Page images
PDF
EPUB

in proof of the number of blue rays which entered into the composition of the light of the great Meteor of Aug. 18, 1783, that the Moon appeared at Brussels quite red, during the Meteor's passage, from the contrast of light.* I have noticed this reddish appearance of the Moon during the combustion of many substances which burn blue, in pyrotechnical exhibitions. In the tail, and in the separated scintillations of the aforesaid great Meteor prismatic colours were observed very variously, by persons in different places.

These appearances seem certainly to favour the hypothesis of M. De Luc, as I have before observed, rather than any mode of explaining them on the known laws of Electricity.t

* Dr. B. refers to a letter of Abbé Mann to Sir Joseph Banks.

+ Some have considered Shooting Stars as bodies projected from the Moon, and ignited in their course. In this case, the peculiarities of their light, at different times, might be caused either by the quality of the air in which they burned, or by the quality of the ignited body. Meteorolites too have been considered as similarly projected from the Moon, and have thence received the appellation of Lunar Stones. And this opinion has gained support by their analysis, which does not correspond with that of any known terrestrial compound. Biot, in his Astronomie Physique, and La Place, in his Sys.

During Thunderstorms however Meteors occasionally come down like Fireballs which sometimes seem very like those described

du Monde, seem rather of this opinion. The altitude of what are called Falling Stars, above the Earth's surface, has never been well ascertained, though it might easily be done by geometrical observation; at least, in many cases, where the Meteor could be identified, as seen in different places. They are not seen below clouds; and, indeed, none, except the larger and brilliant kind, are usually observed when there are many clouds about: but this may arise from the state of the atmosphere necessary to their production being incompatible with the existence of much cloud. M. De Luc mentioned to me his having seen them from the top of high mountains, and that they then appeared at a very great distance. From observations which I have made, they certainly vary in the height, as well as the length, of their course. It is not impossible, but that if Meteorolites were observed to fall at night, they might be always found to be accompanied by some fiery phaenomenon of this kind. The almost horizontal motion of some large Meteors, would be no objection to this hypothesis, if they always moved from E. to W. or nearly so; as, when they came into the sphere of the Earth's attraction, their motion might be spent, and they would then receive an apparent motion compounded of the opposite of the Earth's rotatory motion, and the attraction to the centre. An analysis of several meteoric stones may be found in Sowerby's Brit. Mineral. vol. ii. p. 18. A catalogue of many of them, and of the places where they fell, was made and published in France; there are also many accounts of them in several numbers of the Philosophical Magazine.

above. And the electric Stars or balls of fire called Fires of St. Elmo, which alight on the masts and rigging of ships at sea before Storms, seem certainly referable rather to electric sparks of some sort than to burning gases.

There are several dissimilar appearances, which may be mentioned in this place, as subjects worthy of the future investigation of natural philosophers, which seem referable to Electricity and which appear to hold a middle nature between the Fiery Meteors above described, and known electrical phaenomena. There are, occasionally, stationary Meteors, simple Accensions, which appear in cloudy Skies, and last scarcely a moment. There are also luminous portions of clouds occasionally, of less intensity of light, which are faint and glimmering, like luminous nebulae; and others, which have a rapid motion, that may be said to have the same relation to moving Meteors above, which the pale light about plants, before noticed, bears to the well known phaenomenon which occurs below called the Ignis Fatuus, Jack with a Lantern, or Will with a Wisp, the power of which to lead astray the benighted traveller into boggy quagmires, by presenting the appearance of a moving lantern, is well

known, and of which many curious stories are

recorded.*

SECTION VI.

Of the Electricity of the Air.

WHAT has hitherto been said of the Electricity of the atmosphere, related chiefly to that of clouds. In serene weather, however, and in the absence of all clouds, the air has shown signs of being electrified, by means of kites raised in the air, and other electrometers.† That air should, at different times and places, have positive and negative charges, is not at all surprising; but the circumstances under which such charges have taken place do not appear to have been sufficiently attended to.

* "She was pinched and pulled, she said,
And he by Friar's Lantern led."

Milton.

The late M. Edward Forster mentioned to me that this phaenomenon used to be common in the marshes between Walthamstow and Tottenham, but it has now disappeared there.

+ See Becaria Elec.; also Cavallo Comp. Treat. Elec. Letters of Abbé Nollet, in Phil. Trans. &c.

During very clear weather, the air has generally been found to have a positive Electricity, and the exceptions to this rule have generally happened when either a strong Wind has blown, or when there have been clouds in the vicinity of the electrometer. That large electrified clouds throw the surrounding air into an opposite state, has already been stated; and some circumstances have induced an opinion, that there are alternate portions of air with different Electricities round the electrified clouds. And it becomes a question for future solution, whether, when air is found electrified positively, there be not a counter charge somewhere else? Possibly the whole Atmosphere, and the Earth too, may have electric polarity. That the electric state of the atmosphere varies much at different times, is beyond a question, from the facts above stated; but the causes of its irregularities, which, in fact, involves the causes of clouds, &c. is as yet a mystery. There have been found alternations in the electric state of the atmosphere, that is, rapid changes from a positive to a negative state, and vice versa. These circumstances were observed near the Appennines, when a strong Wind blew, and when clouds hung about the tops of those

« PreviousContinue »