can do. As far as I could judge by the eye, it is from forty to fifty feet in height. An open circular lanthorn of ten Corinthian pillars, surmounted by a conical roof of stone, and containing two standing figures, rests on a square base, presenting an open arch on each side, which is in its turn supported by a solid pedestal, exhibiting on each of its four sides a bas relief corresponding to the respective arch. There is great spirit and fine grouping in the bas reliefs, which represent battles of cavalry and infantry. The standing figures, before mentioned, to whose honour the mausoleum may be supposed to have been erected, are in the civil garb and there is an ease and repose in their attitudes, corresponding with the grave, calm expression of the heads, of which necessary appendage the merciless French Itineraire has guillotined them without warrant. The colour of the freestone of which it is built is as fresh as that of the castle of Tarascon. The building is constructed with a thorough knowledge of what the human eye requires, tapering and becoming more light towards its conical top. It is also of size sufficient for all purposes of effect, though not too large for a private monument. The situation in which these relics stand is sufficient to add beauty to objects of less merit. They are placed, as I mentioned, on a cultivated rising ground, at the foot of the wild gray rocks which ran parallel to the former day's route, and which assume from this spot a more castellated appearance than when viewed from the road. On the other side a fine and boundless view opens into the great plain of Avignon and the Rhone, almost perplexing to the eye by its variety and number of objects: in which we distinguished Avignon itself, and Mont Ventou many leagues behind it, rising in height apparently undiminished, with light hazy clouds sailing along its middle, and backed by the wild Dauphiné mountains near Château Grignan. We could also distinguish Beaucaire, Tarascon, and a large part of the former day's route, to the extreme left; and the right opened into various vistas of the hilly country which we had to cross in our road to Marseilles. The whole scene was lighted up and perfumed by the effects of the shower of rain which had fallen in the night, and without which a summer-landscape in this country is a dusty mass oppressive to the eyes. The thyme and lavender on which we sat, and the mulberries and standard-peaches which shaded us, seemed, as well as the vineyards, to be actually growing; and the catching lights were thrown in such a manner, as to make every distant object successively distinct. After a couple of hours' survey, we took leave of the ancient Glanum Livii, convinced that we had as yet seen nothing more perfect in its way than their tout ensemble, when combined with the surrounding scenery.

S. The etchings are the slightest and most unsatisfactory em bellishments that we have lately seen prefixed to any book: but a separate publication of more finished drawings and engravings, in illustration of this Itinerary, is advertized at the end as in preparation. The plates are to be thirty in number, executed in the line-manner by Messrs. Cook and Mr. Allen.

ART.

ART. VII. Philosophical Transactions of the Royal Society of London, for the Year 1822, Part I. 29 Plates. 2. sewed. Nicol and Son.

4to.

ASTRONOMY, PHILOSOPHY, &c.

Pp. 266., and

THE Bakerian Lecture. An Account of Experiments to determine the Amount of the Dip of the Magnetic Needle in London, in August, 1821; with Remarks on the Instruments which are usually employed in such Determinations. By Captain Sabine, of the Artillery. The great attention which, in late years, has been paid to the science of magnetism, and the near approach that has now been made towards a complete developement of the laws of this hitherto mysterious power, render it important that we should possess more accurate data than we have yet had of the several changes to which the inclination and declination of the magnetic needle are subject; for which purpose more accurate instruments, and better means of observation, than those at present employed, are indispensably necessary. The horizontal needle, when carefully constructed, is perhaps as perfect as we can desire, the only impediment to its correct action being the friction on the point of support; which, however, by means of a carefully polished agate cap, and a fine steel point, is so far reduced (the needle itself being made as light as it can be consistently with its length) that it will during a succession of trials return to the same point of bearing with considerable precision, and thus indicate with sufficient certainty the plane of the magnetic meridian.

Besides the accurate determination of the plane of the mag netic meridian, however, we have constantly occasion to know also the natural line of magnetic direction; which, it is well ascertained, is at this time in London inclined to the plane of the horizon at an angle exceeding 70°, and which angle increases, as we proceed northwards, till the natural direction of the needle is vertical: while in proceeding southwards it decreases till the needle becomes horizontal, then dips again in the opposite direction, viz. to the south, and ultimately again becomes vertical. The instrument by which observations of this kind are made is called a dipping needle; and it has even been known to be exceedingly uncertain in its action, notwithstanding that it has engaged the attention of many eminent artists: a succession of observations, even with the best instruments, being liable to differences of 10′ or 20'. Consequently, some uncertainty has always attended this kind of results and determination.

The object of the present paper is to explain the construction of a dipping needle on a somewhat different principle, by 1 which

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which the dip itself is not actually exhibited, but is to be computed from the observed results. Capt. S. observes:

The needle is a parallelopipedon of eleven inches and a half in length, four-tenths in breadth, and one-twentieth in thickness; the ends are rounded; and a line marked on the face of the needle, passing through the centre to the extremities, answers the purpose of an index.

"The cylindrical axis on which the needle revolves is of bell metal, terminated, where it rests on the agate planes, by cylinders of less diameter; the finer these terminations are made, so long as they do not bend with the weight of the needle, the more accurate will be the oscillations; small grooves in the thicker part of the axis receive the Y's, which raise and lower the needle on its supports, and ensure that the same parts of the axis rest in each observation on the planes.

