the observer must behold them when he is situated upon the surface of the earth. We may hence perceive the reasons why, when the sun is invisible, the densest clouds appear to be at the horizon; why the clouds which are driven by the wind towards the zenith decrease in magnitude the higher they rise; why, when the sun is momentarily hid by the clouds, the rays appear to accumulate at the point where he is situated, although, in reality, they are parallel to one another. The explanation of the beautiful phenomenon of the Aurora Borealis is capable also of receiving a slight modification.

Upon the Meteors which play round the Earth.

PROFESSOR MEINECKE, in a paper read to the Society of Natural History at Halle, in Germany, proves, in various ways, the existence of an inferior or subterrestial atmosphere. He considers himself borne out by reasons, which he alleges to conclude with certainty, that an atmosphere which can penetrate to the depth of 20 geographical miles is already compressed at a less depth, to a degree at which, without being liquid, forms a fluid equivalent to water. Hence there results to the interior parts of the earth an atmosphere, in comparison with which the common atmosphere will appear to be of a very light description, which, as is known, is equal to a column of water 30 feet high, or thereabouts.

It is to this mass of interior air, which pervades the pores of fossils, exists in the cavities and abysses of the earth, and forms likewise a portion of the elementary parts of fossils, that Professor Meinecke attributes the greater part of meteors; while the mass of light air universally disseminated in the form of a vapour, and which is called atmosphere, contributes but in a very small degree to their appearance. As he attributes the barometrical phenomena to the subterrestial atmosphere, he denies at the same time the influence of the moon upon the weather.

On the Change of the Place of the Fixed Stars.

It appears from a paper by Mr. Pond, Astronomer Royal, lately IT read before the Royal Society, that he thinks that his observations lead to the conclusion that some variation, either continued or periodical, takes place in the siderial system, which, producing but very small deviations in a finite portion of time, has hitherto escaped notice. The nature of this motion appears to be such, that the stars are now mostly found a considerable quantity to the southward of their computed places. With respect to the laws by which these motions are governed, Mr. Pond admits that his observations are not sufficiently exact to throw any light upon the subject.

Astronomical Telescope.

THE Astronomical Telescope which was invented by Kepler is represented by the following figure.

It consists of two convex glasses placed in a tube; the one next the object is called the object-glass, and the one next the eye, as BE, the eye-glass, which is always of a much less focal length than the object-glass. The focal length of the object-glass is a little greater than the tube into which it is screwed, and the eye-glass is fixed into a small tube, which can be moved out and in at the other extremity of the longer tube. When such an instrument is directed to a distant object, and distinct vision obtained by adjusting the tube containing the eye-glass, the magnified object is formed by the rays ABC and DEC, which come from the extremities of the visible field through the middle of the object-glass, and produce an inverted image nearly in the principal focus of the eye-glass, through which this image is viewed as by a simple microscope, and therefore still remains apparently inverted. This, however is not considered a disadvantage by astronomers, and therefore it has received the name of the astronomical telescope.

In order to find the magnifying power, we must divide the focal length of the object-glass by that of the eye-glass: this quotient is consequently the greater as the focal length of the object-glass is greater, and as that of the eye-glass is smaller; but the power of the instrument cannot be increased at pleasure by lessening the focal length of the eye-glass, because the object-glass would not furnish light enough to render the view distinct, if the magnifying power were too great.

Galilean Telescope.

The Galilean Telescope received its name from its having been first used by Galileo, and differs in no respect from the astronomical telescope, excepting in the substitution of a concave eye-glass in place of a convex eye-glass. This eye-glass is placed between the object-glass and its principal focus, and receives the converging rays before they form the image.

The magnifying power of this telescope will be equal to the focal length of the object-glass divided by that of the eye-glass; for the extreme pencils which were formerly made to converge, are now made to diverge.

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This telescope possesses some advantages over the astronomical telescope. 1st. It has the 1st. It has the same magnifying power under shorter length, the length of the telescope being equal to the difference of the focal lengths of the object-glass and the eye-glass, whereas in the astronomical telescope it is equal to their sum. 2d. It gives us an erect view of the object without using three eyeglasses, which must occasion a great loss of light, even if the lenses are ground to a perfect figure. 3d. There is less absorption of light, in consequence of the rays passing through a less thickness of eye-glass; and 4th, the vision is much more perfect; a circumstance which no doubt arises from the rays never coming to a positive focus, and never crossing one another in a condensed state during the whole of their progress through the instrument. For it cannot be doubted that the rays of light, notwithstanding their extreme tenuity, interfere with one another, and produce an indistinctness of vision.

The disadvantages of the Galilean telescope arise solely from its limited field of view since the lateral pencils now diverge from the axis of the lenses, the field of view depends solely on the diameter of the pupil of the eye, and as this cannot be increased at pleasure, there are no means of remedying this evil in the Galilean telescope.

The imperfections of the common refracting telescope with a single object-glass, are so great, that in order to obtain a high magnifying power, it is necessary to use object glasses of a great focal length, such as those of Huygens, some of which were not less than 24 feet in length.*

It follows from the theory of aberration, arising from the spherical figure of the surface, that the apparent indistinctness of a given object seen through a refracting telescope, is directly as the area of the object-glass, and inversely as the square of the focal distance of the eye-glass. In like manner, it may be shown, that the apparent brightness of a given object is directly as the square of the lineal aperture of the object-glass, and inversely as the square of its mag⚫nifying power. Hence, it follows, that in refracting telescopes of different lengths, a given object will appear equally bright, and equally distinct, when their linear apertures, and the focal distances of the eye-glasses, are as the square roots of the focal distances of their object-glasses, and consequently their magnifying powers will be as the square roots of the focal distances of the object-glasses.

Upon these principles, we may compute the focal distance of the eye-glasses suited to object-glasses of different focal lengths, provided we have once ascertained by experiment the highest magnifying power that an object-glass of a given focal length will bear with perfect distinctness. One of Huygen's best telescopes, 30 feet long, had an aperture of three inches, with an eye-glass three inches and three-tenths in focal length; and from this telescope, as a standard, the editors of his Dioptrics computed the table given in p. 211 of that work, and reprinted by Dr. Smith in the 148th page of the first

*This excellent astronomer while in England presented the Royal Society with two object-glasses, one of which had a focal length of 120, and the other of 123 feet.

volume of his Optics. It appears, however, from the Astroscopia Compendiari of Huygens, that he possessed a telescope superior even to this, whose object-glass was 34 feet in focal length, and which had a magnifying power of 163 times, with an eye-glass of 24 inches focal length.

In order to render the common refracting telescope as perfect as possible, without making it achromatic, the spherical aberration should be reduced by grinding the exterior surface of the object-glass to a radius equal to five-ninths of its focal length, and the interior surface to a radius equal to five times its focal length. In the eyeglasses, the radius of the surface next the object should be nine times its focal length, and that of the surface next the eye, threefifths of the same distance.

When the object to which the telescope is directed is luminous, such as the Sun, and Jupiter, and Venus, when they are near the earth, considerable advantage may be derived from the use of red or green eye-glasses; or, if the telescope is large, from the interposition of plane pieces of green and red glass.

On Meteors, or Falling Stars.

THESE bodies appear to be of different magnitudes, and even of various forms, though this last circumstance may perhaps be the effect of optical deception. In general they seem to be globular, continuing visible only for a few seconds, and moving with great velocity. Their course is on some occasions in a straight line, and on others curvilineal, rendered more distinct by the tail or luminous train which they leave behind them; and before disappearing they are sometimes separated into several smaller bodies, accompanied with an explosion resembling thunder, more or less loud according to their magnitude or distance. It was long supposed, and has now been proved by the most incontrovertible evidence, that these explosions are followed by a shower of solid bodies of a stony or metallic substance, some of which have appeared luminous even in their descent after the explosion, and have been taken up before they had time to cool. This last phenomenon, indeed; is of comparatively rare occurrence. Thousands of small meteors, as various in magnitude and brilliancy as the fixed stars, have been seen in all seasons, and in almost every variety of weather, unaccompanied either with explosions, or the deposition of solid substances; nor is it certain that even the larger and more luminous meteors, such as that of 1783, described by Cavallo, or one in 1811, an account of which was given by Professor Pictet in the Bibliothéque Britannique for May, 1811, are always followed by a fall of meteoric stones. On the other hand, these stones have sometimes been observed to fall after a loud detonation, when no meteor was visible, though this may perhaps be accounted for, from its having been obscured either by the superior light of the sun, or the intervention of clouds. But however this

may be, the appearance of large meteors, and the fall of meteoric stones, or, as they have very improperly been called, aeroliths, are phenomena that appear to be closely connected, and this is almost all that is known upon the subject. Whether they are all of the same origin, but varying in appearance, in consequence either of their different distances, or of some peculiar state of the atmosphere, or whether they are essentially different in their nature, are questions to which, in the present state of metereological science, no answer can be given. As prognostics of the weather, they have in general been supposed to predict wind, as appears from various passages in ancient authors; and it is also commonly believed, that the wind which follows will blow from the point of the compass towards which the meteor is observed to move. One at least of the various hypotheses which have been proposed to account for these phenomena is interesting, inasmuch as it appears to explain, in certain cases, the connection between the motion of the meteor and the direction of the wind.

The hypothesis to which we allude, is that which ascribes meteors to certain vapours arising from the earth, and becoming ignited in the higher regions of the atmosphere. The origin of this opinion may be traced to Aristotle; but from the discoveries in chemistry, of which that author was in a great measure ignorant, it has assumed, in the hands of the modern philosopher, a more definite form. Halley, and after him De Luc, has endeavoured, on this principle, to account for some at least of the circumstances attending the appearance of luminous meteors. The latter supposes that falling stars proceed from a phosphoric fluid, ascending from some spot of the surface of the earth, which becomes visible only when, by decomposition in the higher regions, it takes fire, and light is disengaged. If such a fluid can be supposed to rise in a continued column, without mixing with the atmosphere, or being dispersed by wind, there is no difficulty in conceiving how it may produce the appearance of a falling star. When the upper extremity of the column has reached such a height as to be in a great measure above the region of the clouds and moisture, it may, from the dryness of the air, take fire spontaneously, as phosphorus is known to do when exposed to the atmosphere in its ordinary state; and ignition having once commenced, it may be communicated backward to successive portions of the column, till it arrives at a portion of the atmosphere sufficiently moist to extinguish it, or at the same point where the column itself has been broken and separated. In these circumstances, it is obvious that the appearance would be precisely that of a falling star; and Mr. Forster has ingeniously applied the hypothesis to account for the apparent relation between such phenomena and succeeding gales of wind. It has been long known that different, and even opposite currents of wind, may exist at different heights in the atmosphere at the same time; and the author just referred to, has found, from various experiments and observations, that when the wind near the surface of the earth changes, it frequently blows from the same point from which the current above had previously blown. He observes, therefore, that