On the evening of February 2, Jupiter was passing near the star B. A. C. 303 (73 Piscium), and the opportunity was taken at the observatory of Harvard College to compare photometrically the third satellite of the planet, with the star. Three observers took part in the work, and four sets of measurements, each consisting of eight single comparisons, were made. The result obtained was that the star was fainter than the satellite by 0.38 magnitudes of POGSON'S logarithmic scale. For the magnitude of the star we have 6.16 by the mean of the available estimates on record, and 6.17 by the observations made at this observatory with the meridian photometer. The resulting magnitude of the satellite is 5.28 or 5.29, in close agreement with the value, 5.24, found by a very different method, in the Annals of the Observatory, Vol. XI, p. 276.

Saturn.-A letter from Prof. HASTINGS, of the Johns Hopkins University, puts an important point in regard to the nature of Saturn's rings (C, the dusky ring, B, the inner bright ring, A, the outer ring) so briefly that it may be quoted. "The ring C projected on Saturn is dark; if the ring were composed of particles having the same albedo as the planet it would appear neither bright nor dark, as it would return to us by reflection the exact equivalent of what it intercepts. But the albedo of B is greater than that of the planet since it is the brightest part of the system. Hence either the material of C is different from that of B, or the planet is self-luminous."

Possible planets beyond Neptune.-Prof. FORBES, of Edinburg, and Mr. D. P. TODD, of Washington, have, during the year, published accounts of their theoretical proofs of the existence of a planet or planets beyond Neptune. Prof. FORBES founds his conclusions upon the statistical distribution of the aphelia of comets. These have long been known to be grouped at certain distances from the sun, with empty spaces so far as known between groups. There is a group of eleven comets whose aphelia are at Jupiter's mean distance, 5, and another group of six comets about Neptune's distance, 30.

Another well marked group of seven comets have aphelion distances of about 100.

The theory of the introdution of comets into the solar system, proposed a few years since by Prof. Newton, of Yale College, is made the basis of the investigation, and according to this, any comet which is drawn into our system from outside and its elements so changed as to make it a permanent member of the system must have been so influenced by a planet which was somewhere near the aphelion point of this comet's orbit at the time of its introdution.

If the aphelion points of the seven comets of the last group described be marked on a globe, it is found that they lie on or near a great circle. Prof. FORBES further finds that a planet revolving at a distance 100 in a periodic time of 1,000 years might have been at the points so marked during the period of two revolutions of the comets of this group, and from

this and other reasons it is concluded that there is such a planet revolving at the distance of about 100, which is at present in R. A. 11h 40m N. P. D. 87°.

Similar considerations are applied to the next group of comets of aphelion distances about 300, and evidence of a disturbing planet at this distance also is adduced. Prof. FORBES has applied the method he has used to the case of the comets which we know to revolve near Neptune, and, without any knowledge of Neptune's real position, he found that the aphelion positions of these comets indicated the existence of a planet in longitude 45°. In fact, Neptune is now in longitude 48°. From all of which Prof. FORBES concludes the existence of at least one ultra-Neptunian planet in the position indicated, i. e., for 1880, R. A. 11h 40m N. P. D. 87°.

The publication of Prof. FORBES's results has caused Mr. TODD, of Washington, to print the results of some investigations he has made in the motions of Uranus and Neptune as compared with Newcomb's tables of these planets. Proceeding by a graphical method he was led to the conclusion that there must be a planet exterior to Neptune revolving in an orbit inclined about 110, whose ascending node is at longitude 1030, and whose longitude is about 1730, or, for 1881, March, its position is R. A. 11h 37m N. P. D. 87°.

In other words, Prof. FORBES and Mr. TODD, proceeding on utterly different data, have arrived at identically the same result. In each case the data are insufficient and the exact agreement is accidental. Still it is noteworthy.

It is extremely doubtful if such a planet would be recognizable by its disk at this distance from the sun. Still, Mr. TODD made a search for it with the 26-inch telescope at Washington, without success however. The true method of research would be the accurate mapping of this portion of the sky-a work of several years.

Minor Planets.-The following discoveries were made in 1879 :

Frigga (77) was refound by PETERS on July 17, 1879, after having been at large for several years. Prof. PETERS suggests that Frigga may be variable.

The Jahrbuch for 1882 gives some details regarding the first 173 asteroids, which we condense as follows: Out of these 173, there are 120 which have been satisfactorily observed at 5 (or more) oppositions, 11 which have been observed at 4 oppositions, 10 at 3, 14 at 2, and 18 at 1 only. Of these last 18, some are new discoveries and only Dike (BORELLY), and Scylla (PALISA) are yet to be rediscovered. Juewa (WATSON) has been lately found, after having been lost six years.

The Moon.-The lunar photographs taken with the 13-inch reflector at the University Observatory, Oxford, have been utilized for finding a new value for the semi-diameter of the Moon. The process adopted was very nearly the same as that employed by WICHMAN, and the resulting mean semi-diameter is 15' 34" .175 +0.069. Mr. NEISON has, from a careful discussion of nearly 1,100 observations, made at Greenwich, Oxford (RADCLIFFE), and Washington, deduced the value

15' 33".37+4".10÷(1+0.70 x aperture in inches). This empirical formula, he adds, agrees closely with the theoretical formula, founded on the assumpution that the differences between the semi-diameters obtained with instruments of different aperture vary as the diffraction discs and the amount of light. Applying the formula to a 13-inch reflector, and adding the photographic irradiation, which Mr. NIESON states his experiments indicate to be about + 0.3, the photographic semi-diameter should be 15' 34".08.

