ferved the duft to rife at the place; that there had been feveral flashes of lightning fome time before from the fouth-eaft, whereas the accident happened to the north-west of where he stood. The diftance, in a right line across the river, might be between two and three hundred yards. He was fenfible of no fhock, nor uncommon fenfation of any kind. I went next morning to examine if there were any marks of putrefaction on the horfes, and to obferve the ftate of the blood-veffels, &c. after the fkin had been taken off; but a gentleman of the neighbourhood, who kept a pack of hounds, had already feized on them.

Several other phænomena happened on that day, probably all proceeding from the fame caufe; fome of which I fhall beg leave to mention.

The fhepherd belonging to the farm of Lennel-Hill was in a neighbouring field, tending his flock, when he obferved a lamb drop down; and faid, he felt at the fame time as if fire had pailed over his face (this was his own expreffion,) although the lightning and claps of thunder were then at a great diftance from him. He ran up immediately, but found the lamb quite dead; nor did he perceive the leaft convulfive motion, nor fymptom of life remaining, although the moment before it appeared to be in perfect health. He bled it with his knife, and the blood flowed freely. This, he told me, happened about a quarter of an hour before the explofion which killed Lauder; and it was not above three hundred yards diftant from the fpot. He was only a few yards from the lamb when it fell down. The earth was not torn up, nor did he obferve any duft rife.

Thomas Fofter, a celebrated fifher in Coldstream, and another man, were ftanding in the middle of the

Tweed, fishing for falmon with the rod, when they fuddenly heard a loud noife; and turning round to fee from whence it came, they found themselves caught in a violent whirlwind, which Fofter told me felt fultry and hot, and almost prevented them from breathing. It was not without much difficulty they could reach the bank, where they fat down, exhausted with fatigue, and greatly alarmed; however it lafted but a very fhort time, and was fucceeded by a perfect calin. This happened about an hour before the explosion.

A woman, making hay near the banks of the river, fell fuddenly to the ground, and called out to her companions that he had received a violent blow on the foot, and could not imagine from whence it came. This I had not from the woman herself, but from Mr Turnbull, a very refpectable farmer. Mr Bell, our minifter, nephew of Thomfon the Poet, and poffeffed of all the candour and ingenuity of his uncle, told me, that, walking in his,garden, a little before Lauder's accident, he feveral times felt a fenfible tremor in the ground. He likewife told me (what I find I had forgot to mention in the proper place,) that he had obferved on Lauder's body a zig-zag line, of about an inch and a quarter broad, which extended from his chin down to his right thigh, and had followed nearly the line of the buttons of his waistcoat. The fkin was burnt white and hard.

Thefe, Sir, are all the circumftances I have been able to collect that are well authenticated; and I fhall not trouble you with reports that are not. From the whole, it would appear, that the earth had acquired a great fuper-abundance of electrical matter, which was every where endeavouring to fly off into the atmosphere. Perhaps it might be accounted for from the exceffive

drynefa

13

gate the caufes: all I wanted was to give you fome account of the ef fects; and your own reflections will lead you much farther than any thing I could suggest.

I have the honour to be, with the greatest respect, &c. P. BRYDONE.

A

FTER having related the refult of fome magnetical experiments, Mr Cavallo adds the following thoughts concerning the application of thofe obfervations towards accounting for the variation of the magnetic needle.

This wonderful phænomenon has, fince it was firft difcovered, employed the thoughts of very able philofophers; many hypothefes having been offered, not only for its explanation, but even to foretel the future variations in various parts of the world. I need not detain this learned Society with a particular hiftory of those hypothefes, but fhall only obferve in general, that nei ther their predictions have answered, nor any of them was founded upon evident principles. The fuppofition of a large magnet being inclofed within the body of the earth, and of its relatively moving with refpect to the outward fhell or cruft; the fuppofition of there being four moveable magnetic poles within the earth; the hypothefis of a magnetic power, partly within and partly without the furface of the earth; together with feveral other hypothefes on the fame fubject, are not only unwarranted by actual experiments, but do neither feem analogous to the other operations of nature. The late ingenious Mr Canton, Member of this Society, was the firft who endeavoured to account for the daily variation of the

magnetic needle by the heating and cooling of the magnetic bodies in different parts of the earth's furface; which was in confequence of his having first observed, that the action of the magnet on the needle was diminished by heating, and in. creafed by cooling.

Following Mr Canton's judicious method of deriving the expla nation of natural appearances from properties actually proved by experiments, I think that the increase and diminution of magnetic attraction, by heating and cooling of the magnet, as obferved by Mr Canton, together with the refult of the experiments recited in this Paper, feem fully fufficient to account for the general variation of the needle.

If we collect under one point of view all the caufes hitherto afcertained, which can increase or diminifh the attraction between magnetic bodies, we fall find that the attraction between the magnet and iron, or between magnet and magnet, is increafed by cooling, by a regeneration of iron or phlogiftication of its calx, and within certain limitations by the action of acids upon the iron; that this attraction is diminished by heating, and by the decompofition of iron; and, laftly, that it is probably annihilated by a very great degree of heat.

