains of this magnificent structure is to be seen in the carved stones with which the south east porch of St John's church is built, now greatly decayed. The king's garment full of stabs was preserved here after the reformation.

The town was anciently provided with a stone bridge over the river, which an inundation swept away; but a new and very fine one has lately been built, the most beautiful structure of the kind in North Britain, and was designed and executed by Mr Smeaton. Its length is 900 feet; the breadth (the only blemish) 22 within the parapets. The piers are founded 10 feet beneath the bed of the river, upon oaken and beechen piles, and the stones laid in puzzolano, and cramped with iron. There are nine arches, of which the centre is 75 feet in diameter. This noble work opens a communication with all the different great roads of the kingdom, and was completed at the expence of 26,000l. Of this the com missioners of forfeited estates, by his majesty's permission, gave 11,000l.; Perth 2000l.; private subscribers 47 561.; the royal boroughs 500l. But still this great work would have met with a check for want of money, had not the earl of Kinnoul, with his characteristic public spirit, advanced the remaining sum, and taken the se

curity

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