Spots in the Sun—Diminution of the Sun—Parhelia, or Mock Suns—Eclipses—Halo, or Corona; and similar Appearances—Falling or Shooting Star—Volcanoes in the Moon.
Spots in the Sun—Diminution of the Sun—Parhelia, or Mock Suns—Eclipses—Halo, or Corona; and similar Appearances—Falling or Shooting Star—Volcanoes in the Moon.
Spots in the Sun.—The following account of the spots in the sun is taken from a French paper.
“The spots were seen for the first time in 1611; and nearly about the same time by J. Fabricius, at Wittenberg, by the Jesuit Scheiner, and by Galileo. This great man watched their course with so much attention, and so well developed their phenomena, that very little has been since added to thedescriptions which he gave, except more precise measures. The spots of the sun are at present viewed with astronomical telescopes, in which the great brilliancy of that luminary is mitigated, and not effaced, by the coloured glass placed between the telescope and the eye. There are in the interior of the telescope, at the focus of the object, some very fine threads stretched crosswise, and moveable parallel to each other, by means of which the distance of the spot from the nearest border of the sun’s disk may be ascertained, which determines its position on the disk at the moment of observation. By following in this manner the same spot for several days, it is perceived to change its place. Its size also varies much. The spots sometimes grow thinner, and disperse from one day to another: and hence it is, that, though in one month rather a large number was visible, in the following only two are to be seen. But during the whole time of their presence they pursue a regular course, of which the aspects are common to all.
“When they first come in sight, they appear on the sun’s border, like a slender thread. In proportion as they advance towards the middle of the disk, they appear, from day to day, to enlarge in the direction of their movement. They then decrease periodically; and if they last long enough to traverse the whole disk, they go off by the opposite side, narrowing to a single thread. These appearances are evidently such as a small body, adhering to a spherical surface, and revolving with or upon that surface, must present. The diminution of the spots, in proportion as they approximate the borders of the disk, results from this—that they then project more obliquely, and are only seen sidewise; but when in the middle of the disk, they are seen in their full extent. In fine, upon comparing the direction and rapidity of their course, it soon becomes evident, that the supposition of their adhering to the body of the sun is the only admissible one. On thus tracing the route of all those which appear, it is ascertained that they move in courses exactly parallel, describing circles which all have their centre on a common axis, passing through the centre of the sun. The size of these circles varies on different points of the disk, according to the same laws as on a sphere; and the rate of movement is modified in such a way, that all the circles are run through in equal times. This perfect concordance of revolution in spots so changeable in other respects, evidently shews that they must be attached to one and the same round body, which makes them revolve altogether with a common motion. Hence it has been concluded, that the sun revolves upon itself with the general motion of these spots, that is, in twenty-five days and a half, in like manner as our earth revolves in twenty-four hours. The samecalculation, applied to the spots which have been discovered on the other planets, has in like manner made us acquainted with their rotation.
PARHELIA, OR MOCK SUN.—Page 673.
THE IGNIS FATUUS, Will-with-a-Wisp, or Jack-with-a-Lantern.—Page 644.
“As to the nature of these solar spots, it is absolutely unknown. Herschel is of opinion, that luminous clouds float in the inflamed atmosphere of this luminary, as clouds of vapour float in ours. He supposes that the body of the sun is opaque and dark; and that the black spots observed there at intervals, are merely the summits of very elevated mountains, which the solar clouds permit us to see between their openings. Other astronomers think that the globe of the sun is on fire, and that the spots are merely immense scoria, launched on the surface of that mass by some terrible explosions, of which our terrestrial volcanoes afford but a feeble picture. But whatever may be thought of these conjectures, it seems sufficient for us to know, that the solar spots are trifling compared with the immense mass of that body; and that the eruptions, of which they are perhaps the effect, take place at too great a distance from our earth to produce the least effect upon it. Generally speaking, the physical state of our little world is incomparably more stable and steady than its moral state.”
Diminution of the Sun.—Baron Lindeneau, who recently published a work on the diminution of the solar mass, says, that the sun may have been imperceptibly subject to successive diminution since the science of astronomy has been cultivated. Baron Lindeneau supposes the sun’s diameter to be 800,000 miles, 4,204,000,000 feet, or nearly 2000 seconds. We have not, he observes, hitherto possessed any instrument for measuring the diameter of the heavenly bodies to a second. The sun may therefore diminish 12,000 of its diameter, or 2,102,000 feet, without the possibility of being perceived. Supposing the sun to diminish daily two feet, it would requirethree thousandyears to render the diminution of a second of its diameter visible.
Account of those singular Appearances, called,Parhelia, or Mock Suns.—
As when two suns appear in th’ azure sky,Mounted in Phœbus’ chariot fierie bright:Both darting forth fair beams to each man’s eye;And both adorn’d with lamps of flaming light,All that behold such strange prodigious sight,Not knowing nature’s work, nor what to weene,Are wrapt with wonder, and with rare affrighte.Spenser.
A Parhelion is a meteor in form of a bright light, appearing on one side of the sun. Phenomena of this kind have been mentioned both by the ancients and moderns. Aristotleobserves, that in general they are seen only when the sun is near the horizon, though he takes notice of two that were seen in Bosphorus from morning till evening; and Pliny has related the times when such phenomena were observed at Rome. Gassendi says, that in 1635-1636 he often saw one mock sun. Two were observed by M. de la Hire in 1689; and the same number by Cassini in 1693; by Mr. Grey in 1700, and by Dr. Halley in 1702; but the most celebrated phenomena of this kind were seen at Rome by Scheiner; by Muschenbroek at Utrecht; and by Hevelius at Ledan. By the two former, four mock suns were observed; and by the latter, seven. Parhelia are apparently of the same size with the sun, though not always of the same brightness, nor even of the same shape; and when a number appear at once, there is some difference in both respects among them. Externally they are tinged with colours like the rainbow; and many have a long fiery tail opposite the sun, but paler towards the extremity. Parhelia are generally accompanied with coronas, some of which are tinged with rainbow colours, but others are white. (SeeHalo.) They differ in number and size; but all agree in breadth, which is that of the apparent diameter of the sun. A very large white circle, parallel to the horizon, generally passes through all the parhelia; and, if it were entire, it would go through the centre of the sun. Sometimes there are arcs of lesser circles concentric to this, touching those coloured circles which surround the sun. They are also tinged with colours, and contain other parhelia. Other circles are said to have been obliquely situated with respect to all these. The order of the colours in these circles is the same as in the rainbow; but on the inside, with respect to the sun, they are red, as is also observed in many haloes. Parhelia have been visible for one, two, three, and four hours together; and in North America, they are said to continue some days, and to be visible from sunrise to sunset. When the parhelia disappear, it sometimes rains, or snow falls in the form of oblong spiculæ, as Maraldi, Weidler, Krafft, and others, have observed; and because the air in North America abounds with such frozen spiculæ, which are even visible to the eye, according to Ellis and Middleton, such particles have been thought to be the cause of all coronas and parhelia.
