the electricity produced by the friction of the mercury against the glass was discharged through the vapour with sparks so bright as to be visible in day-light.

When the minutest quantity of rare air was introduced into the mercurial vacuum, the colour of the electrical light changed from green to sea-green, and by increasing the quantity, to blue and purple: in low temperatures, the vacuum became a much better conductor.

The results were precisely the same, when a difficultly fusible amalgam of mercury and tin was used as when pure mercury was employed, and in a vacuum above fused tin, the same phenomena were also exhibited. Electrical and magnetic repulsions and attractions took place as they would have done in air. It was ascertained" that the feebleness of the light in the more perfect vacuum was not owing merely to a smaller quantity of electricity passing through it; for the same discharge which produced a faint green light in the upper part of the tube, produced a bright purple light in the lower part, and a strong spark in the atmosphere.'

Pure olive oil and chloride of antimony were severally tried in the vacuum, and it was found " that the light produced by the electricity passing through the vapour of the chloride was much more brilliant than that produced by it in passing through the vapour of the oil; and in the last it was more brilliant than in the vapour of mercury at common temperatures: the lights were of different colours, being of a pure white in the vapour of the chloride, and of a red, inclined to purple, in that of the oil; and in both cases permanent elastic fluid was produced by its transmission."

Sir H. Davy observes, "The law of the diminution of the density of vapours by diminution of temperature has not been accurately ascertained; but I have no doubt, from the experiments of Mr. Dalton, and some I have made myself, that it is represented by a geometrical progression; the decrements of temperature being in arithmetical progression; and in three pure fluids that I operated upon (water, chloride of phosphorus, and sulphuret of carbon), the ratio seemed nearly uniform for the same number of degrees below the boiling point; and (taking intervals of 20 degrees of temperature) 369416. Upon this datum, Sir Humphry was obliged to Mr. Babbage for the calculation, that considering the elastic force of vapour of water at 52° to be equal to raise by its pressure about 45 of an inch of mercury; the relative strengths of vapour will be, reckoning the boiling points all from 52°, for mercury at 600°, 000015615; for oil at 540°, 0016819; for chloride of antimony at 340°, 01692; and for tin at 5000°, 37015 preceded by 48 zeros. These numbers are given to show how minute the quantity of matter must be in vapours where its effects are distinct upon electrical phe

nomena, especially with respect to mercury artificially cooled, and in vapours from comparatively fixed substances.

The diminution of the temperature of the torricellian vacuum, to as low as about 20°, appeared to diminish its power of transmitting electricity; but between 20° above and 20° below zero, the lowest temperature that could be produced by pounded ice and muriate of lime, the power seemed stationary, and nearly the same as that of the vacuum above tin. "At all temperatures below 200°, the mercurial vacuum was a much worse conductor than highly rarefied air."

"It is evident from these general results," the author continues, "that the light (and probably the heat) generated in electrical discharges depends principally on some properties or substances belonging to the ponderable matter through which it passes; but they prove likewise that space, where there is no appreciable quantity of this matter, is capable of exhibiting electrical phenomena; and, under this point of view, they are favourable to the idea of the phenomena of electricity being produced by a highly subtile fluid or fluids, of which the particles are repulsive, with respect to each other, and attractive of the particles of other matter."

To this succeed some further observations on the nature of electrical phenomena and their relations, which are terminated by a remark, that the luminous appearances of electrical action must be considered as secondary, while the uniform exertions of attractions and repulsions, under all circumstances, point them out as primary and invariable phenomena of electricity. This valuable communication is then concluded by the important statement, that recently distilled mercury which has been afterwards boiled and cooled in the atmosphere, and which presents a perfectly smooth surface in a barometer tube, emits air when strongly heated in vacuo; and an instance is given in which the metal was observed to imbibe air.

IX. Croonian Lecture.-On the Anatomical Structure of the Eye; illustrated by Microscopical Drawings, executed by F. Bauer, Esq. By Sir Everard Home, Bart. VPRS.

The contents of this lecture would be unintelligible without the engravings, in which the structure of the visual organ is minutely and beautifully delineated.

X. A Letter from John Pond, Esq. Astronomer Royal, to Sir H. Davy, Bart, PRS. relative to a Derangement in the Mural Circle at the Royal Observatory.

As the amount of error occasioned by this derangement has been stated by Mr. Pond in the Preface to the Greenwich Observations for 1820; and as the derangement itself has been rectified by Mr. Troughton, it is unnecessary to abridge this letter. XI. On the Finite Extent of the Atmosphere. By Dr. Wollaston, VPRS.

This admirable paper is printed entire in the last number of the Annals.*

XII. On the Expansion in a Series of the Attraction of a Spheroid. By James Ivory, MA. FRS.

This elaborate paper does not admit of profitable abridgment. The author suggests, in the conclusion, that Laplace's theory of the figure of the planets "will probably be found to hinge on this proposition, that a spheroid, whether homogeneous or heterogeneous, cannot be in equilibrium by means of a rotatory motion about an axis, and the joint effect of the attraction of its own particles, and of the other bodies in the system, unless its radius be a function of three rectangular co-ordinates."

XIII. On the late extraordinary Depression of the Barometer. By Luke Howard, Esq. FRS.

Mr. Howard has already stated the amount of this remarkable depression in the Annals for February last, p. 160. Among other observations respecting it in this paper, which is illustrated by a plate of the autographic curve of the barometric variations, are the following: "It will be seen that this great depression was preceded by abrupt changes, fluctuating for 30 days, chiefly between 29.5 and 30 inches, during a continuance of stormy weather; and that the depression itself was 14 or 15 days in progress from the point of 30 inches, to that from which it finally rose in three days. The rain for these two months is 10-10 inches, a quantity without precedent in the same space of time at London; that is to say, without one on record."

