Sir T. Maclear saw the same thing happen to the same star on October 23, 1831:

"Previous to the contact of the Moon and star nothing particular occurred; but at that moment, and when I might expect the star to immerge, it advanced upon the Moon's limb for about 3 seconds, and to rather more than the star's apparent diameter, and then instantly disappeared."

"This phenomenon seems to be owing to the greater proportionate refrangibility of the white lunar light, than that of the red light of the star, elevating her apparent disc at the time and point of contact d"

In 1699 La Hire endeavoured to explain the apparition of stars on the Moon's disc by supposing that the true disc is accompanied by a parasitic light, or, as it was formerly termed, a circle of dissipation, which enlarges the star's apparent diameter, and through which it shews itself before passing behind the opaque part of the lunar globe. Arago accepted this theory with the explanation that the observer's eye-piece must be in imperfect focus, and that so the false disc is caused. The fact that some have and some have not seen the phenomenon he considered confirmatory of this explanation e.

The present state of the question is that we do not possess any certain explanation of the phenomenon.

A remarkable occurrence was noticed by Mr. Ralph Copeland, on the occasion of the occultation of κ Cancri on April 26, 1863

"About three-fourths of the light disappeared in the usual instantaneous manner; and after an interval of (as near as I can judge) rather more than half a second, the remaining portion disappeared."

Dawes regarded this as a decisive indication that the star was double, though he failed to verify this surmise. On Oct. 30,

c Mem. R.A.S., vol. v. p. 373. 1833. d Smyth.

Pop. Ast., vol. ii. p. 348, Eng. ed. For other remarks on this phenomenon, see papers by Airy in Mem. R.A.S., vol. xxviii. p. 173, 1860, and Month. Not., vol. xix. p. 208 (April 1859), and one

by Stevelly discussing the Diffraction hypothesis in Brit. Assoc. Rep., 1845; Transactions of the Sections, p. 5. Also one by Plummer in Month. Not., vol. xxxiii. p. 345 (March 1873).

Month. Not., vol. xxiii. p. 221 (May 1863).

1863, I watched the emersion of y1 Orionis, and it was unquestionably not instantaneous.

An occultation of the planet Jupiter took place on January 2, 1857. A dark shadowy streak which appeared projected on the planet, from the edge of the Moon, was seen by several observers.

OCCULTATION OF JUPITER BY THE MOON: January 2, 1857. (Lassell.)

Mr. W. Simms, Sen. thus described it :--

"The only remarkable appearance noticed by me during the emersion was the very positive line by which the Moon's limb was marked upon the planet; dark as the mark of a black-lead pencil close to the limb, and gradually softened off as the distance increased."

A representation of this appearance, from a drawing by Lassell, is annexed [Fig. 172].

An occultation of the planet Saturn by the Moon took place on May 8, 1859. Dawes thus described it :

"At the disappearance, the dark edge of the Moon was sharply defined on the rings and ball of the planet, without the slightest distortion of their figure. There was no extension of light along the Moon's limb. Even the satellites disappeared without the slightest warning, and precisely at the edge which was faintly visible.

At the reappearance I could not perceive any dark shading contiguous to the Moon's bright edge, such as was seen by myself and several other observers on Jupiter on January 2, 1858 [Qy. 1857]. The dark belt south of the planet's equator was clearly defined up to the very edge; and there was no distortion of any kind, either of the rings or ball.

"The very pale greenish hue of Saturn contrasted strikingly with the brilliant yellowish light of the Moon "."

Month. Not., vol. xvii. p. 81 (Jan.

1857).

h Month. Not., vol. xix. p. 241 (May

1859). Other observations will be found at p. 238 of the same volume.

Mr. W. Simms, Jun. did see a dark shading on the planet contiguous to the Moon's bright edge; but in 1857 he failed to notice it.

The occultation of Saturn on April 9, 1883, was observed by Mr. L. W. Loomis, who remarked on the impression being vividly conveyed that the Moon was very much nearer to the eye than Saturn. The successive disappearance of the rings was an extremely interesting phenomenon.

OCCULTATION OF SATURN BY THE MOON: April 9, 1883. (L. W. Loomis.)

In an occultation of Saturn on Oct. 30, 1825, Messrs. R. Comfield and J. Wallis plainly saw both one ansa and the ball flattened.

The earliest record which we have of an occultation is that of an occultation of Mars by the Moon, mentioned by Aristotle *. Kepler found that it occurred on the night of April 4, 357 B.C.1 Instances are on record of one planet occulting another, but these are of very rare occurrence. Kepler states that he watched an occultation of Jupiter by Mars on January 9, 1591. He also

i Mem. R.A.S., vol. ii. p. 457. 1826.

k De Cœlo, lib. ii. cap 12.

1 Ad Vitell. Paralipom., p. 307.

mentions that Mostlin witnessed an occultation of Mars by Venus on October 3, 1590. Mercury was occulted by Venus on May 17, 1737 m. As these observations, with the exception of the last, were made before the invention of the telescope, it is possible that the one planet was not actually in front of the other, but only that they were so close together as to have had the appearance of being one object: as was the case with Venus and Jupiter on July 21, 1859.

Sometimes stars are occulted by planets. J. D. Cassini mentions the occultation of a star in Aquarius by Mars on October 1, 1672".

m Phil. Trans., vol. xl. p. 394. 1738.

n See a paper on Occultations by A. C.

Twining in Amer. Journ. of Science, 2nd
Ser., vol. xxvi. p. 15. July, 1858.

BOOK III.

PHYSICAL AND MISCELLANEOUS ASTRONOMICAL PHENOMENA.

CHAPTER I.

THE TIDES.

"O ye seas and floods, bless ye the LORD: praise Him, and magnify
Him for ever."— Benedicite.

Introduction.-Physical cause of the Tides.-Attractive force exercised by the Moon. By the Sun.-Spring Tides.-Neap Tides.-Summary of the principal facts.-Priming and Lagging.—Diurnal Inequality.

ANY inhabitants of a maritime country like Great Britain

MAN

have some acquaintance with the phenomena now to come under consideration, but beyond a vague notion that the Moon has something to do with the tides, very few people have an intelligent idea of the way in which the tides are produced.

These phenomena are very frequently attributed to the attraction of the Moon, whereby the waters of the ocean are drawn towards that side of the Earth on which our satellite happens to be situated; in fact, that it is high water when the Moon is on or near the meridian of the place of observation.

This, though to a great extent true, by no means adequately

a See a paper by the late Sir J. Lubbock, in the Companion to the Almanac for 1830, p. 49. And reference should also be made to an important and exhaustive Memoir on "Tides and Waves"

by Sir G. B. Airy, in Encycl. Metrop., vol. v. p. 241. There are maps of co-tidal lines around the British Isles, and over the World generally, which will be found of interest.