the alteration of the style in England), and to the meridian of the Royal Observatory at Greenwich, are as follow:

Number c is the period of 437 days, in which the 3 innermost satellites return very nearly to the same situation in respect of one another, and of Jupiter's shadow, in millesimals of a circle; and must be corrected by the equation of number в, under a contrary title. The second satellite has a synodical equation of 16 or 17′ in time, whose revolution is, in this period, to be subtracted, if number c be less than 500; added, if greater. The first and third satellites have also small synodical equations, returning in the same period, that of the first satellite being about 3', of the third about 2′ in time; both to be added, if number c be less than 500; subtracted, if greater. The orbit of the third satellite is manifestly eccentric, as well as that of the 4th. Its apojovium in 1728 was about 10° of Y, and moves forward 35° in 20 years its greatest equation is about 15′ in the satellite's orbit, or 7′ in time. The apojovium of the fourth satellite in 1728, was in 12° 30′ of, and moves forward about 12° in 20 years: its greatest equation is 53′ in the satellite's orbit, or 59' in time.

I found no reason to make any alteration in the semi-durations of the eclipses of the first satellite from Mr. Pound's tables.

The greatest semi-durations of the eclipses of the 2d, 3d, and 4th satellites in the nodes, are 1h 27m, 1h 47m, and 2h 24m* respectively. The nodes of the 2d satellite seem to be at rest in about 50° of and ; but the inclination of its orbit varies from 2° 50′ to 3° 52′': it was least in 1668, greatest in 1715, and seems to have been at its greatest and least once in the intermediate years. I suppose it at the least in 1730. The nodes of the 3d satellite in 1727, were in 161° of and Я, and move forward about 24o in 20 years: the inclination of its orbit in 1695 was 3o, and has been increasing ever since: it seems as if it would get to its maximum about 1765, and would then be about 3° 24′. The nodes of the 4th satellite in 1730 were in 134° of and , and move forward 2o in 12 years: the inclination of its orbit is about 2° 40′, and does not seem to vary above one or two minutes either way. From these elements it will be easy

The semi-durations of the eclipses of the 4th satellite will be about 2′ more at the ascending, and 2' less at the descending node, on account of the eccentricity of its orbit.-Orig.

for any person moderately skilled in such matters to construct tables of the motions of the satellites in the method of Mr. Pound, which may be seen in the latter part of Halley's tables.

XXI. Dissertationem hanc de Zoophytis, Regia Societati Scientiarum Anglia legendam et judicandam præbet Job Baster, M.D., &c. p. 108..

Dr. Baster's notions relative to corals and corallines being now entirely exploded, it is unnecessary to insert the present dissertation. We have therefore only preserved the part relative to the figures represented on the accompanying plate.

Fig. 6, pl. 14, shows a branch of a zoophyte called corallina abietis forma. About this branch, which was kept from Sept. 1758 to February 1759, was collected a kind of bark or concretion of sordes. For at least five months fresh water was supplied once a day, the old being thrown away; and although it had not grown much, yet it emitted small lateral branches in various parts which were all beset with polypes.

Fig. 7 shows the same branch viewed by a lens. The letters of reference are the same in both figures; viz. A, the trunk of the sertularia, which was seated on an oyster-shell; B, some lateial branches, which were here and there produced while he kept the coralline, and which were from the beginning beset with polypes; c, the tip of a branch, recently produced, and perfectly free both from sordes and polypes; D, a larger kind of polypes, being the beginnings of the zoophyte called corallina tubularia; E, a very small species of polype shown in a magnified state in the first part of Dr. B.'s work called Opuscula Subseciva, at tab. 3. f. A, B, C. Fig. 8, the summit c of the two former figures viewed microscopically. A, the place where it has been pulled from the trunk; B, two lateral branches with polypes proceeding from them, as if from cells, and expanding their arms at d; e, cellules, in which the polypes, when their arms are contracted, entirely hide themselves, appearing at that time like white spots.

