great plague of London in 1665, the deaths from the plague were, in June, 590; in July, 4129; in August, 20,046; in September, 26,230; in October, 14,373; in November, 3449; while in December, they were less than 1000.

The exact nature of the disease is unknown. A poison whose characters evade all chemical and microscopical examination, is absorbed, and alters at once, or after a short stage of incubation, the composition of the blood and the condition of the tissues.

With respect to treatment, little can be done to arrest the progress of the disease in any individual case. The patient should, if possible, be removed at once from the source of the disease; he should be exposed freely to fresh air; his secretions should be duly regulated, and his strength supported as far as possible. Friction with olive oil has been strongly recommended, but subsequent experience has not confirmed the first reports in its favour. But although treatment is comparatively valueless, much may be done towards guarding against the attacks of the disease. There can be little doubt that it is in consequence of the free external use of cold water, perfect cleanliness, moderate habits of life, and superior ventilation, that European (especially English) residents in the infected cities of the Levant are comparatively exempt from this disease. It is very possible that inunction of the body with olive oil may be (as has been asserted) a useful prophylactic agent, although it fails to cure the disease. It is almost needless to add, that all unnecessary communication with the sick, or contact with clothes or other matter that may have been infected with the poison, should be as much as possible avoided.

PLAICE (Platessa vulgaris), a species of Flounder (q. v.), much resembling the common flounder, but rather broader in proportion to its length; the upper surface of the body and the fins olive-brown, marked with large bright orange spots; a row of similar spots on the dorsal fin and on the anal fin; no tubercular asperities on any part of the body, but a curved row of bony tubercles on the eye-side of the head. The P. inhabits sandy and muddy banks, not in very deep water, and is very abundant on most parts of the British coasts, as well as on those of continental Europe. Like the common flounder, it often ascends slow rivers to some distance from the sea, and it has even been found to thrive well when transferred to fresh-water ponds. It feeds on worms, molluscs, small crustaceans, and young fishes. It has been known to attain the weight of fifteen pounds, but a P. of seven or eight pounds is accounted large. It is taken both by lines and trawl-nets. It is in considerable esteem for the table, although so plentiful in the British markets, that it is in general very cheap.

PLAID, a woollen garment, in the form of a large scarf, to wrap round the body, and used chiefly among the rural population of Scotland. See

TARTAN.

PLAIN, in Geography, is an extensive tract of country which, on the whole, preserves a nearly uniform elevation. When referred to the level of the sea, plains may be distinguished into low plains or lowlands, and elevated plains called plateaux or Table-lands (q. v.). Plains differ much in appearance, according to the nature of their soil and climate, from the frightful sandy wastes of Africa, to the luxuriant fertility of the South American silvas. They are occasionally crossed by hills of moderate altitude, which, however, are generally detached, and exhibit no connection with

any neighbouring mountain system. These hills often, as in the North American plains, degenerate into mere undulations, perfectly uniform in structure. The term 'plains is, in a limited sense, confined to the plains of Western Europe: those of other parts of the world receiving special designations, and differing from each other in many important points; thus, we have the Steppes (q. v.) of Eastern Europe and Asia; the Deserts (q. v.) of Arabia and Africa; the Savannahs (q. v.) and Prairies (q. v.) of North America; and the Llanos (q. v), Pampas (q. v.), and Silvas (q. v.) of South America The chief plains of Europe are, the country stretching from the foot of the Carpathians in Galicia to the Ural Mountains (including Poland and Russia), the drainage-area of the Danube in Hungary, and the portion of Europe which is bounded by the Elbe, the Harz Mountains, France, and the sea. Plains of comparatively small extent, but presenting the necessary characteristics in perfection, are found in almost all countries.

