HISTORY OF PHYSICAL ASTRONOMY.

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during the greater part of the time of its continuance, but seemed to go offvery suddenly; so that when the disk of the moon approached the concaveline of the sun’s disk, they seemed to run together like two contiguousdrops of water on a table, when they touch one another.”* A similarremark was made by the Kev. Mr. Irvine, of Elgin, relative to the annulareclipse of 1748. In his account of the eclipse, as cited by Short, he statesthat the moon’s body seemed to pass quicker about the time of the annulus,than at any other time during the eclipse f.

The observations of solar eclipses in more recent times have, in general,afforded similar indications of an enlargement of the apparent magnitudeof the sun from the irradiation of his light.

The dark ligament observed by Capt. Jacob during the annular eclipse ofOctober 9, 1847, is obviously an effect of irradiation, for when the moon iswholly within the solar disk, except at the point of interior contact whereit still projects in a very small degree over the sun’s limb, the enlarge-ment produced by irradiation will be wholly wanting at this point, whereas,on the other hand, the rest of the limb being uncovered by the moon willexhibit its usual appearance. The sudden deficiency of light arising fromthis cause will manifestly produce an appearance resembling a dark liga-ment or neck, uniting the limbs of the sun and moon at the point wherethe contact of the two bodies is about to take place J.

Prof. Powell’s explanation of the beads and threads witnessed at theformation and dissolution of the annulus, is perhaps less liable to objectionthan any other that has hitherto been proposed. It is based upon twoprinciples, the real existence of which has been established beyond doubtby observation and experiment. These are, the rapid degradation of thesun’s light towards the margin of his disk, and the law of irradiation invirtue of which the intensity of its influence increases with the brightnessof the luminous object. Admitting these two principles, it is easy to seethat a series of fissures in the moon’s limb will give rise to so many patchesof light, which cannot fail to acquire a more elongated appearance as themoon advances upon the solar disk.

It is to the inferior brightness of the sun towards the margin of hisdisk, and the consequent diminution of the effect produced by his irradi-ation, that Prof. Powell ascribes the protuberance on the moon’s limbtowards the point of her interior contact with the sun, observed by Mr.Baily during the annular eclipse of 1836, when the ring was just formed,and when it was about to be dissolved. In this case it is clear that thesolar light, being less intense towards the point of contact than at theother parts where it borders upon the moon’s limb, will encroach less byirradiation upon the dark body of the moon than elsewhere; and that hencewill arise an apparent protuberance of the contour of the lunar disk, whichwhich will be most sensible at the point where the limbs of the sun andmoon are nearest to each other §.

The foregoing modes of explaining the phenomena observed during annu-lar eclipses are not wanting in plausibility, but they cannot be said to afforda satisfactory account of all the observations. A difficulty which attendsthese, as well as all other explanations of the same nature, consists in thefact of the phenomena being visible in some cases and not in others, al-though sought for with the utmost care. In some instances the absence of

* Phil. Trans., 1737, p. 183. f Phil. Trans., 1748, p. 594.

I Lalande was the first who gave this explanation of the dark ligament.

§ Monthly Proc. Ast. Soc., vol. viii., p. 28.