(902.) Now this is precisely what actually happens. The meteors of the 12th-14th of November, 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 same, the apex only differing; B Camelopardali 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 excentricity, 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. (903.) 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 groupes 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 groupes have been encountered. (904.) No other plausible explanation of these highly characteristic features (the annual periodicity, and divergence from a common apex, always alike for each respective epoch) has been even attempted, and accordingly the opinion is generally gaining ground among astronomers that shooting stars belong to their department of science, and great interest is excited in their observation and the further development of their laws. The most connected and systematic series of observations of them, having for their object to trace out their relative paths with respect to the earth, are those of Benzenberg and Brandes, who, by noting the instants and apparent places of appearance and extinction, as well as the precise apparent paths among the stars, of individual meteors, from the extremities of a measured base line nearly 50,000 feet in length, were led to conclude that their heights at the instant of their appearance and disappearance vary from 16 miles to 140, and their relative velocities from 18 to 36 miles per second, velocities so great as clearly to indicate an independent planetary circulation round the sun. [A very remarkable meteor or bolide, which appeared on the 19th August, 1847, was observed at Dieppe and at Paris, with sufficient precision to admit of calculation of the elements of its orbit in absolute space. This calculation has been performed by M. Petit, director of the observatory of Toulouse, and the following hyperbolic elements of its orbit round the sun are stated by him (Astr. Nachr. 701.) as its result; viz., Semimajor axis = −0·3240083; excentricity=3.95130; perihelion distance = 0·95626; inclination to plane of the earth's equator, 18° 20′ 18′′; ascending node on the same plane, 10° 34′ 48′′; motion direct. According to this calculation, the body would have occupied no less than 37340 years in travelling from the distance of the nearest fixed star supposed to have a parallax of 1′′]. (905.) It is by no means inconceivable that the earth approaching to such as differ but little from it in direction and velocity, may have attached many of them to it as permanent satellites, and of these there may be some so large, and of such texture and solidity, as to shine by reflected light, and become visible (such, at least, as are very near the earth) for a brief moment, suffering extinction by plunging into the earth's shadow; in other words, undergoing total eclipse. Sir John Lubbock is of opinion that such is the case, and has given geometrical formulæ for calculating their distances from observations of this nature. The observations of M. Petit would lead us to believe in the existence of at least one such body, revolving round the carth, as a satellite, in about 3 hours 20 minutes, and therefore at a distance equal to 2.513 radii of the earth from its center, or 5000 miles above its surface.† Phil. Mag., Lond. Ed. Dub. 1848, p. 80. † Comptes Rendus, Oct. 12. 1846, and Aug. 9. 1847. (905 a.) In art. 400. the generation of heat by friction is suggested as affording a possible explanation of the supply of solar heat, without actual combustion. A very old doctrine, advocated on grounds anything rather than reasonable or even plausible by Bacon, but afterwards worked into a circumstantial and elaborate theory by the elder Seguin, which makes heat to consist in a continual, rapid, vibratory or gyratory motion of the particles of bodies, has of late been put forward into great prominence by Messrs. Mayer, Joule and Thomson. According to this theory motion once generated, or however originating, is never destroyed, but continues to subsist in the form of "vis viva" among the molecules of bodies, even when by their impact or mutual obstruction they appear to have been brought to rest. The "vis viva " only takes another form, and is disseminated, as increased vibratory or gyratory movement, among their molecules; as such it is heat, or light, or both, and is communicated to the molecules of the luminiferous ether, and so distributed throughout that ether, constituting the phænomena of radiant light and heat. Granting a few postulates (not very easy of conception, and still less so of admission when conceived,) this theory is not without its plausibility, and certainly does (on its own conventions) give a consistent account of the production of heat by friction and impact. It has been applied by Messrs. Watherson and Thomson to explain the evolution of solar light and heat, as follows. According to the former, the meteorolites which, revolving in very excentric or cometic orbits, arrive within the limits of the solar atmosphere are precipitated on the sun's surface in such abundance, and with such velocity, as to generate in the way above described the totality of the emitted radiants. Prof. Thomson, undismayed as would appear by the perpetual battery thus kept up on the sun's surface (on every square foot of which, on Mr. Watherson's view of the subject, a weight of matter equal to 5 lbs. would require to be delivered per hour with a velocity of 390 miles per second, covering the whole surface * On this point see a paper by the Author on the absorption of light, Lond. and Ed. Phil. Mag. and Journ., 3rd series, vol. ii. No. 18, Dec. 1833. with stony or other solid material, to the depth of 12 feet per annum, if of the density of granite,) prefers to consider the nebula of the zodiacal light in a vaporous state as continually subsiding into the sun, by gradual spiral approach, until suddenly meeting with greatly increased resistance in its atmosphere (as arriving in a state of more rapid revolution) by friction on the external envelope or photosphere of its surface (art. 389.), produces there the heat and light actually observed; whereas the theory of Mr. Watherson would place its origin on the solid surface itself, contrary to the observed fact.* Our readers will judge for themselves what degree of support the telescopic aspect of the sun's surface as described in arts. 386-395, and especially (387 a), Note G, affords. The quantity of matter annually required to be deposited on the sun, whether in a pulverulent, liquid, or vaporous form, by Prof. Thomson's theory, is nearly double of that called for by Mr. Watherson's, viz., 24 feet of granite per annum, i.e. a mile in 260 years; so that the sun's apparent diameter would be increasing at the rate of about 1" per 100,000 years on this hypothesis. In the "Manuel de la Science, ou Annuaire du Cosmos" for 1859, by the Abbé Moigno (a work of high interest, and, generally speaking, of great impartiality in the discussion of claims to scientific priority), pp. 85, 6, 7, 2me partie, this article is so translated (probably for want of a perfect appretiation of the force of the expressions used in it) as to convey an unqualified adhesion to the theory in question and to M. Seguin's doctrine. This, however (especially the latter, as stated at length in Pt. I. pp. 224. et seq.), 1 am very far from prepared to give-and the English reader will, I presume, consider the terms employed quite sufficiently guarded, even as respects the general principle; to say nothing of the specialties of M. Seguin's theory.-[Note added, 1859.] - RULES JULIAN PERIOD. - TABLE OF CHRONOLOGICAL ERAS. TIME. BY ECLIPSES. (906). TIME, like distance, may be measured by comparison with standards of any length, and all that is requisite for ascertaining correctly the length of any interval, is to be able to apply the standard to the interval throughout its whole extent, without overlapping on the one hand, or leaving unmeasured vacancies on the other; to determine, without the possible error of a unit, the number of integer standards which the interval admits of being interposed between its beginning and end; and to estimate precisely the fraction, over and above an integer, which remains when all the possible integers are subtracted. (907). But though all standard units of time are equally possible, theoretically speaking, yet all are not, practically, equally convenient. The solar day is a natural interval which the wants and occupations of man in every state of society force upon him, and compel him to adopt as his fundamental unit of time. Its length as estimated from the departure of the sun from a given meridian, and its next return to the same, is subject, it is true, to an annual fluctuation in excess and defect of its mean value, amounting at its |