day, 365 values of half-tide level were obtained for the year, varying from 25 inches above the mean to 10 inches below, or to a total extent of 35 inches. These 365 values were tabulated and grouped successively as follows:

1st. The averages for the calendar months respectively were obtained. These do not indicate any sensible annual or semiannual tide.

2nd. The same figures were next grouped according to the lunations, so as to obtain the mean height for every day of the moon's age. The figures showed that there was no sensible tide following these periods.

3rd. Next they were grouped according to the declination, so as to obtain the mean height on the days when the moon was crossing the equator from north to south, and on each successive day till she returned to the same position. From this it appeared that the water was slightly higher when the moon was in north than when she was in south declination; and a similar result was obtained by treating the values for the year 1867 in the same manner, but the difference (1 to 2 inches) was too small to justify any conclusion.

4th. They were then grouped according to the moon's distance from the earth. The means for 1868-69 showed a slight elevation when the moon was near apogee, and a depression when near perigee; but this result was not confirmed for 1867.

From these trials it was concluded that there were no sensible tides of long period due to solar or lunar influence, and that the causes of the variations must be sought for elsewhere. So far, however, the result has been only negative. The range was too great to be attributed to atmospheric pressure, and the want of any persistent elevation during the south-west monsoon showed that they could not be attributed (directly, at least) to local winds.

An extraordinary rise took place in June 1869 to the extent, when at the maximum, of 25 inches. The sea-level was unusually high for about seven days, during which there was great heat and an unsettled appearance in the weather, but no definite atmospheric disturbance. A similar effect was observed at about the same period in 1868, but not in 1870.

ASTRONOMY.

On the Present State of the Question relative to Lunar Activity or Quiescence. By W. R. BIRT, F.R.A.S.

From the time of Schröter the question of change on the moon's surface has been more or less agitated; the 'Selenotopographische Fragmente' contain numerous instances of what he considered to be changes of a temporary character, and a few of a more permanent nature, as the formation of new craters. It is, however, notorious that he failed to establish the fact of a decided change in any one instance; nor is this to be wondered at when we consider the paucity of the materials he had at his command. Notwithstanding the comparative neglect into which the observations recorded in the 'Fragments' have fallen and the judgment passed upon them by some of the best known selenographers, there can be no question that they embody the results of zealous and persevering attention to the moon's surface, and ought not to be passed over in the examination of any given spot the history of which we are desirous of becoming aquainted with during the earliest period of descriptive observational selenography.

The labours of Schröter's successors, Lohrmann and Beer and Mädler, have added greatly to the number of objects, either as delineated on their maps or referred to in their letterpress. Lohrmann appears to have carefully studied Schröter's results, as we find him quoting the measures obtained by Schröter in several instances. On examining the results of the two greatest selenographical works of the present century and comparing the one with the other, we find precisely the same kind of phenomena presenting themselves, which in a great measure perplexed Schröter; but as Lohrmann and Mädler worked independently of each other, and Mädler evidently had a very low idea of the value of the preceding labours of Schröter, these plienomena passed unnoticed at the time. Upon consult

ing the three works for elucidating the history of any given object, such results as these are frequently obtained. An object is found in Schröter designated by a Greek or other character, and its appearance described in his text. This object may be altogether omitted by Lohrmann, but given on Beer and Mädler's map; and objects are by no means rare which may be found on Lohrmann, but omitted by Beer and Mädler, and vice versa.

