seen that, under the most favourable circumstances, the Sun cannot remain totally eclipsed for more than a few minutes.

The duration of the obscuration in a total eclipse of the Sun varies, cæteris paribus, with the latitude of the place of observation, being greatest under the equator. Du Séjours found that, under the most favourable circumstances, the greatest possible duration of the total phase at the equator was 7m 58, and that at the latitude of Paris it was 6m 108.

The duration of an annular eclipse is greatest when the Moon is in apogee and the Sun in perigee, for then the apparent diameter of the Sun is the greatest, whilst that of the Moon is the least possible, and consequently the excess of the former over the latter, upon which the annulus depends, is then at a maximum.

The perigean diameter of the diameter of the Moon = 29′ 22′′.

Sun = 32′ 35′′. The apogean

This then is the arc described by the Moon during the continuance of the annular phase. Du Séjour found by calculation that the greatest possible duration of the annular phase under the equator was 12m 24, and that at the latitude of Paris it was 9m 568.

It may be desirable just briefly to point out the reasons why the greatest possible duration of an annular eclipse exceeds that of a total one. They are 2 in number: 1st. Because the excess of the perigean diameter of the Sun over the apogean diameter of the Moon (=3′ 13′′) is greater than the excess of the perigean diameter of the Moon over the apogean diameter of the Sun (=2′ 1′′). 2nd. Because the motion of the Moon in apogee is much slower than it is in perigee.

From the above remarks it will be readily understood that though so many solar eclipses happen from time to time, yet the occurrence of an annular or total one at any particular locality, is a very rare phenomenon. Thus, according to Halley, no total eclipse had been observed at London between March 20,

Mém. Acad. des Sciences, 1777, p. 318.

i Ibid., p. 316.

h Ibid., p. 317.

1140, and April 22, 1715 (0. s.), though at different times during that interval the shadow of the Moon had frequently passed over other parts of Great Britain *.

The calculation of eclipses is a matter of considerable complexity. A paper by Woolhouse, in the supplement to the Nautical Almanac for 1836, and the chapters in Loomis's well-known work', may be named as the best guides in our language m.

* Phil. Trans., vol. xxix. p. 245. 1715.

Practical Astronomy.

m It is recorded by Rittenhouse that in his early days he calculated eclipses on his plough-handle.

CHAPTER II.

ECLIPSES OF THE SUN.

Grandeur of a Total Eclipse of the Sun-How regarded in ancient times.—Effects of the progress of Science.-Chief phenomena seen in connexion with Total Eclipses.-Change in the colour of the sky.-The obscurity which prevails.— Effect noticed by Piola.-Physical explanation.-Baily's Beads.-Extract from Baily's original memoir.—Probably due to irradiation.—Supposed to have been first noticed by Halley in 1715.—His description.-The Corona.—Hypothesis advanced to explain its origin.—Probably caused by an atmosphere around the Sun.-Remarks by Grant.-First alluded to by Philostratus.-Then by Plutarch. Corona visible during Annular Eclipses.-The Red Flames.-Remarks by Dawes.— Physical cause unknown.-First mentioned by Stannyan.-Note by Flamsteed.Observations of Vassenius.-Aspect presented by the Moon.-Remarks by Arago.

A

TOTAL eclipse of the Sun is a most imposing spectacle, especially when viewed from the summit of a lofty mountain. Words can but inadequately describe the grandeur and magnificence of the scene. On all sides indications are afforded that something unusual is taking place. At the moment of totality the darkness is sometimes so intense that the brighter stars and planets are seen, birds go to roost, flowers close, and the face of nature assumes an unearthly cadaverous hue; not the least striking thing is the sudden and frequently considerable fall that takes place in the temperature of the atmosphere as the time of the greatest obscuration draws near.

