the propofitions depend on each other, we cannot not give an abstract of it.

IV. Of the cause of the variation of the obliquity of the ecliptic ; by Colin M Laurin, late fellow of the royal fociety, and proJellor of mathematics in the university of Edinburgh.

The variation of the obliquity of the ecliptic has been the fubject of many difputes among aftronomers; it is alfo difputed among those who hold it liable to change, whether that change be regular or irregular. It is certain, that the obliquity of the ecliptic is at present found to be above a third part of a degree lefs than Ptolemy has given it us: and if we enter into a detail of all the obfervations of it which were made from Ptolemy's time down to Tycho, we shall fee that later observers have generally found it lefs than those who preceded them. We fay generally, for this has not always been the cafe, there being fome inftances' where, on the contrary, later obfervations have made it a few minutes greater than fome preceding ones.

They who will have the obliquity of the ecliptic to have been always the fame as at prefent, remove all thefe difficulties, by imputing to the inaccuracy of the obfervations of the antients, and the imperfection of their inftruments, all the differences that are found between them and the moderns in this affair. Others think this is treating the antients with too little refpect; for, tho' it appears by Ptolemy, that they did not pretend to obferve more nicely than to the fixth part of a degree, and their inftruments, as far as we have any account of them, fall far fhort of those made ufe of by the moderns; yet it can hardly be imagined they could be fo greatly miftaken in a matter so easy to be known, and that fo highly merited their

attention.

With regard to thofe who hold, that the obliquity of the ecliptic is fubject to a fort of libration, increafing in fome ages, and decreafing in others, they feem to attribute a greater degree of exactnels to the obfervations upon which this opinion is founded, than they have fufficient reafon for.

Those who maintain, that the obliquity of the ecliptic has been always gradually decreasing, the planes of the ecliptic and equator flowing continually nearer to coincidence, found their opinion not only on a comparifon between the obfervations of the antients and moderns, but alfo on the fpheroidical figure of

the earth.

The obliquity of the ecliptic being equal to the diftance between either tropic and the equator, or half the diftance between the two tropics, the best way of finding it, because

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least liable to error from refraction, is to take the altitude of the fun when he is in the tropic of Cancer; the altitude of the pole, and confequently its complement, the altitude of the equator being first accurately known in the place of obfervation: this gives us the diftance of the tropic from the equator directly. There is another way of finding this, and the only one made use of by the antients; which is, to obferve the meridian altitude of the fun in each tropic: but then allowance must be made for refraction, which increases the meridian altitude of the fun, when in the winter folftice, more than it does when in the fummer folftice. For want of knowing this the antients thought the diftance of the tropics less than it really was: they alfo fuppofed the fun's parallax too great, which increafed the error; but these errors of the antients hinder us not from making ufe of their observations in this difquifition; because we know their confequences, and can therefore tell how to make proper allowances for them. Now after correcting the errors arifing from refraction and parallax, the obliquity of the ecliptic, according to the obfervations of the antients, appears to be about a third part of a degree greater than what the modern aftronomers have found it.

According to the obfervations of the moderns, the obliquity of the ecliptic varies at the rate of 55" in feventy-nine years, which variation Mr. M'Laurin has endeavoured to account for in the following manner.

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Sir Ifaac Newton has found, that the fun and planets gra'vitate towards each other mutually; that it is neither the fun (according to the Copernican fyftem) nor the earth (according to Ptolemy) that is the center of the fyftem, or fixed point; but the center of gravity of the whole fyftem. That the fun therefore moves about this center: and that when Jupiter and Saturn, the two biggeft planets, are in the fame right line, on the fame fide of the fun; the center of the fun will be almost a diameter of the fun diftant from this fixed point. Hence, tho' we fuppofe, the earth to move always in the fame plane, the fun will appear to have differ⚫ent declinations at the time of the folftice: and, as the obliquity of the ecliptic is determined always from the declination of the fun at the foiftice, it will therefore appear to vary, but not in a manner that will accrefce and produce any fenfible change in our feafons; but it will increase and decreate a little within fmall limits, and its variation will principally depend on the pofition of Jupiter and Saturn to the fun and earth, and will nearly return to the fame mag

nitude,

nitude, when these return to the fame pofition, with refpect to one another, and to the folftice. Jupiter has most effect in producing this variation; and if it be found to have a connection or dependance on his pofition to the fun and earth, it will be an indication that this is the true caufe of the phænomenon.'

The author farther obferves, that if the orbit of the earth was perpendicular to the orbit of Jupiter, this variation would be much more confiderable than it is, and might amount to above half a degree, or a diameter of the fun. But because the angle contained between the orbits of Jupiter and the earth is small, the variation thence arifing must be also small, but may amount to somewhat more than a minute.

This theory of Mr. M'Laurin certainly deserves to be farther profecuted, and accurately determined; for an error of a minute in a matter of fuch moment as the fun's declination, is of great confequence in navigation, geography, and aftronomy. But it does not appear fufficient to account for the different obliquities of the ecliptic, as obferved by the antient and modern aftronomers; for can we with juftice suppose, that the antients could have committed an error of above twenty minutes in their observations? An annual diminution therefore of the angle intercepted between the ecliptic and equator must be fuppofed, if we will allow the antients to have had eyes and inftruments, and to have used any tolerable care in their obfervations.

