Hence the masses of the Sun and Earth are to each other as the numbers 1479560.5 and 4.486113; therefore the 1 mass of the earth isthat of the 329809' Sun being unity. M. de la Place calculated the masses of Mars and Venus from the secular diminution of the obliquity of the ecliptic, and from the mean acceleration of the moon's motion. The mass of Mercury he obtained from its volume, supposing the densities of that planet and of the Earth reciprocally as their mean distance from the Sun, a rule which holds with respect to the Earth, Jupiter, and Saturn. The following table exhibits the masses of the different planets, that of the Sun being unity;

The densities of bodies are proportional to their masses divided by their bulks; and when bodies are nearly spherical, their bulks are as the cubes of their semi-diameters, of course the densities in that case are as the masses divided by the cubes of the semi-diameters.

PLANETS, motion of the. Each of the primary planets bend their course about the centre of the Sun, and are accelerated in their motions as they approach to him, and retarded as they recede from him; so that a ray drawn from any one of them to the Sun, always describes equal spaces, or areas, in equal times: whence it follows that the power which bends their way into a curve line, must be directed to the Sun. This power is no other than that of gravitation, which we have already proved to increase, as the square of the planet's distance from the Sun decreases. See GRAVITATION,

&c. But the universality of this law still further appears, by comparing the motions of the different planets: for the power which acts on a planet near the Sun is manifestly greater than that which acts on a planet more remote; both because it moves with greater velocity, and because it moves in a lesser orbit, which has more curvature, and separates further from its tangent, in arcs of the same length, than in a greater orbit. By comparing the motion of the planets, the velocity of a nearer planet is found to be greater than that of one more remote, in the proportion of the square root of the number which expresses the greater distance to the square root of that which expresses the lesser distance; so that if one planet was four times further from the Sun than another, the velocity of the first would be half the velocity of the latter; and the near planet would describe an arc in one minute, equal to the arc described by the other planet in two minutes; and though the curvature of the orbits were the same, the nearer planet would describe, by its gravity, four times as much space as the other would describe in the same time; so that the gravity of the nearer planet would appear to be quadruple, from the consideration of its greater velocity only. But be sides this, as the radius of the lesser orbit is supposed to be four times less than the radius of the other, the lesser orbit must be four times more curved; and the extremity of a small arc of the same length will be four times further below the tangent, drawn at the other extremity, in the lesser orbit than in the greater; so that though the velocities were equal, the gravity of the nearer planet would, on this account, only be found to be quadruple. Hence, on both these accounts together, the greater velocity of the nearer planet, and the greater curvature of its orbit, its gravity towards the Sun must be supposed sixteen times greater, though its distance from the Sun is only four times less than that of the other; that is, when the distances are as 1 to 4, the gravities are reciprocally as the squares of these numbers, or as 16 to 1. And in the same manner as this principle governs the motions of the primary planets of the great solar system, acts at their surfaces, and keeps their parts together; so it governs also the motions of the satellites, or secondary planets, in the lesser systems, of which the greater is composed, and is extended around them, decreasing in the, same manner as the

squares of the distances increase. The comets are evidently governed by the same law, since they descend with an accelerated motion, as they approach towards the Sun, and ascend again with a retarded motion, bending their way about the Sun, and describing equal areas in equal times, by rays drawn from them to his centre. See ASTRONOMY.

PLANETARIUM, an astronomical machine, contrived to represent the motions, orbits, &c. of the planets, as they really are in nature, or according to the Copernican system. A very remarkable machine of this sort was invented by Huygens, which is still preserved among the curiosities of the University at Leyden. In this planetarium, the five primary planets perform their revolutions about the Sun, and the moon performs her revolution about the Earth, in the same time that they are really performed in the heavens. Also the orbits of the Moon and planets are represented with their true proportions, eccentricity, position, and declination from the ecliptic, or orbit of the Earth. So that by this machine, the situation of the planets, with the conjunctions, oppositions, &c. may be known, not only for the present time, but for any other time, either past or yet to come, as in a perpetual ephemeris. There was exhibited in London, viz. in the year 1791, a still much more complete planetarium of this sort, called "a planetarium, or astronomical machine, which exhibits the most remarkable phenomena, motions, and revolutions of the aniverse, invented, and partly executed, by the celebrated M. Hahn, member of the academy of sciences at Erfurt; but finished and completed by M. A. de Mylius." This is a most stupendous and elaborate machine, consisting of the solar system in general, with all the orbits and planets in their due proportions and positions: as also the several particular planetary systems of such as have satellites, as of the Earth, Jupiter, &c.; the whole kept in continual motion by a chronometer, or grand eight-day clock: by which all these systems are made perpetually to perform all their motions exactly as in nature, exhibiting at all times the true and real motions, positions, aspects, phenomena, &c. of all the celestial bodies, even to the very diurnal rotation of the planets, and the unequal motions in their elliptic orbits. A description was published of this most superb machine, and it was purchased and sent as one of the presents to the Emperor of

China, in the embassy of Lord Macartney.

We shall now give a description of one of these machines in common use.

Fig. 1, Plate Planetarium, is an elevation of the mechanism of a planetarium; and fig. 2, a plan of the same. A, (fig. 1.) is a bail of brass representing the sun, supported by a wire screwed to a bridge, b, fixed beneath the board, BB, which supports the whole instrument; a is the section of an endless screw, which has a small handle on the end of its spindle to turn it by; it gives motion to a worm wheel, 60, of sixty teeth, the arbor of this wheel is a tube, and goes over the central wire sustaining the Sun; to its upper end is fixed the frame, EE, containing the wheel-work, and carrying the Earth, ✪, and Moon, Y. The plan (fig. 2.) is this frame of wheels, the upper plate of the frame being removed, d is the first wheel of sixty-four teeth, fixed fast to the central wire of the Sun, and having no motion, it works with another of sixty. four, on the same arbor, hh, with several others to be hereafter described; it turns another, f, of sixty-four, on whose arbor, g, the Earth is fixed; as d is fixed, and the next wheel, with its frame, EE, rolls round it, and is thereby turned upon its own axis; the wheel, f, which is on the other side, will have no motion on its axis, and the axis of the Earth fixed to it will remain parallel to itself, while it describes an orbit round the Sun, by the mo tion of the frame, EE. The next wheel, 60, upon the arbor, h, turns a pinion, 14, of fourteen teeth, (not seen in the plan) by the intervention of a wheel, 64, which does not alter its velocity; the arbor of the pinion is a tube, and fitted upon the central wire; at it upper end it supports the planet, Mercury, . The third wheel from the bottom on the arbor, h, has forty teeth, and by the wheel, 56, communi-. cates motion to a small wheel, of twentyfour, which has the planet, Venus, ?, fixed to its tubular arbor. The upper wheel of the arbor, h, has seventy-four teeth, and turns a pinion of six, on a tube, concentric with g, and with it the moon. There is a small wheel of fourteen teeth between the wheel and pinion, but it does not alter the velocity; k, (fig. 1.) is a thin brass ring seen edgewise, which has a wire diametrically across it, on which it turns as an axis, to set it at any given obliquity to the axis, g, supporting the Earth; the wire is fixed into a short tube, which turns stiffly in a hole made in the upper plate of the frame, EE, and thus the circle can be

turned round, while its plane continues oblique to the axis g; this ring represents the plane of the Moon's orbit, and is engraved with the different phases of the Moon. The Moon is not fixed to the arm which turns it, but its stem slides up and down in a short tube fixed to the arm, and rests upon the ring, so as to describe a parallel lane to it. On the end of the frame, E E, a pillar is erected, to support a small semicircular piece of brass, m, inclosing the Earth, and showing the line of light and darkness. Nisa tube screwed fast to the board, B B, by a flanch at the lower end: it fits the outside of the tube of the wheel, 60, beneath the board, and thus steadies the whole frame as it turns round; upon this tube long arms are fitted, carrying Mars, Jupiter, and the other superior planets;

but as there is no wheel-work to turn these, they are omitted in the plate. This instrument is defective in not having the diurnal motion of the earth upon its axis shown, and the rotation of the Moon's nodes; there have been instruments made, which show all these motions, and those of the superior planets with their satellites; but they are so complicated, that it would far exceed the limits of our plates to describe them.

The numbers of the teeth of the wheels of this planetarium are not correctly calculated to produce true revolutions of the planets introduced in it, as the fixed wheel, 64, and the wheel 64, on the axis, h, are equal; the latter and all the wheels on h h, revolve once in a tropical year; the wheels which turn Mercury are 60, upon h, turning 14, that is of

a tropical year, or 85.223185 days; this period, which is intended to be the tropical revolution of Mercury, viz. 87d. 23h. 14m. 35s. is made the synodical revolution by the mechanism, by reason of the wheel-work being carried round the Sun again in a year, by the frame, E, representing the Earth's radius vector; so that the planet Mercury goes from conjunction with the Earth to conjunction again, instead of going through the ecliptic only in this period, and the imperfection of the wheel work is rendered still more imperfect by its position, which ought to have been on a stationary bar, to have produced the true calculated effect; this error causes it to make just one revolution in a year more than intended. The tropical period of Venus is also turned into a synodic one, by the same fault in the position of the wheel-work; besides, the period itself being erroneous,

14

For Mercury, instead of 1, (or), put 2, in which case the wheel, 63, will produce 33 3 revolutions; and the Earth's arm will carry the 20 round oval in a year, making together, 4.15 revolutions of Mercury for one of the Earth's, which is very near the truth, producing one tropical revolution in 88d. Oh. 14m. 388.

For Venus, instead of 24, put 33, and one revolution per annum will be produced by the motion of the Earth's arm, and of another by the wheels, making together, 1.625 in each year, or one tropical revolution in 224d. 18h. 21m.

278.

The true synodic periods are:-of Mercury, 115.877d., and of Venus, 583.923d.; therefore the said periods, by the present wheel-work, are too short by more than thirty days in Mercury, and in Venus, by 364d., and upwards.

PLANIMETRY, that part of geometry which considers lines and plain figures, without considering their height or depth. See SURVEYING, &c.

::

PLANISPHERE signifies a projection of the sphere, and its various circles on a plane in which sense maps, wherein are exhibited the meridians, and other circles of the sphere, are planispheres. See MAP, SPHERE, &c.

PLANISPHERE, is more particularly used for an astronomical instrument used in observing the motions of the heavenly bodies. It consists of a projection of the celestial sphere upon a plane, representing the stars, constellations, &c. in their proper order; some being projected on the meridian, and others on the equator.

The use of the planisphere is to represent the face of the heavens for any day and hour; find, on the lesser moveable

plate, the month and day proposed, and turn the plate to the given day of the month standing against the hour and minute required; and the plate will then represent the face of the heavens, by showing what stars are then rising in the meridian, or what setting. 2. To know at what hour and minute any star rises or sets, &c. Turn the moveable plate till the given star reaches the horizon east or west, and against the given day, on the moveable plate, is the hour and minute on the exterior or immoveable one; and in the same manner may most of the problems usually resolved by the celestial globe be determined.

PLANT, in botany, an organic vegetable body, consisting of roots and other parts. Whether capable either of sensation, or of spontaneous motion, is not yet fully ascertained. It attaches itself to other bodies, in such a manner as to derive nourishment from them, and to propagate itself by seeds. The constituent parts of plants are the roots, stems, branches, rind, or bark, leaves, flowers, and seeds; which greatly vary, both in figure and size, according to the nature of particular trees, shrubs, &c. Their various appearances have induced botanists to divide the vegetable kingdom into orders, classes, genera, species, and varities; for an account of which see BOTANY.

According to the Linnæan system, plants take their denominations from the sex of their flowers, in the following manner-1. Hermaphrodite plants, are such as upon the same root bear flowers that are all hermaphrodite, as in most ge

nera.

2. Androgynous, male and female, such as upon the same root bear both male and female flowers, as in the class Monacia. 3. Male, such as upon the same root bear male flowers only, as in the class Diœcia. 4. Female, such as upon the same root bear female flowers only, as in the class Diccia. 5. Polygamous, such as, either in the same individual plant, or in different individual plants of the same species, have hermaphrodite flowers, and flowers of either or both sexes, as in the class Polygamia. All plants, however minute, are propagated by seed; and so easy is their cultivation, that in many instances they may be reared by parting their roots, or depositing layers, cuttings, &c. of the parent stock, in such soils as are most congenial to their nature. Hence some botanists consider them as somewhat analogous to animals: a conjecture that is strongly cor

roborated by the regular circulation of the sap throughout all their parts; and by the sleep of plants, or the faculty which some possess, of assuming at night a position different from that in which they appear during the day. In the second volume of the Manchester Transactions, we find some speculations on the perceptive power of vegetables, by Dr. Percival, who attempts to show, by the several analogies of organization, life, instinct, spontaneity, and self-motion, that plants, like animals, are endued both with the powers of perception and enjoyment. The attempt, though ingeniously supported, however, fails to convince. That there is an analogy between animals and vege tables is certain; but we cannot from thence conclude, that they either perceive or enjoy. Botanists have, it is true, derived from anatomy and physiology almost all the terms employed in the description of plants. But we cannot from thence conclude, that their organization, though it bears an analogy to that of animals, is the sign of a living principle, if to this principle we annex the idea of perception. Yet so fully is our author convinced of the truth of it, that he does not think it extravagant to suppose, that, in some future period perception may be discovered to extend even beyond the limits now assigned to vegetable life.

