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 unsatisfactory. No telescope should be selected for this purpose less than a 34 feet Achromatic, with powers varying from 40 to 180 or 200 times.* A power of 150 is a very good medium for inspecting all the more interesting phenomena of the heavens. With this power, distinct and satisfactory views may be obtained of the solar spots, the phases of Mercury, Venus, and Mars, the belts, and sometimes the spots of Jupiter, and the shadows of his satel lites, the ring and some of the moons and belts of Saturn, the spots of Mars, the minute hills and cavities of the moon, several of the double stars, and many of the most remarkable nebula. To perceive distinctly the division of Saturn's ring, requires a power of at least 200 times. In exhibiting such objects to the young, especially when the lower powers are used, some attention is requisite to adjust the instrument to distinct vision, as their eyes are generally more convex than the eyes of persons advanced in life, and those who are short-sighted will require an adjustment different from that of others. Unless this circumstance be attended to, their views of celestial phenomena will frequently be unsatisfactory and obscure. In exhibiting the surface of the moon, the period of half-moon, or a day or two before or after it, should generally be selected; as it is only at such periods that the shadows of the mountains and vales, and the circular ridges, can be most distinctly perceived. At the time of full moon, its hemisphere presents only a variegated appearance of darker and brighter streaks, and no shadows are discernible; so that, from the telescopic appearance of the full moon, we could scarcely determine whether or not its surface were diversified with mountains and vales.

Previous to exhibiting the moon through a telescope, it may be proper to give the observers an idea of some particular objects they will see, on which their attention should be fixed, and from which they should deduce certain conclusions. For, a view of the moon, for the first time, through a powerful telescope, is apt to overpower the eye, and to produce a confused and indistinct perception. As one of the peculiarities of the lunar surface consists in the numerous cavities, and plains surrounded with circu

• An Achromatic telescope of this description, with an object-glass, 45 inches focal distance, and about three inches diameter, with 4 or 5 magnify. ing powers, with a brass tube mounted on a brass tripod, may be purchased in London, for 25 guineas.

lar ridges of mountains, and insulated mountains rising from a level surface-an idea of the shadows and circumstances by which these objects are indicated should be previously communi. eated. This may be done by means of a saucer, the top of a small circular box, or any other object which may represent a plain surrounded by a circular ridge. In the middle of any of these objects may be placed a small peg to represent a mountain. Then placing a candle at the distance of a foot or two, so as to shine obliquely upon the objects, the inside of the circular dish - farthest from the candle will be seen enlightened, while a considerable portion of the bottom will be covered by the shadow thrown upon it by the side next the candle, and the shadow of the peg will be seen verging towards the enlightened side. This previous exhibition will give them an idea of the form of some of the mountains and vales on the lunar surface, and enable them to appreciate the nature of those striking inequalities which appear near the boundary between the dark and enlightened parts of the moon. Other objects which diversify the moon's surface may be represented and illustrated in a similar manner, and sufficient time should be allowed to every observer for taking a minute inspection of all the varieties on the lunar disk. The solur spots may be viewed with ease, by interposing a coloured glass between the eye and the image of the sun; but, in looking through the telescope in the ordinary way, they can be perceived by only one individual at a time. In order to exhibit them to a company of 30 or 40 persons at once, the image of the sun may be thrown on a white wall or screen. I have generally exhibited them in the following manner. To a 34 feet Achromatic telescope, I apply a diagonal eye-piece, which has a plain metallic speculum placed at half a right angle to the axis of the telescope. By this eye-piece, after the room has been darkened as much as possible, the image of the sun and his spots is thrown upon the roof of the apartment, which forms a beautiful circle of light, and exhibits all the spots which then happen to diversify his surface. His apparent diurnal motion is also represented, along with the motions of any thin fleeces of clouds which may happen to cross his disk. In this way, too, the proportional magnitudes of the spots may be measured, and compared with the diameter of the sun, and, of course, their real magnitudes ascertained.

In illustrating the phenomena of the planetary system by means of orreries, planetariums, and lunariums, great care should be taken to guard the young against the false and imperfect concep tions of the magnitudes and distances of the planets, which such instruments have a tendency to convey. No orrery, of a portable

size, can represent, at the same time, both the proportional dis tances and relative magnitudes of the different planets. Even those large machines designated Eidouraniums and Transparent Orreries afford no correct views of these particulars; and some of them convey very erroneous and distorted conceptions of the relations of the solar system, where it is the chief design to dazzle the eye with a splendid show. In some of these exhibitions I have seen the stars represented as if they had been scattered through different parts of the planetary system.-An orrery representing the proportional distances and magnitudes of the sun and planets would require to be more than three miles in diameter; and, even on this scale, Jupiter would be less than 3 inches diameter, the Earth a quarter of an inch, or about the size of a small pea, and Mercury only about the dimensions of the head of a small pin, while the sun would require to be represented by a ball 30 inches in diameter in which case all the planets would be invisible from the centre of the system. To correct, in some measure, the erroneous ideas which a common orrery is apt to convey, the magnitudes and distances should be separately represented. Suppose a celestial globe, 18 inches in diameter, to represent the Sun, Jupiter will be represented by a ball about 1 inch diameter, Saturn by one of 13 inch, Herschel by one of about inch, the Earth by a ball of inch, or somewhat less than a small pea, Venus by a ball of nearly the same size, Mars by a globule of about inch, Mercury by a globule of, and the Moon by a still smaller globule of inch in diameter. These three last might be represented by three different sizes of pin-heads. When balls of these sizes are placed adjacent to an 18-inch globe, and compared with it, an impressive idea is conveyed of the astonish. ing magnitude of the sun, which is 500 times greater than all the planets, satellites, and comets, taken together. The proportional distances may be represented as follows. At one end of a table

9 feet in length, fix a ball upon a pillar to represent the sun; at 2 inches from the sun's ball, place another to represent Mercury; at 3 inches, Venus; at 5 inches, the Earth; at 7 inches, Mars; at 25 inches, Jupiter; at 47 inches, or about 4 feet, Saturn; and, at 95 inches, or about 8 feet from the sun's ball, place one to represent Herschel. This will convey a pretty correct idea of the proportional distances from the sun of the principal primary planets. The distances of Ceres, Pallas, Juno, and Vesta, might likewise be represented, if-judged expedient; but as their orbits' are more eccentric than those of the other planets, and some of them cross each other, they cannot be accurately represented. When orreries or telescopes cannot be procured for exhibiting the

celestial motions and phenomena to which I have alluded, some of these objects, such as the rings of Saturn, the belts and moons of Jupiter, the phases of Venus, the Moon, and some of the constellations, may be represented in a dark room by means of the phantasmagoria. But the representations made by this instru. ment form but a rude and paltry substitute for the exhibitions presented by the orrery and the telescope, and need never be resorted to, except for amusement, where these instruments can be obtained.

It might next be expedient to communicate to the pupil an idea of the nature of a parallax, to prepare him for understanding the mode by which the distances and magnitudes of the heavenly bodies are ascertained. This might be done by fixing a pole or staff, with a pointed top, in a garden or large area, opposite a wall or hedge, FG, Fig. 1, and, desiring one of the pupils to take his station at A, and another at B, and to direct their eyes to the points on the wall which appear in a line with the top of the pole, when the one stationed at A will perceive it to coincide with the Fig. 1.

point C, and the other stationed at B will perceive it at D. They may be told that CD is the parallax, or the difference of the apparent place of the pole P, when viewed from the positions A and B, which is measured by the angle CPD; and that, if the distance between A and B were measured, and the number of degrees or minutes in the angle CPD or APB ascertained, the distance between the pole and any of the stations can be easily determined. This may be easily applied to the case of the heavenly bodies by means of such a diagram as Fig. 2, where HIK represents the Earth, M the Moon, P a planet, and ST a quadrant of the starry heavens. It is evident, that, if the moon be viewed from the surface of the earth at H, she will appear in the

heavens at the point a; but if she be viewed from the centre C, she will be seen at the point b, the angle a M b being the angle of parallax. This angle being found, which is the same as the 21. bovenger od porn act engin sir T

angle H M C, and the base line HC, or the earth's semidiameter being known, which is nearly 4000 miles-the length of the line HM, or the distance of the moon, can be easily determined. It may be proper also to state that the farther any heavenly body is distant from the earth, the less is its parallax. Hence the parallaxes of the sun and planets are all much less than that of the moon, which is the nearest celestial body to the earth. Thus, the parallax c d of the planet P is less than that of the Moon, M, and the same principle likewise holds true with respect to all terrestrial objects. This subject may soon be rendered quite plain to the pupil, by familiar illustrations, in connection with a few instructions on the nature and properties of triangles, and the first principles of trigonometry.

I have been somewhat particular in some of the hints thrown out above, because it is of some importance that the young should have clear and impressive conceptions of every object presented to their view, in every step of their progress on this subject, and