PLAINSONG, or CANTO FERMO (Ital.), a name given by the Church of Rome to the ecelesiastical chant. It is an extremely simple melody, admitting only notes of equal value, rarely extending beyond the compass of an octave, and never exceeding nine notes, the staff on which the notes are placed consisting of only four lines. The clefs are C and F. St Ambrose is considered to have been the inventor or systematiser of Plainsong. His labours consisted in selecting from the extremely complicated system of the Greeks a set of scales sufficiently few and simple for a very rude people. During the two centuries succeeding the death of Ambrose, his institutions fell into utter confusion. Gregory the Great revived and perfected them, recasting them into an Antiphony, or authorised body of ecclesiastical music, and brought Plainsong into the state in which it is yet used in the Roman church. See AMBROSIAN CHANT and GREGORIAN CHANT.

PLAINTIFF, in English and Irish Law, is the name given to the person who institutes and maintains a civil action or suit against another, who is called the Defendant. In Scotland, a plaintiff is called a Pursuer. But in both countries, many proceedings and applications of a civil nature are commenced by petition; and hence the party taking the initiative is called the Petitioner.

PLAN, a word frequently applied to all kinds of architectural drawings, but which ought to be limited to those which represent the horizontal sections of the various floors of buildings. Plans shew the disposition of the apartments and walls, with the situation of the fireplaces, cupboards, doors, &c.; they, in fact, represent the different stories as they actually appear as seen from above, when the walls are built two or three feet above the level of each floor.

PLANARIA, a genus of worms placed by Cuvier among Entozoa, although not parasites, but inha bitants of stagnant waters, because of their great resemblance to some of the entozoic parasites, and particularly to flukes. The species are numerons. Some inhabit fresh, and others salt water; they feed on small annelids, molluscs, &c. They are generally found creeping among confervæ, or on the stems of plants. Many of the larger marine species are able to swim freely by flappings of the broad margins of their bodies. The body of a planaria seems to be entirely gelatinous; but M. de Quatrefages has detected under the skin an arrangement of muscular fibres. Two red specks in the fore-part of the body of many species have been supposed to be eyes; but there is no proof of

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it. Planariæ are hermaphrodite, but copulate for mutual impregnation. Their power of multiplication by division is very great; if an individual be cut in pieces, each piece continues to live and feel, and even if it be the end of the tail, as soon as the first moment of pain and irritation has passed, begins to move in the same direction as that in which the entire animal was advancing, as if the body was actuated throughout by the same impulse; and, moreover, every division, even if it is not more than the eighth or tenth part of the creature, will become complete and perfect in all its organs.'Rymer Jones.

PLANE, in Geometry, is a surface without curvature, and the test of it is, that any two points whatever being taken in the surface, the straight line which joins them lies wholly in the surface. When two planes cross or intersect one another, their common section is a straight line; and the inclination of the planes to each other is measured by taking any point in their common section, and drawing from it two straight lines, one in each plane, perpendicular to the common section; the angle contained by these lines is the angle of inclination of the planes. When the angle is a right angle, the planes are perpendicular to each other.

PLANE (Platanus), a genus of trees, the sole genus of the natural order Platanacea, regarded by many as a sub-order of Amentacea (q. v.). The flowers are in globose stalked catkins; the ovary is one-celled, and contains one or two pendulous

the south of Europe. Many fine trees exist in England, but they were at one time much more numerous, great part having died in the end of last century, probably from some disease similar to the potato disease. The injury often done to the young leaves by late frosts, and the insufficient duration of the summer for the proper ripening of the wood, render the P. less suitable for Scotland; yet there is a tree at Gordon Castle 66 feet high. No tree better endures the atmosphere of a large city, and there are no finer trees within the precincts of London than the P. trees which are to be seen in some places there. In its native regions the P. attains an immense size. One tree, banks of the Bosporus, is 141 feet in circumwhich grows in the meadow of Buyukdere on the ference at the base-its trunk being apparently formed of several which have grown together extends its branches 45 feet from the trunk, and is believed to be more than 2000 years old. The wood of the P., when young, is yellowishwhite; when old, it is brownish, fine grained, takes a high polish, and is esteemed for cabinet-making. A rich alluvial soil and the vicinity of water are most suitable to this tree.-The NORTH AMERICAN P., or BUTTONWOOD (P. occidentalis), is a very similar tree. It is the largest deciduous tree of the United States, and abounds on the banks of the great rivers of the middle states. Its timber is not very valuable, and is very liable to decay. It is sometimes called the Cotton Tree, from the wool which, as in the former species, covers the under side of the young leaves, and which, being cast off, floats about on the wind. A tree of this species on the bank of the Thames, in Chelsea Hospital gardens, is 115 feet high, with a trunk five feet in diameter. -The name P.-tree is commonly given in Scotland to the Sycamore (Acer pseudo-platanus), which resembles the true planes in its foliage.

