Pagina-afbeeldingen
PDF
ePub

Kev'd Sir:-You have doubtless been sensible of the late frequent appearance of the Aurora Borealis, and that some of them have been unusually bright and extensive. From the 27th of November there have been six or seven, and two or three very remarkable. That on the 27th exceeded, in several circumstances, any that I remember, and is mentioned in the Paper as being “prodigious” great, as far south ward as Lancaster, in Pennsylvania. But I am told that of December 21st was much more extraordinary in the latter part of the night,

This Phenomenon sensibly affects the minds of people in general, and by many is considered as portentious; which opinion they more readily embrace, as it has never been clearly explained from natural causes. But there may be no more reason to conclude this striking appearance is miraculous, or portentious, than we now believe Comets are, but is produced by some cause consonant to the established Laws of nature. Observing pretty carefully the late Auroræ, and taking notice of some very curious and unnsual appearances, especially in those of November 27th and December 21st, I have thought more of them than ever before, and can not but wonder that they have no more engaged the attention of Philosophers; that they have not given us the Physical cause, or, at least, been more particular, in Philosophical Treatises, in considering so curious a Phenomenon.

It appears to me, however, that no general principles can be applied without depending too much upon mere Hypotheses, until some facts are ascertained with regard to the Aurora, by observations and experiments. I doubt not but you will be ready, Sir, to encourage inquiries of this kind. I therefore take the liberty to mention to you some observations and experiments, and my reasons for them, which I apprehend might lead to a more certain solution than can otherwise be made. Many others may doubtless be added. I shall submit their propriety to your judgment, and should be glad to know your opinion.

It seems that the Aurora has been, and, I believe, before the improvements made in Electricity, was generally, attributed

to sulphurous and nitrous exhalations. Mr. De Mairan, F.R.S., indeed, investigated a new cause in the year 1734 (Philos. Trans., V. 8, p. 539). He supposed it to be the effect of the Solar Atmosphere making a descent at certain times and seasons of the year; that the Zodiacal Light, as it is called, spread on each side of the sun toward the Poles of the Earth in the form of Pyramids, and extended beyond the annual orbit of the Earth, blending itself with our Atmosphere, which, being heterogeneous, produced the several appearances of the Aurora. Mr. De Mairan makes the frequent appearing of the Aurora periodical. From the history of meteors he has computed twenty-two returns from the year 400 to 1716 ; makes the several Auroræ from 1707 to 1710, after their nonappearance of twenty years, but one return. But the present knowledge of the Atmospheres of the Sun and Planets must overturn his Hypothesis.

But the different phenomena of the Aurora may be, perhaps, more satisfactorily accounted for from Electricity than in any other way. There are several experiments that exhibit appearances in some respects similar, such as the approach of certain bodies to a large prime conductor when fully charged, and several experiments in an exhausted receiver. But experiments have not been made, that I have ever known of, sufficient to account for the several Phenomena, or for some of the most material; so that a solution from Electricity will be attended with its difficulties, until further discoveries are made.

The observations and experiments which I apprehend might be of great service in investigating the true cause and solving the greatest present difficulties of the Aurora are these:

1. That there be accurate observations made of every Aurora, for some considerable space of time, at least for twelve months, at some suitable distances North and South, and the two extremes as far distant as may be.

2. Let the state of the Air be nicely ascertained, both as to its density and heat, by a Barometer and Thermometer, and the course of the wind at the time and for some days before and after.

3. Let it be determined, by an Electrical Apparatus, if one electrified, and if it is, whether it be positive or negative clectricity. And,

4. Let the number, situation, figure, and motion of the Clouds in every part of the Hemisphere, and the appearance of the Atmosphere in different parts, be noted with as much accuracy as may be.

By comparing such observations and experiments made in different Latitudes, several material things may be known from which Ilypotheses may be formed with greater certainty. The observation of the Aurora at different distances, and especially if the extreme distances are considerable, will determine, with sufficient exactness, in what regions of the Atmosphere those illuminated particles are, which at present, I presume, is very uncertain. If they are very low, or not higher than the Clouls, or than watery exhalations commonly rise, the elevation of the light from the horizon will be different at different distances, and a light that was low in the horizon far north would not be seen at any considerable distance south. And any person that has a little acquaintance with the Constellations may easily describe the height and extent of the Aurora, the length and direction of the corruscations, at different times and under all its changes, by the stars, for the Aurora is rarely seen but when the air is tolerably clear.