A small brass sphere traverses on a steel screw, inserted in the lower edge of the needle, as nearly as possible in the perpendicular to the index-line passing through the axis of motion; by this mechanism, the centre of gravity of the needle, with the screw and sphere, may be made to fall more or less below the axis of motion, according as the sphere is screwed nearer to or more distant from the needle, and according as spheres of greater or less diameter are employed.

The object proposed in thus separating the centres of motion and gravity, is to give to the needle a force arising from its own weight to assist that of magnetism in overcoming the inequalities of the axis, and thus to cause the needle to return, after oscillation, with more certainty to the same point of the divided limb, than it would do were the centres strictly coincident.

The centres of motion and of gravity not coinciding, the position which the needle assumes, when placed in the magnetic meridian, is not that of the dip: but the dip is deducible, by an easy calculation, from observations made with such a needle, according to the following directions.

'If the needle has been carefully made, and the screw inserted truly as described, the centres of motion and of gravity will be disposed as in the lever of a balance, when a right line joining them will be a perpendicular to the horizontal passing through the extremities (or to the index-line); this condition is not indeed a necessary one, but it is desirable to be accomplished, because it shortens the observations, as well as the calculation, from whence the dip is deduced; its fulfilment may be ascertained with great precision by placing the needle on the agate planes before magnetism is imparted to it, and observing whether it returns to a -horizontal direction, after oscillation in each position of the axis; if it does not, it may be made to do so at this time with no great trouble.'

When a needle is accurately adjusted in the manner above described, two observations are sufficient for determining the dip; the one being made with the face of the instrument to

the

the east, and the other with the face to the west: the mean of the cotangents of the angles thus obtained being equal to the cotangent of the dip.-The needle in this case owes its direction to two forces; viz. gravity acting on the balanceweight in a vertical line, and the powers of magnetism in the natural direction of the dip. Let W denote the power of the former, and M that of the latter; let also F be the angle observed when the weight is below the axis, and when it is above, as shewn by the two observations; ard let d be the angle of the dip sought. Then, from the obvious principles of mechanics,

as stated in the proposition.

Another method of determining the dip, from approximate observations on it, is by counting the number of vibrations, which such a needle makes when in the plane of the meridian, and when perpendicular to that plane; and a third, by counting the vibrations in the meridian in both cases, but in one instance using the needle in its dipping position, and then int a horizontal position like the common compass. The former method was suggested by Laplace, and the latter by Captain Sabine himself; the needle and the mode of adjustment first described being due to Professor Meyer.

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Captain Sabine used all the three methods, and the mean results are extremely satisfactory, viz.

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By 10 experiments with Meyer's needle, dip
By Laplace's method,

By Captain Sabine's mode,

Mean

We may therefore rely on this as exhibiting very nearly the actual dip in London for August, 1821; and a similar course of accurate experiments, made in different parts of the globe, and at different periods, could not fail to furnish such data as would in all probability establish the theory of terrestrial magnetism, on a basis nearly as certain as that of any other branch of natural philosophy. T 4

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On the Concentric Adjustment of a Triple Object-glass. By William Hyde Wollaston, M. D.-This article depends so much on the illustrations furnished by figures and diagrams, that we cannot attempt to explain its object without them.

Extract of a Letter from Captain Basil Hall, R. N., to Dr. Wollaston, containing Observations of a Comet seen at Valparaiso. These observations, which were made from April 8. 1821, to the 3d of May, and were thirteen in number, were taken by Captain Hall, assisted by Lieut. William Robertson, and Mr. Henry Foster, midshipman, a very ingenious officer, and a most excellent and accurate observer; whose merits and long service in his present rank deserve the early atten>tion of those to whom promotion belongs. The comet, we are told, was always so near the horizon before it became visible, that on no evening, during the whole month in which it was observed, could its right ascension and declination be measured more than once:

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On its first appearance, the comet was of a dull white colour: the tail seemed to be split, or to have a dark streak between its sides. On the second evening, the tail subtended an angle of 79, reaching to Ceti: the northern part of the tail was the longest. On the third, the appearance was much the same. It was hid till the seventh by clouds: the tail then appeared shorter, and the nucleus less bright; changes which at the time were ascribed to the interference of the moon's light; but which, I think, must have arisen from the increased distance of the comet. The tail at first was nearly at right angles to the horizon, but at each succeeding night it inclined more to the south. The time of its appearance was always very short, and that time was generally occupied with the adjustment of the micrometer, so that I was not enabled to draw it so frequently as I could have wished; but the few accompanying sketches will give some idea of its appearance.

Elements of Captain Hall's Comet. By J. Brinkley, D.D. Andrew's Professor of Astronomy in the University of Dublin.

At the time when Dr. Brinkley first laid this paper before the Royal Society, he was not aware that the comet had been observed in Europe, and his elements of it were drawn wholly from the observations furnished by Captain Hall. From them he deduced that the inclination of its orbit was 106° 44', and its perihelion distance .093; and from these circumstances he was led to conjecture that it might be the same comet which was visible in 1593, and had been computed by Lacaille. It appears, however, that the comet had been seen by several astronomers in Europe before it passed its perihelion; and that the above elements required correction by comparison with these European results. Dr. Brinkley remarks in a note: