In November, 1796, the magnetic variation at St. Helena was 15° 48′ 34′′ weft, and the daily variation 3′35′′. The magnetic needle was obferved to be stationary from about fix o'clock in the evening, till fix in the morning, when it began to move; and the weft variation increafed, and came to its maximum at about eight o'clock.

Mr. M. obferved, both at Bencoolen and at St. Helena, a vertical motion in the magnetic needle, fo that he found it fometimes below, and at other times above its ufual fituation. The difference amounted to fix or eight minutes. He is therefore of opinion, that the dip or inclination of the needle is likewife fubject to a diurnal variation; to which opinion we have great reafon to affent.

XIX. On the Corundum Stone from Afia. By the Right Hon. Charles Greville, F. R. S.

The mineral, which forms the fubject of this long and elaborate paper, has been fome years known in Europe under the name of Adamantine Spar. In Dr. Woodward's catalogue of foreign foffils, published about the year 1719, it is called Nella Corivindum, or Tella Convindum, or Nella Corivendum.

From a very fatisfactory account received by the author of this paper, from Sir Charles Oakley, Bart. then Governor of Madras, it appears that this mineral is found in a particular mine, near a place called Caranel, on the fouth fide the Cavery; the distance of the pits from the river is upwards of four miles. It is dug by a particular caft of people, who fell it to the glass venders, who again fell it throughout the whole country, where it is ufed, when pulverized, for grinding and polifhing chryftals, and all gems, diamonds excepted.

From China, by the favour of Captain Cumming, Mr. Greville received fpecimens of, and information relative to, the fame mineral, which has long been in ufe in that country, for the fame purposes of grinding and polishing.

Omitting the various mineralogical obfervations, with which this paper is replete, we fhall extract the following more interefting particulars, which relate to the Corundum only.

The fpecific gravity of this mineral varies from 2,768 to

4,18.

Its colour is generally brownifh, or inclining to purple. Its hardness exceeds that of most hard ftones, fo that when pulverized, it will grind or cut cryftals, agates, &c.

Mr. Klaproth, in his first analysis of the Corundum, imagined he had detected in it a new genus of earth; but his sub-. fequent analyfis proved, that this fuppofed new earth was no

thing more than argillaceous earth. He found that 100 parts of the India Corundum confift of 89,5 of argillaceous earth, 5,5 of filiceous earth, and 1,25 of oxide of iron. And in 100 parts of Corundum, from China, he found 84 of argillaceous earth, 6,5 of filiceous earth, and 7,5 of oxide of iron.

The moft ufual form of the Corundum is a regular hexædral prifm, and, in general, the furface of the cryftal is rough, with little luftre; but it has likewife been found crystallized under a variety of other forms, which forms are particularly defcribed in the paper, and are delineated in a plate.

XX. An Inquiry concerning the chemical Properties that have been attributed to Light. By Benjamin Count of Rumford, F. R. S. M. R. I. A.

Count Rumford has, in a former publication, expreffed his doubt of the existence of those properties in light, which have been attributed to it, and he is of opinion,

"That all thofe visible changes produced in bodies by exposure to the action of the fun's rays, are effected, not by any chemical combination of the matter of light with fuch bodies, but merely by the heat which is generated, or excited, by the light that is abforbed by them."

He now gives an account of fuch experiments as he has lately made, with a view to determine this important question; which experiments, though upon the whole not very conclufive, are nevertheless deferving of notice.

Moft of thofe experiments were made with a folution of gold in aqua regia, evaporated to drynefs, and then diffolved in diftilled water; as alfo with a fimilar aqueous folution of nitrate of filver: and the Count liberally acknowledges his being induced to engage in those investigations, from the perufal of Mrs. Fulhame's ingenious Effay on Combustion.

