velocities, (if they could be supposed to exist) are not the motions of mere points or small luminous balls, but the motions of immense globes, many thousands of times larger than the earth-that a hundred millions of such globes are visible from our abode, besides the myriads that may be hid from human view in the unexplored regions of space-and that it is impossible to conceive how all these innumerable globes, of different magnitudes, at different distances, and moving with different velocities, could be so adjusted as to finish their diurnal revolutions at the same moment, while many of them are at the same time impelled by other forces in a contrary direction. He may be reminded that the Creator, who formed the universe, is possessed of INFINITE WISDOM-that wisdom consists in proportionating means to ends, or in selecting the most appropriate arrangements in order to accomplish an important purpose that to make the whole frame of Universal Nature move round the earth every day, merely to produce the alternate succession of day and night, is repugnant to every idea we ought to entertain of the Wisdom and Intelligence of the Divine Mind, since the same effect can be produced by a simple rotation of the earth in twentyfour hours; and since we find that Jupiter and Saturn, and other globes much larger than ours, move round their axes in a shorter period-that in all the other works of Omnipotence, means apparently the most simple are selected to accomplish the most grand and magnificent designsand that there is no example known to us, throughout the universe, of a larger body revolving around a smaller. When such considerations are fully and familiarly illustrated, the pupil will soon be made clearly to perceive, that the rotation of the earth must necessarily be admitted, and that it will fully account for all the diversity of diurnal motion which appear in the sun and moon, the planets and the stars.

The annual revolution of the earth, and its position in the solar system, might be proved and illustrated by such considerations as the following:-that if this motion did not exist, the motions of all the planets would present a scene of inextricable confusion, consisting of direct and retrograde motions, and looped curves, so anomalous and irregular, as to be inconsistent with every thing like harmony, order, or intelligence-that Mercury and Venus are observed to have two conjunctions with the sun, but no opposition; which could not happen unless the orbits of

these planets lay within the orbit of the earth-that Mars, Jupiter, and the other superior planets, have each their conjunctions with and oppositions to the sun, which could not be unless they were exterior to the orbit of the earththat the greatest elongation of Mercury from the sun is only about 20 degrees, and that of Venus 47; but if the earth were the centre of their motions, as the Ptolemaic system supposes, they might sometimes be seen 180 degrees from the sun, which never happens-that some of the planets appear much larger and brighter at one time than at another, on account of their different distances from the earth; but, on the other hypothesis, their brilliancy should be always the same-that Mercury and Venus, in their superior conjunctions with the sun, are sometimes hid behind his body, and in their inferior conjunctions sometimes appear to pass across the sun's disk, like round black spots, which would be impossible according to the Ptolemaic system; and, in short, that the times in which the conjunctions, oppositions, stations, and retrogradations happen, are not such as they would be if the earth were at rest, but precisely such as would happen, if the earth move along with all the other planets, in the stations and periods assigned them in the system which has the sun for its centre. From such considerations, when properly explained, the annual motion of the earth, and and its relative position in the system, may be clearly demonstrated, and the pupil made to perceive the beauty and harmony of the celestial motions, and the necessity of having the great source of light and heat placed in the centre of the system. For as the sun is intended to cheer and irradiate surrounding worlds, it is from the centre alone that these agencies can be communicated, in a uniform and equable manner, to the planets in every part of their orbits. Were the earth the centre, and the sun and planets revolving around it, the planets, when nearest the sun, would be scorched with excessive heat, and when farthest distant, frozen with excessive cold.

There is another consideration by which the earth's annual revolution and its position in the system are demonstrated; and that is, that the planets Mercury and Venus, when viewed through good telescopes, are found to assume different phases, in different parts of their orbits; sometimes appearing gibbous, sometimes like a half moon, and at other times like a crescent, and a full enlightened hemisphere, which could never happen if they revolved

round the earth as their centre, and if the earth was not placed in an orbit exterior to that of Venus. I have sometimes illustrated this argument, with peculiar effect, by means of an equatorial telescope, and a common planetari

By the equatorial telescope, with a power of 60 or 80 times, most of the stars of the first magnitude, and some of those of the second, may be seen even at noonday. Venus may be seen by this instrument, in the day time, during the space of nineteen months, with the interruption of only about thirteen days at the time of her superior conjunction, and three days at the time of her inferior, so that the phase she exhibits may be seen almost every clear day. Having placed the Earth and Venus in their true positions on the planetarium, by means of an Ephemeris or the Nautical Almanac, I desire the pupil to place his eye in a line with the balls representing these planets, and to mark the phase of Venus as seen from the earth-whether a crescent, a half moon, or a gibbous phase. I then adjust the equatorial telescope for Venus, if she is within the range of our view, and show him the planet with the same phase in the heavens. This exhibition never fails to gratify every observer, and to produce conviction. But it can seldom be made, if we must wait till the planet be visible to the naked eye, and capable of being viewed by a common telescope; for it is sometimes invisible to the naked eye, for nearly one half of its course from one conjunction to another. Besides, the phases of this planet are more distinctly marked in the day time, when near the meridian, than either in the morning or evening, when at a low altitude, in which case it appears glaring and undefined, on account of the brilliancy of its light, and the undulating vapors near the horizon, through which it is seen. As actual observations on the planets in the heavens make a deeper and more convincing impression on the mind of a young person, than mere diagrams or verbal explanations, I consider an equatorial telescope, in conjunction with a celestial globe and an orrery, as essentially necessary to every teacher of astronomy; as, independently of its use, now hinted at, it is the best and most comprehensive instrument for conveying an idea of the practical operations of this science. It may be made to serve the general purposes of a transit instrument, a quadrant, an equal altitude instrument, a theodolite, an azimuth instrument, a level, and an accurate universal sundial. It serves for taking the right ascensions and declinations of the

