of the war which was declared in the following spring. It is well known that an indefinite apprehension of a more dreadful catastrophe lately pervaded both continents, in anticipation of Biela's comet of 1832.

The nucleus of the comet of 1811, according to observations made near Boston, was 2,617 miles in diameter, corresponding nearly to the size of the Moon. The brilliancy with which it shone, was equal to one tenth of that of the Moon. The envelope, or aeriform covering, surrounding the nucleus, was 24,000 miles thick, about five hundred times as thick as the atmosphere which encircles the Earth; making the diameter of the comet, including its envelope, 50,617 miles. It had a very luminous tail, whose greates length was one hundred million of miles.

This comet moved, in its perihelion, with an almost inconceivable velocityfifteen hundred times greater than that of a ball bursting from the mouth of a cannon. According to Regiomontanus, the comet of 1472 moved over an are of 120° in one day. Brydone observed a comet at Palermo in 1770, which passed through 50° of a great circle in the heavens in 24 hours. Another comet, which appeared in 1759, passed over 41o in the same time. The conjecture of Dr. Halley therefore seems highly probable, that if a body of such a size, having any considerable density, and moving with such a velocity, were to strike our Earth, it would instantly reduce it to chaos, mingling its elements in ruin.

The transient effect of a comet passing near the Earth, could scarcely amount to any great convulsion, says Dr. Brewster: but if the Earth were actually to receive a shock from one of these bodies, the consequences would be awful. A new direction would be given to its rotary motion, and it would revolve around a new axis. The seas, forsaking their beds, would be hurried, by their centrifugal force, to the new equatorial regions: islands and continents, the abodes of men and animals, would be covered by the universal rush of the waters to the new equator, and every vestige of human industry and genius would be at once destroyed.

The chances against such an event, however, are so very numerous, that there is no reason to dread its occurrence. The French government, not long since, called the attention of some of her ablest mathematicians and astronomers to the solution of this problem; that is, to determine, upon mathematical principles, how many chances of collision the Earth was exposed to. After a mature examination, they reported,-"We have found that, of 281,000,000 of chances, there is only one unfavourable, there exists but one which can produce a collision between the two bodies."

"Admitting, then," say they, "for a moment, that the comets which may strike the Earth with their nucleuses, would annihilate the whole human race; the danger of death to each individual, resulting from the ap

Describe this comet. Give some examples of the velocity of comets. What would probably be the effect upon the Earth, should a comet strike it? What does Dr. Breweter say would be the effect of a comet passing near the Earth? But if the Earth were actually to receive a shock from a comet, what does he say would be the results? How did the French mathematicians and astronomers find the chances of a collision be tween the Earth and comets to stand? What, then, on the supposition that a stroke of a comet would annihilate the whole human race, is the danger of death to each individual, resulting from the appearance of an unknown comet ?

pearance of an unknown comet, would be exactly equal to the risk he would run, if in an urn there was only one single white ball among a total number of 281,000,000 balls, and that his condemnation to death would be the inevitable consequence of the white ball being prorluced at the first drawing."

We have before stated that comets, unlike the planets, observe no one direction in their orbits, but approacî to, and recede from their great centre of attraction, in every possible direction. Nothing can be more sublime, or better calculated to fill the mind with profound astonishment, than to contemplate the revolution of comets, while in that part of their orbits which comes within the sphere of the telescope. Some seem to come up from the immeasurable depths below the ecliptic, and, having doubled the heavens' mighty cape, again plunge downward with their fiery trains,

"On the long travel of a thousand years.”

Others appear to come down from the zenith of the universe to double their perihelion about the Sun, and then reascend far above all human vision.

Others are dashing through the solar system in all possible directions, and apparently without any undisturbed or undisturbing path prescribed by him who guides and sustains them all.

Until within a few years, it was universally believed that the periods of their revolutions must necessarily be of prodigious length; but within a few years, two comets have been discovered, whose revolutions are performed, comparatively, within our own neighbourhood. To distinguish them from the more remote, they are denominated the comets of a short period. The first was discovered in the constellation Aquarius, by two French astronomers, in the year 1786. The same comet was again observed by Miss Caroline Herschel, in the constellation Cygnus, in 1795, and again in 1805. In 1818, Professor Encke determined the dimensions of its orbit, and the period of its sidereal revolution; for which reason it has been called "Encke's Comet."

This comet performs its revolution around the Sun in about 3 years and 4 months,* in an elliptical orbit which lies wholly within the orbit of Jupiter. Its mean distance from the Sun is 212 millions of miles; the eccentricity of its orbit is 179

* Owing to the disturbing influences of the surrounding planets, the periodic return of his comet, like that of all others, is liable to be hastened or retarded several days. Its period varies from about 1203 to 1212 days.

