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 earth, 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. 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. iii, No. 18, Dec. 1833.

per

with stony or other solid material, to the depth of 12 feet 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. affords to either of these explanations.

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.

FIRST INTRODUCTION.

REFORMATION.

-

HOW OBIRREGULARITIES AT ITS

GREGORIAN

REFORMED BY AUGUSTUS. SOLAR AND LUNAR CYCLES. INDICTION. JULIAN PERIOD. - TABLE OF CHRONOLOGICAL ERAS. FOR CALCULATING THE DAYS ELAPSED BETWEEN GIVEN DATES. EQUINOCTIAL TIME. FIXATION OF ANCIENT DATES BY

ECLIPSES.

RULES

(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

maximum to full half a minute. But except for astronomical purposes, this is too small a change to interfere in the slightest degree with its use, or to attract any attention, and the tacit substitution of its mean for its true (or variable) value may be considered as having been made from the earliest ages, by the ignorance of mankind that any such fluctuation existed.

*

(908). The time occupied by one complete rotation of the earth on its axis, or the mean sidereal day, may be shewn, on dynamical principles, to be subject to no variation from any external cause, and although its duration would be shortened by contraction in the dimensions of the globe itself, such as might arise from the gradual escape of its internal heat, and consequent refrigeration and shrinking of the whole mass, yet theory, on the one hand, has rendered it. almost certain that this cause cannot have effected any perceptible amount of change during the history of the human. race; and, on the other, the comparison of ancient and modern observations affords every corroboration to this conclusion. From such comparisons, Laplace has concluded that the sidereal day has not changed by so much as one hundredth of a second since the time of Hipparchus. The mean sidereal day therefore possesses in perfection the essential quality of a standard unit, that of complete invariability. The same is true of the mean sidereal year, if estimated upon an average sufficiently large to compensate the minute fluctuations arising from the periodical variations of the major axis of the earth's orbit due to planetary perturbation (Art. 668.).

(909.) The mean solar day is an immediate derivative of the sidereal day and year, being connected with them by the same relation which determines the synodic from the sidereal revolutions of any two planets or other revolving bodies (Art. 418.). The exact determination of the ratio of the sidereal to the solar day, which is a point of the utmost importance in astronomy, is however, in some degree, complicated by the effect of precession, which renders it necessary

The true sidereal day is variable by the effect of nutation; but the variation (an excessively minute fraction of the whole) compensates itself in a revolution of the moon's nodes.