Mr. Wales says, that at Churchill, in Hudson’s Bay, the rising of the sun is always preceded by two long streams of red light, one on each side, and about twenty degrees distant from him. These rise as the sun rises; and as they grow longer, they begin to bend towards each other, till they meet directly over the sun, just as he rises, forming there a parhelion, or mock sun. These two streams of light, he says,seem to have their source in two other parhelia, which rise with the true sun; and in winter, when the sun never rises above the haze or fog, which he says is constantly seen near the horizon, all these accompany him the whole day, and set with him. Once or twice he saw a fourth parhelion, directly under the sun; but this is not common. These facts being constant, are very valuable, and may throw great light on the theory of these remarkable phenomena. Sometimes parhelia appear in a different manner; as when three suns have been seen in the same vertical circle, well defined, and touching one another. The true sun was in the middle, and the lowest touched the horizon, and they set one after the other. This appearance was seen by Maleziew, in 1722. Other appearances similar to this are recited by Mr. Muschenbroek. Sometimes the sun has risen or set with a luminous tail projecting from him, of the same breadth with his diameter, and perpendicular to the horizon. Such an appearance was seen by Cassini in 1672 and 1692; by De la Hire in 1702; and by Mr. Ellis in Hudson’s Bay. As M. Feuilée was walking on the banks of the river La Plata, he saw the sun rising over the river, with a luminous tail projecting downwards, which continued till he was six degrees high. Paraselæ, or mock moons, have also been seen, accompanied with tails and coloured circles, like those which accompany the parhelia. An account of several, and a particular description of a fine appearance of this kind, may be seen in Muschenbroek.
The following account of this phenomenon is extracted from a pamphlet, entitled, ‘Somewhat written by occasion of Three Sunnes’ seene, at Tregorie, in Cornwall, the 22nd of December last; with other memorable occurrents in other places. Imprinted 1622: 20 pages small 4to.’
“Since this strange apparition, namely, upon the 10th of January last, there happened in Devonshire, yet not farre from the other place, being on the edge of Cornwalle, another wonder, which, did as much affrighte the eares of men, as this did their eyes: for in the afternoone of that day, being the Thursday after Twelfth-day, there were heard in the aire unusuall cracks or claps of thunder, resembling in all points the sound of many drums together, sometimes beating charges, sometimes retreats, sometimes marches, and all other points of warre: which, after it had continued a good time, it seemed that the same thunder did most lively expresse many volleyes of small-shot, and afterwards the like volleyes of ordnance, with so great and yet so distinct noyse, that many of them who dwelt neare the sea, went toward the shore to see what it might meane, as verily supposing there had beene some sea fight neere upon that coast. These severall fearfull noyses were againe and againe renewed in the same order, till atlength with an horrible and extraordinary cracke of thunder, there fell in a ground of one Robert Pierce, where there were divers workemen planting apple-trees, (which ground lay neere the house of one Master George Chidley,) a thunder-bolt, if I may so call it, being a stone of three foot and an halfe in length, of two foot and an halfe in breadth, and one foot and an halfe in thicknesse, the substance whereof was in hardnesse and colour not much unlike a flint, as appeares by many pieces thereof, which are shewed up and downe by many credible and honest gentlemen, who, with their own hands, brake them off from the maine stone. After the fall of this stone, which with the weight thereof was cleane buried in the ground above a yard deepe, the thunder ceased, and people began as much to won—at that which they now saw, as they had lately done at that, which with so much feare and amazement they had heard.”
Observations onEclipses of the Sun and Moon.—
Give me the ways of wand’ring stars to know,The depths of heav’n above and earth below;Teach me the various labours of the Moon,And whence proceed th’ Eclipses of the Sun.Virg. Georg.ii
The deprivation of the light of the sun, or some heavenly body, by the interposition of another heavenly body between our sight and it is, called an Eclipse. Thus, eclipses of the sun happen by the moon’s intervening between it and the earth; by which means the shadow of the moon falls upon the earth, when the latitude of the moon does not prevent it, by elevating her orb above, or depressing it below the earth. On the other hand, an eclipse of the moon can only happen when the earth is interposed between the sun and it; for then, if the latitude of the moon does not prevent it, the shadow of the earth may fall on the moon, and thereby cause either a partial, or total eclipse. A total eclipse of the sun or moon, is when their whole bodies are obscured; and a partial one, is when part only of their bodies is darkened: again, a central eclipse is when it is not only total, but the eclipsed body passes through the centre of the shadow.
As total solar eclipses are by no means common, we shall give an interesting description of one, by Dr. Stukeley, sent to his friend, the celebrated Dr. Edmund Halley.
“According to my promise, I send you what I observed of the solar eclipse, though I fear it will not be of any great use to you. I was not prepared with any instruments for measuring time or the like, and proposed to myself only to watch all the appearances that nature would present to the naked eye upon so remarkable an occasion, and which generally are overlooked, or but grossly regarded. I chose for my station a place called Haradon Hill, two miles eastward fromAmsbury, and full east from the opening of Stonehenge avenue, to which it is as the point of view. Before me lay the vast plain where that celebrated work stands, and I knew that the eclipse would appear directly over it; besides, I had the advantage of a very extensive prospect every way, this being the highest hill hereabouts, and nearest the middle of the shadow. Full west of me, and beyond Stonehenge, is a pretty copped hill, like the top of a cone, lifting itself above the horizon; this is Clay-hill, near Warminster, twenty miles distant, and near the central line of darkness, which must come from thence, so that I could have notice enough beforehand of its approach. Abraham Sturgis and Stephen Ewens, both of this place, and sensible men, were with me. Though it was very cloudy, yet now and then we had gleams of sunshine, rather more than I could perceive at any other place around us. These two persons, looking through smoked glasses, while I was taking some bearings of the country with a circumferentor, both confidently affirmed the eclipse was begun, when, by my watch, I found it just half an hour after five; and accordingly from thence the progress of it was visible, and very often to the naked eye; the thin clouds doing the office of glasses. From the time of the sun’s body being half covered, there was a very conspicuous circular iris round the sun, with perfect colours. On all sides we beheld the shepherds hurrying their flocks into fold, the darkness coming on; for they expected nothing less than a total eclipse for an hour and a quarter.
“When the sun looked very sharp like a new moon, the sky was pretty clear in that spot; but soon after a thicker cloud covered it, at which time the iris vanished; the copped hill before-mentioned grew very dark, together with the horizon on both sides, that is, to the north and south, and looked blue, just as it appears at the declension of day. We had scarcely time to tell them, when Salisbury steeple, six miles off southward, became very black; the copped hill was quite lost, and a most gloomy night with full career came upon us: at this instant we lost sight of the sun, whose place among the clouds was hitherto sufficiently distinguishable, but now not the least trace of it was to be found, any more than if really absent: then I saw by my watch, though with difficulty, and only by help of some light from the northern quarter, that it was six hours thirty-five minutes: just before this, the whole compass of the heavens and earth looked of a lurid complexion, properly speaking, for it was black and blue, only on the earth upon the horizon the blue prevailed; there was likewise in the heavens, among the clouds, much green interspersed, so that the whole appearance was really very dreadful, and as symptoms of sickening nature.