What is the direction of comets in their orbits? What has been, until within a few years, the universal opinion in regard to the length of the times of their revolution? Why does not the same opinion prevail now? What are these two comets denominated? Relate the history of the discovery of the first. Why is it called Encke's comet? What is the time of the revolution of Encke's comet? What is the form of its orbit, and what its position with regard to the orbit of Jupiter? What is this comet's mean distance from the Sun? What is the eccentricity of its orbit ?

millions of miles; consequently it is 358 millions of miles nearer the Sun in its perihelion, than it is in its aphelion. It was visible throughout the United States in 1825, when it presented a fine appearance. It was also observed at its next return in 1828; but its last return to its perihelion, on the 6th of May, 1832, was invisible in the United States, on account of its great southern declination.

The second "Comet of a short period," was observed in 1772; and was seen again in 1805. It was not until its re-appearance in 1826, that astronomers were able to determine the elements of its orbit, and the exact period of its revolution. This was successfully accomplished by M. Biela of Josephstadt; hence it is called Biela's Comet. According to observations made upon it in 1805, by the celebrated Dr. Olbers, its diameter, including its envelope, is 42,280 miles. It is a curious fact, that the path of Biela's comet passes very near to that of the Earth; so near, that at the moment the centre of the comet is at the point nearest to the Earth's path, the matter of the comet extends beyond that path, and includes a portion within it. Thus, if the Earth were at that point of its orbit which is nearest to the path of the comet, at the same moment that the comet should be at that point of its orbit which is nearest to the path of the Earth, the Earth would be enveloped in the nebulous atmosphere of the comet.

With respect to the effect which might be produced upon our atmosphere by such a circumstance, it is impossible to offer any thing but the most vague conjecture. Sir John Herschel was able to distinguish stars as minute as the 16th or 17th magnitude through the body of the comet! Hence it seems reasonable to infer, that the nebulous matter of which it is composed, must be infinitely more attenuated than our atmosphere; so that for every particle of cometary matter which we should inhale, we should inspire millions of particles of atmospheric air.

This is the comet which was to come into llision with the Earth, and to blot it out from the Solar System. In returning to its perihelion, November 26th, 1832, it was computed that it would cross the Earth's orbit at a distance of

How much nearer the Sun, then, is the comet, when in its perihelion than when in its aphelion? In what years has this comet been seen in the United States? Why was i not visible in the United States at the time of its return in 1832? Relate the history of the discovery of the second comet of a short period? Why is it called Biela's comet? What, according to the observations of Dr. Olbers in 1805, was the diameter of Biela's comet, ineluding the envelope? How near does the path of Biela's comet lie to that of the Earth? What would be the effect upon our atmosphere should the nebulous atmosphere of the comet envelope it? What reason have we to suppose that it is more attenuated than our atmosphere? It was predicted that this comet would come into collision with the Earth; what were the grounds of probability that such an event wild take place, and why did it not 7

only 18,500 miles. It is evident that if the Earth had been in that part of her orbit at the same time with the comet, our atmosphere would have mingled with the atmosphere of the comet, and the two bodies, perhaps, have come in contact. But the comet passed the Earth's orbit on the 29th of October, in the 8th degree of Sagittarius, and the Earth did not arrive at that point until the 30th of November, which was 32 days afterwards.

If we multiply the number of hours in 32 days, by 68,000 (the velocity of the Earth per hour,) we shall find that the Earth was more than 52,000,000 miles behind the comet when it crossed her orbit. Its nearest approach to the Earth, at any time, was about 51 millions of miles; its nearest approach to the Sun, was about 83 millions of miles. Its mean distance from the Sun, or half the longest axis of its orbit, is 337 millions of miles. Its eccentricity is 253 millions of miles; consequently, it is 507 millions of miles nearer the Sun in its perihelion than it is in its aphelion. The period of its sidereal revolution is 2,460 days, or about 63 years.

Although the comets of Encke and Biela are objects of very great interest, yet their short periods, the limited space within which their motion is circumscribed, and consequently the very slight disturbance which they sustain from the attraction of the planets, render them of less interest to physical astronomy than those of longer periods.

They do not, like them, rush from the invisible and inaccessible depths of space, and, after sweeping our system, depart to distances with the conception of which the imagination itself is confounded. They possess none A that grandeur which is connected with whatever appears to break through the fixed order of the universe. It is reserved for the comet of Halley alone to afford the proudest triumph to those powers of calculation by which we are enabled to follow it in the depths of space, two thousand millions of miles beyond the extreme verge of the solar system; and, notwithstanding disturbances which render each succeeding period of its return different from the last, to foretel that return with precision.

The following representation of the entire orbit of Biela's comet, was obtained from the Astronomer Royal of the Greenwich Observatory. It shows not only the space and position it occupies in the solar system, but the points where its orbitersects all the planetary orbits through which it passes. By us, it is seen that its perihelion lies between the orbits of the Earth and Venus, while its aphelion extends a little beyond that of Jupiter.

What was its nearest approach to the Earth at any time? What its nearest approach to the Sun? What its mean distance from the Sun? What its eccentricity? What, then, is the difference between its perihelion and aphelion distances? What is the period of its sidereal revolution? Why are the comets of Encke and Biela objects of less inteTest to physical astronomy than those of longer periods? What is the situation of the orbit of Biela's comet in the solar system 7