departs, therefore, from the sun to thirty-five times the distance of the CHAP. XII. earth, and afterward approaches nearly twice as near the sun as the earth is, thus describing an ellipse extremely elongated. same. "The intervals of its return to its perihelion are not constantly the That between 1531 and 1607 was three months longer than that between 1607 and 1682; and this last was 18 months shorter than the one between 1682 and 1759. It appears, therefore, that the motions of comets are subject to perturbations, like those of the planets, and to a much more sensible degree. "Elements of the Orbits of the three Comets, which have appeared according to prediction, taken from the work of Professor Littrow. Halley. Encke. Biela. 3350 2490 Longitude of the ascending node, 54 "The comets of Encke and Biela move according to the order of the signs of the zodiac, or have their motions direct; the motion of that of Halley is retrograde. "Comets, in passing among and near the planets, are materially drawn aside from their courses, and in some cases have their orbits entirely changed. This is remarkably the case with Jupiter, which seems, by some strange fatality, to be constantly in their way, and to serve as a perpetual stumbling-block to them. In the case of the remarkable conet of 1770, which was found by Lexell to revolve in a moderate ellipse in the period of about five years, and whose return was predicted by him accordingly, the prediction was disappointed by the comet actually getting entangled among the satellites of Jupiter, and being completely thrown out of its orbit by the attraction of that planet, and forced into a much larger ellipse. By this extraordinary renconter, the motions of the satellites suffered not the least perceptible derangement— a sufficient proof of the smallness of the comet's mass." The comet of 1456, represented as having a tail of 60° in length, is now found to be Halley's comet, which has made several returns in 1531, 1607, 1682, 1759, and recently, in 1835. In 1607 the tail was said to have been over 30° in length; but in 1835 the tail did not exceed 120 Does it lose substance, or does the matter composing the tail condense? or, have we received only exaggerated and distorted accounts from the earlier times, such as fear, superstition, and awe, always put forth? We ask these questions, but cannot answer them. M* Jupiter, and his satel lites, a great stumbling. block to the comets. CHAP. XII. The following cut represents the appearance of the comet of 1819. Fears en comets may "Professor Kendall, in his Uranography, speaking of the fears occatertained, by sioned by comets, says: "Another source of apprehension, with regard some, that to comets, arises from the possibility of their striking our earth. It is quite probable that even in the historical period the earth has been into enveloped in the tail of a comet. It is not likely that the effect would collision with be sensible at the time. The actual shock of the head of a comet against the earth is extremely improbable. It is not likely to happen once in a million of years. ultimately come our earth. "If such a shock should occur, the consequences might perhaps be very trivial. It is quite possible that many of the comets are not heavier than a single mountain on the surface of the earth. It is well known that the size of mountains on the earth is illustrated by comparing them to particles of dust on a common globe." CHAP. XIII. FOR the facts as contained in the subject matter of this chapter, we must depend wholly on authority; for that reason we give only a compilation, made in as brief a manner as the nature of the subject will admit. In the first part of this work it was soon discovered that the fixed stars were more remote than the sun or planets; and now, having determined their distances, we may make further inquiries as to the distances to the stars, which will give some index by which to judge of their magnitudes, nature, CHAP. XIII. and peculiarities. measure to to "It would be idle to inquire whether the fixed stars have a sensible Base from parallax, when observed from different parts of the earth. We have which already had abundant evidence that their distance is almost infinite. It the stars.. is only by taking the longest base accessible to us, that we can hope to arrive at any satisfactory result. "Accordingly, we employ the major axis of the earth's orbit, which is nearly 200 millions of miles in extent. By observing a star from the two extremities of this axis, at intervals of six months, and applying a correction for all the small inequalities, the effect of which we have calculated, we shall know whether the longitude and latitude are the same or not at these two epochs. "It is obvious, indeed, that the star must appear more elevated above Annual the plane of the ecliptic when the earth is in the part of its orbit which parallax. is nearest to the star, and more depressed when the contrary takes place. The visual rays drawn from the earth to the star, in these two positions, differ from the straight line drawn from the star to the center of the earth's orbit; and the angle which either of them forms with this straight line, is called the annual parallax. "As the earth does not pass suddenly from one point of its orbit to the opposite, but proceeds gradually, if we observe the positions of a star at the intermediate epochs, we ought, if the annual parallax is sensible, to see its effects developed in the same gradual manner. For example, if the star is placed at the pole of the ecliptic, the visual rays drawn from it to the earth, will form a conical surface, having its apex at the star, and for its base, the earth's orbit. This conical surface being produced beyond the star, will form another opposite to the first, and the intersection of this last with the celestial sphere, will constitute a small ellipse, in which the star will always appear diametrically opposite to the earth, and in the prolongation of the visual rays drawn to the apex of the cones. The effect of a sensible parallax. one second. "But notwithstanding all the pains that have been taken to multiply The annual observations, and all the care that has been used to render them per- parallaxmust fectly exact, we have been able to discover nothing which indicates, be less than with certainty, even the existence of an annual parallax, to say nothing of its magnitude. Yet the precision of modern observations is such, that if this parallax were only 1", it is altogether probable that it would not have escaped the multiplied efforts of observers, and especially those of Dr. Bradley, who made many observations to discover it, and who, in this undertaking, fell