of the shot. In this way of considering the matter, the flight of the bird represents the motion of the earth, and the flight of the shot the motion of the light proceeding from the object."

Many philosophers have attempted, not only to compare the light of the stars with that of the sun, but also to ascertain their distances by comparisons of this kind.

The Rev. Mr. Mitchell, in an elaborate and ingenious paper in the Transactions of the Royal Society, states, that our sun would still appear as luminous as the star Sirius, although removed to 400,000 times his present distance; and that the fixed stars cannot be nearer than this, if they be equal to the sun in lustre and magnitude, and that they are so is the opinion of the most celebrated astronomers of the present day. Euler, who has already been mentioned, makes the light of the sun equal to 6,500 candles at one foot distance; the moon equal to one candle at 74 feet distance; Venus to one at 421 feet; and Jupiter to one at 1320 feet. From this comparison it follows, the light of the sun exceeds that of the moon 364,000 times.It is therefore no wonder that the attempts which have been made by some philosophers to condense the light of the moon by lenses, have been attended with so little success. For, should one of the largest of these lenses even increase the light of the moon one thousand times, still, in this increased state, it will be three hundred and sixty-four times less than the intensity of the common light of the sun. The intensity of light has been found to vary as the square of the distance; for, if an object be placed one foot distant from a candle, it will receive four times more light than when it is removed to double the distance; nine times more than when it is removed to three times the distance, and so on. The refraction, &c. of light will be noticed when treating of the atmosphere.

OF THE AURORA BOREALIS.

Silent from the north

A blaze of meteors shoots: ensweeping first
The lower skies, they all at once converge
High to the crown of heaven, and all at once,
Relapsing quick, as quickly reascend,

And mix, and thwart, extinguish, and renew,
All ether coursing in a blaze of light.

THOMSON.

The Aurora Borealis, or Northern Lights, are luminous meteors, which sometimes appear in the northern part of the heavens in the winter season, and particularly in frosty weather. They are usually of a reddish colour, inclining to yellow, but they frequently send out coruscations of pale whitish light. These seem to rise from the horizon in a pyramidical form, and move backwards and forwards with a tremulous undulating motion; but on some occasions they shoot to the zenith with the greatest velocity, and then form themselves

into the most whimsical figures. This has led M. Godin to suppose, that most of the extraordinary meteors and prodigies which are stated in history to have been seen in the skies, such as battles, and the like, may probably enough have been produced by particular forms assumed by Aurora Borealis.

From look to look, contagious thro' the crowd
The panic runs, and into wondrous shapes
Th' appearance throws: armies in meet array
Throng'd with aërial spears and steeds of fire,
Till the long lines of full-extended war,
In bleeding fight commixt, the sanguine flood
Rolls a broad slaughter o'er the plains of heaven.
As thus they scan the visionary scene,
On all sides swells the superstitious din,
Incontinent, and busy Frenzy talks

Of blood and battle, cities overturn'd,

And late at night in swallowing earthquake sunk,
Or hideous wrapt in fierce ascending flame;
Of sallow famine, inundation, storm;

Of pestilence, and every great distress;

Empires subvers'd, when ruling Fate has struck
Th' unalterable hour: even Nature's self

Is deem'd to totter on the brink of time.
Not so the man of philosophic eye,
And inspect sage; the waving brightness he
Curious surveys, inquisitive to know
The causes and materials, yet unfix'd,
Of this appearance, beautiful and new.

THOMSON.

This kind of meteor never appears near the equator; but has frequently been seen towards the south pole, as well as the north.

Forster, in the account of his voyage round the world with Captain Cook, says he observed them for several nights together, in sharp frosty weather, and that they had much the same appearance as those observed in the north, except that they were of a lighter colour.

In the Shetland Islands these phenomena are the constant attendants of clear evenings, and afford great relief to the inhabitants in the long and gloomy nights of winter experienced in this part of the

world.

The same kind of appearances are also seen in the northern parts of Sweden and Lapland, where they are particularly beautiful, and afford light to travellers during the whole night.

By dancing meteors then, that ceaseless shake
A waving blaze refracted o'er the heavens,
And vivid moons, and stars that keener play
With keener lustre from the glossy waste.
Even in the depth of Polar Night they find
A wondrous day: enough to light the chase,
Or guide their daring steps to Finland fairs.

THOMSON.

In Hudson's Bay the Aurora Borealis spread a variegated splendour

over the whole sky, not to be defaced even by the splendour of the full moon. In the north-east parts of Siberia these northern lights are observed to begin with single bright pillars, rising in the north, and almost at the same time in the north-east, which gradually in creasing, comprehend a large space of the heavens, rush about from place to place, with incredible velocity, and finally almost cover the whole sky up to the zenith, producing an appearance, as if a vast tent were expanded in the heavens, glittering with gold, rubies, and saphires. A more beautiful spectacle cannot be painted; but no person could behold it for the first time without terror. For, however grand the illumination may appear, it is attended with as much hissing, crackling, and tumult, as if the largest fire-works were playing off. The hunters who pursue the white and blue foxes on the confines of the icy sea, are often overtaken in their courses by these northern lights; their dogs are then so much frightened, that they will not move, but lie obstinately on the ground till the noise has passed.

It is chiefly in the arctic regions that the Aurora Borealis are most striking in their appearance. In England it is only their extremities that are seen, and even these have been noticed very seldom for there are none recorded in our annals between the appearance of the remarkable one of Nov. 14th, 1574, and the surprising one of March 6th, 1716, which appeared for three nights successively. This one was visible from the west of Ireland to the confines of Russia and east of Poland, and from the 30th degree of latitude, over almost all the north of Europe; and in all places, at the same time, it exhibited the same wonderful appearances. Father Boscovich calculated the height of an Aurora Borealis which appeared on the 16th December, 1737, and found that it was 825 miles; and the celebrated Bergman, from a mean of thirty computations, makes the average height of the · Aurora Borealis 70 Swedish or 469 English miles. But Euler and some other philosophers suppose their height to be several thousand miles.

Aurora Borealis were long considered by the ignorant and superstitious as portending war, pestilence, and famine. And this was not only the opinion of the inhabitants of the northern islands, but even the inhabitants of this country were alarmed at their appearance.

When the splendid Aurora Borealis of 1716 first made its appearance, it was viewed with the greatest consternation by the vulgar; and considered by them as marking the introduction of a foreign race of princes into this country." Since that time, these meteors have been so common that they have not excited any particular interest, and are now viewed with the greatest indifference by all classes of society.

Many conjectures have been made, at various periods, respecting the cause of this phenomenon; but since the identity of lightning

* George the First came to the throne of Great Britain on the 1st of August, 1714.

with the electric fluid has been ascertained, philosophers have been naturally led to seek for the explication of aërial meteors in the prin ciples of electricity; and there is now little doubt but most of them, and especially Aurora Borealis, are electrical phenomena. Besides the more obvious and known appearances which constitute a resemblance between this meteor and the electric matter by which lightning is produced, it has been observed, that the Aurora Borealis produces a very sensible fluctuation in the magnetic needle; and that when it has extended lower than usual into the atmosphere, the flashes have been attended with various sounds of rumbling and hissing, especially in Russia and the other northern parts of Europe.

Mr. Canton, soon after he had obtained electricity from the clouds, offered a conjecture that the Aurora is occasioned by the flashing of electric fire from positive towards negative clouds at a great distance, through the upper part of the atmosphere, where the resistance is least; and this appears chiefly in the northern regions, as the alteration in the heat of the air in those parts is the greatest.

Signior Beccaria supposes that there is a constant and regular circulation of the electric fluid from north to south; and he thinks, that the Aurora Borealis may be this electric matter, performing its circulation in such a state of the atmosphere as renders it visible.Dr. Franklin thinks, that the electric fire discharged into the polar regions, from many miles of vapour raised from the ocean between the tropics, accounts for the Aurora Borealis; and that this phenomenon appears first, where it is first put in motion, namely, in the northern regions, and the appearance proceeds southward, though the fire really moves in the opposite direction. Several other eminent philosophers have advanced conjectures respecting the cause of this phenomenon, but our limits will not permit us to insert them.*

There is another luminous appearance occasionally seen in the heavens after sun-set, and before sun-rise, which somewhat resembles the milky way, but of a fainter light. This phenomenon is called the zodiacal light, because it is only to be seen in the zodiac. Its figure resembles an inverted cone or pyramid, with its base toward the sun, and its axis lying along the zodiac, somewhat inclined to the horizon. It was first discovered by Cassini, in the year 1683; but there is some reason to think it had been observed before that period. The length of this phenomenon, taken from the sun upwards to its vertex, varies from 45 degrees to 100, and even 120 degrees. The season most favourable for observing it, is the beginning of March after sun-set. But its aspect is very different in different years. One of the most brilliant appearances of it was observed at Paris on the 16th of February, 1769.

* Seamen who have often visited the north seas consider the appearance of Aurora Borealis as indicative of a gale of wind.

Various opinions have been advanced respecting the cause of the zodiacal light, as well as the Aurora Borealis; but the greater number of philosophers agree in ascribing both phenomena to the same cause, namely, the electric fluid.

But the celebrated M. de Mairan, who has written a treatise expressly on the Aurora Borealis, supposes that the zodiacal light causes the Aurora Borealis; and that this light is nothing more than the sun's atmosphere, which is thrown off by means of the rotation of that luminary on his axis to such a distance, as to strike on the upper part of the earth's atmosphere, and produce the luminous appearance which we call the zodiacal light. And as this is chiefly collected towards the polar regions, by means of the diurnal revolution of the earth, it will produce the Aurora Borealis.

OF RAINBOWS, PARHELIA,* &c.

Refracted from yon eastern cloud,

Bestriding earth, the grand etherial bow;
Shoots up immense; and every hue unfolds,
In fair proportions running from the red,
To where the violet fades into the sky.
Here, awful Newton, the dissolving clouds
Form, fronting on the sun thy showery prism;
And to the sage-instructed eye unfold

The various twine of light, by thee disclosed
From the white mingling maze.

THOMSON.

Besides the Aurora Borealis, there are several other beautiful phenomena occasionally seen in the heavens. Among these may be ranked the Rainbow; which is, unquestionably, the most beautiful meteor with which we are acquainted.

It is never seen but in rainy weather, and in that point of the heavens which is opposite to the sun, being occasioned by the refraction and reflection of his rays falling on the drops of rain as they descend to the earth. There are frequently two bows to be seen at the same time-an interior and an exterior one. The interior is the brightest, being formed by the rays of the sun falling on the upper parts of the drops of rain; for a ray of light entering the upper part of a drop of rain, will, by refraction, be thrown upon the inner part of the spherical surface of that drop, whence it will be reflected to the lower part of the drop, where, undergoing a second refraction, it will be bent toward the eye of the spectator. Hence, the rays which fall upon the interior bow come to the eye after two refractions and one reflection; and the colours of this bow from the upper part, are red, orange, yellow, green, blue, indigo, and violet. The exterior bow is

* Although we have given a short account and representation of the Rainbow in the supplementary part of this work, we have deemed it necessary to give a still more popular account of it here.