showers, except the last, the Geminids are also annually recurrent, and this character was noticed and the radiant-point of the shower was determined simultaneously by Mr. Greg in England, and by Professor Twining in America, on December 12, 1863. Indications of periodicity and of early notices of years of maxima in these appearances have been sought for, with some success, in catalogues of meteor showers by Prof. Newton P and Prof. Kirkwood with the probable result announced by Kirkwood that the meteors of April, October, and December revolve in periods respectively of 28§, 271, and 29 years, while the January meteor ring has a suspected period of about 13 years.

Many of the astronomical views concerning shooting stars adopted before the first predicted return of the November meteors in 1866-67 were due to a valuable memoir by Olbers', in which in the place of orbits approximately circular like those conceived by Biot, and in the contemporaneous paper on shooting stars above referred to, by Arago, they were assumed to move rather in comet-like or very elongated orbits. The 33-year cycle of the November meteors was pointed out and thus explained by Olbers, who also ventured to predict a probable great return of the November meteors about the year 1867, which prediction, as well as the grounds upon which it rested, was verified by the

event.

Subdividing the recorded instances of great showers of shooting stars according to the months of the year, we obtain the following results:

We thus find, and it is worthy of especial remark, that the coincidence to which I have already adverted in the case of aëro

P Silliman's Journal, 2nd Ser., vol. xxxvi. p. 145, July 1863.

a Proceedings of the American Philosophical Society, vol. xi. p. 299, March 4,

1870, and vol. xiii. p. 501, Nov. 21, 1873.

r Schumacher's Jahrbuch, 1837, p. 36.

lites, fireballs and other meteors also obtains with the showers of shooting stars-namely, that the Earth encounters a larger number of these bodies in passing from aphelion to perihelion, or between July and January, than in passing from perihelion to aphelion, or between January and July.

In concluding this chapter, brief reference may be made to the apparent magnitudes of meteors. From many thousands of observations recorded in various published catalogues it would appear that the following is something like the relative brightness of these bodies :

The numbers show a definite increase of 7.8 per cent. There is an enormous excess of faint meteors as compared with the more brilliant forms of these phenomena *.

An article in The Observatory, vol. ii. p. 20 (May 1878), may be consulted for further details.

* A catalogue of 221 meteoric showers is given in Arago's Ast. Pop., vol. iv.

Pp. 292-314. Also a catalogue, extending from 538-1223 A.D., by Chasles, in Compt. Rend., vol. i. pp. 499–509. 1841. For an account of Quetelet's Catalogue see p. 628, post.

SS

CHAPTER IV.

THE THEORY OF METEORS.

Meteors are planetary bodies.-Their periodicity.—Meteoric orbits.—Researches of Newton and Adams.-Orbit of the meteors of November 13.-Identity of the orbits of comets and meteors.-The meteor showers of Nov. 13 and 27.—Recent progress of Meteoric Astronomy.-Table of the chief radiant points.

IT

T has been mentioned in a previous chapter that it is to some extent doubtful whether aërolites, fireballs and shooting stars are manifestations of identical phenomena or whether they belong to distinct classes of bodies. There is much evidence to warrant the assumption of identity, and it will be convenient to adopt this view during our further consideration of the subject.

Many theories have been propounded to explain luminous meteors, but they were usually based on few observations, and later researches did not support them. But in recent years a theory has been framed which so well accords with observed facts that it has received universal recognition.

Meteors are diminutive planetary bodies revolving round the sun in orbits similar to those pursued by comets. These orbits intersect the annual path of the Earth, and hence it follows that whenever the Earth passes through these points of intersection there is a rencontre with the meteoric particles, which are thereupon propelled into our atmosphere with great velocity and are ignited by the friction generated by the force of impact. Fireballs of ordinary noiseless character and shooting stars are entirely consumed and dissipated before reaching the lower regions of the atmosphere, while aërolites are meteors which succeed in penetrating completely through the air strata and ultimately fall upon the Earth's surface.