XIV. On the anomalous Magnetic Action of Hot Iron between the White and Blood-red Heat. By Peter Barlow, Esq. of the Royal Military Academy. (Communicated by Major Thomas Colby, of the Royal Engineers, FRS.)

Certain theoretical results relative to the magnetic action of iron, obtained by Mr. Bonnycastle, induced Mr. Barlow to ascertain the relative attraction which different species of iron and steel had for the magnet. The following are the results of his experiments for that purpose, assuming the tangents of the angles formed by the deviation of the needle when acted upon by equal-sized bars of the several descriptions of metal placed

* The following investigation of the same subject by a method entirely different from Dr. Wollaston's, has been pointed out in a contemporary Journal:

"The highest portions of the atmosphere, which is carried round in 23 hours and 56 minutes, by the rotation of the earth about its axis, would be projected into space, if their centrifugal force at that distance were not less than their gravitation towards the centre. But the centrifugal force is directly as the distance, while the power of gravity is as its square. Consequently when the centrifugal force at the distance of 6.6 radii of the earth is augmented as many times, the corresponding gravitation is diminished by its square, or 43.7 times, their relative proportion being thus changed to 289. Now the centrifugal force being only the 289th part of gravity at the surface of the equator, it will, therefore, just balance this power at the distance of 6.6 radii from the centre, or at the elevation of 22,200 miles."(Leslie on Meteorology, Supplement to the Encyclopædia Britannica, vol. v. p. 325.)

in the direction of the dip, as the measure of the disturbing

It being obvious from these experiments that the intensity of the magnetic power was in proportion to the softness of the metal, the author became desirous of determining the magnetic relations of each variety when rendered perfectly soft by being heated in a furnace. With this view, bars of each substance, of equal size, were rendered white-hot, when it was found that their powers, as was anticipated, agreed nearly with each other.

"While carrying on these experiments," says Mr. Barlow, "it had been observed, both by Mr. Bonnycastle and myself, that between the white heat of the metal, when all magnetic action was lost, and the blood-red heat, at which it was the strongest, there was an intermediate state in which the iron attracted the needle the contrary way to what it did when it was cold, viz. if the bar and compass were so situated that the north end of the needle was drawn towards it when cold, the south end was attracted during the interval above alluded to, or while the iron was passing through the shades of colours, denoted by the workman the bright-red and red heat."

After noticing the results hitherto obtained relative to the magnetic action of heated iron, and showing how the contradictory statements on the subject may be reconciled, by supposing that the observations were made with iron at different degrees of heat, Mr. Barlow proceeds to describe some preliminary experiments I on the anomalous attraction of heated iron which takes place while the metal retains the bright-red and red heat;" and he then gives a table containing the results of a regular series of experiments on the subject, amounting in number to 38. These experiments were all made with bars of cast and of malleable iron inclined in the direction of the dipping needle, and, what is somewhat unappropriately called, the negative attraction was found to be the greatest where the natural attraction was the least; that is, opposite the middle of the bar, or in the plane of no attraction. With the bar inclined at right angles to its former position, the results were not so strongly marked as in the experiments just mentioned.

Mr. Barlow shows from experiment, that these singular effects

on the compass needle were not caused by the heat itself, independently of the iron, and modestly terminates his communication with the following remarks:

"The only probable explanation which I can offer by way of accounting for these anomalies is, that the iron cooling faster towards its extremities than towards its centre, a part of the bar will become magnetic before the other part, and thereby cause a different species of attraction; but I must acknowledge that this will not satisfactorily explain all the observed phenomena. The results, however, are stated precisely as they were noted down during the experiments, and others more competent than myself will probably be able to deduce the theory of them."

XV. Observations for ascertaining the Length of the Pendulum at Madras in the East Indies, Lat. 13° 4' 9.1" N, with the Conclusions drawn from the Same. By John Goldingham, Esq. FRS.

The pendulum and accompanying apparatus, with which these observations were made, were precisely the same, in all their parts, as those used by Capt. Kater at the different stations in the trigonometrical survey of England, and which have been described in the Philosophical Transactions for 1819. The results obtained with them were as follows:

By the first series of observations, the length of the seconds pendulum at Madras was 39-026323087 inches; by the second series, 39-026280447 inches.

"The mean of both is 39-026302 inches, being, according to Sir George Shuckburgh's scale, the length of the seconds pendulum by these experiments at Madras in lat. 13° 4′ 9-1" N. at the level of the sea, in vacuo, and at a temperature of 70° of Fahrenheit."

"Then comparing this length with 39-142213 inches, the length in latitude 51° 31′ 8.4" N. as before stated, the diminution of gravity from the pole to the equator will be ·0052894, and the ellipticity nearly."

I

297.56

XVI. Account of an Assemblage of Fossil Teeth and Bones discovered in a Cave at Kirkdale, in Yorkshire. By the Rev. W. Buckland, Professor of Geology in the University of Oxford, &c. &e.

This highly interesting paper has already appeared in the Annals.

XVII. Communication of a curious Appearance lately observed upon the Moon. By the Rev. Fearon Failows. (In a Letter addressed to John Barrow, Esq. FRS.)

Mr. Fallows, who is the astronomer at the new observatory founded at the Cape of Good Hope, observed on Nov. 28, 1821, a whitish spot on the dark part of the moon's limb, sufficiently luminous to be seen with the naked eye, and which now and then seemed to flash with considerable lustre. When examined