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ART. I. Signor MONTICELLI'S Report to the Royal Academy of Sciences at Naples, upon the Eruption of Vesuvius in December 1817.

THIS eruption of Mount Vesuvius began on the 22nd, and

terminated on the 26th of December last. On the 23rd I was at Resina, and on the 24th at Torre del' Annunciata, so that I had an opportunity of observing the two currents of Java, one of which ran towards the plain of Pedimentina, the other towards Mauro. On the 24th, I remarked that the small conical hillock which stood near the centre of the edge of the crater had disappeared; it seemed swallowed up by the same. ignivomous aperture which raised it in 1816. The other smaller hillock upon the western ridge of the crater had also fallen in, and was swallowed up by a very large rent upon that side of the Volcano.-Instead of these hillocks, I found the recent lava curiously disposed in the manner of a wall, fortifying, as it were, the antient crater upon the east and west sides; convex, and very irregular upon the north and south. Of this wall some parts are quite even and regular, looking exactly like our terraces; the whole was extremely hot, and apparently incandescent in the interior, as seen through some of the holes and fissures. I have little doubt that parts of these walls were hollow, not only from this appearance, but from the sound occasioned by throwing a large stone upon any P


part of them. Upon the south, all former appearances are destroyed, and there has been produced a very gently inclined plain, covered with fine sand; indeed it would have been impossible here to have recognized the former edge of the crater, were it not for two large blocks of stone which were thrown up in the eruption of 1812, and which, though much changed by the action of two small fumarolee underneath them, which have burned since the year 1815, still serve as landmarks. This plain is often traversed by long fissures more or less perpendicular, running east and west.

On the second of March we counted round the crater fourteen apertures, most of which were still smoking; one of them was circular, and about two feet in diameter; it was perfectly quiet, and appeared of an unfathomable depth. The largest of them is on the northern side of the crater, at a little distance from the great fissure which rent the cone asunder during the eruption of 1813, and which has been intirely obliterated, or at least covered by the late formation of lava. Upon the north-east side, a little above the sandy plain, is the new crater, which poured forth the lava that cut the cone of the volcano, and took the direction of Mauro. This lava spread round the antient Somma, and upon the east side of that mountain descended through a wood, and passing before a house belonging to the Prince of Ottaiano, reached to within a very short distance of the principal street of Mauro. On the 26th of December, while we were observing the progress of the torrent, from a small wood of oaks near the Prince's Casino, we were suddenly surprised and alarmed by the motion of the ground we were standing upon, and immediately afterwards, three small jets of flame made their appearance at a few feet only from us; we therefore hurried away to a place of safety, expecting a repetition of the same phænomenon, but we only observed jets of smoke here and there in the wood.

Whilst observing Vesuvius on the 24th of December, I remarked lava flowing from five apertures, which augmented the current that formerly issued from the south side of the cone previous to the destruction of Torre del Greco, and in which

were small apertures emitting flame, and rapidly appearing and disappearing in succession. The light was very intense and splendid.

On the north of the great fissure of the crater above alluded to, the recent lava assumed the aspect of basaltic columns.

On the 27th of December, a cavern near Mauro was covered with a white incrustation of salt, sublimed from below; its quantity was so considerable, that 50 or 60 people made a profitable occupation of collecting it; for this purpose they either broke the stones, or scraped off the saline matter, and replaced them in their former situations, and a day or two afterwards they became again covered as before. We often saw the deposition of this sublimate, which I am induced to believe required the presence of air for its formation, for it only existed near the surface, or in cavities open to the access of atmospheric air. The same observation applies to the beautiful specimens of sublimed oxide of iron (fer oligiste). Various other sublimates were deposited upon the lava, but in much smaller quantity; their colours were chiefly yellow, red, and green; they were most abundant near the large crater; the yellow and red were deliquescent; but the yellow and green permanent. The smell of muriatic acid, though frequently perceived near the large burning orifice of the mountain, was never observed in the lava of Mauro.

The sand ejected during this eruption was of two kinds; one red and in large grains, found upon the west ridge of the mountain; the other of a colour approaching to violet, and much finer.

On the 25th the air was dark; there was not a breath of wind; but the sea on the coast was extremely agitated. In the evening there was a hail storm accompanied by red sand.

ART. II. Instructions for the Adjustments and Use of the Instruments intended for the Northern Expeditions.*

Captain Kater's Directions for the use of the Instruments executed under his Superintendance.


NE of the many objects of scientific research which present themselves on the present occasion, is the length of the pendulum vibrating seconds in a high northern latitude; and from the excellence of the instruments provided, we may con. fidently hope for results on this and on various other subjects, far more satisfactory than any that have yet been obtained.

A Clock is sent out with each Expedition, the pendulum of which, cast in one solid mass, vibrates on a blunt knife edge resting in longitudinal sections, of hollow cylinders of agate. The points to be determined are, the number of vibrations made by the pendulum of this clock in a certain known interval of time; the arc in which the vibrations are performed; the temperature; the height of the barometer; the latitude and longitude of the place of observation; and (if practicable) its elevation above the level of the sea.

A Transit accompanies each clock, the adjustments of which


To place the vertical wire perpendicular to the horizon; and The line of collimation at right angles to the axis. The level requires no correction, it having been permanently adjusted by the maker.

Slide the eye piece in or out till the wires are seen distinctly. Direct the telescope to some distant and well defined small object, and turn the milled head which is on the side of the transit till the object is seen with perfect distinctness. the level on the axis, and bring the bubble to the middle by


These Instructions were printed by desire of the Council of the Royal Society, and copies were distributed among the proper persons employed on the above occasion.

the screw which elevates or depresses one of the Ys. The axis of the transit will then be parallel to the horizon.

Having brought the object to the central vertical wire by means of the screws which act horizontally on one of the Ys, observe whether the same part of the object is covered by the wire whilst the telescope is elevated or depressed, and if not, correct half the apparent deviation by turning round the cell which contains the wires.

The vertical wire covering some well defined distant object, take the instrument out of the Ys, and carefully invert it, when, if the wire no longer covers the same part of the object, correct half the error by means of the screws which act horizontally upon the wires, unscrewing the one and screwing the other; and the remaining half by the screws which act horizontally on one of the Ys. Repeat this operation till the vertical wire covers the same part of the object in both positions of the telescope; the line of collimation will then be perpendicular to the axis.

These adjustments once made will seldom vary.

Of the Observations necessary to obtain the Number of Vibrations made by the Pendulum of the Clock during a certain interval.

Screw the triangular support of the clock very firmly together, and having taken off the head, fix the clock-case to the support by the screws for that purpose. Bring the bubble of the level which is near the agates, to the middle by means of the screw which acts on the piece projecting behind from the triangular support, taking particular care that the three legs. of the support rest on a very firm foundation, as on the stability of this will depend in a great measure the accuracy of the results. Next, see that the fork which is connected by a joint to the crutch is perpendicular, as it would otherwise be liable to injury.

The pendulum is now to be taken from its case, and carefully passed up through the aperture which is made to admit it at the back of the clock, and gently lodged on the brass near the agate. In this part of the operation the most minute

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