Were the results of the labours of Julius Schmidt during a period of nearly thirty years given to the public, there can be no doubt that our knowledge of selenography would be greatly advanced. His chart must contain a large proportion of the objects previously recorded by Schröter, Lohrmann, and Beer and Mädler; and, judging from the instances already alluded to, of apparent omissions by one or other of the above-named observers, it is highly probable that the number of such instances would be much increased. The value of his measures (4000) of the altitudes of lunar mountains for comparison with or addition to those of Schröter and Mädler cannot admit of a doubt. His published catalogue of rills is very valuable in this respect. It is to Schmidt that we are indebted for one of the most important announcements bearing on the subject of lunar activity, that of a change in the crater Linné, "which," says Mädler (Report Brit. Assoc. 1868, p. 517), "has hitherto offered the only authentic example of an admitted change." He had previously said (same Report), "what has lately been observed in the crater Linné proves at all events that there real changes have taken place, and that too under circumstances even visible to us." Further on, however, the great selenographer remarks that on the 10th of May, 1867, his eye having undergone an operation for cataract, he attempted an observation of Linné in the heliometer of the observatory at Bonn, and found it shaped exactly, and with the same throw of shadow, as he remembered to have seen it in 1831. "The event," he says, "of whatever nature it might have been, must have passed away without leaving any trace observable by me." The doubt still hanging over this object is well known, and it may be regarded as furnishing at least one of the instances of the present state of the question of activity. The uncertainty attaching to the question of change in this particular instance mainly arises from the difficulty of deciding upon the accuracy or otherwise of the delineations of Lohrmann and Beer and Mädler, although both describe it as showing a diameter of five or six English miles. Generally speaking, the observations between October 1866 and July 1870 all agree in its present appearance, differing greatly from that which it must have presented according to the delineations and descriptions of the two selenographers just named; also that no change of a physical character has taken place in it during the 3 years it has been under constant observation.

It has been supposed that photography would solve all such difficulties, and that photograms of the lunar surface taken under similar angles of illumination and visual ray would agree with each other; but here, again, precisely the same difficulties present themselves which perplexed Schröter, and which have been met with in comparing Lohrmann's and Beer and Mädler's works. Objects figured by the earlier selenographers occur on some photograms, but not on others of about the same phase of illumination. There appears to be an agency capable of affecting the visibility of objects, rendering them indistinct or invisible on some occasions, while on others they are distinctly seen on the photograms. Whatever operations may have taken place in the crater Linné, producing phenomena the recurrence of which is rare, in all the examples above mentioned, from Schröter's time to the present we have phenomena of a different character, exceedingly difficult of explanation, and constituting an important element in the solution of the question of present activity or quiescence; for unless it be fully proved that all these instances depend upon changes of visual and illuminating angles, a strong suspicion will exist of their being more immediately connected with the moon itself. To effect such a proof, however, is a matter of no small difficulty. Mädler alludes to the performance of calculations of the most varied kind as necessary for the delineation of lunar features; and in the case before us the calculation of several elements for each separate observation (and they are very numerous) is absolutely essential for the purpose of referring the phenomena observed to changes of illumination and visual ray. Calculations of this kind have not yet been made to any great extent,

and the consequence is that the entire question remains involved in doubt. During the last seventeen months as many as 1227 observations of the spots on Plato alone have been made; and although the varying state of the earth's atmosphere affects in no slight degree the visibility of such delicate objects, phenomena are presenting themselves which call for a much more rigorous treatment than has yet been accorded to them. The affirmation of change on, or quiescence of, the moon's surface must depend, not upon the accumulation of desultory and undiscussed observations, but upon such as are undertaken on a well-arranged system and discussed with reference to every known agency capable of affecting them. The present state of the question is therefore one of doubt, one that calls for observation of the most vigorous character and discussion of the most rigorous nature to settle it. Observation of late has been tending towards a registration of minute detail detected on the moon's surface, but discussion in various ways is behind the requirements of selenography, and until it can keep pace with observation the doubt alluded to above must remain,

On the Distribution of Cometic Perihelia. By A. S. DAVIS, B.A.

The hypothesis that those comets whose orbits are undistinguishable from parabolas are moving in hyperbolic, non-periodic orbits, leads to the following theoretical law for the distribution of their perihelia. The number of comets within any distance from the sun is proportional to that distance. This follows from an investigation contained in a paper on cometary orbits published in the Philosophical Magazine for September 1870. The first object of the present paper was to show that the actual distribution of the perihelia of parabolic and hyperbolic comets is probably in accordance with this law. For this purpose the numbers of comets having perihelion distances lying respectively between 0-0 and 0.1, between 0·1 and 0.2, and so on, were ascertained. It was found that, instead of these numbers being nearly equal, they were respectively 11, 10, 14, 17, 11, 33, 18, 23, 21, 24, 15, 10, 8, 4, 5, 1, 1. It was then shown that this want of agreement with the theoretical law of distribution might arise from the fact that the probability that a comet will be observed depends upon the magnitude of its perihelion distance, those comets being most likely to be observed which have perihelion distances rather less than the radius of the earth's orbit. That this cause does produce a considerable effect upon the apparent manner of distribution of perihelia was shown thus:-The known comets were arranged in three groups containing respectively those which appeared before 1750, between 1750 and 1800, and between 1800 and 1865. The numbers representing the distribution of perihelia for these three groups were respectively

1, 3, 2, 8, 3, 16, 4, 7, 3, 6, 2, &c.,

4, 2, 6, 3, 7, 7, 7, 10, 6, 7, 5, 2, 1, &c.,

6, 5, 6, 6, 1, 10, 7, 6, 12, 11, 8, 8, 7, 4, &c.,

showing that the distribution of the perihelia of a later group are much more nearly in accordance with the theoretical law than that of an earlier group, owing doubtless to the fact that the probability that a comet would be detected was formerly more dependent upon the magnitude of its perihelion distance than it now is. It seems probable that if all the comets which visit the sun were observed, the distribution of their perihelia would be nearly in accordance with the above-stated theoretical law. Such an accordance the author thought would be an argument in favour of the theory that the parabolic comets are non-periodic.

In the second part of the paper the author showed that a statement made by Prof. Kirkwood regarding the distribution of cometic perihelia was incorrect. Prof. Kirkwood, on finding that the longitudes of the perihelia of a large proportion of those comets with very small perihelion distances do not differ greatly from the longitude of the solar apex, concluded that this fact was due to a crowding of the perihelia about the solar apex, produced, he thought, by the sun's motion in space, Prof. Kirkwood had not shown that the latitudes of the perihelia were nearly the same as the latitude of the solar apex. The author found, by marking upon a celestial globe the positions of the perihelia of all parabolic and hyperbolic comets with

perihelion distances less than 5, that the perihelia of comets with very small perihelion distances exhibit no greater tendency to crowd about the solar apex than comets with larger perihelion distances.

On Solar Spots observed during the past Eleven Years.
By the Rev. FREDERICK HOWLETT, F.R.A.S.

The paper was illustrated by numerous very carefully executed drawings, enlarged from others which had been micrometrically observed and drawn at the telescope, chiefly by means of projecting the sun's image on a screen. It was well known how rich the years 1859 and 1860 were in solar spots; and the eleven-year period was again being strikingly corroborated by the number and size of the groups and individual spots of the present year, and which may be expected to prevail until 1871. Magnificent groups which appeared in the sun's northern hemisphere in March, April, and August, in almost precisely the same heliographic latitude and longitude, would apparently seem to evince that the disturbing causes, whatever they were, had localized themselves on the disk-not, however, without long intervals of comparative repose. The forms assumed by the faculæ were described by the author, who felt convinced that they were attached, for the time being, to the photosphere, and that they were not clouds floating above it; otherwise they would frequently impinge on the penumbræ in ways very different from those in which, in point of fact, they are seen to occur. If they consist of simply photospheric matter, however, it would seem to be in some compressed or otherwise peculiar manner; inasmuch as the coarser mottling, so plainly to be distinguished on all other parts of the sun's surface, can never be detected on the faculæ, and especially on those masses enclosed more or less at times within the receding margins of the penumbra. Dr. Huggins, however, has detected the finer or rice-grain specks of light in some of the more diffused forms of the faculæ. There is apparently no direct relationship between the amount of solar-spot disturbance and the terrestrial magnetic storms. The author, however, has suggested the possibility of there being some degree of correlation between groups of a peculiarly cyclonic arrangement and unusual magnetic disturbances; none, or next to none, of the spots had been found to possess any tendency to rotate as it were on an axis-as has, however, been occasionally witnessed by other observers. An instance was given (illustrated by a drawing) how a diffused penumbral speck was observed to draw in towards the neighbouring umbra of a solar spot at the rate of 12 seconds of arc in four hours, which is equivalent to about 660 yards per second (and closely similar to observations of the same kind by Chacornac). As the speck drew in towards the umbra it assumed a continually more narrow and wedge-shaped form (the apex towards the direction of advance), and which, therefore, might well be taken to indicate that down-rush into the umbra aforesaid insisted upon by Mr. Norman Lockyer. Assuming, as the author does, that the spots are depressions in the solar photosphere, filled up by the solar gaseous atmosphere, this is evinced, 1st, by the ordinary testimony of the eye; 2ndly, by the stereoscopic effect obtained by Mr. De La Rue's photographs of spots taken at intervals of about two days; 3rdly, by the foreshortening of the penumbra of a neat circular spot, alternately on the right and left side, as it first comes on, and then passes off the disk-a phenomenon first noticed by Dr. Wilson in the last century; and 4thly, by the elegant spectro-barometric evidence (as the author termed it), whereby the progressive thickening of the dark solar absorption-lines, as they pass successively over the spectrum of the photosphere, penumbra, and umbra, seemed to prove an increasing density and depth of an absorbing solar atmosphere. It is, however, urged by Kirchhoff, Donati, and some others (and in a measure admitted by Browning) that like effects similar to those four above enumerated might be produced were the spots cloudy condensations, and not depressions. The author called attention to a delicate way (not readily to be noticed without projecting the sun's image on a screen) in which a fine trailing serpentine arrangement of minute specks of penumbral matter may be sometimes seen either following in the wake of a large spot, or meandering amidst a group of spots, indicating the resolution of two or more forces, partly, perhaps, cyclonic, and partly centrifugal, as connected with the sun's axial rotation.