Thefe truths being premifed, it must be confidered, firit, that, according to innumerable observations

and

and daily experience, the body of the earth contains almost every where ferruginous bodies in various states and bulks; fecondly, that the magnetic needle must be attracted by all thofe bodies, and its fituation or direction must be determined by all thofe attractions confidered together, viz. from their common center of action; thirdly, that by removing or altering the degrees of attraction of fome of thofe bodies which are fituated on one fide of the magnetic meridian, more than of thofe fituated on the other fide, the above-mentioned common center of attractions, and, of course, the direction of the magnetic needle muft be altered, which in fact is the variation of the needle; and, laftly, that this alteration in the attractions of fome of the ferruginous bodies in the earth muft undoubtedly take place, it being occafioned by the parts of the earth being irregularly heated and cooled, by the action of volcanoes which decompofe or otherwife alter large maffes of ferruginous fubftance, by earthquakes which remove ferruginous bodies from their original places, and we may add also, by the aurora borealis; for though we are as yet ignorant of the cause of that surprising phanomenon, it is however certain, that the magnetic needle has been frequently disturbed when the aurora borealis appeared very ftrong.

The magnetic needle, therefore, being neceffarily affected by thofe causes, it feems unneceffary to have recourfe to other hypothetical caufes which are not established on actual experience.

In order to exemplify this explanation of the variation in a familiar manner; I made the following experiment, with the account of which I fhall conclude this Paper. Four earthen veffels were difpofed round the magnetic needle, two near its fouth, and the other two near its north pole, but not at equal distances. In one of thofe veffels there was pla ced a natural magnet, the fecond contained feveral fmall bits of magnetie fteel mixed with earth; and in each of the other two there were put about four ounces of iron filings. Things being thus difpofed, and left undisturbed for about half an hour, the needle remained unaltered. Then the pieces of magnetic steel and earth were stirred with a stick, in confequence of which the needle was agitated. After this, fome diluted vitriolic acid was poured upon the filings in one of the vessels, the action of which attracted the needle that way; but whilft the needle remained in that fituation, fome diluted vitriolic acid was poured upon the iron filings in the other veffel, which flood on the other fide, in confequence of which the needle went back again towards its former direction. Whilft the efferveffences were going on in the two veffels, the magnet in the firft veffel was heated by means of boiling water, which occafioned another alteration in the direction of the magnetic needle; and thus, by altering the ftate of the ferruginous fubftances in the veffels, the needle's direction was altered, in evident imitation of the natural variation. T. CAVALO."*

Τ

I per

I perceived the 19th and 20th of this month. The phænomena of nature, especially those that fall under the infpection of the aftronomer, are to be viewed, not only. with the usual attention to facts as they occur, but with the eye of reafon and experience. In this we are, however, not allowed to depart from plain appearances; tho' their origin and fignification fhould be indicated by the most characterizing features. Thus, when we fee, on the furface of the moon, a great number of elevations, from half a mile to a mile and an hålf in height, we are strictly intitled to call them Mountains; but when we attend to their particular fhape, in which many of them resemble the eraters of our volcanos, and thence argue, that they owe their origin to the fame caufe which has modelled many of these, we may be faid to fee by analogy, or with the eye of reafon. Now, in this latter cafe, though it may be convenient, in fpeaking of phænomena, to ufe expreffions that can only be juftified by reafoning upon the facts themfelves, it will certainly be the fafeft way not to neglect a full defcription of them, that it may appear to others how far we have been authorized to use the mental eye. This being premised, I may fafely proseed to give my obfervations.

April 19, 1787, 10h. 36' fidereal time. I perceive three volcanos 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 fhews 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

15

M. Méchain difcovered at Paris the 10th of this month.

April 20, 1787, 10 h. 2' fidereal time.

The volcano burns with greater violence than laft night. I believe its diameter cannot be lefs than 3", by comparing it with that of the Georgian planet: as Jupiter was near at hand, I turned the telescope to his third fatellite, and estimated the diameter of the burning part of the volcano to be equal to at least twice that of the fatellite. Hence we may compute, that the fhining or burning matter must be above three miles in diameter. It is of an irregular round figure, and very fharply defined on the edges. The other two volcanos are much farther towards the center of the moon, and resemble large, pretty faint nebulae, that are gradually much brighter in the middle; but no well-defined luminous fpot can be difcerned in them. Thefe three fpots are plainly to be diftinguished from the reft of the marks upon the moon; for the reflection of the fun's rays from the earth is, in its prefent fituation, fufficiently bright, with a ten-feet reflector, to flew the moon's fpots, even the darkest of them: nor did I perceive any fimilar phænomena laft lunation, tho I then viewed the fame places with the fame inftrument.

The appearance of what I have called the actual fire or eruption of a volcano, exactly resembled a fmall piece of burning charcoal, when it is covered by a very thin coat of white afhes, which frequently adhere to it when it has been fome time ignited; and it had a degree of brightness, about as ftrong as that with which fuch a coal would be feen to glow in faint daylight.

All the adjacent parts of the volca, nic mountain feemed to be faintly illuminated by the eruption, and were gradually