“Now I perceived we were involved in total and palpable darkness, as I may aptly call it; for though it came quickly, yet I was so intent, that I could perceive its steps, and feel it as it were drop upon us, and fall on the right shoulder (we looking westward) like a great dark mantle, or coverlet of a bed, thrown over us, or like the drawing of a curtain on that side. The horses we held in our hands were very sensible of it, and crowded close to us, startling with great surprise; and as much as I could see of the men’s faces that stood by me, they had a horrible aspect. At this instant I looked around me, not without exclamations of admiration, and could discern colours in the heavens, but the earth had lost its blue, and was wholly black. For some time, among the clouds, there were visible streaks of rays, tending to the place of the sun as their centre; but immediately after, the whole appearance of earth and sky was entirely black: of all things I ever saw in my life, or can by imagination fancy, it was a sight the most tremendous.
“Towards the north-west, whence the eclipse came, I could not in the least find any distinction in the horizon between heaven and earth, for a good breadth of about sixty degrees, or more; nor the town of Amsbury underneath us, nor scarcely the ground we trod on. I turned myself round several times during this total darkness, and remarked at a good distance from the west on both sides, that is, to the north and south, the horizon very perfectly; the earth being black, the lower parts of the heavens light; for the darkness above hung over us like a canopy, almost reaching the horizon in those parts, or as if made with skirts of a lighter colour; so that the upper edges of all the hills were as a black line, and I knew them very distinctly by their shape or profile; and northward, I saw perfectly, that the interval of light and darkness in the horizon was between Martinsal-hill and St. Ann’s-hill; but southward it was more indefinite. I do not mean that the verge of the shadow passed between those hills, which were but twelve miles distant from us; but, so far I could distinguish the horizon; beyond it not at all. The reason of it was this; the elevation of ground I was upon gave me an opportunity of seeing the light of the heavens beyond the shadow; nevertheless, this verge of light looked of a dead yellowish, and greenish colour; it was broader to the north than south; but the southern was of a tawny colour: at this time behind us, or eastward toward London, it was dark too, where otherwise I could see the hills beyond Andover; for the foremost end of the shadow was past thither; so that the whole horizon was now divided into four parts of unequal bulk, and degrees of light and dark; the part to the north-west broadest and blackest, to the south-west lightest and longest. All the change I could perceiveduring the totality, was, that the horizon by degrees drew into two parts, light and dark: the northern hemisphere growing still longer, lighter, and broader; and the two opposite dark parts uniting into one, and swallowing up the southern enlightened part.
“As at the beginning the shade came feelingly upon our right shoulders, so now the light from the north, where it opened as it were; though I could discern no defined light or shade upon the earth that way, which I earnestly watched for, yet it was manifestly by degrees, and with oscillation, going back a little, and quickly advancing further, till at length, upon the first lucid point appearing in the heavens, where the sun was, I could distinguish pretty plainly a rim of light running alongside of us a good while together, or sweeping by at our elbows from west to east. Just then, having reason to suppose the totality ended with us, I looked on my watch, and found it to be full three minutes and a half more. Now the hill-tops changed their black into blue again, and I could distinguish an horizon where the centre of darkness was before: the men cried out, they saw the copped-hill again, which they had eagerly looked for; but still it continued dark to the south-east, yet I cannot say that ever the horizon that way was undistinguishable. Immediately we heard the larks chirping, and singing very briskly, for joy of the restored luminary, after all things had been hushed into a most profound and universal silence. The heavens and earth now appeared exactly like morning before sunrise, of a greyish cast, but rather more blue interspersed; and the earth, so far as the verge of the hill reached, was of a dark green, or russet colour.
“As soon as the sun emerged, the clouds grew thicker, and the light was very little amended for a minute or more, like a cloudy morning slowly advancing. After about the middle of the totality, and so after the emersion of the sun, we saw Venus very plainly, but no other star. Salisbury steeple now appeared; but the clouds never removing, we could take no account of it afterwards; but in the evening it lightened very much. I hastened home to write this letter, and the impression was so vivid upon my mind, that I am sure, I could for some days after have written the same account of it, and very precisely. After supper I made a drawing of it from my imagination, upon the same paper on which I had taken a prospect of the country before.
“I must confess to you, that I was (I believe) the only person in England, that regretted not the cloudiness of the day, which added so much to the solemnity of the sight, and which incomparably exceeded, in my apprehension, that of 1715, which I saw very perfectly from the top of Bostonsteeple, in Lincolnshire, where the air was very clear; but the night of this was more complete and dreadful: there, indeed, I saw both sides of the shadow come from a great distance, and pass beyond us to a considerable extent; but this eclipse had much more of variety and majestic terror; so that I cannot but felicitate myself upon the opportunity of seeing these two rare accidents of nature, in so different a manner. Yet I should willingly have lost this pleasure, for your more valuable advantage of perfecting the noble theory of the celestial bodies, which, last time, you gave the world so nice a calculation of; and I wish the sky had now as much favoured us for an addition to your honour and great skill, which I doubt not to be as exact in this as before.”
We now proceed to describeThe Halo, or Corona; and similar Appearances.—An Halo is a luminous circle surrounding the sun, moon, planets, or fixed stars. Occasionally these circles are white, and sometimes they are coloured like the rainbow. Sometimes one only is visible, and at others several concentric halos appear at the same time. Mr. Huygens observed red next the sun, and a pale blue outwards. Sometimes they are red on the inside, and white on the outside. In France, one was observed in 1683, the middle of which was white; after which followed a border of red, next to it was blue, then green, and the outermost circle was a bright red. In 1728, one was seen of a pale red outwardly, then followed yellow, and then green, terminated by a white. In Holland, M. Muschenbroek says, fifty may be seen in the day-time, almost every year; but they are difficult to be observed, except the eye be so situated, that not the body of the sun, but only the neighbouring parts of the heavens, can be seen. Mr. Middleton says, that this phenomenon is very frequent in North America; for that there is generally one or two about the sun every week, and as many about the moon every month. Halos round the sun are very frequent in Russia. M. Æpinus says, that from the 23d of April, 1758, to the 20th of September, he himself had observed no less than twenty-six, and that he has sometimes seen twice as many in the same space of time.
