The computations relative to the comet observed by Captain Hall were finished in the middle of October last, and the results immediately sent to London for the purpose of being laid before the Royal Society. The second part of the Transactions for 1821 did not reach me till after the communication had been read at the Royal Society. In that second part, I was much surprised to find the elements of the same comet computed by Mr. Rumker, from the observations made by Dr. Olbers, before the passage through perihelion..

Subsequently, the "Conn. des Tems," for 1824, reached me, which contains Observations made at Paris, and Elements by M. Nicollet; also a notice of the comet having been observed in several places of Europe. It certainly is highly creditable to those observers who discovered, under very difficult circumstances, the comet in its approach to the sun.

By the addition of Captain Hall's observations after the passage through perihelion, we are enabled to obtain very exact elements.

The errors of my Elements, when applied to observations before perihelion, and the errors of Mr. Rumker's and of M. Nicollet's Elements, when applied to Captain Hall's observations after perihelion, are considerable.

Therefore, I have farther corrected my Elements, by using Dr. Olbers' observations of January 30. with those of Captain Hall of April 8. and May 3.

The new elements (C) are perihelion distance,091677.

These circumstances shew the great difficulty of making a correct determination of a comet's orbit; and what liberty we may allow ourselves in comparing the elements of different comets, with the view of establishing their identity. Between January 21. and May 3. this comet described above 300° about the Sun; consequently, (says Dr. B.) as a parabola represents the orbit, with so much exactness, it follows that the period of its revolution must be very considerable.

A Letter from John Pond, Esq., Astronomer Royal, to Sir Humphry Davy, Bart., President of the Royal Society, relative to a Derangement in the Mural Circle at the Royal Observatory.

Certain discrepancies having been detected in the observations made with the new mural circle at the Royal Observatory of Greenwich, some anxiety was manifested to ascertain the cause, but this was very difficult till the error had so far

increased

increased as to render itself obvious. The object of Mr. Pond's letter is, first, to inform astronomers of the fact; and, secondly, to explain the cause, and to report that the defect has been corrected. He observes:

Those who are acquainted with the construction of the Greenwich mural circle are aware, that though the telescope may be applied to every part of the circle, yet, when fixed for observation, the principle of the instrument requires that the tube, especially at its extremities, should be so firmly fixed to the circle as to form one piece with it: to accomplish this, connecting braces are attached at each end of the telescope. It now appears that these braces have, in progress of time, become insecure, owing to the screws which fastened them having given way. The effect of this; will be, to permit the ends to bend from the centre instead of retaining, as they ought to do, an invariable position with respect to the circle. Under these circumstances, when the telescope is directed to the zenith, the position may be considered as free from error; but when the instrument is moved either towards the north or south horizon, should either extremity bend more than the other, an error will take place, and will increase from the zenith towards the horizon, but in what exact proportion, remains to be determined by future observations.

The cause of this error being thus ascertained, Mr. Troughton has applied additional braces to connect the telescope with the circle, sufficiently strong, I should conceive, to prevent the possibility of such an accident for the future.'

How far the observations that have been made since this derangement first commenced, and during its progressive increase, are susceptible of correction, may perhaps appear doubtful: but it is at all events satisfactory to learn that the cause of the errors has been discovered, and a sufficient remedy applied.

On the Finite Extent of the Atmosphere. By W. H. Wollaston, M. D.-The finite or infinite divisibility of matter is a subject that has led to various physical and metaphysical arguments and controversies, without having produced any very satisfactory conclusions. In the present paper, the question is placed under a new point of view, but we are not sure that it will be considered as more conclusive than any that has preceded it.

The principle assumed appears to be that, if matter were infinitely divisible, an elastic medium such as the atmosphere must be of indefinite extent, and consequently that the several bodies of the solar system would attract, about their respective centres, atmospheres that would be proportional to their several attractive powers: therefore, that the solar atmosphere must have at the surface of that body, and at a considerable

1

considerable height above it, an atmosphere of very superior. density to ours at the surface of the earth; the same remark applying, but in a less degree, to the planet Jupiter.

The mass of the Sun being considered as 330,000 times that of the earth, the distance at which its attractive power is equal to that of terrestrial gravity at the surface will be about 575 times the earth's radius, or 5.15 times the solar diameter; which answers to an angular distance of 1° 21', from the Sun's centre; where the refractive power of the solar atmosphere is equal to that of the earth at its surface, and where it would produce a refraction of more than a degree to a ray of light. passing through it. Dr. Wollaston shews that Venus may be observed within this distance of the Sun; and consequently, if the latter body possessed such an atmosphere, it would become known by the refraction which it would produce on the rays of light passing from Venus to the earth: whereas the observations demonstrate that the place of this planet, under these circumstances, agrees with the computed place to a fraction of a minute. Hence the author infers that, at the distance in question, the density of the Sun's atmosphere is not such as it would be if each body in the system possessed an atmosphere proportional to its own attractive power: but, as this must be the case if the elastic matter of the atmosphere were infinitely divisible, it is concluded that matter is not infinitely divisible; and, therefore, that the atmosphere of the earth is of a finite and limited height, and may be peculiar to our planet.

Some doubt, however, may hang over this deduction with respect to the Sun, on account of the probable extreme heat near his surface, which may produce a rarefaction far exceed ing any thing that we can imagine; and, therefore, the author, without insisting much on this deduction, applies the same kind of observation, and draws the same inference from the satellites of Jupiter.

