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CHARACTERS OF THE PLANETS.

(I.) The character of the SUN O (a circle) feems to be merely a delineation of this glorious centre of the fyftem; and needs no engraving to render it intelligible. The character (a circle with a knob in the middle) imitating a buckler, has been attributed to the fun; fo has alfo a cone. But a circle feems greatly preferable.

(II.)

the character for the Moon, is nearly defcriptive of her appearance when partly enlightened, therefore needs no explanation.

(III.) MERCURY is fignified by the caduceus (), an attribute conftantly attendant on the heathen deity of this name.

(IV,) VENUS is expreffed by (?) a mirror with a handle; aptly hinting the perfonal attention of the goddess of beauty. The ancient mirrors were polished brass.

(V.) MARS (3) a fpear and fhield, as god of war.

(VI.) JUPITER (4) the first letter of his Greek name (Zeus) with a stroke of abbreviation.

(VII.) SATURN () a fickle; originally a fcythe, which refers to his Greek name Cronos (Time), whofe effect is, to cut down all things without remorfe.

(VIII.) The earth is marked by a circle with a line cross it.

(IX.) The character adopted at present for the Georgium Sidus (Mr. HERSCHEL's difcovery) is an H with a globe pendent from the middle of the cross ftroke.

V.

B b

PLATE

PLATE XI.

COMET S.

Fig. 1. reprefents the famous comet of 1680, which, partly becaufe of its own remarkableness, and partly because it had the fortune to be observed by the ableft aftronomers and mathematicians, is among the most notorious of thefe phenomena; and may be confidered as having laid the foundation for a juft theory of their principles and motions.

It advanced fo near to the fun, that at its perihelion it was not above a fixth part of the diameter of that luminary from the furface thereof. NEWTON thought it fuftained a heat two thousand times greater than that of red-hot iron. HALLEY fays, this comet, in paffing its fouthern node, came within the length of the fun's femidiameter of the orbit of our earth; had the earth been then in that part of her orbit, the attraction would have changed as well the plane of the terreftrial orbit, as the length of the year. Who can compute the power of the fhock, had it been commiffioned to ftrike us! The further limit of its orbit is calculated to reach fourteen times the diftance of Saturn.

This plan fhews the path of the comet while visible to the inhabitants of earth; advancing in Nov. 1680, returning, (and from the fituation of the earth feen much fooner) from December to March 1681. The apparent length of its tail increases according to the ftation from whence it is feen. It is many millions of miles in real extent.

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Fig. 2. Comet feen 1744. From a drawing made at Cambridge. '

Fig. 3, The tail of a comet fo fituated, and feen fore-fhortened, turning downwards, that it fomewhat refembles a beard.

Fig. 4 and 5. Comets, whofe tails may be likened to a flaming fword. Many other forms fuggeft themselves to a fertile imagination; that thefe vapours may vary in their. properties is very fuppofable; but the general panic they have heretofore occafioned, together with the ftation from whence they were viewed, has been the most fruitful fource of tremendous and terrific appellations.

The tails of comets are fuppofed to be atmofpherical vapours furrounding thefe bodies, which are no fooner arrived fufficiently near the fun to feel his influence, than they bccome exceedingly rarified; and are extended proportionally to the heat of their fituation. They are extremely fine and pellucid, and permit the stars to be feen through them. The caufe of their afcent oppofite to the fun, is a difficulty, which feems beft anfwered by faying, they follow the impulfe of the folar rays; which, ftreaming from the fun, carry with them thefe extremely volatile particles through the free æther.

The vapours of the cometary tails have been calculated by obfervation, to be fo long as forty, fixty, or even eighty millions of miles.

Comets, like planets, appear from the earth to be direct, flower than their real motion, retrograde, and ftationary, during fo much of their courfe as can be feen.

LECTURE

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