No papers on the Moon published during the past year exceed in importance those by Mr. G. H. DARWIN on the secular effects of tidal friction on the configuration of a planet and its satellite. In the first paper the theory of the deformation of a viscous or imperfectly elastic spheroid under the attraction of satellites is investigated. Numerical calculations show that bodily tides in the Earth are at present very small. From this point Mr. DARWIN was led to consider the perturbed rotation of such a spheroid, and the reaction on the perturbing bodies. He has in the A. N. 2294 given the following summary of the results arrived at

through these investigations, which, no doubt, mark the beginning of a new era in our knowledge of the past history of the solar system:

1. The lunar period must have been shorter in the past, and may be traced back from the present 27.3 days, until initially the Moon revolved round the Earth in from two to four hours.

2. The inclination of the orbit to the "proper plane" must have been larger in the past, and may be traced back from the present 5° 9′ until it was 6 or 7°. This was a maximum inclination, and in the more remote past the inclination was less and initially was very small

or zero.

3. The inclination of the proper plane to the ecliptic must have been greater in the past, and may be traced back from its present 8" until it was in very early times about 11° 45'. It is possible that initially this inclination was less, and that the 11° 45′ was a maximum value.

4. The eccentricity of the orbit must have been smaller in the past. Either at one time it had a minimum value, before which it had a maximum value, and, again, earlier it was very small or zero, or else the maximum value never occurred, and the eccentricity has always been increasing. The history of the eccentricity depends on the nature of the tides in the Earth, but the former of these alternatives seems the more probable.

At the present time the Earth rotates in twenty-four hours, its equator is inclined at an angle of 9" to a plane which Mr. DARWIN calls "the proper plane of the Earth" (the mean equator). This proper plane is inclined at an angle of 23° 28′ to the ecliptic, and its intersection with the ecliptic is the equinoctial line. He finds that the frictional tides in the Earth have caused changes which may be summarized as follows:

5. The day must have been shorter in the past, and it may be traced back from its present value of twenty-four hours until initially it was from two to four hours in length. It was then identical with the Moon's period of revolution as described in (1).

6. The inclination of the equator to the Earth's proper plane must have been larger in the past, and may be traced back from the present value of 9" until it was about 20 45'. This was a maximum inclination, and in the more remote past the inclination was less, and initially it was very small or zero.

7. The inclination of the Earth's proper plane to the ecliptic must have been smaller in the past, and may be traced back from its present value of 23° 28' until initially it was 11° 45', or, perhaps, somewhat less. It was then identical with the proper plane of the lunar orbit, and this is true whether or not 11° 45' was a maximum inclination of the lunar proper plane to the ecliptic as described in (1).

The preceding statements may be subject to varieties of detail, according to the nature of the tides raised in the Earth, but the above is a summary of what appears to be the most probable course of evolution.

The hypothesis which is suggested as most probable is, that the more recent changes in the system have been principally due to oceanic tidal friction, and that the more ancient changes were produced by bodily tidal friction. The initial state of the Moon, nearly in contact with the Earth, and always opposite the same face of it, suggests that the Moon was produced by the rupture in consequence of rapid rotation or other causes of a primeval planet, whose mass was made up of the present Moon and Earth. It is a remarkable coincidence, that the shortest period of revolution of a fluid mass of the same mean destiny as the Earth, which is consistent with an ellipsoidal form of equilibrium, is two hours and twenty-four minutes; and that if the Moon were to revolve about the Earth with this periodic time, the surfaces of the two bodies would be almost in contact with one another. The theory gives an interesting explanation of the rapid movement of the inner satellite of Mars.

OBSERVATORIES.

The buildings of the new observatory of the University of Strasburg have been completed, and a description and plates of them made by the director, Dr. WINNECKE. The principal instruments are:

1. A meridian circle by REPSOLD; aperture 162 millimeters (6.4 inches). 2. An altazimuth by REPSOLD; aperture 136 millimeters (5.4 inches). 3. A refractor by MERZ; aperture 487 millimeters (20.2 inches). 4. An orbit-sweeper by REPSOLD; aperture 163 millimeters (6.4 inches).

There are three buildings-one large one containing the residences of the astronomers, and two smaller.

On the main building is the large iron dome, some 39 feet in diameter. There are two novelties in its construction: First, the shutters to the dome are made in two parts, and extend from horizon to horizon. They are opened symmetrically by screws, and leave the whole 180° open at once. Secondly, the dome, which weighs about 93,000 pounds, is turned by means of heavy weights, which are wound up from time to time as required. A touch from the observer will release these, and their fall will turn the dome right or left as required. The working of this dome will be attentively regarded, as most of the difficulties of such structures appear to have been successfully avoided. In windy situations the whole slit must not be opened at the same time, but the principle of these shutters properly modified will serve even in this case. The dome of the new Naval Observatory at Washington is to be of the turret construction, by which form the difficulties of shutters can be evaded.

The 27-inch refractor for the Vienna observatory has been finished by Mr. GRUBB, and is now mounted, and is said to be satisfactory. An elaborate description of it is published by Mr. GRUBB as a reprint from Engineering.

Mr. A. A. COMMON has mounted his new 36-inch silver-on-glass equa torial at Ealing, near London. It is described in "The Observatory,"