Similar, in some respects, to the halo, was the remarkable appearance which M. Bouguer describes, as observed on the top of Mount Pichinca, in the Cordilleras. When the sun was just rising behind them, so as to appear white, each of them saw his own shadow projected upon it, and no other. The distance was such, that all the parts of the shadow were easily distinguishable, as the arms, the legs, and the head; but what surprised them most was, that the head was adorned witha kind of glory, consisting of three or four small concentric crowns, of a very lively colour, each exhibiting all the varieties of the primary rainbow, and having the circle of red on the outside. The intervals between these circles continued equal, though the diameters of them all were constantly changing. The last of them was very faint; and at a considerable distance was another great white circle, which surrounded the whole. This phenomenon never appeared but in a cloud consisting of frozen particles, and never in drops of rain like the rainbow. When the sun was not in the horizon, only part of the white circle was visible, as M. Bouquer frequently observed afterwards. Similar to this curious appearance, was one seen by Dr. M’Fait in Scotland; who observed a rainbow round his shadow in the mist, when he was upon an eminence above it. In this situation the whole country round seemed buried under a vast deluge, and nothing but the tops of distant hills appeared here and there above the flood. In those upper regions, the air, he says, is at that time very pure and agreeable. At another time he observed a double range of colours round his shadow. The colours of the outermost range were broad and very distinct, and every where about two feet distant from the shadow. Then there was a darkish interval, and after that another narrower range of colours, closely surrounding the shadow, which was very much contracted. He thinks that these ranges of colours are caused by the inflection of the rays of light, the same that occasions the ring of light which surrounds the shadow of all bodies, observed by M. Maraldi, and others.
We next proceed to the phenomenon generally calledFalling or Shooting Star.—This is a luminous meteor, darting rapidly through the air, and resembling a star falling from the heavens. The explication of this phenomenon had puzzled all philosophers, till the modern discoveries in electricity led to the most probable account of it. Signior Beccari makes it pretty evident, that it is an electrical appearance, and recites the following fact in proof of his opinion. About an hour after sunset, he, and some friends that were with him, observed a falling star directing its course towards them, and apparently growing larger and larger, but it disappeared not far from them. When it vanished, it left their faces, hands, and clothes, with the earth, and all the neighbouring objects, suddenly illuminated with a diffused and lambent light, but not attended with any noise. During their surprise at this appearance, a servant informed them, that he had seen a light shine suddenly in the garden, and especially upon the streams which he was throwing to water it. All these appearances were evidently electrical; and Beccari wasconfirmed in his conjecture, that electricity was the cause of them, by the quantity of electric matter which he had seen gradually advancing towards a kite he had elevated, which had very much the appearance of a falling star. Sometimes, also, he saw a kind of glory round the kite, which followed it when it changed its place, but left some light, for a small space of time, in the place it had quitted.
Captain Bagnold says, whilst passing through the straits of Bahama, in the autumn of 1799, he witnessed the following singular atmospheric phenomenon.
“It was a fine star-light morning, about two o’clock, the atmosphere remarkably clear, with a light air from the north-east; the sky to windward, from north-north-east to south-south-east, was illuminated by a profusion of those meteors, vulgarly denominated falling stars, but of a description far more vivid than those usually seen in the higher latitudes; the head of each was an oblong ignited mass, followed by a long luminous tail, which, after three or four seconds, gradually vanished. They were formed, to all appearance, in the air, at an elevation of from thirty-five to sixty-four degrees, none being observed in the zenith, and few to commence nearer the horizon than the first-mentioned angles. At the mean of these elevations, the greatest numbers were seen darting in different directions, forming portions of a large curve, all slightly inclined to the horizon. Multitudes were constantly visible at the same moment, and they succeeded each other so rapidly, that the eye of the spectator was kept in motion between the above points of the compass. In about ten minutes they became less frequent, and at length ceased altogether.
“The apparent distance of this phenomenon would, by a seaman, be estimated at fifteen or twenty miles; and if it really was what I have always considered it, namely, a nocturnal shower of meteoric stones, it was perhaps fortunate for all on board, that we were not within the sphere of its action: whatever it was, never shall I forget the splendour of the spectacle.”—SeeHumboldt’s Personal Narrative, volume III. page 331, 335.
We close this chapter withAn Account of Three Volcanoes in the Moon; by Dr. Herschel.
“It will be necessary to say a few words by way of introduction to the account I have to give of some appearances upon the moon. The phenomena of nature, especially those that fall under the inspection of the astronomer, are to be viewed, not only with the usual attention to facts as they occur, but with the eye of reason and experience. In this we are, however, not allowed to depart from plain appearances,though their origin and signification should be indicated by the most characterizing features. Thus, when we see on the surface of the moon a great number of elevations, from half a mile to a mile and a half in height, we are strictly entitled to call them mountains; but when we attend to their particular shape, in which many of them resemble the craters of our volcanoes, and thence argue that they owe their origin to the same cause which has modelled many of these, we may be said to see by analogy, or with the eye of reason. Now, in this latter case, though it may be convenient, in speaking of phenomena, to use expressions that can only be justified by reasoning upon the facts themselves, it will certainly be the safest way not to neglect a full description of them, that it may appear to others how far we have been authorized to use the mental eye. This being premised, I may safely proceed to give my observations.
“April 19th, 1787, 10h. 36′, sidereal time: I perceive three volcanoes in different places of the dark part of the new moon. Two of them are either already nearly extinct, or otherwise in a state of going to break out; which, perhaps, may be decided next lunation. The third shews an actual eruption of fire, or luminous matter. I measured the distance of the crater from the northern limb of the moon, and found it 3′ 57″.3. Its light is much brighter than the nucleus of the comet which M. Mechain discovered at Paris the 10th of this month.—April 20th, 1787, 10h. 2′, sidereal time: The volcano burns with greater violence than last night. I believe its diameter cannot be less than 3″, by comparing it with that of the Georgian planet: as Jupiter was near at hand, I turned the telescope to his third satellite, and estimated the diameter of the burning part of the volcano to be equal to at least twice that of the satellite. Hence we may compute that the shining or burning matter must be above three miles in diameter. It is of an irregular round figure, and very sharply defined on the edges. The other two volcanoes are much farther towards the centre of the moon, and resemble large pretty faint nebulæ, that are gradually much brighter in the middle; but no well-defined luminous spot can be discerned in them. These three spots are plainly to be distinguished from the rest of the marks upon the moon; for the reflection of the sun’s rays from the earth is, in its present situation, sufficiently bright, with a ten-feet reflector, to shew the moon’s spots, even the darkest of them; nor did I perceive any similar phenomena last lunation, though I then viewed the same places with the same instrument.
“The appearance of what I have called the actual fire, or eruption of a volcano, exactly resembled a small piece of burning charcoal, when it is covered by a very thin coat ofwhite ashes, which frequently adhere to it after it has been some time ignited; and it had a degree of brightness about as strong as that with which such a coal would be seen to glow in faint daylight. All the adjacent parts of the volcanic mountain seemed to be faintly illuminated by the eruption, and were gradually more obscure as they lay at a greater distance from the crater.
“This eruption resembled much that which I saw on the fourth of May, in the year 1783; an account of which, with many remarkable particulars relating to volcanic mountains in the moon, I shall take an early opportunity of communicating to the Royal Society. It differed, however, considerably in magnitude and brightness; for the volcano of the year 1783, though much brighter than that which is now burning, was not near so large in the dimensions of its eruption; the former seen in the telescope resembled a star of the fourth magnitude, as it appears to the natural eye: this, on the contrary, shews a visible disk of luminous matter, very different from the sparkling brightness of star-light.”