unexpectedly upon the phenomena of aberration and nutation. These admirable discoveries have themselves served to show, by the perfect agreement which is thus found to take *Subject to be explained hereafter. CHAP. XIII. place among observations, that it is hardly to be supposed that the annual parallax can amount to 1". The numerous observations of the pole star, recently employed in measuring an arc of the meridian through France, have been attended with a similar result, as to the amount of the annual parallax. From all this we may conclude, that as yet there are strong reasons for believing that the annual parallax is less than 1", at least with respect to the stars hitherto observed. "Thus the semidiameter of the earth's orbit, seen from the nearest star, would not appear to subtend an angle of 1"; and to an observer placed at this distance, our sun, with the whole planetary system, would occupy a space scarcely exceeding the thickness of a spider's thread. Conclusion "If these results do not make known the distance of the stars from to be drawn the earth, they at least teach us the limit beyond which the stars must these necessarily be situated. If we conceive a right-angled triangle, having from facts. stars. for its base half the major axis of the earth's orbit, and for its vertex an angle of 1", the distance of this vertex from the earth, or the length of the visual ray, will be expressed by 212207, the radius of the earth's orbit being unity; and as this radius contains 23987 times the semidiameter of the earth, it follows that if the annual parallax of a star were only 1", its distance from the earth would be equal to 5090209309 radii of the earth, or 20086868036404 miles; that is, more than 20 billions. But if the annual parallax is less than 1", the stars are beyond the limit which we have assigned. Changes "It is evident that the stars undergo considerable changes, since these in individual changes are sensible even at the distance at which we are placed. There are some which gradually lose their light, as the star of Ursa Major. Others, as of Cetus, become more brilliant. Finally, there are some which have been observed to assume suddenly a new splendor, and then gradually fade away. Such was the new star which appeared in 1572, A new star. in the constellation Cassiopeia. It became all at once so brilliant that it surpassed the brightest stars, and even Venus and Jupiter when nearest the earth. It could be seen at midday. Gradually this great brilliancy began to diminish, and the star disappeared in sixteen months from the time it was first seen, without having changed its place in the heavens. Its color, during this time, suffered great variations. At first it was of a dazzling white, like Venus; then of a reddish yellow, like Mars and Aldebaran; and lastly, of a leaden white, like Saturn. Another star which appeared suddenly in 1604, in the constellation Serpentarius, presented similar variations, and disappeared after several months. These phenomena seem to indicate vast flames which burst forth suddenly in these great bodies. Who knows that our sun may not be subject to similar changes, by which great revolutions have perhaps taken place in the state of our globe, and are yet to take place. "Some stars, without entirely disappearing, exhibit variations not less remarkable. Their light increases and decreases alternately in regular periods. They are called for this reason variable stars. Such is the Another new star. ¡Periodical changes. star Algol, in the head of Medusa, which has a period of about three CHAP. XIII. days; of Cepheus, which has one of five days; ß of Lyra, six ; μ of Antinous, seven; of Cetus, 334; and many others. Attempts "Several attempts have been made to explain these periodical variations. It is supposed that the stars which are subject to them, are, like to explain all the other stars, self-luminous bodies, or true suns, turning on their periodical axes, and having their surfaces partly covered with dark spots, which changes. may be supposed to present themselves to us at certain times only, in consequence of their rotation. Other astronomers have attempted to account for the facts under consideration, by supposing these stars to have a form extremely oblate, by which a great difference would take place in the light emitted by them under different aspects. Lastly, it has been supposed that the effect in question is owing to large opake bodies, revolving about these stars, and occasionally intercepting a part of their light. Time and the multiplication of observations may perhaps decide which of these hypotheses is the true one. Order in "One of the best methods of observing these phenomena is to compare the stars together, designating them by letters or numbers, and dispos- these obsering them in the order of their brilliancy. If we find, by observation, vations. that this order changes, it is a proof that one of the stars thus compared, has likewise changed; and a few trials of this kind will enable us to ascertain which it is that has undergone a variation. In this manner, we can only compare each star with those which are in the neighborhood, and visible at the same time. But by afterward comparing these with others, we can, by a series of intermediate terms, connect together the most distant extremes. This method, which is now practiced, is far preferable to that of the ancient astronomers, who classed the stars after a very vague comparison, according to what they called the order of their magnitudes, but which was, in reality, nothing but that of their brightness, estimated in a very imperfect manner. 66 Suggestion "By comparing the places of some of the fixed stars, as determined from ancient and modern observations, Dr. Halley discovered that they of Dr.Halley. had a proper motion, which could not arise from parallax, precession, or aberration. This remarkable circumstance was afterward noticed by Cassini and Le Monnier, and was completely confirmed by Tobias Mayer, who compared the places of 80 stars, as determined by Roemer, with his own observations, and found that the greater part of them had a proper motion. He suggested that the change of place might arise from a progressive motion of the sun toward one quarter of the heavens; but as the result of his observation did not accord with his theory, he remarks that many centuries must elapse before the true cause of this motion could be explained. "The probability of a progressive motion of the sun was suggested upon theoretical principles by the late Dr. Wilson of Glasgow; and Lalande deduced a similar opinion from the rotatory motion of the sun, by supposing, that the same mechanical force which gives it a motion |