PLANE, a tool used for rendering the surface of wood smooth and level. It consists of an oblong block of wood or metal (the latter is only just coming into use), with an opening through the centre; this opening is square on the upper side, and is always large enough to admit the cutting instrument; it diminishes down to a mere slit on the under side, merely wide enough to allow the cutting edge of the plane-iron and the shaving of wood which it cuts off to pass through. The form

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Plane Tree (Platanus orientalis). ovules. The species of P. are few; natives of temperate climates in the northern hemisphere; tall trees, with smooth whitish bark, which annually scales off in large pieces, and large palmate deciduous leaves. The catkins are small, and curiously placed one above another on the same stalk; they are pendulous, with long stalks, and give plane trees a very peculiar appearance, especially in winter, when they remain after the leaves have fallen.-The ORIENTAL P. (P. orientalis), a native of Greece and the East, was much admired and planted, both by the Greeks and the Romans, as an ornamental tree; no other tree, indeed, commanding equal admiration; and, for centuries, the youth of Greece assembled under the shade of planes, in the groves of Academus and elsewhere, to receive lessons in philosophy. To this day, the P. is generally planted for shade and ornament in

α

Fig. 1.

of this opening will be seen at a, fig. 1, which repre sents the section of a common jack-plane. The essential part of the tool is the plane-iron, a piece of steel with a chisel-shaped edge, and a slot in its centre for a large headed screw to work and to attach to it a strengthening plate. Fig. 2 shews the plane-iron, and fig. 3 the same with the strengthening plate attached; these are shewn in their proper position at bd in the section fig. 1, and they are held in place by the hard-wood wedge (fig. 4), seen also in the section at c. By driving in the wedge, the irons are held very firmly in their place, and they are so adjusted that only the fine

planets. The name planet is of considerable antiquity, and was applied to these dependents of the sun to distinguish them from the myriads of luminous bodies which stud the sky, and which present to the naked eye no indication of change of place (see STARS). The planets at present known are, in the order of their distance from the sun, Mercury, Venus, the Earth, Mars, the Planetoids (q. v.), Jupiter, Saturn, Uranus, and Neptune. Six of these, Mercury, Venus, the Earth (which was not, however, then reckoned a planet), Mars, Jupiter, and Saturn, were known to the ancients; Uranus was discovered by Sir William Herschel (q. v.) in 1781 ; and Neptune, after having its position and elements determined theoretically by Leverrier and Adams, was discovered by M. Challis, and afterwards by Dr Galle, in 1846. The Planetoids, which now number 81, have all been discovered during the present century. Five of the planets, the Earth, Jupiter, Saturn, Uranus, and Neptune, are attended by one or more satellites; Uranus (generally), Neptune, almost the whole of the Planetoids, and all the satellites except the Moon, are invisible to the naked eye. The visible planets can be at once distinguished from the fixed stars by their clear steady light, while the latter have a sparkling or twinkling appearance. The comparative proximity of the planets may be proved by examining them through a telescope of moderate power, when they appear as round luminous disks, while the fixed stars exhibit no increase of magnitude. The planets, as observed from the Earth, move sometimes from west to east, sometimes from east to west, and for some time remain stationary at the point where progression ends and retrogression commences. This irregularity in their movements was very puzzling to the ancient astronomers, who invented various hypotheses to account for it. See PTOLEMAIC SYSTEM and EPICYCLE. The system of Copernicus, by assuming the sun, and not the earth, as the centre of the system, explained with admirable simplicity what seemed before a maze of confusion.

