on the same day and during the same hours in 1832, and as extraordinary flights of shooting stars were seen at many places both in Europe and America on the 13th of November, 1834, 1835, and 1836, tending also from a fixed point in the constellation Leo, it has been conjectured, with much apparent probability, that this nebula or group of bodies performs its revolution round the sun in a period of about 182 days, in an elliptical orbit, whose major axis is 119 millions of miles; and that its aphelion distance, where it comes in contact with the earth's atmosphere, is about 95 millions of miles, or nearly the same with the mean distance of the earth from the sun. This body must have met with disturbances after 1799, which prevented it from encountering the earth for 32 years, and it may again deviate from its path from the same cause.

As early as the year 1833, Professor Olmsted, of Yale College, in the United States of America, had conjectured that the phenomenon of shooting stars originated in the zodiacal light, and his subsequent observations, continued for three successive years, have tended to confirm him in this opinion. He agrees with La Place in thinking that the zodiacal light is a nebulous body, revolving in the plane of the solar equator. In fact, this light stretches beyond the earth's orbit, making an angle of about 74° with the plane of the ecliptic, and, according to observation, it is sometimes. seen in the dawn, and sometimes in the twilight, like an inferior planet. It was seen by Professor Olmsted for several weeks previous to the 13th of November, in the morning dawn, with an elongation (N. 231) of from 60° to 90° west of the sun. It then by degrees withdrew from the morning sky, and appeared in the evenings immediately after twilight, rising like a pyramid through the constellations Capricornus and Aquarius to an elongation of more than 90° eastward of the sun. A change like this, taking place annually about the 13th of November, has led the Professor to believe that it is to the zodiacal light we are indebted

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for those splendid exhibitions of falling stars which take place at that season.

The orbit already described is that which he formerly assigned to this nebulous or cometary body, but he is now of opinion that it has a period of something less than a year, which would not only account for the shooting stars of the 13th of November, but would also account for those that happen at all seasons, and for some very great showers of them that have taken place on two occasions near the end of April. In the position assigned to this orbit by Professor Olmsted, showers of shooting stars may happen in November and April. A very able memoir has been published by M. Biot, in which that great philosopher shows that, in his opinion also, meteoric showers are owing to the zodiacal light coming into periodic contact with the atmosphere of the earth. Which of these conjectures may be nearest the truth, time alone can show; but certain it is that the recurrence of this phenomenon at the same season for seven successive years proves that it can arise from no accidental

cause.

It is now well ascertained that great showers of shooting stars occur also on the 12th of August, so that the earth's atmosphere comes into contact with a zone of these small bodies twice in the year. But, as shooting stars are seen almost every night when the sky is clear, Sir John Lubbock has thought it probable that some of these bodies may have come so near, that the attraction of the earth has overcome that of the sun, and caused them to revolve as satellites round it. Should that be the case, they might shine by the reflected light of the sun, and suddenly cease to be visible on entering the earth's shadow. The splitting of the falling stars like a rocket, and the trains of light, may be accounted for by supposing the stars to graze the surface of the shadow before being eclipsed; and the disappearance would be more or less rapid, according to the breadth of the penumbra traversed. The calculations of M. Petit, Director

of the Observatory of Toulouse, not only render probable the existence of small satellites, but tend to establish the identity of a body revolving round the earth in three hours and twenty minutes. It is evident that in this case the same satellite would be seen very often, and a very few would be sufficient to account for their nightly appearance. It is possible, however, that some shooting stars may belong to one class, and some to the other, since one group may be revolving about the sun, and another round the earth. In the case of a satellite shooting star, geometry furnishes the means of ascertaining its exact distance from the spectator, or from the centre of the earth, if the time and place of its disappearance be known with regard to the neighbouring stars.

SECTION XXXVIII.

Diffusion of Matter through Space-Gravitation-Its Velocity-Simplicity of its Laws-Gravitation independent of the Magnitude and Distances of the Bodies-Not impeded by the intervention of any Substance-Its Intensity invariable-General Laws-Recapitulation and Conclusion.

THE known quantity of matter bears a very small proportion to the immensity of space. Large as the bodies are, the distances which separate them are immeasurably greater; but, as design is manifest in every part of creation, it is probable that, if the various systems in the universe had been nearer to one another, their mutual disturbances would have been inconsistent with the harmony and stability of the whole. It is clear that space is not pervaded by atmospheric air, since its resistance would, long ere this, have destroyed the velocity of the planets; neither can we affirm it to be a void, since it seems to be replete with ether, and traversed in all directions by light, heat, gravitation, and possibly by influences whereof we can form no idea.

Whatever the laws may be that obtain in the more distant regions of creation, we are assured that one alone regulates the motions, not only of our own system, but also of the binary systems of the fixed stars; and, as general laws form the ultimate object of philosophical research, we cannot conclude these remarks without considering the nature of gravitation—that extraordinary power, whose effects we have been endeavouring to trace through some of their mazes. It was at one time imagined that the acceleration in the moon's mean motion was occasioned by the successive transmission of the gravitating force. It has been proved that, in order to produce this effect, its velocity must be about fifty millions of times greater than that of light, which flies

at the rate of 200,000 miles in a second. Its action, even at the distance of the sun, may therefore be regarded as instantaneous; yet so remote are the nearest of the fixed stars, that it may be doubted whether the sun has any sensible influence on them.

The curves in which the celestial bodies move by the force of gravitation are only lines of the second order. The attraction of spheroids, according to any other law of force than that of gravitation, would be much more complicated; and, as it is easy to prove that matter might have been moved according to an infinite variety of laws, it may be concluded that gravitation must have been selected by Divine Wisdom out of an infinity of others, as being the most simple, and that which gives the greatest stability to the celestial motions.

It is a singular result of the simplicity of the laws of nature, which admit only of the observation and comparison of ratios, that the gravitation and theory of the motions of the celestial bodies are independent of their absolute magnitudes and distances. Consequently, if all the bodies of the solar system, their mutual distances, and their velocities, were to diminish proportionally, they would describe curves in all respects similar to those in which they now move; and the system might be successively reduced to the smallest sensible dimensions, and still exhibit the same appearances. We learn by experience that a very different law of attraction prevails when the particles of matter are placed within inappreciable distances from each other, as in chemical and capillary attraction, the attraction of cohesion, and molecular repulsion; yet it has been shown that in all probability not only these, but even gravitation itself is only a particular case of the still more general principle of electric action.

The action of the gravitating force is not impeded by the intervention even of the densest substances. If the attraction of the sun for the centre of the earth, and of the hemisphere