more rapid than that of Mars, he will be overtaken and passed by the earth. Hence Mars will have two stationary and one retrograde appearance. Suppose the earth to be at E when Mars is at M, he will be seen in the heavens among the fixed stars at m; and for some time before the earth has arrived at E, and after it has passed E, he will appear nearly in the same point m, viz. he will be stationary. While the earth moves through the part E te of its orbit, if Mars stood still at M, he would appear to move in a retrograde direction through the arch m Prn, in the heavens, and would again be stationary at n; but if during the time the earth moves from E to e, Mars moves from M to O, the arch of retrogradation would be nearly m Pr.

The same manner of reasoning may be applied to Jupiter and all the superior planets.

The Motion of the Fixed Stars.

All the fixed stars, except the polar star, appear to have a diurnal motion from east to west: this arises from the diurnal motion of the earth on its axis from west to east. The fixed stars have also a small apparent motion about their real places, arising from the velocity of the earth in its orbit, combined with the motion of light. This motion is called the aberration of the fixed stars, and was discovered by Dr. Bradley. They vary in their situations by the precession of the equinoxes; hence, their longitudes, &c. vary considerably in a series of years, which renders it necessary to have new plates engraven for our celestial globes at least once in about fifty years.

Dr. Maskeline observes, that many, if not all the fixed stars, have small motions among themselves, which are called their proper motions; the cause and laws of which are hid, for the present, in almost equal obscurity.

Of the Magnitudes, Distance, Number, and Appearance of the Fixed Stars.

The magnitude of the fixed stars will probably forever remain unknown; all that we can have any reason to expect is a mere approximation founded on conjecture. From a comparison of the light afforded by a fixed star, and that of the sun, it has been concluded that the magnitudes of the stars do not differ materially from that of the sun. The different apparent magnitudes of the stars is supposed to arise from their different distances; for the young astronomer must not imagine that all the fixed stars are placed in a concave hemisphere, as they appear in the heavens, or on a convex surface, as they are represented on a celestial globe.

From a series of accurate observations by Dr. Bradley on Draconis, he inferred that its annual parallax did not amount

to a single second; that is, the diameter of the earth's annual orbit, which is not less than 190 millions of miles, would not form an angle at this star of one second in magnitude; or, that it appeared in the same point in the heavens during the earth's annual course round the sun.

The same author calculates the distance of Draconis from พ the earth to be 400,000 times that of the sun, or 38,000,000, 000,000 miles; and the distance of the nearest fixed star from the earth to be 40,000 times the diameter of the earth's orbit, or 7,600,000,000,000, miles. These distances are so immensely great, that it is impossible for the fixed stars to shine by the light of the sun reflected from their surfaces: they must, therefore, be of the same nature with the sun; and, like him, shine by their own light.

The number of the fixed stars is almost infinite, though the number which may be seen by the naked eye in the whole heavens does not exceed, and perhaps falls short of 3000, comprehending all the stars from the first to the sixth magnitude inclusive; but a good telescope, directed almost indifferently to any point in the heavens, discovers multitudes of stars invisible to the naked eye. That bright irregular zone, the milky way, has been very carefully examined by Dr. Herschel; who has, in the space of a quarter of an hour, seen 116000 stars pass through the field of view of a telescope of only 15' aperture. The fixed stars are the only marks by which astronomers are enabled to judge of the course of the moveable ones, because they do not vary their relative situations. Thus, in contemplating any number of fixed stars, which to our view form a triangle, a four sided figure, or any other, we shall find that they always retain the same relative situation, and that they have had the same situation for some thousands of years, viz. from the earliest records of authentic history. But as there are few general rules without some exceptions, so this general inference is likewise subject to restrictions. Several stars whose situations were formerly marked with precision, are no longer to be found; new ones have also been discovered, which were unknown to the ancients; while numbers seem gradually to vanish, and others appear to have a periodical increase and decrease of magnitude. Dr. Herschel, in the Philosophical Transactions for 1783, has given a large collection of stars which were formerly seen, but are now lost, together with a catalogue of variable stars, and of new stars.—KEITH.

On Comets.

Comets are a class of celestial bodies, which appear occasionally in the heavens. They exhibit no visible or defined

disc, but shine with a pale and cloudy light, accompanied with a tail or train turned from the sun. They are found in every part of the heavens, and move in all possible directions.

When examined through a good telescope, a comet resembles a mass of aqueous vapours encircling an opaque nucleus of different degrees of darkness in different comets, though sometimes, as in the case of several discovered by Dr. Herschel, no nucleus can be seen. As the comet advances towards the sun, its faint and nebulous light becomes more brilliant, and its luminous train gradually increases in length. When it reaches its perihelion, the intensity of its light, and the length of its tail, reach their maximum, and sometimes it shines with all the splendour of Venus. During its retreat from the perihelion, it is shorn of its splendour, it gradually resumes its nebulous appearance, and its tail decreases in magnitude till it reaches such a distance from the earth, that the attenuated light of the sun which it reflects, ceases to make an impression on the organ of sight. Traversing unseen the remote portion of its orbit, the comet wheels its ethereal course far beyond the limits of our system. What region it there visits, or upon what destination it is sent, the limited powers of man are unable to discover. After the lapse of years, we perceive it again returning to our system, and tracing a portion of the same orbit round the sun, which it had formerly described.

It would be a waste of time to detail the various wild and extravagant opinions which have been entertained respecting these interesting stars. During the ages of barbarism and superstition they were regarded as the harbingers of awful convulsions, both in the political, and in the physical world. Wars, pestilence, and famine, the dethronement of kings, the fall of nations, and the more alarming convulsions of the globe, were the dreadful evils which they presented to the diseased and terrified imaginations of men. As the light of knowledge dissipated these gloomy apprehensions, the absurdities of licentious speculators supplied their place, and all the ingenuity of conjecture was exhausted in assigning some rational office to these wandering planets. Even at the beginning of the eighteenth century, the friend and companion of Newton regarded them as the abode of the damned. Anxious to know more than what is revealed, the fancy of speculative theologians strove to discover the frightful regions in which vice was to suffer its merited punishment; and the interior caverns of the earth had, in general, been regarded as the awful prison house in which the Almighty was to dispense the severities of justice. Mr. Whiston, however, outstripped all his predecessors in fertility of invention. He pretended not only to fix the residence of the damned, but also the nature of their punishment. Wheeled from the remotest limits of the system, the chilling regions of