antly the most remarkable and interesting is a central eclipse of the sun. If it be total, the spectacle it offers is most imposing: the light of day is gradually withdrawn to such a degree that the brighter planets, such as Venus and Jupiter, and the stars of the first magnitude, become visible to the naked eye. We see, however, a faint light of the sun behind the disk of the moon. Sometimes, as has been stated, when the apparent magnitude of the moon is a little less than that of the sun, the disk of the moon conceals the entire disk of the sun, except only a thin luminous ring surrounding it. This is a phenomenon of very rare occurrence, and only to be seen at particular places on the earth. An instance of it occurred on the 7th of September, 1820. It commenced to be visible at the north latitude of 80°, in Hudson's bay, near the eastern coast of New North Wales. It was visible next in the direction of the northeast of Greenland, at the mouth of the Wesel, at Bremen, in the gulf of Venice, and in Arabia deserta, and ceased near the Persian gulf. While this eclipse was produced in these different places, the observers who were on the same meridians, but further south, saw only a partial eclipse, and others, still further south, saw no eclipse at all, the contrary took place with observers on the same meridians farther north, to all of whom the eclipse was annular.

It was during a phenomenon of this kind that Schröter imagined he saw the solar light coming through an immense opening in the moon. Other observers, however, who saw, or imagined they saw, luminous spots on the dark hemisphere of the moon, in a solar eclipse, ascribed them to lunar volcanoes. As to the existence of these luminous spots on the dark hemisphere of the moon, rendered manifest in a total eclipse of the sun, we have the testimony of so many astronomers, among whom, besides Schröter, may be mentioned Sir William Herschel and Kater, that we can scarcely doubt their reality. The causes which may produce them have only been explained in the two ways above mentioned, namely, either by the supposed existence of active volcanoes, on the moon, or perforations through the moon, through which the sun's light passes.

The following description of a total eclipse of the sun, given by Halley, who observed it, is quoted by Arago, and will be read with interest:

'I send you, according to promise, my observations of the solar eclipse, though I fear they will not be of much use to you. Not being furnished with the necessary instruments for measuring time, I confined my views to examining the spectacle presented by nature under such extraordinary circumstances, a spectacle which has hitherto been neglected or imperfectly studied. I chose for my point of observation a place called Haradowhill, two miles from Amesbury, and east of the avenue of Stonehenge, of which it closes the vista. In front is that celebrated edifice upon which I knew that the eclipse would be directed. I had, moreover, the advantage of a very extensive prospect in every direction, being on the loftiest hill in the neighborhood, and that nearest to the centre of the shadow. To the west, beyond Stonehenge, is another rather steep hill, rising like the summit of a cone above the horizon. This is Clay hill, adjoining Westminster, (?) and situated near the central line of darkness which was to set out from this point, so that I could be aware in time of its approach. I had with me Abraham Sturges and Stephen Evans, both natives of the country, and able men. The sky, though overcast, gave out some straggling rays of the sun, that enabled me to see around us. My two companions looked through the blackened glasses, while I made some reconnaissance of the country. It was half-past five by my watch when they informed me that the eclipse was begun. We watched its progress, therefore, with the naked eye, as the clouds performed for us the service of colored glasses. At the moment when the sun was half obscured, a very evident circular rainbow

formed at its circumference, with perfect colors. As the darkness increased, we saw the shepherds on all sides hastening to fold their flocks, for they expected a total eclipse of an hour and a quarter duration.

“When the sun assumed the appearance of the new moon, the sky was tolerably clear, but it was soon covered with deeper clouds. The rainbow then vanished, the steep hill I have named became very obscure, and on each side, that is, north and south, the horizon exhibited a blue tint, like that which it possesses in summer toward the close of day. Scarcely had we time to count ten, when Salisbury spire, six miles to the south, was enveloped in darkness. The hill disappeared entirely, and the deepest night spread around us. We lost sight of the sun, whose place till then we had been able to distinguish in the clouds, but whose trace we could now no more discover than if it had never existed. "By my watch, which I could scarcely discern by some light that reached us from the north, it was thirty-five minutes past six. Shortly before, the sky and the earth had assumed, literally speaking, a livid tint, for it was a mixture of black and blue, only the latter predominated on the earth and at the horizon. There was also much black diffused through the clouds, so that the whole picture presented an awful aspect, that seemed to announce the death of nature. "We were now enveloped in a total and palpable darkness, if I may be allowed the expression. It came on rapidly, but I watched so attentively, that I could perceive its progress. It came upon us like rain, falling on our left shoulders (we were looking to the west), or like a great black cloak thrown over us, or like a curtain drawn from that side. The horses we held by the bridle seemed deeply struck by it, and pressed to us with marks of extreme surprise. As well as I could perceive, the countenances of my friends wore a horrible aspect. It was not without an involuntary exclamation of wonder I looked around me at this moment. I distinguished colors in the sun, but the earth had lost all its blue, and was entirely black. A few rays shot through the clouds for a moment, but immediately afterward the earth and the sky appeared totally black. It was the most awful sight I had ever beheld in my life.

"Northwest of the point whence the eclipse came on, it was impossible for me to distinguish in the least degree the earth from the sky, for a breadth of sixty degrees or more. We looked in vain for the town of Amesbury, situated below us; scarcely could we see the ground under our feet. I turned frequently during the total darkness, and observed that, at a considerable distance to the west, the horizon was perfect on both sides, that is, to the north and to the south; the earth was black, and the lower part of the sky clear; the obscurity, which extended to the horizon in those points, seemed like a canopy over our heads, adorned with fringes of a lighter color, so that the upper edges of all the hills, which I recognised perfectly by their outlines, formed a black line. I saw perfectly that the interval between light and darkness, observable in the earth, was between Mortinsol (?) and St. Anne; but to the south it was less distinctly marked.

"I do not mean to say that the line of shadow passed between these two hills, which were twelve miles distant from us; but as far as I could distinguish the horizon, there was none behind, and for this reason: My elevated position enabled me to see the light of the sky behind the shadow; still, that yellowish green line of light I saw was broader toward the north than toward the south, where it was of a tan color. At this period it was too black behind us, that is, to the east, looking toward London, to enable me to see the hills beyond Andover, for the anterior extremity of the shadow lay beyond that place. The horizon was then divided into four parts, differing in extent, in light, and in darkness. The broadest and least black was to the northwest, and

Whenever the moon is less than seventeen degrees from its node at a time when it is in conjunction with the sun, there must be a solar eclipse; and whenever it is less than twelve degrees from its node at the time of full moon, there must be a lunar eclipse. Within these limits the less the distance of the moon from its node, the greater will be the number of digits eclipsed, whether of the sun or moon.

THE AURORA BOREALIS.

Origin of the Name.-Produced by Electricity.-General Phenomena of Auroras. Various Examples of this Meteor.-Biot's Excursion to the Shetland Isles to observe the Aurora.-Lottin's Observations in 1838-'9.-Various Auroras seen by him.-Theory of Biot to explain these Meteors.Objections to it.-Hypothesis of Faraday.-Auroras seen on the Polar Voyage of Captain Franklin.