During the early history of mankind, a total eclipse of the Sun was invariably regarded with a feeling of indescribable terror, as an indication of the anger of the offended Deity, or the presage of some impending calamity; and various instances are on record of the (supposed) extraordinary effects produced by so unusual an event. In a more advanced state of society, when Science had

begun to diffuse her genial influence over the human mind, these vain apprehensions gave place to juster and more ennobling views of nature; and eclipses generally came to be looked upon as necessary consequences flowing from the uniform operation of fixed laws, and differing from the ordinary phenomena of nature only in their less frequent occurrence. To astronomers they have in all ages proved valuable in the highest degree, as tests of great delicacy for ascertaining the accuracy of their calculations relative to the place of the Moon, and hence deducing a further improvement of the intricate theory of her movements. In modern times, when the physical constitution of the celestial bodies has attracted the attention of many eminent astronomers, observations of eclipses have disclosed several interesting facts, which have thrown considerable light on some important points of inquiry respecting the Sun and Moon "."

Among the Hindùs a singular custom is said to exist. When during a solar eclipse the black disc of our satellite is seen advancing over the Sun, the natives believe that the jaws of some monster are gradually eating it up. They then commence beating gongs, and rending the air with the most discordant screams of terror and shouts of vengeance. For a time their efforts are productive of no good result-the eclipse still progresses. At length, however, the terrific uproar has the desired effect on the voracious monster; it appears to pause, and then, like a fish that has nearly swallowed a bait and then rejects it, it gradually disgorges the fiery mouthful. When the Sun is quite clear of the great dragon's mouth, a shout of joy is raised, and the poor natives disperse, extremely self-satisfied on account of their having (as they suppose) so successfully relieved their deity from his late perils. Times have now happily altered somewhat. We do not look on a total eclipse of the Sun as a dire calamity, but merely as one of the ordinary effects resulting from the due working of those laws by which a Divine Being wills to govern the universe.

An eclipse of the Sun may be either partial, annular, or total: it is partial when only a portion of the Moon's disc intervenes between the Sun and the observer on the Earth; annular, when

a Grant, Hist. Phys. Ast., p. 359.

the Moon's apparent diameter is less than the Sun's, so that when the former is projected on the latter it is not sufficiently large completely to cover it,-an annulus, or ring of the Sun, being left unobscured; and total when the Moon's apparent diameter is greater than that of the Sun, which is, therefore, wholly obscured. In an annular eclipse, when the centre of the Sun and Moon exactly coincide, it is said to be central and annular—the Sun appearing, for an instant only, as a brilliant ring of light around the dark body of the Moon.

I shall now proceed to describe the principal phenomena which are usually witnessed in connexion with solar eclipses. Not the least remarkable is the almost invariable change of colour which the sky undergoes. Halley, in his account of the eclipse of 1715, says: "When the eclipse was about 10 digits (that is, when about of the solar diameter was immersed), the face and colour of the sky began to change from a perfect serene azure blue to a more dusky livid colour, intermixed with a tinge of purple, and grew darker and darker till the total immersion of the Sun"."

It has also been found that whilst the sky changes colour during the progress of an eclipse, similar effects are also produced upon terrestrial objects. This seems to have been noticed as far back as 840 A.D. Kepler mentions that during the solar eclipse which happened in the autumn of 1590, the reapers in Styria noticed that everything had a yellow tinged. Similar effects have also been described in modern times".

The darkness which prevails during a total eclipse of the Sun is not usually so considerable as might be expected. It is, however, subject to much variation. Ferrer, speaking of the eclipse of 1806, says, "that at the time of total obscuration as much light remained as is equal to that afforded by a full Moon." In general it has been found that the darkness is sufficiently great to prevent persons from reading, though exceptions to this rule have

b Phil. Trans., vol. xxix. p. 247. 1715. Arago gives an elaborate explanation of this. Pop. Ast., vol. ii. p. 358, Eng. ed.

Ad Vitellionem Paralipomena, p. 294.

a Ibid., p. 303.

e Mem. R.A.S., vol. xv. xxii.; Annuaire, 1846, p. 291, &c.

266.

Trans. Amer. Phil. Soc., vol. vi. p.