V. Concerning the fudden and surprising changes obferved on the furface of Jupiter's body. By the fame.

More furprifing phænomena have been obferved on the body of Jupiter than on any other planet in the folar fyftem. Among these the several belts obferved by Caffini to rife in an hour's time, are not the least extraordinary. Spots of very dif ferent forms have alfo very frequently been feen upon that planet's body. Thefe Mr. McLaurin suspects to be owing to tides analogous to thofe we call fpring tides. But with regard to that famous phænomenon mentioned by Caffini, namely, That fome of his fpots revolve in leis time than others; this ingenious author is of opinion, that it is better not to attempt any explication till the fame be confirmed by more

obfervations.

VI. Obfervations on thunder and electricity. By Ebenezer M'Fait, M. D.

Mr. Franklin, after fhewing that lightning and electrical fire are the fame, and that fharp points attract and discharge

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the electrical matter moft copioufly, fuppofes, that a very sharp pointed rod, fixed to the extremity of the top-maft of a ship, with a wire conducted down from the foot of the rod, round one of the fhrouds, and over the fhip's fide into the fea, would filently lead off the electrical fire, and fave the ship from thunder in hot countries; and that by a fimilar method buildings might be preferved. But Dr. M'Fait is of opinion, from feveral experiments made with iron rods during a thunder ftorm, that a more fimple and eafy way of protecting mafts and fpires from thunder, is to fix horizontally, on the highest parts of them, a flat round piece of wood, of a foot diameter, or more, in order to prevent the electrical matter from fixing on them and accumulating.

Our author is alfo of opinion, that the lightning and the aurora borealis are of the fame materials. For, he obferves, that in hot countries ftreamers are rarely feen, because they are kindle into thunder and flashes of lightning: that thunder difturbs the motion of the magnetic needle, and it has lately. been found in Sweden, that ftreamers do the fame. Whence he concludes, that thunder, electricity, magnetifm, and the aurora borealis are wonderfully related.

VII. Some phænomena obfervable in foggy weather, by the fame.

In Mr. Bouguer's account of his voyage to Peru, in order to measure a degree at the equator, to determine that famous problem relating to the true figure of the earth, he tells us, that when they were upon the tops of the Cordilleras, they often faw their fhadows projected on the clouds beneath them; and that the head was adorned with a glory, like that painted round the heads of faints in pictures; it was compofed of three or four concentric crowns of a very lively colour, and each with the fame varieties as a primary rainbow, the red being outward. The intervals between thefe circles were equal, but the laft circle fainter; and,' adds that ingenious writer, at a distance we faw a large white circle which environed the whole. It was a kind of apotheofis to each spectator.' The fame phænomenon, Dr. M Fait tell us, he has often obferved, when he has afcended the mountains above the fog, where the fun, fhining, projected his fhadow on the mist.

VIII. The measures of Scotland compared with thofe of England. By James Gray.

From feveral experiments made by Mr. Gray it appears, that the Scotch pint contains 103, the wheat firlot 2197, and the bear firlot 2150 cubic inches: that the Scotch ounce

is equal to 476 Troy grains, or four grains lefs than the Englifb-Troy ounce that the Scotch foot is equal to 125, and the Scotch ell, according to the ftandard of Edinburgh, equal to 37% English inches: that the Scotch mile contains 5952 Englifh feet, and the Scotch acre 55353% fquare feet.

IX. A differtation on the fexes of plants, by Charles Alfton, M. D. king's botanist in Scotland, fellow of the royal college of phyficians, and professor of medicine and botany in the univerfity of Edinburgh.

There being a pamphlet, entitled, A differtation on botany, by the fame author, from whence this differtation is extracted, the reader will hereafter find an account of it in our article relating to that pamphlet.

X. Remarks on chemical folutions and precipitations, by Andrew Plumier, M. D. &c.

The hiftory of chemical folutions and precipitations is very extensive, and has, perhaps, never been pursued with that attention and accurary the importance of it deferves. Indeed the many fingular and furprifing appearances obfervable in the actions of menftruums and precipitants, feem to render an attempt to explain, in a fatisfactory manner, the caufes which produce thofe effects, almoft impoffible. But this should not deter us from purfuing thete enquiries; for they not only often lead to discoveries of the utmost importance to fociety, but it is by accumulating as many particular inftances as poffible, that we can ever hope to discover the ultimate phyfical caufe of fuch various effects, and the laws by which it acts in particular circumstances.

The paper before us, which contains many ufeful obfervations on chemical folutions and precipitations, is conducted in the following manner: the author lays down his remarks by way of propofitions, and after each, mentions the facts or experiments which gave occafion to the remark, or which confirm or illuftrate the propofition. But as an extract from it, without inferting the experiments (which would extend this article too far) would be of little ufe, we must refer the reader to the paper itself.

XI. Experiments on neutral falts, compounded of different acid liquors, and alcaline falts, fixed and volatile. By the fame. This paper is a fupplement to the former, the author, when that paper was read, not having finished the neceflary expe

riments,