Having wetted pieces of ribband, of fine linen, of cotton, &c. with one or the other of the above-mentioned, folutions, he held them fingly over the flame of a wax candle, and, in a few seconds of time, a ftain appeared in that part of the ribband which ftood just over the flame. The tinge was indelible; that produced from the folution of gold was of a deep purple, inclining to crimson; that from the folution of filver was of a very dark orange colour, or rather of a yellowish brown.

When ribbands, thus moistened, were held not over, but by the fide of the flame, no fpot appeared, excepting indeed when they were brought exceedingly near.

Pieces of ribbands, and likewife parcels of magnefia alba, moistened with the folution of gold, were expofed to the fun, both covered and uncovered, either moist, or after having been

ried;

dried; and it was found, that the uncovered pieces acquired in a few hours time the deep purple colour; whereas, those that had been under cover, and had confequently remained in darknefs, fhowed no fuch colour; alfo, that the pieces which had been expofed in a dry state, did not affume the coloured ap pearance near fo foon, or fo deep, as thofe which had been expofed in a moift ftate.

Count R. likewife revived gold from the above-mentioned folution, by only expofing it, together with pieces of charcoal, in a glass tube, to the direct rays of a bright fun. But it is remarkable, that when a fimilar preparation was exposed not to the light, but only to a degree of heat equal to that of the fun's rays, the gold was likewife completely revived. Similar experiments, made with the folution of filver, were attended with fimilar refults.

This author alfo obtained a revivification of gold and filver from their folutions, by mixing them with effential oils, and expofing the mixtures to the heat of boiling water; but the experiment did not fucceed when fpirit of wine was used inftead of effential oils.

XXI. Experiments to determine the Density of the Earth. By Henry Cavendish, Efq. F. R. S. and A. S.

A method formerly contrived by the Rev. John Michell, has lately been put in practice by the ingenious author of this excellent paper.

"The apparatus is very fimple; it confifts of a wooden arm, fix feet long, made fo as to unite great ftrength with little weight. This arm is fufpended in an horizontal pofition, by a flender wire 40 inches long, and to each extremity is hung a leaden ball, about two inches in diameter; and the whole is inclosed in a narrow wooden cafe, to defend it from the wind.

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"As no more force is required to make this arm turn round on its centre, than what is neceffary to twift the fufpending wire, it is plain, that if the wire is fufficiently flender, the moft minute force, fuch as the attraction of a leaden weight, a few inches in diameter, will be fufficient to draw the arm fenfibly afide. The weights which Mr. Michell intended to ufe were eight inches in diameter. One of thefe weights was to be placed on one fide the cafe, oppofite to one of the balls, and as near it as could conveniently be done, and the other on the other fide, oppofite to the other ball, fo that the attraction of both thefe weights would confpire in drawing the arm afide; and, when its polition, as affected by thefe weights, was afcertained, the weights were, to be removed to the other fide of the cafe, fo as to draw the arm the contrary way, and the pofition of the arm was to be again determined; and, confequently, half the difference of thefe pofitions would show how much the arm was drawn afide by the attraction of the weights,

"In order to determine from hence the density of the earth, it is necessary to ascertain what force is required to draw the arm afide through a given fpace. This Mr. Michell intended to do by putting the arm in motion, and obferving the time of its vibrations, from which it may easily be computed."

Thefe paragraphs are followed by the particular description of the apparatus, which is delineated in two plates, and of the various objections, difficulties, &c. which Mr. Cavendish's knowledge and addrefs naturally detected, and in great meafure obviated. The experiments are then clearly and particularly ftated, together with the calculations that are derived from them; the conclufion of which we shall fubjoin in the author's own words.