On Shooting-stars. By the Rev. R. MAIN, M.A., F.R.S., F.R.A.S. The author presented to the Association the observations of meteors and shootingstars made almost exclusively by Mr. Lucas, at the Observatory, during the past year. The whole number of meteors seen and observed during the year is about 300, of which, of course, a considerable number belong to the November and August groups. Next to these, the month of October seems to be the most fruitful, while, on the contrary, in January, February, and March scarcely any were seen. This, however, may be due to the cloudy state of the sky which generally prevails at Oxford in the early part of the year. Of those observed, more than fifty were as bright or brighter than stars of the first magnitude; six were as bright as Jupiter; one, observed on November 4, 1869, was estimated as equal to one-sixth of the full moon; and one, on December 29, as equal to one quarter, the sky being overspread in the latter case by light clouds. Two remarkable meteors were seen on November 6, one of which was observed (by a person not connected with the Observatory) to burst with noise near the north-west horizon. On July 8 one was seen to burst, though the sky was so overcast with nimbus cloud that no star could be seen.

On the Laws of Star-grouping. By R. A. PROCtor.

The aim of this paper was to show that the stars are grouped together in varied forms, separated by comparatively barren distances, and that the observed peculiarities of stellar distribution are due to real laws of aggregation and segregation.

ELECTRICITY AND MAGNETISM.

On Faure's Battery. By C. BECKER.

Faure's element is a modification of that known as Bunsen's, the poles consisting of carbon in strong nitric acid and amalgamated zinc in dilute sulphuric acid. In Bunsen's ordinary form of carbon element the carbon pole is immersed in a vessel holding a considerable quantity of nitric acid, which, as it becomes deoxidized by the electrolytic action of the current, liberates nitrous-acid gas, which rises into the air, rendering it unwholesome to breathe and destructive to most metallic apparatus in its neighbourhood. The purpose of Faure's battery is to obviate those drawbacks. This is effected by confining the nitric acid inside the carbon pole, and allowing only sufficient acid to percolate through it in order to keep up the necessary electrolytic action of the element. The carbon pole is made in the form of an ordinary bottle, and is provided with a carbon or platinum stopper to which the bindingscrew of the pole is attached. This bottle, which at once fulfils the functions of pole and porous diaphragm, is placed concentrically in the interior of a cylinder of amalgamated zinc; and the whole is contained in an earthenware jar. When set up for action the bottle is nearly filled with nitric acid, and the space containing the zinc between the bottle and the outer jar to the required height with sulphuric acid. The slight liberation of gas within the bottle causes a sufficient pressure to be exerted upon the nitric acid to force it gradually through the carbon.

In this way the exterior of the carbon pole remains immersed in a very thin layer of nitric acid immediately opposite the zinc, which is in course of dissolution of the dilute sulphuric acid. In point of constancy the element is superior either to Bunsen's or Grove's, because the body of nitric acid remaining protected within the bottle does not become weakened as in the case with those forms of element in which the fluids are exposed in larger quantities and separated by porous diaphragms. It acts also entirely without any disengagement of gas into the air, so that it may be used in any room without disagreeable consequences. A variety of forms might no doubt be given to these elements, which would enable them to fulfil the desired object. Those exhibited were designed and manufactured by Messrs. Elliot Brothers, and are found to be convenient for experimental purposes as well as for use in telegraph offices.