The planetary orbits differ considerably in their degrees of excentricity, the Planetoids, Mars, and Mercury being most, and the larger planets least excentric. No two planets move exactly in the same plane, though, as a general rule, the planes of the larger planets most nearly coincide with that of the ecliptic. The latter are consequently always to be found within a small strip of the heavens extending on both sides of the ecliptic; while the others have a far wider range, Pallas, one of them, having the angular elevation of its orbit no less than 34 35 above the ecliptic. According to Kepler's Laws (q. v.), the nearer a planet is to the sun the shorter is the time of its revolution. The arrangement of the planets in the solar system bears no known relation to their relative size or weight, for though Mercury, Venus, and the Earth follow the same order in size and distance from the sun, yet Mars, which is further from the sun, is much less than either the Earth or Venus, and the Planetoids, which are still further off, are the least of all. Jupiter, which is next in order, is by far the largest, being about 14 times as large as all the others together; and as we proceed further outwards, the planets become smaller and smaller, Saturn being less than Jupiter, Uranus than Saturn, and Neptune than Uranus.

With reference to their distance from the sun, as compared with that of the Earth, the planets are divided into superior and inferior; Mercury and Venus are consequently the only inferior' planets, all the others being superior.' The inferior planets must always be on the same side of the Earth as the sun is, and can never be above the horizon of

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any place (not in a very high latitude) at midnight; they are always invisible at their superior and inferior conjunctions, except when, at the latter, a Transit (q. v.) takes place. The superior planets are likewise invisible at conjunction, but when in opposition they are seen with the greatest distinctness, being then due south at midnight. The time which elapses from one conjunction to its corresponding conjunction is called the synodic period of a planet, and in the case of the inferior planets must always be greater than the true period of revolution.

Mercury, the planet which is nearest the sun, is also, with the exception of the Planetoids, the smallest (being only 3 times the size of the moon), and performs its revolution round the sun in the shortest time. Its greatest elongation is never more than 28° 45', and consequently it is never above the horizon more than two hours after sunset, or the same time before sunrise; on this account, and from its small apparent size (5′′ to 12), it is seldom distinctly observable by the naked eye. It shines with a peculiarly vivid white or rose-coloured light, and exhibits no spots.Venus, the next in order of distance and period, is to us the most brilliant of all the planets. Its orbit is more nearly a circle than any of the others, and when at its inferior conjunction, it approaches nearer the Earth than any other planet. Its apparent angular dimensions thence vary from 10" at the superior, to 70" at the inferior conjunction. Its greatest elongation varies from 45° to 47° 12', and therefore it can never be above the horizon for much more than three hours after sunset, or the same time before sunrise. While moving from the inferior to the superior conjunction, Venus is a morning star, and during the other half of its synodic period an evening star. When this planet is at an elongation of 40°, its brilliancy is greatest, far surpassing that of the other planets, and rendering a minute examination through the telescope impossible. At this period it sometimes becomes visible in the daytime, and after sunset is so bright as to throw a distinct shadow. Astronomers have repeatedly attempted to ascertain the nature and characteristics of its surface, but its brightness so dazzles the eyes as to render the correctness of their observations at best doubtful. From the changes in the position of dusky patches on its surface, which have been frequently noticed, it is concluded that it revolves on its axis, and that its equator is inclined to the plane of its orbit at an angle of 75°, but many astronomers (Sir John Herschel included) profess to doubt these conclusions. Both Venus and Mercury necessarily exhibit phases like the moon.-The Earth, the next planet in order, will be found under its own name; it has a single satellite, the Moon (q. v.).-Mars, the first of the superior planets, is much inferior in size to the two previous, its volume being about 4th of the Earth's, and, after Mercury, its orbit is much more excentric than those of the other planets. When it is nearest to the Earth (i. e., in opposition), its apparent angular diameter is 30"; but when furthest from it (i. e., in conjunction), its diameter is not more than 4". Mars is less known than the rest of the superior planets, owing to its not possess ing a satellite, by the motions of which its attractive force (and hence its mass and density) could be estimated. It shines with a fiery red light, and is a brilliant object in the heavens at midnight when near opposition; when seen through the telescope its surface appears to be covered with irregular blotches, some of them of a reddish, others of a greenish colour, while at each pole is a spot of dazzling white. The red spots are surmised to be