11

Since the mass of Jupiter is full 309 times that of the earth, the distance at which his attraction is equal to gravity must be as √309, or about 17.6 times the earth's radius. And since his diameter is nearly eleven times greater than that of the earth, 17.6 ➡1.6 times his own radius will be the distance from his centre, at which an atmosphere equal to our own should occasion a refrac tion exceeding one degree. To the fourth satellite this distance would subtend an angle of about 3° 37', so that an increase of density to 3 times our common atmosphere, would be more than sufficient to render the fourth satellite visible to us when behind the centre of the planet, and consequently to make it appear on both (or all) sides at the same time.

The

The space of about six miles in depth, within which this increase of density would take place, according to known laws of barometric pressure, would not subtend to our eye so much as

300

of a second, a quantity not to be regarded in an estimate, where so much latitude has been allowed for all imaginable sources of error.

Now though, with reference to the solar atmosphere, some degree of doubt may be entertained in consequence of the possible effects of heat which cannot be appreciated, it is evident that no error from this source can be apprehended in regard to Jupiter; and as this planet certainly has not its due share of an infinitely divisible atmosphere, the universal prevalence of such a medium cannot be maintained; while, on the contrary, all the' phenomena accord entirely with the supposition that the earth's' atmosphere is of finite extent, limited by the weight of ultimate atoms of definite magnitude no longer divisible by repulsion of their parts.

་ ་ ་ ་ ་

On the Expansion in a Series of the Attraction of a Spheroid. By James Ivory, M. A. - The purpose of this paper is to make some observations on the developement of the attractions of spheroids, and on the differential equation which takes place at their surfaces. The subject is too refined, and too much involved in the most abstruse parts of analysis, to admit of an intelligible abridgment.

On the late extraordinary Depression of the Barometer By Luke Howard, Esq. —The close of the last year was remarkable for the very low state of the barometer, not only in London but in all parts of England: whether it extended to the Continent, we are not informed, but in this country the fact was very satisfactorily ascertained, in consequence of a pro position made about two years ago by Mr. Bevin (well known as an ingenious civil engineer) for keeping a register of the state of this instrument in various parts of England, and inviting philosophical observers to take their notes at the same hour and on certain days. This proposition was circulated through "The Philosophical Magazine," in which work many of the observations were published; and thus a fact, which might otherwise have been supposed to be partial, has been indisputably shewn to be nearly general throughout this kingdom.

of Mr. Howard's statement of the oscillations in his barometer, Dec. 24th, being the time of greatest depression, will be interesting to some of our meteorological readers: Be

On the evening of December 24. I found the barometer at 28.20 in., the wind being moderate at S.E., with steady rain, the temp. without, at 8 P. M., 45°. Water boiled freely at 210°.

Finding

Finding the depression still to continue, I took a portable barometer, on Sir H. Englefield's construction, and having ascertained its height to be, at 11 P. M., 27.96 in., I set it up in my chamber on the first floor. At 5 A. M., the 25th, this instrument gave 27.83 in., and I have reason to think it did not go much lower the rain had ceased early in the night, and it had become somewhat star-light, with a calm air, and hazy cirrostrati above: soon after five, however, the wind rose again, bringing some rain, apparently from N.W., but there was no tempest that I had opportunity to observe, though it might have blown hard during the few hours I slept. The pencil of my clock-barometer travelled precisely to twotenths below the bottom of the scale, having made a continous downward sweep of nearly an inch and four-tenths in 24 hours: it appears to have turned to rise abruptly, and by 8 A. M. was again on the point of passing 28 inches. In the 24 hours preceding this time, there had fallen eight-tenths of an inch of rain; in the 24 hours following it there fell none, nor was the wind, which blew from S. W., at all strong; indeed it was calm all the middle part of the day, with sunshine and cirrus above: evaporation was very perceptible, and the night, up to 10 P.M., star-light. The barometer, at 8 P. M. the 25th, was at 28.40 in. In the early morning of the 27th, not having yet reached 29 in., it turned to fall again, with the wind at S. and S. W., after S. E, we had again some heavy rain with hail about noon, and by midnight the quick. silver reached 28.07, or .06 in., where it stood, or rather made minute oscillations, during the twelve hours following, a thing I should scarcely have thought possible in our climate.'

A diminution of pressure to the amount of one-twentieth of the whole, in the course of twenty-four hours, is perhaps an unexampled fact in meteorology; and it is highly interesting not only as connected with that science but on general philosophical principles. What can occasion this extraordinary diminution in pressure? Does the disturbance, to which it owes its origin, takes place in the upper or the lower regions of the atmosphere; and to what natural agent can we attri bute it? These and other similar questions naturally suggest themselves to the mind of the philosophic reader: but at present, we apprehend, they are not likely to receive very satisfactory answers.

On the anomalous Magnetic Action of Hot Iron between the White and Blood-red Heats. By Peter Barlow, Esq., of the Royal Military Academy. -These experiments appear to have been undertaken in consequence of certain theoretical results connected with Mr. Barlow's previous discoveries of the laws of magnetic action; that is, it became necessary, in order to reduce those laws to theory, to be able to ascertain the relative magnetic powers of different species of iron and

steel.