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 collision 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 ben seen in the United States? Why was it not visible in the United States at the time of its return in 1932? Relate the history of the discovery of the second comet of a short period? Why is it called Biela's coinet? What, according to the observations of Dr. Olbers in 1805, was the diameter of Biela's comet, including 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 would take place, and why did it not?

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 of 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, not• withstanding 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 orbit intersects all the planetary orbits through which it passes. By this, 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 mteTest to physical astronomy than those of longer periods? What is the situation of the orbit of Biela's comet in the solar system?

This diagram not only exhibits the course of the comet at its last return, but also denotes its future positions on the first day of every year during its next revolution. It is also apparent that it will return to its perihelion again in the autumn of 1839, but not so immediately in our vicinity as to be the proper cause of alarm. To be able to predict the very day and circumstances of the return of such a bodiless and eccentric wanderer, after the lapse of so many years, evinces a perfection of the astronomical calculus that may justly challenge our admiration.

"The re-appearance of this comet," says Herschel, "whose return in 1832 was made the subject of elaborate calculations by mathematicians of the first eminence, did not disappoint the expectation of astronomers. It is hardly possible to imagine any thing more striking than the appearance, after the lapse of nearly seven years, of such an all but imperceptible cloud or wisp of vapour, true, however, to its predicted time and place, and obeying laws like those which regulate the planets."

Herschel, whose Observatory is at Slough, England, observed the daily progress of this comet from the 24th of September, until its disappearance, compared its actual position from day to day with its calculated position, and found them to agree within four or five ininutes of time in right ascension, and within a few seconds of declination. Its position, then, as represented on a planisphere which the author prépared for his pupils, and af terwards published, was true to within a less space than one third of its projected diameter. Like some others that have been observed this comet has no luminous train by which it can be easily recognized by the naked eye, except when it is very near the Sun This is the reason why it was not more generally observed at its late return.

Although this comet is usually denominated "Biela's comet," yet it seems that M. Gambart, director of the Observatory at Marseilles, is equally entitled to the honour of identifying it with the comet of 1772, and of 1805. Ile discovered it only 10 days after Biela, and immediately set about calcu lating its elements from his own observations, which are thought to equal, if they do not surpass, in point of accuracy, those of every other as

tronomer.

Up to the beginning of the 17th century, no correct notions had been entertained in respect to the paths of comets Kepler's first conjecture was that they moved in straight lines; but as that did not agree with observation, he next concluded that they were parabolic curves, having the Sun near the vertex, and running indefinitely into the regions of space at both extremities. There was nothing in the observations of the earlier astronomers to fix their identity, or to lead him to suspect that any one of them had ever been seen before; much less that they formed a part of the solar

When will this comet retun again? How much did its actual position from day to day, as observed by Herschel, differ from its calulated position? Why was it not more generally observed at its late return? What astronomer besides Biela identified it with the comet of 1772 and 1805? What were the opinions of astronomers in regard to the paths of comets, up to the beginning of the 17th century? What were Kepler's opinions on this subject?

system, revolving about the Sun in elliptical orbits that returned into themselves.

This grand discovery was reserved for one of the most industrious and sagacious astronomers that ever lived-this was Dr. Halley, the contemporary and friend of Newton. When the comet of 1682 made its appearance, he set himself about observing it with great care, and found there was a wonderful resemblance between it and three other comets that he found recorded, the comets of 1456, of 1531, and 1607. The times of their appearance had been nearly at equal and regular intervals; their perihelion distances were nearly the same; and he finally proved them to be one and the same comet, performing its circuit around the Sun in a period varying a little from 76 years. This is therefore called Halley's comet. It is the very same comet that filled the eastern world with so much consternation in 1456, and became an object of such abhorrence to the church of Rome.

Of all the comets which have been observed since the Christian era, only three have had their elements so well determined that astronomers are able to fix the period of their revolution, and to predict the time and circumstances of their appearance. These three are, Encke's, whose last revolution about the Sun was performed in 1212 days; Biela's, whose period was 2461 days; and Halley's, which is now accomplishing its broad circuit in about 28,000 days, Encke's and Halley's will return to their perihelion the present year (1835), and Biela's in 1839.

Halley's comet, true to its predicted time and place, is now (Oct. 1835,) visible in the evening sky. But we behold none of those phenomena which threw our ancestors of the middle ages into agonies of superstitious terrour, We sce not the cometa horrendo magnitudinis, as it appeared in 1305, nor that tail of enormous length which, in 1456, extended over two thirds of the interval between the horizon and the zenith, nor even a star as brilliant as was the same comet in 1682, with its tail of 30°.

Its mean distance from the Sun is 1,713,700.000 miles; the eccentricity of its orbit is 1,658,000,000 miles; consequently it is 3.316,000.000 miles farther from the Sun in its aphelion than it is in its perihelion. In the latter case, its distance from himi is only 55.700,000 miles; but in the former, it is 3,371,700 000 miles Therefore, though its aphelion distance be great, its mean distance is less than that of Herschel; and great as is the aphelion distance, it is but a very small fraction less than one five-thousandth part of that distance from the Sun, beyond which the very nearest of the fixed stars must be situated; and, as the determination of their distance is nega

Who first discovered the identity of comets? Relate the manner by which he came to this discovery. How many of all the comets observed since the Christian era, have had their elements so well determined, that astronomers are able to fix the period of their revolutions, and to predict the time and circumstances of their appearance? What comets are these? In what time do they accomplish their revolutions? When will they, severally, return to their perihelion? What comet is now (Oct. 1835) visible? What are

the mean, and the aphelion and perihelion distances of Halley's comet from the Sun ? What part of the distance beyond which the nearest of the fixed stars must be pla ced, is its aphelion distance ?