With the meteors there prevails, as we have already seen, a periodicity: this will be found on examination to countenance the theory of their being planetary in their nature; and the wellknown experiment of igniting tinder by compressing air in a fire syringe removes the notion of self-ignition from the domain of fanciful speculation.

With reference to their periodicity, Sir J. Herschel says:

"It is impossible to attribute such a recurrence of identical dates of very remarkable phenomena to accident. Annual periodicity, irrespective of geographical position, refers us at once to the place occupied by the Earth in its annual orbit, and leads directly to the conclusion that at that place it incurs a liability to frequent encounters or concurrences with a stream of meteors in their progress of circulation around the Sun. Let us test this idea, by pursuing it into some of its consequences. In the first place, then, supposing the Earth to plunge in its yearly circuit into a uniform ring of innumerable small meteoric planets, of such breadth as would be traversed by it in one or two days; since, during this small time, the motions, whether of the Earth or of each individual meteor, may be taken as uniform and rectilinear, and those of all the latter (at the place and time) parallel, or very nearly so, it will follow that the relative motion of the meteors, referred to the Earth as at rest, will be also uniform, rectilinear, and parallel. Viewed, therefore, from the centre of the Earth (or from any point of the circumference, if we neglect the diurnal velocity, as very small compared with the annual), they will all appear to diverge from a common point, fixed in relation to the celestial sphere, as if emanating from a sidereal apex.

"Now this is precisely what happens. The meteors of the 12th-14th of Nov., or at least the vast majority of them, describe apparently arcs of great circles, passing through or near y Leonis. No matter what the situation of that star, with respect to the horizon or to its East and West points, may be at the time of observation, the paths of the meteors all appear to diverge from that star. On the 9th-11th of August, the geometrical fact is the a Outlines of Astronomy, 11th Ed., p. 651.

same, the apex only differing; B Camelopardi being for that epoch the point of divergence. As we need not suppose the meteoric ring coincident in its plane with the ecliptic, and as for a ring of meteors we may substitute an elliptic annulus of any reasonable eccentricity, so that both the velocity and direction of each meteor may differ to any extent from the Earth's, there is nothing in the great and obvious difference in latitude of these apices at all militating against the conclusion.

"If the meteors be uniformly distributed in such a ring or elliptic annulus, the Earth's encounter with them in every revolution will be certain, if it occur once. But if the ring be broken -if it be a succession of groups revolving in an ellipse in a period not identical with that of the Earth, years may pass without a rencontre; and when such happen, they may differ to any extent in their intensity of character, according as richer or poorer groups have been encountered."

We will now consider the character of meteor orbits, and in order to form a clear conception of the matter it may be necessary to go back a few years and trace the developments leading up to the present theory.

b

In November, 1833, there was witnessed, as has already been stated, a grand display of meteors ("shooting stars"), a less grand one in 1832, and 33 years before that, namely in 1799, another very magnificent one. Availing himself of a comprehensive catalogue of recorded appearances of meteor showers compiled by A. Quetelet in 1836-39°, a learned American astronomer, Prof. H. A. Newton, set himself the task of searching out all the ancient records he could find of such displays: he found that more than a dozen had been taken note of by historians, beginning with 902 A.D., and that in all cases the intervals were either ±rd

b In this chapter the word "meteors" is intended to apply generally, to aërolites, fireballs, and shooting stars; in fact to all the allied, and probably identical, forms of meteoric apparitions.

c Nouveaux Mémoires de l'Académie Royale des Sciences, vol. xii. 1839.

d His papers appear in Silliman's

Journal, 2nd Ser., vol. xxxvii. p. 377, and vol. xxxviii. p. 53, May and July 1864. The periodic dates of the November and of some other annual meteor showers had been discussed in a previous paper in the same Journal, vol. xxxvi. p. 145, July 1863.