heavenly bodies, and for conveying a clear idea of these operations. It may be made to point to any phenomenon in the heavens whose declination and right ascension are known; and, in this way, the planets Mercury, Herschel, Ceres, Pallas, Juno, and Vesta, a small comet, or any other body not easily distinguished by the naked eye, may be readily pointed out.*

The cause of the variety of seasons may next be explained and illustrated. It is difficult, if not impossible, by mere diagrams and verbal explanations, to convey a clear idea on this subject; and therefore, some appropriate machinery must be resorted to, in order to assist the mind in forming its conceptions on this point. The difficulty is, to conceive how the sun can enlighten the North Pole without intermission, during one half of the year, and the South Pole during the other, while the poles of the earth never shift their position, but are directed invariably to the same points of the heavens. This is frequently attempted to be illustrated by means of a brass hoop with a candle placed in its centre, and a small terrestrial globe carried round it, having its axis inclined to the brass circle, which is intended to represent the orbit of the earth. But this exhibition requires some dexterity to conduct it aright, and after all is not quite satisfactory. An orrery, having all the requisite movements by wheel work, and where the Earth moves with its axis parallel to itself, and inclined to the plane of the ecliptic, is the best instrument for illustrating all the variety of the seasons. When such a machine cannot be procured for this purpose, its place may be supplied by a neat little instrument, called a Tellurian, which has been manufactured for many years past by Messrs. Jones, Holburn, London, and may be purchased for about thirty shillings. This instrument consists of a brass ball representing the sun- -which may be occasionally screwed off, and a lamp substituted in its place an ivory ball representing the earth, having the circles of the sphere drawn upon it, a small ball representing the moon, and about eight wheels, pinions, and circles. It exhibits the annual motion of the earth, and the moon revolving around it, with its different phases, the cause of

* A small Equatorial, having the Horizontal, Declination, and Equatorial circles about six inches diameter, surmounted with a twenty inch achromatic telescope, with magnifying powers of from 30 to 80 times, may be procured for about fifteen or sixteen guineas, which will serve every general purpose in teaching astronomy.

eclipses, the retrograde motion of the moon's nodes, and the inclination of its orbit to the plane of the ecliptic. The earth is moveable on an axis inclined 23 degrees to the ecliptic, and its axis preserves its parallelism during its course round the sun. The seasons are exhibited on this instrument as follows:-The index, which points out the sun's place and the day of the month, is placed at the 21st March, the time of the vernal equinox, and the north and south poles of the earth are placed exactly under the terminator, or boundary between light and darkness. When the machinery is moved by the hand till the index points to the 21st June, the time of the summer solstice, then the North Polar regions appear within the boundary of light, and the South Polar within the boundary of darkness. Turning the machine till the index points to September 23d, both poles again appear on the boundary of light and darkness. Moving it on to December 21st, the Arctic circle appears in darkness, and the Antarctic in the light. During these motions, the earth's axis keeps parallel to itself, pointing uniformly in the same direction. This exhibition is quite satisfactory and convincing; the only objection to the instrument is, that it is small,-about eight or nine inches diameter-and, consequently, will admit only four or five individuals at a time to inspect its movements with distinctness.

A full and specific description should next be given of all the facts connected with the solar system—the distances and magnitudes of the sun and planets-their annual and diurnal revolutions-the solar spots-the belts and satel lites of Jupiter-the rings of Saturn-the phases of Venus -the spots of Mars, and the mountains and cavities of the Moon. After which some details might be given of the facts which have been ascertained respecting comets, variable stars, double and treble stars, new stars, stars once visible which have disappeared, and the numerous nebula which are dispersed through different regions of the heavens. The pupils should now be gratified with a view of some of these objects through good telescopes. A telescope, magnifying about 30 times, will show the satellites of Jupiter, the crescent of Venus, the solar spots, and the rugged appearance of the Moon. With a magnifying power of 60 or 70, the ring of Saturn, the belts of Jupiter, the shadows of the lunar mountains and cavities, and all the phases of Venus, may be distinguished. But the views of these objects obtained by such magnifying powers are