Mr. Good, the learned author of the translation of Lucretius, delivered in the spring of the present year, before the Medical Society of London, a discourse "On the general Structure and Physiology of Plants compared with those of Animals, and the mutual Convertibility of their Organic Elements," which contained much interesting matter, and many curious and ingenious speculations. He began by assuming, what indeed is the basis of the sexual system, that every thing that has life is produced from an egg; that the egg of the plant is its seed. The seed is sometimes naked, and sometimes covered with a pericarp, which is of various forms and structures: the seed itself consists internally of a corculum, or little heart, and externally of a parenchymatous substance, called a cotyledon, which is necessary for the germination and future growth of the seed, and may be denominated its lungs, or placentule. The corculum is the "punctum saliens" of vegetable life, and to this the cotyledon is subservient. The corcle consists of an ascending and descending part: the former is called its plumule, which gives birth to the trunk and branches; from

the latter spring the root and radicles. The position of the corcle in the seed, which is always in the vicinity of the eye, is a cicatrix, or umbilicus, remaining after the separation of the funis from the pericarp, to which the seed has been attached. The first radicle elongates, and pushes into the earth, before the plumule evinces any change: like the cotyledon, the radicles consist chiefly of lymphatics and air-vessels, which serve to separate the water from the soil, in order that the oxygen may be separated from the water. Hence originates the root, the most important part of the plant. The solid parts of the trunk of the plant are the cortex, or outer bark; the liber, or inner bark; the alburnum, or soft wood; lignum or hard wood; and medulla, or pitch. These lie in concentric circles; and the trunk enlarges, by the formation of a new liber, or inner bark, every year; the whole of the liber, excepting indeed its outermost layer, which is transformed into cortex, becoming the alburnum of the next, and the alburnum becoming the lignum. Hence a mark of any sort, as the initials of a name, which has penetrated through the outer into the inner bark, must in a long process of years be transferred to the central parts of the trunk. Independently of these more solid parts of the trunk, we generally meet with some portion of parenchyma and cellular substance: the vessels contained in this may be compared to arteries and veins, air vessels, and lymphatics. The lymphatics lie immediately under the cuticle, and in the cuticle, and by branching different ways are enabled to perform the alternating economy of inhalation and exhalation below these lie the arteries, which rise immediately from the root, and communicate nutriment in a perpendicular direction: interior to these lie the reducent vessels, or veins, which are softer and more numerous, and in young shoots run down through the cellular texture and the pith. Between the arteries and veins are situated the air vessels.

"The lymphatics of a plant may be often seen with great ease by merely stripping off the cuticle with a delicate hand, and then subjecting it to a microscope; and in the course of the examination, we are also frequently able to trace the existence of a great multitude of valves, by the action of which the apertures of the lymphatics are commonly found closed. Whether the other systems of vegetable vessels possess the same mechanism, VOL. X.

we have not been able to determine decisively: the following experiment, however, should induce us to conclude that they do. If we take the stem of a common balsamine, or of various other plants, and cut it horizontally at its lower end, and plunge it, so cut, into a decoction of Brazil wood or any other coloured fluid, we shall perceive that the arteries, or adducent vessels, as also the air-vessels, will become filled or injected by an absorption of the coloured liquor, but that the veins, or reduced vessels, will not become filled; of course evincing an obstacle in this direction to the ascent of the coloured fluid. But if we invert the stem, and in like manner cut horizontally the extremity which till now was uppermost, and plunge it so cut into the same fluid, we shall then perceive that the veins will become injected, or suffer the fluid to ascend; but rather the arteries will not; proving clearly the same kind of obstacle in the course of the arteries in this direction, which was proved to exist in the veins in the opposite direction; and which reverse obstacles we can scarcely ascribe to any other cause than the existence of valves.

"By this double set of vessels, moreover, possessed of an opposite power, and acting in an opposite direction, the one to convey the sap or vegetable blood forwards, and the other to bring it backwards, we are able very sufficiently to establish the phenomenon of a circulatory system."

The author admits that no experiments, nor observations, have been able to detect the existence of muscular or nervous fibres in vegetables; but notwithstanding this, in answer to those who maintain the necessity of a regular and alternate contraction and dilatation for the production of a circulatory system, both in animals and vegetables, he says, "still must we admit the competency of other powers to produce the same result, while we reflect on the facility with which the human cutis or skin, an organ destitute of all muscular fibres whatever, contracts and relaxes generally on the application of a variety of other powers; powers different in their nature, and in their effect palpable to the external senses: whilst we recal to mind that it is contracted by austere, and relaxed by oleaginous preparations; constringed by cold, and dilated by warmth: and that the opposite passions of the mind have a still more powerful influence on the same organ, since fear, apprehension, horror, will not only freeze and corrugate

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