"By a mean of the experiments made with the wire firft used, the denfity of the earth comes out 5,48 times greater than that of water; and by a mean of those made with the latter wire, it comes out the fame; and the extreme difference of the refults of the 23 obfervations made with this wire, is only 0,75; fo that the extreme refults do not differ from the mean by more than 0,38, or of the whole, and therefore the denfity fhould feem to be determined hereby, to great exactness. It, indeed, may be objected, that as the refult appears to be influenced by the current of air, or some other caufe, the laws of which we are not well acquainted with, this caufe may perhaps act always, or com. monly, in the fame direction, and thereby make a confiderable error in the refult. But yet, as the experiments were tried in various weathers, and with confiderable variety in the difference of temperature of the weights and air, and with the arm refting at different diftances from the fides of the cafe, it feems very unlikely that this caufe should act fo uniformly in the fame way, as to make the error of the mean refult nearly equal to the difference between this and the extreme; and therefore, it feems very unlikely that the denfity of the earth should differ from 5,48 by fo much as of the whole.

"Another objection, perhaps, may be made to thefe experiments, namely, that it is uncertain whether, in these fmall distances, the force of gravity follows exactly the fame law as in greater diftances. There is no reafon, however, to think that any irregularity of this kind takes place, until the bodies come within the action of what is called the attraction of cohesion, and which feems to extend only to very minute diftances. With a view to fee whether the refult could be affected by this attraction, I made the 9th, 10th, 11th, and 15th experiments, in which the balls were made to rest as close to the fides of the cafe as they could; but there is no difference to be depended on, between the refults under that circumftance, and when the balls are placed in any other part of the cafe.

"According to the experiments made by Dr. Maskelyne, on the attraction of the hill Schehallien, the denfity of the earth is 4 times that of water; which differs rather more from the preceding determination than I fhould have expected. But I forbear entering into any confideration of which determination is most to be depended on, till I have

I have examined more carefully how much the preceding determina tion is affected by irregularities whofe quantity I cannot measure."

XXII. An improved Solution of a Problem in phyfical Aftronomy; by which fwiftly converging Series are obtained, which are useful in computing the Perturbations of the Motions of the Earth, Mars, and Venus, by their mutual Attraction. To which is added, an Appendix, containing an eafy Method of obtaining the Sums of many flowly converging Series which arife in taking the Fluents of Binomial Surds, &c. By the Rev. John Hellins, F. R. S. Vicar of Potter's Pury, in Northamptonshire.

The contents of this valuable article are, a letter to the Rev. Dr. Maskelyne, the problem, and a long appendix; but it is not in our power to give our readers a better idea of the fubject, than is expreffed in the title; unlefs, indeed, we were to tranfcribe the greater part of the paper.

XXIII. Account of a Subftance found in a Clay-Pit; and of the Effect of the Mere of Difs, upon various Subftances immerfed in it. By Mr. Benjamin Wiseman, of Difs in Norfolk. With an Analysis of the Water of the faid Mere. By Charles Hatchett, Efq. F. R. S.

"The fubfance," fays Mr. Wifeman, " I have inclosed, was found near Difs, in a body of clay, from five to eight feet below the furface of the foil. All the pieces I obferved laid nearly in a horizontal direction; and varied in fize, from two or three ounces, to as many pounds. The colour of the fubftance, when taken fresh from the claypit, was like that of chocolate; it cuts eafily, and has the striated appearance of rotten wood. The picces were of no particular form; in general, they were broad and flat, but I do not recollect to have met with a piece that was more than two inches in thickness: it breaks into laminæ, between which are the remains of various kinds of shell. The fpecific gravity of this fubftance, dried in the fhade, is 1,588; it burns freely, giving out a great quantity of smoke, with a strong ful phureous fmell.

"By a chemical analyfis, which I cannot confider as very accurate, one hundred grains appear to contain 41,3 grains of inflammable matter, including a fmall quantity of water; 20 grains of mild calcareous earth; 2 grains of iron; and 36,7 grains of earth that ap pears to be filex.

"The Mere of Difs contains about eight acres, and is of various depths, to 24 feet: from its fituation with refpect to the town, it may naturally be fupposed to contain a vast quantity of mud, as it has received the filth of the streets for ages. In fummer, the water turns green; and the vegetable matter that fwims on its furface, when expofed to the rays of the fun, affords vast quantities of oxygen gas."

Mr.