XXII. Of an Uncommon Phenomenon in Dorsetshire. By John Stephens, M. A. p. 119.

In August 1751, the air, having been for some time remarkably hot and dry, was changed of a sudden by a heavy fall of rain, and a high south-west wind. the cliffs near Charmouth, in the western part of Dorsetshire, presently after this alteration of the atmosphere, began to smoke, and soon after they burned, with a visible though a subtle flame for several days successively; and continued to smoke, and sometimes to burn at intervals, till the approach of winter: nay, ever since that time, especially after any great fall of rain, thunder and lightning, or a high south-west wind (which drives the sea with great violence against the 3 Z

VOL. XI.

cliffs, and beats off large pieces of them) the cliffs continue to smoke, and sometimes to burn with a visible flame; which during the summer-months is frequently observed in the night-time. On examining these cliffs, in the year 1759, Mr. S. discovered a great quantity of pyrites, not in any regular strata, but interspersed in large masses through the earth, and which proved to be martial; of marcasites, which yielded near one tenth part of common sulphur; of cornua ammonis of different sizes, and other shells, but of the bivalve class, which were crusted over, and as it were mineralized with the pyritical matter; of belemnites, also crusted over with the like substance: and the cliffs, for near 2 miles long, and from the surface, to 35 or 40 feet deep, even to the rocks at high water mark, were one bed of a dark coloured loam, strongly charged with bitumen. He found also a dark coloured substance, resembling coal-cinder; some of which being powdered, and washed in distilled rain water, on filtrating the water, and evaporating it slowly to a pellicule, its salts shoot into fine crystals, and appear to be no more than a martial vitriol: one ounce of this cinderlike substance yields one drachm of salt. He gathered up about 100lb. weight of the different kinds of those pyritæ, marcasites, &c. which were laid in a heap, exposed to the air, and every day sprinkled with water: the consequence was, that in about 10 days time they grew hot, soon after caught fire, burned for se veral hours, and fell into dust. Hence therefore it is imagined that these martial and sulphureous fossils, by being exposed to the air and wet, by being agitated by the beating of the sea, and by being electrified as it were by the subtle flame of the lightning, take fire, which is favoured by the bituminous particles contained in the loam, and burn till all their phlogiston is consumed, and their iron or martial earth is dissolved in the acid of sulphur; which constitutes the martial vitriol, found to be near the one-eighth part of this cinder-like matter.

When the cliffs were observed to burn in the night-time, the flame was plainly perceived by a spectator at a distance; but when he drew near to the place seemingly on fire, he could perceive a smoke, but no flame. In the daytime nothing but a smoke was perceived, except the sun shined, when the cliffs appeared at a distance as if they were covered with pieces of glass, which reflected the sun's meridional rays; but on drawing near to the places where these luminous appearances were perceived, they disappeared, and the cliffs seemed to be covered with sinoke, which stunk of a bituminous and sulphureous matter.

Mr. S. had also been an eye witness of the saine kind of flame arising from the Lodes in Cornwall, especially such as contained a great quantity of mundic and martial pyrites. Three times he had seen this flame arise from the earth in the night, and once in the middle of the day. In the night a person standing at a little distance, would imagine that the place was all on fire, and even on drawing near the same he perceives himself surrounded with flame, but is not

hurt; and in 4 or 5 minutes time, he perceives this flame to decrease, and fall into the earth. In the day-time the flame is of a different colour, and not much unlike the flame which arises from a furnace. There are several mines discovered in this county by these mineral fires, where there were no symptoms of such mines before: but it is generally observed that they abound with marcasite and pyrites. These mineral flames, arising from ignited pyrites, are frequently discovered in the bottom of mines and coal-pits; and are often detrimental, and sometimes destructive to the miners; which made the late Dr. Woodward and others imagine that they were vapours arising from an abyss.

From what has been said Mr. S. draws the following conclusions.