CURIOSITIES RESPECTING VARIOUS PHENOMENA, OR APPEARANCES IN NATURE.—(Concluded.)
The Aurora Borealis.
The Aurora Borealis, sometimes called Streamers, is an extraordinary meteor, or luminous appearance, shewing itself in the night time in the northern part of the heavens; and most usually in frosty weather. It is generally of a reddish colour, inclining to yellow, and sends out frequent corruscations of pale light, which seem to rise from the horizon in a pyramidical undulating form, and shoot with great velocity up to the zenith. The Aurora Borealis appears frequently in form of an arch, chiefly in the spring and autumn, after a dry year. The arch is partly bright, partly dark, but generally transparent: and the matter of which it consists, is also found to have no effect on rays of light which pass through it.Dr. Hamilton observes, that he could plainly discern the smallest speck in the Pleiades through the density of those clouds which formed the Aurora Borealis in 1763, without the least diminution of its splendour, or increase of twinkling.
AURORA BOREALIS.—Page 684.
This is an extraordinary appearance of the Aurora Borealis, observedby Captain Parry in his expedition to the Arctic regions.
AURORA BOREALIS.—Page 684.
This is an aspect of the Aurora Borealis sometimes observed in Scotland.The view embraces a portion of Loch Leven, with the island and the castlein which the unfortunate Mary Queen of Scots was imprisoned.
This kind of meteor, which is more uncommon as we approach towards the equator, is almost constant during the long winter, and appears with the greatest lustre in the polar regions. In the Shetland isles, the “Merry Dancers,” as the northern lights are there called, are the constant attendants of clear evenings, and afford great relief amidst the gloom of the long winter nights. They commonly appear at twilight, near the horizon, of a dun colour, approaching to yellow; they sometimes continue in that state for several hours, without any perceptible motion; and sometimes they break out into streams of stronger light, spreading into columns, and altering slowly into ten thousand different shapes, and varying their colours from all the tints of yellow, to the most obscure russet. They often cover the whole hemisphere, and then exhibit the most brilliant appearance. Their motions at this time are most amazingly quick; and they astonish the spectator with the rapid changes of their form. They break out in places where none were seen before, skimming briskly among the heavens, are suddenly extinguished, and are succeeded by a uniform dusky tract. This again is brilliantly illuminated in the same manner, and as suddenly left a dark space. In some nights, they assume the appearance of large columns, on one side of the deepest yellow, and on the other, gradually changing, till it becomes undistinguished from the sky. They have generally a strong tremulous motion from one end to the other, and this continues till the whole vanishes.
As for us, who see only the extremities of these northern phenomena, we can have but a faint idea of their splendour and motions. According to the state of the atmosphere, they differ in hue; and sometimes assuming the colour of blood, they make a dreadful appearance. The rustic sages who observe them, become prophetic, and terrify the spectators with alarms of war, pestilence, and famine. Nor, indeed, were these superstitious presages peculiar to the northern islands: appearances of a similar nature are of ancient date; and they were distinguished by the appellations of “phasmata,” “trabes,” and “balides,” according to their forms and colours. In old times they were either more rare, or less frequently noticed: they were supposed to portend great events, and the timid imagination formed of them aërial conflicts.
In the northern latitudes of Sweden and Lapland, the Auroræ Boreales are not only singularly beautiful in their appearance, but they afford travellers, by their almost constant effulgence, a very beautiful light during the whole night. InHudson’s Bay the Aurora Borealis diffuses a variegated splendour, which is said to equal that of the full moon. In the northeastern parts of Siberia, according to the description of Gmelin, these northern lights are observed to “begin with single bright pillars, rising in the north, and almost at the same time in the north-east, which, gradually increasing, comprehend a large space of the heavens, rush about from place to place with incredible velocity, and, finally, almost cover the whole sky up to the zenith, and produce an appearance as if a vast tent were expanded in the heavens, glittering with gold, rubies, and sapphire. A more beautiful spectacle cannot be painted; but whoever should see such a northern light for the first time, could not behold it without terror. For, however fine the illumination may be, it is attended, as I have learned from the relation of many persons, with such a hissing, crackling, and rushing noise through the air, as if the largest fire-works were played off. To describe what they then hear, they make use of the expression, ‘The raging host is passing.’ The hunters, who pursue the white and blue foxes in the confines of the Icy Sea, are often alarmed in their course by these northern lights. Their dogs are then so much frightened, that they will not move, but lie obstinately on the ground, till the noise has passed. Commonly, clear and calm weather follows this kind of northern lights. This account has been confirmed by the uniform testimony of many, who have spent part of several years in these northern regions, and inhabited different countries from the Yenisei to the Lena; so that no doubt of its truth can remain. This seems, indeed, to be the real birth-place of the Aurora Borealis.”
A person who resided seven years at Hudson’s Bay, confirms M. Gmelin’s relation of the fine appearance and brilliant colours of the northern lights, and particularly of their rushing noise, which he affirms he has frequently heard, and he compares it to the sound produced by whirling round a stick swiftly at the end of a string. A similar noise has likewise been noticed in Sweden. Mr. Nairne also, being in Northampton at the time when the northern lights were remarkably bright, is confident he heard a hissing or whizzing sound. Mr. Belknap, of Dover, in New Hampshire, North America, testifies to this fact. M. Cavallo says, that the cracking noise is distinctly audible, and that he has heard it more than once. Similar lights, called Auroræ Australes, have been long since observed towards the south pole, and their existence has been lately ascertained by Mr. Forster, who assures us, that in his voyage round the world with Captain Cook, he observed them in high southern latitudes, though attended with phenomena somewhat different from those which are seen here.
On February 17, 1773, in south latitude 58°, “a beautiful phenomenon (he says) was observed during the preceding night, which appeared again this and several following nights. It consisted of long columns of a clear white light, shooting up from the horizon to the eastward, almost to the zenith, and gradually spreading on the whole southern part of the sky. The columns were sometimes bent sideways at their upper extremities; and though in most respects similar to the northern lights (Aurora Borealis) of our hemisphere, yet they differed from them in being always of a whitish colour, whereas ours assume various tints, especially those of a fiery and purple hue. The sky was generally clear when they appeared, and the air sharp and cold, the thermometer standing at the freezing point.”
The periods of the appearance of these northern lights are very inconstant. In some years they occur very frequently, and in others they are more rare; and it has been observed, that they are more common about the time of the equinoxes than at other seasons of the year. Dr. Halley (see Philos. Trans. No. 347, p. 406,) has collected together several observations, which form a kind of history of this phenomenon. After having particularly described the various circumstances which attended that observed by himself, and many others, in March, 1716, and which was singularly brilliant, he proceeds with informing us, that the first account of similar phenomena recorded in the English annals, is that of the appearance noticed January 30, 1560, and called, Burning Spears, by the author of a book entitled, “A Description of Meteors,” by W. F. D.D.; reprinted at London, in 1654. The next appearance of a like kind, recorded by Stow, occurred on October 7, 1564. In 1574, as Camden and Stow inform us, an Aurora Borealis was seen for two successive nights, viz. on the 14th and 15th of November, with appearances similar to those observed in 1716, and which are now commonly noticed. The same phenomenon was twice seen in Brabant, in 1575, viz. on the 13th of February, and the 28th of September; and the circumstances attending it were described by Cornelius Gemma, who compares them to “spears, fortified cities, and armies fighting in the air.” In the year 1580, M. Masline observed these phasmata, as he calls them, at Baknang, in the county of Wirtemberg, in Germany, no less than seven times in the space of twelve months; and again at several different times, in 1581. On September 2d, 1621, the same phenomenon was seen over all France; and it was particularly described by Gassendus, in his “Physics,” who gave it the name of Aurora Borealis. Another was seen all over Germany, in November, 1623, and was described by Kepler. Since that time, for more than eighty years, we have no accountof any such phenomenon, either at home or abroad. In 1707, Mr. Neve observed one of small continuance in Ireland; and in the same year, a similar appearance was seen by Romer, at Copenhagen; and during an interval of eighteen months, in the years 1707 and 1708, this sort of light had been seen no less than five times.
Hence it should seem, (says Dr. Halley,) that the air or earth, or both, are not at all times disposed to produce this phenomenon, though it is possible it may happen in the day-time, in bright moonshine, or in cloudy weather, and so pass unobserved. Dr. Halley further observes, that the Aurora Borealis of 1716, which he described, was visible from the west of Ireland to the confines of Russia, and to the east of Poland; extending at least near thirty degrees of longitude, and from about the fiftieth degree of north latitude, over almost all the north of Europe; and in all places at the same time, it exhibited appearances similar to those which he observed in London. He regrets, however, that he was unable to determine its height, for want of contemporary observations at different places.
Father Boscovich has determined the height of an Aurora Borealis, observed on the 16th of December, 1737, by the Marquis of Poleni, to have been eight hundred and twenty-five miles; and Mr. Bergman, from a mean of thirty computations, makes the average height of the Aurora Borealis to be seventy-two Swedish, or (supposing a Swedish mile to be about six and a half English miles) four hundred and sixty-eight English miles. Euler supposes the height to be several thousands of miles; and Mairan also assigns to these phenomena a very elevated region, the far greater number of them being, according to him, about two hundred leagues above the surface of the earth. Dr. Blagden, speaking of the height of some fiery meteors, (Phil. Trans. vol. lxxiv. p. 227,) says, “that the Aurora Borealis appears to occupy as high, if not a higher region, above the surface of the earth, as may be judged from the very distant countries to which it has been visible at the same time:” he adds, that “the great accumulation of electric matter seems to lie beyond the verge of our atmosphere, as estimated by the cessation of twilight.” But as it is difficult to make such observations on this phenomenon as are sufficient to afford a just estimate of its altitude, they must be subject to considerable variation, and to material error.
Dr. Blagden informs us, that instances are recorded, in which the northern lights have been seen to join, and form luminous balls, darting about with great velocity, and even leaving a train behind them like the common fire-balls. This ingenious author, however, conjecturing that distinct regionsare allotted to the electrical phenomena of our atmosphere, assigns the appearance of fire-balls to that region which lies beyond the limits of our crepuscular atmosphere; and a greater elevation above the earth, to that accumulation of electricity in a lighter and less condensed form, which produces the wonderfully diversified streams and coruscations of the Aurora Borealis.
CURIOSITIES RESPECTING GALVANISM.
Galvani, a professor of anatomy in the university of Bologna, was one day making experiments on electricity. In his laboratory, near the machine, were some frogs that had been flayed; the limbs of which became convulsed every time a spark was drawn from the apparatus. Galvani, surprised at this phenomenon, made it a subject of investigation, and discovered that metals, applied to the nerves and muscles of these animals, occasioned powerful and sudden contractions, when disposed in a certain manner. He gave the name of Animal Electricity to this order of new phenomena, from the analogy that he considered existing between these effects and those produced by electricity.
The name, Animal Electricity, has been superseded, notwithstanding the great analogy that exists between the effects of electricity and of Galvanism, in favour of the latter term; which is not only applicable to the generality of the phenomena, but likewise serves to perpetuate the memory of the discoverer.
In order to give rise to galvanic effects, it is necessary to establish a communication between two points of one series of nervous and muscular organs. In this manner a circle is formed, one arch of which consists of the animal parts, rendered the subject of experiment, while the other arch is composed of exciting instruments, which generally consists of those animal parts called supporters; others, destined to establish a communication between the latter, are called conductors. To form a complete galvanic circle, take the thigh of a frog, deprived of its skin; detach the crural nerve, as far as the knee; put it on a piece of zinc; lay the muscles of the leg on a piece of silver; then finish the exciting arch, and complete the galvanic circle by establishing a communicationby means of the two supporters, by iron or copper wire, pewter, or lead. The instant that the communicators touch the two supporters, a part of the animal arch formed by the two supporters will be convulsed. Although this disposition of the animal parts, and of galvanic instruments, be most favourable to the development of the phenomena, yet the composition of the animal and excitatory arch may be much varied. Thus contractions are obtained, by placing the two supporters under the nerve, and leaving the muscle out of the circle; which proves that nerves essentially constitute the animal arch.
It is not necessary for nerves to be entire, in order to produce contractions. They take place whether the organs be tied or cut through, provided there exists a simple contiguity between the divided ends. This proves that we cannot strictly conclude what happens in muscular action, from that which takes place in galvanic phenomena; since, if a nerve be tied or divided, the muscles on which the energy is distributed lose the power of action.
The cuticle is an obstacle to galvanic effects; they are always feebly manifested in parts covered by it. When it is moist, fine, and delicate, the effect is not entirely interrupted. Humboldt, after having detached the cuticle from the posterior part of the neck and back, by means of two blisters, applied plates of metal to the bare cutis, and, at the moment of establishing a communication, he experienced sharp prickings, accompanied with a serosanguinous discharge.
If a plate of zinc be placed under the tongue, and a flat piece of silver on its superior surface, on making them touch each other, an acerb taste will be perceived, accompanied with a slight trembling.
The exciting arch may be constructed with two or three metals, or even one metal only; with alloys, amalgams, or other metallic or mineral combinations, carbonated substances, &c. It is observed, that metals, which are in general the most powerful exciters, induce contractions so much the more as they have an extent of surface. Metals are all more or less excitants; and it has been noticed that zinc, gold, silver, and pewter, are of the highest rank; then copper, lead, nickel, antimony, &c.
Galvanic susceptibility is exhausted by too long-continued exercise, and is recruited by repose. Immersion of nerves in alkohol and opiate solutions diminishes, and even destroys, this susceptibility; in the same manner, doubtless, as the immoderate use of these substances in the living man, blunts, and induces paralysis in muscular action. Immersion in oxygenated muriatic acid, revests the fatigued parts, in being acted on by the stimulus. Animals killed by the repeateddischarge of an electric battery, acquire an increase of galvanic susceptibility; and this property subsists unchanged in animals destroyed by submersions in mercury, pure hydrogen gas, azote, and ammoniac; and finally, it is totally annihilated in animals suffocated by the vapour of charcoal.