By observing the state of the air, which can not well be done but hy keeping a diary for the whole time, many valuable purposes may be answered. From the state of the air may he conjecturel, with greater certainty, what those corpuscles are which exhibit the luminous appearances. For the height to which different corpuscles will rise by exhalation must be in proportion to their specific gravities and the degree of heat acting upon them, by which they become expanded and exhaleil. The course of the wind, at the time and before and after the Aurora, and the changes that take place in the air, may discover, in some degree, how far the current of the air, in different directions, may be concerned in producing the Phenomena. And inductions may be found, whether attrition, or the combination of heterogeneous particles, or the discharge of some fluid from corpuscles already overcharged to those which are destitute, or have not a due proportion, by those grand agents in nature, attraction and repulsion, are the cause of the appearances.

The Electrical Apparatus will likewise be of singular service, if it should be considerably electrified, especially in the winter and far northward, for it may from thence be almost demonstrated that the electrical fluid is principally concerned in the Aurora. It will also be necessary to take particular notice of those clouds that are above the horizon, as from their form, situation, and motion the current of the air in that part and region of the Atmosphere may be known, and whether the electrical fluid is discharged from them, or from one to another, or whether the Aurora really has, or has not, any connection with them.

It has generally been observed, when the Aurora is bright, that there is a long, dark cloud in the north, a little above the the horizon, but I believe there are some instances in which there has been none at all. Dr. Winthrop supposes it is not a proper cloud, but only an optical deception, because it is so rare that the stars are commonly seen through it. But in some of the late Auroræ the clouds not only entirely obscured the stars, but appeared sufficiently condensed to contain a large quantity of rain.

When we consider the velocity with which the lucid streams commonly shoot up, it is reasonable to suppose they can not be very high in the atmosphere—should they be in the upper regions of it, their velocity and length must be amazingly great, and the condensation of those corpuscles which appear luminous can not easily be accounted for. And that there is a collection or condensation of certain corpuscles in the Aurora is very evident to me, for it sometimes entirely obscures stars of the first magnitude. On the 27th of November, the Aurora, in the eastern part of the Hemisphere, appeared, for a considerable space, of a fiery red color, and entirely obscured Capella, which was then about 40° above the horizon, and must therefore be much denser than the atmosphere of a Comet.

imagined, for that would immediately expand. And tho’ dry air, at a great distance from the Earth, may be highly elecчисvvлт, аши а виален у галепей оr солтаеllѕеи шау, аритеSS, give or receive the electrical fluid to or from clouds or vapors, yet I know of no experiment by which it appears that the electrical fluid generates air to any sensible degree. President Langdon supposes it may be generated air, or condensed vapor, and tells me he has seen the Aurora down below the horizon, even to the very ground; it appeared bright between him and some objects, such as trees and houses, not far distant. This, I think, could not be Aurora Borealis, but Aurora quoquoversum, as the spectator happened to be situated. And indeed this must be the case, if the Aurora is very low in the atmosphere, unless we suppose the southern sides of the corpuscles only are illuminated. The Doctor assigns electricity as the only cause, but, in attempting to account for the several Phenomena, found insuperable difficulties in the way, unless he depended upon very doubtful Hypotheses.

It appears to me the most probable that the illuminated particles are usually pretty high in the atmosphere, and perhaps much higher than is generally imagined. The Aurora of the 27th of November is mentioned in the newspapers as being prodigiously great at Lancaster, “illuminating the greatest part of the hemisphere.” If it was really as extensive as this description, it must have been much greater there than it was here. For I am certain, after I saw it, which was ten minutes after 7 o'clock, it did not extend further south than the great star in Orion's shoulder, which had just risen, the two stars in Perseus' foot, the cluster in the head of Medusa, the head of Cassiopia, which was almost in the meridian, the Swan, the great star of Lyra, and so to the horizon near the head of Hercules, then setting; which did not include more two-thirds of the northern hemisphere. But I am inclined to think, as it was exceeding bright and extensive, it might be mentioned in the paper as being greater than it really was.

That the Aurora, however, may be seen at a very great distance, not only north and south, but east and west, appears to me highly probable. The remarkable Aurora seen in Europe, December 5, 1737, I am apprehensive, was seen here. Prof. Weidler says, at Wittemburg, the sky was remarkably red

« VorigeDoorgaan »