1. That all subterraneous fires, even those of Hecla, Vesuvius, and Ætna, together with those observed in the mines and coal-pits, are caused by the heat and fixing of pyrites and marcasites. 2. That the waters of our hot baths derive

their heat from passing over a bed of ignited pyrites. Indeed the solid contents of those waters do evidently prove this assertion, being nothing more than such particles of the pyrites as are soluble in water. 3. That these mineral flames will be more or less subtle, according to the minuteness of the particles of the combustible matter, and the quantity of phlogiston which they contain. 4. That the convulsive motions and tremblings of the earth are caused by the heat of the burning pyrites expanding the air contained in its bowels. This is clearly proved by their causing, immediately after, an eruption of the earth, which generally discharges a dark coloured, cinder-like and frothy matter. And, 5. That those places, where the earth contains the greatest quantity of pyrites and marcasites, will be most liable to these convulsive motions and tremblings, no other natural cause contradictory.

XXIII. Additional Observations on some Plates of White Glass found at Herculaneum, and on the Use of Glass in Windows. By J. Nixon, A. M., F. R. S. p. 123.

In a paper which Mr. N. presented to the Society about 2 years before, he offered his thoughts on some plates of white glass found in the ruins of Herculaneum. He now adds some more observations, with a view partly to explain and support what he then delivered, and partly to communicate such new informations as he has since received relating to the same subject.

Mr. N. observed, on the authorities produced by Mons. Renaudot, that glass plates were not applied for magnifying objects in optical experiments, till the beginning of the 13th century: but on reviewing his dissertation, Mr. N. finds he sinks the antiquity of that usage a century lower than this. Mons. Renaudot

observes that Mabillon mentions a manuscript he saw in an abbey in the diocese of Freisingen, where Ptolemy was represented observing the stars with a tube, like our modern perspective glasses. This manuscript is said to have been written in the beginning of the 13th century. Mabillon does not mention that the tube had glasses; neither indeed was that circumstance easily discoverable. Perhaps such tubes were then used only to preserve and direct the sight, or to render it more distinct, by singling out the particular object looked at, and shutting out all the rays reflected from others, whose proximity might have rendered the image less precise.

As it appears that neither the lapis specularis nor glass was used for windows before Seneca's time; and it cannot be supposed that the Romans, a people of so refined a taste in other instances, would suffer their apartments to be exposed to the free entrance of winds, &c. it may be reasonably asked, what supplied the place of those materials before? To satisfy this inquiry, it is to be observed that several other materials are mentioned by ancient writers, as serving the purpose before us; such as thin hides or skins, like our parchment, mentioned by Philoponus. Pliny likewise informs us that the horns of the urus being cut into thin laminæ, were transparent, and supplied in some measure the use of our lanterns; and we may probably conclude, from the analogy of things, that they served for window-lights also; especially as we meet with windows made of horn (corneum specular) in Tertullian, who wrote within less than 200 years after Pliny. To these may be added the vela, made of hair-cloth, or pieces of hides, which Pitiscus, on the authority of Ulpian, says were in use before the invention of windows of the lapis specularis or glass.

Mr. N. took notice of the natural connection there seemed to subsist between the using of plates of glass for adorning the inside of apartments in ancient times, and the employing them for introducing light into those apartments. This observation has been supported by a letter he received from the Abbate Venuti at Rome, dated December 30, 1759, wherein he informs him that he had lately read in some anecdotes of Cardinal Maximi, That as they were digging among the ruins on mount Cælius in the last century, they found a room belonging to an antique dwelling-house, that had all its sides within ornamented with plates of glass, some of them tinged with various colours, others of their own natural hue, which was dusky, occasioned by the thickness of the mass of which they consisted. There were likewise in the same apartment window-frames composed of marble, and glazed with laminæ of glass.'

Presuming the evidence to be undeniable which Mr. N. has produced in his dissertation, to prove the use of glass in windows to have been as early as the 3d century, if not before, he deems it not unacceptable to the curious in antiquity to observe the slow progress this very commodious invention made towards the