Galvanic susceptibility is extinct in the muscles of animals of warm blood, in proportion as vital heat is dissipated; sometimes even when life is terminated in convulsions, contractibility cannot be put into action, although warmth be not completely gone, as though the vital property were consumed by the convulsions amidst which the animals had expired. In those of cold blood, on the contrary, it is more durable. The thighs of frogs, long after being separated from every thing, and even to the instant of incipient putrefaction, are influenced by galvanic stimuli; doubtless, because irritability, in these animals, is less intimately connected with respiration, and life more divided among the different organs, which have less occasion to act on each other for the execution of its phenomena. The galvanic chain does not produce sensible actions (that is, contractions) until the moment it is completed, by establishing a communication with the parts constituting it. During the time it is complete, that is, throughout the whole space of time that the communication remains established, every thing remains tranquil; nevertheless, galvanic influence is not suspended; in fact, excitability is evidently increased or diminished, in muscles that have been long continued in the galvanic chain, according to the difference of the reciprocal situation of the connecting metals.
If silver has been applied to the nerves, and zinc to the muscles, the irritability of the latter increases in proportion to the time they have remained in the chain. By this method, the thighs of frogs have been revivified in some degree, and afterwards became sensible to stimuli that before had ceased to act on them. By distributing the metals in an inverse manner, applying zinc to the nerves, and silver to the muscles, an effect absolutely contrary is observed; and the muscles that possessed the most lively irritability when placed in the chain, seem to be rendered entirely paralytic if they remain long in this situation.
This difference evidently depends on the direction of the galvanic fluid, determined towards the muscles or nerves, according to the manner in which these metals are disposed; and this is of some importance to be known for the application of galvanic means to the cure of diseases.
M. Volta’s apparatus is as follows:—Raise a pile, by placing a plate of zinc, a flat piece of wet card, and a plate of silver, successively; then a second piece of zinc, &c. until the elevation is several feet high; for the effects are greaterin proportion to its height; then touch both extremities of the pile, at the same instant, with one piece of iron wire: at the moment of contact, a spark is excited from the extremities of the pile, and luminous points are often perceived at different heights, where the zinc and silver come into mutual contact. The zinc end of this pile appears to be negatively electrified; that formed by the silver, on the contrary, indicates marks of positive electricity.
If we touch both extremities of the pile, after having dipped our hands into water, or, what is better, a saline solution, a commotion, followed by a disagreeable pricking in the fingers and elbow, is felt.
If we place, in a tube filled with water, and hermetically closed by two corks, the extremities of two wires of the same metal, which are in contact at the other extremity, one with the summit, the other with the base of the pile; these ends, even when separated only by the space of a few lines, experience evident changes at the instant the extremities of the pile are touched: the wire in contact with that part of the pile composed of zinc, becomes covered with bullæ of hydrogen gas; that which touches the extremity formed by silver, becomes oxydated. Fourcroy attributes this phenomenon to the decomposition of water by the galvanic fluid, which abandons the oxygen to the iron that touches the positive extremity of the pile; then conducts the other gas invisibly to the end of the other wire, there to be disengaged.
From the numerous experiments of Mr. Davy, many new and important facts have been established, and Galvanism has been found to be one of the most powerful agents in chemistry. By its influence, platina wire has been melted; gold, silver, copper, and most of the metals, have easily been burnt! the fixed alkalis, and many of the earths, have been made to appear as consisting of a metallic base and oxygen; compound substances, which were before extremely difficult to decompose, are now, by the aid of Galvanism, easily resolved into their constituent.
CURIOSITIES RESPECTING MAGNETISM.
Magnetismis supposed to have been first rendered useful about the end of the twelfth, or at least very early in the thirteenth, century, by John de Gioja, a handicraft of Naples, who noticed the peculiar attraction of metals, and iron in particular, towards certain masses of rude ore; the touch of which communicated to other substances of a ferruginous nature, especially iron or steel bars, the property of attraction: these touched bars he observed to have a peculiar and similar tendency towards one particular point; that when suspended in equilibrio, by means of threads around their centres, they invariably turned towards the same point; and that, when placed in a row, however adversely directed, they soon disposed themselves in perfectly parallel order. In this instance he improved upon the property long known to, but not comprehended or applied to use by, the ancients, who considered the loadstone simply as a rude species of iron ore, and curious only so far as it might serve to amuse.
Gioja being possessed of a quick understanding, and of a strong mind, was not long in further ascertaining the more sensible purposes to which the magnet might be appropriated. He accordingly fixed various magnets upon pivots, supporting their centres in such a manner as allowed the bars to traverse freely. Finding that, however situated within the reach of observation and comparison, they all had the same tendency, he naturally concluded them to be governed by some attraction, which might be ultimately ascertained and acted upon. He therefore removed into various parts of Italy, to satisfy himself whether or not the extraordinaryimpulse which agitated these bars, that had been magnetized by friction, existed only in the vicinity of Naples, or was general. The result of his researches appears to be, that the influence was general, but that the magnets were rendered extremely variable, and fluctuated much, when near large masses of iron. The experiments of Gioja gave birth to many others, and at length to a trial of the magnetic influence on the surface of the water. To establish this, a vessel was moored out at sea, in a direction corresponding with that of the magnet; and a boat, having a magnet equipoised on a pivot at its centre, was sent out at night in the exact line indicated thereby; which, being duly followed, carried them close to the vessel that was at anchor. Thus the active power of attraction appeared to be established on both elements, and in the course of time the magnet was fixed to a card, marked with thirty-two points, whereby the mariner’s compass was presented to us. The points to which the magnet always turned itself, being generally in correspondence with the meridian of the place where it acted, occasioned the extremities of the bars to be called poles. Succeeding experiments proved, that the magnetic bar never retained an exactly horizontal position; but that one of its poles invariably formed an angle with any perfect level, over which it was placed: this was not so very measurable in a short bar, but in one of a yard in length was formed to give several degrees of inclination. This, which is called “the dip of the needle,” (or magnet,) seems to indicate that the attracting power is placed within the earth. What that attracting power is, we cannot determine; some consider it to be a fluid, while others conjecture it to be an immense mass of loadstone, situated somewhere about the north pole. The difficulty is, however, considerably increased by the known fact of the needles of compasses not always pointing due north; but in many places varying greatly from the meridional lines respectively; and from each other at different times and places.
The facility with which a meridional line may be drawn by solar observation, and especially by taking an azimuth, fortunately enables navigators to establish the variation between the true northern direction, and that indicated by the magnet attached to the card of the compass. Nevertheless, we have great reason to believe, that, for want either of accurate knowledge of the prevalent variations, or from inattention thereto, many vessels, of which no tidings were ever heard, have been cast away; it being obvious, that a false indication of the northern point, in many places amounting to nearly the extent of twenty-five degrees, must produce so important an error in a vessel’s course, as to subject her to destruction on those very shoals, rocks, &c. of which the navigator unhappilythinks he steers perfectly clear. To obviate such danger, as far as possible, all modern sea-charts have the variations of the compass in their several parts duly noted down; and in reckoning upon the course steered by the compass, an allowance is usually made for the difference between the apparent course by the compass, and the real course, as ascertained by celestial observation. Under circumstances so completely contradictory, the principle of magnetism must remain unknown: we know not of any hypothesis which strikes conviction on our minds, or which seems to convey any adequate idea of the origin, ormodus operandi, of this wondrous influence. All we can treat of is, the effect; also of the appearances which guide our practice, and of the manner in which the attractive power may be generated and increased. In regard to the latter point, namely, the generation and increase of the magnetic attraction, we shall endeavour to give a brief but distinct view of what relates thereto: observing, that where volcanic eruptions are frequent, and in those latitudes where the Aurora Borealis is distinctly seen, the needle or magnet is sensibly affected.
Previously to earthquakes, as well as during their action, and while the northern lights are in full display, no reliance can be placed on the compass; the card of which will appear much agitated. This has given rise to the opinion held by some, that the power is a fluid: to this, however, there appear so many objections, that we are more disposed to reject than to favour it, although under the necessity of confessing, that we are not able to offer one that may account satisfactorily for the various phenomena attendant upon magnetism.
We have already stated, that every magnet has two poles; that is, one end is called the north, the other the south pole: the former being considered as capable of attraction; the other, as we shall infer from the subjoined explanations, being far more inert, if at all possessed of an attractive power. When two magnets are brought together with their north poles in contact, they will, instead of cohering, be obviously repelled to a distance corresponding with their respective powers of attraction, when applied individually to unmagnetized needles. The south poles will, in like manner, repel each other; but the north pole of one, and the south pole of the other, will, when approximated, be evidently attracted, and will cohere so as to sustain considerable weights. Iron is the only metal, hitherto known, which is capable of receiving and communicating the magnetic power; but quiet, and the absence of contact, in some respects, are indispensably necessary towards its perfect retention. Thus, when a bar has been impregnated, however abundantly, with the magneticprinciple, if it be heated or hammered, the power of attraction will be dissipated; or if a tube filled with iron filings have their surface magnetized, by shaking the tube the magnetic influence will likewise be lost. In some respects the magnetic influence resembles caloric; for it very rapidly communicates to iron, devoid of magnetism, a certain portion of its own powers; which, however, appear to be reproduced instantaneously. As various small fires under one large vessel will thereby heat it, and cause the water it contains to boil, though neither of them individually would produce that effect; so, many weak magnets may, by being united, communicate a power equal to its own, and be made to create an accumulated power, larger than that contained by either of them individually.
There is, however, a seeming contradiction to be found in some authors, who recommend that the weakest magnets should be first applied,—and those more forcible, in succession, according to the power they may possess; the reason assigned being, that the weaker magnets would else, in all probability, draw off some of the accumulated power from the new magnet. But of this there appears no danger, since experience proves that magnets rather gain than lose efficiency by contact, not only with each other, but even with common iron. In fact, the magnetic power may at any time be created by various means: the friction of two pieces of flat and polished bars of iron, will cause them for a short time to attract and to suspend light weights. Soft iron is more easily influenced, but steel will retain the influence longer. Lightning, electricity, and galvanism, being all of the same nature, equally render iron magnetic. It is also peculiar, that when two or more magnets are left for any time with their several north poles in contact, the whole will be thereby weakened; whereas, by leaving a piece of common iron attached to a magnet, the latter will acquire strength. It is also well known that some pieces of steel quickly receive the magnetic influence, while others require considerable labour, and after all are scarcely impregnated. The oxide of iron cannot be impregnated, and those bars that have been so, when they become partially oxydized, lose their power. Hence we see the necessity of preserving the needles of compasses from rust.
Magnets have the power to act notwithstanding the intervention of substances in any degree porous between them and the body to be acted upon: thus, if a needle be put on a sheet of paper, and a magnet be drawn under it, the needle will follow the course of the magnet. The peculiar affinity of the loadstone for iron, is employed with great success, by those who work in precious metals, for the separation of filings, &c. of iron from the smaller particles of gold, &c. A magnetbeing dipped into the vessel, in which the whole are blended, will attract all ferruginous particles.
To communicate the magnetic power to a needle, let it be placed horizontally; and with a magnet in each hand, let the north pole of one, and the south pole of the other, be brought obliquely in contact over the centre of the needle: draw them asunder, taking care to press firmly, and preserving the same angle or inclination to the very ends of the needles, which should be supported by two magnets, whose ends ought to correspond in polarity with those of the needle. Observe to carry the magnets you press with clear away from the ends of the needle, at least a foot therefrom; repeat the friction in the same manner several times, perhaps six, eight, or ten, and the needle will be permanently magnetized; and, as we have already stated, by using other magnets in succession, the powers of the needle will be proportionably increased.
But no effect will result from the friction if the bars are rusty, or, indeed, not highly polished; their angles must be perfect, and their several sides and ends completely flat. It is, perhaps, one of the most curious of the phenomena attendant on this occult property, that the centre of every magnet is devoid of attraction; yet, that when a needle is placed in a line with a magnet, and within the influence of its pole, that needle almost becomes magnetic, or rather, a conductor, possessing a certain portion of attractive power: and it is no less extraordinary, that the magnet retains its power even in the exhausted receiver of an air-pump; which seems to be a formidable objection to its being influenced by any fluid. Perhaps the opinion entertained by many of our most popular lecturers on this subject, viz. that the earth itself is the great attractor, may be nearest the truth. We are the more inclined towards such an hypothesis, knowing that, at the true magnetic equator, the needle does not dip; and from the well-ascertained fact, that bars of iron, placed for a length of time exactly perpendicular, receive a strong magnetic power, their lower ends repelling the south, but attracting the north poles of magnets applied to them respectively. The direction of the dipping needle was ascertained by one Robert Norman, about two hundred and fifty years ago. He suspended a small magnetic needle, by means of a fine thread round its centre, so as to balance perfectly, over a large magnet: the south pole of the former was instantly attracted by the north pole of the latter. He found, that so long as the needle was held exactly centrical, at about two inches above the magnet, it remained horizontal; but so soon as withdrawn a little more towards one end than the other of the magnet, the equilibrium was destroyed, and that pole of the needle which was nearest to either pole of the magnet was instantlyattracted, and pointed downwards thereto. By the magnetic equator, we mean a circle passing round the earth at right angles with the magnetic poles, which do not correspond with the geographical poles, as may be fully understood by the indications of all compasses to points differing from the latter; and as the indications of compasses vary so much both at different times and places, we may reasonably conclude, that the magnetic poles are not fixed. The variation of the dipping-needle has not, in our latitude at least, varied more than half a degree since its depressive tendency was first discovered by Norman.